US 5,356,775 AGrant
Primary Structure for Functional Expression From Complementary DNA of a Mammalian ATP-sensitive Potassium Channel
Issue Date:1994-10-18
•10 Claims
•15 Drawing Sheets
Abstract
This invention is directed to the cloning of the gene which encodes an ATP-sensitive K.sup.+ channel in rat outer medulla cells, isolated cDNA sequences which encode said ATP-sensitive K.sup.+ channels, isolated proteins produced by said cDNA sequences, and agents capable of binding to said proteins. Further included in the invention are methods for identifying other members of the family of ATP-sensitive potassium channels (the ROMK1 family of channel proteins), identifying, isolating, and cloning the genes which encode ROMK1 associated polypeptides, identifying agents capable of binding to other members of the family, modulating expression of said family of ATP-sensitive potassium channels, and modulating the activity of said family of ATP-sensitive potassium channels. Additionally, included in the invention are methods for identifying drugs which function as K.sub.ATP channel openers and K.sub.ATP channel closers.
Metadata
Assignee
- Brigham & Women's Hospital
Inventors
- Steven C. Hebert
- Kevin Ho
Application Information
Application Number:US 9211780
Filing Date:1992-07-29
Priority Date:1992-07-29
Art Unit:184
Classifications
IPC:
C12N 1512C12N 1510C12N 1563
Field of Search:
43553693569.1;172.3;320.1;252.3;240.2;623.511;24;56
Patent Drawings (15 sheets)
Description
FIELD OF THE INVENTION
This invention relates to the cloning of a member of a previously uncloned family of potassium channels K.sub.ATP channel). Specifically, the present invention discloses the cloning of ROMK1 from rat kidney outer medulla cells. Based on this disclosure, the present invention provides: 1) isolated ROMK1 cDNA sequence; 2) isolated ROMK1 protein; 3) agents capable of binding to the ROMK1 protein; and 4) methods for a) identifying other members of this potassium channel family, b) identifying agents capable of binding to members of this family, c) modulating the expression of ROMK1, d) modulating the activity of ROMK1, and e) identifying and/or testing for drugs which function as K.sub.ATP channel openers and K.sub.ATP channel closers.
BRIEF DESCRIPTION OF THE BACKGROUND ART
ATP-sensitive K.sup.+ channels (K.sub.ATP) comprise a distinct family of potassium channels based on their biophysical, functional, and pharmacological characteristics. Five classes of K.sub.ATP channels are recognized based in part on differences in single channel conductance, ATP-sensitivity, pharmacology, and ion selectivity. Major properties exhibited by K.sub.ATP channels, best exemplified by the extensively characterized Type 1 channels from pancreatic beta-cells, include: reversible inhibition by intracellular ATP; rapid loss of channel activity in membrane patches following excision (channel rundown); MgATP-dependent maintenance of channel activity or reactivation following rundown in excised patches; inward rectification; and limited voltage-dependence in contrast to voltage-gated ion channels (Ashcroft et al., Cellular Signalling 2:197 (1990); Ashcroft, F. M., Ann. Rev. Neurosci. 11:97 (1988)).
Since the initial description of K.sub.ATP channels in 1983 by Noma in cardiac muscle (Noma, A., Nature 305:147 (1983)), there has been tremendous interest in the role of these metabolically-regulated channels in diverse physiological and pathophysiological processes involving a wide variety of both excitable (pancreatic .beta.-cells, central neurons, cardiac, skeletal and smooth muscle cells) and non-excitable cells (renal tubular and respiratory epithelial cells) (Ashcroft et al., Cellular Signalling 2:197 (1990); Ashcroft, F. M., Ann. Rev. Neurosci. 11:97 (1988); Lang et al., Physiol. Rev. 72:1 (1992); Kunelmann et al., Pflugers Arch. 414:297 (1989)). In part, this attention has resulted from the demonstration that sulfonylureas (e.g., tolbutamide, glibenclamide) specifically inhibit and that potassium channel openers (PCOs, e.g., diazolxide, cromakalim, pinacidil, nicroandil) activate these channels in a tissue-specific manner (de Weille et al., Pfluger Arch. 414:S80 (1989); Sanguinetti, M. C., Hypertension 19:228 (1992)). Inhibition of K.sub.ATP channels by glucose metabolism or by sulfonylureas results in cellular depolarization and the release of insulin from pancreatic .beta.-cells (Ashcroft et al., Biochem. Soc. Trans. 18:109 (1990)) and .gamma.-aminobutyric acid (GABA) from substantia nigra cells, the latter being involved in seizure control (Amoroso et al., Science 247:852 (1990)). These channels also appear to be involved in ischemia-induced alterations in cardiac myocyte electrical activity and in the regulation of smooth muscle tone. Channel activation by hypoxia, metabolic insult, or PCOs is thought to result in action potential shortening together with both antiarrhythmic and proarrhythmic effects in cardiac muscle (Nichols et al., Am. J. Physiol. 261:H1675 (1991)) and in vascular smooth muscle relaxation (Amoroso et al., Science 247:852 (1990)).
The successful molecular characterization of voltage-gated ion channels (K.sup.+, Na.sup.+ and Ca.sup.2+) (Perney et al., Curr. Opin. Cell Biol. 3:663 (1991); Stuihmer, W., Annu. Rev. Biophys. Chem. 20:65 (1991); Miller, R. J., J. Biol. Chem. 267:1403 (1992)), cyclic nucleotide-activated channels (Kaupp et al., Nature 342:762 (1989); Dhallan et al., Nature 347:184 (1990)) and more recently of a Ca.sup.2+ -activated K.sup.+ channel component (Atkinson et al., Science 253:551 (1991)) has greatly advanced our understanding of ion channel structure-function relationships and regulation and has revealed both common and distinctive features of each ion channel family. Voltage-gated Na.sup.+ and Ca.sup.2+ channel proteins contain four internal homologous domains with each domain consisting of six transmembrane segments and a pore-forming H5 region, while K.sup.+ channels are a tetrameric complex of polypeptides, each containing only one of these domains. To date, all K.sup.+ channels that have been cloned belong to either the superfamily of voltage-gated and second messenger-gated channels (Jan et al., Cell 69:715 (1992)) or to a class of channels composed of proteins with only a single membrane-spanning segment (Takumi et al., Science 242:1042 (1988)). The isolation of a K.sub.ATP channel protein or a cDNA clone, however, has remained elusive. Screening of cDNA libraries by Shaker sequence-derived oligonucleotide probes has resulted in the discovery of new members of the Shaker K.sup.+ channel family, but not in the identification of a K.sub.ATP channel. Moreover, approaches based on the affinity labelling of proteins from brain and a .beta.-cell line using sulfonylurea analog have not yielded functional channel proteins (Bernardi et al., Proc. Natl. Acad. Sci. USA 85:981 6 (1988); Aguilar-Bryan et al., J. Biol. Chem. 265:8218 (1990)). Thus given the unavailability of structural information, it has not been possible to directly address issues regarding K.sub.ATP channel gating and regulation by ATP, phosphorylation, and G protein interactions, the types and number of channel regulatory sites, the nature of the K.sub.ATP channel ion-conducting pore, and the mechanisms of action of pharmaceutical agents.
SUMMARY OF THE INVENTION
The present invention is based on the cloning of a member of a previously uncloned family of potassium channels. Specifically, the present invention discloses the cloning, cDNA sequence, and amino acid sequence of ROMK1, a K.sub.ATP channel isolated from rat kidney outer medulla cells.
Based on this disclosure, the present invention provides isolated ROMK1 cDNA, vectors containing ROMK1, vectors capable of expressing ROMK1, and hosts transformed with vectors capable of expressing ROMK1.
The invention further provides methods of obtaining other members of this novel family of potassium channel proteins, hereinafter the ROMK1 family of channel proteins. Specifically, by using the sequence disclosed herein as a probe or as primers, and techniques such as PCR cloning and colony/plaque hybridization, one skilled in the art can obtain other members of this unique family of channel proteins.
The invention further provides isolated ROMK1 protein. Such a protein can be purified from natural sources or from cells which are engineered to express ROMK1 using standard purification techniques such as immunoaffinity chromatography.
In yet another embodiment, the invention provides for the identification, separation, purification and cloning of the genes which encode polypeptides which are associated with the ROMK1 family of channel proteins (hereinafter ROMK1 associated polypeptides).
Using the purified ROMK1 protein, the present invention provides methods of obtaining and identifying agents capable of binding to ROMK1. Specifically, such agents include antibodies, peptides, carbohydrates and pharmaceutical agents. The invention further provide detectably labeled, immobilized and toxin-conjugated forms of these agents.
The present invention further provides DNA constructs which transcribe a message which is capable of hybridizing to the message encoding ROMK1. In members of the ROMK1 family of channel proteins, such constructs are generated by placing the ROMK1 sequence in an expression vector which contains a promoter capable of transcribing the antisense strand of the ROMK1 sequence. Using such a construct, the present invention provides methods of modulating the expression of ROMK1.
The present invention further provides methods of modulating the activity of ROMK1 in a cell. Specifically, agents which are capable of binding to ROMK1 are provided to a cell expressing ROMK1. The binding of such an agent to ROMK1 can be used either to activate or inhibit the activity of ROMK1.
In addition, the present invention provides methods for identifying ROMK1-associated polypeptides that are involved in regulating or modulating ROMK1 function.
The present invention further provides methods of selectively killing cells expressing ROMK1. Specifically, a cell expressing ROMK1 can be selectively killed by providing to the cell an agent capable of binding ROMK1 which is conjugated to a cytotoxin.
The invention further provides for screening technology for the identification and/or testing of drugs for their ability to function as K.sub.ATP channel openers and K.sub.ATP channel closers.
DESCRIPTION OF THE FIGURES
FIG. 1. Ba.sup.2+ -sensitive potassium currents (I.sub.K(Ba)) expressed in Xenopus oocytes injected with rat kidney ISOM (inner stripe of outer medulla) poly(A).sup.+ RNA or mRNA transcribed in vitro from ROMK1 cDNA.
(A) Representative current tracings evoked by 450 ms voltage steps ranging from -80 mV to 0 mV in 20 mV increments from a holding potential (V.sub.H) of -60 mV in oocytes injected with .about.25 ng ISOM poly(A).sup.+ RNA or 50 nl H.sub.2 O [K.sup.+ ].sub.ext =50 mM. Addition of 5 mM Ba.sup.2+ resulted in marked inhibition of currents expressed in RNA-injected oocytes to the level of background currents in H.sub.2 O-injected controls over the range of test potentials used.
(B) Current-voltage relationships of I.sub.K(Ba) in a representative ISOM poly(A).sup.+ RNA-injected oocyte ( ) and a H.sub.2 O-injected control (.circle.). RNA-injected oocytes exhibited a mean I.sub.K(Ba) =-335.+-.19 nA, n=3] in comparison to H.sub.2 O-injected controls with a mean I.sub.K(Ba) =-86.+-.8 nA, n=28 (V.sub.m =-75 mV, [K.sub.+ ].sub.ext 32 50 mM). Both current amplitudes and V.sub.m were measured 75 ms after initiation of test pulses; Ba.sup.2+ -sensitive current (K.sub.K(Ba)) amplitudes were obtained by determining the difference in current amplitudes before and after the addition of 5 mM Ba.sup.2+ at each test potential and plotted as a function of observed V.sub.m.
(C) External K.sup.+ -dependence of reversal potentials for I.sub.K(Ba) in poly(A).sup.+ RNA-injected oocytes. [K.sup.+ ].sub.ext was varied by replacement with NMDG.sup.+ ( , n=7, (mM) 2KCl, 96 NMDG.sup.+, 10 KCl, 88 NMDG.sup.+, 30 KCl, 68 NMDG.sup.+, 90 KCl, 8 NMDG.sup.+) or with Na.sup.+ (.circle., n=4, (mM) 2 KCl, 96 NaCl, 50 KCl, 48 NaCl) in perfusion solutions containing 0.3 CaCl.sub.2, 1 MgCl.sub.2, 5 HEPES, pH 7.6. Points represent mean values for E.sub.rev (error bars, SEM) obtained from the number of oocytes indicated. E.sub.rev varied linearly with log [K.sup.+ ].sub.ext, a linear regression line fitted to the data shows a slope of 50 mV per tenfold change in external K.sup.+.
(D) I.sub.K(Ba) expressed in Xenopus oocytes injected with pools (#1-8) of size-fractionated ISOM poly(A).sup.+ RNA ranging from .about.2 to 4 kb (hatched bars) compared with H.sub.2 O-injected controls (open bar); bars represent mean I.sub.K(Ba) values from 3 to 12 oocytes. Currents were evoked by 450 ms test potentials from a V.sub.H =-50 mV. Oocytes injected with mRNA of .about.2 to 3 kb (pool #3) exhibited a mean inward I.sub.K(Ba) =-1307.+-.120 nA, n=4; H.sub.2 O-injected controls showed a I.sub.K(Ba) .revreaction.-83.+-.14 nA, n=8 (V.sub.m =-75 mV, [K.sub.+ ].sub.ext .revreaction.50 mM).
(E) Representative current tracings in oocytes injected with .about.0.3 ng ROMK1 mRNA or H.sub.2 O evoked by 450 ms test pulses from a V.sub.H =-45 mV to test potentials from -60 mV to 0 mV in 20 mV increments in the presence or absence of 10 mM Ba.sub.2+, [K.sub.+ ].sub.ext =50 mM. ROMK1 currents were blocked by 10 mM Ba.sup.2+ at the test potentials used. 10 mM TEA.sup.+ showed no inhibition of ROMK1 currents (dam not shown).
(F) ROMK1 1.sub.K(Ba) currents exhibit high K.sup.+ selectivity. [K.sup.+ ].sub.ext was changed by replacement with Na.sup.+ in perfusion solutions containing (mM) 0.3 CaCl.sub.2, 1 MgCl.sub.2, 5 HEPES, pH 8.0 as follows: 2 KCl, 96 NaCl; 5 KCl, 93 NaCl; 10 KCl, 88 NaCl; 30 KCl, 68 NaCl; 50 KCl, 48 NaCl. The E.sub.rev of ROMK1 I.sub.K(Ba) currents were dependent on [K.sup.+ ].sub.ext ; the linear regression line has a slope of 63 mV per tenfold change in external K.sup.+. Mean values and standard errors (error bars) for E.sub.rev represent combined data from 5 oocytes.
FIG. 2. Single channel characteristics of ATP-sensitive ROMK1 K.sup.+ channels expressed in Xenopus oocytes. Channel activities were recorded in inside-out patches excised from oocyte membranes.
(A) and (B) Current tracings recorded in the presence of 150 mM KCl or 50 mM KCl in the pipette solution. Holding potentials (V.sub.H) are indicated on the left side of each tracing. Horizontal bars indicate closed channel current levels. Downward deflections represent inward currents; upward deflections represent outward currents. Three active channels are present in A; only one active channel is present in B.
(C) Current-voltage relationships of currents recorded in the presence of pipette solutions containing 150 mM KCl (.circle.) or 50 mM KCl ( ) clearly display inward rectification. Unitary slope conductances of inward currents were 44 pS and 32 pS for 150 mM KCl and 50 mM KCl-containing pipette solutions, respectively. The corresponding reversal potentials extrapolated from regression lines fitted to inward currents were .about.0 mV (150 mM KCl) and -28 mV (50 mM KCl) indicating high K.sup.+ selectivity.
(D) A continuous recording of both channel rundown in ATP-free bath solution and channel recovery in the same excised patch following addition of bath solution containing 0.025 mM ATP. Changes made in bath solution ATP concentration are indicated above the current recording: ATP-free (solid line), 0.025 mM ATP (dashed line). Recordings were taken at V.sub.H =-80 mV. Interruptions were introduced into the recording shown in order to display the entire current tracing. Expanded current tracings taken from the continuous recording are indicated by numbers and arrowheads.
(E) The inhibitory effect of 2.5 mM ATP on channel activity. Changes in bath solution ATP concentration are indicated above the current recording; 0.025 mM ATP (dashed line), 2.5 mM ATP (solid line). The inside-out patch was continuously perfused with bath solution containing 0.025 mM ATP; an increase in ATP concentration to 2.5 mM was achieved at the cytosolic side of the membrane patch using a puffer pipette over the time interval shown and resulted in inhibition of channel activity. Exposure of the patch to 0.025 mM ATP-containing bath solution after removal of the puffer pipette from the bath resulted in restoration of channel activity. The recording was taken at V.sub.H =-40 mV. Expanded current tracings taken from the recording at various time intervals are indicated by arrows and numbers. The pipette solution contained (mM): 150 or 50 KCl, 1 CaCl.sub.2, 1 MgCl.sub.2, 5 HEPES (pH 7.4). The bath solution contained (mM): 0,0.025 or 2.5 MgATP, 150 KCl, 1 MgCl.sub.2 , 5 EGTA, 5 HEPES (pH 7.4).
FIG. 3. Nucleotide sequence of ROMK1 cDNA. Protein coding region begins with the ATG in position 210 and ends with the stop codon, TGA, in position 1323, deduced amino acid sequence is shown in three-letter code. 5' and 3' untranslated regions show nucleotides in lower case; translated region have nucleotides in upper case. Potential membrane spanning regions [M0, M 1 and M2] are underlined. The putative channel pore-forming region [H5-like] is boxed. The Walker Type A and Type B, ATP-binding sequences, are shown in bold type.
FIG. 4. Comparison of Kyte-Doolittle hydropathy histogram plots of ROMK1 [K.sub.ATP family] and two members of the voltage-dependent potassium channel family, RCK4 and Slo. The major hydrophilic regions are dark shaded. Note completely different topology of ROMK1 compared to the voltage-gated channels. Also note the four hydrophilic regions in ROMK1 [potential membrane spanning regions M0, M1 and M2 and the H5-like region].
FIG. 5. Homology of ROMK1 with the voltage-gated potassium channel H5 and S4 regions.
(A) H5 homology. Shaker proteins are shown above the mammalian homologue for the four classes of voltage-gated potassium channels. Eag and Slo are shown above ROMK1 at the bottom. Comparisons are made to Shaker A; identical amino acids are shaded; conservative changes are boxed.
(B) Homology of ROMK1 M0 region with the S4 regions of the voltage-gated potassium [Shaker A, Shab 11 and Shaw 2] and voltage-gated sodium (Na.sup.+ brain 1 ] channels and similar regions in the ligand-gated channels [cGMP-gated]. Regions of identity or conserved changes are boxed.
FIG. 6. Model of ROMK1 protein. Membrane spanning regions M1 and M2 are shown in the membrane together with the pore-forming region, P, and the H5-like region. A single glycosylation site is shown on the extracellular face of the protein between M 1 and the H5-like region. The ATP-binding site is indicated as the Walker type A, P-loop. The partially hydrophobic region, M0, is placed outside the membrane on the intracellular side, however, this region could be partly associated with the membrane. "P" in an open circle represented a putative cyclic AMP-dependent protein kinase phosphorylation site; "P" in a solid circle represents a putative protein kinase C phosphorylation site. [open circles--hydrophobic amino acids; striped circle--non-charged, polar amino acids; solid circles--negatively charged amino acids; solid triangles--positively charged amino acids; open triangles--proline].
FIG. 7. In vitro translation of synthetic messenger RNA (cRNA) made from ROMK1 cDNA.
(A) Translation using rabbit reticulocyte lysate without (lane a) or with (lane b) dog pancreatic microsome membranes. Lanes c and d are water controls without and with membranes, respectively. ROMK1 translates to a 45 kDA protein that is glycoslyated in the presence of membranes.
(B) Effect of the glycosidase, Endo H, on in vitro translation of ROMK1 cRNA. Lanes a-d demonstrate that ROMK1 protein is a glycoprotein. Lanes e-f are translation-glycosylation controls.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the cloning of a member of a previously uncloned family of potassium channels. Specifically, the present invention discloses the cloning, cDNA sequence, and amino acid sequence of ROMK1, K.sub.ATP channel isolated from rat outer medulla cells.
Based on this disclosure, the present invention provides the nucleotide sequence encoding ROMK1 (Sequence ID No. 2). In detail, the present invention provides the previously unknown DNA sequence identified as Sequence ID No. 1.
The invention further provides vectors containing sequences encoding ROMK1. Such vectors included, but are not limited to, plasmid, phage, retrovirus and baculovirus vectors. One skilled in the art can readily place the sequences of the present invention into any known suitable vector using routine procedures.
The invention further includes vectors containing all of the elements necessary to express the ROMK1 sequence, or members of the ROMK1 family of channel proteins. Such elements can include, but are not limited to, promoters, ribosome binding sites, polyadenylation signals, termination signals, and capping signals. As above, one skilled in the art can readily generate such expression vectors using known methods.
The invention further provides methods of obtaining other members of this novel family (the ROMK1 family) of potassium channel.
As used herein, a potassium channel is said to be "a member of the ROMK1 family of channel protein" if it shares significant homology to one or more regions of the ROMK1 protein. Specifically, by using the sequence disclosed herein as a probe or as primers, and techniques such as PCR cloning and colony/plaque hybridization, one skilled in the art can obtain other members of the ROMK1 family of channel proteins as well as genomic sequences encoding the ROMK1 family members.
As used herein, a protein is said to "share significant homology" if the two proteins, contains regions which process greater than 50% homology.
Region specific primers or probes derived from Sequence ID No. 1 can be used to prime DNA synthesis and PCR amplification, as well as to identify colonies containing cloned DNA encoding a member of the ROMK1 family using known methods (Innis et al., PCR Protocols, Academic Press, San Diego, Calif. (1990)).
When using primers derived from ROMK1 for amplification, one skilled in the art will recognize that by employing high stringency condition, annealing at 50.degree.-60.degree. C., sequences which are greater than 75% homologous to the primer will be amplified. By employing lower stringency conditions, annealing at 35.degree.-37.degree. C., sequences which are greater than 40-50% homologous to the primer will be amplified.
When using DNA probes derived from ROMK1 for colony/plaque hybridization, one skilled in the art will recognize that by employing high stringency condition, hybridization at 50.degree.-65.degree. C. 5X SSPC, 50% formamide, wash at 50.degree.-65.degree. C., 0.5X SSPC, sequences having regions which are greater than 90% homologous to the probe can be obtained, and by employing lower stringency conditions, hybridization at 35.degree.-37.degree. C., 5X SSPC, 40-45% formamide, wash at 42.degree. C., SSPC, sequences having regions which are greater than 35-45% homologous to the probe will be obtained.
Any tissue can be used as the source for the genomic DNA or RNA encoding members of the ROMK1 family of potassium channels. However, with respect to RNA, the most preferred source is tissues which express elevated levels of the desired potassium channel family member. In the present invention, oocyte injection and two-electrode whole oocyte clamping and patch clamping of single channels was used to identify expression from such a tissue source. However, using the sequences disclosed herein, it is now possible to identify such cells using the ROMK1 sequence as a probe in northern blot or in situ hybridization procedures, thus eliminating the necessity of the procedures employed to clone the first member of this family and eliminating the need to obtain RNA/DNA from a tissue which expresses elevated levels of ROMK1 protein.
The present invention further provides methods of identifying cells or tissues which express a member of the ROMK1 family of channel proteins. In detail, a probe comprising the DNA sequence of Sequence ID No. 1, a fragment thereof, or a DNA sequence encoding another member of the ROMK1 family of channel proteins can be used as a probe or amplification primer to detect cells which express a message homologous to the probe or primer. One skilled in the art can readily adapt currently available nucleic acid amplification or detection techniques so that it employs probes or primers based on the sequences encoding a member of the ROMK1 family.
The materials for use in the invention are ideally suited for the preparation of a kit. Specifically, the invention provides a kit compartmentalized to receive in close confinement, one or more containers which comprises: (a) a first container comprising one or more probes or amplification primers based on the ROMK1 sequence; and (b) one or more other containers comprising one or more of the following: a sample reservoir, wash reagents, reagents capable of detecting presence of bound probe from the first container, or reagents capable of amplifying sequences hybridizing to the amplification primers.
In detail, a compartmentalized kit includes any kit in which reagents are contained in separate containers. Such containers include small glass containers, plastic containers or strips of plastic or paper. Such containers allow one to efficiently transfer reagents from one compartment to another compartment such that the samples and reagents are not cross-contaminated and the agents or solutions of each container can be added in a quantitative fashion from one compartment to another. Such containers will include a container which will accept the test sample, a container which contains the probe or primers used in the assay, containers which contain wash reagents (such as phosphate buffered saline, Tris-buffers, etc.), and containers which contain the reagents used to detect the bound probe or amplified product.
Types of detection reagents include labelled secondary probes, or in the alternative, if the primary probe is labelled, the enzymatic, or antibody binding reagents which are capable of reacting with the labelled probe. One skilled in the art will readily recognize that probes and amplification primers based on the sequence disclosed in the present invention can be readily incorporated into one of the established kit formats which are well known in the art.
In one example, a first container may contain a hybridization probe. Other containers may contain reagents useful in the localization of labelled probes, such as enzyme substrates. Still other containers may contain buffers, etc.
The present invention further provides functional derivatives of the ROMK1 sequence. As used herein, the term "functional derivative" is used to define any DNA sequence which is derived from the original DNA sequence and which still possesses at least one of the biological activities present in the parent molecule. A functional derivative can be an insertion, deletion, or a substitution of one or more bases in the original DNA sequence. Functional derivatives of ROMK1 therefore include, but are not limited to derivatives which have altered ATP binding, altered potassium gating capacity, and altered cofactor requirements.
Functional derivatives of Sequence ID No. 1 having an altered nucleic acid sequence can be prepared by mutagenesis of the DNA. This can be accomplished using one of the mutagenesis procedures known in the art.
Preparation of functional derivatives of Sequence ID No. 1 are preferably achieved by site-directed mutagenesis. Site-directed mutagenesis allows the production of functional derivatives of Sequence ID No. 1 through the use of a specific oligonucleotide which contains the desired mutated DNA sequence.
Site-directed mutagenesis typically employs a phage vector that exists in both a single-stranded and double-stranded form. Typical vectors useful in site-directed mutagenesis include vectors such as the M 13 phage, as disclosed by Messing et al., Third Cleveland Symposium on Macromolecules and Recombinant DNA, Editor A. Walton, Elsevier, Amsterdam (1981), the disclosure of which is incorporated herein by reference. These phage are commercially available and their use is generally well known to those skilled in the art. Alternatively, plasmid vectors containing a single-stranded phage origin of replication (Veira et al., Meth. Enzymol. 153:3 (1987)) may be employed to obtain single-stranded DNA.
In general, site-directed mutagenesis in accordance herewith is performed by first obtaining a single-stranded vector that includes within its sequence the DNA sequence which is to be altered. An oligonucleotide primer bearing the desired mutated sequence is prepared, generally synthetically, for example by the method of Crea et al., Proc. Natl. Acad. Sci. (USA) 75:5765 (1978). The primer is then annealed with the single-stranded vector containing the sequence which is to be altered, and the created vector is incubated with a DNA-polymerizing enzyme such as E. coli polymerase I Klenow fragment in an appropriate reaction buffer. The polymerase will complete the synthesis of a mutation-bearing strand. Thus, the second strand will contain the desired mutation. This heteroduplex vector is then used to transform appropriate cells, such as JM101 cells, and clones are selected that contain recombinant vectors bearing the mutated sequence.
While the site for introducing a sequence variation is predetermined, the mutation per se need not be predetermined. For example, to optimize the performance of a mutation at a given site, random mutagenesis may be conducted at a target region and the newly generated sequences can be screened for the optimal combination of desired activity.
It is difficult to predict the exact effect of the substitution, deletion, or insertion in advance of doing so. However, one skilled in the art will recognize that the functionality of the derivative can be evaluated by routine screening assays. For example, mRNA encoded by a functional derivative made by site-directed mutagenesis can be injected into an oocyte as described in the Examples and the oocyte tested for channel activity.
Using genomic sequence corresponding to the ROMK1 family of channel proteins (the isolation of which is described above), the present invention further provides methods of directing the expression of a heterologous gene in the same temporal and spatial manner as the ROMK1 channel protein. In detail, using chromosome walking techniques and DNA sequence analysis, the regulatory elements responsible for the tissue specific expression of ROMK1 can readily be obtained by one skilled in the art. These elements, such as, but not limited to, tissue specific promoters and enhancers, can readily be operably linked to a heterologous gene such that the heterologous gene is expressed in a cell in the same temporal and spatial manner as the ROMK1 gene.
The invention further provides an isolated protein of the ROMK1 family of channel proteins. Specifically, the present invention provides a protein comprising the amino acid sequence of Sequence ID No. 2.
Any eukaryotic organism can be used as a source for a protein which is a member of the ROMK1 protein, or the genes encoding same, so long as the source organism naturally expresses such a protein or contains genes encoding same. As used herein, "source organism" refers to the original organism from which the amino acid or DNA sequence of the protein is derived, regardless of the organism the protein is expressed in and ultimately isolated from. For example, a member of the ROMK1 family of channel proteins expressed in hamster cells is of human origin as long as the amino acid sequence is that of a human protein which is a member of this family. The most preferred source organism for ROMK1 protein is rats or humans.
A variety of methodologies known in the art can be utilized to obtain a member of the ROMK1 family of channel proteins. In one method, the protein is purified from tissues or cells which naturally produce the protein. One skilled in the art can readily follow known methods for isolating proteins in order to obtain a member of the ROMK1 protein family, free of natural contaminants. These include, but are not limited to, immunochromatography, HPLC, size-exclusion chromatography, ion-exchange chromatography, and immuno-affinity chromatography.
In another embodiment, a member of the ROMK1 family of channel proteins is purified from cells which have been altered to express the desired protein.
As used herein, a cell is said to be "altered to express a desired protein" when the cell, through genetic manipulation, is made to produce a protein which it normally does not produce or which the cell normally produces at low levels. One skilled in the art can readily adapt procedures for introducing and expressing either genomic, cDNA, or synthetic sequences into either eukaryotic or prokaryotic cells in order to generate a cell which produces a member of the ROMK1 family of channel proteins.
There are a variety of sources for DNA encoding members of the ROMK1 family of channel proteins. One of these is the sequence of the ROMK1 protein depicted in Sequence ID No. 2. The DNA can be isolated as described herein from a source such as human, or, alternatively, the sequence encoding the ROMK1 family member can be synthesized utilizing the DNA sequences disclosed herein.
Any host/vector system can be used to express a member of the ROMK1 family of channel proteins. These include, but are not limited to, eukaryotic hosts such as Hela cells, Cv-1 cell, COS cells, and Sf9 cells, as well as prokaryotic host such as E. coli and B. subtills. The most preferred cells are those which do not normally express the member of the ROMK1 family of channel proteins or which expresses such a protein at low levels.
The members of the ROMK1 family of channel proteins, as well as the example of this family which is depicted in Sequence ID No. 2 can be used in a variety procedures and methods known in the art which are currently applied to other proteins.
For example, the ROMK1 family member is used to generate an antibody which is capable of binding to the channel protein.
The antibodies of the present invention include monoclonal and polyclonal antibodies, as well fragments of these antibodies, and humanized forms. Humanized forms of the antibodies of the present invention may be generated using one of the procedures known in the art such as chimerization or CDR grafting. Fragments of the antibodies of the present invention include, but are not limited to, the Fab, the Fab2, and the Fd fragment.
The invention also provides hybridomas which are capable of producing the above-described antibodies. A hybridoma is an immortalized cell line which is capable of secreting a specific monoclonal antibody.
In general, techniques for preparing polyclonal and monoclonal antibodies as well as hybridomas capable of producing the desired antibody are well known in the art (Campbell, A. M., "Monoclonal Antibody Technology: Laboratory Techniques in Biochemistry and Molecular Biology," Elsevier Science Publishers, Amsterdam, The Netherlands (1984); St. Groth et al., J. Immunol. Methods 35:1-21 (1980)).
Any animal (mouse, rabbit, etc.) which is known to produce antibodies can be immunized with the pseudogene polypeptide. Methods for immunization are well known in the art. Such methods include subcutaneous or interperitoneal injection of the polypeptide. One skilled in the art will recognize that the amount of the ROMK1 family of channel proteins used for immunization will vary based on the animal which is immunized, the antigenicity of the peptide and the site of injection.
The protein which is used as an immunogen may be modified or administered in an adjuvant in order to increase the protein's antigenicity. Methods of increasing the antigenicity of a protein are well known in the art and include, but are not limited to coupling the antigen with a heterologous protein (such as globulin or .beta.-galactosidase) or through the inclusion of an adjuvant during immunization.
For monoclonal antibodies, spleen cells from the immunized animals are removed, fused with myeloma cells, such as SP2/0-Ag 14 myeloma cells, and allowed to become monoclonal antibody producing hybridoma cells.
Any one of a number of methods well known in the art can be used to identify the hybridoma cell which produces an antibody with the desired characteristics. These include screening the hybridomas with an ELISA assay, western blot analysis, or radioimmunoassay (Lutz et al., Exp. Cell Res. 175:109-124 (1988)).
Hybridomas secreting the desired antibodies are cloned and the class and subclass is determined using procedures known in the art (Campbell, A. M., Monoclonal Antibody Technology: Laboratory Techniques in Biochemistry and Molecular Biology, Elsevier Science Publishers, Amsterdam, The Netherlands (1984)).
For polyclonal antibodies, antibody containing antisera is isolated from the immunized animal and is screened for the presence of antibodies with the desired specificity using one of the above-described procedures.
The present invention further provides the above-described antibodies in detectably labelled form. Antibodies can be detectably labelled through the use of radioisotopes, affinity labels (such as biotin, avidin, etc.), enzymatic labels (such as horseradish peroxidase, alkaline phosphatase, etc.) fluorescent labels (such as FITC or rhodamine, etc.), paramagnetic atoms, etc. Procedures for accomplishing such labelling are well-known in the art, for example see (Sternberger, L. A. et al., J. Histochem. Cytochem. 18:315 (1970); Bayer, E. A. et al., Meth. Enzym. 62:308 (1979); Engval, E. et al., Immunol. 109:129 (1972); Goding, J. W. J. Immnunol. Meth. 13:215 (1976)).
The labeled antibodies of the present invention can be used for in vitro, in vivo, and in situ assays to identify cells or tissues which express a member of the ROMK1 family of channel proteins or to identify samples containing the protein.
The present invention further provides the above-described antibodies immobilized on a solid support. Examples of such solid supports include plastics such as polycarbonate, complex carbohydrates such as agarose and sepharose, acrylic resins and such as polyacrylamide and latex beads. Techniques for coupling antibodies to such solid supports are well known in the art (Weir, D. M. et al., "Handbook of Experimental Immunology" 4th Ed., Blackwell Scientific Publications, Oxford, England, Chapter 10 (1986); Jacoby, W. D. et al., Meth. Enzym. 34 Academic Press, N.Y. (1974)). The immobilized antibodies of the present invention can be used for in vitro, in vivo, and in situ assays as well as for immunoaffinity purification of the ROMK1 channel protein family member.
The present invention further provides the antibodies of the present invention conjugated to a cytotoxin. Examples of such cytotoxins include, but are not limited to, Ricin A chain, diphtheria toxin, cholera toxin, as well as radionuclides. Methods of conjugation antibodies, there use, and the types of toxins currently employed is disclosed in Golbert et at., Cancer Diagnosis and Therapy with Radiolabeled Antibodies pp. 259-280, Oxford Press, N.Y.(1987); Vitetta et al., Science 219:644 (1983), and Pastan et al., Cell 47:641-648 (1986).
The present invention further provides methods of identify the expression of a member of the ROMK1 family of channel proteins in a test sample.
In detail, the methods comprise incubating a test sample with one or more of the antibodies of the present invention and assaying for binding of the antibody to the test sample.
Conditions for incubating an antibody with a test sample vary. Incubation conditions depend on the format employed in the assay, the detection methods employed, and the type and nature of the antibody used in the assay. One skilled in the art will recognize that any one of the commonly available immunological assay formats (such as radioimmunoassays, enzyme-linked immunosorbent assays, diffusion based Ouchterlony, or rocket immunofluorescent assays) can readily be adapted to employ the antibodies of the present invention. Examples of such assays can be found in Chard, T. "An Introduction to Radioimmunoassay and Related Techniques" Elsevier Science Publishers, Amsterdam, The Netherlands (1986); Bullock, G. R. et al., "Techniques in Immunocytochemistry," Academic Press, Orlando, Fla. Vol. 1 (1982), Vol. 2 (1983), Vol. 3 (1985); Tijssen, P., "Practice and Theory of Enzyme Immunoassays: Laboratory Techniques in Biochemistry and Molecular Biology," Elsevier Science Publishers, Amsterdam, The Netherlands (1985).
The test samples of the present invention include cells, protein or membrane extracts of cells, or biological fluids such as blood, serum, plasma, or urine. The test sample used in the above-described method will vary based on the assay format, nature of the detection method and the tissues, cells or extracts used as the sample to be assayed. Methods for preparing protein extracts or membrane extracts of cells are well known in the an and can be readily be adapted in order to obtain a sample which is capable with the system utilized.
In another embodiment of the present invention, kits are provided which contain the necessary reagents to carry out the previously described assays.
Specifically, the invention provides a compartmentalized kit to receive, in close confinement, one or more containers which comprises: (a) a first container comprising one of the antibodies of the present invention; and (b) one or more other containers comprising one or more of the following: wash reagents, reagents capable of detecting presence of bound antibodies.
In detail, a compartmentalized kit includes any kit in which reagents are contained in separate containers. Such containers include small glass containers, plastic containers or strips of plastic or paper. Such containers allows one to efficiently transfer reagents from one compartment to another compartment such that the samples and reagents are not cross-contaminated, and the agents or solutions of each container can be added in a quantitative fashion from one compartment to another. Such containers will include a container which will accept the test sample, a container which contains the antibodies used in the assay, containers which contain wash reagents (such as phosphate buffered saline, Tris-buffers, etc.), and containers which contain the reagents used to detect the bound antibody.
Types of detection reagents include labelled secondary antibodies, or in the alternative, if the primary antibody is labelled, the enzymatic, or antibody binding reagents which are capable of reacting with the labelled antibody. One skilled in the art will readily recognize that the disclosed antibodies of the present invention can readily be incorporated into one of the established kit formats which are well known in the art.
The present invention further provides anti-ROMK1 anti-idiotypic antibodies. In detail, the anti-ROMK1 antibodies are suitable for use in generating anti-idiotypic antibodies.
Anti-idiotypic antibodies can be generated by any of the methods described above using one of the antibodies of the present invention as an immunogen, for example see Hellstrom et al., U.S. Pat. No. 4,918,164, or Shecter et al., Anti-Idiotypes, Receptor, and Molecular Mimicry p 73, D. S. Linthicum et al., eds. New York (1988). One skilled in the art can readily adapt known methods in order to generate the anti-idiotypic antibodies capable of binding to the antigen binding site of the anti-pseudogene peptide antibodies of the present invention.
Using the purified ROMK1 protein, the present invention provides methods of obtaining and identifying agents capable of binding to ROMK1. Specifically, such agents include antibodies (described above), peptides, carbohydrates, pharmaceutical agents and the like.
In detail, said method comprises:
(a) contacting an agent with a purified member of the ROMK1 family of channel proteins; and
(b) determining whether the agent binds to said protein.
The agents screened in the above assay can be, but are not limited to, peptides, carbohydrates, vitamin derivatives, or other pharmaceutical agents. The agents can be selected and screened at random or rationally selected or designed using protein modeling techniques.
For random screening, agents such as peptides, carbohydrates, pharmaceutical agents and the like are selected at random and are assayed for their ability to bind to the ROMK1 family member as outlined above.
Alternatively, agents may be rationally selected or designed. As used herein, an agent is said to be "rationally selected or designed" when the agent is chosen based on the configuration of the ROMK1 family of channel proteins. For example, one skilled in the art can readily adapt currently available procedures to generate peptides, pharmaceutical agents and the like capable of binding to a specific peptide sequence in order to generate rationally designed antipeptide peptides', for example see Hurby et al., "Application of Synthetic Peptides: Antisense Peptides", In Synthetic Peptides, A User's Guide, W. H. Freeman, N.Y., pp. 289-307 (1992), and Kaspczak et al., Biochemistry 28:9230-8 (1989), or pharmaceutical agents, or the like.
Anti-peptide peptides can be generated in one of two fashions. First, the anti-peptide peptides can be generated by replacing the basic amino acid residues found in the ROMk1 protein with acidic residues, while maintaining hydrophobic and uncharged polar groups. For example, lysine, arginine, and/or histidine residues are replaced with aspartic acid or glutamic acid and glutamic acid residues are replaced by lysine, arginine or histidine.
Alternatively, the anti-peptide peptides of the present invention can be generated by synthesizing and expressing a peptide encoded by the antisense strand of the DNA which encodes the ROMK1 family member. Peptides produced in this fashion are, in general, similar to those described above since codons complementary to those coding for basic residues generally code for acidic residues.
The cloning of ROMK1 now makes possible the screening capability which enables the identification of agonists (K.sub.ATP channel openers) and antagonists (K.sub.ATP channel closers) of the ROMK1 family of channel proteins. K.sub.ATP channel openers are smooth muscle relaxants, functioning as vasodilators, vasospasmolytics, and other smooth muscle spasmolytic. As vasodilators, these compounds have use as dilators of peripheral vasculature, coronary arteries, renal vasculature, cerebral vasculature, and mesenteric vasculature. As vasospasmolytics, these compounds have use in the treatment of coronary artery spasm, peripheral vascular spasm, cerebral vascular spasm and impotence. Other smooth muscle spasmolytics have use as bronchodilators, in the control of urinary bladder and gall bladder spasm, and in the control of esophageal, gastric, and intestinal smooth muscle spasm.
K.sub.ATP channel closers function in the pancreas to enhance release of insulin, in the kidney as diuretics and renal epithelial anti-ischemic agents, as hypertensive agents for promoting vasoconstriction for use in hypotensive states as antiarrhythmic agents, and as agents for modifying cardiac muscle contractility.
Other uses for K.sub.ATP channel agonists or antagonists include cardiac antiarrhythmic agents, agents effective in reducing ischemia-induced cardiac damage, agents effective in reducing ischemia-induced brain damage, anticonvulsants, hair growth promoting agents, and agents effective in preventing or reducing skeletal muscle damage or fatigue.
The invention further provides detectably labeled, immobilized and toxin conjugated forms of these agents. Such modified agents can be generated using the procedures described above for modifying antibodies.
The present invention further provides methods for modulating the activity of ROMK1, or a member of the ROMK1 family of channel proteins. Specifically, the activity of a member of the ROMK1 family of channel proteins can be modulated by providing to cells expressing such a protein an agent capable of binding to the channel protein. Such agents include, but are not limited to, the anti-ROMK1 antibodies and the antipeptide peptides of the present invention. By providing such an agent to a cell, the activity of the ROMK1 family member can either be blocked or stimulated. Such agents can be used to treat any of the physiological or pathophysiological conditions which are associated with the ROMK1 class of potassium channels, for example see Weston, A. H., Pflugers Arch. 414 (Suppl. 1):S99-S105 (1989), Tsuura, et al., Diabetes 41:861-865 (1992), and Escande, D. Pflugers Arch. 414 (Suppl 1):S93-S98 (1989).
The present invention further provides methods for modulating the expression of ROMK1, or a member of the ROMK1 family of channel proteins. Specifically, anti-sense RNA expression is used to disrupt the translation of the mRNA encoding the ROMK1 protein.
In detail, a cell is modified using routine procedures such that if expresses an antisense mRNA, an mRNA which is complementary to mRNA encoding the ROMK1 family member. By constitutively or inducibly expressing the antisense RNA, the translation of the ROMK1 family member mRNA can be regulated. Such antisense technology has been successfully applied to regulate the expression of poly(ADP-ribose) polymerase (Ding et al., J. Biol. Chem. 267:12804-12812 (1992)) as well as other proteins.
The present invention further provides methods of selectively killing cells expressing ROMK1. Specifically, a cell expressing ROMK1 can be selectively killed by providing to the cell an agent capable of binding ROMK1 which is conjugated to a cytotoxin.
The present invention further provides methods for generating chimerio or transgenic animals 1) in which the animal contains one or more exogenously supplied genes which are expressed in the same temporal and spatial manner as a member of the ROMK1 family of channel proteins, or 2) in which the member of the ROMK1 family of channel proteins has been deleted. Such chimeric and transgenic animals are useful in further elucidation the mechanisms of potassium channel function as well as their effect an animal physiology. These transgenic and chimerio animals are produced by utilization of techniques which are well known and well described in the technical literature.
The cloning of the gene for ROMK1, and the ROMK1 family of channel proteins also permits one to identify individuals with one or more defects in the genes encoding the ROMK1 family of channel proteins and correct the defect using gene therapy regimens known to the art. For example, the ROMK1 cDNA can be inserted in an expression vector such as, but not limited to, recombinant retroviruses (Collins, F. A., Science 256:774-779 (1992)). Cells exposed to this recombinant virus become infected and stably incorporate the new ROMK1 gene. In addition, there are vector-free approaches including encapsulating the ROMK1 DNA into various types of liposomes which are then taken up by cells or complexing the ROMK1 DNA to a protein normally taken up by the cell into which the new gene is to be transferred.
By the term "ROMK1 associated polypeptides" is intended such other distinct polypeptides as are intimately associated with the ROMK1 family of channel proteins. Although a single polypeptide type generally provides the structure for an ion channel pore [the pore itself may be a homomeric complex], this polypeptide may be intimately associated with other distinct polypeptides to form an ion channel complex (heteromeric complexes of polypeptides). For example, voltage gated Na.sup.+ and Ca.sup.2+ channel complexes are composed of three and five distinct protein subunits, respectively (Catterall, W. A., Science 242:50-61 (1988)) and the epithelial amiloride-sensitive Na.sup.+ channel is a complex of 10 distinct subunits. These polypeptide subunits may be involved in the regulation of channel activity by regulating phosphorylation, G-protein binding, nucleotide binding, etc. Such subunits provide the target for the development of ROMK1 channel function modifying agents.
These other associated ion channel subunits can be identified during standard protein purification under non-denaturing conditions (Catterall, W. A. Science 242:50-61 (1988); Agnew et at., Proc. Natl. Acad. Sci. USA 77:639 (1980); Hartshorne et at., J. Biol. Chem. 259:1667 (1984))with the specificity of the protein complex identified by using an antibody to one of the subunits. Thus, an antibody to ROMK1 is used by one skilled in the art to identify ROMK1 associated subunits. Alternatively, one uses one or more of a variety of homobifunctional cross-linking reagents (Brenner et al., Cell 40:183-190 (1985); Oettgen et al., Nature 320:272-275 (1986))to link the ROMK1 protein to polypeptides with which it is in intimate contact and then immunoprecipitates the complex using an antibody to ROMK1. The cross-linked product is split into its individual polypeptides and the non-ROMK1 peptides identified and sequenced. Partial sequencing of the polypeptides [e.g., to provide a 15-20 mer fragment] is used to construct a nucleotide probe that is utilized to screen a cDNA library by high stringency hybridization. In this manner the genes encoding the ROMK1 associated polypeptides are cloned.
Once cloned, the genes for, and the ROMK1 associated polypeptides, may be utilized in like manner to the ROMK1 family of channel proteins, following procedures as described above.
Having now generally described the invention, the same will be understood by means of specific examples which are, however, not intended to be limiting unless otherwise specified.
EXAMPLES
We report here the cloning, functional characterization, and amino acid sequence of a K.sub.ATP channel cDNA, ROMK1, isolated by expression cloning. Patch clamp studies have recently demonstrated that the predominant potassium channels present in apical membranes of thick ascending limb epithelial cells are K.sub.ATP channels which are thought to mediate K.sup.+ recycling in this nephron segment (Bliech et al., Pflugers Arch. 415:449 (1990); Wang et al., Am. J. Physiol. 258:F244 (1990)); similar channels in principal cells of the cortical collecting tubule may be involved in K.sup.+ secretion (Wang et al., J. Gen. Physiol. 98:35 (1991); Wang et al., Am. J. Physiol. 259:F494 (1990)). Given these findings, the inner stripe of the outer medulla of rat kidney which contains an abundance of medullary thick ascending limb segments was used as a source of poly(A).sup.+ RNA for cDNA library construction. We have functionally expressed the cDNA in Xenopus oocytes and have shown that it is sufficient for encoding a K.sup.+ channel which exhibits the basic properties of K.sub.ATP channels. The predicted protein with a calculated M.sub.r of 43 kDa shows marked structural divergence when compared to voltage-gated K.sup.+ channel proteins. In contrast to these latter channels, the ROMK1 protein does not possess the characteristic structural motif of six membrane-spanning segments nor does it contain a typical S4 segment. Remarkably, however, the protein demonstrates the conservation of an amino acid segment which is homologous to the pore-forming H5 region of voltage-gated K.sup.+ channels.
Expression Cloning of Complementary DNA
Two-electrode voltage clamping was used to study Ba.sup.2+ -sensitive potassium currents (I.sub.K(Ba) expressed in Xenopus laevis oocytes injected with poly(A).sup.+ RNA was isolated from the inner stripe of outer medulla (ISOM) of rat kidneys (FIG. 1 ). The criterion of Ba.sup.2+ -sensitivity was used as a means to selectively examine expressed potassium currents. RNA-injected oocytes displayed large inward Ba.sup.2+ -sensitive currents when compared with H.sub.2 O -injected oocytes at a membrane potential (V.sub.m) of -75 mV with 50 mM external K.sup.+ ([K.sup.+ ].sub.ext) (FIGS. 1A, 1B). The I.sub.K(Ba) current expressed in RNA-injected oocytes was carried selectively by potassium ions as shown by the dependence of the observed reversal potentials (E.sub.rev ) for these currents on external potassium concentration (FIG. 1C). Size-fractionated mRNA with a size of .about.2-3 kb yielded a highly augmented mean inward I.sub.K(Ba) sixteen-fold greater than that of H.sub.2 O-injected controls when injected into oocytes (FIG. 1D) and was therefore used to construct a directional cDNA library in the vector pSPORT1. RNA transcribed in vitro from pooled clones using T7 RNA polymerase was injected into Xenopus oocytes and tested for I.sub.K(Ba) currents using two-electrode voltage clamping. Functional screening of cRNA led to the identification of a 2.1 kb cDNA, ROMK1. Oocytes injected with ROMK1 cRNA (.about.0.3 ng/oocyte) exhibited a mean 1.sub.K(Ba) of -1408.+-.179 nA, n=10 (FIG. 1E) at V.sub.m =-60 mV ([K.sup.+].sub.ext =50 mM K.sup.+). Characterization of the ionic selectivity of ROMK1 currents in Na.sup.+ /K.sup.+ ion substitution experiments (FIG. 1F) using two-electrode voltage clamping demonstrated that ROMK1 encodes for a protein exhibiting high K.sup.+ selectivity.
Characterization of ROMK1 Currents
Single channel recordings were obtained in oocytes injected with ROMK1 mRNA following vitelline membrane removal using standard patch clamp techniques to further characterize the properties of ROMK1 K.sup.+ currents (FIG. 2). Inside-out patches excised from ROMK1 mRNA-injected oocytes displayed inwardly rectifying currents in the presence of symmetrical K.sup.+ solutions (150 mM KCl) (FIGS. 2A, 2C). At negative membrane potentials, the amplitude of inward currents increased linearly with increasing hyperpolarization relative to E.sub.rev (E.sub.rev =O mV for symmetrical K.sup.+ solutions). Inward currents showed a unitary slope conductance of 44 pS. In contrast, depolarizing membrane potentials resulted in outward currents with significantly smaller amplitudes. Moreover, the slope conductances of outward currents exhibited non-linear characteristics with increasing depolarization. Similar inwardly rectifying currents were also observed in the cell-attached patch configuration. The K.sup.+ selectivity of these currents was confirmed by varying the pipette solution K.sup.+ concentration ([K.sup.+ ].sub.ext). Reversal potentials obtained with pipette solutions containing either 150 mM KCl or 50 mM KCl+100 mM NaCl were in complete agreement with the Nernstian predictions for E.sub.rev under these conditions (FIGS. 2B, 2C); the permeability ratio (P.sub.Na /P.sub.K) of 0.025 for these channels indicates high K.sup.+ selectivity. Unlike voltage-gated K.sup.+ channels, the open probability (P.sub.0) for the expressed channels showed only slight voltage-dependence at membrane potentials in the physiological range; P.sub.0 increased with depolarizing test potentials (P.sub.0 =0.40, 0;61, 0.74, 0.71 at V.sub.m =-90, -70, -50, -30 mV, respectively). These electrophysiological characteristics are comparable to those of Type 1 K.sub.ATP channels, which have been characterized in pancreatic .beta.-cells, cardiac myocytes, as well as skeletal and smooth muscle cells. Type 1 channels exhibit inwardly rectifying currents when exposed to symmetrical K.sup.+ solutions, single channel conductances of .about.50-80 pS (symmetrical 140 mM K.sup.+ solutions), relatively little voltage-dependence (Ashcroft et al., Cellular Signalling 2:197 (1990); Ashcroft, F. M., Ann. Rev. Neurosci. 11:97 (1988); Qin et al., Am. J. Physiol. 257:H1624 (1989)), and high K.sup.+ -selectivity (Ashcroft et al., Cellular Signalling 2:197 (1990)). Epithelial K.sub.ATP channels (classified as Type 3) present in renal tubular cells from the thick ascending limb of Henle and the conical collecting duct (Bliech et al., Pflugers Arch. 415:449 (1990); Wang et al., Am. J. Physiol. 258:F244 (1990); Wang et al., Am. J. Physiol. 259:F494 (1990)) have single conductances of .about.25-60 pS and share similar characteristics.
The effect of intracellular ATP on ROMK1 currents was also investigated using the inside-out patch configuration. Channel activity spontaneously decreased several minutes after removal of MgATP from the external bathing solution (0.025 mM.fwdarw.0 mM MgATP) (channel rundown) (FIG. 2D). Partial restoration of channel activity following complete channel rundown by the readdition of 25 .mu.M ATP was observed in some cases. In contrast, channel activity was routinely maintained for 30 minutes or longer by excision of patches directly into a bath solution containing 25 .mu.M MgATP. Application of high concentrations of ATP (1.0-2.5 mM), on the other hand, to the cytosolic face of inside-out patches initially excised into a bath containing 25 .mu.M ATP reproducibly resulted in a rapid and marked inhibition of channel activity (FIG. 2E). This inhibition by ATP was found to be reversible as channel activity could be restored following wash-out of the high ATP-containing solution. Channels with these properties and electrophysiological characteristics were never observed in either cell-attached or inside-out patches from H.sub.2 O-injected control oocytes; in contrast, several channels of this type were typically active in patches from ROMK1 mRNA-injected oocytes. The incidence of ROMK1 channels in patches was 94% (63 of 67 patches) which was consistent with the high channel density observed in these experiments. This dual role of ATP as an inhibitor and as a requirement for the maintenance of channel activity has been well demonstrated for both Type 1 and renal Type 3 K.sub.ATP channels. In channels of both types, channel rundown rapidly occurs in excised membrane patches; this loss of channel activity is reversed or prevented, at least partially, by the addition of MgATP (Ashcroft et al., Cellular Signalling 2:197 (1990); Ashcroft, F. M., Ann. Rev. Neurosci. 11:97 (1988); Nichols et al., Am. J. Physiol. 261:H1675 (1991); Wang et al., J. Gen. Physiol. 98:35 (1991)). On the other hand, the channel activity of Type 1 and renal Type 3 K.sub.ATP channels (0.2-0.5 mM ATP) (Bleich et al., Pflugers Arch. 415:449 (1990); Wang et al., J. Gen. Physiol. 98:35 (1991)) is reversibly inhibited by intracellular ATP although that Ki for renal Type 3 K.sub.ATP channels (0.2-0.5 mM ATP) (Bleich et al., Pflugers Arch. 415:449 (1990); Wang et al., J. Gen. Physiol. 98:35 (1991)) is significantly higher than that for Type 1 channels in pancreatic .beta.-cells (.about.10-20 .mu.M) or in cardiac myocytes (.about.17-100 .mu.M); Type 2 K.sub.ATP channels which are found in central neurons (e.g., ventromedial hypothalamic nucleus), show less sensitivity to ATP with a Ki of .about.2-3 mM (Ashcroft et al., Cellular Signalling 2:197 (1990)).
Structural Features
The 2069 bp ROMK1 cDNA was bidirectionally sequenced yielding a single long open reading frame of 1,173 nucleotides downstream from three in-frame termination codons. This coding region encodes a protein of 391 amino acids with a calculated M.sub.T of 43 kDa (FIG. 3). The initiation codon was assigned to the first in-frame ATG which is contained within a strong Kozak initiation consensus sequence AGCAUGG (Kozak, M., J. Biol. Chem. 266:19867 (1991)). The size of this putative polypeptide was confirmed by in vitro translation experiments of ROMK1-specific mRNA using rabbit reticulocyte lysate which yielded a 45 kDa product by SDS polyacrylamide gel electrophoresis (FIG. 7). A 47 kDa glycosylated product obtained by translation in the presence of canine pancreatic microsomes is in agreement with the single glycosylation site predicted for the putative protein (FIG. 7). A long 3'-untranslated sequence of 726 nucleotides follows the coding region and contains two polyadenylation signal sequences of the form ATTAAA (Manley, J. L., Biochemica et Biophys. Acta 950:1 (1988)). These hexanucleotides occur in close proximity to each other and are located upstream from a poly(A) tail.
Sequence comparison using the GenBank, European Molecular Biology Laboratory (EMBL) and SWISSPROT databases revealed no significant similarities. In fact, hydropathy analysis suggests a strikingly unique topology for ROMK1 unlike any potassium channel protein cloned to date, namely, the presence of only two major hydrophobic regions M1 and M2 (FIG. 4). When Kyte-Doolittle hydropathy plots of ROMK1 are compared with those of potassium channel proteins from the Shaker superfamily, the M1 and M2 hydrophobic peaks together with two peaks of lesser hydrophobicity (M0, H5) appear reminiscent of the peaks corresponding to the S4, S5, H5 and S6 segments in voltage-gated potassium channels. Sequence analysis, however, reveals that only the amino acid sequence associated with the third peak (H5), which is flanked by the M1 and M2 regions, shares extensive homology with known potassium channel regions. Importantly this amino acid segment is remarkably similar to the pore-forming H5 region of potassium channels (MacKinnon et al., Science 245:1382 (1989); MacKinnon et al., Science 250:276 (1990); Yellen et al., Science 251:939 (1991); Hartmann et al., Science 251:942 (1991); Yool et al., Nature 349:700 (1991)) (FIG. 5). It has been suggested that the channel pore in voltage-gated potassium channels consists of an eight-stranded .beta.-barrel structure formed by the H5 segments from each of the four component polypeptides (Yellen et al., Science 251:939 (1991); Miller, C., Science 252:1092 (1991)). A comparison of ROMK1 and Shaker H5 sequences reveals 7 identical amino acids (28%) and 4 conserved substitutions (16% ) in a segment of 25 amino acids. The degree of homology is greatest (59%) in a core of 17 amino acids within this region having 7 identical (41%) and 3 conserved (18%) amino acids. Interestingly, this amino acid core is also shared to a comparable degree by Eag (Warmke et al., Science 252:1560 (1991)) and Slo (Atkinson et al., Science 253:551 (1991)), the two major potassium channel structural proteins showing significant divergence from the Shaker family. In voltage-gated potassium channels this highly conserved segment within the H5 region, the P segment (Durell et al., Biophys. J. 62:238 (1992)) (delimited by residues 431 and 449 in the Shaker H4 channel), has been shown to form the ion conduction pathway (MacKinnon et al., Science 250:276 (1990); Yellen et al., Science 251:939 (1991)). Single amino acid substitutions introduced into this segment by site-directed mutagenesis result in loss of channel function (MacKinnon et al., Neuron 767 (1990)) or changes in single channel conductance and ion selectivity (Yool et al., Nature 349:700 (1991); Kirsch et al., Biophys. J. 62:136 (1992)). In ROMK1 two amino acid substitutions are notable. A valine residue at position 140 replaces a highly conserved threonine residue at the corresponding position in Shaker sequences. In Shaker B, introduction of a mutation, T441S (threonine to serine), at this position dramatically increased the NH.sub.4.sup.+ permeability (P.sub.NH4 /P.sub.K =0.85) of the mutant without reducing its ability to exclude Na.sup.+ (Yool et al., Nature 349:700 (1991)). An identical mutation in Shaker IR resulted in a dramatic reduction in the binding affinity of intracellular TEA.sup.+ (Yellen et al., Science 251:939 (1991)). Moreover, an isoleucine (Ile 142) in ROMK1 occupies a position (443 in Shaker A) which is associated with changes in the ratio of Rb.sup.+ /K.sup.+ conductance as displayed by DRK1 and NGK2 (Kirsch et al., Biophys. J. 62:136 (1992)). The finding of an H5 region in ROMK1 suggests a common ancestry with other potassium channels. And yet, the degree of amino acid identity shared between ROMK1 and Shaker proteins within this region when compared to that exhibited by the most divergent members of the Shaker family (35-40%, excluding Eag and Slo (Swanson et al., Neuron 4:929 (1990); Wei et al., Science 248:599 (1990)) emphasizes the distinct nature of ROMK1.
Apart from the P segment that forms the ion permeation pathway, the remainder of the S5-S6 linker in Shaker proteins is believed to form the external vestibule surrounding the channel pore (MacKinnon et al., Science 245:1382 (1989); MaeKinnon et al., Neuron5:767 (1990); MacKinnon et al., Science 250:276 (1990); Yellen et al., Science 251:939 (1991)). Comparable regions in the M1-M2 linker of ROMK1 show significant differences. Positions immediately adjacent to the P segment (corresponding to residues 431 and 449 in Shaker A) which affect the ability of both external TEA.sup.+ and charybdotoxin (CTX) to inhibit channel activity in Shaker proteins are occupied by serine (Ser 130) and phenylalanine (Phe 148) in ROMK1. Both amino acids might be expected to decrease the affinity of either toxins for ROMK1 based on results obtained for analogous substitutions in Shaker mutants (MacKinnon et al., Neuron 5:767 (1990); MacKinnon et al., Science 250:276 (1990)). Replacement of threonine 449 in Shaker H4 by tyrosine and valine decreased channel sensitivity to CTX and TEA.sup.+, respectively; channel affinity for both toxins was also reduced by substituting an asparagine for aspartic acid at position 431. On the other hand, the glutamic acid residues at positions 123 and 151 in ROMK1 would be expected to favor the binding of CTX specifically given the demonstrated electrostatic influence exerted by negatively charged residues located in similar positions near the external mouth of the Shaker H4 pore (MacKinnon et al., Neuron 5:767 (1990)).
ROMK1 also possesses a single potential N-linked glycosylation site at position 117 which is consistent with the predicted extracellular location of this segment. Interestingly, this glycosylation site occurs within a cluster of four proline residues which potentially may form a loop-like structure. Putative N-linked glycosylation sites are also present in the analogous S5-P links of Sha12, mSha1, and Eag channel proteins; like ROMK1, the two sites in the Eag segment also occur in a short stretch of amino acids containing several proline residues.
Of equal significance to the presence of an H5 region in ROMK1 is the remarkable absence of regions corresponding to the S1, S2, S3 and S4 segments present in all K.sup.+, Na.sup.+, and Ca.sup.2+ voltage-gated ion channels. The absence of a typical S4 segment is particularly noteworthy. The highly conserved S4 segment is thought to form the voltage sensor in these channels (Catterall, W. A., Science 242:50 (1988); Papazian et al., Nature 349:305 (1991); Liman et al., Nature 353:752 (1991)). Using the ROMK1 H5 region as a reference point for alignment, the minor hydrophobic peak, M0, which would be expected to correspond to the S4 segment shows no characteristic S4 motif of repeating positively-charged residues at every third position (FIG. 6). Of the seven potential sites for basic residues in Shaker channel S4 sequences, every other of these sites is occupied by an uncharged residue (Phe, Thr, Trp) in the M0 segment of ROMK1, and two of the remaining four sites are occupied by negatively-charged amino acids. The M0 sequence contains only two positive charges in appropriate positions. It is possible that these substitutions may account for the relative lack of voltage-dependence exhibited by ROMK1 gating if the M0 segment represents an S4 region counterpart. The Shaw channel which displays significantly less voltage-sensitivity than other shaker channels may be analogous. The S4 segment of this channel contains only four positive charges, and two negative charges are present in positions that would be typically occupied by positive charges in other Shaker channels (Wei et al., Science 248:599 (1990)). Overall, the M0 region displays limited homology with K.sup.+ and Na.sup.+ channel S4 regions (36% with Shaker A, 24% with Na.sup.+ brain I, 4th domain).
In view of the inhibitory effect of ATP on ROMK1 channel activity, the amino acid sequence was also examined for nucleotide-binding motifs. Many but not all ATP-binding proteins contain an amino acid motif representing a phosphate-binding loop (P-loop) which has been suggested to be the only known region of homology common to all ATP/GTP-binding protein superfamily members (Saraste et al., TIBS 15:430 91990)). Furthermore, it often has been found that this Walker Type A consensus motif, GX.sub.4 GKX.sub.7 (I/V) as initially proposed (Walker et al., EMBO J. 1:945 (1982)), adopts distinct characteristics in individual protein families (e.g. GXPGXGKGT for adenylate kinases) (Saraste et al., TIBS 15:430 91990)). In ROMK1, a single motif of this type occurs following the M2 region (FIG. 3) in a segment with predicted secondary structure (small .beta.-sheet--turn containing Gly 228 and Lys 229-.alpha.-helix) not unlike that (small .beta.-sheet--turn -.alpha.-helix) predicted for phosphate-binding loops in adenylate kinase and some other ATP-binding proteins (Chin et al., J. Biol. Chem. 263:11718 (1988)). A number of these latter proteins also share a less conserved Walker Type B sequence (Walker et al., EMBO J. 1:945 (1982)), (H/K/R)X.sub.5-8 .PHI..chi..PHI..sub.2 (D/E) (.PHI.=hydrophobic residue) (Chin et al., J. Biol. Chem. 263:11718 (1988)) which is predicted to form a hydrophobic .beta.-strand ending with a negatively charged residue. A candidate .beta.-strand segment, VVFLD, is located the expected distance away from the P-loop motif toward the C-terminus in ROMK1. No homology was found, however, between the region containing these sequences in ROMK1 and the nucleotide-binding domains of ATP-binding cassette (ABC) superfamily members which include the CFTR protein (Hyde et al., Nature 346:362 (1990)). Taken together, these putative motif sequences in ROMK1 would predict a single ATP-binding site per polypeptide. A number of potential phosphorylation sites (Pearson et al., Methods Enzymol. 200:62 (1991)) for protein kinase C and cAMP-dependent protein kinase occur in close proximity to this nucleotide binding site. In addition, a high density of basic amino acids is also clustered near this site; all of the sixteen charged amino acids occurring in the segment delimited by residues 181 and 232 are positively charged.
Discussion
Patch clamp recordings of excised inside-out patches from injected Xenopus oocytes demonstrate that mRNA derived from ROMK1 cDNA alone is sufficient for the expression at high levels of a potassium channel that exhibits the basic characteristics of ATP-sensitive potassium channels. The topology of the ROMK1 protein suggested by local hydropathy analysis is strikingly novel and represents a major departure from the basic structural design of six membrane-spanning segments characteristic of the superfamily of voltage-gated and second messenger-gated ion channels (Jan et al., Cell 69:715 (1992)). Interestingly, however, ROMK1 shares specific structural features with other potassium channels which provides evidence for a common origin.
We propose that segments M1 and M2 are membrane-spanning and flank the pore-forming H5 region of ROMK1. The conformation of this latter region is likely to closely resemble that of Shaker channel H5 regions given the remarkably tight structural constraints in these segments as demonstrated by the effects of even subtle single amino acid substitutions on pore function. It is reasonable, then, that the channel formed by ROMK1 proteins is at minimum a tetrameric complex as in the case of Shaker channels. Also by analogy to voltage-gated potassium channels, glutamic acid residues (Glu 123 and Glu 151) may form a ring of negatively charged residues in the external vestibule of the ROMK1 pore composed of the M 1-P and P-M2 linkers. The only predicted glycosylation site for ROMK1 (Asn 117) is consistent with the extracellular location of the M 1-P linker. In contrast, both the highly charged hydrophilic N-terminal and C-terminal segments of the protein are probably cytoplasmic. The topology of the amphipathic M0 segment is unclear. Both the absence of a hydrophobic signal sequence and the predominance of positively-charged residues in the N-terminal segment (Hartmann et al., Proc. Natl. Acad. Sci. USA 86:5786 (1989)) would suggest that this end of the protein is located in the cytoplasm therefore making it unlikely that M0 spans the membrane completely. However, given the homology of the M0 sequence to Shaker S4 sequences, there remains the possibility that M0 interacts with the membrane in some manner. If the M0 segment were to fulfill a structural requirement in the protein, the modest voltage-dependence of ROMK1 gating would be consistent. In fact, S4-like sequences have been found in cyclic nucleotide-activated channels (Kaupp et al., Nature 342:762 (1989); Dhallan et al., Nature 347:184 (1990)) which are not voltage-gated. It has been proposed that the S4 segment arose in an ancestral channel and serves an essential structural function, apart from its role as a voltage-sensor, in the underlying core architecture of ion channels (Jan et al., Nature 345:672 (1990)). A similar argument may be relevant to the M0 region in ROMK1.
In the recently revised model of voltage-gated potassium channel structure proposed by Durell and Guy (Durell et al., Biophys. J. 62:238 (1992)), the Shaker channel is postulated to be composed of an outer ring of sixteen .alpha.-helices (segments S1, S2, S3 and S5), a middle ring of eight .alpha.-helices (segments S4 and S6), and an innermost .beta.-barrel structure composed of P segments. The ROMK1 channel protein which has a maximum of three putative transmembrane segments would clearly have a different structure given its lack of regions corresponding to segments S1, S2, and S3. One possible model would consist of a .beta.-barrel of P segments supported by a surrounding framework of eight transmembrane segments (M 1 and M2). The marked structural differences exhibited by ROMK1 would suggest that the ancestral gene encoding for this channel diverged early on during evolution from genes encoding voltage-gated and cyclic nucleotide-gated ion channels. Interestingly, it has been recently suggested that members of a superfamily of putative channel proteins (Baker et al., Cell 60:185 (1990)) which have six putative transmembrane segments and which may form tetrameric complexes (lens fibre major intrinsic protein (MIP), Drosophila neurogenic gene bib, E. coli glycerol facilitator (glpF), and soybean nodulin 26 (nod26)) may have evolved by the gene duplication of a single structural motif containing three membrane-spanning segments (Wistow et al., TIBS 16:170 (1991)).
The primary structure of ROMK1 also provides a possible insight into the role of ATP in regulating channel activity. The inhibition of K.sub.ATP channels by intracellular ATP is generally felt to result from the binding of ATP to a channel regulatory site(s) without the need for hydrolysis (Ashcroft et al., Cellular Signalling 2:197 (1990); Nichols et al., Am. J. Physiol. 261:H1675 (1991); Nichols et al., J. Gen. Physiol. 94:693 (1989)), although alternative models have been proposed (Ribalet et al., J. Gen. Physiol. 94:693 (1989)). In ROMK1, a single putative ATP-binding site identified by a Walker Type A nucleotide binding motif occurs in the long cytoplasmic stretch of amino acids following M2 in a region with predicted .alpha.-helical and .beta.-sheet structure. The relative position of the nucleotide-binding site immediately following the hydrophobic domains of ROMK1 is similar to that of the single cyclic nucleotide-binding site in cAMP- and cGMP-gated channels. Moreover, these latter channels and K.sub.ATP channels both share steep nucleotide-dependent gating kinetics with Hill coefficients of about 2 (Kaupp et al., Nature 342:762 (1989); Nichols et al., Biophys. J. 60:1164 (1991)). A recent kinetic model for the K.sub.ATP channel, which features the sequential binding of ATP to multiple sites, suggests that the channel consists of four monomers with each monomer containing a single ATP-binding site (Nichols et al., Biophys. J. 60:1164 (1991)). This would be consistent with a tetrameric model of the ROMK1 channel which would predict a total of four ATP-binding sites. Likewise, the cGMP-gated channel, which has been found to be cooperatively activated by three or more cGMP molecules, is thought to be a homo-oligomeric complex with each component having a single cGMP-binding site (Kaupp et al., Nature 342:762 (1989)); evidence suggests that cyclic nucleotide-gated channels share a common structure with Shaker channels (Jan et al., Nature 345:672 (1990)).
On the other hand, the loss of K.sub. ATP channel activity in isolated membrane patches (channel rundown) is at least partially reversed by MgATP but not by nonhydrolyzable analogues which suggests a role for phosphorylation in maintaining channel activity (Ribalet et al., J. Gen. Physiol. 94:693 (1989); Takano et al., Am. J. Physiol. 258:H45 (1990); Fiondlay, I., Pflugers Arch. 410:313 (1987)). Protein kinases A and C (Ribalet et at., J. Gen. Physiol. 94:693 (1989); Wolheim et at., EMBO J. 7:2443 (1988); De Weille et al., Proc. Natl. Acad. Sci. USA 86:2971 (1989)) have been found to modulate K.sub.ATP channel activity in different cell types. Likewise, the regulation of channel activity by PKA, PKC, and endogenous protein kinases has been demonstrated for both delayed rectifier (Walsh et al., Science 242:67 (1988); Rehem et al., Biochemistry 28:6455 (1989); Busch et al., Science 255:1705 (1992)) and Ca.sup.2+ -activated potassium channels (Reinhart et al., J. Neurosci. 11:1627 (1991); Chung et al., Science 253:560 (1991)). The presence of several potential phosphorylation sites for protein kinases A and C near the putative ATP-binding site in the ROMK1 protein is therefore intriguing. Clustering of multiple phosphorylation sites to a specific cytoplasmic domain has been noted in the .alpha. subunit of the rat brain Na.sup.+ channel (Rossie et at., J. Biol. Chem. 262:17530 (1987)), in subunits of some ligand-gated ion channels (e.g., muscle and neuronal acetylcholine receptors (AChR), .gamma.-aminobutyric acid (GABA) and glycine receptors) (Swope et al., FASEB J. 6:2514 (1992)), and in the R domain of the cFTR protein (Riordan et al., Science 245:1066 (1989)). In ROMK1, the apparent close association of phosphorylation sites and the putative nucleotide-binding sites raises the possibility that both types of sites may exert an effect on or participate in a common mechanism affecting channel opening and closure. In the "ball and chain" model of inactivation initially proposed for NA.sup.+ channels (Armstrong et al., J. Gen. Physiol. 70:567 (1977)), the movement of a cytoplasmic domain results in the occlusion of the ion channel pore and therefore channel inactivation. Such a mechanism has been demonstrated in Shaker channels involving an N-terminal segment consisting of a cluster of positively-charged amino acids and a hydrophobic domain (Hoshi et al., Science 250:533 (1990); Zagbotta et al., Science 250:568 (1990)); an analogous role has been proposed for the highly charged R domain of the CFTR protein (Riordan et al., Science 245:1066 (1989)). The finding that cytoplasmic proteolytic treatment of Shaker and NA.sup.+ channels either disrupts or slows inactivation is therefore consistent with the role of a cytoplasmic domain in channel inactivation. Similarly in pancreatic .beta.-cell K.sub.ATP channels, internal trypsin treatment results in an increase in channel activity, as well as, a reduction in sensitivity to inhibition by ATP (Trube et al., in Secretion and Its Control, G. S. Oxford and C. M. Armstrong, Eds. (Rockefeller University Press, New York, 1989), vol. 44, pp. 84-95.). A potential candidate for such an inactivation domain in ROMK1 is the highly-charged cytoplasmic segment following M2 (residues 181-232) which contains the putative phosphorylation sites and P-loop. Other possibilities include the N-terminal (residues 1-57) and C-terminal (residues 331-391) segments in which 35% and 39%, respectively, of the amino acids are charged. All three clusters of potential trypsin sites in ROMK1 overlap these three amino acid segments. The knowledge of the ROMK1 amino acid sequence enables the design of experiments which address these issues and many others.
Claims
What is claimed:
1. An isolated nucleic acid molecule encoding a peptide consisting essentially of the amino acid sequence depicted in Sequence ID No. 2.
2. The isolated nucleic acid molecule of claim 1 wherein said sequence consists essentially of the nucleotide sequence depicted in Sequence ID No. 1.
3. The isolated nucleic acid molecule of claim 1 or 2 wherein said sequence further consists essentially of a vector selected from the group consisting of plasmids, phage, retrovirus, baculovirus and integration elements.
4. The vector of claim 3 wherein said vector is an expression vector.
5. An isolated nucleic acid molecule, which is capable of hybridizing to the isolated nucleic acid molecule of claim 1 or 2, wherein said hybridization occurs at about 5.degree.-65.degree. C. and in 5X SSPC and 50% formamide or equivalent hybridization conditions thereto.
6. A method of isolating the nucleic acid molecule of claim 5 comprising the steps of: a) contacting a genomic or cDNA library with a probe comprising the sequence depicted in Sequence ID No. 1; b) identifying sequences within said library which are capable of hybridizing to said probe, and c) purifying said sequence, wherein said hybridization occurs at about 50.degree.-65.degree. C. in about 5X SSPC and 50% formamide or equivalent hybridization conditions thereto.
7. The method of claim 6 wherein said probe is detectably labeled.
8. A method of using the isolated nucleic acid molecule of sequence ID No. 1, or a sequence which hybridizes under stringent condition to said sequence ID No. 1, to produce a peptide consisting essentially of the amino acid sequence of Sequence ID No. 2, comprising the steps of: a) transforming a host with a DNA sequence capable of encoding said peptide, b) incubating said host under conditions which allows said sequence to be express; and c) isolating said peptide from said host.
9. The method of claim 8 wherein said DNA sequence comprises the nucleotide sequence of Sequence ID No. 1.
10. The method of claim 8 wherein said host is selected from the group consisting of bacteria, yeast, fungi, mammalian cells, plant cells, and insect cells.
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