US 5,783,405 AGrant
Rapid Screening Method for Effectors of Signal Transduction
Issue Date:1998-07-21
•12 Claims
•5 Drawing Sheets
Abstract
Methods for assessing libraries of candidate modulators of intracellular signaling transmission pathways are described. The methods assess the ability of candidates from the library to inhibit the binding of peptides which represent participants in the signaling pathways as either a signal-generating protein or its cognate partner binding at a noncatalytic site. Specific peptides useful in this regard have been identified.
Metadata
Assignee
- Terrapin Technologies, Inc.
Inventors
- Daria Mochly-Rosen
- Dorit Ron
- Lawrence M. Kauvar
- Eugene W. Napolitano
Application Information
Application Number:US 5419646
Filing Date:1995-10-10
Priority Date:1995-06-07
Art Unit:184
Classifications
IPC:
C12Q 148C12Q 168
Field of Search:
4356;7.8;15
Patent Drawings (5 sheets)
Description
TECHNICAL FIELD
The invention relates to intracellular signal transduction. More specifically, it concerns methods to identify modulators of intracellular signal transduction by assessing the ability of candidate modulators to affect the interaction between a catalytically active signal-generating protein and a cognate binding protein involved in modulating the signal transduction function. The invention also concerns specific embodiments of such methods and the components thereof.
BACKGROUND ART
PCT Application WO 95/21252 described above, discloses and claims peptide compositions which alter the activity of a signal-generating protein with respect to its cognate protein wherein the cognate protein contains at least one WD-40 region which putatively interacts with the signal-generating protein. The peptide compositions mimic the WD-40 regions, thus competing with the interaction of the cognate with the signal-generating protein. This competition results either in inhibiting the signal-generation or activating it.
One specifically exemplified signal-generating protein is protein kinase C (PKC); the illustrated cognate receptor for activated kinase C (RACK), in this case specific for .beta.PKC, was designated RACK1. The gene encoding RACK1 was cloned and sequenced, showing that RACK1 contains the requisite WD-40 regions.
The above PCT application and parent applications herein further describe methods to identify additional pairs of signal-generating proteins and their cognates and methods for recognizing WD-40 sequences in the cognates. These applications also note that such interactions can be used as a system to identify additional molecules that bind the signal-generating protein by measuring the effect of candidate binding molecules on the interaction between the signal-generating protein and either its cognate per se or the polypeptide compositions that mimic the WD-40 regions of the cognate.
In the present invention, several specific peptides have been identified that bind either to the signal-generating protein or to the cognate protein in a signal-affecting manner. The use of the signal-generating protein/cognate system to assay for modulators of signal transduction in assays which are independent of the purity of these participants is described. The PKC enzyme system is illustrated as a specific embodiment. In addition, peptides which reside on the signal-generating protein, as well as those which reside on the cognate or mimics thereof, can be used to modulate the signal-generating interactions.
DISCLOSURE OF THE INVENTION
The present invention is directed to an efficient assay system to identify modulators of intracellular signaling pathways. Because the method takes advantage of inherent biological specificity, it can be conducted on impure preparations of the participants in the signal pathway--the signal-generating protein and its cognate receptor controlling the signal pathway. The assay is conducted by assessing the interaction between the signal-generating protein and its cognate either by measuring binding directly or by measuring a physiological or metabolic effect. The measurement is made in the presence and in the absence of a candidate modulator. Successful candidates which agonize the signal effect an increase in a metabolic or physiological output; antagonists effect a decrease. Both antagonists and agonists compete for binding between cognate and signal-generating protein.
Among successful candidates will be peptides which mimic regions on either the signal-generating protein or the cognate as well as nonpeptide small molecules. Due to their ease of identification, these peptides are particularly useful in alternate forms of the screening assays that detect binding between the peptide and the signal-generating or cognate protein. Although the assay methods disclosed may not all be suitable for direct screening of large chemical libraries, they do enable a sophisticated screening of candidates that can be combined with other techniques for selecting leads.
The methods described herein may involve peptides derived from the cognate or signal-generating protein. By "derived from" we mean that such peptides are either found in the cognate or signal-generating protein, or are modified by a limited number of conservative changes. Preferably the conservative changes represent less than about 20%, more preferably less than about 10%, and most preferably less than about 5% of the residues in the identified native sequence. One way to identify a suitable peptide is to compare sequences among species as is described in Example 4.
The invention is also directed to methods to screen libraries of candidate modulators using the above-described methods and to peptides representative of sites on the signal-generating protein and cognate which are themselves useful in these assays as well as in other applications involving the relevant interaction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows, diagrammatically, the known general primary sequence and domains of various PKC isoenzyme families.
FIG. 2 shows the result of experiments demonstrating that PKC mediated effects on contraction of cardiac myocytes is inhibited by a fragment of the regulatory domain of .epsilon.PKC but not by a corresponding fragment of .delta.PKC.
FIG. 3 shows the specific effect of an 8 amino acid peptide derived from the RACK-binding site in the regulatory domain of .epsilon.PKC on the contraction rate of cardiac myocytes; an analogous peptide from the .beta.PKC has no effect.
FIG. 4 shows the specific effect of peptides derived from the RACK-binding site of .beta.PKC on maturation of Xenopus oocytes.
FIG. 5 shows the effects of peptides derived from RACK1 on PKC mediated maturation of Xenopus oocytes.
FIG. 6 shows the effect of various peptides on the binding of .theta.PKC V1 fragment to RACK1 in vitro.
FIG. 7 shows the differential effects of the peptide T2 on translocation of .theta.PKC V1 fragment and .beta.PKC in intact cells.
MODES OF CARRYING OUT THE INVENTION
The invention is, perhaps, best understood as a generalization of an illustrative interacting signal-generating pair, wherein the signal-generating protein is PKC and the cognate is an appropriate RACK. A peptide that mimics a PKC binding element contained in the RACK or in the PKC or that mimics a RACK-binding element contained in the PKC can be used as a component of assays relevant to the signal pathway.
PKCs represent a family of signal-generating isoenzymes, at least several of which are present in each cell type. Upon activation by a suitable agent, typically phosphatidylserine (PS) and diacylglycerol (DAG), and in some cases calcium ion, a PKC is translocated subcellularly, generally from the soluble fraction to another location in the cell that is associated with the particulate fraction. Each isoenzyme in this family apparently has a cognate anchoring protein at the appropriate location associated with the physiological or metabolic effect of the activation of each particular isoenzyme. Thus, for example, one or a subset of PKCs contained in cardiac myocytes, when activated, results in a slowing of the contraction rate.
One or a subset of PKCs contained in Xenopus oocytes, when activated, effect maturation of the egg. One or a subset of PKCs, when inhibited at the catalytic site, blocks T-lymphocyte activation. See, Birdchall et al. (1994) J. Pharm. Expt'l Ther. 268:922. The interaction of a particular PKC isoenzyme with its cognate RACK is required for the metabolic or physiological effect; therefore interference with this interaction will modulate that effect. Alternatively, the effect of the modulation may be agonistic if the interaction of the modulator promotes a conformational change in the signal generating partner corresponding to that normally occurring only upon the concurrent binding of activators (e.g., PS or DAG) and cognate protein, or otherwise results in signal activation.
The known PKC isoenzymes can be divided into three major groups as shown in FIG. 1. All of the isoenzymes, regardless of group, contain a protein kinase domain represented by two constant (C) and two variable (V) regions. The regions which are responsible for the enzymatic activity are highly homologous or constant; the C4 region toward the carboxy terminus is thought to contain the catalytic site; the C3 regions upstream are responsible for binding ATP.
Upstream of the protein kinase domain in each case is a regulatory domain. All of the three families, the conventional (cPKC), the novel (nPKC) and atypical (aPKC) contain variable regions at the N-terminus designated V1, and constant regions immediately downstream marked C1. The C1 regions are thought to be involved in activation by phosphatidylserine, diacylglycerol, or pharmacological mimics such as phorbol esters. The C2 region is found only in the cPKC family and is thought to be the site for activation by calcium. However, the picture may not be quite so simple as C1 regions may also be involved in calcium binding, and the atypical class respond poorly to agents such as phorbol esters.
Nevertheless, it now appears clear that sequences is within the regions shown as the regulatory domain are responsible for the interaction of the particular PKC with its cognate RACK. They may also contain a RACK-mimicking region, called a pseudo-RACK site, that prevents binding of PKC to its RACK when the PKC has not been activated. This situation is analogous to the pseudosubstrate sequence which is located elsewhere in the primary sequence and blocks the catalytic site prior to activation of the PKC. It is shown hereinbelow that the relevant regions are specific for the particular isoenzyme involved in a designated signal-generation event.
In the parent applications herein, published as PCT W095/21252, the cognate RACK1 protein which interacts with .beta.PKC (a member of the cPKC family) was cloned and the WD-40 regions putatively responsible for binding to the .beta.PKC were identified through structural analogy. One of these WD-40 peptides was found to induce the kinase activity of PKC in the absence of PKC activators; both this peptide and another representing a WD-40 region rendered the .beta.PKC susceptible to proteolysis, a characteristic of activated PKC forms. All of these peptides were also shown to inhibit the binding of .beta.PKC to RACK1. In principle, the WD-40 regions of the appropriate RACK can serve as antagonists or agonists of the signal generation associated with the corresponding PKC. As described, an assay which shows the effects of members of a library of candidate modulators on interaction between the relevant PKC and its cognate or the relevant PKC and a WD-40 domain derived from said cognate can be used as a screening assay to identify modulators of this signal pathway.
In the illustrative work described below, similar modulation of signal generation is achieved as before by supplying, to a reaction mixture containing PKC, WD-40 peptides derived from the relevant RACK or pseudo-RACK peptides from the PKC regulatory domain which themselves mimic the RACK's binding domains, and examining the effect of a candidate on binding or signal generation. Similarly, the RACK-binding peptides derived from PKC can be used as assay reagents in combination with the appropriate RACK to screen for modulators of the signal-generating pathway by virtue of the ability of the successful candidate to affect the binding of the cognate protein to the signal-generating protein.
Thus, in summary, various counterpart interactions can be tested; in no case are purified components required:
In general, the present invention is directed to screening methods to identify modulators of particular signal pathways. Each assay will involve identifying a cognate protein that binds sufficiently specifically to a catalytically active signal-generating protein, via a noncatalytic site, to permit assay in impure preparations. The interaction of these two components is observed in the presence and absence of a candidate modulator. Depending on the assay system chosen, the interaction and its modification can be observed in a variety of ways, including intracellular binding assays affecting an observable parameter; either a physiological readout, such as change in subcellular distribution, or an artificial construct, such as transcription of a reporter gene, can be used. In no case, however, are purified reagents required, although it may be convenient in some cases, for example, to utilize the peptides identified as illustrated below which represent regions of the signal-generating protein (illustrated by PKC) or its cognate binding protein (represented by the relevant RACK) that are responsible for interaction.
As further described below, the peptides which can be substituted for one or the other component in the assay method are themselves identifiable through conduct of the assay. Thus, the ability of a peptide to affect the interaction of the cognate protein and the signal-generating protein will identify it as a useful component of the assay, as well as a modulator of the signal pathway per se. Once the appropriate peptides are identified, the individual labeled peptides could be used to assess the level of binding. The labeled peptide may represent a region of the signal-generating peptide measured against a composition containing the cognate protein or, conversely, a peptide representing a portion of the cognate protein measured against the composition containing the signal-generating protein. These compositions may be whole cells or cell-free extracts or partially purified extracts.
It will be apparent that when a peptide is chosen as one component of the assay, the screening tests are preferably performed by measuring only binding per se.
Alternatively, both the signal-generating protein and the cognate protein may be contained in a crude preparation and the method for assessing their interaction may include measuring localization of the signal-generating protein within the preparation per se or measuring a metabolic effect of the interaction, such as, for example, maturation of Xenopus oocytes or effect on the contraction rate of cardiac myocytes. The particular method of assessing the interaction will, of course, be appropriate to the partners in the interaction, and can readily be ascertained by taking advantage of the specificity of the signal pathways and their components as illustrated below.
Thus, for convenience, the assays to identify modulating candidate compounds will be described as measuring the effect of the candidate on the "binding" of the counterpart components in the reaction mixture. It will be understood that in the instance where both the cognate protein and the signal-generating protein are the active components of the composition participating in the assay, binding may be measured not only directly, but also by the resulting metabolic or physiological effects.
The examples below illustrate the signal generation specificity in the PKC isoenzyme families. They also illustrate the identification of peptides that may serve as counterpart members of a signal modulation assay.
These examples are intended to illustrate but not to limit the invention.
EXAMPLE 1
Specificity of Negative Chronotropy for .epsilon.PKC Translocation
Neonatal rat cardiac myocytes were used in this assay. These cells, when prepared in culture, exhibit contractions at approximately the rate of 40-50/15 sec., and it is known that the phorbol ester, 4-.beta. phorbol 12-myristate-13-acetate (PMA) reduces the contraction rate (Johnson, J. A. et al., Circ Res (1995) 76:654-653). Previous work has also shown that treating cardiac myocytes with PMA or with norepinephrine (NE) causes translocation of .alpha.PKC to the nuclear boundary, .beta.IPKC to the interior of the nuclei, .delta.PKC to the fibrillar and perinuclear structures, and .epsilon.PKC to cross-striated structures (Disatnik, M.-H. et al., Exp Cell Res (1994) 210:287-297). It has also been shown that exogenously added activated PKCs bind similarly (Mochly-Rosen, D. et al., Molec Biol Cell (1990) 1:693-706). Since the location to which the various isoenzymes are translocated are different, it has been suggested that the variable regions specific for each isoenzyme (Nishizuka, Y., Nature (1988) 334:661-665) should contain at least part of the specific RACK binding site (Disatnik, M.-H. et al., Exp Cell Res (1994) 210:287-297). Furthermore, it has been suggested that the V1 region of .epsilon.PKC determines its substrate specificity (Pears, C. et al., Biochem J (1991) 276:257-260).
To show that only translocation of the corresponding isoenzyme is inhibited by one of its fragments--e.g., only translocation of .epsilon.PKC is inhibited by an .epsilon.PKC-V1 fragment, cells cultured on chamber slides were permeabilized with saponin (50 .mu.g/ml) in the absence or presence of 100 .mu.g/ml rat recombinant .epsilon.PKC-V1 or .delta.PKC-V1 fragments containing amino acids 2-144 in each case. Cellular functions, including cell viability, spontaneous and stimulated contraction rates, gene expression and hypertrophy are unaffected by the saponin treatment.
These fragments were prepared by amplifying the relevant portion of the gene from a cDNA library (Stratagene). A FLAG.TM. epitope (DYKDDDK) (SEQ ID NO:1) was engineered at the 5' end of the fragment and the 0.45 kb PCR fragment was subcloned into pMAL-C2 vector (New England Biolabs) for overexpression as a fusion protein with maltose binding protein in E. coli. Protein purification and Factor Xa proteolysis of the fusion proteins was as described by Ron, D. et al., Proc Natl Acad Sci USA (1994) 91:839-843.
The intracellular concentration of each fragment was approximately 300 DM or about 3% of the extracellular concentration as determined by quantitative Western blot of washed and extracted cells.
After the .epsilon.PKC-V1 or .delta.PKC-V1 fragments were administered by permeabilization, the cells were incubated with either 4-.alpha. or 4-.beta. PMA. (4.alpha. PMA is not active and is used as a control.) The cells were then fixed with methanol and acetone and PKC isoenzyme localization was determined by immunofluorescence; the antisera used to detect .delta.PKC and .epsilon.PKC do not recognize the administered fragments. Multiple fields of cells for each treatment group and for PKC isoenzymes .alpha., .beta.I, .delta., and .epsilon. were observed and the data were presented as a percentage of cells having the tested enzyme at the activated site. When the cells were treated with 100 nM PMA for five minutes, it was apparent that neither .delta.PKC-V1 nor .epsilon.PKC-V1 had any effect on translocation of the .alpha. or .beta. isoenzymes whereas each of the .delta. and .epsilon. fragments specifically inhibited the translocation of the corresponding isoenzyme, but not the other isoenzyme. An additional experiment measuring translocation of .epsilon.PKC at the much lower level of 3 nM PMA also showed complete inhibition by the .epsilon. fragment. It has previously been shown that 3 nM PMA is only marginally effective in translocation of PKC isoenzymes other than the .epsilon. form (Johnson, J. A. et al., Circ Res (1995) 76:654-663).
The localization of .epsilon.PKC to the cross-striated structures suggested that the .epsilon. isoenzyme might mediate the effect of PMA on the contraction rate. Cells were cultured and permeabilized with saponin in the presence or absence of 150 .mu.g/ml .delta. or .epsilon.PKC-V1 fragments described above. Basal contraction rates were monitored for 10 min, and the cells were then treated with 3 nM PMA. The rate of contraction was monitored over the next 20 minutes. The results are shown in FIG. 2.
As shown, in cells where no fragment was added, the contraction rate is reduced almost to zero within 15 min of the addition of PMA. Similarly, in cells where the .delta.PKC fragment is added, the contraction rate is thus reduced. However, in cells where the .epsilon.PKC fragment was added, the contraction rate is maintained. Thus, the .epsilon.PKC-V1 fragments specifically prevented PMA-induced inhibition of spontaneous contraction. These data, combined with the data described above with respect to translocation and the fact that the .epsilon.PKC-V1 fragment does not affect the catalytic activity of .epsilon.PKC in vitro, demonstrate that the translocation of .epsilon.PKC is an essential step in signaling the chronotropic effect of PMA and that this signaling is inhibited by a fragment containing the V1 region.
The effect of PMA in reducing the contraction rate can be mimicked by controlling the .alpha.1 and .beta.1 adrenergic receptors of the myocytes, providing a more physiologically relevant phenomenon. If both the .alpha.1 and .beta.1 receptors are activated with NE, an increase in contraction rate occurs; when both receptors are inhibited, NE no longer has this effect. If the .alpha.1 receptor is inhibited alone by prazosin, the initial increase in contraction rate is higher; if the .beta.1 receptor alone is inhibited, the contraction rate decreases.
When either the .delta. or .epsilon. fragments described above is substituted for the known inhibitors of the .alpha.1 and .beta.1 receptors, the behavior of the cells in response to NE is unaffected by the presence of the .delta. fragment; however, addition of the .epsilon. fragment gives a response similar to that obtained in the presence of prazosin. These data are consistent with the role of the .epsilon. fragment in controlling contraction rate since the .alpha.1 receptor (inhibited by prazosin) mediates PKC translocation.
FIG. 3 shows the results of a similar experiment using stimulation with 3 nM PMA, and providing peptides of less than 10 residues that block localization of .epsilon.PKC and .beta.PKC using a 0.1 .mu.M .epsilon.PKC-derived peptide .epsilon.V1-2 (sequence EAVSLKPT (SEQ ID NO:2)) or 20 .mu.M of a .beta.PKC-derived peptide .beta.C2-4 (sequence SLNPEWNET (SEQ ID NO:3)). As shown in FIG. 3, stimulation with 3 nM PMA without adding peptides to the permeabilized cells or in the presence of 20 .mu.M of the .beta.PKC localization inhibitor results in negative chronotropy as above.
EXAMPLE 2
Specific Inhibition of .beta.PKC Translocation by C2-Derived Peptides
The parent application herein described binding sites on a particular RACK, RACK1, which are responsible for binding .beta.PKC. It is recognized that if the binding site on PKC is identified, peptides that mimic this binding site could also serve as modulators of .beta.PKC translocation and function. Furthermore, it should be noted that PKC may itself contain pseudo-RACK peptide sequences that mimic the binding sites on RACK and regulate the exposure of the binding site for RACK on PKC. The following experiments do not distinguish between these possibilities; nevertheless, whichever function on the PKC sequence is represented, mimics of the sequence will be effective modulators of the relevant signal pathway.
The cPKC class of isozymes comprises the only members of the PKC general family that contains C2 regions. Other C2-containing proteins such as synaptotagmin and phospholipase C.gamma. also bind to a mixture of RACKs prepared from cell particulate fractions. It has also been demonstrated that recombinant fragments of synaptotagmin containing the C2 homologous region bind to mixtures of RACKs and inhibit PKC binding to RACKs (Mochly-Rosen, D. et al., Biochemistry (1992) 31:8120-8124).
The following experiments demonstrate that certain peptides residing in the C2 region of .beta.PKC are able to inhibit translocation of .beta.PKC and the maturation of Xenopus oocytes.
The following .beta.PKC-derived peptides were prepared:
.beta.C2-1 (SEQ ID NO:4): KQKTKTIK (210-217);
.beta.C2-2 (SEQ ID NO:5): MDPNGLSDPYVKL (186-198);
.beta.C2-3 (SEQ ID NO:6): IPDPKSE (201-207);
.beta.C2-4 (SEQ ID NO:3): SLNPEWNET (218-226);
Scrambled .beta.C2-1 (SEQ ID NO:7): TKQKKITK;
Control Peptide (SEQ ID NO:8): LQKAGVDG (266-271).
Recombinantly produced fragments of .beta.PKC were expressed as fusion proteins with GST: Fusion L9 includes the V1 region, the pseudosubstrate sequence, and the C1 and V2 regions (residues 3-182) of .beta.PKC. L10 includes the V1 region, the pseudosubstrate sequence and the first cysteine repeat from the C1 region, as well as the entire C2 and V3 regions (residues 3-76 and 143-339). The numbering is as described in Luo, J.-H. et al., J Biol Chem (1993) 248:3715-3719.
Standard overlay assays were performed by blotting RACK1 onto nitrocellulose as described by Mochly-Rosen, D. et al., Proc Natl Acad Sci USA (1991) 88:3997-4000. Strips of the nitrocellulose sheet containing 0.1-1 .mu.g RACK1 per strip were incubated in overlay buffer with or without the test fragment added at approximately 10 .mu.M. Addition was in the presence or absence of 50 .mu.g/ml phosphatidyl serine (PS) and 1 mM calcium. The mixture was further incubated for 30 min at room temperature. The strips were then washed and binding of fragment of L9 or L10 to RACK1 was detected with anti-GST polyclonal antibodies followed by labeling with anti-rabbit horseradish peroxidase-linked antibodies and development by addition of substrate.
Using this assay, L10, but not L9 was found to bind RACK1. The PKC activators phosphatidyl serine and calcium did not increase the binding of L10 to RACK1, although these activators are necessary for the binding of intact PKC to RACK1. Thus, these data are consistent with the suggestion that the PKC activators are required to expose the RACK binding site in the intact PKC; this site is already exposed in the C2-containing fragment L10.
To determine whether L10 would inhibit the binding of intact .beta.PKC to RACK1, RACK1 was immobilized on an amylose column and .beta.PKC binding in the presence of PS, DAG and calcium and in the presence of L10 or L9 was determined. In the presence of L10, .beta.PKC binding to RACK1 was completely inhibited; however, this was not true of L9. Similar results were obtained in an overlay assay.
Similar overlay assays were conducted using the above-listed peptides as candidate inhibitors for the binding of L10 to RACK1. The C2-derived peptides .beta.C2-1, .beta.C2-2 and pC2-4 peptides were successful in inhibiting binding of L10 to RACK1; however .beta.C2-3 and scrambled .beta.C2-1 were not.
In addition to the foregoing cell-free assays, the association of .beta.PKC with RACK1 and the ability of peptides derived from the C2 region to interrupt this interaction was tested in rat neonatal cardiac myocytes in culture. The presence of RACK1 in these cells was confirmed by immunostaining. RACK1 was found at perinuclear structures and throughout the cytosol. Treating with NE or PMA did not alter these locations. It was also demonstrated that activated .beta.II PKC, but not C2-less isoenzymes .delta. or .epsilon.PKC, colocalized with RACK1.
The C2-derived peptides that had been shown to inhibit .beta.PKC binding to RACK1 in vitro were then tested for their ability to inhibit activation-induced translocation in myocytes.
The myocytes were exposed to 100 nM PMA for 15 min after transient permeabilization with saponin (50 .mu.g/ml) in the presence and absence of the test peptides. 80% of the cells that had not been treated with peptides showed localization of .beta.1 PKC to perinuclear structures. However, when .beta.C2-1, .beta.C2-2 or .beta.C2-4 at 10 .mu.M extracellular concentration had been supplied to the permeabilized cells, translocation of both .beta.I PKC and .beta.II PKC isoenzymes was inhibited by 65-95%. .beta.C2-4 was the most effective. Control peptides described above did not affect translocation.
Consistent with the results in Example 1, treating nonpermeabilized cardiac myocytes with 100 nM PMA resulted in translocation of .epsilon.PKC from the nucleus to the perinuclear and cross-striated structures and of .delta.PKC from the perinuclear and fibrilar cytosolic structures in 80% and 90% of the cells respectively. Permeabilization and treatment of the cells with the C2 peptides derived from .beta.PKC had no effect on the translocation of these C2-less isozymes.
While the chronotropy of myocytes is not affected by .beta.PKC isoenzymes, the insulin-induced maturation of Xenopus oocytes is mediated by the .beta. form. Insulin treatment of these oocytes results in translocation of .beta.PKC and maturation is delayed by the PKC-specific catalytic inhibitor pseudosubserate peptide. PKC translocation is blocked by injection of purified RACKs or a peptide corresponding to the PKC binding site on RACKs. (Smith, B. L. et al., Biochem Biophys Res Commun (1992) 188:1235-1240; Ron, D. et al., J Biol Chem (1994) 269:21395-21398).
Accordingly, the maturation of Xenopus oocytes was used as an alternative assay system to test the function of the peptides derived from the C2 region described above. In this assay, oocytes were injected with 50 .mu.M of the test peptide one hour before insulin treatment (8.25 .mu.g/ml). Insulin-induced oocyte maturation was then determined by monitoring the appearance of a white spot in the animal pole of the oocyte that is indicative of germinal vesicle breakdown in maturation. 10-15 oocytes were included per assay and oocytes were scored for 35 hours after treatment. As expected, .beta.C2-1, .beta.C2-2 and .beta.C2-4 supplied in the range of 5 .mu.M-500 .mu.M significantly delayed oocyte maturation in a dose-dependent manner. The control peptides did not. The association of this effect with the prevention of translocation of .beta.PKC to the particulate fraction in Xenopus oocytes was confirmed in a separate experiment. The peptide .beta.C2-4 inhibited .beta.PKC translocation but not .theta.PKC in T-Jurkat cells. FIG. 4 shows the effect of these various peptides on Xenopus oocyte maturation.
EXAMPLE 3
Agonist Effect of Interacting Peptides
The oocyte maturation assay described above was also used to test the effect of various peptides derived from the PKC/RACK1 pair. Peptide I, derived from RACK1, as expected, inhibits the maturation of Xenopus oocytes presumably by interfering with the binding of .beta.PKC1 to RACK1. On the other hand, a short peptide, rVI derived from the sixth WD-40 repeat in RACK1 enhances maturation. Ron, D., Mochly-Rosen, D., J. Biol. Chem. (1994) 269:21395-21398 This result is shown in FIG. 5. This peptide is believed to interfere with the RACK-mimicking site on PKC which normally covers the RACK-binding site in the absence of activation.
EXAMPLE 4
Interaction Peptides Derived from .theta.PKC
.theta.PKC is a member of the nPKC family and lacks a C2 region. Comparison of variable sequences of .epsilon.PKC to other isozymes reveals regions of maximum disparity. Of these regions, some are strongly conserved across vast phylogenetic spans, e.g., from mammals to the invertebrate Aplysia. Isozyme specific sequences that are strongly conserved by evolution are probable sites for binding cognate proteins. Comparing .delta.PKC to .theta.PKC in the analogous region allowed identification of a .theta.-specific peptide expected to interfere with PKC binding to a RACK. Peptides with these characteristics from the V1 region of .theta.PKC were prepared and tested for their ability to inhibit the binding of .theta.PKC V1 fragment to RACK1 in vitro. The results are shown in FIG. 6. Of a multiplicity of peptides tested, both from other regions of the .theta.PKC isoenzyme and from alternative isoenzymes in the family, only .theta.V1 derived peptides .theta.V1-1 and .theta.V1-2, having the amino acid sequences GLSNFDCG (SEQ ID NO:9) (.theta.PKC residues 8-15) and YVESENGQMYI (SEQ ID NO:10) (.theta.PKC residues 36-46), respectively, were able to affect the interaction negatively. As expected, peptides rIII and rVI derived from the WD-40 regions of RACK1 were also effective.
The peptides .theta.V1-1 and .theta.V1-2 could inhibit the translocation of .theta.PKC to the cellular particulate fraction in T-Jurkat cells. The specificity of this interaction is shown in FIG. 7 which shows inhibition by the .theta.PKC V1 peptide (T2) of the translocation of .theta.PKC but not of .theta.PKC.
Sequences derived from other PKC regions.
V3 region. The V3 or hinge region separates the regulatory and catalytic domains. This region contains the sites of proteolysis by trypsin and calpain. The lack of conservation of the V3 domain between the different PKC isozymes suggests that this section may also be at least, in part, involved in targeting the PKC isozymes to their anchoring proteins. Furthermore, it has been demonstrated that regions within the V3 of .alpha.PKC mediate the translocation of that isozyme to the nucleus (James G., and Olson E. J. Cell Biol. 116:863-873, 1992). The V3 region of .theta.PKC was found to bind to RACK1 in vitro. Therefore, the V3 region could affect not only the targeting of the activated isozymes (in which the V3-region is exposed) but could also regulate the enzyme susceptibilities to proteolysis.
V5. The amino acid sequences of the .beta.PKC isoforms .beta.IPKC and .beta.IIPKC are identical except for variability within the V5 region (35 amino acids for .beta.IPKC and 38 amino acids for .beta.IIPKC). Upon activation, .beta.IPKC and .beta.IIPKC translocate to different localization sites in the cell (Disatnik M.-H., Buraggi G., Mochly-Rosen D. Exp. Cell Res. (1994) 210:287-297). This difference in localization of isozymes that are almost identical can be explained by the importance of the V5 region in mediating their targeting. Moreover, .beta.IIPKC was found to selectively translocate to the nucleus upon proliferative stimulation where it selectively phosphorylated the nuclear envelope protein Iamin B.sub.1. (Murray N. R. Burns D. J. Fields A. P. (1994) J. Biol. Chem. 269:21385-21390).
PKC-related proteins. Recently the human cDNAs encoding two novel protein kinases have been cloned. These proteins termed PRK1 and PRK2 (protein kinase C related kinase 1 and 2) show high homology to each other and some homology to the regulatory region of PKC (Palmer R. H., Ridden J., P. J. Parker Eur. J. Biochem. (1995) 227:344-351. Since the sequences within the regulatory domain of PKC are responsible for the interaction between a PKC and its anchoring proteins, sequences from PRK1 and PRK2 which show homology to functionally important sequences within the regulatory domain of PKC, are likely to be of biological importance.
Sequences from other isozymes and related proteins that meet the same isozyme selectivity/evolutionary conservation criteria include the following:
Peptides derived from the V1 region of PKC isozymes (Human):
Peptides derived from the V3 region of PKC isozymes (Human):
Peptides derived from the V5 region of PKC isozymes (Human):
Peptides derived deom protein kinase C related proteins (Human):
.mu.V1-1, .mu.V1-2 derived from .mu.PKC were picked because they aligned with .epsilon.V1-2 and .theta.V1-2 and part of .theta.V1-1 respectively. .lambda.V1-1 and .lambda.V1-2 from .lambda.PKC were picked based on their alignment with .epsilon.V1-2 and part of .epsilon.V1-3 and .theta.V1-2 respectively. .zeta.V1-1, .zeta.V1-2, .zeta.V1-3 derived from .zeta.PKC were picked according to their homology to: .epsilon.V1-2, .theta.V1-2, and .epsilon.V1-3 respectively. PRK1-1 and PRK2-2 were identified according to their homology to .beta.C2-1. PRK1-2 and PRK2-2 were identified according to their homology to the biologically active .epsilon.PKC-derived peptide .epsilon.V1-3 and part of .epsilon.V1-2. PRK1-3 and PRK2-3 were picked according to their alignment with the peptide .epsilon.V1-5.
The peptide sequences were generated by aligning the human PKC sequences and the human PRK1 and PRK2 sequences using the MegAlign DNASTAR Inc. program. The sequences were aligned by using the clustal method. The algorithm groups sequences into clusters by examining the distances between all pairs. The clusters are aligned, first individually, then collectively to produce an overall alignment. (Higgins D. G. and Sharp, P. M. (1989). CABIOS, Vol 5, No 2, 151-153). The matrix for the alignment was PAM250 (percent accepted mutation 250-2.5 mutations per residue). This matrix allows only high stringency alignments.
Claims
We claim:
1. A method to ascertain a modulator of intracellular signal transduction mediated by the signal-generating protein, protein kinase C (PKC), from a library of candidate molecules, which method comprises providing the cognate receptor for activated kinase C (RACK) or a peptide derived from said PKC or the cognate RACK which binds specifically to the catalytically active PKC via a noncatalytic site of said PKC; testing the candidate molecules of said library for the ability to modulate positively or negatively the binding of said cognate RACK or said peptide to the PKC in an assay not dependent on purity of the cognate RACK or PKC; and ascertaining as said modulator any candidate molecule that enhances or diminishes said binding.
2. The method of claim 1 wherein the testing is by a method which comprises contacting the PKC with the cognate RACK or peptide in the presence and absence of candidate molecule; measuring the binding of the cognate RACK or peptide to PKC in the presence and absence of candidate molecules; comparing the binding of cognate RACK or peptide to PKC in the presence and absence of candidate molecule; and identifying as a modulator a candidate molecule wherein the binding of the cognate RACK or peptide to the PKC is reduced or enhanced in the presence of said candidate molecule as compared to its absence.
3. The method of claim 2 wherein the binding is measured by measuring a metabolic effect.
4. The method of claim 3 wherein said metabolic effect is the maturation of a Xenopus oocyte, or induction of negative chronotropy in cardiac myocytes by an inducer, or transcription of a reporter gene, or subcellular translocation of a catalytically active signal-generating protein.
5. The method of claim 2 wherein the binding is measured by measuring a metabolic effect.
6. The method of claim 5 wherein said metabolic effect is subcellular translocation of catalytically active PKC.
7. The method of claim 2 wherein said peptide is derived from the PKC.
8. The method of claim 1 wherein said peptide is derived from the PKC.
9. The method of claim 8 wherein the peptide is selected from the group consisting of .beta.C2-1, .beta.C2-2, .beta.C2-4, .theta.V1-1, .theta.V1-2, .theta.V1-3, .theta.V1-4, .delta.V1-1, .delta.V1-2, .delta.V1-3, .delta.V1-4, .epsilon.V1-1, .epsilon.V1-2, .epsilon.V1-3, .epsilon.V1-4, .epsilon.V1-5, .epsilon.V1-6, .epsilon.V1-7, .eta.V1-1, .eta.V1-2, .eta.V1-3, .eta.V1-4, .eta.V1-5, .eta.V1-6, .eta.V1-7, .mu.V1-1, .mu.V1-2, .lambda.V1-1, .lambda.V1-2, .zeta.V1-1, .zeta.V1-2 and .zeta.V1-3.
10. The method of claim 1 wherein said library of candidate molecules comprises peptides derived from the PKC or from the cognate RACK.
11. The method of claim 1 wherein said library of candidate molecules comprises nonpeptide molecules.
12. The peptide .beta.C2-1, .beta.C2-2, .beta.C2-4, .theta.V1-1, .theta.V1-2, .theta.V1-3, .theta.V1-4, .delta.V1-1, .delta.V1-2, .delta.V1-3, .theta.V1-4, .epsilon.V1-1, .epsilon.V1-2, .epsilon.V1-3, .epsilon.V1-4, .epsilon.V1-5, .epsilon.V1-6, .epsilon.V1-7, .eta.V1-1, .eta.V1-2, .eta.V1-3, .eta.V1-4, .eta.V1-5, .eta.V1-6, .eta.V1-7, .mu.V1-1, .mu.V1-2, .lambda.V1-1, .lambda.V1-2, .zeta.V1-1, .zeta.V1-2 or .zeta.V1-3.
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