AGONISTS AND ANTAGONISTS OF PROTEIN-KINASE-C FUNCTION, DERIVED FROM ITS BINDING-PROTEINS

Physical association between proteins involved in signal transduction is required for their functions. Therefore, identification of the inte racting sites in the signaling molecules can lead to the development o f means to modulate these interactions. We applied this approach to st udy signal transduction by protein kinase C (PKC). We have previously identified potential PKC binding sites in two PKC binding proteins (an nexin I and RACK1). Peptides derived from these sequences inhibit PKC binding to RACK1 in vitro. Here, we tested the ability of two of these peptides, I (KGDYEKILVALCGGN) and rVI (DIINALCF), to affect PKC-media ted function in vivo. The peptides were microinjected into Xenopus ooc ytes, and insulin-induced beta PKC translocation and oocyte maturation were examined. The peptides had opposite activities on oocyte; peptid e I inhibited whereas peptide rVI stimulated insulin-induced Xenopus o ocyte maturation. As expected, beta PKC translocation from the cytosol to the particulate fraction of the Xenopus oocytes was inhibited afte r microinjection of peptide I and induced after microinjection of pept ide rVI. Moreover, peptide rVI caused translocation of beta PRC and oo cyte maturation without hormone stimulation, In the absence of PKC act ivators, peptide rVI but not peptide I, activated PKC in vitro as demo nstrated in three assays: increased sensitivity to Arg-C endopeptidase , PRC autophosphorylation, and histone phosphorylation. Therefore, alt hough peptides I and rVI have sequence homology, one mimicked hormone- induced PRC-mediated function whereas the other inhibited this hormone -induced function. The molecular mechanisms underlying these opposing effects of the peptides are discussed.