Binding Specificity for RACK1 Resides in the V5 Region of βII Protein Kinase C

Identification of selective anchoring proteins responsible for specialized localization of specific signaling proteins has led to the identification of new inhibitors of signal transduction, inhibitors of anchoring protein-ligand interactions. RACK1, the firstreceptor for activated C kinase identified in our lab, is a selective anchoring protein for βII protein kinase C (βIIPKC). We previously found that at least part of the RACK1-binding site resides in the C2 domain of βIIPKC (Ron, D., Luo, J., and Mochly-Rosen, D. (1995) J. Biol. Chem.270, 24180–24187). Here we show that the V5 domain also contains part of the RACK1-binding site in βIIPKC. In neonatal rat cardiac myocytes, the βIIV5-3 peptide (amino acids 645–650 in βIIPKC) selectively inhibited phorbol 12-myristate 13-acetate (PMA)-induced translocation of βIIPKC and not βIPKC. In addition, the βIIV5-3 peptide inhibited cardiac myocyte hypertrophy in PMA-treated cells. Interestingly, βIV5-3 (646–651 in βIPKC), a selective translocation inhibitor of βIPKC, also inhibited PMA-induced cardiac myocyte hypertrophy, demonstrating that both βI- and βIIPKC are essential for this cardiac function. Therefore, the βIIV5 domain contains part of the RACK1-binding site in βIIPKC; a peptide corresponding to this site is a selective inhibitor of βIIPKC and, hence, enables the identification of βIIPKC-selective functions.