Translocation inhibitors define specificity of protein kinase C isoenzymes in pancreatic beta-cells

The protein kinase C (PKC) family consists of 11 isoenzymes. Following activation, each isoenzyme translocates and binds to a specific receptor for activated C kinase (RACK) (Mochly-Rosen, D. (1995) Science 268, 247-251) that provides an anchoring site in close proximity to the isoenzyme's specific substrate. Pancreatic islet cells contain at least six PKC isoenzymes (Knutson, K. L., and Hoenig, M. (1994) Endocrinology 135, 881-886). Although PKC activation enhances Insulin release, the specific function of each isoenzyme is unknown. Here we show that following stimulation with glucose, alphaPKC and epsilonPKC translocate to the cell's periphery, while deltaPKC and zetaPKC translocate to perinuclear sites. betaC2-4, a peptide derived from the RACK1-binding site in the C2 domain of betaPKC, inhibits translocation of alphaPKC and reduces Insulin response to glucose. Likewise, epsilonV1-2, an epsilonPKC-derived peptide containing the site for its specific RACK, inhibits translocation of epsilonPKC and reduces Insulin response to glucose. Inhibition of islet-glucose metabolism with mannoheptulose blocks translocation of both alphaPKC and epsilonPKC and diminishes Insulin response to glucose while calcium-free buffer inhibits translocation of alphaPKC but not epsilonPKC and lowers Insulin response by 50%. These findings illustrate the unique ability of specific translocation inhibitors to elucidate the isoenzyme-specific functions of PKC in complex signal transduction pathways.