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 (a) under bar ctivated (C) under bar (k) under bar inase (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, alpha PKC and epsilon PKC translocate to the cell's periphery, while delta PKC and zeta PKC translocate to perinuclear sites. beta C2-4, a peptide derived from the RACK1 binding site in the C2 domain of beta PKC, inhibits translocation of alpha PKC and reduces insulin response to glucose. Likewise, epsilon V1-2, an epsilon PKC derived peptide containing the site for its specific RACK, inhibits translocation of epsilon PKC and reduces insulin response to glucose. Inhibition of islet-glucose metabolism with mannoheptulose blocks translocation of both alpha PKC and epsilon PKC and diminishes insulin response to glucose while calcium-free buffer inhibits translocation of alpha PKC but not epsilon PKC 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.