Secretagogue-dependent phosphorylation of phogrin, an insulin granule membrane protein tyrosine phosphatase homologue

Phogrin, a 60/64 kDa integral membrane protein localized to dense-core secr etory granules of neuroendocrine cells, was found to be reversibly phosphor ylated in intact pancreatic beta-cells, Phosphorylation occurred in respons e to a variety of secretory stimuli, including glucose and depolarizing con centrations of K+. In MIN6 cells, the glucose dose-response and time course of phogrin phosphorylation paralleled that of insulin secretion. Like secr etion, glucose- or K+-stimulated phosphorylation required the presence of C a2+. The calmodulin antagonist W-7 and the Ca2+/calmodulin-dependent kinase II inhibitor KN-93 dose-dependently inhibited both phosphorylation and sec retion, while the 'inactive' analogue KN-92 was effective only at significa ntly higher concentrations. Phosphorylation of phogrin was also stimulated in cells exposed to forskolin, an effect presumably mediated by protein kin ase A (cAMP-dependent protein kinase), Under these conditions, phogrin phos phorylation could be dissociated from the secretory response. In MIN6 cells , as in pancreatic islets, cAMP potentiates rather than initiates insulin r elease. Thus our observations are consistent with a role for phogrin phosph orylation in the signal-transduction pathway at a site proximal to the exoc ytic event itself, possibly regulating secretory-granule mobilization and r ecruitment to the exocytic site.