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 secretory granules of neuroendocrine cells, was found to be reversibly phosphorylated in intact pancreatic β-cells. Phosphorylation occurred in response to a variety of secretory stimuli, including glucose and depolarizing concentrations of K⁺. In MIN6 cells, the glucose dose-response and time course of phogrin phosphorylation paralleled that of insulin secretion. Like secretion, glucose- or K⁺-stimulated phosphorylation required the presence of Ca²⁺. The calmodulin antagonist W-7 and the Ca²⁺/calmodulin-dependent kinase II inhibitor KN-93 dose-dependently inhibited both phosphorylation and secretion, while the ‘inactive’ analogue KN-92 was effective only at significantly higher concentrations. Phosphorylation of phogrin was also stimulated in cells exposed to forskolin, an effect presumably mediated by protein kinase A (cAMP-dependent protein kinase). Under these conditions, phogrin phosphorylation could be dissociated from the secretory response. In MIN6 cells, as in pancreatic islets, cAMP potentiates rather than initiates insulin release. Thus our observations are consistent with a role for phogrin phosphorylation in the signal-transduction pathway at a site proximal to the exocytic event itself, possibly regulating secretory-granule mobilization and recruitment to the exocytic site.