Secretagogue-dependent phosphorylation of the insulin granule membrane protein phogrin is mediated by cAMP-dependent protein kinase

Phogrin, a 60/64-kDa integral membrane protein of dense-core granules in neuroendocrine cells, is phosphorylated in a Ca2+-sensitive manner in response to secretagogue stimulation of pancreatic beta -cells. Phosphorylation of the phogrin cytosolic domain by beta -cell homogenates was Ca2+-independent but stimulated by cAMP. Recombinant protein kinase A (PKA) could phosphorylate phogrin directly. High performance liquid chromatography analysis of tryptic phosphopeptides, combined with site-directed mutagenesis of candidate sites, revealed the presence of two phosphorylation sites at Ser-680 and Thr-699, located in the juxtamembrane region between the transmembrane span and the protein-tyrosine phosphatase homology domain of phogrin. Full-length wild-type phogrin, as well as mutant versions where Ser-680 and Thr-699 had been replaced either by alanines or by aspartic acid residues, were targeted to secretory granules in transfected AtT20 neuroendocrine cells. Stimulation of these cells with a range of secretagogues, including K+, BaCl2, and forskolin, demonstrated that the in vivo phosphorylation sites are the same as those identified in vitro. In MIN6 beta -cells, the PKA inhibitor H-89 prevented Ca2+-dependent phogrin phosphorylation in response to glucose, suggesting that Ca2+ exerts its effect on phogrin phosphorylation through regulating the activity of PKA.