| Phogrin is expressed in cells with stimulus-coupled peptide hormone secretion, including neurons and pancreatic beta cells, in which it is localized to the insulin-containing dense-core vesicles. By sequence, phogrin is a member of the family of receptor-like protein tyrosine phosphatases, but it contains substitutions in conserved catalytic sequences and no significant enzymatic activity for phogrin has ever been reported. We report here that phogrin is able to dephosphorylate specific inositol phospholipids, including PI(3)P and PI(4,5)P2 but not PI(3,4,5)P3. The PIPase activity of phogrin was low but measurable when evaluated by ability of a catalytic domain fusion protein to hydrolyze soluble short-chain PIPs. Unlike most PIPases, which are cytoplasmic proteins that associate with membranes, mature phogrin is a transmembrane protein. When the transmembrane form of phogrin was overexpressed in mammalian cells it reduced plasma membrane PI(4,5)P2 levels in a dose-dependent manner. When purified and assayed in vitro, the PIPase activity of the transmembrane form was 75 fold greater than that of the catalytic domain fusion protein. The in vitro and in vivo PIPase activity of phogrin depended on the catalytic site cysteine, was inhibited by phosphorylation by PKA, and correlated with inhibition of glucose-stimulated insulin secretion. Many secretory vesicle-associated proteins have domains that bind PI(3)P and/or PI(4,5)P2, and both PI(3)P and PI(4,5)P2 have been implicated in the organization and function of secretory and endocytic vesicles. We propose that phogrin can function as a phosphatidylinositol phosphatase, that this activity is regulated by phosphorylation by PKA, and that it contributes maintaining subcellular differences in levels of PIP that are important for regulating stimulus-coupled exocytosis of insulin. |
