Cdk5 stimulates secretion via munc18-1 Cyclin-dependent Kinase 5 Associated with p39 Promotes Munc18-1 Phosphorylation and Ca-dependent Exocytosis

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine protein kinase that requires association with a regulatory protein, p35 or p39, to form an active enzyme. Munc18-1 plays an essential role in membrane fusion and its function is regulated by phosphorylation. We report here that both p35 and p39 are expressed in insulin-secreting ß-cells, where they exhibited individual subcellular distributions and associated with membranous organelles of different densities. Overexpression of Cdk5, p35 or p39 showed that Cdk5 and p39 augmented Ca 2+-induced insulin exocytosis. Suppression of p39 and Cdk5, but not of p35, by antisense oligonucleotides selectively inhibited insulin exocytosis. Transient transfection of primary ß-cells with munc18-1 templates mutated in potential Cdk5 or PKC phosphorylation sites, in combination with Cdk5 and the different Cdk5 activators, suggested that Cdk5/p39 promoted Ca 2+-dependent insulin secretion from primary β-cells by phosphorylating munc18-1 at a biochemical step immediately prior to vesicle fusion.