Interaction of ATP Sensor, cAMP Sensor, Ca2+ Sensor, and Voltage-dependent Ca2+ Channel in Insulin Granule Exocytosis

Abstract ATP, cAMP, and Ca2+ are the major signals in the regulation of insulin granule exocytosis in pancreatic β cells. The sensors and regulators of these signals have been characterized individually. The ATP-sensitive K+ channel, acting as the ATP sensor, couples cell metabolism to membrane potential. cAMP-GEFII, acting as a cAMP sensor, mediates cAMP-dependent, protein kinase A-independent exocytosis, which requires interaction with both Piccolo as a Ca2+ sensor and Rim2 as a Rab3 effector. l-type voltage-dependent Ca2+ channels (VDCCs) regulate Ca2+ influx. In the present study, we demonstrate interactions of these molecules. Sulfonylurea receptor 1, a subunit of ATP-sensitive K+ channels, interacts specifically with cAMP-GEFII through nucleotide-binding fold 1, and the interaction is decreased by a high concentration of cAMP. Localization of cAMP-GEFII overlaps with that of Rim2 in plasma membrane of insulin-secreting MIN6 cells. Localization of Rab3 co-incides with that of Rim2. Rim2 mutant lacking the Rab3 binding region, when overexpressed in MIN6 cells, is localized exclusively in cytoplasm, and impairs cAMP-dependent exocytosis in MIN6 cells. In addition, Rim2 and Piccolo bind directly to the α11.2-subunit of VDCC. These results indicate that ATP sensor, cAMP sensor, Ca2+ sensor, and VDCC interact with each other, which further suggests that ATP, cAMP, and Ca2+ signals in insulin granule exocytosis are integrated in a specialized domain of pancreatic β cells to facilitate stimulus-secretion coupling.