Rab11 and its effector Rip11 participate in regulation of insulin granule exocytosis

Rab GTPases and their effectors play important roles in membrane trafficking between cellular compartments in eukaryotic cells. In the present study, we examined the roles of Rab11B and its effectors in insulin secretion in pancreatic beta-cells. In the mouse insulin-secreting cell line MIN6, Rab11 was co-localized with insulin-containing granules, and over-expression of the GTP- or the GDP-bound form of Rab11B significantly inhibited regulated secretion, indicating involvement of Rab11B in regulated insulin secretion. To determine the downstream signal of Rab11-mediated insulin secretion, we examined the effects of various Rab11-interacting proteins on insulin secretion, and found that Rip11 is involved in cAMP-potentiated insulin secretion but not in glucose-induced insulin secretion. Analyses by immunocytochemistry and subcellular fractionation revealed Rip11 to be co-localized with insulin granules. The inhibitory effect of the Rip11 mutant was not altered in MIN6 cells lacking Epac2, which mediates protein kinase A (PKA)-independent potentiation of insulin secretion, compared with wild-type MIN6 cells. In addition, Rip11 was found to be phosphorylated by PKA in MIN6 cells. The present study shows that both Rab11 and its effector Rip11 participate in insulin granule exocytosis and that Rip11, as a substrate of PKA, regulates the potentiation of exocytosis by cAMP in pancreatic beta-cells.