title: Exophilin8 transiently clusters insulin granules at the actin-rich cell cortex prior to exocytosis.
abstract: Exophilin8/MyRIP/Slac2-c is an effector protein of the small GTPase Rab27a and is specifically localized on retinal melanosomes and secretory granules. We investigated the role of exophilin8 in insulin granule trafficking. Exogenous expression of exophilin8 in pancreatic β cells or their cell line, MIN6, polarized (exophilin8-positive) insulin granules at the cell corners, where both cortical actin and the microtubule plus-end-binding protein, EB1, were present. Mutation analyses indicated that the ability of exophilin8 to act as a linker between Rab27a and myosin Va is essential for its granule-clustering activity. Moreover, exophilin8 and exophilin8-associated insulin granules were markedly stable and immobile. Total internal reflection fluorescence microscopy indicated that exophilin8 restricts the motion of insulin granules at a region deeper than that where another Rab27a effector, granuphilin, accumulates docked granules directly attached to the plasma membrane. However, the exophilin8-induced immobility of insulin granules was eliminated upon secretagogue stimulation and did not inhibit evoked exocytosis. Furthermore, exophilin8 depletion prevents insulin granules from being transported close to the plasma membrane and inhibits their fusion. These findings indicate that exophilin8 transiently traps insulin granules into the cortical actin network close to the microtubule plus-ends and supplies them for release during the stimulation.
authors: Laboratory of Molecular Endocrinology and Metabolism, Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi 371-8512, Japan.
authors: Kouichi
authors: K
authors: Mizuno
authors: José S
authors: JS
authors: Ramalho
authors: Tetsuro
authors: T
authors: Izumi
mesh_terms: D000199:Actins / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D005089:Exocytosis; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism; D051379:Mice; D018613:Microscopy, Confocal; D050505:Mutant Proteins / Q000378:metabolism; D021381:Protein Transport; D034622:RNA Interference; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D022142:Secretory Vesicles / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*; D020691:rab GTP-Binding Proteins / Q000378:metabolism; D000075745:rab27 GTP-Binding Proteins
chemical_list: D000199:Actins; D007328:Insulin; C521067:MYRIP protein, mouse; D050505:Mutant Proteins; D011993:Recombinant Fusion Proteins; D033921:Vesicular Transport Proteins; C485184:enhanced green fluorescent protein; D000075745:rab27 GTP-Binding Proteins; D049452:Green Fluorescent Proteins; C512962:Rab27a protein, mouse; D020691:rab GTP-Binding Proteins