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Melanophilin accelerates insulin granule fusion without predocking to the plasma membrane

posted on 29.09.2020 by Ada Admin, Hao Wang, Kouichi Mizuno, Noriko Takahashi, Eri Kobayashi, Jun Shirakawa, Yasuo Terauchi, Haruo Kasai, Katsuhide Okunishi, Tetsuro Izumi
Direct observation of fluorescence-labeled secretory granule exocytosis in living pancreatic β cells has revealed heterogeneous prefusion behaviors: some granules dwell beneath the plasma membrane before fusion, while others fuse immediately once they are recruited to the plasma membrane. Although the former mode seems to follow sequential docking-priming-fusion steps as found in synaptic vesicle exocytosis, the latter mode, which is unique to secretory granule exocytosis, has not been explored well. Here, we show that melanophilin, one of the effectors of the monomeric GTPase Rab27 on the granule membrane, is involved in such an accelerated mode of exocytosis. Both melanophilin-mutated leaden mouse and melanophilin-downregulated human pancreatic β cells exhibit impaired glucose-stimulated insulin secretion, with a specific reduction in fusion events that bypass stable docking to the plasma membrane. Upon stimulus-induced [Ca2+]i rise, melanophilin mediates this type of fusion by dissociating granules from myosin-Va and actin in the actin cortex and by associating them with a fusion-competent, open form of syntaxin-4 on the plasma membrane. These findings provide the hitherto unknown mechanism to support sustainable exocytosis by which granules are recruited from the cell interior and fuse promptly without stable predocking to the plasma membrane.


This work was supported by JSPS KAKENHI grants JP26670133, JP14F04104, and JP16K15211 to T.I., JP18K14647 and JP20K15742 to H.W., and JST-CREST grant JPMJCR1652 to H.K. It was also supported by grants from Uehara Memorial Foundation, Kobayashi International Scholarship Foundation, Novartis Research Grants, Pfizer Academic Contributions, Astellas Research Support, MSD Scholarship Donation, and Sanofi Scholarship Donation to T.I.