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Updated: Feb 27, 2026

Imaging FITC-dextran as a Reporter for Regulated Exocytosis
Published on: June 20, 2018
Exophilin-8 assembles secretory granules for exocytosis in the actin cortex via interaction with RIM-BP2 and
Fushun Fan1, Kohichi Matsunaga1, Hao Wang1
1Laboratory of Molecular Endocrinology and Metabolism, Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi, Japan.
Abstract:
Exophilin-8 has been reported to play a role in anchoring secretory granules within the actin cortex, due to its direct binding activities to Rab27 on the granule membrane and to F-actin and its motor protein, myosin-Va. Here, we show that exophilin-8 accumulates granules in the cortical F-actin network not by direct interaction with myosin-Va, but by indirect interaction with a specific form of myosin-VIIa through its previously unknown binding partner, RIM-BP2. RIM-BP2 also associates with exocytic machinery, Cav1.3, RIM, and Munc13-1. Disruption of the exophilin-8-RIM-BP2-myosin-VIIa complex by ablation or knockdown of each component markedly decreases both the peripheral accumulation and exocytosis of granules. Furthermore, exophilin-8-null mouse pancreatic islets lose polarized granule localization at the β-cell periphery and exhibit impaired insulin secretion. This newly identified complex acts as a physical and functional scaffold and provides a mechanism supporting a releasable pool of granules within the F-actin network beneath the plasma membrane.
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