Annexin A6 mediates calcium-dependent secretion of exosomes

Exosomes are a subtype of extracellular vesicles (EV) that are secreted upon fusion of multivesicular bodies (MVBs) with the plasma membrane. In order to identify proteins responsible for this fusion event, we developed sensitive cellular and cell-free assays to measure exosome secretion. Our results suggest that exosome secretion is coupled to Ca²⁺-dependent plasma membrane repair. Using a targeted proteomics approach, we identify annexin A6 (ANXA6), a well-known plasma membrane repair protein, as a MVB binding protein and demonstrate that it is required for Ca²⁺-dependent exosome secretion both in intact and in permeabilized cells. Our results suggest that cells employ MVBs as a membrane source for plasma membrane lesion repair during physiological mechanical stress or treatment with a bacterial pore-forming toxin, and that this repair process results in the coincident secretion of exosomes. We propose that this unconventional secretion process may contribute significantly to the heterogeneity of EVs in biological fluids.