Exocytosis is often viewed as a relatively simple process that involves the fusion of a vesicle with the plasma membrane, the instantaneous release of cargo and the collapse of the vesicle. However, this classical view is increasingly being challenged, and recent studies have shown that post-fusion events are crucial for secretion of certain cargos. On page , Manfred Frick and co-workers now provide evidence that the secretion of pulmonary surfactant – a bulky lipoprotein complex – requires active extrusion mechanisms that involve actin and myosin II. Surfactant does not readily diffuse out of its secretory vesicles (called lamellar bodies) following their fusion with the plasma membrane and fusion pore opening. Instead, lamellar bodies become coated with actin after they have fused with the plasma membrane, and this process is required for surfactant release. Preventing actin coat formation inhibits secretion from these vesicles. Rho activation is required for actin coating, and formins mediate actin nucleation on fused vesicles. Furthermore, the authors show that contraction of the actin coat, which is mediated by myosin II, is required to actively compress the vesicles and expel their cargo. These findings provide evidence that the active ‘squeezing’ of vesicles through compression of an actin coat is necessary for bulky cargos to be released from vesicles.
