Actin puts a squeeze on vesicles

Recent evidence suggests that actin is recruited to, and forms a coat around, secretory vesicles after their fusion to the plasma membrane. It has been hypothesised that compression of the actin coat provides the force necessary for the release of vesicular cargo, although an underlying mechanism has thus far remained elusive. On page 1193, Manfred Frick and colleagues use live-cell imaging to deconstruct the sequence of events governing the release of surfactant from large secretory vesicles in alveolar cells termed lamellar bodies (LBs). They show that ROCK1 and myosin light chain kinase 1 (MLCK1) are recruited to actin coats on LBs fused to the plasma membrane, and are necessary for actin coat compression. However, the subsequent activation of myosin II by ROCK1 and MLCK1 was not sufficient for coat contraction. Surprisingly, the authors discovered that ROCK1 was required to fine tune the activity of the actin-filament-severing protein cofilin-1. In keeping with the idea that regulated actin depolymerisation could drive coat compression, recruitment of the actin crosslinker α-actinin to LBs was also shown to be necessary for coat contraction. Taken together, these findings elucidate the kinetics of and key players in actin coat contraction and demonstrate that the interplay between actin depolymerisation and crosslinking can indeed provide the physical force required for cargo expulsion from LBs, thereby providing exciting new insights into the final step of exocytosis.