Coordinated oscillations in cortical actin and Ca2+ correlate with cycles of vesicle secretion

The actin cortex both facilitates and hinders the exocytosis of secretory granules. How cells consolidate these two opposing roles was not well understood. Here we show that antigen activation of mast cells induces oscillations in Ca2+ and PtdIns(4,5)P-2 lipid levels that in turn drive cyclic recruitment of N-WASP and cortical actin level oscillations. Experimental and computational analysis argues that vesicle fusion correlates with the observed actin and Ca2+ level oscillations. A vesicle secretion cycle starts with the capture of vesicles by actin when cortical F-actin levels are high, followed by vesicle passage through the cortex when F-actin levels are low, and vesicle fusion with the plasma membrane when Ca2+ levels subsequently increase. Thus, cells employ oscillating levels of Ca2+, PtdIns(4,5)P-2 and cortical F-actin to increase secretion efficiency, explaining how the actin cortex can function as a carrier as well as barrier for vesicle secretion.