Calmodulin regulates the disassembly of cortical F-actin in mast cells butis not required for secretion

Secretion is dependent on a rise in cytosolic Ca2+ concentration and is ass ociated with dramatic changes in actin organization. The actin cortex may a ct as a barrier between secretory vesicles and plasma membrane. Thus, disas sembly of this cortex should precede late steps of exocytosis. Here we inve stigate regulation of both the actin cytoskeleton and secretion by calmodul in. Ca2+, together with ATP, induces cortical F-actin disassembly in permea bilized rat peritoneal mast cells. This effect is strongly inhibited by rem oving endogenous calmodulin (using calmodulin inhibitory peptides), and inc reased by exogenous calmodulin. Neither treatment, however, affects secreti on. Low concentrations (similar to 1 mu M) of a specific inhibitor of myosi n light chain kinase, ML-7, prevent F-actin disassembly, but not secretion. In contrast, a myosin inhibitor affecting both conventional and unconventi onal myosins, BDM, decreases cortical disassembly as well as secretion. Obs ervations of fluorescein-calmodulin, introduced into permeabilized cells, c onfirmed a strong (Ca2+-independent) association of calmodulin with the act in cortex. In addition, fluorescein-calmodulin enters the nuclei in a Ca2+- dependent manner. In conclusion, calmodulin promotes myosin II-based contra ction of the membrane cytoskeleton, which is a prerequisite for its disasse mbly. The late steps of exocytosis, however, require neither calmodulin nor cortical F-actin disassembly, but may be modulated by unconventional myosi n(s). (C) 2000 Harcourt Publishers Ltd.