It is well known that actin can associate with intracellular membranes to drive endocytosis and the rocketing motion of bacteria, virions and some organelles and to regulate synaptic vesicle plasticity. Actin also has been extensively reported to be involved at several steps of exocytosis; however, it has typically been described as functioning either within the actin cortex or by providing actin tracks for organelle transport. Increasingly, actin filament coats or rings have been directly localized on the surface of the exocytic organelle. Here, we suggest a common mechanism for actin-based regulation of large secretory granules whereby organelle-associated actomyosin II contraction either directly expels secretory content or stabilizes the exocytosing organelle.
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