Stages of regulated exocytosis

Despite its unique features of spatial organization, Ca2+ regulation and speed, neurotransmitter secretion is a paradigm for studies of membrane fusion because it shares homologous proteins and common mechanisms with constitutive exocytosis in all eukaryotic cells. Recent advances have expanded knowledge of the number of gene products required for neurosecretion, and a current major challenge is to determine their mechanisms of action and the stages at which they function in the multistep exocytic pathway. This review discusses progress in this direction from in vivo and in vitro studies that have characterized roles for specific proteins in post-docking steps proximal to membrane fusion.