The functions of Munc18-1 in regulated exocytosis.

The activation of regulated exocytosis occurs by a rise in cytosolic Ca(2+) concentration. Synaptotagmins act as the Ca(2+) sensors, whereas the machinery that allows fusion of secretory vesicles with the plasma membrane consists of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, including syntaxin 1, SNAP-25, and VAMP. Within the pathway leading to exocytosis, there is an essential requirement for a member of the conserved Sec1/Munc18 (SM) protein family, which in neurotransmitter and neurohormone release in mammalian cells is Munc18-1. The exact role of Munc18-1 and the steps within exocytosis in which it acts have been intensively investigated. Current evidence suggests that Munc18-1 acts via distinct modes of interactions with syntaxin 1 and the other SNARE proteins and influences all of the steps leading to exocytosis, including vesicle recruitment, tethering, docking, priming, and membrane fusion.