Fig. 2.

Steps in regulated exocytosis. Newly formed secretory vesicles are transported toward the cell periphery using microtubules and their associated motors kinesin and dynein (a). At the cell periphery, the secretory vesicles associate with the actin cytoskeleton by recruiting and activating myosin motors. Vesicles at the plasma membrane may dock using multiple mechanisms (b). The initial tether may be initiated by the exocyst complex, whereas the docking may be achieved by the interactions between the SNARE complex and Munc18. Both processes are coordinated by small GTPases, such as Rab3a, Rab8, and RalA (b). In order to become fusion-competent, docked secretory vesicles have to undergo priming that is mediated by proteins such as Mun13 and CAPS, which displaces Mun18 from the SNARE complex and facilitates the binding of complexin (c). Upon increase of Ca⁺⁺ levels of, synaptotagmin, binds to the phospholipid at the plasma membrane, displaces complexin enabling the SNARE complex to promote the fusion of the bilayers (d). After fusion, vesicles can undergo full fusion, kiss and run/cavicapture or compound exocytosis