V- and T-SNAREs in Neuronal Exocytosis: a Need for Additional Components to Define Sites of Release

Synaptic vesicle recycling is a specialized form of membrane recycling which takes place in all cells between early endosomes and the plasmalemma. Synaptic vesicles exocytosis is highly regulated and occurs only at presynaptic active zones. In contrast, exocytosis of endosome-derived vesicles of the housekeeping recycling pathway takes place constitutively and throughout the cell surface. Since v- and t-SNAREs play a key role in membrane interactions leading to fusion, unique v- and t-SNAREs may be implicated in synaptic vesicle exocytosis. It was found, however, that the same v-SNAREs of the synaptobrevin family are found both on synaptic vesicles and on endosome-derived vesicles which undergo constitutive fusion. Likewise, t-SNAREs which act as plasmalemmal receptors for synaptic vesicles are not restricted to synaptic active zones. Thus, v- and t-SNAREs interactions may define which organelles can fuse with the plasmalemma, but require additional components to define properties of the exocytotic reaction which are specific for distinct classes of secretory organelles.