, 2010, Rathore et al., 2010, Schollmeier et al., 2011 and Yu et al., 2013). The proposal that SM proteins act in fusion by enabling phospholipid mixing, riding on the assembling SNARE complexes, is at present only that—a hypothesis. Alternative, not necessarily mutually exclusive hypotheses are also plausible. Early on, the notion that SM proteins primarily inhibit fusion obtained significant support (Yang et al., 2000 and Wu et al., 2001),

but more recent experiments with in vivo and in vitro fusion reactions have argued against this notion (e.g., see Schollmeier et al., 2011 and Rathore et al., 2010). More recently, the idea that Munc18-1 and other SM proteins catalyze SNARE complex assembly, possibly by nucleating it, has received significant attention. This idea accounts for the binding of SM proteins to SNARE complexes. However, a see more sole function of SM proteins in promoting SNARE complex assembly is difficult to reconcile with the essential role of SM proteins in fusion. A decreased rate of SNARE complex assembly in the absence of an SM protein should decrease the rate of fusion but not block fusion. Moreover, deletion of Sec1p in yeast blocks fusion but not SNARE complex assembly (Grote et al.,

2000). Finally, other proteins are known to promote SNARE complex assembly (e.g., Munc13 [Ma et al., 2013] and synucleins [Burré et al., 2010]). selleckchem Although these data argue against a primary function of SM proteins as SNARE complex assembly catalysts, it is quite possible that SM proteins also act to promote SNARE complex assembly. Besides the notion that SM proteins mediate fusion and/or catalyze SNARE complex assembly, a third plausible hypothesis for how SM proteins activate fusion is that they spatially organize assembled SNARE complexes many around the fusion site (Rizo et al., 2006). This esthetically pleasing hypothesis also accounts for the binding of SM proteins to assembled

SNARE complexes and could be related to a lipid-mixing activity of SM proteins. However, fusion mediated by only one to three SNARE complexes (van den Bogaart et al., 2010 and Mohrmann et al., 2010) presumably still requires Munc18-1, and thus according to this hypothesis an SM protein would organize isolated SNARE complexes for fusion. Differentiating between these hypotheses and testing them will require novel assays that monitor lipid mixing with a high temporal resolution in an SM protein-dependent manner. Thus, answering the question of SM protein function in fusion remains a major challenge whose resolution requires not only a reductionist approach using liposome fusion, but also physiological tests of normal fusion reactions in a living cell. As outlined above, the Ca2+ sensor synaptotagmin and its assistant complexin transduce the presynaptic Ca2+ signal for release. These proteins probably act on a primed fusion machinery that is ready to go. However, the precise interplay of these two key players is incompletely understood.