Counting the SNAREs needed for membrane fusion

SNARE proteins mediate most membrane fusion reactions in eukaryotic cells, and are thereby vital for many functions, including synaptic transmission, cell growth, and cytokinesis. However, until recently it was still unclear how many of these proteins are required to catalyze membrane fusion in vivo. In a recent Science paper, Mohrmann et al. (2010) elegantly show that in chromaffin cells, three SNARE complexes are required for the synchronous release of granules, whereas only a single SNARE complex is sufficient for overall release.