Snapin accelerates exocytosis at low intracellular calcium concentration in mouse chromaffin cells

Snapin associates with SNAP-25 and with assembled SNARE complexes, stabilizing the coupling between Synaptotagmin-1 and SNAP-25. Deletion of Snapin reduces releasable pools of vesicles in chromaffin cells and reduces synchronous release of neurotransmitter in cortical neurons. Snapin deletion leads to a deficit in exocytosis at low calcium concentration with no change in the threshold calcium concentration for exocytosis in chromaffin cells. In order to determine whether Snapin deletion alters release rates or calcium dependence, we examined the effect of overexpression of wild type Snapin on readily releasable pool kinetics and pool size in mouse chromaffin cells. Modest increases in intracellular calcium induced by flash-photolysis unmasked a rapidly releasing component of secretion which was enhanced when Snapin was overexpressed. This result indicates that Snapin allows rapid release at lower intracellular calcium levels at which release of the remaining RRP occurs more slowly.