Synaptotagmin 1 is required for vesicular Ca 2+ /H + ‐antiport activity

A low‐affinity Ca 2+ /H + ‐antiport was described in the membrane of mammalian brain synaptic vesicles. Electrophysiological studies showed that this antiport contributes to the extreme brevity of excitation‐release coupling in rapid synapses. Synaptotagmin‐1, a vesicular protein interacting with membranes upon low‐affinity Ca 2+ ‐binding, plays a major role in excitation‐release coupling, by synchronizing calcium entry with fast neurotransmitter release. Here, we report that synaptotagmin‐1 is necessary for expression of the vesicular Ca 2+ /H + ‐antiport. We measured Ca 2+ /H + ‐antiport activity in vesicles and granules of pheochromocytoma PC12 cells by three methods: (i) Ca 2+ ‐induced dissipation of the vesicular H + ‐gradient; (ii) bafilomycin‐sensitive calcium accumulation and (iii) pH ‐jump‐induced calcium accumulation. The results were congruent and highly significant: Ca 2+ /H + ‐antiport activity is detectable only in acidic organelles expressing functional synaptotagmin–1. In contrast, synaptotagmin‐1‐deficient cells – and cells where transgenically encoded synaptotagmin‐1 was acutely photo‐inactivated – were devoid of any Ca 2+ /H + ‐antiport activity. Therefore, in addition to its previously described functions, synaptotagmin‐1 is involved in a rapid vesicular Ca 2+ sequestration through a Ca 2+ /H + antiport.