Dynamic Ca2+-dependent stimulation of vesicle fusion by membrane-anchored synaptotagmin 1.

In neurons, synaptotagmin 1 (Syt1) is thought to mediate the fusion of synaptic vesicles with the plasma membrane when presynaptic Ca2+ levels rise. However, in vitro reconstitution experiments have failed to recapitulate key characteristics of Ca2+-triggered membrane fusion. Using an in vitro single-vesicle fusion assay, we found that membrane-anchored Syt1 enhanced Ca2+ sensitivity and fusion speed. This stimulatory activity of membrane-anchored Syt1 dropped as the Ca2+ level rose beyond physiological levels. Thus, Syt1 requires the membrane anchor to stimulate vesicle fusion at physiological Ca2+ levels and may function as a dynamic presynaptic Ca2+ sensor to control the probability of neurotransmitter release.