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.