Dynamic Binding Mode of a Synaptotagmin-1-SNARE Complex in Solution

Rapid neurotransmitter release depends on the Ca(2+)-sensor Synaptotagmin-1 and the SNARE complex formed by synaptobrevin, syntaxin-1 and SNAP-25. How Synaptotagmin-1 triggers release remains unclear, in part because elucidating high-resolution structures of Synaptotagmin-1-SNARE complexes has been challenging. An NMR approach based on lanthanide-induced pseudocontact shifts now reveals a dynamic binding mode where basic residues in the concave side of the Synaptotagmin-1 C(2)B domain β-sandwich interact with a polyacidic region of the SNARE complex formed by syntaxin-1 and SNAP-25. The physiological relevance of this dynamic structural model is supported by mutations in basic residues of Synaptotagmin-1 that markedly impair SNARE-complex binding in vitro and Synaptotagmin-1 function in neurons. Mutations with milder effects on binding have correspondingly milder effects on Synaptotagmin-1 function. Our results support a model whereby their dynamic interaction facilitates cooperation between synaptotagmin-1 and the SNAREs in inducing membrane fusion.