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Synaptotagmin-1 protein binding energetics, conformations, and interactions with anionic membranes
SNARE proteins are the molecular motors that drive vesicle fusion in synaptic neurotransmission. A large class of proteins regulate the SNARE machinery for the rapid, on-demand fusion necessary for synchronous neurotransmitter release. In particular, Synaptotagmin-1 (Syt1) has been established as the primary calcium ion (Ca2+) sensor that triggers neurotransmitter release. The tandem C2 domains (C2A and C2B) of Syt1 both bind Ca2+ and exhibit functionally-critical, Ca2+ dependent interactions with the plasma membrane, but the precise mechanisms of Syt1-triggered neurotransmission remain an open question.
In this talk, I will discuss recent surface forces apparatus (SFA) measurements, in which we directly measure the binding interactions of membrane-anchored Syt1 with an anionic membrane that mimics the plasma membrane. We find that wild-type Syt1 binds more strongly in the presence of Ca2+. Correlated molecular rearrangements measured during confinement are more prevalent in Ca2+, suggesting that Syt1 binds first through its C2B domain, then reorients the C2 domains into a preferred final binding state. Systematic site-directed mutagenesis of Syt1 modulates the membrane binding energies, allowing us to elucidate molecular mechanisms for Syt1-membrane binding : a strong membrane-binding role for C2B, and an auxiliary role for C2A.