Partial metal ion saturation of C2 domains primes Syt1-membrane interactions

Synaptotagmin 1 (Syt1) is an integral membrane protein that acts as a Ca sensor of neurotransmitter release. How the Ca-sensing function of Syt1 is coupled to its interactions with anionic membranes and synaptic fusion machinery is not well understood. Here, we investigated the dynamics and membrane-binding properties of Syt1 under conditions where its highest affinity Ca sites, which are thought to drive the initial membrane recruitment, are selectively populated by divalent metal ions. To create such protein states for the Ca-sensing C2 domains of Syt1, we exploited the unique chemistry of Pb, a xenobiotic metal ion that is isostructural and isofunctional to Ca. NMR experiments revealed that binding of a single metal ion results in the loss of conformational plasticity of the C2 domain loop regions that are involved in both coordinating metal ions and membrane interactions. In the C2A domain, a single metal ion is sufficient to drive its weak association with PtdSer-containing membranes; in C2B, it enhances the interactions with the signaling lipid PtdIns(4,5)P. In full-length Syt1, both C2 domains associate with PtdSer-containing membranes, with the depth of insertion modulated by the occupancy of the metal ion sites. Our data suggest that Syt1 adopts a shallow membrane-bound state upon initial recruitment of its C2 domains to the membranes. The properties of this state, such as conformationally restricted loop regions and positioning of C2 domains in close proximity to anionic lipid headgroups, “prime” Syt1 for binding a full complement of metal ions required for activation of protein function.