Non-Native Metal Ion Reveals the Role of Electrostatics in Synaptotagmin 1–Membrane Interactions

4 years ago

Non-Native Metal Ion Reveals the Role of Electrostatics in Synaptotagmin 1–Membrane Interactions

P. John Hart, Bin Her, Sachin Katti, Tatyana I. Igumenova, David S. Cafiso, Sarah B. Nyenhuis, Alexander B. Taylor, Atul K. Srivastava

C2 domains are independently folded modules that often target their host proteins to anionic membranes in a Ca²⁺-dependent manner. In these cases, membrane association is triggered by Ca²⁺ binding to the negatively charged loop region of the C2 domain. Here, we used a non-native metal ion, Cd²⁺, in lieu of Ca²⁺ to gain insight into the contributions made by long-range Coulombic interactions and direct metal ion-lipid bridging to membrane binding. Using X-ray crystallography, NMR, Förster resonance energy transfer, and vesicle cosedimentation assays, we demonstrate that, although Cd²⁺ binds to the loop region of C2A/B domains of synaptotagmin 1 with high affinity, long-range Coulombic interactions are too weak to support membrane binding of individual domains. We attribute this behavior to two factors: the stoichiometry of Cd²⁺ binding to the loop regions of the C2A and C2B domains and the impaired ability of Cd²⁺ to directly coordinate the lipids. In contrast, electron paramagnetic resonance experiments revealed that Cd²⁺ does support membrane binding of the C2 domains in full-length synaptotagmin 1, where the high local lipid concentrations that result from membrane tethering can partially compensate for lack of a full complement of divalent metal ions and specific lipid coordination in Cd²⁺-complexed C2A/B domains. Our data suggest that long-range Coulombic interactions alone can drive the initial association of C2A/B with anionic membranes and that Ca²⁺ further augments membrane binding by the formation of metal ion–lipid coordination bonds and additional Ca²⁺ ion binding to the C2 domain loop regions.

Publisher URL: http://dx.doi.org/10.1021/acs.biochem.7b00188

DOI: 10.1021/acs.biochem.7b00188