Stepwise membrane binding of the C2 domains of the extended synaptotagmins revealed by optical tweezers

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Abstract
Extended synaptotagmins (E-Syts) mediate lipid exchange between the endoplasmic reticulum (ER) and the plasma membrane (PM). Anchored on ER, E-Syts bind the PM via an array of C2 domains in a Ca2+- and lipid-dependent manner, drawing the two membranes close to facilitate lipid exchange. How these C2 domains bind the PM and regulate the ER-PM distance have not been well understood. Here, we applied optical tweezers to dissect PM membrane binding by E-Syt1 and E-Syt2. We detected Ca2+- and lipid-dependent membrane binding kinetics of both E-Syts and determined the binding energies and rates of individual C2 domains or pairs. We incorporated these parameters in a theoretical model to recapitulate various properties of E-Syt-mediated membrane contacts observed in vivo, including their equilibrium distances and probabilities. Our methods can be applied to study other proteins containing multiple membrane-binding domains linked by disordered polypeptides.