Hydrophobic Contributions to the Membrane Docking of Synaptotagmin 7 C2A Domain: Mechanistic Contrast between Isoforms 1 and 7

Synaptotagmin (Syt) triggers Ca2+-dependent membrane fusion via its tandem C2 domains, C2A and C2B. The 17 known human isoforms are active in different secretory cell types, including neurons (Syt1 and others) and pancreatic beta cells (Syt7 and others). Here, quantitative fluorescence measurements reveal notable differences in the membrane docking mechanisms of Syt1 C2A and Syt7 C2A to vesicles comprised of physiological lipid mixtures. In agreement with previous studies, the Ca2+ sensitivity of membrane binding is much higher for Syt7 C2A. We report here for the first time that this increased sensitivity is due to the slower target membrane dissociation of Syt7 C2A. Association and dissociation rate constants for Syt7 C2A are found to be similar to 2-fold and similar to 60-fold slower than Sytl C2A, respectively. Furthermore, the membrane dissociation of Syt7 C2A but not Sytl C2A is slowed by Na2SO4 and trehalose, solutes that enhance the hydrophobic effect. Overall, the simplest model consistent with these findings proposes that Syt7 C2A first docks electrostatically to the target membrane surface and then inserts into the bilayer via a slow hydrophobic mechanism. In contrast, the membrane docking of Sytl C2A is known to be predominantly electrostatic. Thus, these two highly homologous domains exhibit distinct mechanisms of membrane binding correlated with their known differences in function.