Hydrophobic Contributions
to the Membrane Docking
of Synaptotagmin 7 C2A Domain: Mechanistic Contrast between Isoforms
1 and 7
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Abstract
Synaptotagmin (Syt) triggers Ca<sup>2+</sup>-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 β 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 Ca<sup>2+</sup> 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 ∼2-fold and ∼60-fold slower than Syt1 C2A,
respectively. Furthermore, the membrane dissociation of Syt7 C2A but
not Syt1 C2A is slowed by Na<sub>2</sub>SO<sub>4</sub> 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 Syt1 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