Synaptotagmin perturbs the structure of phospholipid bilayers

Area of Science:

• Neuroscience
• Biophysics
• Molecular Biology

Background:

• Synaptotagmin I (syt) is a key Ca2+ sensor regulating neuronal exocytosis.
• Its cytoplasmic C2 domains (C2A and C2B) mediate Ca2+-dependent membrane interactions.

Purpose of the Study:

• To visualize the interaction of syt C2AB with lipid bilayers using Atomic Force Microscopy (AFM).
• To investigate the role of Ca2+ and anionic phospholipids in syt binding and bilayer perturbation.

Main Methods:

• Atomic Force Microscopy (AFM) to image syt C2AB on fluid lipid bilayers.
• Controlled variation of anionic phospholipid (phosphatidylserine) content.

Main Results:

• Syt C2AB binding to bilayers necessitates Ca2+ and phosphatidylserine (PS).
• Syt C2AB formed aggregates on bilayers, with size correlating to PS concentration.
• Dissociation of syt C2AB left residual indentations (1.81 nm depth) in the lipid bilayer.
• Mutations affecting Ca2+ binding or C2B charge impaired syt binding and indentation formation.

Conclusions:

• Syt C2AB binding to membranes is Ca2+- and PS-dependent, forming aggregates.
• Syt-induced bilayer indentations suggest membrane perturbation is integral to its function.
• These findings propose a mechanism for syt's role in promoting membrane fusion during exocytosis.