Synaptotagmin-1 (SYT-1) is a C2-domain family protein that is widely believed to play an important role in membrane fusion [1, 2]. SYT-1 is known to act as a Ca2+ sensor in this process  owing to the presence of two Ca2+ binding loops which may bind up to 5 Ca2+ ions [4,5]. Experimental studies suggest that post Ca2+ binding, two domains of SYT-1, C2A and C2B insert into anionic bilayers to a third of the lipid monolayer depth . This Ca2+ dependent lipid bilayer binding is considered to be the primary step for SYT-1 functionality as it may either cause localized membrane tubulation  or bring two membranes into close proximity  in either case lowering the energy barrier for fusion. This work focuses on the factors that influence the insertion of SYT-1 into the lipid bilayer, specifically on the role of Ca2+ and presence of acidic lipids in bilayer. The two SYT-1 domains, C2A and C2B are studied individually interacting with the lipid bilayer in presence and absence of bound Ca2+. Interaction energies between the bilayer and SYT-1 are evaluated using atomistic molecular dynamics simulations. The simulations show that C2A and C2B domains bound to Ca2+ insert into lipid bilayers with a composition of ~ 35% acidic lipids while no insertion is observed for SYT-1 interacting with neutral lipids. The electrostatic charge on the protein surface is mapped for different stages using the APBS technique. To understand the role of hydrophobicity in the process, the inserting residues in both C2A and C2B domain are mutated to a residue with a lower hydropathy index. Results show a loss of insertion in this case, suggesting the importance of hydrophobic nature of the C2A/C2B tip residues in the overall process.
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