Synaptotagmin (SYT) and SNAREs have been identified as critical proteins that regulate membrane fusion in concert with Ca2+ [1, 2]. SYT, a C2-domain family protein is thought to act as a Ca2+ sensor, although there is still debate as to the precise mechanism through which SYT enhances membrane fusion. In one hypothesis, SYT penetrates the negatively charged membrane bilayer after binding Ca2+, causing tubulation of the bilayer, thereby lowering the activation energy barrier for fusion . Another hypothesis is that SYT mediates Ca2+ interactions with SNARE proteins and facilitates the fusion process by providing Ca2+ ions to the synaptic vesicle fusion apparatus [4, 5]. The primary role of SNAREs is in bridging the apposed synaptic vesicle and plasma membrane, by forming a core fusion complex. .
In this work, molecular dynamics simulations used to understand the interactions of SYT with phospholipid bilayers and SNARE proteins in an effort to elucidate the process of membrane fusion at the atomic level. The penetration of SYT into a bilayer containing acidic phospholipids is studied in the presence and absence of Ca2+. The potential of mean force (PMF) between SYT and phospholipid bilayers is calculated using the adaptive biasing technique  and shows that Ca2+ binding to SYT is required creates favorable protein bilayer interactions that lead to the insertion of SYT. The effect of SYT binding on local water structure and bilayer properties is determined. Simulations are also used to understand the hypothesized binding of SYT to the tv-SNARE complex during membrane fusion. The energy required for dissociation of the SNARE complex is determined and compared to recent atomic force microscopy experiments.
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