(721f) Understanding the Role of Calcium In Membrane Fusion
Membrane fusion is a localized event which occurs at a sub-millisecond timescale when two distinct lipid bilayers fuse with each other, subsequently opening a channel and allowing the transfer of contents. This process is a critical step in a variety of cellular functions, including exocytosis, fertilization, and neurotransmitter release. The process of membrane fusion is thermodynamically unfavorable, and several factors are required to overcome the activation energy barrier[3,4]. Ca2+ has been identified as one such factor that regulates membrane fusion, although the precise mechanism through which Ca2+ triggers membrane fusion remains unclear [5-7].
This work is focused on understanding the role of Ca2+ in the fusion process at the atomic level. Using molecular dynamics simulations and a variety of techniques such as adaptive force bias and steered molecular dynamics, the interactions of Ca2+, Mg2+ and Ba2+ with phospholipid bilayers, and the membrane fusion protein snaptotagamin (SYT) are studied. These simulations show Ca2+ has the unique ability to bridge apposed phospholipid head groups, while Mg2+ and Ba2+ do not. Potential of mean force calculations are used to show this propensity for Ca2+ to bridge opposed bilayers is due to an optimum balance of Ca2+-phospholipid and Ca2+ water interactions. The effect of cation binding to phospholipid head groups on the local structure of water and the free energy barriers to phospholipid rearrangement are discussed.
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