(102d) Interactions of Ca2+ with Phospholipids: Insights Into Mechanisms of Membrane Fusion

Prior x-ray diffraction, light scattering, photon correlation spectroscopy, and atomic force microscopy experiments suggest that SNARE-induced membrane fusion in cells proceeds as a result of calcium bridging opposing bilayers, which leads to the release of water from hydrated Ca²⁺ ions as well as the loosely coordinated water at PO-lipid head groups [1]. It is hypothesized that the combined effect of local dehydration of phosphate head groups, as a result of Ca²⁺ bridging, leads to a destabilization of lipid bilayers and resulting membrane fusion. This hypothesis was tested in the current study by performing atomistic molecular dynamic simulations in the isobaric-isothermal ensemble on dimethylphosphate anions (DMP^-) and Ca²⁺ in water at 298 K and 1.01 bar. Examination of molecular configurations obtained from the molecular dynamics simulations reveals extensive formation of DMP-Ca-DMP bridges. It was also observed that two Ca²⁺ are able to combine with two DMP^- to form a ?ring complex? by bridging the anionic oxygens in DMP^-[2]. Average distances between Ca²⁺ and the anionic oxygens of bridged DMP^- are calculated as 2.92 Å, which is in close agreement with the 2.8 Å separation between vesicles reported by light scattering experiments of tv-SNARE induced membrane fusion[1]. Simulations also show that upon calcium bridging to DMP^-, water is removed from the anionic oxygens of DMP^- as well as the hydrated calcium ions.

The hypothesis that Ca²⁺ can bridge the head groups of apposed bilayers is tested with NPT molecular dynamics simulations of apposed bilayers containing either pure DMPC bilayers or mixed DMPC/POPS bilayers composed of 128 lipids of different compositions. Simulations are preformed with a 0.3 nm initial spacing between the bilayers, corresponding to the closet approach detected by light scattering experiments of tv-SNARE mediated vesicle fusion. Calcium bridging of apposing bilayers was observed after 10 ns. Simulations show structural changes in the lipid bilayers as well as the water molecules associated with the lipids upon calcium bridging the head groups.

[1] Jeremic, A., Cho, W-J., Jena, B.; J. Biol. Phys. & Chem. 4:139-142 (2004)

[2] Potoff, J., Issa, Z., Manke, C., Jena, B., Cell Biol. Int. 32:361-366 (2008)