Intermembrane contact affects calcium binding to phospholipid vesicles.

Abstract

Binding of Ca2+ to liposomes composed of phosphatidylserine (PtdSer) was analyzed by potentiometric titrations. Ca2+ binding to large unilamellar PtdSer vesicles was saturable at a stoichiometry of 1:2 (Ca2+/PtdSer). At approximately 6 X 10(-4) M [Ca2+]free, the binding curve exhibited a discontinuity that can be attributed to the formation of a Ca2+/PtdSer complex with a higher affinity. When both Ca2+ and Mg2+ are present, depending on the relative concentrations, Mg2+ can either complete or can enhance Ca2+ binding. Concomitant to the enhanced binding, the vesicle suspension was found to aggregate, suggesting that close contact of membranes is a prerequisite for the abrupt change in affinity. This concept was tested by binding studies with liposomes of mixed composition. It was found that the incorporation of 50 mol% phosphatidylethanolamine (PtdEtn) into PtdSer liposomes produced a similar binding pattern to that of pure PtdSer with a saturable stoichiometry of 1:2 (Ca2+/PtdSer). However, incorporation of 50 mol% phosphatidylcholine (PtdCho) completely abolished the discontinuous shift in affinity and apparent saturation was reached at a stoichiometry of 1:4 (Ca2+/PtdSer). In addition, Ca2+ binding to PtdSer liposomes with 10 mol% galactosylcerebroside was not altered when compared to pure PtdSer, whereas 10 mol% of the glycolipid GL-4 abolished the increased binding. The results are closely correlated with recent findings on the role of the membrane composition in Ca2+-induced fusion of liposomes and argue in favor of a specific Ca2+/PtdSer complex (with 1:2 stoichiometry) forming only at points of close contact between membranes and serving as the trigger for membrane fusion.

The Charge Properties of Phospholipid Nanodiscs

“…Also, our results disagree with stability and binding studies (53)(54)(55) that suggested that specific binding occurs between Ca 2þ and the phosphate portion of the PL. Perhaps the relative instability and heterogeneity (in size and possibly composition) of the liposome preparations and the different solvent conditions used (specifically the Ca 2þ concentrations) contribute to these differences.…”

Surface dielectric constant, surface hydrophobicity and membrane fusion

“…Consequently, the surface dielectric constant reduced to lower values, as low as 4. These results suggest that when these strong fusogenic ions interact with acidic lipid membranes, they may form nearly anhydrous complexes of cation and lipid polar groups [5]. They may form a Ca2+-interbridged lipid complex [6,7].…”

Root Plasma Membrane Lipid Changes in Relation to Water Transport in Pepper: a Response to NaCl and CaCl2 Treatment

“…When calcium is present, it is partially bound to PS in the membrane, contributing to the charge stabilization of the membrane; however, a decrease of calcium can perturb the biophysical properties of the membrane and producing its destabilization. It has been shown that sodium binding to PS produces dehydration of the membrane, and, finally, disruption of the membrane integrity (Ekerdt and Papahadjopoulos, 1982). Therefore, it is feasible that one response of the root cells to high salinity concentrations would be the increase of the PS level in the membrane, to achieve more calcium binding to the surface of the membrane, thus contributing to its stabilization.…”