Control of membrane fusion by phospholipid head groups. I. Phosphatidate/phosphatidylinositol specificity

Diffusion of aminoglycoside antibiotics across the outer membrane of Escherichia coli

The diffusion of aminoglycoside antibiotics (gentamicin, kanamycin, streptomycin, fradiomycin, lividomycin, and mannosylparomomycin) through porin pores was examined in vitro by the liposome swelling technique, using vesicle membranes reconstituted from phospholipids and purified porin trimers. Results showed that aminoglycoside antibiotics diffuse through porin-pores very efficiently, as rapidly as hexoses and disaccharides, despite the fact that the molecular weights of the aminoglycosides used were higher than or close to the exclusion limit of porin pores. The susceptibility to aminoglycoside antibiotics of mutant strains producing 3 to 4% of porin was not significantly different from that of a strain producing a wild-type quantity of porin. These results were interpreted to mean that aminoglycoside antibiotics diffuse through porin-pores very efficiently. Therefore, the diffusion of these drugs through the mutant outer membranes producing 3 to 4% of porin is not a rate-limiting step for aminoglycoside diffusion and its action.

Synexin facilitates fusion of specific phospholipid membranes at divalent cation concentrations found intracellularly

The effect of synexin (an adrenal medullary protein) on the kinetics of Ca2+- and Mg2+-mediated membrane fusion was examined. Membrane fusion was studied by monitoring intermixing of the aqueous contents of phospholipid vesicles. Synexin facilitated Ca2+-mediated, but not Mg2+-mediated, fusion of phosphatidate/phosphatidylethanolamine (1:3) and phosphatidate/phosphatidylserine/phosphatidylethanolamine/cholesterol (1:2:3:2) vesicles. The threshold concentration of Ca2+ for fusion was decreased to approximately equal to 10 microM in the presence of synexin at 6 micrograms/ml and 1.5 mM Mg2+ in vesicle suspensions containing 50 microM lipid. This effect of synexin was drastically inhibited by including 25% phosphatidylcholine (mol/mol) in the vesicle membrane. It is proposed that the Ca2+-dependent lipid-specific enhancement of membrane fusion by synexin contributes to an increase in the sensitivity of specific intracellular membranes to Ca2+ with respect to fusion.

Intermembrane contact affects calcium binding to phospholipid vesicles

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.