[Effect of pH of the medium on the liposome fusion process after treatment with alpha-latrotoxin]

alpha-Latrotoxin is a presynaptic toxin isolated from the venom of the black widow spider which induces fusion of negatively charged liposomes composed of phosphatidyl choline:phosphatidyl ethanolamine:cardiolipin at a 2:3:5 molar ratio, respectively. The vesicle fusion was monitored by mixing the water content of the liposomes as well as by mixing their lipid bilayers. The rate and efficiency of liposome fusion increased with a decrease in pH. The extent of fusion increased significantly at mild acidic pH 6.0, the increase in the initial fusion rate being especially well pronounced. Neutralization of the medium in the course of alpha-latrotoxin-induced fusion sharply decelerated the process, whereas repeated acidification activated it. This finding suggests that protein protonation plays a role in fusion reactions. Using the fluorescent probe ANS to monitor the hydrophobicity of this protein as a function of pH, it was shown that the decrease in pH results in the exposition of the hydrophobic domains. The putative role of protein conformation changes in activation of liposome fusion is discussed.