Agglutination and fusion of globoside GL-4 containing phospholipid vesicles mediated by lectins and calcium ion

Abstract

The interaction of 5 N-acetylgalactosamine (GalNAc) specific lectins with the glycosphingolipid globoside GL-4, inserted into phospholipid vesicles composed of phosphatidylethanolamine and phosphatidic acid were investigated, with respect to their ability to induce vesicle agglutination, fusion and destabilization. The following lectins were used: soybean agglutinin (SBA); Sophora japonica agglutinin (SJA); Helix pomatia agglutinin (HPA); Ricinus communis agglutinin II (RCAII); and Codium fragile agglutinin (CFA). SBA and SJA caused rapid vesicle agglutination while HPA, CFA and RCAII were ineffective. In the presence of RCAII, but not HPA and CFA, the addition of Ca2+ caused vesicle agglutination which was specifically inhibited by the haptenic sugar GalNAc, while EDTA dissociated the vesicle complex. RCAII/Ca2+-induced vesicle agglutination was accomplished by binding of Ca2+ to RCAII after the lectin/receptor interaction. The rate of SBA-induced vesicle agglutination was increased in the presence of Ca2+, independent of the order of Ca2+ addition, and was not reversed by EDTA, indicating that the mechanism by which Ca2+ stimulated agglutination in this case was different from that observed in the presence of RCAII. In contrast to RCAII/Ca2+, SBA/Ca2+ induced fusion of the vesicles, which occurred only when Ca2+ was added after lectin addition. Close approach of adjacent bilayer was accomplished by nonspecific interactions of SBA with the bilayer after lectin binding to the receptor as revealed by a limited extent of SBA-induced fusion and an enhanced membrane permeability upon lectin binding. The phenomena observed can be explained in terms of a Ca2+-modulated reorientation of the carbohydrate head group, causing it to adopt a more perpendicular orientation with respect to the plane of the bilayer. Evidently Ca2+ exerted its effect indirectly, and the involvement of (local) bilayer dehydration in this process, as a result of the Ca2+/phosphatidic acid interaction, is discussed.