Influence of phospholipid chain length on verotoxin/globotriaosyl ceramide binding in model membranes: Comparison of a supported bilayer film and liposomes

The importance of the surrounding lipid environment on the availability of glycolipid carbohydrate for ligand binding was demonstrated by studying the influence of phosphatidylcholine fatty acid chain length on binding of verotoxins (VT1 and VT2c) to their specific cell surface receptor, globotriaosylceramide (Gb(3)) in the presence of auxiliary lipids both in a microtitre plate surface bilayer film and in a liposome membrane model system. In the microtitre assay, both VT1 and VT2c binding to Gb(3) was increased as a function of decreasing PC acyl chain length likely resulting in increased Gb(3) exposure. In the liposome assay VT1 binding was similarly modulated, however the effect on VT2c binding was more complex and did not follow a simple function of increased carbohydrate exposure. Earlier work established that C22:1 and C18:1Gb(3) fatty acid homologues were the preferred Gb(3) receptor isoforms in the microtitre assay for VT1 and VT2c respectively. This selectivity was maintained in C16PC containing liposomes, but in C14PC liposomes, binding to C22:1Gb(3) (but not C18:1Gb(3)) was elevated such that this Gb(3) species now became the preferred receptor for both toxins. This change in verotoxin/Gb(3) homologue binding selectivity in the presence of C14PC did not occur in the microtitre bilayer format. These results are consistent with our proposal that these toxins recognize different epitopes on the Gb(3) oligosaccharide. We infer that relative availability of these epitopes for toxin binding in an artificial bilayer is influenced not only by the exposure due to the discrepancy between the fatty acyl chain lengths of Gb(3) and PC, but by the physical mode of presentation of the bilayer structure. Such acyl chain length differences have a more marked effect in a supported bilayer film whereas only the largest discrepancies affect Gb(3) receptor function in liposomes. The basis of phospholipid modulation of glycolipid carbohydrate accessibility for receptor function is likely complex and will involve phase separation, gel/liquid crystalline transition, packing and lateral mobility within the bilayer, suggesting that such parameters should be considered in the assessment of glycolipid receptor function in cells.