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 availabilit y of glycolipid carbohydrate for ligand binding was demonstrated by st udying the influence of phosphatidylcholine fatty acid chain length on binding of verotoxins (VT1 and VT2c) to their specific cell surface r eceptor, globotriaosylceramide (Gb(3)) in the presence of auxiliary li pids both in a microtitre plate surface bilayer film and in a liposome membrane model system. In the microtitre assay, both VT1 and VT2c bin ding to Gb(3) was increased as a function of decreasing PC acyl chain length likely resulting in increased Gb(3) exposure. In the liposome a ssay VT1 binding was similarly modulated, however the effect on VT2c b inding was more complex and did not follow a simple function of increa sed carbohydrate exposure. Earlier work established that C22:1 and C18 :1Gb(3) fatty acid homologues were the preferred Gb(3) receptor isofor ms in the microtitre assay for VT1 and VT2c respectively. This selecti vity was maintained in C16PC containing liposomes, but in C14PC liposo mes, 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 p resence 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 relativ e availability of these epitopes for toxin binding in an artificial bi layer is influenced not only by the exposure due to the discrepancy be tween the fatty acyl chain lengths of Gb(3) and PC, but by the physica l mode of presentation of the bilayer structure. Such acyl chain lengt h differences have a more marked effect in a supported bilayer film wh ereas only the largest discrepancies affect Gb(3) receptor function in liposomes. The basis of phospholipid modulation of glycolipid carbohy drate accessibility for receptor function is likely complex and will i nvolve phase separation, gel/liquid crystalline transition, packing an d lateral mobility within the bilayer, suggesting that such parameters should be considered in the assessment of glycolipid receptor functio n in cells.