Investigating the interaction of the toxin ricin and its B-chain with immobilised glycolipids in supported phospholipid membranes by surface plasmon resonance

Abstract

The affinity of the plant toxin ricin to glycolipid receptor molecules is important in mediating toxicity to the cell. The interactions of ricin and its B-chain with glycolipids in artificial lipid membranes were monitored using surface plasmon resonance (SPR). Liposomes containing GM1, GM2, GD1b, asialo-GM1, globotriaosylceramide, lactosylceramide and galactosylceramide glycolipids were fused to the sensor surface in order to form lipid films. The affinity of intact ricin and its B-chain differed markedly and was also pH dependent. The binding of the intact toxin was low at neutral pH and increased upon acidification whereas the B-chain had greater and more specific binding at both pH tested. The discrepancy in affinity between the intact toxin and the B-chain may depend on the overall structure of the toxin, giving low availability of the binding sites in the intact toxin. The apparent kinetic constants were determined for GM1, GD1b and asialo-GM1 where the binding between the B-chain and the gangliosides was significant. The association constants were in the order of 0.3×10 6–1.0×10 7 per M. The chain length of the carbohydrate moiety also influenced the binding. A higher affinity was observed with increasing chain length and this was likely due to the availability of the carbohydrate residues. Longer chains are available for binding to the ligand, whereas shorter chains may not sufficiently extend from the lipid layer.