Ricinus communis toxin interacts specifically with GM 1 ganglioside incorporated into planar lipid bilayers

Dehydration of model membranes induced by lectins from Ricinus communis and Viscum album

The effects of ribosome-inactivating proteins (RIPs) from Ricinus communis and from Viscum album on the water permeability, Pf, and the surface dielectric constant, epsilon, of model membranes were studied. Pf was calculated from microelectrode measurements of the ion concentration distribution in the immediate vicinity of a planar membrane, and epsilon was obtained from the fluorescence of dansyl phosphatidylethanolamine incorporated into unilamellar vesicles. Pf and epsilon of fully saturated phosphatidylcholine membranes were affected only in the presence of a lectin receptor (monosialoganglioside, GM1) in the bilayer. It is suggested that the membrane area occupied by clustered lectin-receptor complexes is markedly less permeable to water. Protein binding to the receptor was not a prelude for hydrophobic lipid-protein interactions when the membranes were formed from a mixture of natural phospholipids with a high content of unsaturated fatty acids. These membranes, characterized by a high initial water permeability, were found to interact with the RIPs unspecifically. From a decrease of both Pf and epsilon it was concluded that not only water partitioning but also protein adsorption correlates with looser packing of polyunsaturated lipids at the lipid-water interface.

Effect of gangliosides on binding, internalization and cytotoxic activity of ricin

The only gangliosides in Burkitt's lymphoma EB-3 cells is GM3. Treatment of Burkitt's lymphoma EB-3 cells with gangliosides GM1 or GM3 results in their binding to and partial incorporation into the cell membrane. About 25% of cell-associated ganglioside GM1 can interact with the ricin. However, such an increase in the number of binding sites does not enhance but rather decreases the cytotoxic effect of ricin. A similar protective effect was observed when the cells were pretreated with ganglioside GM3. In contrast, the increase in ricin biding sites caused by pretreatment of the cells with neuraminidase was accompanied by increase in ricin cytotoxicity. These differences may be related to observed differences in the rate of ricin-endocytosis by native and ganglioside-treated cells.

Interaction of ricin and its constituent polypeptides with dipalmitoylphosphatidylcholine vesicles

The interaction of ricin and of its constituent polypeptides, the A- and B-chain, with dipalmitoylphosphatidylcholine (DPPC) vesicles was investigated. The A- and B-chain were individually associated with DPPC vesicles, although the intact ricin was not associated. The maximum binding and association constants were evaluated to be 154 micrograms per mg of DPPC and Ka = 2.30 X 10(5) M-1 for the A-chain, and 87 micrograms per mg of DPPC and Ka = 14.5 X 10(5) M-1 for the B-chain, respectively. The A-chain could induce the phase transition release of carboxyfluorescein from DPPC vesicles to a greater extent than the B-chain, whereas the release induced by the intact ricin was negligible. The evidence indicated that the hydrophobic regions on the A-chain and on the B-chain were buried inside when the two chains constituted the intact ricin molecule through one interchain disulfide bond, and that the A-chain caused perturbation of the DPPC bilayer at the phase transition temperature with consequent leakage of carboxyfluorescein.

Immunoglobulin-lipid interaction. A model membrane study

We recently presented evidence (Vandenbranden, M., De Coen, J.L., Jeener, R., Kanarek, L. and Ruysschaert, J.M. (1981) Mol. Immunol. 8, 621-631) for the existence of two conformational rabbit serum IgG immunoglobulin isomers. In the present report, their capacity to interact with lipid is investigated in model membranes. (1) One isomer, IgG(H), behaves like several intrinsic membrane proteins: it induces a large surface pressure increase when injected under a lipid monolayer in the close packed state and increases by 20-fold the conductance of a planar bilayer. The other isomer, IgG(S) doesn't interact significantly with the lipids. (2) IgG(H) marked a preference for monolayers made of lipids with a negatively charged polar headgroup and bearing unsaturations in their acyl chains. Penetration is stronger with lipid monolayer in the fluid state than in the rigid state. (3) As previously shown (Vanderbranden, M., De Coen, J.L., Jeener, R., Kanarek, L. and Ruysschaert, J.M. (1918) Mol. Immunol. 18, 621-631), circular dichroïsm spectra and antigen precipitation tests don't allow to detect any difference in the overall secondary conformation and antigen recognition properties of the two isomers. (4) Papaïn cleavage of the hinge region suppresses the hydrophobic properties of IgG towards lipid monolayers. (5) The hypothesis of a binding of the hinge region with the lipid bilayer is discussed.