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, P-f, and the surface diele ctric constant, epsilon, of model membranes were studied. P-f was calculate d 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 unil amellar vesicles. P-f and epsilon of fully saturated phosphatidylcholine me mbranes were affected only in the presence of a lectin receptor (monosialog anglioside, GM1) in the bilayer. It is suggested that the membrane area occ upied by clustered lectin-receptor complexes is markedly less permeable to water. Protein binding to the receptor was not a prelude for hydrophobic li pid-protein interactions when the membranes were formed from a mixture of n atural phospholipids with a high content of unsaturated fatty acids. These membranes, characterized by a high initial water permeability, were found t o interact with the RIPs unspecifically, From a decrease of both P-f and ep silon it was concluded that not only water partitioning but also protein ad sorption correlates with looser packing of polyunsaturated lipids at the li pid-water interface.