Interaction of the toxic plant protein--ricin, with model membranes. A fluorescence method of study]

The fluorescence method has been used to investigate ricin and its isolated subunits interaction with some model membranes. Three liposome types were used as a model of biological membrane: 1) liposomes constructed from lecithin and cholesterol (9:1, M:M) 2) from ganglioside receptors GM1 and 3) from the mixture of GM1, lecithin and cholesterol (1:9:1). Interaction of the protein with liposome evokes changes in the parameters of both intrinsic protein fluorescence and fluorescence of the covalently bound dansyl. Binding constants were calculated from a decrease of the intrinsic fluorescence intensity as well as from the changes in the dansyl rotation anisotropy. Measurements were carried out at neutral and acidic pH. There was good correlation of the results obtained by different methods. It was shown that association constants were different for intact ricin and its subunits. The constants also depend on liposome composition and pH of the solution. The present study has demonstrated that interaction of ricin with liposome is accounted for not only by receptor centers but also by other hydrophobic regions of ricin that are inaccessible in the native toxin and may represent the region of the subunits interaction.