The effect of multivalent binding on the lateral phase separation of adhesive lipids

The relationship between the clustering of adhesive lipids and the binding of multivalent ligands has been investigated using a simple model system. Pyrene perfluoroalkyl lipids capped with mannose and biotin were found to extensively phase separate in phospholipid bilayers containing >25% mol/mol cholesterol. This clustering of adhesive lipids into these artificial lipid rafts'' did not enhance multivalent binding in either case, conversely weakening the initial interaction between avidin and phase-separated biotin-capped lipid (at 1% mol/mol). The affinity of concanavalin A for vesicle-bound mannose lipid (K similar to 2500 M-1) was low and insensitive to the extent of adhesive lipid clustering, fitting an intervesicular monovalent binding model. Although the initial binding of avidin to vesicle-bound biotin lipid was also weak (K up to 8 x 106 M-1), avidin maximised the adhesive interaction by either clustering or dispersing these biotinylated lipids. This reorganisation gave up to a similar to 500-fold increase in affinity, consistent with multivalent intramembrane binding of avidin to biotinylated lipids. These observations suggest that in natural systems, high receptor densities in lipid rafts may not be designed solely to generate high affinities for multivalent ligands but also to allow multivalent ligands to induce signal transduction or endocytosis by increasing or decreasing receptor density in lipid rafts.