LIPID-BINDING CHARACTERISTICS OF THE POLYBASIC CARBOXY-TERMINAL SEQUENCE OF K-RAS4B

We have examined the association with lipid vesicles of fluorescent li pidated peptides based on the farnesylated, polybasic carboxy-terminal region of mature K-ras4B, which functions physiologically as an auton omous plasma membrane-targeting motif. While the peptides bind to neut ral lipid (phosphatidylcholine/phosphatidylethanolamine) vesicles with relatively low affinity, the vesicle-binding affinity increases expon entially as increasing amounts of anionic lipids art: incorporated int o the vesicle bilayers. Competitive vesicle-binding experiments reveal that the K-ras4B carboxy-terminal sequence accordingly discriminates strongly between lipid surfaces of differing surface charge, such that two lipid bilayers differing in anionic lipid content by 10 mol % wil l show a 45-fold preferential accumulation of the lipidated peptide in the more negatively charged surface. At the same time, the carboxyl-t erminal region of K-ras4B exhibits no preferential binding to particul ar anionic lipids, including the polyanionic species phosphatidylinosi tol-4'-phosphate and phosphatidylinositol-4',5'-bisphosphate, beyond t hat predicted on the basis of surface-charge effects. The K-ras4B carb oxyl-terminal sequence dissociates rapidly (with half-times of seconds or less) from lipid bilayers containing up to 40 mol % anionic lipid. These results suggest that the targeting of the mature K-ras4B carbox y-terminus to the plasma membrane, if it is based on interactions with plasma membrane lipids, is not mediated by a kinetic-trapping mechani sm or by specific binding to particular anionic lipids but may rest on the sensitive surface potential-sensing function of this region of th e protein.