Influence of the lipidation motif on the partitioning and association of N-Ras in model membrane subdomains.

In a combined chemical biological and biophysical approach using time-lapse tapping-mode atomic force microscopy, we studied the partitioning of differently lipidated N-Ras proteins with various membrane-localization motifs into lipid domains of canonical model raft mixtures. The results provide direct evidence that partitioning of N-Ras occurs preferentially into liquid-disordered lipid domains, independent of the lipid anchor system. N-Ras proteins bearing at least one farnesyl group have a comparable membrane partitioning behavior and show diffusion of the protein into the liquid-disordered/liquid-ordered phase boundary region, thus leading to a decrease of the unfavorable line tension between domains. In addition, except for the monofarnesylated N-Ras, strong intermolecular interactions foster self-association and formation of nanoclusters at the domain boundaries and may serve as an important vehicle for association processes and nanoclustering, which has also been observed in in vivo studies. No significant changes of the localization between GDP- and GTP-loaded N-Ras could be detected. Conversely, the nonbiological dual-hexadecylated N-Ras exhibits a time-independent incorporation into the bulk liquid-disordered phase to maintain high conformational entropy of its lipid chains.