Lipid modifications of a ras peptide exhibit altered packing and mobility versus host membrane as detected by²H solid-state NMR

The human N-ras protein binds to cellular membranes by insertion of two covalently bound posttranslational lipid modifications, which is crucial for its function in signal transduction and cell proliferation. Mutations in ras may lead to unregulated cell growth and eventually cancer, making it an important therapeutic target. Here we have investigated the molecular details of the membrane binding mechanism. A heptapeptide derived from the C-terminus of the human N-ras protein was synthesized including two hexadecyl modifications. Solid-state ²H NMR was used to determine the packing and molecular dynamics of the ras lipid chains as well as the phospholipid matrix. Separately labeling the chains of the peptide and the phospholipids with ²H enabled us to obtain atomically resolved parameters relevant to their structural dynamics. While the presence of ras only marginally affected the packing of DMPC membranes, dramatically lower order parameters (S_CD) were observed for the ras acyl chains indicating modified packing properties. Essentially identical projected lengths of the 16:0 ras chains and the 14:0 DMPC chains were found, implying that the polypeptide backbone is located at the lipid-water interface. Dynamical properties of both the ras and phospholipid chains were determined from spin-lattice ²H relaxation (R _1Z) measurements. Plots of R_1Z rates versus the corresponding squared segmental order parameters revealed striking differences. We propose the ras peptide is confined to microdomains containing DMPC chains which are in exchange with the bulk bilayer on the ²H NMR time scale (∼10^-5 s). Compared to the host DMPC matrix, the ras lipid modifications are extremely flexible and undergo relatively large amplitude motions. It is hypothesized that this flexibility is a requirement for the optimal anchoring of lipid-modified proteins to cellular membranes.