Chemoenzymatic Synthesis of N-Ras Lipopeptides

For the study of biological phenomena influenced by the plasma-membrane-bound Ras proteins and other lipidated proteins, characteristic peptides which embody the correct lipid modifications of their parent proteins (palmitoyl thioesters and farnesyl thioethers), as well as analogues thereof, may serve as suitable tools. For the construction of such acid- and base-labile peptide conjugates, the enzyme-labile p-acetoxybenzyloxycarbonyl (AcOZ) urethane blocking group was developed. The acetate moiety within the AcOZ group is easily saponified by treatment with acetyl esterase or lipase. After cleavage of the acetate group the resulting quinone methide spontaneously fragments, resulting in the liberation of the desired peptide or peptide conjugates. This enzymatic protecting group technique formed the key step in the synthesis of the characteristic S-palmitoylated and S-farnesylated C-terminus of the human N-Ras protein. Deprotections are so mild that no undesired side reactions of the lipid conjugates are observed (i.e., no hydrolysis or β-elimination of the thioester and no acid-mediated attack on the double bonds of the farnesyl group). The combination of enzymatic protecting group techniques with classical chemical methods allowed access to various fluorescent-labeled and differently lipid-modified Ras lipopeptides. Their application in biological experiments enabeled the study of the structural requirements for the acylation of Ras sequence motifs in vivo and gave insight into the subcellular site at which these modifications occur. The results indicate that the plasma membrane is a major site of cellular S-acylation. This supports a mechanism for the selective subcellular localization of lipidated proteins, including the Ras proteins themselves, by kinetic targeting to the plasma membrane.