KRAS Can Bind to FTase Despite Disruption of the CAAX Binding Site.

Protein prenylation is a post-translational modification promoting membrane association where isoprenoid lipids attach to C-terminal cysteines of eukaryotic proteins such as Ras and Rho GTPases, nucleus lamins, and G-protein subunits. Three enzymes catalyze this process: farnesyltransferase (FTase) and geranylgeranyltransferase type I and II (GGTase I and RabGGTase). FTase and GGTase-I recognize C-terminal CaaX motifs, of which the terminal amino acid confers specificity. Due to its involvement in oncogenic Ras activation, FTase has become a major anticancer target for drug development. Although first-generation FTase inhibitors failed in clinical trials in many cancers due to compensatory geranylgeranylation of KRAS and NRAS, they remain effective against HRAS-driven tumors and other pathologies, such as Hutchinson-Gilford progeria syndrome. The FTase inhibitor A-176120 was reported to compete with farnesyl and not KRAS. However, our crystallographic and biochemical analyses reveal that A-176120 sterically interferes with the engagement of the KRAS CAAX motif, reducing, but not abolishing, its binding to FTase.