Potent inhibition of human tumor p21(ras) farnesyltransferase by A₁A₂- lacking p21(ras) CA₁A₂X peptidomimetics

The ras oncogene product p21(ras) requires farnesylation and subsequent plasma membrane association for its transforming activity. This key post- translational modification is catalyzed by p21(ras) farnesyltransferase, which transfers farnesyl from farnesylpyrophosphate to the cysteine of the CA₁A₂X carboxyl-terminal tetrapeptide of p21(ras). In the present report, we describe potent inhibition of p21(ras) farnesyltransferase by CA₁A₂X peptidomimetics containing no peptidic amide bonds. We synthesized a series of CA₁A₂X analogues where the 2 aliphatic amino acids A₁ and A₂ were replaced by a hydrophobic spacer, 3-aminomethylbenzoic acid (AMBA). The peptidomimetic Cys-AMBA-Met, inhibits p21(ras) farnesyltransferase from human colon carcinoma (COLO-205) and Burkitt's lymphoma (Daudi) with IC₅₀ values of 60 and 120 nM, respectively. Cys-AMBA-Met is 3-, 8-, and 9-fold (COLO- 205) and 2-, 5-, and 7-fold (Daudi) more potent than the corresponding tetrapeptides of p21(K(B)-ras) (CVIM), p21(N-ras) (CVVM), and p21(K(A)-ras) (CIIM), respectively. Replacing methionine at the X position with negatively charged glutamate reduces its ability to inhibit the enzyme, whereas positively charged lysine at this position abolishes the inhibitory character of the peptidomimetic. A hydrophobic moiety at the X position, as in Cys- AMBA-Phe, retains potent inhibitory activity. Leucine in the X position of CA₁A₂X is a post-translational signal for protein geranylgeranylation rather than farnesylation, and, as expected, Cys-AMBA-Leu does not inhibit the enzyme. Furthermore, CVIM, CVVM, and CIIM are farnesylated by human p21(ras) farnesyltransferases and inhibit these enzymes by serving as alternative substrates. In contrast, the peptidomimetics described here are true p21(ras) farnesyltransferase inhibitors since none is farnesylated by this enzyme.