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^KB-ras (CVIM), p21^N-ras (CVVM), and p21^KA-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.