A mutant form of human protein farnesyltransferase exhibits increased resistance to farnesyltransferase inhibitors

Protein farnesyltransferase (FTase) is a key enzyme responsible for the lip id modification of a large and important number of proteins including Ras. Recent demonstrations that inhibitors of this enzyme block, the growth of a variety of human tumors point to the importance of this enzyme in human tu mor formation. In this paper, we report that a mutant form of human FTase, Y361L, exhibits increased resistance to farnesyltransferase inhibitors, par ticularly a tricyclic: compound, SCH56582, which is a competitive inhibitor of FTase with respect to the CAAX (where C is cysteine, A is an aliphatic amino acid, and X is the C-terminal residue that is preferentially serine, cysteine, methionine, glutamine or alanine) substrates. The Y361L mutant ma intains FTase activity toward substrates ending with CIIS. However, the mut ant also exhibits an increased affinity for peptides terminating with CIIL, a motif that is recognized by geranylgeranyltransferase I (GGTase I). The Y361L mutant also demonstrates activity with Ha-Ras and Cdc42Hs proteins, s ubstrates of FTase and GGTase I, respectively. In addition, the Y361L mutan t shows a marked sensitivity to a zinc chelator HPH-5 suggesting that the m utant has altered zinc coordination. These results demonstrate that a singl e amino acid change at a residue at the active site can lead to the generat ion of a mutant resistant to FTase inhibitors. Such a mutant may be valuabl e for the study of the effects of FTase inhibitors on tumor cells.