Enzymic characterization of fission yeast farnesyl transferase: recognition of the -CAAL motif at the C-terminus.

The enzyme farnesyl transferase (FTase) catalyzes the posttranslational modification of Ras and other Ras family proteins with a C15 farnesyl group. The target proteins have a consensus -CAAX motif (X, any amino acid except leucine) at the C-terminus. Since proteins that have leucine as the C-terminal amino acid X are modified with a C20 geranylgeranyl group, it is thought that the C-terminal leucine is the signal (-CAAL motif) for selection of isoprenoid molecules. Here, we report the presence of multiple FTase activities in the fission yeast Schizosaccharomyces pombe, each seeming to correspond to a particular protein known to be modified by the farnesyl group in vivo. Using enzymic activities specific to S. pombe Ras1, we found similar affinities for FTases in the wild-type (EVSTKCCVIC) and mutant Ras1 peptide, in which the C-terminal amino acid is replaced by leucine (EVSTKCCVIL). These results suggest that recognition and selection of the correct isoprenoid group by the FTases require other amino acid sequences of the target protein in addition to the C-terminal -CAAX motif.