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Interestingly, these same mutations affected the isoprenoid selectivity of the mutant enzymes. Moreover, the isoprenoid selectivity was observed to be affected by the CaaX peptide substrate used. These observations of interdependent selectivity in yeast FTase are consistent with the observation that the substrates directly interact in a ternary complex {10, 11). C. MUTATIONS AFFECTING CATALYTIC TURNOVER Eleven conserved residues in the ^ subunit of human FTase, which he in the active site region, were mutated and their steady state kinetic properties analyzed {30).

Figure 10 illustrates the FTase residues within 4 A of the CaaX motif of the peptide in the ternary complex. Three a-subunit and eight j8-subunit residues are found within this distance: Y131a, Y166a:, Q167a, A98i8, 899)8, W102i8, H149i8, P152i8, £198)8, R202i8, and ¥361/3. Of these interactions, there are only two direct FTase-peptide hydrogen bonds: Q167Q: side chain to the C-terminal carboxylate and R202j8 to the isoleucine a2 backbone carbonyl oxygen. All other hydrogen bonds between the peptide and the enzyme are water mediated.

F. STRUCTURAL BASIS FOR PEPTIDE SUBSTRATE SPECIFICITY FTase is selective for protein substrates that have a cysteine residue at the fourth position from the C terminus. Peptide competition studies have shown that the cysteine residue must be at this position and not the third or fifth position from the C terminus (3, 38). The structural basis for this length specificity has been proposed from the binding of Caia2X peptides (7^). Peptide binding to zinc-depleted FTase indicates that the conformation of the a2X portion of the peptide is independent of thiol(ate)-zinc coordination.