Characterization of Recombinant Human Farnesyl-Protein Transferase: Cloning, Expression, Farnesyl Diphosphate Binding, and Functional Homology with Yeast Prenyl-Protein Transferases

We have isolated cDN As encoding the α and β subunits of human farnesyl-protein transferase (FPTase). The proteins encoded by these two cDNAs are 93–95% identical to the corresponding subunits of bovine and rat FPTase and show regions of homology with proteins encoded by Saccharomyces cerevisiae prenyl-protein transferase genes. Human FPTase expressed in Escherichia coli from a translationally coupled operon had kinetic properties similar to those of FPTase isolated from bovine brain. Examination of farnesyl diphosphate binding indicated that while neither individual subunit was capable of isoprenoid binding, a radiolabeled farnesyl diphosphate analog could be specifically photo-cross-linked to the α subunit of FPTase holoenzyme. To further analyze subunit structure–function and to detect functional similarities with yeast prenyl-protein transferases (FPTase and two geranylgeranyl-protein transferases), amino acid changes homologous to those found in mutant yeast prenyl-protein transferase subunits were made in the subunits of human FPTase. Substitutions in either the α or β subunits that decrease the activity of yeast prenyl-protein transferases were also observed to impair human FPTase. Kinetic analyses showed that these mutant human FPTases have K_m and k_cat values that are altered with respect to wild-type human FPTase.