PLANT FARNESYLTRANSFERASE CAN RESTORE YEAST RAS SIGNALING AND MATING

Farnesyltransferase (FTase) is a heterodimeric enzyme that modifies a group of proteins, including Pas, in mammals and yeasts. Plant FTase a lpha and beta subunits were cloned from tomato and expressed in the ye ast Saccharomyces cerevisiae to assess their functional conservation i n farnesylating Ras and a-factor proteins, which are important for cel l growth and mating. The tomato FTase beta subunit (LeFTB) alone was u nable to complement the growth defect of ram1 Delta mutant yeast strai ns in which the chromosomal FTase beta subunit gene was deleted, but c oexpression of LeFTB with the plant a subunit gene (LeFTA) restored no rmal growth, Ras membrane association, and mating. LeFTB contains a no vel 66-amino-acid sequence domain whose deletion reduces the efficienc y of tomato FTase to restore normal growth to yeast raml Delta strains , Coexpression of LeFTA and LeFTB in either yeast or insect cells yiel ded a functional enzyme that correctly farnesylated CaaX-motif-contain ing peptides. Despite their low degree of sequence homology, yeast and plant FTases shared similar in vivo and in vitro substrate specificit ies, demonstrating that this enzymatic modification of proteins with i ntermediates from the isoprenoid biosynthesis pathway is conserved in evolutionarily divergent eukaryotes.