Farnesyltransferase (FTase) is a heterodimeric enzyme that modifies a group of proteins, including Ras, in mammals and yeasts. Plant FTase ␣ and  subunits were cloned from tomato and expressed in the yeast Saccharomyces cerevisiae to assess their functional conservation in farnesylating Ras and a-factor proteins, which are important for cell growth and mating. The tomato FTase  subunit (LeFTB) alone was unable to complement the growth defect of ram1⌬ mutant yeast strains in which the chromosomal FTase  subunit gene was deleted, but coexpression of LeFTB with the plant ␣ subunit gene (LeFTA) restored normal growth, Ras membrane association, and mating. LeFTB contains a novel 66-amino-acid sequence domain whose deletion reduces the efficiency of tomato FTase to restore normal growth to yeast ram1⌬ strains. Coexpression of LeFTA and LeFTB in either yeast or insect cells yielded a functional enzyme that correctly farnesylated CaaX-motifcontaining peptides. Despite their low degree of sequence homology, yeast and plant FTases shared similar in vivo and in vitro substrate specificities, demonstrating that this enzymatic modification of proteins with intermediates from the isoprenoid biosynthesis pathway is conserved in evolutionarily divergent eukaryotes.Modification of various proteins by the C 15 isoprene farnesyl or the C 20 isoprene geranylgeranyl groups facilitates their adherence to membranes and, for some proteins that have been studied in detail, is also required for their function (34,44,54). Protein prenylation was discovered almost 2 decades ago as a modification of peptide mating pheromones produced by fungi (28,43). During the last 8 years, a growing number of proteins have been discovered that are prenylated. Interestingly, many of these proteins play key roles in the regulation of cell division, cell growth, and signal transduction processes. These proteins include normal and oncogenic forms of Ras, the yeast a-factor mating pheromone, certain ␥ subunits of heterotrimeric G proteins, proteins in the vertebrate visual system, hepatitis delta virus large antigen, and type I inositol triphosphate 5Ј-phosphatase (1,2,11,16,18,23,25,26,30,36,45,52).All known farnesylated proteins have a common C-terminal sequence motif known as the CaaX box (C, cysteine; a, usually an aliphatic amino acid; X, any amino acid). Yeast and mammalian protein farnesyltransferases (FTases) attach the farnesyl group from farnesyl pyrophosphate via a thioether linkage to the cysteines of CaaX sequences in which X is serine, methionine, cysteine, alanine, or glutamine (44). In most cases, farnesylation is followed by proteolytic removal of the C-terminal three amino acids and methylation of the free carboxyl group of cysteine (2,15,22).In mammals and the yeast Saccharomyces cerevisiae, protein FTase is a heterodimeric enzyme composed of an ␣ and a  subunit (the yeast subunits are designated Ram2p and Ram1p, respectively) (21,24,40,41,45). A second protein prenyltransferase, geranylgeranyltransferase type I (GGTase-I), uses geranyl...