GENES ENCODING FARNESYL CYSTEINE CARBOXYL METHYLTRANSFERASE IN SCHIZOSACCHAROMYCES-POMBE AND XENOPUS-LAEVIS

The mam4 mutation of Schizosaccharomyces pombe causes mating deficienc y in h(-) cells but not in h(+) cells. h(-) cells defective in mam4 do not secrete active mating pheromone M-factor. We cloned mam4 by compl ementation. The mam4 gene encodes a protein of 236 amino acids, with s everal potential membrane-spanning domains, which is 44% identical wit h farnesyl cysteine carboxyl methyltransferase encoded by STE14 and re quired for the modification of a-factor in Saccharomyces cerevisiae. A nalysis of membrane fractions revealed that mnm4 is responsible for th e methyltransferase activity in S. pombe. Cells defective in mam4 prod uced farnesylated but unmethylated cysteine and small peptides but no intact M-factor. These observations strongly suggest that the mnm4 gen e product is farnesyl cysteine carboxyl methyltransferase that modifie s M-factor. Furthermore, transcomplementation of S. pombe mam4 allowed us to isolate an apparent homolog of mam4 from Xenopus laevis (Xmam4) . In addition to its sequence similarity to S. pombe mam4, the product of Xmam4 was shown to have a farnesyl cysteine carboxyl methyltransfe rase activity in S. pombe cells. The isolation of a vertebrate gene en coding farnesyl cysteine carboxyl methyltransferase opens the way to i n-depth studies of the role of methylation in a large body of proteins , including Ras superfamily proteins.