GENETIC-EVIDENCE FOR IN-VIVO CROSS-SPECIFICITY OF THE CAAX-BOX PROTEIN PRENYLTRANSFERASES FARNESYLTRANSFERASE AND GERANYLGERANYLTRANSFERASE-I IN SACCHAROMYCES-CEREVISIAE

Two protein prenyltransferase enzymes, farnesyltransferase (FTase) and geranylgeranyltransferase-I (GGTase-I), catalyze the covalent attachm ent of a farnesyl or geranylgeranyl lipid group to the cysteine of a C aaX sequence (cysteine [C], two aliphatic amino acids [aa], and any am ino acid [X]). In vitro studies reported here confirm previous reports that CaaX proteins with a C-terminal serine are farnesylated by FTase and those with a C-terminal leucine are geranylgeranylated by GGTase- I. In addition, we found that FTase can farnesylate CaaX proteins with a C-terminal leucine and can transfer,a geranylgeranyl group to some CaaX proteins. Genetic data indicate that FTase and GGTase-I have the same substrate preferences in vivo as in vitro and also show that each enzyme can prenylate some of the preferred substrates of the other en zyme in vivo. Specifically, the viability of yeast cells lacking FTase is due to prenylation of Ras proteins by GGTase-I. Although this GGTa se-I dependent prenylation of Ras is sufficient for growth, it is not sufficient for mutationally activated Ras proteins to exert deleteriou s effects on growth. The dependence of the activated Ras phenotype on FTase can be bypassed by replacing the C-terminal serine with leucine. This altered form of Ras appears to be prenylated by both GGTase-I an d FTase, since it produces an activated phenotype in a strain lacking either FTase or GGTase-I. Yeast cells can grow in the absence of GGTas e-I as long as two essential substrates are overexpressed, but their g rowth is slow. Such strains are dependent on FTase for viability and a re able to grow faster when FTase is overproduced, suggesting that FTa se can prenylate the essential substrates of GGTase-I when they are ov erproduced.