Phosphatidylcholine synthesis in yeast

The two pathways for the biosynthesis of phosphatidylcholine, by way of phosphocholine and by methylation of phosphatidylethanolamine, in wild-type yeast (Sacchammyces cereviSCae) and in the yeast mutant GL7 have been compared. The mutant requires for growth a sterol, unsaturated fatty acids, and methionine. The uptake of labeled choline or labeled methionine and their conversion to phosphatidylcholine were determined in both cell types. The activities of the major enzymes for both pathways were assayed in vitro. We find that the methylation pathway is predominant in both wild-type and mutant cells though the overall activity of the choline pathway is lower in the yeast mutant. The methionine analogue ethionine inhibits the growth of the mutant more strongly than growth of wild-type yeast. Ethionine, while a powerful inhibitor of phosphatidylcholine synthesis by methylation, stimulates the choline pathway in both cell types. -Chin, J., and K. Bloch. Phosphatidylcholine synthesis in yeast. J. Lipid Res. 1988. 29: 9-14. Supplementary key words N-methylation ethionine CDP-choline phosphatidylethanolamine Eukaryotic cells synthesize phosphatidylcholine (PC) by two independent routes. The pathway starting with choline and leading to phosphatidylcholine by way of CDPcholine was discovered by Kennedy and Weiss (1). In the alternative or methylation pathway, phosphatidylcholine arises by three successive transfers of methyl-groups from S-adenosylmethionine to phosphatidylethanolamine (PE), as shown first by Bremer and Greenberg (2). The relative contribution of the two pathways to PC synthesis varies among organisms and in higher eukaryotes from one tissue to another. In animal tissues, the pathway starting with choline predominates (3) whereas in yeast PC may be entirely derived from the methylation of PE (4). We have recently shown that in the yeast mutant GL7, a strain that requires exogenous sterol for growth, certain reactions of phospholipid biosynthesis, e.g., of PC and phosphatidylinositol (PI), are stimulated by ergosterol (5). The question therefore arose whether one or both of the alternate routes to PC are sterol-controlled. We had already shown that in GL7 cells raised on ergosterol, PE-PC transmethylation occurs at a faster rate than in mutant cells supplied with cholesterol (6). Here we describe experiments comparing the rates of the two pathways for PC synthesis in the mutant with the corresponding rates in wild-type yeast.' MATERIALS AND METHODS