Redundant systems of phosphatidic acid biosynthesis via acylation of glycerol-3-phosphate or dihydroxyacetone phosphate in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae lipid particles harbor two acyltransf erases, Gat1p and Slc1p, which catalyze subsequent steps of acylation requi red for the formation of phosphatidic acid. Both enzymes are also component s of the endoplasmic reticulum, but this compartment contains additional ac yltransferase(s) involved in the biosynthesis of phosphatidic acid (K. Athe nstaedt and G. Daum, J. Bacteriol. 179:7611-7616, 1997). Using the gat1 mut ant strain TTA1, we show here that Gat1p present in both subcellular fracti ons accepts glycerol-3-phosphate and dihydroxyacetone phosphate as a substr ate. Similarly, the additional acyltransferase(s) present in the endoplasmi c reticulum can acylate both precursors. In contrast, yeast mitochondria ha rbor an enzyme(s) that significantly prefers dihydroxyacetone phosphate as a substrate for acylation, suggesting that at least one additional independ ent acyltransferase is present in this organelle. Surprisingly, enzymatic a ctivity of 1-acyldihydroxyacetone phosphate reductase, which is required fo r the conversion of 1-acyldihydroxyacetone phosphate to 1-acylglycerol-3-ph osphate (lysophosphatidic acid), is detectable only in lipid particles and the endoplasmic reticulum and not in mitochondria. In vivo labeling of wild -type cells with [2-H-3, U-C-14]glycerol revealed that both glycerol-3-phos phate and dihydroxyacetone phosphate can be incorporated as a backbone of g lycerolipids. In the gat1 mutant and the 1-acylglycerol-3-phosphate acyltra nsferase slc1 mutant, the dihydroxyacetone phosphate pathway of phosphatidi c acid biosynthesis is slightly preferred as compared to the wild type. Thu s, mutations of the major acyltransferases Gat1p and Slc1p lead to an incre ased contribution of mitochondrial acyltransferase(s) to glycerolipid synth esis due to their substrate preference for dihydroxyacetone phosphate.