1-acyldihydroxyacetone-phosphate reductase (Ayr1p) of the yeast Saccharomyces cerevisiae encoded by the open reading frame YIL124w is a major component of lipid particles

Biosynthesis of phosphatidic acid through the dihydroxyacetone phosphate pa thway requires NADPH-dependent reduction of the intermediate 1-acyldihydrox yacetone phosphate before the second step of acylation. Studies with isolat ed subcellular fractions of the yeast Saccharomyces cerevisiae revealed tha t lipid particles and the endoplasmic reticulum harbor 1-acyldihydroxyaceto ne-phosphate reductase (ADR) activity, Deletion of the open reading frame Y IL124w (in the following named AYR1) abolished reduction of 1-acyldihydroxy acetone phosphate in lipid particles, whereas ADR activity in microsomes of the deletion strain was decreased approximately 3-fold as compared with th e wild-type level, This result indicates that (i) both lipid particles and microsomes harbor Ayr1p, which was confirmed by immunological detection of the protein in these two cellular compartments, and (ii) microsomes contain at least one additional ADR activity. As a consequence of this redundancy, deletion of AYR1 neither results in an obvious growth phenotype nor affect s the lipid composition of a haploid deletion strain. When a heterozygous A YR1(+)/layr1(-) diploid strain was subjected to sporulation; however, spore s bearing the ary1 defect failed to germinate, suggesting that Ayr1p plays an essential role at this stage. Overexpression of Ayr1p at a 5- to 10-fold level of wild type caused growth arrest. Heterologous expression of Ayr1p in Escherichia coli resulted in gain of ADR activity in the prokaryote, con firming that YIL124w is the structural gene of the enzyme and does not enco de a regulatory or auxiliary component required for reduction of 1-acyldihy droxyacetone phosphate. Taken together, these results identified Ayr1p of t he yeast as the first ADR from any organism at the molecular level.