REGULATION OF PHOSPHATIDIC-ACID BIOSYNTHETIC-ENZYMES IN SACCHAROMYCES-CEREVISIAE

Phosphatidic acid is the biosynthetic precursor of all glycerolipids. To understand how phosphatidic acid biosynthesis is controlled in Sacc haromyces cerevisiae, we studied the regulation of three enzyme activi ties involved in the synthesis of this glycerolipid precursor, i.e., g lycerophosphate acyltransferase (GPAT), dihydroxyacetone phosphate acy ltransferase (DHAPAT), and acyl DHAP reductase. GPAT activity was incr eased 3-fold, while DHAPAT activity was increased up to 9-fold in wild type cells grown in a nonfermentable carbon source compared to that o f glucose-grown cells. The ratio of GPAT/DHAPAT activity was 12 in glu cose-grown cells but only 4 in cells grown in glycerol/ethanol. In the previously characterized tpa1 mutant, (T. S. Tillman and R. M. Bell. 1986. J. Biol. Chem. 261: 9144-9149), GPAT was decreased 2-fold and DH APAT 27-fold compared to activities in the wild type. Acyl DHAP reduct ase activity in both wild type and tpa1 cells grown on a nonfermentabl e carbon source was increased approximately 2-fold over that of glucos e-grown cells. All three enzymatic activities increased as wild type c ells grown on glucose entered the stationary phase of growth. Therefor e, GPAT, DHAPAT, and acyl DHAP reductase activities appear to be regul ated by the respiratory state of the cell. None of the activities ties was affected to a great extent by inositol, which is a key regulator of many enzymes involved in the synthesis of PtdOH-derived phospholipi ds in S. cerevisiae, nor by deletion of the mitochondrial genome. Thes e data show that i) the PtdOH biosynthetic enzymes GPAT, DHAPAT, and a cyl DHAP reductase are increased during respirative growth but are not affected by inositol; ii) the extent of derepression differs for the acyltransferases GPAT and DHAPAT in the wild type cell; and iii) the e xtent of reduction of the two enzyme activities is strikingly differen t in the tpa1 mutant. The results suggest that GPAT and DHAPAT are pro bably different enzymes, and that both the G3P and acyl DHAP pathways may be important for glycerolipid synthesis in yeast.