BIOSYNTHESIS OF PHOSPHATIDIC-ACID IN LIPID PARTICLES AND ENDOPLASMIC-RETICULUM OF SACCHAROMYCES-CEREVISIAE

Lipid particles of the yeast Saccharomyces cerevisiae harbor two enzym es that stepwise acylate glycerol-3-phosphate to phosphatidic acid, a key intermediate in lipid biosynthesis. In lipid particles of the slc1 disruptant YMN5 (M. M. Nagiec et al. J. Biol. Chem. 268:22156-22163, 1993) acylation stops after the first step, resulting in the accumulat ion of lysophosphatidic acid, Two-dimensional gel electrophoresis conf irmed that Slc1p is a component of lipid particles, Lipid particles of a second mutant strain,TTA1 (T. S. Tillman and R. M. Bell, J. Biol. C hem. 261:9144-9149, 1986), which harbors a point mutation in the GAT g ene, are essentially devoid of glycerol-3-phosphate acyltransferase ac tivity in vitro., Synthesis of phosphatidic acid is reconstituted by c ombining lipid particles from YMN5 and TTA1. These results indicate th at two distinct enzymes are necessary for phosphatidic acid synthesis in lipid particles: the first step, acylation of glycerol-3-phosphate, is catalyzed by a putative Gat1p; the second step, acylation of lysop hosphatidic acid, requires Slc1p. Surprisingly, YMN5 and TTA1 mutants grow like the corresponding wild types because the endoplasmic reticul um of both mutants has the capacity to form a reduced but significant amount of phosphatidic acid. As a consequence, an slc1 gat1 double mut ant is also viable, Lipid particles from this double mutant fail compl etely to acylate glycerol-3-phosphate, whereas endoplasmic reticulum m embranes harbor residual enzyme activities to synthesize phosphatidic acid, Thus, yeast contains at least two independent systems of phospha tidic acid biosynthesis.