Identification of a Non-mitochondrial Phosphatidylserine Decarboxylase Activity (PSD2) in the Yeast Saccharomyces cerevisiae(*)

Phosphatidylserine decarboxylase (PSD1) plays a central role in the biosynthesis of aminophospholipids in both prokaryotes and eukaryotes by catalyzing the synthesis of phosphatidylethanolamine. Recent reports (Trotter, P. J., Pedretti, J., and Voelker, D. R.(1993) J. Biol. Chem.268, 21416-21424; Clancey, C. J., Chang, S.-C., and Dowhan, W.(1993) J. Biol. Chem. 268, 24580-24590) described the cloning of a yeast structural gene for this enzyme (PSD1) and the creation of the null allele. Based on the phenotype of strains containing a null allele for PSD1 (psd1-Δ1::TRP1) it was hypothesized that yeast have a second phosphatidylserine decarboxylase. The present studies demonstrate the presence of a second enzyme activity (denoted PSD2) which, depending on the method of evaluation, accounts for 4-12% of the total cellular phosphatidylserine decarboxylase activity found in wild type. Recessive mutations resulting in loss of this enzyme activity (denoted psd2) in cells containing the psd1-Δ1::TRP1 null allele also result in ethanolamine auxotrophy. When incubated with [3H]serine these double mutants accumulate label in phosphatidylserine, while very little (<5%) is converted to phosphatidylethanolamine. In addition, these mutants have a 70% decrease in the amount of total phosphatidylethanolamine even when grown in the presence of exogenous ethanolamine. Strains containing psd1 or psd2 mutations were utilized for the subcellular localization of the PSD2 enzyme activity. Unlike the PSD1 activity, the PSD2 enzyme activity does not localize to the mitochondria, but to a low density subcellular compartment with fractionation properties similar to both vacuoles and Golgi.