IDENTIFICATION OF A NONMITOCHONDRIAL PHOSPHATIDYLSERINE DECARBOXYLASEACTIVITY (PSD2) IN THE YEAST SACCHAROMYCES-CEREVISIAE

Phosphatidylserine decarboxylase (PSD1) plays a central role in the bi osynthesis 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 st ructural gene for this enzyme (PSD1) and the creation of the null alle le, Based on the phenotype of strains containing a null allele for PSD 1 (psd1-Delta 1::TRP1) it was hypothesized that yeast have a second ph osphatidylserine decarboxylase, The present studies demonstrate the pr esence of a second enzyme activity (denoted PSD2) which, depending on the method of evaluation, accounts for 4-12% of the total cellular pho sphatidylserine decarboxylase activity found in wild type, Recessive m utations resulting in loss of this enzyme activity (denoted psd2) in c ells containing the psd1-Delta 1::TRP1 null allele also result in etha nolamine auxotrophy. When incubated with [H-3] serine these double mut ants accumulate label in phosphatidylserine, while very little (<5%) i s converted to phosphatidylethanolamine. In addition, these mutants ha ve a similar to 70% decrease in the amount of total phosphatidylethano lamine even when grown in the presence of exogenous ethanolamine. Stra ins containing psd1 or psd2 mutations were utilized for the subcellula r 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 s imilar to both vacuoles and Golgi.