Molecular Genetic Research on Organellar Specific Synthesis and Intracellular Translocation of Membrane Phospholipids.

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1) Phosphatidylserine (PS) synthase as well as phosphatidylinositol (PI) synthase has been formerly assumed to be localized in bothmitochondria and microsome in Saccharomyces cerevisiae. Our density gradient centrifugation analyses of crude cell extracts, however, revealed that the mitochondrial localization of these two enzymes is unlikely.2) We constructed a series of chol'-lacZ fusion genes and examined subcellular localization of their products. The fusion proteins with PS synthase N-terminals of more than 129 amino acids were found to be distributed among subcellular fractions in the similar manner with the PS synthase, whereas those with PS synthase N-terminals of less than 114 amino acids were found in soluble fraction. This result suggests that the region between 114th to 129th residue, which is rich in hydrophobic amino acids, is of critical importance for microsomal binding of PS synthase.3) We obtained two mutants with lowered PS decarboxylase activity, one of which carried a mutation that could be classified to pet and was defective in PS decarboxylase activity when it was grown in the presence of myo-inositol.4) The PS decarboxylase activity of a fission yeast Schizosaccharomyces pombe was less than one-tenth of that of S. cerevisiae. This and other lines of evidence suggest that decarboxylation of PS does not so much contribute to phosphatidylethanolamine formation in the fission yeast as known in the budding yeast.