Mitochondrial Phosphatidylserine Decarboxylase from Higher Plants. Functional Complementation in Yeast, Localization in Plants, and Overexpression in Arabidopsis

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Abstract
Plants are known to synthesize ethanolamine (Etn) moieties by decarboxylation of free serine (Ser), but there is also some evidence for phosphatidyl-Ser (Ptd-Ser) decarboxylation. Database searches identified diverse plant cDNAs and an Arabidopsis gene encoding 50-kD proteins homologous to yeast (Saccharomyces cerevisiae) and mammalian mitochondrial Ptd-Ser decarboxylases (PSDs). Like the latter, the plant proteins have putative mitochondrial targeting and inner membrane sorting sequences and contain near the C terminus a Glycine-Serine-Threonine motif corresponding to the site of proteolysis and catalytic pyruvoyl residue formation. A truncated tomato (Lycopersicon esculentum) cDNA lacking the targeting sequence and a chimeric construct in which the targeting and sorting sequences were replaced by those from yeast PSD1 both complemented the Etn requirement of a yeast psd1 psd2 mutant, and PSD activity was detected in the mitochondria of the complemented cells. Immunoblot analysis of potato (Solanum tuberosum) mitochondria demonstrated that PSD is located in mitochondrial membranes, and mRNA analysis in Arabidopsis showed that the mitochondrial PSD gene is expressed at low levels throughout the plant. An Arabidopsis knockup mutant grew normally but had 6- to 13-fold more mitochondrial PSD mRNA and 9-fold more mitochondrial PSD activity. Total membrane PSD activity was, however, unchanged in the mutant, showing mitochondrial activity to be a minor part of the total. These results establish that plants can synthesize Etn moieties via a phospholipid pathway and have both mitochondrial and extramitochondrial PSDs. They also indicate that mitochondrial PSD is an important housekeeping enzyme whose expression is strongly regulated at the transcriptional level.