Respiratory Deficiency Mediates the Regulation of CHO1-encoded Phosphatidylserine Synthase by mRNA Stability in Saccharomyces cerevisiae

Hyeon-Son Choi,George M Carman

doi:10.1074/jbc.m705098200

Abstract

The CHO1-encoded phosphatidylserine synthase (CDP-diacylglycerol:l-serine O-phosphatidyltransferase, EC 2.7.8.8) is one of the most highly regulated phospholipid biosynthetic enzymes in the yeast Saccharomyces cerevisiae. CHO1 expression is regulated by nutrient availability through a regulatory circuit involving a UAS(INO) cis-acting element in the CHO1 promoter, the positive transcription factors Ino2p and Ino4p, and the transcriptional repressor Opi1p. In this work, we examined the post-transcriptional regulation of CHO1 by mRNA stability. CHO1 mRNA was stabilized in mutants defective in deadenylation (ccr4Delta), mRNA decapping (dcp1), and the 5'-3'-exonuclease (xrn1), indicating that the CHO1 transcript is primarily degraded through the general 5'-3' mRNA decay pathway. In respiratory-sufficient cells, the CHO1 transcript was moderately stable with a half-life of 12 min. However, the CHO1 transcript was stabilized to a half-life of >45 min in respiratory-deficient (rho(-) and rho(o)) cells, the cox4Delta mutant defective in the cytochrome c oxidase, and wild type cells treated with KCN (a cytochrome c oxidase inhibitor). The increased CHO1 mRNA stability in response to respiratory deficiency caused increases in CHO1 mRNA abundance, phosphatidylserine synthase protein and activity, and the synthesis of phosphatidylserine in vivo. Respiratory deficiency also caused increases in the activities of CDP-diacylglycerol synthase, phosphatidylserine decarboxylase, and the phospholipid methyltransferases. Phosphatidylinositol synthase and choline kinase activities were not affected by respiratory deficiency. This work advances our understanding of phosphatidylserine synthase regulation and underscores the importance of mitochondrial respiration to the regulation of phospholipid synthesis in S. cerevisiae.

Highlights

The yeast Saccharomyces cerevisiae (4 – 6)
Respiratory Deficiency Mediates CHO1 mRNA Stability covered that the stabilization of CHO1 mRNA was not mediated by components of the Kennedy pathway, but instead it was mediated by a defect in mitochondrial respiration
That CHO1 mRNA was stabilized in these mutants indicated that the CHO1 transcript is degraded by the general decay pathway (71)

Summary

Introduction

PS serves as the precursor for the synthesis of the most abundant membrane phospholipids PE (20 –32%) and PC (35– 55%) that are synthesized by the de novo CDP-DAG pathway (Fig. 1) (5, 9). Both the CDP-DAG and Kennedy pathways contribute to the synthesis of PC regardless of whether choline is supplemented to the growth medium (14 –19). The biochemical regulation of PS synthase activity governs the partitioning of the substrate CDP-DAG between PS and PI; the inhibition of PS synthase activity favors PI synthesis (Fig. 1) (4). The expression of the PS synthase (CHO1) gene is regulated by the supplementation of water-soluble phospholipid precursors (e.g. inositol) (28 –31), zinc deprivation (32), and by growth phase (33, 34). This work underscores the importance of respiration to the regulation of phospholipid synthesis and advances our understanding of PS synthase regulation in yeast

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