S-Adenosyl-L-homocysteine Hydrolase, Key Enzyme of Methylation Metabolism, Regulates Phosphatidylcholine Synthesis and Triacylglycerol Homeostasis in Yeast: IMPLICATIONS FOR HOMOCYSTEINE AS A RISK FACTOR OF ATHEROSCLEROSIS*

摘要

In eukaryotes, S-adenosyl-l-homocysteine hydrolase(Sah1) offers a single way for degradation ofS-adenosyl-l-homocysteine, a product and potentcompetitive inhibitor of S-adenosyl-l-methionine(AdoMet)-dependent methyltransferases. Denovophosphatidylcholine(PC)synthesisrequiresthreeAdoMet-dependentmethylation steps. Here we show that down-regulation of SAH1expression in yeast leads to accumulation ofS-adenosyl-l-homocysteine and decreased de novoPC synthesis in vivo. This decrease is accompanied by an increase intriacylglycerol (TG) levels, demonstrating that Sah1-regulated methylation hasa major impact on cellular lipid homeostasis. TG accumulation is also observedin cho2 and opi3 mutants defective in methylation ofphosphatidylethanolamine to PC, confirming that PC de novo synthesisand TG synthesis are metabolically coupled through the efficiency of thephospholipid methylation reaction. Indeed, because both types of lipids sharephosphatidic acid as a precursor, we find in cells with down-regulated Sah1activity major alterations in the expression of the INO1 gene as wellas in the localization of Opi1, a negative regulatory factor of phospholipidsynthesis, which binds and is retained in the endoplasmic reticulum membraneby phosphatidic acid in conjunction with VAMP/synaptobrevin-associatedprotein, Scs2. The addition of homocysteine, by the reversal of theSah1-catalyzed reaction, also leads to TG accumulation in yeast, providing anattractive model for the role of homocysteine as a risk factor ofatherosclerosis in humans.