Department of Biology, McMaster University, Hamilton, Ontario L8S 4A8, Canada.
J Biol Chem. 2010 Sep 17;285(38):29147-55. doi: 10.1074/jbc.M110.112151. Epub 2010 Jul 22.
Three sequential methylations of phosphoethanolamine (PEA) are required for the synthesis of phosphocholine (PCho) in plants. A cDNA encoding an N-methyltransferase that catalyzes the last two methylation steps was cloned from Arabidopsis by heterologous complementation of a Saccharomyces cerevisiae cho2, opi3 mutant. The cDNA encodes phosphomethylethanolamine N-methyltransferase (PMEAMT), a polypeptide of 475 amino acids that is organized as two tandem methyltransferase domains. PMEAMT shows 87% amino acid identity to a related enzyme, phosphoethanolamine N-methyltransferase, an enzyme in plants that catalyzes all three methylations of PEA to PCho. PMEAMT cannot use PEA as a substrate, but assays using phosphomethylethanolamine as a substrate result in both phosphodimethylethanolamine and PCho as products. PMEAMT is inhibited by the reaction products PCho and S-adenosyl-l-homocysteine, a property reported for phosphoethanolamine N-methyltransferase from various plants. An Arabidopsis mutant with a T-DNA insertion associated with locus At1g48600 showed no transcripts encoding PMEAMT. Shotgun lipidomic analyses of leaves of atpmeamt and wild-type plants generated phospholipid profiles showing the content of phosphatidylmethylethanolamine to be altered relative to wild type with the content of a 34:3 lipid molecular species 2-fold higher in mutant plants. In S. cerevisiae, an increase in PtdMEA in membranes is associated with reduced viability. This raises a question regarding the role of PMEAMT in plants and whether it serves to prevent the accumulation of PtdMEA to potentially deleterious levels.
植物中磷酸胆碱(PCho)的合成需要磷酸乙醇胺(PEA)的三个连续甲基化。通过异源互补酿酒酵母 cho2、opi3 突变体,从拟南芥中克隆了编码催化最后两个甲基化步骤的 N-甲基转移酶的 cDNA。该 cDNA 编码磷酸甲乙醇胺 N-甲基转移酶(PMEAMT),这是一种 475 个氨基酸的多肽,组织成两个串联甲基转移酶结构域。PMEAMT 与一种相关酶,即磷酸乙醇胺 N-甲基转移酶,具有 87%的氨基酸同一性,该酶在植物中催化 PEA 的所有三个甲基化形成 PCho。PMEAMT 不能将 PEA 作为底物,但使用磷酸甲乙醇胺作为底物的测定结果表明同时产生磷酸二甲乙醇胺和 PCho。PMEAMT 被反应产物 PCho 和 S-腺苷-L-同型半胱氨酸抑制,这是各种植物中报道的磷酸乙醇胺 N-甲基转移酶的特性。带有与 At1g48600 基因座相关的 T-DNA 插入的拟南芥突变体显示不出编码 PMEAMT 的转录物。对 atpmeamt 和野生型植物叶片的鸟枪法脂质组学分析生成了磷脂谱,显示与野生型相比,磷酸甲乙醇胺的含量发生了变化,突变体植物中 34:3 脂质分子种类的含量增加了 2 倍。在酿酒酵母中,膜中 PtdMEA 的增加与生存能力降低有关。这就提出了一个关于 PMEAMT 在植物中的作用以及它是否有助于防止 PtdMEA 积累到潜在有害水平的问题。