Saccharomyces cerevisiae Eukaryotic Elongation Factor 1A (eEF1A) Is Methylated at Lys-390 by a METTL21-Like Methyltransferase

The human methyltransferases (MTases) METTL21A and VCP-KMT (METTL21D) were recently shown to methylate single lysine residues in Hsp70 proteins and in VCP, respectively. The yet uncharacterized MTase encoded by the YNL024C gene in Saccharomyces cerevisiae shows high sequence similarity to METTL21A and VCP-KMT, as well as to their uncharacterized paralogues METTL21B and METTL21C. Despite being most similar to METTL21A, the Ynl024c protein does not methylate yeast Hsp70 proteins, which were found to be unmethylated on the relevant lysine residue. Eukaryotic translation elongation factor eEF1A in yeast has been reported to contain four methylated lysine residues (Lys30, Lys79, Lys318 and Lys390), and we here show that the YNL024C gene is required for methylation of eEF1A at Lys390, the only of these methylations for which the responsible MTase has not yet been identified. Lys390 was found in a partially monomethylated state in wild-type yeast cells but was exclusively unmethylated in a ynl024cΔ strain, and over-expression of Ynl024c caused a dramatic increase in Lys390 methylation, with trimethylation becoming the predominant state. Our results demonstrate that Ynl024c is the enzyme responsible for methylation of eEF1A at Lys390, and in accordance with prior naming of similar enzymes, we suggest that Ynl024c is renamed to Efm6 (Elongation factor MTase 6).人类甲基转移酶（methyltransferases, MTases）METTL21A与VCP-KMT（METTL21D）近期被证实可分别对热休克蛋白70（Hsp70）家族蛋白及VCP中的单个赖氨酸残基进行甲基化修饰。酿酒酵母（Saccharomyces cerevisiae）中由YNL024C基因编码的、尚未被功能表征的甲基转移酶，与METTL21A、VCP-KMT以及二者尚未被表征的旁系同源蛋白METTL21B和METTL21C均具有较高的序列相似性。尽管Ynl024c蛋白与METTL21A的序列相似性最高，但它无法对酿酒酵母Hsp70蛋白进行甲基化修饰——研究发现这类蛋白的对应赖氨酸残基本身并未发生甲基化。有研究报道，酿酒酵母中的真核翻译延伸因子eEF1A含有四个甲基化赖氨酸残基（Lys30、Lys79、Lys318和Lys390）；本研究证实，YNL024C基因是eEF1A在Lys390位点发生甲基化修饰所必需的，而这四个甲基化位点中，仅有Lys390的对应甲基转移酶尚未被鉴定出来。在野生型酿酒酵母细胞中，Lys390位点呈部分单甲基化状态；而在ynl024cΔ突变菌株中，该位点则完全未发生甲基化。此外，过表达Ynl024c可使Lys390位点的甲基化水平显著升高，三甲基化成为该位点的主要修饰形式。本研究结果证实，Ynl024c正是负责eEF1A在Lys390位点发生甲基化的酶；参照同类酶的命名规则，我们建议将Ynl024c重命名为Efm6（Elongation factor MTase 6）。