School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, Queensland, Australia.
PLoS One. 2013 Sep 30;8(9):e75669. doi: 10.1371/journal.pone.0075669. eCollection 2013.
The hnRNP A/B paralogs A1, A2/B1 and A3 are key components of the nuclear 40S hnRNP core particles. Despite a high degree of sequence similarity, increasing evidence suggests they perform additional, functionally distinct roles in RNA metabolism. Here we identify and study the functional consequences of differential post-translational modification of hnRNPs A1, A2 and A3. We show that while arginine residues in the RGG box domain of hnRNP A1 and A3 are almost exhaustively, asymmetrically dimethylated, hnRNP A2 is dimethylated at only a single residue (Arg-254) and this modification is conserved across cell types. It has been suggested that arginine methylation regulates the nucleocytoplasmic distribution of hnRNP A/B proteins. However, we show that transfected cells expressing an A2(R254A) point mutant exhibit no difference in subcellular localization. Similarly, immunostaining and mass spectrometry of endogenous hnRNP A2 in transformed cells reveals a naturally-occurring pool of unmethylated protein but an exclusively nuclear pattern of localization. Our results suggest an alternative role for post-translational arginine methylation of hnRNPs and offer further evidence that the hnRNP A/B paralogs are not functionally redundant.
hnRNP A/B 同源物 A1、A2/B1 和 A3 是核 40S hnRNP 核心颗粒的关键组成部分。尽管具有高度的序列相似性，但越来越多的证据表明它们在 RNA 代谢中具有额外的、功能不同的作用。在这里，我们鉴定并研究了 hnRNPs A1、A2 和 A3 差异翻译后修饰的功能后果。我们表明，虽然 hnRNP A1 和 A3 的 RGG 盒结构域中的精氨酸残基几乎完全、不对称地二甲基化，但 hnRNP A2 仅在一个残基（Arg-254）上被二甲基化，并且这种修饰在细胞类型中是保守的。有人提出，精氨酸甲基化调节 hnRNP A/B 蛋白的核质分布。然而，我们表明，转染表达 A2(R254A)点突变的细胞在亚细胞定位上没有差异。同样，对转化细胞中内源性 hnRNP A2 的免疫染色和质谱分析显示存在未甲基化的蛋白质，但存在完全的核定位模式。我们的结果表明 hnRNPs 的翻译后精氨酸甲基化具有替代作用，并进一步证明 hnRNP A/B 同源物在功能上不是冗余的。
