Henry M F, Silver P A
Department of Biological Chemistry and Molecular Phamacology, Harvard Medical School, Boston, Massachusetts, USA.
Mol Cell Biol. 1996 Jul;16(7):3668-78. doi: 10.1128/MCB.16.7.3668.
RNA-binding proteins play many essential roles in the metabolism of nuclear pre-mRNA. As such, they demonstrate a myriad of dynamic behaviors and modifications. In particular, heterogeneous nuclear ribonucleoproteins (hnRNPs) contain the bulk of methylated arginine residues in eukaryotic cells. We have identified the first eukaryotic hnRNP-specific methyltransferase via a genetic screen for proteins that interact with an abundant poly(A)+-RNA-binding protein termed Npl3p. We have previously shown that npl3-1 mutants are temperature sensitive for growth and defective for export of mRNA from the nucleus. New mutants in interacting genes were isolated by their failure to survive in the presence of the npl3-1 allele. Four alleles of the same gene were identified in this manner. Cloning of the cognate gene revealed an encoded protein with similarity to methyltransferases that was termed HMT1 for hnRNP methyltransferase. HMT1 is not required for normal cell viability except when NPL3 is also defective. The Hmt1 protein is located in the nucleus. We demonstrate that Npl3p is methylated by Hmt1p both in vivo and in vitro. These findings now allow further exploration of the function of this previously uncharacterized class of enzymes.
RNA结合蛋白在核内前体mRNA的代谢过程中发挥着许多重要作用。因此，它们表现出无数种动态行为和修饰。特别是，不均一核核糖核蛋白（hnRNPs）在真核细胞中含有大量甲基化精氨酸残基。我们通过对与一种丰富的聚腺苷酸（A）+RNA结合蛋白Npl3p相互作用的蛋白质进行遗传筛选，鉴定出了首个真核生物hnRNP特异性甲基转移酶。我们之前已经表明，npl3-1突变体对生长具有温度敏感性，并且在mRNA从细胞核输出方面存在缺陷。通过在npl3-1等位基因存在的情况下无法存活，分离出了相互作用基因中的新突变体。以这种方式鉴定出了同一基因的四个等位基因。同源基因的克隆揭示了一种编码的与甲基转移酶相似的蛋白质，该蛋白质被命名为hnRNP甲基转移酶HMT1。除了NPL3也存在缺陷时，HMT1对于正常细胞活力并非必需。Hmt1蛋白位于细胞核中。我们证明，Npl3p在体内和体外均被Hmt1p甲基化。这些发现现在使得能够进一步探索这类以前未被表征的酶的功能。
