Duncan K, Umen J G, Guthrie C
Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0448, USA.
Curr Biol. 2000 Jun 15;10(12):687-96. doi: 10.1016/s0960-9822(00)00527-3.
In the nucleus, mRNAs are bound by hnRNP proteins. A subset of hnRNP proteins shuttle between the nucleus and cytoplasm and are believed to promote mRNA export by acting as adaptors between mRNA and the transport machinery. The existence of multiple shuttling hnRNP proteins raises the question of whether differentially regulated, hnRNP-specific mRNA export pathways exist.
We have determined that Tom1p, a conserved protein with a hect (homology to E6-AP carboxyl terminus) E3 ubiquitin ligase domain, is required for efficient mRNA export in S. cerevisiae, yet differentially affects hnRNP protein localization and export. Mutations in tom1 predicted to abolish ubiquitin ligase activity block efficient export of Nab2p and mRNA, causing Nab2p-mRNA complexes to accumulate in a punctate pattern coincident with the nuclear pore complex (NPC). Notably, the subcellular distribution of several other hnRNP proteins is not affected. In particular, Np13p remains mRNA-associated and continues to be efficiently exported in tom1 mutants.
Our results demonstrate that mutations predicted to affect the enzymatic activity of the Tom1p ubiquitin ligase differentially affect export of hnRNP proteins in association with mRNA. We propose the existence of multiple mRNA export pathways, with export of Nab2p-associated mRNAs dependent on a branch of the ubiquitin protein modification pathway.
在细胞核中，mRNA与hnRNP蛋白结合。一部分hnRNP蛋白在细胞核和细胞质之间穿梭，被认为通过充当mRNA与转运机制之间的衔接子来促进mRNA输出。多种穿梭hnRNP蛋白的存在引发了一个问题，即是否存在差异调节的、hnRNP特异性的mRNA输出途径。
我们已经确定，Tom1p是一种具有hect（与E6-AP羧基末端同源）E3泛素连接酶结构域的保守蛋白，是酿酒酵母中高效mRNA输出所必需的，但对hnRNP蛋白的定位和输出有不同影响。预测会消除泛素连接酶活性的tom1突变会阻断Nab2p和mRNA的有效输出，导致Nab2p-mRNA复合物以与核孔复合体（NPC）一致的点状模式积累。值得注意的是，其他几种hnRNP蛋白的亚细胞分布不受影响。特别是，Np13p仍然与mRNA相关，并在tom1突变体中继续有效地输出。
我们的结果表明，预测影响Tom1p泛素连接酶酶活性的突变对与mRNA相关的hnRNP蛋白的输出有不同影响。我们提出存在多种mRNA输出途径，与Nab2p相关的mRNA的输出依赖于泛素蛋白修饰途径的一个分支。
