RNA Regulation Group, Institute of Cell Biology and Neuroscience, Goethe-University Frankfurt, 60438 Frankfurt/Main, Germany;
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520, USA;
Genes Dev. 2016 Mar 1;30(5):553-66. doi: 10.1101/gad.276477.115.
Nuclear export factor 1 (NXF1) exports mRNA to the cytoplasm after recruitment to mRNA by specific adaptor proteins. How and why cells use numerous different export adaptors is poorly understood. Here we critically evaluate members of the SR protein family (SRSF1-7) for their potential to act as NXF1 adaptors that couple pre-mRNA processing to mRNA export. Consistent with this proposal, >1000 endogenous mRNAs required individual SR proteins for nuclear export in vivo. To address the mechanism, transcriptome-wide RNA-binding profiles of NXF1 and SRSF1-7 were determined in parallel by individual-nucleotide-resolution UV cross-linking and immunoprecipitation (iCLIP). Quantitative comparisons of RNA-binding sites showed that NXF1 and SR proteins bind mRNA targets at adjacent sites, indicative of cobinding. SRSF3 emerged as the most potent NXF1 adaptor, conferring sequence specificity to RNA binding by NXF1 in last exons. Interestingly, SRSF3 and SRSF7 were shown to bind different sites in last exons and regulate 3' untranslated region length in an opposing manner. Both SRSF3 and SRSF7 promoted NXF1 recruitment to mRNA. Thus, SRSF3 and SRSF7 couple alternative splicing and polyadenylation to NXF1-mediated mRNA export, thereby controlling the cytoplasmic abundance of transcripts with alternative 3' ends.
核输出因子 1 (NXF1) 在被特定衔接蛋白募集到 mRNA 后，将 mRNA 输出到细胞质。细胞如何以及为何使用许多不同的输出衔接子还知之甚少。在这里，我们批判性地评估了 SR 蛋白家族（SRSF1-7）成员作为 NXF1 衔接子的潜力，它们将前体 mRNA 加工与 mRNA 输出偶联。与这一观点一致的是，>1000 种内源性 mRNA 在体内需要单个 SR 蛋白进行核输出。为了解决这个问题，我们通过单个核苷酸分辨率的紫外线交联和免疫沉淀（iCLIP）平行确定了 NXF1 和 SRSF1-7 的转录组范围的 RNA 结合谱。RNA 结合位点的定量比较表明，NXF1 和 SR 蛋白在相邻的位点结合 mRNA 靶标，表明共结合。SRSF3 是最有效的 NXF1 衔接子，赋予 NXF1 在最后外显子中对 RNA 结合的序列特异性。有趣的是，SRSF3 和 SRSF7 被证明在最后外显子中结合不同的位点，并以相反的方式调节 3'非翻译区长度。SRSF3 和 SRSF7 都促进了 NXF1 对 mRNA 的募集。因此，SRSF3 和 SRSF7 将选择性剪接和多聚腺苷酸化与 NXF1 介导的 mRNA 输出偶联，从而控制具有替代 3'末端的转录物在细胞质中的丰度。
