今天是一篇关于RNA结合蛋白HOS5参与前体mRNA剪接的研究。

题目为：A KH-Domain RNA-Binding Protein Interacts with FIERY2/CTD Phosphatase-Like 1 and Splicing Factors and Is Important for Pre-mRNA Splicing in Arabidopsis

主要内容为：含有KH结构域的RNA结合蛋白HOS5可以与FRY2/CPL1互作，也可以与剪接因子RS40和RS41互作，调控前体mRNA的剪接，尤其在逆境条件下发挥作用。

前体mRNA加工包括5'加帽，剪接和3'聚腺苷酸化，这些对基因表达至关重要，并且与转录紧密联系。RNA聚合酶II（RNAP II）的磷酸化C末端结构域（CTD）可作为前体mRNA加工因子募集的平台，但其他参与招募的组分还不清楚。在拟南芥胁迫信号转导的遗传学研究中，我们分离了一个KH结构域RNA结合蛋白HOS5，在逆境基因调控和胁迫耐受中起重要作用。文章作者发现，HOS5与FRY2/CPL1相互作用，它们在核斑点中也与两个新的剪接因子RS40和RS41相互作用。在fry2突变体中，HOS5不能被募集到核斑点，而是主要定位在核质中。这些基因的突变体具有类似的逆境敏感表型。在盐胁迫条件下，转录组分析鉴定了这些突变体中许多与胁迫相关的基因，有显著的内含子保留。本文的研究显示，除了RNAP II，CTD磷酸酶还可以募集特定的剪接因子和RNA结合蛋白，来调节某些转录本的共转录，以适应环境胁迫。

真核生物基因组编码数以百计的RNA结合蛋白，但大多数这些蛋白质的功能是未知的。在拟南芥胁迫信号转导的遗传研究中，我们鉴定了一个KH结构域RNA结合蛋白HOS5，其在逆境基因调控和胁迫耐受中是所需的。HOS5在体外和体内均与FRY2/CPL1互作。这种互作由FRY2/CPL1的第一个双链RNA结合结构域和HOS5的KH结构域介导。有趣的是，HOS5和FRY2/CPL1也与两个新的富含丝氨酸-精氨酸（SR）的剪接因子RS40和RS41在核散斑中互作。重要的是，FRY2/CPL1对于HOS5募集是必需的。在fry2突变体中，HOS5无法定位于核斑点，但主要定位于核质。hos5突变体在mRNA输出中受损，并且在细胞核中积累了大量的mRNA，特别是在盐胁迫条件下。所有这些基因的拟南芥突变体表现出类似的逆境敏感表型。这些突变体的RNA-seq分析发现，在盐胁迫下在许多胁迫相关基因中，检测到显著的内含子保留，而在正常条件下没有。本研究不仅鉴定了几种新的前体mRNA加工的调节因子，它们对植物胁迫反应具有重要作用；而且还提出，除了RNAP II CTD外，FRY2/CPL1也可能募集特定的因子来调节某些转录本的共转录，以应对环境的挑战。

Eukaryotic genomes encode hundreds of RNA-binding proteins, yet the functions of most of these proteins are unknown. In a genetic study of stress signal transduction in Arabidopsis, we identified a K homology (KH)-domain RNA-binding protein, HOS5 (High Osmotic Stress Gene Expression 5), as required for stress gene regulation and stress tolerance. HOS5 was found to interact with FIERY2/RNA polymerase II (RNAP II) carboxyl terminal domain (CTD) phosphatase-like 1 (FRY2/CPL1) both in vitro and in vivo. This interaction is mediated by the first double-stranded RNA-binding domain of FRY2/CPL1 and the KH domains of HOS5. Interestingly, both HOS5 and FRY2/CPL1 also interact with two novel serine-arginine (SR)-rich splicing factors, RS40 and RS41, in nuclear speckles. Importantly, FRY2/CPL1 is required for the recruitment of HOS5. In fry2 mutants, HOS5 failed to be localized in nuclear speckles but was found mainly in the nucleoplasm. hos5 mutants were impaired in mRNA export and accumulated a significant amount of mRNA in the nuclei, particularly under salt stress conditions. Arabidopsis mutants of all these genes exhibit similar stress-sensitive phenotypes. RNA-seq analyses of these mutants detected significant intron retention in many stress-related genes under salt stress but not under normal conditions. Our study not only identified several novel regulators of pre-mRNA processing as important for plant stress response but also suggested that, in addition to RNAP II CTD that is a well-recognized platform for the recruitment of mRNA processing factors, FRY2/CPL1 may also recruit specific factors to regulate the co-transcriptional processing of certain transcripts to deal with environmental challenges.

Figure 3. HOS5 interacts with FRY2 and two RS proteins