植物生长发育和逆境响应中SR-like蛋白的研究进展

同一基因pre-mRNA经可变剪接(alternative splicing, AS)后能够产生不同的转录本,使得编码的蛋白在细胞中的定位、稳定性和翻译后修饰的功能发生改变,进而增强应答发育及环境胁迫的能力,富含丝氨酸-精氨酸蛋白(serine/arginine-rich proteins, SR proteins/SR)是决定可变剪接效率和准确性的一个重要剪接因子家族。该文在简要介绍SR蛋白概念、分类的基础上,首次系统综述了植物特有的SR蛋白亚家族SR-like(SR45/45a)结构特点、成员构成、亚细胞定位和转录调控功能,尤其是对于非生物胁迫应答过程中相关基因可变剪接的调控机制进行了阐述,并展望了未来植物SR-like可能的前景方向和研究内容。     

SR蛋白家族在RNA剪接中的调控作用

SR蛋白家族成员都具有一个富含丝氨酸/精氨酸(S/R)重复序列的RS结构域,在RNA剪接体的组装和选择性剪接的调控过程中具有重要的作用。绝大多数SR蛋白是生存的必需因子,通过其RS结构域和特有的其他结构域,实现与前体mRNA的特异性序列或其他剪接因子的相互作用,协同完成剪接位点的正确选择或促进剪接体的形成。深入研究SR蛋白家族在RNA选择性剪接中的调控机制,可以促进以疾病治疗或害虫防治为目的的应用研究。该文总结了SR蛋白家族在基础研究和应用方面的进展。     

可变剪接调控因子SRSF2的研究进展

RNA剪接是指从mRNA前体中去除内含子、连接外显子形成成熟mRNA的过程。由于选择不同的剪接位点,可变剪接控制着从单一前体mRNA生成多种成熟mRNA的过程,因此是真核生物中转录后调控基因表达和决定蛋白质多样性的重要层次。SR蛋白家族是参与调控可变剪接的一类重要的剪接因子。SRSF2是SR蛋白家族的一员,具有经典的SR蛋白结构域。SRSF2不仅能够调控可变剪接,还能调控基因的转录过程,在维持胸腺、骨髓等造血系统的正常发育以及维持肝脏代谢稳态中是非常关键的调控因子。大量的研究表明:SRSF2的突变与骨髓增生异常综合征等造血系统疾病密切相关。本文总结了SRSF2最近的研究进展,以期对SRSF2在体内的功能有更全面和深入的理解,并为相关疾病的研究和治疗提供一定的思路。     

玉米苗期SR蛋白基因家族的干旱胁迫应答

基因表达的选择性剪接(Alternative splicing, AS)调控与植物对逆境胁迫应答密切相关, SR蛋白(Serine/ arginine-rich proteins)是其中关键的调节因子。文章对玉米B73参考基因组进行分析显示: 多数SR蛋白家族基因成员启动子区域含有3~8种与发育或胁迫相关的顺式调控元件; 27个基因成员编码碱性蛋白, 其中23个成员的编码蛋白依照其N′端的首个RRM(RNA recognition motif)结构域特征大体上可划分为5个亚组。利用双向分级聚类方法, 对三叶期干旱胁迫下玉米杂交种郑单958及其亲本郑58和昌7-2的SR蛋白基因家族的分析显示, 该基因家族的表达模式具有明显的组织表达特异性和基因型依赖性特征; 其中在干旱胁迫下地下组织以下调表达模式为主, 而地上组织中以上调表达模式为主。在重度干旱胁迫后的3个不同时段复水过程中, 地上和地下组织中SR蛋白基因家族的表达皆以下调表达模式为主。另外, 尽管不同基因成员的表达模式在干旱胁迫及其后的复水过程中存在明显差异, 但普遍存在自身选择性剪接现象。SR蛋白基因家族在玉米干旱胁迫的应答规律, 为从AS-network视角解析玉米的抗逆分子机制提供了新思路。     

转录因子叉头框K1在肿瘤中的作用

叉头框K (forkhead box class K,FOXK)蛋白家族是一类进化保守的转录因子,近年来被认为是多种癌症的关键转录调控因子。FOXK家族成员参与介导了广泛的生物学过程,包括细胞增殖、分化、凋亡、自噬、细胞周期、DNA损伤和肿瘤发生。FOXK1基因作为转录因子参与了多种肿瘤发生、发展等过程的调控,其基因结构及表观遗传学的异常与人类多种疾病的发生发展密切相关。研究发现,FOXK1在大多数恶性肿瘤中充当癌基因的角色,但其相关发病机制尚未明确。因此,对于FOXK1蛋白功能的深入研究,有助于更好地揭示相关疾病的发病机制,为疾病的预防和治疗提供理论依据。本文就国内外FOXK1蛋白在不同肿瘤中的主要作用的研究进展进行综述。     

组蛋白甲基转移酶SMYD家族与肿瘤

组蛋白甲基化修饰是肿瘤表观遗传学修饰异常的研究热点。这种修饰涉及肿瘤细胞的生物学行为,并参与肿瘤发生、发展和病理转归。含有SET结构域和MYND结构域蛋白的SMYD家族,是一组重要的赖氨酸甲基转移酶,主要通过组蛋白或非组蛋白甲基化修饰,调控其下游靶基因和肿瘤关键信号通路,参与肿瘤发生和发展的整个过程。SMYD家族影响肿瘤细胞的增殖、分化、凋亡、血管形成、侵袭和转移以及化疗敏感性等生物学特性。SMYD家族成员作为肿瘤新型分子诊断标志物和治疗靶点,有着巨大的临床应用价值和意义。本文综述了SMYD家族在肿瘤中的转录调控机制、生物学功能、临床研究意义及其作为分子靶点的抗肿瘤新药研究。     

SRp38基因研究进展

SR蛋白在前体mRNA可变剪接调控中发挥重要作用。可变剪接调节因子SRp38作为一种新近发现的具有神经及生殖组织特异性的SR蛋白,有典型的SR蛋白结构特征并能够调控GluR-B、TRK-C以及NCAML1等基因的可变剪接,但与其他SR蛋白不一致的是,SRp38可以在一定条件下(有丝分裂M期,热休克)抑制前体mRNA剪接,从而防止错误剪接的出现。SRp38的RRM结构域可以识别特殊的RNA序列并跟U1snRNP结合,而其RS结构域则参与调控前体mRNA剪接。SRp38的磷酸化状态可以影响其调控功能的发挥,在有丝分裂M期及热休克时,该蛋白质均呈去磷酸化状态。SRp38在爪蟾胚胎神经发育过程中发挥作用并且可以同TLS(translocation liposarcoma)蛋白相互作用,提示其可能通过调节前体mRNA可变剪接在神经系统的发育分化以及在肿瘤的发生中扮演角色。     

mRNA前体的剪接因子

真核生物通过mRNA前体的剪接,包括选择性剪接机制,调控着自身的生长与发育,了解其基本过程和有关参与因子,对进一步探索真核生物基因的表达调控和分子进化都具有极其重要的意义.该文简要综述了mRNA前体剪接的基本过程及有关剪接因子的最新研究进展,介绍了SR蛋白（Ser-Arg rich protein）家族因子、某些新发现的参与形成核不均一核糖核蛋白（heterogeneous nuclear ribonucleoprotein,hnRNP）的因子及部分：RNA解旋酶等在mRNA前体剪接过程中的功能和作用.     

前体mRNA剪接及剪接调节因子SR蛋白和Tra2蛋白

在高等真核生物中,前体mRNA的剪接及其调节是一个复杂的、由多因子参与的过程,它对基因的正常功能的发挥起着重要的作用,任何一种剪接调节因子的异常变化均有可能导致疾病的发生。因此,研究参与前体mRNA剪接调控的相关因子的功能及作用机制,对前体mRNA剪接机制的阐明,无疑是相当必要的。本文着重介绍了两类重要的mRNA剪接调节蛋白——SR蛋白和Tra2蛋白的研究近况,以期对前体mRNA剪接机制的研究的重要性和复杂性有更多的了解。     

多功能核蛋白NONO研究进展北大核心CSCD

NONO(non-POU-domain-containing octamer binding protein)是一种多功能的核蛋白质,作为人类剪切蛋白家族的一员,NONO参与多种生物学事件,如基因转录调控、前体RNA剪接、DNA解螺旋和配对、缺陷RNA核滞留、DNA损伤的修复以及肿瘤发生等过程。尽管NONO参与胞内多种生物过程,但目前,针对NONO生物功能的研究仍处于初级阶段,其相关的临床研究更少。本文就NONO基因的结构及功能研究进展予以综述。     

Alternative splicing during chondrogenesis: modulation of fibronectin exon EIIIA splicing by SR proteins

The alternative exon EIIIA of the fibronectin gene is included in mRNAs produced in undifferentiated mesenchymal cells but excluded from differentiated chondrocytes. As members of the SR protein family of splicing factors have been demonstrated to be involved in the alternative splicing of other mRNAs, the role of SR proteins in chondrogenesis-associated EIIIA splicing was investigated. SR proteins interacted with chick exon EIIIA sequences that are required for exon inclusion in a gel mobility shift assay. Addition of SR proteins to in vitro splicing reactions increased the rate and extent of exon EIIIA inclusion. Co-transfection studies employing cDNAs encoding individual SR proteins revealed that SRp20 decreased mRNA accumulation in HeLa cells, which make A+ mRNA, apparently by interfering with pre-mRNA splicing. Co-transfection studies also demonstrated that SRp40 increased exon EIIIA inclusion in chondrocytes, but not in HeLa cells, suggesting the importance of cellular context for SR protein activity. Immunoblot analysis did not reveal a relative depletion of SRp40 in chondrocytic cells. Possible mechanisms for regulation of EIIIA splicing in particular, and chondrogenesis associated splicing in general, are discussed.     

Alternative Splicing and Control of Apoptotic DNA Fragmentation

It has long been suggested that alternative splicing is involved in regulation of apoptosis by producing mRNA isoforms that encode proteins with distinct and even opposite functions in apoptotic pathways. However, the physiological functions and regulatory mechanisms of such alternative splicing events have been unclear. Recently, it was demonstrated that inactivation of a single SR protein, ASF/SF2, can modulate a specific step in the apoptotic pathway, internucleosomal DNA fragmentation, by regulating ICAD pre-mRNA alternative splicing. These studies have provided new evidence supporting the important role of regulated splicing and SR proteins in the process of apoptosis.     

Concerted regulation of nuclear and cytoplasmic activities of SR proteins by AKT

Serine/arginine-rich (SR) proteins are important regulators of mRNA splicing. Several postsplicing activities have been described for a subset of shuttling SR proteins, including regulation of mRNA export and translation. Using the fibronectin gene to study the links between signal-transduction pathways and SR protein activity, we show that growth factors not only modify the alternative splicing pattern of the fibronectin gene but also alter translation of reporter messenger RNAs in an SR protein-dependent fashion, providing two coregulated levels of isoform-specific amplification. These effects are inhibited by specific small interfering RNAs against SR proteins and are mediated by the AKT kinase, which elicits opposite effects to those evoked by overexpressing SR protein kinases Clk and SRPK. These results show how SR protein activity is modified in response to extracellular stimulation, leading to a concerted regulation of splicing and translation.     

The Protein Kinase Clk/Sty Directly Modulates SR Protein Activity: Both Hyper- and Hypophosphorylation Inhibit Splicing

The splicing of mammalian mRNA precursors requires both protein phosphorylation and dephosphorylation, likely involving modification of members of the SR protein family of splicing factors. Several kinases have been identified that can phosphorylate SR proteins in vitro, and transfection assays have provided evidence that at least one of these, Clk/Sty, can modulate splicing in vivo. But evidence that a specific kinase can directly affect the splicing activity of SR proteins has been lacking. Here, by using purified recombinant Clk/Sty, a catalytically inactive mutant, and individual SR proteins, we show that Clk/Sty directly affects the activity of SR proteins, but not other essential splicing factors, in reconstituted splicing assays. We also provide evidence that both hyper- and hypophosphorylation inhibit SR protein splicing activity, repressing constitutive splicing and switching alternative splice site selection. These findings indicate that Clk/Sty directly and specifically influences the activity of SR protein splicing factors and, importantly, show that both under- and overphosphorylation of SR proteins can modulate splicing.     

Hypophosphorylated ASF/SF2 binds TAP and is present in messenger ribonucleoproteins

Serine/arginine-rich proteins (SR proteins) function in precursor mRNA (pre-mRNA) splicing and may also act as adaptors for mRNA export. SR proteins are dynamically phosphorylated in their RS domain, and differential phosphorylation modulates their splicing activity and subcellular localization. In this study, we investigated the influence of phosphorylation on the function of SR proteins in events occurring during mRNA maturation. Immunoprecipitation experiments showed that the mRNA export receptor TAP associates preferentially with the hypophosphorylated form of shuttling SR proteins, including ASF/SF2. Overexpression of ASF induced subnuclear relocalization of TAP to SR protein-enriched nuclear speckles, suggesting their interaction in vivo. Moreover, the ASF found in a nucleoplasmic fraction rich in heterogeneous nuclear ribonucleoprotein (hnRNP) complexes is hyperphosphorylated, whereas mature messenger RNP (mRNP)-bound ASF is hypophosphorylated. Therefore, hypophosphorylation of ASF in mRNPs coincides with its higher affinity for TAP, suggesting that dephosphorylation of ASF promotes both its incorporation into mRNPs and recruitment of TAP for mRNA export. Thus, the phosphorylation state of RS domains may modulate the function of mammalian shuttling SR proteins during mRNA maturation or export.     

Dephosphorylation-dependent sorting of SR splicing factors during mRNP maturation

SR proteins are a family of sequence-specific RNA binding proteins originally discovered as essential factors for pre-mRNA splicing and recently implicated in mRNA transport, stability, and translation. Here, we used a genetic complementation system derived from conditional knockout mice to address the function and regulation of SR proteins in vivo. We demonstrate that ASF/SF2 and SC35 are each required for cell viability, but, surprisingly, the effector RS domain of ASF/SF2 is dispensable for cell survival in MEFs. Although shuttling SR proteins have been implicated in mRNA export, prevention of ASF/SF2 from shuttling had little impact on mRNA export. We found that shuttling and nonshuttling SR proteins are segregated in an orderly fashion during mRNP maturation, indicating distinct recycling pathways for different SR proteins. We further showed that this process is regulated by differential dephosphorylation of the RS domain, thus revealing a sorting mechanism for mRNP transition from splicing to export.     

A single ancient origin for prototypical serine/arginine-rich splicing factors

Eukaryotic precursor mRNA splicing is a process involving a very complex RNA-protein edifice. Serine/arginine-rich (SR) proteins play essential roles in precursor mRNA constitutive and alternative splicing and have been suggested to be crucial in plant-specific forms of developmental regulation and environmental adaptation. Despite their functional importance, little is known about their origin and evolutionary history. SR splicing factors have a modular organization featuring at least one RNA recognition motif (RRM) domain and a carboxyl-terminal region enriched in serine/arginine dipeptides. To investigate the evolution of SR proteins, we infer phylogenies for more than 12,000 RRM domains representing more than 200 broadly sampled organisms. Our analyses reveal that the RRM domain is not restricted to eukaryotes and that all prototypical SR proteins share a single ancient origin, including the plant-specific SR45 protein. Based on these findings, we propose a scenario for their diversification into four natural families, each corresponding to a main SR architecture, and a dozen subfamilies, of which we profile both sequence conservation and composition. Finally, using operational criteria for computational discovery and classification, we catalog SR proteins in 20 model organisms, with a focus on green algae and land plants. Altogether, our study confirms the homogeneity and antiquity of SR splicing factors while establishing robust phylogenetic relationships between animal and plant proteins, which should enable functional analyses of lesser characterized SR family members, especially in green plants.