TRanscription and EXport (TREX) is a conserved multisubunit complex essential for embryogenesis, organogenesis and cellular differentiation throughout life. By linking transcription, mRNA processing and export together, it exerts a physiologically vital role in the gene expression pathway. In addition, this complex prevents DNA damage and regulates the cell cycle by ensuring optimal gene expression. As the extent of TREX activity in viral infections, amyotrophic lateral sclerosis and cancer emerges, the need for a greater understanding of TREX function becomes evident. A complete elucidation of the composition, function and interactions of the complex will provide the framework for understanding the molecular basis for a variety of diseases. This review details the known composition of TREX, how it is regulated and its cellular functions with an emphasis on mammalian systems.

N6-methyladenosine (m6A) is the most abundant internal modification of eukaryotic mRNA. This modification has previously been shown to alter the export kinetics for mRNAs though the molecular details surrounding this phenomenon remain poorly understood. Recruitment of the TREX mRNA export complex to mRNA is driven by transcription, 5′ capping and pre-mRNA splicing. Here we identify a fourth mechanism in human cells driving the association of TREX with mRNA involving the m6A methylase complex. We show that the m6A complex recruits TREX to m6A modified mRNAs and this process is essential for their efficient export. TREX also stimulates recruitment of the m6A reader protein YTHDC1 to the mRNA and the m6A complex influences the interaction of TREX with YTHDC1. Together our studies reveal a key role for TREX in the export of m6A modified mRNAs.

Summary During gene expression, RNA export factors are mainly known for driving nucleo-cytoplasmic transport. While early studies suggested that the exon junction complex (EJC) provides a binding platform for them, subsequent work proposed that they are only recruited by the cap binding complex to the 5′ end of RNAs, as part of TREX. Using iCLIP, we show that the export receptor Nxf1 and two TREX subunits, Alyref and Chtop, are recruited to the whole mRNA co-transcriptionally via splicing but before 3′ end processing. Consequently, Alyref alters splicing decisions and Chtop regulates alternative polyadenylation. Alyref is recruited to the 5′ end of RNAs by CBC, and our data reveal subsequent binding to RNAs near EJCs. We demonstrate that eIF4A3 stimulates Alyref deposition not only on spliced RNAs close to EJC sites but also on single-exon transcripts. Our study reveals mechanistic insights into the co-transcriptional recruitment of mRNA export factors and how this shapes the human transcriptome.

During gene expression, RNA export factors are mainly known for driving nucleocytoplasmic transport. Whilst early studies suggested that the Exon Junction Complex provides a binding platform for them, subsequent work proposed that they are only recruited by the Cap-Binding Complex to the 5' end of RNAs, as part of TREX. Using iCLIP, we show that the export receptor Nxf1 and two TREX subunits, Alyref and Chtop, are actually recruited to the whole mRNA co-transcriptionally via splicing but before 3'-end processing. Consequently, Alyref alters splicing decisions and Chtop regulates alternative polyadenylation. Alyref is recruited to the 5'-end of RNAs by CBC and our data reveal subsequent binding to RNAs near EJCs. We demonstrate eIF4A3 stimulates Alyref deposition not only on spliced RNAs close to EJC sites, but also single exon transcripts. Our study reveals mechanistic insights into the cotranscriptional recruitment of mRNA export factors and how this shapes the human transcriptome. Kaida et al., 2010;Millevoi et al., 2006;Rot et al., 2017), and Thoc5, which plays a role in mRNA export (Katahira et al., 2013).A major pathway for nuclear mRNA export uses the TREX complex (Strässer et al., 2002) which contains a core THO sub-complex, the RNA helicase Ddx39b, and RNA export adaptors and co-adaptors that Ddx39b loads onto mRNAs (Heath et al., 2016). A major adaptor is Alyref (Stutz et al., 2000), though some shuttling SR proteins can also work as adaptors (Huang and Steitz, 2003). Several co-adaptors have been characterized: Chtop, Thoc5, Cpsf6 and Rbm15 (Chang et al., 2013;Katahira et al., 2009;Ruepp et al., 2009;Zolotukhin et al., 2009). These proteins together recruit the export receptor Nxf1 and stimulate its RNA binding activity, which promotes mRNA export Hautbergue et al., 2008;Viphakone et al., 2015;. The topology of TREX components on RNAs and the molecular mechanisms involved in their deposition are still unclear. It has been suggested that the EJC may serve as a binding platform for RNA export factors (Le Hir et al., 2001;Singh et al., 2012).Yet, these interactions were not seen in other studies which instead revealed a key role for the cap binding complex (CBC, containing Ncbp1 and Ncbp2) in recruiting TREX, consistent with the idea that mRNPs may be exported 5' end first (Cheng et al., 2006;Chi et al., 2013). Thus, the relative contribution of the EJC and CBC to TREX deposition on the RNA in vivo remains unresolved.Here, we address some of these outstanding questions by performing individual nucleotide resolution UV-crosslinking and immunoprecipitation (iCLIP) (Broughton and Pasquinelli, 2013;König et al., 2011;Lee and Ule, 2018) on the mRNA export factors Alyref, Chtop, and Nxf1. Our in vivo results suggest co-transcriptional recruitment of these proteins all along the RNA during splicing but before CPA, which grants them additional roles in gene expression. Alyref can bind inefficiently spliced introns and regulates their splicing, and Chtop binds last exons and participates in APA regulation. We est...