Chtop is a component of the dynamic TREX mRNA export complex

The TREX complex couples nuclear pre‐mRNA processing with mRNA export and contains multiple protein components, including Uap56, Alyref, Cip29 and the multi‐subunit THO complex. Here, we have identified Chtop as a novel TREX component. We show that both Chtop and Alyref activate the ATPase and RNA helicase activities of Uap56 and that Uap56 functions to recruit both Alyref and Chtop onto mRNA. As observed with the THO complex subunit Thoc5, Chtop binds to the NTF2‐like domain of Nxf1, and this interaction requires arginine methylation of Chtop. Using RNAi, we show that co‐knockdown of Alyref and Chtop results in a potent mRNA export block. Chtop binds to Uap56 in a mutually exclusive manner with Alyref, and Chtop binds to Nxf1 in a mutually exclusive manner with Thoc5. However, Chtop, Thoc5 and Nxf1 exist in a single complex in vivo. Together, our data indicate that TREX and Nxf1 undergo dynamic remodelling, driven by the ATPase cycle of Uap56 and post‐translational modifications of Chtop.     

ORC interacts with THSC/TREX-2 and its subunits promote Nxf1 association with mRNP and mRNA export in Drosophila

The origin recognition complex (ORC) of eukaryotes associates with the replication origins and initiates the pre-replication complex assembly. In the literature, there are several reports of interaction of ORC with different RNAs. Here, we demonstrate for the first time a direct interaction of ORC with the THSC/TREX-2 mRNA nuclear export complex. The THSC/TREX-2 was purified from the Drosophila embryonic extract and found to bind with a fraction of the ORC. This interaction occurred via several subunits and was essential for Drosophila viability. Also, ORC was associated with mRNP, which was facilitated by TREX-2. ORC subunits interacted with the Nxf1 receptor mediating the bulk mRNA export. The knockdown of Orc5 led to a drop in the Nxf1 association with mRNP, while Orc3 knockdown increased the level of mRNP-bound Nxf1. The knockdown of Orc5, Orc3 and several other ORC subunits led to an accumulation of mRNA in the nucleus, suggesting that ORC participates in the regulation of the mRNP export.     

Luzp4 defines a new mRNA export pathway in cancer cells

Cancer testis antigens (CTAs) represented a poorly characterized group of proteins whose expression is normally restricted to testis but are frequently up-regulated in cancer cells. Here we show that one CTA, Luzp4, is an mRNA export adaptor. It associates with the TREX mRNA export complex subunit Uap56 and harbours a Uap56 binding motif, conserved in other mRNA export adaptors. Luzp4 binds the principal mRNA export receptor Nxf1, enhances its RNA binding activity and complements Alyref knockdown in vivo. Whilst Luzp4 is up-regulated in a range of tumours, it appears preferentially expressed in melanoma cells where it is required for growth.     

Aly and THO are required for assembly of the human TREX complex and association of TREX components with the spliced mRNA

The mRNA export complex TREX (TREX) is known to contain Aly, UAP56, Tex1 and the THO complex, among which UAP56 is required for TREX assembly. Here, we systematically investigated the role of each human TREX component in TREX assembly and its association with the mRNA. We found that Tex1 is essentially a subunit of the THO complex. Aly, THO and UAP56 are all required for assembly of TREX, in which Aly directly interacts with THO subunits Thoc2 and Thoc5. Both Aly and THO function in linking UAP56 to the cap-binding protein CBP80. Interestingly, association of UAP56 with the spliced mRNA, but not with the pre-mRNA, requires Aly and THO. Unexpectedly, we found that Aly and THO require each other to associate with the spliced mRNA. Consistent with these biochemical results, similar to Aly and UAP56, THO plays critical roles in mRNA export. Together, we propose that Aly, THO and UAP56 form a highly integrated unit to associate with the spliced mRNA and function in mRNA export.