Summary

TREX and TREX-2 transcription export complexes are key factors in eukaryotic gene expression. They facilitate the coupling of transcription and mRNP assembly to the export of these mRNPs out the nuclear pore complexes and into the cytoplasm, where translation takes place.  Coupling is achieved by the interaction of TREX-2 with SAGA, the transcription co-activator, and NPC. This coupling provides a suitable molecular environment for gene expression regulation and produces export-competent mRNPs thus increases the success and the rate of mRNA export. Further evidence to support TREX-2's role in transcription and mRNA export coupling in Saccharomyces cerevisiae is the determination of the structure of TREX-2 where the central region of Sac3 (the CID domain) adopts a long alpha-helix conformation, wrapped by one Cdc31 and two Sus1 molecules

The Sac3-Thp1-Sem1 complex is found on the distal region of TREX-2, and is involved in facilitating the interaction between TREX-2 and the proteins involved in transcription and mRNP assembly. The generation of the crystal structure of the S. cerevisiae Sac3-Thp1-Sem1 complex helped elucidate the role of the complex in mRNA export. It provided information on the interactions between the three proteins of the complex, their assembly, and their binding to nucleic acids. Firstly, the Thp1-Sem1 complex forms, since Sem1 is needed for the stabilization and solubilization of Thp1. Sem1 operates analogously in other complexes that contain PCI folds. This Thp1-Sem1 then interacts with Sac3, forming the complete complex. A conserved mechanism is present within the human homolog, GANP-PCID2-DSS1, in which the PCID2-DSS1 complex forms and then associates with GANP. 

Sac3 and Thp1 both have C-terminal winged helix domains, with a αβααββ motif, that juxtapose through the interactions of several crucial side chains leading to the formation of a platform that mediates nucleic acid binding. The Sac3-Thp1-Sem1 complex has a specificity for uracil and preferentially binds to RNA molecules with a length of 25 bases.