The TREX and TREX-2 transcription export complexes are key factors in eukaryotic gene expression.
The TREX complex is responsible for the recruitment of the main yeast transport factor, Mex67-Mtr2, onto the pre-mRNA, which is necessary for the generation of export-competent complexes. Overall, the complex, itself, is formed through the interactions between Yra1 and the DEAD-box helicase, Sub2, with the THO complex.
The TREX-2 complex, on the other hand, is involved in a process known as "gene gating". In short, this process associates actively transcribing genes with Nuclear Pore Complexes (NPCs). The Saccharomyces cerevisiae TREX-2 complex is composed of 4 proteins: Sac3, Thp1, Sem1, Sus1 and Cdc31; its function is conserved from yeast to humans.
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| Figure 1 |
These five proteins contribute to different functions and form two distinct subregions of the TREX-2 complex. The proximal CTD-Interacting Domain (CID) of TREX-2, i.e. the C-terminal region of Sac3, interacts with Cdc31 and two Sus1 chains (see figure 1), which act in tandem, leading to the localization of the complex to the nuclear surface where it associates with the NPCs. The distal N- and M-regions of the complex bind to Thp1-Sem1 and Mex67-Mtr2, leading to the interaction of the complex with the proteins responsible for transcription and mRNP formation.
On its own, Mex67-Mtr2 has a low affinity for mRNA, yet its association with mRNA is required for the generation of export-competent mRNPs. To overcome this, Mex67-Mtr2 binds to the FG repeats on the N-terminal region of Sac3 via two of its domains: NTF2 and UBA. Sac3 is adjacent to Thp1-Sem1, forming the Sac3-Thp1-Sem1 complex. The adjacent, exposed winged helix domains of Sac3 and Thp1 provide a platform able to bind to the mRNA transcript. Hence, binding of the Sac3-Thp1-Sem1 complex to the mRNA transcript leads to an increased proximity of the transcript of the Mex67-Mtr2 transport factor. The presence of the Yra1-Sub2 complex on the mRNA transcript assists this. Thus, export-competent mRNPs are produced adjacent to the NPCs, due to the localization of TREX-2, resulting in an increase in the rate of mRNA export. This is all illustrated in Figure 2.
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| Figure 2 |
