The THO-TREX complex (suppressors of the transcription defects of hpr1∆ mutants by overexpression-Transcription and Export), discovered by the groups of Andres Aguilera, Robin Reed, and Ed Hurt, is emerging in the literature as a crucial factor in eukaryotic gene expression that couples transcription and pre-mRNA maturation with mRNA export 17-21. It is also known as a potentially important player in tumour development 22,23. However, despite nearly twenty years of studies, the precise molecular function of this complex in gene expression has remained elusive.
Organization of the THO-TREX complex. In human cells, the THO-TREX complex is mainly recruited at the 5'-end of the mRNA via the cap-binding complex (CBC) and this is dependent on pre-mRNA splicing. For simplicity, not all TREX subunits have been depicted.
I have conceived, designed, and performed a study that led to the discovery of the first ever-described molecular function for the THO-TREX complex 24. I have discovered that the human heterodimeric mRNA export receptor Nxf1:p15 naturally exists in a locked conformation that reduces its RNA binding activity. I have shown that the THO-TREX complex functions to remodel Nxf1:p15 and ensure it binds RNA efficiently and that this mechanism is essential to trigger mRNA export in human cells. Additionally, I have shown that depletion of some THO-TREX subunits from human cells by RNA interference (RNAi) results not only in a blockage of mRNA export but also in Nxf1 being trapped in the nucleus in an insoluble fraction that would correspond to chromatin. Defects in mRNA export or in THO-TREX assembly have been associated in the literature for over twenty years with genome instability potentially as a result of delayed mRNA export, but without a precise molecular reason 25-27. This finding may provide one of the missing molecular links explaining this phenomenon and may be useful to understand tumour development.
A molecular function for the THO-TREX complex. I/II. I have discovered that, in human cells, the two subunits Alyref and Thoc5 of the THO-TREX complex act synergistically to unlock the RNA binding domain of the mRNA export receptor Nxf1:p15.
A molecular function for the THO-TREX complex. II/II. Process as known in 2012. I have discovered that in human cells, the THO-TREX complex stimulates efficient transfer of the mRNA to the receptor Nxf1:p15, thereby promoting export of the messenger RNA to the cytoplasm upon completion of its maturation steps. Here, I have designated "D-TREX" the TREX complex containing the RNA helicase Ddx39b and "N-TREX" the subsequent form of the TREX complex in which Nxf1 is present and Ddx39b is absent.
This work has also confirmed that the RNA helicase Ddx39b (a.k.a. Uap56, or Bat1, and termed Sub2p in budding yeast) and the mRNA export receptor Nxf1:p15, key players of the mRNA licensing process required for export, cannot be found associated to the same Alyref-containing complex in vivo. This suggests a rearrangement of the THO-TREX composition which takes place during mRNP biogenesis.
Further reading on THO-TREX function in our review
References used:
Preceding references: 1-5, 6-16
17. Chávez, S. et al. A protein complex containing Tho2, Hpr1, Mft1 and a novel protein, Thp2, connects transcription elongation with mitotic recombination in Saccharomyces cerevisiae. EMBO J 19, 5824–5834 (2000).
18. Strässer, K. et al. TREX is a conserved complex coupling transcription with messenger RNA export. Nature 417, 304–308 (2002).
19. Masuda, S. et al. Recruitment of the human TREX complex to mRNA during splicing. Genes & Development 19, 1512–1517 (2005).
20. Cheng, H. et al. Human mRNA Export Machinery Recruited to the 5′ End of mRNA. Cell 127, 1389–1400 (2006).
21. Rougemaille, M. et al. THO/Sub2p functions to coordinate 3'-end processing with gene-nuclear pore association. Cell 135, 308–321 (2008).
22. Domínguez-Sánchez, M. S., Sáez, C., Japón, M. A., Aguilera, A. & Luna, R. Differential expression of THOC1 and ALY mRNP biogenesis/export factors in human cancers. BMC Cancer 11, 77 (2011).
23. Li, Y. et al. Cancer cells and normal cells differ in their requirements for Thoc1. Cancer Res. 67, 6657–6664 (2007).
24. Viphakone, N., Hautbergue, G.M., Walsh, M.J., Chang, C.T., Holland, A., Folco, E.G., Reed, R. & Wilson, S.A. . TREX exposes the RNA-binding domain of Nxf1 to enable mRNA export. Nature Communications 3, 1006 (2012).
25. Aguilera, A. The connection between transcription and genomic instability. EMBO J 21, 195–201 (2002).
26. Gómez-González, B. et al. Genome-wide function of THO/TREX in active genes prevents R-loop-dependent replication obstacles. EMBO J 30, 3106–3119 (2011).
27. Domínguez-Sánchez, M. S., Barroso, S., Gómez-González, B., Luna, R. & Aguilera, A. Genome instability and transcription elongation impairment in human cells depleted of THO/TREX. PLoS Genet 7, e1002386 (2011).