The role of TREX-2 complex in nucleo-cytoplasmic mRNA export

Abstract

Eukaryotic cells display compartmentalisation within the nucleus and cytoplasm, regulated by nucleo-cytoplasmic transport (NCT). This process is essential for the movement of functional molecules, with the nuclear pore complex (NPC) acting as a selective barrier. While the karyopherin family oversees the transport of most macromolecules, mRNA export follows a distinct route mediated by Mex67-Nxt1 in S. pombe and Nxf1-Nxt1 in humans, recognised as the sole mRNA export receptors with evolutionary conservation. Although current models propose the recruitment of Transcription and Export complexes 1 and 2 (TREX-1 and TREX-2) to mRNAs during transcription elongation, delivering them to the Mex67-Nxt1/NXF1-NXT1 transport receptor, the non-essential nature of Mex67 and Nxt1 genes in S. pombe suggests the existence of alternative mRNA export receptors. In this study, we discovered the intriguing role of the TREX-2 complex as an mRNA export receptor, facilitating the transit of mRNAs through the nucleopore channel. Our results demonstrate that the TREX-2 complex functions as an mRNA export receptor, despite its anchorage to the nucleoplasmic side of the NPC via the extended C-terminal domain of the Sac3 subunit, known as GANP in humans. Our research revealed that the TREX-2's head-domain, consisting of the N-region with FG-like repeats and the mRNA-binding M-region, can extend to the cytoplasmic side of the NPC. Furthermore, we demonstrated a strong correlation between the head-domain of TREX-2 complex and messenger ribonucleoproteins (mRNPs) during nuclear translocation, independent of Mex67. The research posits a dynamic model in which the TREX-2 complex and mRNPs move conjointly through the NPC's FG-repeat framework. While this coordinated migration occurs, the TREX-2's tail-domain is consistently anchored to the nuclear side, signifying an existence of a distinctive pathway for the nuclear export of mRNA, one that does not solely depend on the traditional transport receptor Mex67-Nxt1 (NXF1-NXT1 in human). Furthermore, we explored the functional role of a newly identified protein complex, namely the Sac32-Pci2-Dss1 complex. Our findings indicate that Sac32 shares conserved homology with Sac3, similar to the Sac3 subunit of TREX-2 complex, and its structural arrangement is preserved across a range of eukaryotes from S. pombe to higher organisms. Our investigation revealed that Sac32 predominantly localises in the nucleoplasm, in contrast to TREX-2 complex, which is situated at the nuclear periphery. The deletion of sac32 and pci2 resulted in the accumulation of mRNA within the nucleus, indicating that the Sac32-Pci2-Dss1 complex may function as a mobile version of the spTREX-2 complex, contributing to the mRNA export. Remarkably, our observations indicate that Sac32 is capable of migrating towards the nuclear periphery where it potentially interacts closely with the head-domain of the TREX-2 complex. This suggests a role for Sac32 in facilitating the transfer of mRNPs to the TREX-2 complex for nuclear mRNA export. Finally, our extensive genome-wide analyses revealed that TREX-2 complex binds to mRNAs globally, while TREX-1 complex exhibits both global and selective interactions with mRNA molecules. Additionally, our findings suggest the potential involvement of Sac32 in co-transcriptional mRNA processes. Our study presents the 'lasso model' where the TREX-2 complex is essential for bulk mRNA export, engaging after mRNPs dock at the nuclear basket facilitated by nuclear export factors like TREX-1 complex, Sac32-Pci2-Dss1 complex, or Mex67-Nxt1. The TREX-2's head-domain actively transports mRNPs to the cytoplasm, while the tail-domain remains fixed at the nuclear basket. Our findings introduce a new paradigm for mRNA export, challenging the traditional view of the only mRNA export receptor and revealing a previously uncharacterised transport pathway within the NCT system that operates independently of established shuttle proteins.