Nuclear export of messenger RNA

Abstract

Transport of messenger RNA (mRNA) from the nucleus to the cytoplasm is an essential step of eukaryotic gene expression. In the cell nucleus, a precursor mRNA undergoes a series of processing steps, including capping at the 5' ends, splicing and cleavage/polyadenylation at the 3' ends. During this process, the mRNA associates with a wide variety of proteins, forming a messenger ribonucleoprotein (mRNP) particle. Association with factors involved in nuclear export also occurs during transcription and processing, and thus nuclear export is fully integrated into mRNA maturation. The coupling between mRNA maturation and nuclear export is an important mechanism for providing only fully functional and competent mRNA to the cytoplasmic translational machinery, thereby ensuring accuracy and swiftness of gene expression. This review describes the molecular mechanism of nuclear mRNA export mediated by the principal transport factors, including Tap-p15 and the TREX complex.

Figure 1

A brief overview of mRNA nuclear export. During transcription, protein factors required for capping, splicing and cleavage/polyadenylation are recruited to the nascent transcript, forming an mRNP. The 5' end of the mRNA is capped early in this process via an interaction between the capping enzyme and RNA polymerase II (RNAPII). Factors involved in splicing and cleavage/polyadenylation are also co-transcriptionally loaded onto the pre-mRNA (see also Figure 3). Measurement of the transcript length by the hnRNP C tetramer, which is important for allocating the transcript to the mRNA-specific processing and export pathway, could occur early during transcription. The TREX complex and a subset of the SR proteins, which are engaged in nuclear export, are recruited to the nascent mRNA via interactions with the transcription and processing factors. The nuclear export receptor Tap-p15 (Mex67-Mtr2 in yeast) in turn gains access to the mRNA via interactions with these factors as adaptors. The nuclear export receptor heterodimer facilitates the translocation of mRNPs through its interaction with FG-repeat containing nucleoporins. During the process of the nuclear mRNA biogenesis, the structure and the composition of the mRNP change drastically (see also Figure 4), and these alterations in the physicochemical properties also help the mRNP translocate through the NPC. The mRNA export factors are then dissociated from the mRNP by factors associated with the NPC to prevent the return of the mRNP to the nucleus. The exported mRNA then directs protein translation in the cytoplasm.

Figure 2

Structure and function of the principal mRNA export receptor Tap-p15. (A) Tap consists of an RNA recognition motif (RRM), leucine-rich repeat (LRR), nuclear transport factor 2-like (NTF2L) and ubiquitin-associated (UBA) domains. These domains are interconnected by flexible linkers (thin lines). Both the NTF2L and UBA domains contain FG-repeat-binding sites. Our recent analysis showed that the RNA binding activity of Tap is attributable to the RRM, LRR and NTF2L domains [50]. Adaptor proteins that bind to various domains of Tap are shown on top of the schema; (B) The structure of the NTF2L domain of Tap (green) complexed with p15 (blue). The surface of the NTF2L domain of Tap, which is critical for its RNA-binding activity, is shown in yellow. Note that the RNA- and the FG-repeat binding sites (an FG-repeat peptide in the complex is shown in red) are localized to opposing surfaces. The structural coordinate (accession number; 1JN5) was taken from the PDB database and displayed using the GRASP2 software [51].

Figure 3

Co-transcriptional loading of the TREX complex. The carboxy-terminal domain (CTD) of RNAPII is subjected to transcription-cycle-specific modifications. Ser5 of CTD is phosphorylated when RNAPII is near the promoter. The capping enzyme specifically binds to the Ser5-phosphorylated CTD, allowing capping to occur early during transcription initiation. Ser2 phosphorylation gradually increases during transcription elongation and becomes dephosphorylated near the transcription termination site. Factors involved in cleavage/polyadenylation specifically bind to the Ser2-phosphorylated CTD. Recent data indicate that the THO subcomplex of the yeast TREX complex specifically and directly interacts with the Ser2/Ser5-diphosphorylated CTD [116].

Figure 4

Splicing changes the structure and the composition of an mRNP. The exon-junction complex (EJC) and a subset of SR proteins are deposited on the mRNP upon splicing. Multimerization of the EJC along with the associated SR proteins promotes mRNP packaging and compaction. An earlier study has shown that splicing also recruits the TREX complex to the 5'-end of mRNA [130]. This compaction and the association of the nuclear export factors may promote the translocation of the mRNP through the NPC.