The Great Escape: mRNA Export through the Nuclear Pore Complex

Abstract

Nuclear export of messenger RNA (mRNA) through the nuclear pore complex (NPC) is an indispensable step to ensure protein translation in the cytoplasm of eukaryotic cells. mRNA is not translocated on its own, but it forms ribonuclear particles (mRNPs) in association with proteins that are crucial for its metabolism, some of which; like Mex67/MTR2-NXF1/NXT1; are key players for its translocation to the cytoplasm. In this review, I will summarize our current body of knowledge on the basic characteristics of mRNA export through the NPC. To be granted passage, the mRNP cargo needs to bind transport receptors, which facilitate the nuclear export. During NPC transport, mRNPs undergo compositional and conformational changes. The interactions between mRNP and the central channel of NPC are described; together with the multiple quality control steps that mRNPs undergo at the different rings of the NPC to ensure only proper export of mature transcripts to the cytoplasm. I conclude by mentioning new opportunities that arise from bottom up approaches for a mechanistic understanding of nuclear export.

Keywords: Mex67/MTR2; NPC; NXF1/NXT1; Sub2/UAP56; Yra1/Aly; mRNA; mRNP; nuclear export; nuclear transport.

Figure 1

mRNA export is intertwined with its processing. Correct maturation of export competent mRNPs relies on sequential steps that go hand-in-hand with transcription initiation, capping, and processing. (A) In yeast, the mature mRNP is characterized by loading of Mex67/MTR2 (in blue), the FG-interacting transporter of the bulk nuclear RNA (in red), and presence of multiple RNA binding proteins (in white). Its loading to the RNA is not direct, and requires adaptors which are themselves loaded onto the transcribing RNA while it is produced. (B) Once loaded, the Mex67/MTR2 heterodimer mediates exit of the mRNP from the nucleoplasm via fast, transient interactions with the multiple FG repeats within the central channel of the NPC (in green) (C), eventually enabling the mRNP passage through the selective barrier into the cytoplasm.

Figure 2

Mex67/MTR2. Mex67/MTR2 (domain schemed in (A)) mediates the bulk RNA export in S. cerevisiae. It interacts with FG nucleoporins via its UBA domain (B) and with RNA, with its RRM, LLR and NTF2 domains, that form an extended, positively charged binding surface (C). Panel (B) from [59], and panel (C) from [58].

Figure 3

Preparing the mRNP for export. Mex67/MTR2 loading onto mRNA is the result of a concerted maturation process (schematized in (A)). (i.) THO, member of the TREX complex, anchors Sub2 in semi-open conformation to the nascent mRNP. (ii.) THO supports Sub2 anchoring to the mRNP, which clamps up in a closed conformation. (iii.) Mex67/MTR2 is recruited by Yra1, and its loading on mRNP displaces Sub2. (iv.) Yra1 leaves the mature mRNP following its ubiquitination. The process produces mature mRNP that vary from 15–35 nm in length (B). Panel (B) adapted from [78].

Figure 4

The NPC acts as a double checkpoint of mRNA export quality control. Both members of the NPC (green) and accessory proteins (orange) contribute to successful export of mature mRNP, which is characterized by capped 5′ end, polyadenylated 3′ end, and interaction with Mex67/MTR2 (in shades of blue). Left view: according to the classic model of mRNP export, the NPC basket member TPR/Mlp1 on the nucleoplasmic side (top) supports loading of Nab2 on the mRNP. Once the mRNP is inside the central channel, Mex67/MTR2 interacts with the FG repeats of the nucleoporins (not shown in the figure), mediating passage towards the cytoplasm. At the cytoplasmic side (bottom), anchored onto the radial-oriented Nup159, member of the Nup82 complex, Dpb5 ATPasic activity, stimulated by Gle1 and IP6, contributes to remodel the mRNP, by removing Nab2 and Mex67/MTR2. Right view: recent evidence suggest that Mex67/MTR2 may rather contribute to export as an integral component of the NPC. Together with loading of Nab2 by TRP/MLP (top), Mex67/MTR2 mediates interaction between the FG repeats of the nucleoporins (centre, not shown in the figure), and the mRNP, granting its passage to the cytoplasm. At the cytoplasmic side (bottom), Dpb5 removes the mRNA from Nab2 and Mex67/MTR2. It is less clear whether the latter is actively reimported or remains associated with the NPC at all times. The RNA becomes available for binding to a different set of RBPs.