The eukaryotic translation initiation factor eIF4E is a potent oncogene that promotes the nuclear export and translation of specific transcripts. Here, we have discovered that eIF4E alters the cytoplasmic face of the nuclear pore complex (NPC), which leads to enhanced mRNA export of eIF4E target mRNAs. Specifically, eIF4E substantially reduces the major component of the cytoplasmic fibrils of the NPC, RanBP2, relocalizes an associated nucleoporin, Nup214, and elevates RanBP1 and the RNA export factors, Gle1 and DDX19. Genetic or pharmacological inhibition of eIF4E impedes these effects. RanBP2 overexpression specifically inhibits the eIF4E mRNA export pathway and impairs oncogenic transformation by eIF4E. The RanBP2 cytoplasmic fibrils most likely slow the release and/or recycling of critical export factors to the nucleus. eIF4E overcomes this inhibitory mechanism by indirectly reducing levels of RanBP2. More generally, these results suggest that reprogramming the NPC is a means by which oncogenes can harness the proliferative capacity of the cell.

Recognition of the methyl-7-guanosine (m ⁷ G) cap structure on mRNA is an essential feature of mRNA metabolism and thus gene expression. Eukaryotic translation initiation factor 4E (eIF4E) promotes translation, mRNA export, proliferation, and oncogenic transformation dependent on this cap-binding activity. eIF4E–cap recognition is mediated via complementary charge interactions of the positively charged m ⁷ G cap between the negative π-electron clouds from two aromatic residues. Here, we demonstrate that a variant subfamily, eIF4E3, specifically binds the m ⁷ G cap in the absence of an aromatic sandwich, using instead a different spatial arrangement of residues to provide the necessary electrostatic and van der Waals contacts. Contacts are much more extensive between eIF4E3–cap than other family members. Structural analyses of other cap-binding proteins indicate this recognition mode is atypical. We demonstrate that eIF4E3 relies on this cap-binding activity to act as a tumor suppressor, competing with the growth-promoting functions of eIF4E. In fact, reduced eIF4E3 in high eIF4E cancers suggests that eIF4E3 underlies a clinically relevant inhibitory mechanism that is lost in some malignancies. Taken together, there is more structural plasticity in cap recognition than previously thought, and this is physiologically relevant.

Metastatic lymph node 51 [MLN51 (also known as CASC3)] is a component of the exon junction complex (EJC), which is assembled on spliced mRNAs and plays important roles in post-splicing events. The four proteins of the EJC core, MLN51, MAGOH, Y14 and EIF4AIII shuttle between the cytoplasm and the nucleus. However, unlike the last three, MLN51 is mainly detected in the cytoplasm, suggesting that it plays an additional function in this compartment. In the present study, we show that MLN51 is recruited into cytoplasmic aggregates known as stress granules (SGs) together with the SG-resident proteins, fragile X mental retardation protein (FMRP), poly(A) binding protein (PABP) and poly(A)⁺ RNA. MLN51 specifically associates with SGs via its C-terminal region, which is dispensable for its incorporation in the EJC. MLN51 does not promote SG formation but its silencing, or the overexpression of a mutant lacking its C-terminal region, alters SG assembly. Finally, in human breast carcinomas, MLN51 is sometimes present in cytoplasmic foci also positive for FMRP and PABP, suggesting that SGs formation occurs in malignant tumours.

The exon junction complex (EJC) is loaded onto mRNAs as a consequence of splicing and regulates multiple posttranscriptional events. MLN51, Magoh, Y14, and eIF4A3 form a highly stable EJC core, but where this tetrameric complex is assembled in the cell remains unclear. Here we show that EJC factors are enriched in domains that we term perispeckles and are visible as doughnuts around nuclear speckles. Fluorescence resonance energy transfer analyses and EJC assembly mutants show that perispeckles do not store free subunits, but instead are enriched for assembled cores. At the ultrastructural level, perispeckles are distinct from interchromatin granule clusters that may function as storage sites for splicing factors and intermingle with perichromatin fibrils, where nascent RNAs and active RNA Pol II are present. These results support a model in which perispeckles are major assembly sites for the tetrameric EJC core. This subnuclear territory thus represents an intermediate region important for mRNA maturation, between transcription sites and splicing factor reservoirs and assembly sites.