The Cap-Binding Complex CBC and the Eukaryotic Translation Factor eIF4E: Co-Conspirators in Cap-Dependent RNA Maturation and Translation

Abstract

The translation of RNA into protein is a dynamic process which is heavily regulated during normal cell physiology and can be dysregulated in human malignancies. Its dysregulation can impact selected groups of RNAs, modifying protein levels independently of transcription. Integral to their suitability for translation, RNAs undergo a series of maturation steps including the addition of the m7G cap on the 5′ end of RNAs, splicing, as well as cleavage and polyadenylation (CPA). Importantly, each of these steps can be coopted to modify the transcript signal. Factors that bind the m7G cap escort these RNAs through different steps of maturation and thus govern the physical nature of the final transcript product presented to the translation machinery. Here, we describe these steps and how the major m7G cap-binding factors in mammalian cells, the cap binding complex (CBC) and the eukaryotic translation initiation factor eIF4E, are positioned to chaperone transcripts through RNA maturation, nuclear export, and translation in a transcript-specific manner. To conceptualize a framework for the flow and integration of this genetic information, we discuss RNA maturation models and how these integrate with translation. Finally, we discuss how these processes can be coopted by cancer cells and means to target these in malignancy.

The eukaryotic translation initiation factor eIF4E reprogrammes the splicing machinery and drives alternative splicing

“…The direct model gives rise to considerations of how RNAs are selected to undergo eIF4Edependent AS rather than CBC-dependent, bulk pre-mRNA splicing (Supp Fig8). Indeed, the CBC and eIF4E compete for capped RNAs in the nucleus 19,33 and we observe a CBP20-eIF4E interaction suggesting that capped RNAs could be exchanged between these factors (Mars et al, unpublished observations). The best-described example of this cap competition comes from studies of eIF4E-dependent nuclear RNA export 19 .…”

“…After splicing, eIF4E is positioned to escort the newly spliced capped-RNAs to other processing steps, to the export machinery and/or possibly is exchanged for other nuclear cap-binding proteins such as the nuclear cap-binding complex (CBC). In this way, eIF4E acts as a cap-chaperone 32,33 (Supp Fig8), in much the same manner that the CBC does for bulk pre-mRNA during splicing [53][54][55] . Interestingly, CBC is linked to splicing of the first intron of target transcripts 54 , a preference we did not observe for eIF4Esensitive transcripts (Supp Fig5 and 6).…”

“…eIF4E has also been implicated in nuclear RNA maturation including m 7 G capping as well as cleavage and polyadenylation (CPA) of selected RNAs 20,22,31 . Thus, eIF4E plays broader roles in mRNA maturation; and this can be viewed as eIF4E acting as an m 7 G cap chaperone 32,33 . Similar to its selection of nuclear RNA export targets, selectivity arises due to cis-acting elements in the targets RNAs 20,22 .…”

The search for genetic dark matter and lessons learned from the journey

“…The central dogma of molecular biology positions mRNA as a simple intermediary in a process whereby DNA instructs the cells to generate protein actors to carry out its directives. However, in biological systems, we often observe a disconnect between the transcriptome and the proteome (de Sousa Abreu et al 2009;Mars et al 2021). One example of this arises when transcript levels remain unchanged while protein levels are elevated or reduced.…”

“…When I started my lab over 20 years ago, the idea that there was genetic dark matter was not commonplace. The steps of RNA processing such as the addition of the methyl-7guanosine (m 7 G cap) on the 5 end of RNAs ("capping"), the removal of introns through splicing, the addition of the polyA tail, and the nuclear export of RNAs were viewed as constitutive housekeeping functions (Mars et al 2021). Back then the predominant view was that the only features driving cancer were DNA mutation, dysregulated transcription, and (or) aberrant signalling.…”