Communication Is Key: 5'-3' Interactions that Regulate mRNA Translation and Turnover

The impact of mRNA poly(A) tail length on eukaryotic translation stages

The poly(A) tail plays an important role in maintaining mRNA stability and influences translation efficiency via binding with PABP. However, the impact of poly(A) tail length on mRNA translation remains incompletely understood. This study explores the effects of poly(A) tail length on human translation. We determined the translation rates in cell lysates using mRNAs with different poly(A) tails. Cap-dependent translation was stimulated by the poly(A) tail, however, it was largely independent of poly(A) tail length, with an exception observed in the case of the 75 nt poly(A) tail. Conversely, cap-independent translation displayed a positive correlation with poly(A) tail length. Examination of translation stages uncovered the dependence of initiation and termination on the presence of the poly(A) tail, but the efficiency of initiation remained unaffected by poly(A) tail extension. Further study unveiled that increased binding of eRFs to the ribosome with the poly(A) tail extension induced more efficient hydrolysis of peptidyl-tRNA. Building upon these findings, we propose a crucial role for the 75 nt poly(A) tail in orchestrating the formation of a double closed-loop mRNA structure within human cells which couples the initiation and termination phases of translation.

Reversible Capture and Release of a Ligand Mediated by a Long-Range Relayed Polarity Switch in a Urea Oligomer

Ethylene-bridged oligoureas characterized by a continuous, switchable chain of hydrogen bonds and carrying a binding site (an N,N′-disubstituted urea) for a hydrogen-bond-accepting ligand (a phosphine oxide) were synthesized. These oligomers show stronger ligand binding when the binding site is located at the hydrogen-bond-donating terminus than when the same binding site is at the hydrogen-bond-accepting terminus. An acidic group at the terminus remote from the binding site allows hydrogen bond polarity, and hence ligand binding ability, to be controlled remotely by a deprotonation/reprotonation cycle. Addition of base induces a remote conformational change that is relayed through up to five urea linkages, reducing the ability of the binding site to retain an intermolecular association to its ligand, which is consequently released into solution. Reprotonation returns the polarity of the oligomer to its original directionality, restoring the function of the remote binding site, which consequently recaptures the ligand. This is the first example of a synthetic molecular structure that relays intermolecular binding information, and these “dynamic foldamer” structures are prototypes of components for chemical systems capable of controlling chemical function from a distance.

RNA-Binding Proteins as Regulators of Internal Initiation of Viral mRNA Translation

Viruses are obligate intracellular parasites that depend on the host’s protein synthesis machinery for translating their mRNAs. The viral mRNA (vRNA) competes with the host mRNA to recruit the translational machinery, including ribosomes, tRNAs, and the limited eukaryotic translation initiation factor (eIFs) pool. Many viruses utilize non-canonical strategies such as targeting host eIFs and RNA elements known as internal ribosome entry sites (IRESs) to reprogram cellular gene expression, ensuring preferential translation of vRNAs. In this review, we discuss vRNA IRES-mediated translation initiation, highlighting the role of RNA-binding proteins (RBPs), other than the canonical translation initiation factors, in regulating their activity.

The viral nucleocapsid protein and the human RNA-binding protein Mex3A promote translation of the Andes orthohantavirus small mRNA

The capped Small segment mRNA (SmRNA) of the Andes orthohantavirus (ANDV) lacks a poly(A) tail. In this study, we characterize the mechanism driving ANDV-SmRNA translation. Results show that the ANDV-nucleocapsid protein (ANDV-N) promotes in vitro translation from capped mRNAs without replacing eukaryotic initiation factor (eIF) 4G. Using an RNA affinity chromatography approach followed by mass spectrometry, we identify the human RNA chaperone Mex3A (hMex3A) as a SmRNA-3’UTR binding protein. Results show that hMex3A enhances SmRNA translation in a 3’UTR dependent manner, either alone or when co-expressed with the ANDV-N. The ANDV-N and hMex3A proteins do not interact in cells, but both proteins interact with eIF4G. The hMex3A–eIF4G interaction showed to be independent of ANDV-infection or ANDV-N expression. Together, our observations suggest that translation of the ANDV SmRNA is enhanced by a 5’-3’ end interaction, mediated by both viral and cellular proteins.

Andes orthohantavirus (ANDV) is endemic in Argentina and Chile and is the primary etiological agent of hantavirus cardiopulmonary syndrome (HCPS) in South America. ANDV is unique among other members of the Hantaviridae family of viruses because of its ability to spread from person to person. The molecular mechanisms driving ANDV protein synthesis remain poorly understood. A previous report showed that translation of the Small segment mRNA (SmRNA) of ANDV relied on both the 5’cap and the 3’untranslated region (UTR) of the SmRNA. In this new study, we further characterize the mechanism by which the 5’ and 3’end of the SmRNA interact to assure viral protein synthesis. We establish that the viral nucleocapsid protein N and the cellular protein hMex3A participate in the process. These observations indicated that both viral and cellular proteins regulate viral gene expression during ANDV infection by enabling the viral mRNA to establish a non-covalent 5’-3’end interaction.

Artificial miRNAs targeting CAG repeat expansion in ORFs cause rapid deadenylation and translation inhibition of mutant transcripts

Polyglutamine (polyQ) diseases are incurable neurological disorders caused by CAG repeat expansion in the open reading frames (ORFs) of specific genes. This type of mutation in the HTT gene is responsible for Huntington’s disease (HD). CAG repeat-targeting artificial miRNAs (art-miRNAs) were shown as attractive therapeutic approach for polyQ disorders as they caused allele-selective decrease in the level of mutant proteins. Here, using polyQ disease models, we aimed to demonstrate how miRNA-based gene expression regulation is dependent on target sequence features. We show that the silencing efficiency and selectivity of art-miRNAs is influenced by the localization of the CAG repeat tract within transcript and the specific sequence context. Furthermore, we aimed to reveal the events leading to downregulation of mutant polyQ proteins and found very rapid activation of translational repression and HTT transcript deadenylation. Slicer-activity of AGO2 was dispensable in this process, as determined in AGO2 knockout cells generated with CRISPR-Cas9 technology. We also showed highly allele-selective downregulation of huntingtin in human HD neural progenitors (NPs). Taken together, art-miRNA activity may serve as a model of the cooperative activity and targeting of ORF regions by endogenous miRNAs.

Determinants of translation efficiency in the evolutionarily-divergent protist Trichomonas vaginalis

BACKGROUND

Trichomonas vaginalis, the causative agent of a prevalent urogenital infection in humans, is an evolutionarily divergent protozoan. Protein-coding genes in T. vaginalis are largely controlled by two core promoter elements, producing mRNAs with short 5' UTRs. The specific mechanisms adopted by T. vaginalis to fine-tune the translation efficiency (TE) of mRNAs remain largely unknown.

RESULTS

Using both computational and experimental approaches, this study investigated two key factors influencing TE in T. vaginalis: codon usage and mRNA secondary structure. Statistical dependence between TE and codon adaptation index (CAI) highlighted the impact of codon usage on mRNA translation in T. vaginalis. A genome-wide interrogation revealed that low structural complexity at the 5' end of mRNA followed closely by a highly structured downstream region correlates with TE variation in this organism. To validate these findings, a synthetic library of 15 synonymous iLOV genes was created, representing five mRNA folding profiles and three codon usage profiles. Fluorescence signals produced by the expression of these synonymous iLOV genes in T. vaginalis were consistent with and validated our in silico predictions.

CONCLUSIONS

This study demonstrates the role of codon usage bias and mRNA secondary structure in TE of T. vaginalis mRNAs, contributing to a better understanding of the factors that influence, and possibly regulate, gene expression in this human pathogen.

Fated for decay: RNA elements targeted by viral endonucleases

For over a decade, studies of messenger RNA regulation have revealed an unprecedented level of connectivity between the RNA pool and global gene expression. These connections are underpinned by a vast array of RNA elements that coordinate RNA-protein and RNA-RNA interactions, each directing mRNA fate from transcription to translation. Consequently, viruses have evolved an arsenal of strategies to target these RNA features and ultimately take control of the pathways they influence, and these strategies contribute to the global shutdown of the host gene expression machinery known as “Host Shutoff”. This takeover of the host cell is mechanistically orchestrated by a number of non-homologous virally encoded endoribonucleases. Recent large-scale screens estimate that over 70 % of the host transcriptome is decimated by the expression of these viral nucleases. While this takeover strategy seems extraordinarily well conserved, each viral endonuclease has evolved to target distinct mRNA elements. Herein, we will explore each of these RNA structures/sequence features that render messenger RNA susceptible or resistant to viral endonuclease cleavage. By further understanding these targeting and escape mechanisms we will continue to unravel untold depths of cellular RNA regulation that further underscores the integral relationship between RNA fate and the fate of the cell.

Cap-independent translation initiation of the unspliced RNA of retroviruses

Retroviruses are a unique family of RNA viruses that utilize a virally encoded reverse transcriptase (RT) to replicate their genomic RNA (gRNA) through a proviral DNA intermediate. The provirus is permanently integrated into the host cell chromosome and is expressed by the host cell transcription, RNA processing, and translation machinery. Retroviral messenger RNAs (mRNAs) entirely resemble a cellular mRNA as they have a 5'cap structure, 5'untranslated region (UTR), an open reading frame (ORF), 3'UTR, and a 3'poly(A) tail. The primary transcription product interacts with the cellular RNA processing machinery and is spliced, exported to the cytoplasm, and translated. However, a proportion of the pre-mRNA subverts typical RNA processing giving rise to the full-length RNA. In the cytoplasm, the full-length retroviral RNA fulfills a dual role acting as mRNA and as the gRNA. Simple retroviruses generate two pools of full-length RNA, one for each purpose. However, complex retroviruses have a single pool of full-length RNA, which is destined for translation or encapsidation. As for eukaryotic mRNAs, translational control of retroviral protein synthesis is mostly exerted at the step of initiation. Interestingly, some retroviral mRNAs, both simple and complex, use a dual mechanism to initiate protein synthesis, a cap-dependent initiation mechanism, or via internal initiation using an internal ribosome entry site (IRES). In this review, we describe and discuss data regarding the molecular mechanism driving the canonical cap-dependent and IRES-mediated translation initiation for retroviral mRNA, focusing the discussion mainly on the most studied retroviral mRNA, the HIV-1 mRNA.

Functional Cyclization of Eukaryotic mRNAs

The closed-loop model of eukaryotic translation states that mRNA is circularized by a chain of the cap-eIF4E-eIF4G-poly(A)-binding protein (PABP)-poly(A) interactions that brings 5' and 3' ends together. This circularization is thought to promote the engagement of terminating ribosomes to a new round of translation at the same mRNA molecule, thus enhancing protein synthesis. Despite the general acceptance and the elegance of the hypothesis, it has never been proved experimentally. Using continuous in situ monitoring of luciferase synthesis in a mammalian in vitro system, we show here that the rate of translation initiation at capped and polyadenylated reporter mRNAs increases after the time required for the first ribosomes to complete mRNA translation. Such acceleration strictly requires the presence of a poly(A)-tail and is abrogated by the addition of poly(A) RNA fragments or m⁷GpppG cap analog to the translation reaction. The optimal functional interaction of mRNA termini requires 5' untranslated region (UTR) and 3' UTR of moderate lengths and provides stronger acceleration, thus a longer poly(A)-tail. Besides, we revealed that the inhibitory effect of the dominant negative R362Q mutant of initiation factor eIF4A diminishes in the course of translation reaction, suggesting a relaxed requirement for ATP. Taken together, our results imply that, upon the functional looping of an mRNA, the recycled ribosomes can be recruited to the start codon of the same mRNA molecule in an eIF4A-independent fashion. This non-canonical closed-loop assisted reinitiation (CLAR) mode provides efficient translation of the functionally circularized mRNAs.