The Molecular Kidnapping: How Reoviruses are Taking Advantage of Our ATXN2L protein to Produce Their Babies

 Mammalian reovirus is a diverse family of virus that has segmented double-stranded RNA genome contained in two protein shells, which distinguishes them from many other viral families. Despite causing disease in wide range of mammalian hosts, reovirus is not typically considered as a human pathogen due to the absence of obvious symptoms or known clinical impact. Because of this, it becomes a perfect 'model organism' for studying cellular processes that can be safely handled in the lab, including translation, which is the main subject of this paper. It is the last step in converting DNA into proteins that are the actual workers of our cells.

In our cells, translation is made possible by the presence of a hat (5'cap) and a tail (poly-A tail) appended to mRNAs (the temporary carrier of genetic information), which helps them to exit the nucleus, protect them from enzymes that can degrade them, and help initiate translation (circularize the mRNA and reload translation machinery). Then mRNA will be converted into protein in the cytoplasm. In comparison, reovirus RNA genome gets translated within viral factories, which are little houses with no walls (often in membrane-less compartments called liquid-liquid phase separation) containing necessary tools for baby viruses to assemble and mature. Reovirus µNS protein forms the scaffold of these houses and interact with the core, where viral mRNAs are transcribed with the hat but without the tail. Surprisingly, these viral mRNAs can be readily translated in infected cells without the assistance and protection of the poly-A tail! This paper seek to find out the host cell factor that reoviruses has 'kidnapped' to produce viral proteins for themselves.

Since µNS protein is the important building block of viral factories where viral translation takes place, the 'manipulated' cellular protein probably interact with µNS protein. Based on this logic, the researchers used a technique called proximity-dependent Biotin identification, which is basically a spray-paint process. µNS protein is tagged with the spray---biotin protein ligase, which will enable the biotinylation of nearby proteins---paint. The 50 painted proteins labeled with biotin are identified and underwent a protein-protein interaction cluster analysis. There's one cluster that matches well with the expectations: proteins associated with SG (stress granule) and translation initiation. Followed by a long process of screening that identified proteins that would lead to failure in viral replication if deleted and if temporarily suppressed through CRISPR and siRNA whole genome screening respectively, a single common candidate that pop out in all filtered results is identified: ATXN2L.

Then the hypothesis that ATXN2L is the essential mediator of viral translation without poly A tail is tested stepwise. If ATXN2L indeed facilitates the translation of viral protein, it should be indispensable for viral replication. The researchers hence used CRISPR/Cas9 gene editing technique, which basically messes up the nucleotides at the specific desired site (gene ATXN2L) and render it nonfunctional, and it resulted in greatly impaired viral replication compared to the control group with undisrupted ATXN2L gene (WT) and with ATXN2L gene re-introduced (KO+). The green fluorescence is the virus-specific the antibody staining, indicating the infectivity. 

Additionally, if ATXN2L plays an essential role in viral translation, it probably interact with viral mRNA in some ways. Therefore, the researchers then tried to confirm this hypothesis and identify the specific interacting region. By creating ATXN2L with different combinations of deletions of regions that potentially participates in RNA binding (LSm and LSmAD) or contacts with poly-A-tail-binding protein (PAM2 motif) (shown in the image below, Δ meaning deletion), they found out that removal of either LSm and LSmAD alone or together significantly decreases ATXN2L's interaction with µNS and viral yield, with LSm being the primary one. This result suggests that RNA binding domains of ATXN2L are required for viral replication and it's highly possible that ATXN2L physically contact with mRNA at these specific sites. 

Then the researchers used Biotin-Streptavidin pull-down to find out if they two indeed interact with each other and where the region of contact is. Biotin is the small molecule that is attached to the viral mRNA and Streptavidin is a protein that acts like a high-powered magnet that can pull out Biotin and the bound mRNA along with proteins associates with it. Researchers tested three versions of viral s4 mRNA (as surrogate or the "representative" one among the ten segments): the normal version (s4 3'UTR), a version with the last ten nucleotides truncated (s4 3'UTR Δ10) and one with the last ten nucleotides scrambled. They chose the last ten nucleotides because they contain a five-nucleotide highly conserved sequence (meaning that it's found among almost all viral mRNAs), which can have evolutionary significance. They found that for both the truncated and scrambled condition, Streptavidin was not able to pull biotinylated mRNA bound by ATXN2L out from the cell lysates whereas with the control condition, they detected ATXN2L binding to the biotinylated mRNA. This suggests that mRNA and ATXN2La indeed interact with each other, and the conserved UCAUC sequence is the critical "landing pad" for ATXN2L, likely mediated by the its LSm and LSmAD domains.

Then researchers also discovered other evidences that indicated ATXN2L's participation in viral protein production, including: the absence of ATXN2L blocked the enlargement of viral factories ( appears to be punctate), which occurs when more viral proteins are contained. In the image below, KO refers to the condition where ATXN2L is knocked out. ATXN2L is also found to associate with the translation complex (polysome), including 80S ribosome as well as translation initiation factors elF4G1 and elf4G3.

Finally and most importantly, the researchers asked if ATXN2L indeed facilitates translation of mRNA WITHOUT poly-A tail. They created two mRNA that can glow when translated (NanoLuciferase translation reporter construct), one with the poly-A tail and the other without, so that they can compare the translation level through detecting luminescence exhibited. They found that the translation level of mRNA with poly-A tail is maintained regardless of the presence of ATXN2L but the translation of mRNA without the tail is impaired significantly by the absence of ATXN2L. Eventually, the researchers arrived at the conclusion that ATXN2L is the protein that's being 'kidnapped' by reovirus to complete their translation as it's doing its normal work in managing cell stress as a SG-associated gene. 

"What is the point of walking me through this extremely long research journey?" you might think. First of all, how RNA viruses translate their mRNA without a poly-A tail has long been an unresolved puzzle. This research helps us to find the molecular bridge that connects the viral factories and mRNAs as well as the probable contact sites between mRNA and ATXN2L. The researchers presented to us a very rigorous and comprehensive way of identifying the protein that's playing the major role in viral translation, from initial screening based on certain criteria and confirming its interacting partner proteins as well as its "jobs" in viral replication from different angles. Additionally, now that the important role of ATXN2L in viral translation is established, it can be used as the potential target of antivirals. If we can develop a drug that blocks either the interaction of ATXN2L with µNS or viral mRNA, the reoviruses will be deprived of their ability to produce viral proteins in our cells. Taking even another step further, since reoviruses are known for their oncolytic potential (preferentially replicate in malignant tumor cells), engineering the interactions among ATXN2L, viral factories and mRNA can also increase the efficiency of delivering therapeutic agents into the cancer cells.  Conceivable next steps of this study can be testing the role of ATXN2L in viral yield in different cell types and families of viruses as well as investigating the specific mechanisms of how ATXN2L facilitate translation, such as circularization of mRNA and reloading the ribosome.

Bibliography:

Primary research article:

Somoulay, Xayathed, et al. “Ataxin-2-like Promotes Translation of Nonpolyadenylated Reovirus MRNA.” Nature Communications, vol. 17, no. 1, 17 Dec. 2025, www.nature.com/articles/s41467-025-67547-1, https://doi.org/10.1038/s41467-025-67547-1. Accessed 21 Apr. 2026.

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