PABP1 Drives the Selective Translation of Influenza A Virus mRNA

Protein turnover regulation is critical for influenza A virus infection

The abundance of a protein is defined by its continuous synthesis and degradation, a process known as protein turnover. Here, we systematically profiled the turnover of proteins in influenza A virus (IAV)-infected cells using a pulse-chase stable isotope labeling by amino acids in cell culture (SILAC)-based approach combined with downstream statistical modeling. We identified 1,798 virus-affected proteins with turnover changes (tVAPs) out of 7,739 detected proteins (data available at pulsechase.innatelab.org). In particular, the affected proteins were involved in RNA transcription, splicing and nuclear transport, protein translation and stability, and energy metabolism. Many tVAPs appeared to be known IAV-interacting proteins that regulate virus propagation, such as KPNA6, PPP6C, and POLR2A. Notably, our analysis identified additional IAV host and restriction factors, such as the splicing factor GPKOW, that exhibit significant turnover rate changes while their total abundance is minimally affected. Overall, we show that protein turnover is a critical factor both for virus replication and antiviral defense.

LINC01197 inhibits influenza A virus replication by serving as a PABPC1 decoy

Influenza A viruses (IAVs) significantly impact animal and human health due to their zoonotic potential. A growing body of evidence indicates that the host's long noncoding RNAs (lncRNAs) play crucial roles in regulating host-virus interactions during IAV infection. However, numerous lncRNAs associated with IAV infection have not been well characterised. Here, in this study, we identify the LINC01197 as an antiviral host factor. LINC01197 was significantly upregulated after IAV infection, which is controlled by the NF-κB pathway. Functional analysis revealed that overexpression of LINC01197 inhibited IAV replication and virus production, while knockdown of LINC01197 facilitated IAV replication. Mechanistically, LINC01197 directly interacts with poly(A) binding protein cytoplasmic 1 (PABPC1), which in turn sequesters and restricts its functions. This work shows that LINC01197 acts as a protein decoy, suppressing IAV replication and playing a key role in controlling IAV replication.

The RBPome of influenza A virus NP-mRNA reveals a role for TDP-43 in viral replication

Genome-wide approaches have significantly advanced our knowledge of the repertoire of RNA-binding proteins (RBPs) that associate with cellular polyadenylated mRNAs within eukaryotic cells. Recent studies focusing on the RBP interactomes of viral mRNAs, notably SARS-Cov-2, have revealed both similarities and differences between the RBP profiles of viral and cellular mRNAs. However, the RBPome of influenza virus mRNAs remains unexplored. Herein, we identify RBPs that associate with the viral mRNA encoding the nucleoprotein (NP) of an influenza A virus. Focusing on TDP-43, we show that it binds several influenza mRNAs beyond the NP-mRNA, and that its depletion results in lower levels of viral mRNAs and proteins within infected cells, and a decreased yield of infectious viral particles. We provide evidence that the viral polymerase recruits TDP-43 onto viral mRNAs through a direct interaction with the disordered C-terminal domain of TDP-43. Notably, other RBPs found to be associated with influenza virus mRNAs also interact with the viral polymerase, which points to a role of the polymerase in orchestrating the assembly of viral messenger ribonucleoproteins.

The Influenza A Virus Replication Cycle: A Comprehensive Review

Influenza A virus (IAV) is the primary causative agent of influenza, colloquially called the flu. Each year, it infects up to a billion people, resulting in hundreds of thousands of human deaths, and causes devastating avian outbreaks with worldwide losses worth billions of dollars. Always present is the possibility that a highly pathogenic novel subtype capable of direct human-to-human transmission will spill over into humans, causing a pandemic as devastating if not more so than the 1918 influenza pandemic. While antiviral drugs for influenza do exist, they target very few aspects of IAV replication and risk becoming obsolete due to antiviral resistance. Antivirals targeting other areas of IAV replication are needed to overcome this resistance and combat the yearly epidemics, which exact a serious toll worldwide. This review aims to summarise the key steps in the IAV replication cycle, along with highlighting areas of research that need more focus.

Strategies of Influenza A Virus to Ensure the Translation of Viral mRNAs

Viruses are obligatorily intracellular pathogens. To generate progeny virus particles, influenza A viruses (IAVs) have to divert the cellular machinery to ensure sufficient translation of viral mRNAs. To this end, several strategies have been exploited by IAVs, such as host gene shutoff, suppression of host innate immune responses, and selective translation of viral mRNAs. Various IAV proteins are responsible for host gene shutoff, e.g., NS1, PA-X, and RdRp, through inhibition of cellular gene transcription, suppression of cellular RNA processing, degradation of cellular RNAs, and blockage of cellular mRNA export from the nucleus. Host shutoff should suppress the innate immune responses and also increase the translation of viral mRNAs indirectly due to the reduced competition from cellular mRNAs for cellular translational machinery. However, many other mechanisms are also responsible for the suppression of innate immune responses by IAV, such as prevention of the detection of the viral RNAs by the RLRs, inhibition of the activities of proteins involved in signaling events of interferon production, and inhibition of the activities of interferon-stimulated genes, mainly through viral NS1, PB1-F2, and PA-X proteins. IAV mRNAs may be selectively translated in favor of cellular mRNAs through interacting with viral and/or cellular proteins, such as NS1, PABPI, and/or IFIT2, in the 5'-UTR of viral mRNAs. This review briefly summarizes the strategies utilized by IAVs to ensure sufficient translation of viral mRNAs focusing on recent developments.

Host adaptive mutations in the 2009 H1N1 pandemic influenza A virus PA gene regulate translation efficiency of viral mRNAs via GRSF1

Avian species are the major natural reservoir from which pandemic influenza A viruses can be introduced to humans. Avian influenza A virus genes, including the three viral polymerase genes, PA, PB1 and PB2, require host-adaptive mutations to allow for viral replication and transmission in humans. Previously, PA from the 2009 pH1N1 viral polymerase was found to harbor host-adaptive mutations leading to enhanced viral polymerase activity. By quantifying translation and mRNA transcription, we found that the 2009 pH1N1 PA, and the associated host-adaptive mutations, led to greater translation efficiency. This was due to enhanced cytosolic accumulation of viral mRNA, which was dependent on the host RNA binding protein GRSF1. Mutations to the GRSF1 binding site in viral mRNA, as well as GRSF1 knockdown, reduced cytosolic accumulation and translation efficiency of viral mRNAs. This study identifies a previously unrecognized mechanism by which host-adaptive mutations in PA regulate viral replication and host adaptation. Importantly, these results provide greater insight into the host adaptation process of IAVs and reveal the importance of GRSF1 in the lifecycle of IAV.

Interaction of the Influenza A Virus NS1 Protein with the 5'-m7G-mRNA·eIF4E·eIF4G1 Complex

The influenza A virus (IAV) is responsible for seasonal epidemics that result in hundreds of thousands of deaths worldwide annually. The non-structural protein 1 (NS1) of the IAV inflicts various antagonistic processes on the host during infection. These processes include inhibition of the host interferon system, inhibition of the apoptotic response, and enhancement of viral mRNA translation, all of which contribute to the overall virulence of the IAV. Although the mechanism by which NS1 stimulates translation is unknown, NS1 has been shown to bind both poly-A binding Protein 1 and eukaryotic initiation factor 4 gamma 1 (eIF4G1), two proteins necessary for cap-dependent translation. We directly analyzed the interaction between NS1 and eIF4G1 within the context of the 5'-m⁷G-mRNA·eIF4E·eIF4G1 complex. Interestingly, our studies show that NS1 can bind this complex in the presence or absence of 5'-m⁷G-mRNA. Additionally, we were interested in investigating whether NS1 interacts with eIF4E directly. Our results indicate that NS1 can bind to eIF4E only in the absence of 5'-m⁷G-mRNA. Considering previous data, we propose that NS1 stimulates translation by binding to eIF4G1 and recruiting the 43S pre-translation initiation complex to the mRNA.