# Human Betacoronavirus OC43 Interferes with the Integrated Stress Response Pathway in Infected Cells
## 1 Introduction  
Eukaryotic cells can rapidly respond to adverse conditions by activating specific stress response mechanisms. One of the conserved signaling pathways activated by various types of stress, including viral infections, is the integrated stress response (ISR) [1]. Its defining feature is the phosphorylation of the alpha subunit of the eukaryotic translation initiation factor 2 (elF2\(\alpha\)) on Serine-51, which causes the transient inhibition of bulk protein synthesis to conserve energy and minimize damage [2]. In mammals, there are four eIF2\(\alpha\) kinases that are activated by different types of stress: heme-regulated inhibitor (HRI) kinase is activated by oxidative stress and heat shock [3; 4], general control nonderepressible 2 (GCN2) kinase is activated by amino acid starvation or ultraviolet damage [5; 6]; double stranded RNA (dsRNA)-activated protein kinase (PKR) is activated by dsRNA [7]; and PKR-like endoplasmic reticulum (ER) kinase (PERK) is activated by ER stress [8]. Although PKR is the kinase primarily dedicated to sensing viral dsRNA replication intermediates, other eIF2\(\alpha\) kinases can also be activated by viral infections that perturb cellular homeostasis [9]. Since viruses rely on host machinery for their protein synthesis, they must evolve mechanisms to block or bypass the ISR to ensure efficient replication. Upon phosphorylation, eIF2\(\alpha\) stably binds the guanine exchange factor eIF2B, which prevents the regeneration of the translation initiation-competent GTP-bound form of eIF2 [2]. This stalls bulk translation initiation, while simultaneously causing the increased translation of a subset of messenger RNAs (mRNAs), some of which contain short upstream open reading frames (uORFs) in front of ORFs that encode stress response factors [10; 11]. The depletion of initiation-competent eIF2 complexes increases leaky scanning, or uORF bypass by the 48S preinitiation complex, which promotes the translation of downstreamORFs [2]. Activating transcription factor 4 (ATF4) is one of the important ISR factors preferentially translated via uORF bypass [12]. This transcriptional activator induces the expression of additional stress response genes, many of which are also encoded by uORF-containing mRNAs. They include another transcription factor C/EBP homologous protein (CHOP) and its downstream target growth arrest and DNA damage-inducible protein 34 (GADD34) [12]. GADD34 (also named protein phosphatase 1 regulatory subunit 15A, PPP1R15A) complexes with protein phosphatase 1 (PP1) and recruits it to deployhydrate eIF2\(\alpha\)[13]. Thus, the main function of GADD34 in the ISR is the negative feedback that terminates phospho-elF2\(\alpha\) mediated translation arrest. Coronaviruses (CoVs) are a family of enveloped single-stranded positive-sense RNA viruses. There are seven CoVs known to infect humans, presenting from common colds to more severe and sometimes fatal respiratory disease [14]. Common cold coronaviruses include HCOV-OC43 (OC43) and HCOV-229E, which have been circulating in the human population for decades, as well as more recently identified HCOV-NL63 and HCOV-HKU1 [15]. The most recent pathogenic CoV, the severe acute respiratory syndrome coronavirus 2 (CoV2), emerged in 2019 and became the cause of the devastating pandemic of coronavirus disease 2019 (COVID-19). CoV2, as well as OC43 and HCOV-HKU1, belong to the Beta-coronavirus genus [14; 15]. In recent years, OC43 emerged as the model Betacoronavirus for studying viral replication and testing the candidate anti-CoV drugs because, unlike CoV2, it is considered safer and can be handled in a more accessible containment level 2 environment [16; 17; 18; 19; 20]. The OC43 virion has five structural proteins. Membrane (M) and envelope (E) proteins are responsible for viral particle formation. Nucleocapsid (N) is an RNA-binding protein that coats the viral genome. There are also two surface glycoproteins: hemagglutinin esterase (HE) and spike (S) [15].