Non-Structural Proteins of Coronaviruses
Coronaviruses have complex RNA-synthesizing machinery composed of many non-structural proteins (NSPs). While one-third of the viral genome encodes for structural proteins such as the spike, envelope, and nucleocapsid proteins, the remaining two-thirds encodes for NSPs. NSP genes are located within the 5’-region of the genome and studies have revealed that NSPs play essential roles in multiple viral processes such as RNA synthesis and processing¹.
The Two Faces of NSP14
NSP14 has recently been identified as an important NSP in SARS-CoV-2 virus replication. NSP14 is a unique bi-functional NSP that contains two functional domains – a 3’ to 5’ exoribonuclease (ExoN) at the N-terminus and a guanine-N7-methyltransferase (N7-MTase) domain at the C-terminus². The ExoN domain allows SARS-CoV-2 to proofread by catalyzing nucleoside monophosphate excision from nucleic acids, which is critical for high-fidelity replication. This is especially important for CoV to maintain its large RNA genome by decreasing the rate of harmful mutations. However, this proofreading capability of CoV has hindered the efficacy of antiviral ribonucleoside/ribonucleotide analogs such as remdesivir. Inactivating ExoN was shown to abolish the viability of SARS-CoV-2 effectively³. This is significant because targeting the ExoN activity of SARS-CoV-2 could potentially be an antiviral target for the treatment of COVID-19.
The N7-MTase domain of NSP14 is involved in mRNA capping. This is done by catalyzing the transfer of a methyl group from S-adenosyl-l-methionine (SAM) to the guanosine triphosphate moiety of the newly synthesized viral RNA, forming methylated capped RNA. This 5’ cap promotes RNA stability and translation by preventing degradation by exonucleases and also prevents detection by host innate antiviral responses4.
NSP14 and Innate Immune Responses
How does NSP14 allow CoV to evade host immune responses? 5’ RNA capping mediated by NSP14 allows CoV to mask the pathogen-associated molecular patterns (PAMPs) on the viral genome from being recognized by host immunity by mimicking eukaryotic RNA structures. Uncapped viral RNA is detected by host pattern recognition receptors, which will activate interferon (IFN)-associated antiviral immune responses5.
NSP14-NSP10 Protein-Protein Interactions
Additionally, NSP14 can form a complex with NSP10 (NSP14/10), where NSP10 aids in enhancing the ExoN activity of NSP14. A recent study found that NSP14/10 plays an essential role in inhibiting host translation, which promotes not only viral replication but also stifles immune responses6, therefore allowing CoV to better evade host immunity. Therefore, NSP14/10 poises as an attractive antiviral target to increase SARS-CoV-2 sensitivity towards host immune responses and increase the potency of existing antiviral pharmaceuticals.
- Yadav, R., Chaudhary, J. K., Jain, N., Chaudhary, P. K., Khanra, S., Dhamija, P., … & Handu, S. (2021). Role of Structural and Non-Structural Proteins and Therapeutic Targets of SARS-CoV-2 for COVID-19. Cells, 10(4), 821. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8067447/
- Tahir, M. (2021). Coronavirus genomic nsp14‐ExoN, structure, role, mechanism, and potential application as a drug target. Journal of Medical Virology, 93(7), 4258-4264. https://pubmed.ncbi.nlm.nih.gov/33837972/
- Ogando, N. S., Zevenhoven-Dobbe, J. C., van der Meer, Y., Bredenbeek, P. J., Posthuma, C. C., & Snijder, E. J. (2020). The enzymatic activity of the nsp14 exoribonuclease is critical for replication of MERS-CoV and SARS-CoV-2. Journal of virology, 94(23), e01246-20. https://journals.asm.org/doi/10.1128/JVI.01246-20
- Decroly, E., Ferron, F., Lescar, J., & Canard, B. (2012). Conventional and unconventional mechanisms for capping viral mRNA. Nature Reviews Microbiology, 10(1), 51-65. https://www.nature.com/articles/nrmicro2675
- Li, J. Y., Zhou, Z. J., Wang, Q., He, Q. N., Zhao, M. Y., Qiu, Y., & Ge, X. Y. (2021). Innate Immunity Evasion Strategies of Highly Pathogenic Coronaviruses: SARS-CoV, MERS-CoV, and SARS-CoV-2. Frontiers in Microbiology, 12. https://www.frontiersin.org/articles/10.3389/fmicb.2021.770656/full
- Hsu, J. C. C., Laurent-Rolle, M., Pawlak, J. B., Wilen, C. B., & Cresswell, P. (2021). Translational shutdown and evasion of the innate immune response by SARS-CoV-2 NSP14 protein. Proceedings of the National Academy of Sciences, 118(24). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8214666/