Crystal structure of the SARS-CoV-2 non-structural protein 9, Nsp9

Dene Littler; Benjamin Gully; Rhys N Colson; Jamie Rossjohn

The sequence of Nsp9 homologues are conserved amongst betacoronoaviruses, yet there remains the potential for functional differences in different viruses. Nsp9COV19 exhibits 97% sequence identity with Nsp9SARS but only 44% sequence identity with Nsp9HCoV. The structure of the HCoV-229E Nsp9 protein suggested a potential oligomeric switch induced upon the formation of an intersubunit disulfide bond. Here, disulfide bond formation shifts the relative orientation of the Nsp9 monomers, which was suggested to promote higher-order oligomerisation (Ponnusamy et al., 2008) . The resultant rod-like higher order Nsp9HCoV assemblies had increased affinity for the RNA oligonucleotides. Cysteine mutants of Nsp9HCoV that are unable to produce the disulfide displayed reduced RNA-binding affinity (Ponnusamy et al., 2008) . The observation of a redox-induced structural switch of Nsp9HCoV led to the hypothesis that Nsp9HCoV may have a functional role in sensing the redox status of the host cell (Ponnusamy et al., 2008) . While the "redox-switch" cysteine responsible for oligomer formation in Nsp9HCoV is conserved amongst different viral Nsp9 homologues the higher-order oligomers were not observed for Nsp9SARS (Ponnusamy et al., 2008) . Because of these potential differences between Nsp9 proteins we sought to further characterise the nature of Nsp9COV19.