Oligomeric state of β-coronavirus non-structural protein 10 stimulators studied by OmniSEC and Small Angle X-ray Scattering

Members of the {beta}-coronavirus family such as SARS-CoV-2, SARS, and MERS have caused pandemics over the last 20 years. Future pandemics are likely and studying the coronavirus family members is necessary for their understanding and treatment. Coronaviruses possess 16 non-structural proteins, many of which are involved in viral replication and other vital functions. Non-structural protein 10 (nsp10) is an essential stimulator of nsp14 and nsp16, modulating RNA proofreading and viral RNA cap formation. Studying nsp10 of pathogenic coronaviruses is central to understanding its multifunctional role. We report the biochemical and biophysical characterisation of full-length nsp10 from MERS, SARS and SARS-CoV-2. Proteins were subjected to a combination of OmniSEC and SEC-MALS to characterise their oligomeric state. Full-length nsp10s were predominantly monomeric in solution, while truncated versions of nsp10 have a higher tendency to oligomerise. Small angle X-ray scattering (SAXS) experiments reveal a globular shape of nsp10 which is conserved in all three coronaviruses, including MERS nsp10, which diverges most from SARS and SARS-CoV-2 nsp10s. In conclusion, unbound nsp10 proteins from SARS, MERS, and SARS-CoV-2 are globular and predominantly monomeric in solution. Additionally, we describe for the first time a functional role of the C-terminus of nsp10 for tight binding to nsp14.