A natural product compound inhibits coronaviral replication in vitro by binding to the conserved Nsp9 SARS-CoV-2 protein

The Nsp9 replicase is a conserved coronaviral protein that acts as an essential accessory component of the multi-subunit viral replication/transcription complex. Nsp9 is the predominant substrate for the essential nucleotidylation activity of Nsp12. Compounds specifically interfering with this viral activity would facilitate its study. Using a native mass spectrometry-based approach to screen a natural product library for Nsp9 binders, we identified an ent-kaurane natural product, oridonin, capable of binding to purified SARS-CoV-2 Nsp9 with micromolar affinities. By determining the crystal structure of the Nsp9-oridonin complex, we showed that oridonin binds through a conserved site near Nsp9's C-terminal GxxxG-helix. In enzymatic assays oridonin's binding to Nsp9 reduces its potential to act as substrate for Nsp12's Nidovirus RdRp-Associated Nucleotidyl transferase (NiRAN) domain. We also showed using in vitro cellular assays oridonin, while cytotoxic at higher doses, has broad antiviral activity, reducing viral titre following infection with either SARS-CoV-2 or, to a lesser extent, MERS-CoV. Accordingly, these preliminary findings suggest the oridonin molecular scaffold may have the potential to be developed into an anti-viral compound to inhibit the function of Nsp9 during coronaviral replication.