Natural product inhibition of SARS-CoV-2 guanine-N7 RNA cap methyltransferase shows potent cellular antiviral efficacy

SARS-CoV-2 virus mimics host mRNA by capping its viral RNA to promote replication and evade host immune sensing. SARS-CoV-2 NSP14 is the N7-guanosine methyltransferase (N7-MTase) responsible for RNA cap-0 formation. Targeting NSP14 for antiviral drug development is an under-explored but promising strategy. Here we conducted a high-throughput screening on natural products library derived from Chinese herbal medicine to discover Emodin as a SARS-CoV-2 NSP14 inhibitor. Exploring Emodin derivatives, Questin was identified with potent cellular inhibitory activity (EC50=249 nM) against SARS-CoV-2, which inhibits NSP14 in an RNA cap competitive manner, making it one the most potent anti-coronaviral natural products. Mechanistically, besides catalyzing viral RNA capping, NSP14 by itself could remodel host transcriptome such as enriching CREBBP, a key host factor in cellular cyclic AMP response pathway, to promote viral infection. As a result, targeting NSP14 by Questin significantly impairs viral Replication & Translation step and reverses host transcriptome remodeled by NSP14. We next validated Questin as a promising lead with significantly improved toxicity upon acute exposure in zebrafish larvae. Taken together, our study not only demonstrates Questin as a potent drug lead for clinical antiviral application, but also highlights multiple antiviral potentials of NSP14 as therapeutic target.