Substrate specificity of SARS-CoV-2 nsp10-nsp16 methyltransferase

The ongoing COVID-19 pandemic exemplifies the general need to better understand viral infections. The positive single strand RNA genome of its causative agent, the SARS coronavirus 2 (SARS-CoV-2) encodes all viral enzymes. In this work, we focus on one particular methyltransferase (MTase), nsp16, which in complex with nsp10 is capable of methylating the first nucleotide of a capped RNA strand at the 2′-O position. This process is part of a viral capping system and is crucial for viral evasion of the innate immune reaction. In light of recently discovered non-canonical RNA caps, we tested various dinucleoside polyphosphate-capped RNAs as substrates for nsp10-nsp16 MTase. We developed an LC-MS-based method and discovered five types of capped RNA (m⁷Gp₃A(G)-, Gp₃A(G)- and Gp₄A-RNA) that are substrates of the nsp10-nsp16 MTase. Our technique is an alternative to the classical isotope labelling approach for measurement of 2′-O-MTase activity. Further, we determined the IC₅₀value of sinefungin (286 ± 66 nM) to illustrate the value of our approach for inhibitor screening. In the future, this approach can be used for screening inhibitors of any type of 2′-O-MTase.