ABSTRACT
There are currently no antiviral therapies specific against SARS-CoV-2, the virus responsible for the global pandemic disease COVID-19. To facilitate structure-based drug design, we conducted an X-ray crystallographic study of the nsp16/nsp10 2'-O-methyltransferase complex that methylates Cap-0 viral mRNAs to improve viral protein translation and to avoid host immune detection. Heterodimer structures are determined with the methyl donor S-adenosylmethionine (SAM), the reaction product S-adenosylhomocysteine (SAH) or the SAH analog sinefungin (SFG). Furthermore, structures of nsp16/nsp10 with the methylated Cap-0 analog (m7GpppA) and SAM or SAH bound were obtained. Comparative analysis revealed flexible loops in open and closed conformations at the m7GpppA binding pocket. Bound sulfates in several structures suggested the location of the phosphates in the ribonucleotide binding groove. Additional nucleotide binding sites were found on the face of the protein opposite the active site. These various sites and the conserved dimer interface could be exploited for development of antiviral inhibitors.