A mechanism for SARS-CoV-2 RNA capping and its inhibition by nucleotide analogue inhibitors

ABSTRACT

Decoration of cap on viral RNA plays essential roles in SARS-CoV-2 proliferation. Here we report a mechanism for SARS-CoV-2 RNA capping and document structural details at atomic resolution. The NiRAN domain in polymerase catalyzes the covalent link of RNA 5’ end to the first residue of nsp9 (termed as RNAylation), thus being an intermediate to form cap core (GpppA) with GTP catalyzed again by NiRAN. We also reveal that triphosphorylated nucleotide analogue inhibitors can be bonded to nsp9 and fit into a previously unknown ‘Nuc-pocket’ in NiRAN, thus inhibiting nsp9 RNAylation and formation of GpppA. S-loop (residues 50-KTN-52) in NiRAN presents a remarkable conformational shift observed in RTC bound with sofosbuvir monophosphate, reasoning an ‘induce-and-lock’ mechanism to design inhibitors. These findings not only improve the understanding of SARS-CoV-2 RNA capping and the mode of action of NAIs, but also provide a strategy to design antiviral drugs. Graphical Structural analyses reveal how proteins from SARS-CoV-2 cooperate and use GTP to form the cap on viral mRNA, and how this process is interrupted by nucleotide analogues that serve as antiviral drugs.