Coronavirus RNA capping pathway

Capping of viral RNAs by conventional or unconventional pathways leads to 5′-end cap structures that allow efficient viral protein synthesis and, in many cases, escape from the innate immune system. Coronaviruses follow the canonical capping pathway, which consists of four sequential enzymatic reactions:

• RTPase, encoded by the nsp13 helicase, hydrolyzing the γ-phosphate of the mRNA
• an as-yet-unidentified guanyltransferase (GTase) adding GMP to the 5′-diphosphate RNA
• nsp14 N7-MTase methylating the guanosine, leading to a cap-0 structure that is essential for efficient translation initiation
• nsp16 2′-0-methyltransferase (2′-0-MTase) carrying out further methylations, leading to cap-1 and cap-2 structures, which are required to efficiently escape the nonself RNA recognition system of the host cell.