This domain covers the NSP16 region of the coronavirus polyprotein. It was originally named NSP13 and later changed to NSP16 to distinguish it from the helicase region []. NSP16 is a 7-methylguanine-triphosphate-adenosine (m7GpppA)-specific, SAM-dependent 2'-O-MTase that has selective RNA binding properties and is a cap-0 binding protein. It contains a highly conserved catalytic tetrad (K-D-K-E) that is a hallmark of RNA 2'O-MTases [, , ]. NSP16 plays a key role in viral replication as it is involved in immune response evasion through the 2'-O-methylation of coronavirus RNA, which is essential for preventing recognition by the host. It mimics the human protein CMTr1 and, unlike CMTr1, it requires NSP10 as a binding partner to activate its enzymatic activity []. Structural analysis in NSP16 from SARS-CoV-2 identified a cryptic pocket (also present in other coronavirus such as SARS-CoV-1 and MERS) not present in human CMTr1, and the fact that NSP16 is one of the most conserved proteins of SARS-CoV-2 and related viruses, make it an interesting target for developing new antiviral treatment for COVID-19 and other diseases caused by coronaviruses [].
