Nsp14

pmid: 33837972

• bifunctional nsp14
• N‐terminal 3′‐to‐5′exoribonuclease (ExoN)
• C‐terminal N7‐methyltransferase (N7‐MTase) domain. - involved in messenger RNA (mRNA) capping
• in 3′‐to‐5′direc-tion, nsp14 catalyzes nucleoside monophosphate excision from nucleic acids, using a mechanism that relies on two divalent metalions and a reactive water molecule.
• exonucleolytic activity is crucial for the Coronavirus replication proofreading activity, a property lacking in other RNA viruses that improve its fidelity replication and has played an essential role in nidoviral evolution and genome expansion - removing mismatched nucleotides from the 3ʹend of the rising RNA strand, - correct errors produced by the RdRp
• The first proofreading protein identified in an RNA virus is the coronavirus ExoN, and targeting ExoN is a promising strategy to develop new coronavirus inhibitors and antiviral sensitizers.

pmid: 33465137

• RNA recombination
• ExoN domain plays a role in the frequency of subgenomic microRNA (sgmRNA) recombination and recombination patterns in virally infected cells and virions in nsp14. Inactivating nsp14‐ExoN in murine hepatitis virus (MHV‐CoV), significantly altered recombination patterns and decreased recombination frequency compared with wild‐type MHV‐CoV.

(pmid: 32638509) Coronaviruses also code for their own 30–50exonuclease—a novelty among RNA viruses. The viral nsp14 protein(also called nsp-14-ExoN) is a 30–50exoribonuclease classified as part of the DE-D-Dh family of exonucleases that influ-ences the fidelity of CoV replication through a proofreading activity (Graepel et al., 2017). This function makesnsp14-ExoN the first RNA proofreading enzyme to be described. Given the large 26–32 kb size of the coronavirus RNAgenome and its surprisingly low mutation rate, nsp14-ExoN contributes in a major way to maintaining fidelity duringreplication (e.g., Ferron et al., 2018). Mutation of the active site of nsp14-ExoN results in up to a 20×decrease in fidelity(Eckerle et al., 2010; Graepel et al., 2017). As discussed above, nsp14 also contains MTase activity for the RNA cappingprocess (Case, Ashbrook, Dermody, & Denison, 2016) and nsp14 is found in complex with the viral nsp10 protein thatupregulates the ExoN and MTase activities (Chen et al., 2011). Thus nsp10 appears to be a broad-based regulator ofCoV RNA modification and quality control. Given the havoc that an unregulated 30–50exoribonuclease can play duringviral infection, nsp14 is clearly carefully controlled by protein–protein complexes and does not appear to significantlyimpact the overall degradation of RNAs in the infected cell.