This model represents the non-structural protein 8 (Nsp8) region of alphacoronaviruses that include Feline infectious peritonitis virus (FCoV), Human coronavirus NL63 (HCoV-NL63), and Porcine epidemic diarrhea coronavirus (PEDV), among others. CoVs utilize a multi-subunit replication/transcription machinery. A set of non-structural proteins (Nsps) generated as cleavage products of the ORF1a and ORF1ab viral polyproteins assemble to facilitate viral replication and transcription. Upon processing of the Nsp7-10 region by protease M (Mpro), the released four small proteins Nsp7, Nsp8, Nsp9, and Nsp10 form functional complexes with CoV core enzymes and thereby stimulate replication. Most importantly, a complex of Nsp8 with Nsp7 has been shown to activate and confer processivity to the RNA-synthesizing activity of Nsp12, the RNA-dependent RNA-polymerase (RdRp); in SARS-CoV, point mutations in the genes encoding Nsp8 and Nsp7 have been shown to delay virus growth. Nsp8 and Nsp7 cooperate in activating the primer-dependent activity of the Nsp12 RdRp such that the level of their association may constitute a limiting factor for obtaining a high RNA polymerase activity. The subsequent Nsp7/Nsp8/Nsp12 polymerase complex is then able to associate with an active bifunctional Nsp14, which includes N-terminal 3' to 5' exoribonuclease (ExoN) and C-terminal N7-guanine cap methyltransferase (N7-MTase) activities, thus representing a unique coronavirus Nsp assembly that incorporates RdRp, exoribonuclease, and N7-MTase activities. Interaction of Nsp8 with Nsp7 appears to be conserved across the coronavirus family, making these proteins interesting drug targets. Nsp8 has a novel 'golf-club' fold composed of an N-terminal 'shaft' domain and a C-terminal 'head' domain. The shaft domain contains three helices, one of which is very long, while the head domain contains another three helices and seven beta-strands, forming an alpha/beta fold. FCoV Nsp8 forms a 1:2 heterotrimer with Nsp7; the Nsp7/Nsp8 complex functions as a noncanonical RNA polymerase capable of synthesizing RNA of up to the template length.
Comment:these three residues have been shown to have an important role in RNA synthesis for SARS-CoV Nsp8; the Pro and Arg have been postulated to be involved in interactions with Nsp12, while the Lys may be critical for Nsp8-RNA interactions
Jiyao W et al.(2023). "The conserved domain database in 2023.",