This model represents the non-structural protein 7 (Nsp7) of alphacoronaviruses that include Feline infectious peritonitis virus (FCoV), Human coronavirus NL63 (HCoV-NL63), and Porcine transmissible gastroenteritis coronavirus (TGEV), 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 stimulate replication. Most importantly, a complex of Nsp7 with Nsp8 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 NSP7- or NSP8-coding region have been shown to delay virus growth. Nsp7 and Nsp8 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 Nsp7 with Nsp8 appears to be conserved across the coronavirus family, making these proteins interesting drug targets. Nsp7 has a 4-helical bundle conformation which is strongly affected by its interaction with Nsp8, especially where it concerns alpha-helix 4. FCoV Nsp7 forms a 2:1 heterotrimer with Nsp8; the Nsp7/Nsp8 complex functions as a noncanonical RNA polymerase capable of synthesizing RNA of up to template length.
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