This model represents the non-structural protein 9 (Nsp9) of alphacoronaviruses, including Porcine epidemic diarrhea virus (PEDV), Porcine transmissible gastroenteritis coronavirus (TGEV), and Human coronavirus 229E. CoVs utilize a multi-subunit replication/transcription machinery assembled from a set of non-structural proteins (Nsps) generated as cleavage products of the ORF1a and ORF1ab viral polyproteins. All of these Nsps, except for Nsp1 and Nsp2, are considered essential for transcription, replication, and translation of the viral RNA. Nsp9, with Nsp7, Nsp8, and Nsp10, localizes within the replication complex. Nsp9 is an essential single-stranded RNA-binding protein for coronavirus replication; it shares structural similarity to the oligosaccharide-binding (OB) fold, which is characteristic of proteins that bind to ssDNA or ssRNA. Nsp9 requires dimerization for binding and orienting RNA for subsequent use by the replicase machinery. CoV Nsp9s have diverse forms of dimerization that promote their biological function, which may help elucidate the mechanism underlying CoVs replication and contribute to the development of antiviral drugs. Generally, dimers are formed via interaction of the parallel alpha-helices containing the protein-protein interaction motif GXXXG; additionally, the N-finger region may also play a critical role in dimerization as seen in porcine delta coronavirus (PDCoV) Nsp9. As a member of the replication complex, Nsp9 may not have a specific RNA-binding sequence but may act in conjunction with other Nsps as a processivity factor, as shown by mutation studies indicating that Nsp9 is a key ingredient that intimately engages other proteins in the replicase complex to mediate efficient virus transcription and replication.
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