C-terminal SARS-Unique Domain (SUD) of non-structural protein 3 (Nsp3) from Rousettus bat coronavirus HKU9 and related betacoronaviruses in the D lineage
This subfamily contains the SUD-C of Rousettus bat coronavirus (CoV) HKU9 non-structural protein 3 (Nsp3) and other Nsp3s from betacoronaviruses in the nobecovirus subgenera (D lineage). Non-structural protein 3 (Nsp3) is a large multi-functional multi-domain protein that is an essential component of the replication/transcription complex (RTC), which carries out RNA synthesis, RNA processing, and interference with the host cell innate immune system. Nsp3 of SARS coronavirus includes a SARS-unique domain (SUD) consisting of three globular domains separated by short linker peptide segments: SUD-N, SUD-M, and SUD-C. SUD-N and SUD-M are macro domains which bind G-quadruplexes (unusual nucleic-acid structures formed by consecutive guanosine nucleotides). SUD is not as specific to SARS CoV as originally thought and is also found in Rousettus bat CoV HKU9 and related bat CoVs. Similar to SARS SUD-C, Rousettus bat CoV HKU9 SUD-C (HKU9 C), also adopts a frataxin-like fold that has structural similarity to DNA-binding domains of DNA-modifying enzymes. However, there is little sequence similarity between the two domains. SARS SUD-C has been shown to bind to single-stranded RNA and recognize purine bases more strongly than pyrimidine bases; it also regulates the RNA binding behavior of the SARS SUD-M macrodomain. It is not known whether HKU9 C functions in the same way.
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