ORF8

ORF8 is a gene that encodes a viral accessory protein, Betacoronavirus NS8 protein, in coronaviruses of the subgenus Sarbecovirus. It is one of the least well conserved and most variable parts of the genome. In some viruses, a deletion splits the region into two smaller open reading frames, called ORF8a and ORF8b - a feature present in many SARS-CoV viral isolates from later in the SARS epidemic, as well as in some bat coronaviruses. For this reason the full-length gene and its protein are sometimes called ORF8ab. The full-length gene, exemplified in SARS-CoV-2, encodes a protein with an immunoglobulin domain of unknown function, possibly involving interactions with the host immune system. It is similar in structure to the ORF7a protein, suggesting it may have originated through gene duplication.

ORF8 in SARS-CoV-2 encodes a protein of 121 amino acid residues with an N-terminal signal sequence. ORF8 forms a dimer that is covalently linked by disulfide bonds. It has an immunoglobulin-like domain with distant similarity to the ORF7a protein. Despite a similar overall fold, an insertion in ORF8 likely is responsible for different protein-protein interactions and creates an additional dimerization interface. Unlike ORF7a, ORF8 lacks a transmembrane helix and is therefore not a transmembrane protein, though it has been suggested it might have a membrane-anchored form.

ORF8 in SARS-CoV and SARS-CoV-2 are very divergent, with less than 20% sequence identity. The full-length ORF8 in SARS-CoV encodes a protein of 122 residues. In many SARS-CoV isolates it is split into ORF8a and ORF8b, separately expressing 39-residue ORF8a and 84-residue ORF8b proteins. It has been suggested that the ORF8a and ORF8b proteins may form a protein complex. The cysteine residue responsible for dimerization of the SARS-CoV-2 protein is not conserved in the SARS-CoV sequence. The ORF8ab protein has also been reported to form disulfide-linked multimers.

The full-length SARS-CoV ORF8ab protein is post-translationally modified by N-glycosylation, which is predicted to be conserved in the SARS-CoV-2 protein. Under experimental conditions, both 8b and 8ab are ubiquitinated.

Along with the genes for other accessory proteins, the ORF8 gene is located near those encoding the structural proteins, at the 5' end of the coronavirus RNA genome. Along with ORF6, ORF7a, and ORF7b, ORF8 is located between the membrane (M) and nucleocapsid (N) genes. The SARS-CoV-2 ORF8 protein has a signal sequence for trafficking to the endoplasmic reticulum (ER) and has been experimentally localized to the ER. It is probably a secreted protein.

There are variable reports in the literature regarding the localization of SARS-CoV ORF8a, ORF8b, or ORF8ab proteins. It is unclear if ORF8b is expressed at significant levels under natural conditions. The full-length ORF8ab appears to localize to the ER.

The function of the ORF8 protein is unknown. It is not essential for viral replication in either SARS-CoV or SARS-CoV-2, though there is conflicting evidence on whether loss of ORF8 affects the efficiency of viral replication.

A function often suggested for ORF8 protein is interacting with the host immune system. The SARS-CoV-2 protein is thought to have a role in immunomodulation via immune evasion or suppressing host immune responses. It has been reported to be a type I interferon antagonist and to downregulate class I MHC. The SARS-CoV-2 ORF8 protein is highly immunogenic and high levels of antibodies to the protein have been found in patients with or recovered from COVID-19. A study indicates that ORF8 is a transcription inhibitor.