| Betacoronavirus NS8 protein | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 TheSARS-CoV-2 ORF8protein dimer. Internaldisulfide bonds are shown in yellow and the intermolecular disulfide is shown in magenta. FromPDB:7JTL.[1] | |||||||||
| Identifiers | |||||||||
| Symbol | bCoV_NS8 | ||||||||
| Pfam | PF12093 | ||||||||
| InterPro | IPR022722 | ||||||||
| |||||||||
ORF8 is agene that encodes aviral accessory protein,Betacoronavirus NS8 protein, incoronaviruses of thesubgenusSarbecovirus. It is one of the least wellconserved and most variable parts of thegenome.[2][3][4][5] In some viruses, adeletion splits the region into two smalleropen reading frames, calledORF8a andORF8b - a feature present in manySARS-CoV viral isolates from later in theSARS epidemic, as well as in somebat coronaviruses.[4][3] For this reason the full-length gene and its protein are sometimes calledORF8ab.[3][6] The full-length gene, exemplified inSARS-CoV-2, encodes a protein with animmunoglobulin domain of unknown function, possibly involving interactions with the hostimmune system.[4][3][1] It is similar in structure to theORF7a protein, suggesting it may have originated throughgene duplication.[7][8]
ORF8 in SARS-CoV-2 encodes a protein of 121amino acid residues with anN-terminalsignal sequence.[4] ORF8 forms adimer that iscovalently linked bydisulfide bonds.[1] It has animmunoglobulin-likedomain with distant similarity to theORF7a protein.[1][2] Despite a similar overall fold, aninsertion in ORF8 likely is responsible for differentprotein-protein interactions and creates an additional dimerization interface.[1][2] Unlike ORF7a, ORF8 lacks atransmembrane helix and is therefore not atransmembrane protein,[1][4] though it has been suggested it might have a membrane-anchored form.[3]
ORF8 in SARS-CoV and SARS-CoV-2 are very divergent, with less than 20%sequence identity.[1] 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.[6] It has been suggested that the ORF8a and ORF8b proteins may form aprotein complex.[2][9] Thecysteine residue responsible for dimerization of the SARS-CoV-2 protein is not conserved in the SARS-CoV sequence.[1] The ORF8ab protein has also been reported to form disulfide-linkedmultimers.[10]
The full-length SARS-CoV ORF8ab protein ispost-translationally modified byN-glycosylation,[6] which is predicted to be conserved in the SARS-CoV-2 protein.[1] Under experimental conditions, both 8b and 8ab areubiquitinated.[6]
Along with the genes for other accessory proteins, the ORF8 gene is located near those encoding the structural proteins, at the5' end of the coronavirus RNA genome. Along withORF6,ORF7a, andORF7b, ORF8 is located between themembrane (M) andnucleocapsid (N) genes.[6][4] The SARS-CoV-2 ORF8 protein has a signal sequence fortrafficking to theendoplasmic reticulum (ER)[4] and has been experimentallylocalized to the ER.[11] It is probably asecreted protein.[4][3]
There are variable reports in the literature regarding the localization of SARS-CoV ORF8a, ORF8b, or ORF8ab proteins.[6] It is unclear if ORF8b is expressed at significant levels under natural conditions.[10][12] The full-length ORF8ab appears to localize to the ER.[12]
The function of the ORF8 protein is unknown. It is notessential forviral replication in either SARS-CoV[6] or SARS-CoV-2,[4] though there is conflicting evidence on whether loss of ORF8 affects the efficiency ofviral replication.[13]
A function often suggested for ORF8 protein is interacting with the hostimmune system.[13] The SARS-CoV-2 protein is thought to have a role inimmunomodulation viaimmune evasion or suppressing host immune responses.[4][1][3] It has been reported to be atype I interferon antagonist and to downregulateclass I MHC.[4][3] The SARS-CoV-2 ORF8 protein is highlyimmunogenic and high levels ofantibodies to the protein have been found in patients with or recovered fromCOVID-19.[4][14] A study indicates that ORF8 is atranscription inhibitor.[15][16]
It has been suggested that the SARS-CoV ORF8a protein assembles into multimers and forms aviroporin.[17]
Theevolutionary history of ORF8 is complex. It is among the leastconserved regions of theSarbecovirus genome.[3][2][4] It is subject to frequentmutations and deletions, and has been described as "hypervariable" and arecombination hotspot.[3] It has been suggested thatRNA secondary structures in the region are associated withgenomic instability.[3][19]
In SARS-CoV, the ORF8 region is thought to have originated throughrecombination among ancestralbat coronaviruses.[3][6][5][20] Among the most distinctive features of this region in SARS-CoV is the emergence of a 29-nucleotidedeletion that split the full-lengthopen reading frame into two smaller ORFs, ORF8a and ORF8b.Viral isolates from early in theSARS epidemic have a full-length, intact ORF8, but the split structure emerged later in the epidemic.[3][6] Similar split structures have since been observed inbat coronaviruses.[21] Mutations and deletions have also been seen inSARS-CoV-2 variants.[2][19] Based on observations in SARS-CoV, it has been suggested that changes in ORF8 may be related tohost adaptation, but it is possible that ORF8 does not affectfitness in human hosts.[19][5] In SARS-CoV, a highdN/dS ratio has been observed in ORF8, consistent withpositive selection or withrelaxed selection.[5]
ORF8 encodes a protein whoseimmunoglobulin domain (Ig) has distant similarity to that ofORF7a.[1] It has been suggested that ORF8 likely have evolved from ORF7a throughgene duplication,[2][7][8] though somebioinformatics analyses suggest the similarity may be too low to support duplication, which is relatively uncommon in viruses.[19] Immunoglobulin domains are uncommon in coronaviruses; other than the subset ofbetacoronaviruses with ORF8 and ORF7a, only a small number of batalphacoronaviruses have been identified as containing likely Ig domains, while they are absent fromgammacoronaviruses anddeltacoronaviruses.[2][8] ORF8 is notably absent inMERS-CoV.[8] The beta and alpha Ig domains may be independent acquisitions, where ORF8 and ORF7a may have been acquired from host proteins.[2] It is also possible that the absence of ORF8 reflects gene loss in those lineages.[8]
