This entry includes the ORF8 immunoglobulin (Ig) domain proteins of Bat SARS coronavirus HKU3-1, which have been classified previously as type III ORF8's.ORF8 is an accessory protein that is not shared by all members of subgenus sarbecovirus. The presence and location of ORF8 in the SARS-CoV-2 genome has led its classification with SARS-CoV [, ]. ORF8 is a potential pathogenicity factor which evolves rapidly to counter the immune response and facilitate the transmission between hosts []. ORF8 has been suggested to be one of the relevant genes in the study of human adaptation of the virus [, ].The ORF8 protein is a fast-evolving protein in SARS-related CoVs, with a tendency to recombine and undergo deletions. During the early phases of the SARS (SARS-CoV) epidemic in 2002, human isolates were found to possess a unique continuous ORF8 with 366 nucleotides and a predicted protein with 122 amino acids. During the middle and late phases of the SARS epidemic, two functional ORFs (ORF8a and ORF8b) were emerged; they are predicted to encode two small proteins, 8a with 39 amino acids and 8b with 84 amino acids. Interestingly, SARS-CoV-2 ORF8 has not undergone any significantly measurable deletion events, so its function as a full-length protein might be more important to its pathogenicity []. ORF8 plays a role in modulating host immune response []which may act by down-regulating major histocompatibility complex class I (MHC-I) []. It may inhibit expression of some members of the IFN-stimulated gene (ISG) family including hosts IGF2BP1/ZBP1, MX1 and MX2, and DHX58 []. ORF8 also binds to IL17RA receptor, leading to IL17 pathway activation and an increased secretion of pro-inflammatory factors, contributing to cytokine storm during COVID-19 infection [].
This is a family of unknown function found in SARS and SARS-like coronaviruses. It includes uncharacterised protein 14 from SARS coronavirus 2 (SARS-CoV-2), Human SARS coronavirus (SARS-CoV) and Bat coronavirus Rp3/2004 (SARS-like coronavirus Rp3) []. In SARS-CoV, Orf14 is completely contained within the ORF encoding the nucleocapsid protein (N) []. In SARS-CoV-2 uncharacterised protein 14 is predicted to contain one transmembrane helix.
This is a family of proteins found in SARS and SARS-like coronaviruses. It includes Protein 9b from SARS coronavirus 2 (SARS-CoV-2), Human SARS coronavirus (SARS-CoV) and Bat coronaviruses.Protein 9b is one of 8 accessory proteins in SARS-CoV []. The gene (ORF 9b, also known as ORF13) that encodes this protein is included within the nucleocapsid (N) gene (alternative ORF) []. Data suggest that protein 9b is a structural component of SARS-CoV virions and functions as an unusual lipid binding protein [, ].SARS-CoV ORF-9b has been shown to localise to the outer mitochondrial membrane and to target mitochondrial antiviral signalling proteins (MAVS), suppressing innate immunity [, , ]. Antibodies against SARS-CoV ORF-9b have been found in patients, demonstrating that it is produced during infection [, ].Protein 9b from SARS CoV comprises 98 amino acids. Its structure has a novel fold which forms a dimeric tent-like beta structure with an amphipathic surface, and a central hydrophobic cavity that binds lipidmolecules []. This cavity is likely to be involved in membrane attachment [].Protein 9b is a group-specific protein of SARS coronavirus (CoV). The sequence of ORF-9b is well conserved in different SARS isolates, however, there is little homology between Protein 9b from SARS-CoV and the I protein (Protein 9b homologue) present in other coronaviruses [, , ].
This entry represents the SUD-C domain of Rousettus bat coronavirus (CoV) HKU9 non-structural protein 3 (NSP3) and other NSP3s from betacoronaviruses in the nobecovirus subgenera (D lineage).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 [].
This entry represents the SUD-C of Middle East respiratory syndrome-related (MERS) coronavirus (CoV) NSP3 and other NSP3s from betacoronaviruses in the merbecovirus subgenera (C lineage), including several bat-CoVs such as Tylonycteris bat CoV HKU4, Pipistrellus bat CoV HKU5, and Hypsugo bat CoV HKU25.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 MERS and related bat coronaviruses. Similar to SARS SUD-C, Tylonycteris bat-CoV HKU4 SUD-C (HKU4 C), a member of the MERS SUD-C group, 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 recognise purine bases more strongly than pyrimidine bases; it also regulates the RNA binding behaviour of the SARS SUD-M macrodomain. It is not known whether MERS SUD-C or HKU4 C functions in the same way [, ].
This superfamily represents a domain found in SARS and bat coronaviruses, which is about 70 amino acids in length. PL2pro is a domain of the non-structural protein NSP3, found associated with various other coronavirus proteins due to the polyprotein nature of most viral translation. The domain performs three of the cleavages required to separate the translated polyprotein into its distinct proteins []. Structurally, this domain consists of two α-helices and seven β-strands arranged into an antiparallel β-sheet.
MHV NSP3 contains a DPUP that is located N-terminal to the ubiquitin-like domain 2 (Ubl2) and papain-like protease 2 (PLP2) catalytic domain. It is structurally similar to the Severe Acute Respiratory Syndrome (SARS) CoV unique domain C (SUD-C), adopting a frataxin-like fold that has structural similarity to DNA-binding domains of DNA-modifying enzymes. SUD-C is also located N-terminal to Ubl2 and PLP2 in SARS NSP3, similar to the DPUP of MHV NSP3; however, unlike DPUP, it is preceded by SUD-N and SUD-M macrodomains that are absent in MHV NSP3. Though structurally similar, there is little sequence similarity between DPUP and SUD-C. 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 DPUP functions in the same way [].This entry represents the DPUP (domain preceding Ubl2 and PLP2) of murine hepatitis virus (MHV) non-structural protein 3 (NSP3) and other NSP3s from betacoronaviruses in the embecovirus subgenera (A lineage), including human CoV OC43, rabbit CoV HKU14 and porcine hemagglutinating encephalomyelitis virus (HEV), among others.
This entry includes the ORF8 gene products (also known as NS8, accessory protein 8) from human SARS coronavirus (SARS-CoV), SARS-CoV-2, Bat coronavirus HKU3 and pangolin coronaviruses [].ORF8 is an accessory protein that is not shared by all members of subgenus sarbecovirus. The presence and location of ORF8 in the SARS-CoV-2 genome has led its classification with SARS-CoV [, ]. ORF8 is a potential pathogenicity factor which evolves rapidly to counter the immune response and facilitate the transmission between hosts []. ORF8 has been suggested to be one of the relevant genes in the study of human adaptation of the virus [, ].The ORF8 protein is a fast-evolving protein in SARS-related CoVs, with a tendency to recombine and undergo deletions. During the early phases of the SARS (SARS-CoV) epidemic in 2002, human isolates were found to possess a unique continuous ORF8 with 366 nucleotides and a predicted protein with 122 amino acids. During the middle and late phases of the SARS epidemic, two functional ORFs (ORF8a and ORF8b) were emerged; they are predicted to encode two small proteins, 8a with 39 amino acids and 8b with 84 amino acids. Interestingly, SARS-CoV-2 ORF8 has not undergone any significantly measurable deletion events, so its function as a full-length protein might be more important to its pathogenicity []. ORF8 plays a role in modulating host immune response []which may act by down-regulating major histocompatibility complex class I (MHC-I) []. It may inhibit expression of some members of the IFN-stimulated gene (ISG) family including hosts IGF2BP1/ZBP1, MX1 and MX2, and DHX58 []. ORF8 also binds to IL17RA receptor, leading to IL17 pathway activation and an increased secretion of pro-inflammatory factors, contributing to cytokine storm during COVID-19 infection [].
This subfamily includes the ORF8 immunoglobulin (Ig) domain proteins of bat coronavirus Rf1 (Bat SARS CoV Rf1) and Bat CoV 273/2005, which have been classified previously as type II ORF8 proteins.ORF8 is an accessory protein that is not shared by all members of subgenus sarbecovirus. The presence and location of ORF8 in the SARS-CoV-2 genome has led its classification with SARS-CoV [, ]. ORF8 is a potential pathogenicity factor which evolves rapidly to counter the immune response and facilitate the transmission between hosts []. ORF8 has been suggested to be one of the relevant genes in the study of human adaptation of the virus [, ].The ORF8 protein is a fast-evolving protein in SARS-related CoVs, with a tendency to recombine and undergo deletions. During the early phases of the SARS (SARS-CoV) epidemic in 2002, human isolates were found to possess a unique continuous ORF8 with 366 nucleotides and a predicted protein with 122 amino acids. During the middle and late phases of the SARS epidemic, two functional ORFs (ORF8a and ORF8b) were emerged; they are predicted to encode two small proteins, 8a with 39 amino acids and 8b with 84 amino acids. Interestingly, SARS-CoV-2 ORF8 has not undergone any significantly measurable deletion events, so its function as a full-length protein might be more important to its pathogenicity []. ORF8 plays a role in modulating host immune response []which may act by down-regulating major histocompatibility complex class I (MHC-I) []. It may inhibit expression of some members of the IFN-stimulated gene (ISG) family including hosts IGF2BP1/ZBP1, MX1 and MX2, and DHX58 []. ORF8 also binds to IL17RA receptor, leading to IL17 pathway activation and an increased secretion of pro-inflammatory factors, contributing to cytokine storm during COVID-19 infection [].
This superfamily represents the N-terminal region of the SUD domain (SUD-N or Mac2) found in non-structural protein NSP3, the product of ORF1a in group 2 (beta) coronaviruses. It is found in human SARS-CoV and SARS-CoV-2 polyprotein 1a and 1ab, and in related coronavirus polyproteins [].Non-structural protein Nsp3 contains at least seven different functional modules within its 1922-amino-acid polypeptide chain. One of these is the so-called SARS (severe acute respiratory syndrome)-unique domain (SUD), a stretch of about 338 residues that is completely absent from any other coronavirus. The SUD domain may be responsible for the high pathogenicity of the SARS coronavirus, compared to other viruses of this family [, ]. Later, the NSP3 of MHV was shown by X-ray crystallography to contain a SUD-C-like fold, so it is no longer appropriate to call this domain "SARS-unique". This region has been renamed into "Domain Preceding Ubl2 and PL2pro"(DPUP) []. NSP3 has been shown to bind to viral RNA, nucleocapsid protein, as well as other viral proteins, and participates in polyprotein processing. It is a multifunctional protein comprising up to 16 different domains and regions []. SUD(core) exhibits a two-domain architecture. The N-terminal subdomain (SUD-N) and the C-terminal subdomain of SUDcore, also named middle SUD subdomain, or SUD-M [, ]. SUD-N has been shown to be dispensable for the SARS-CoV replication/transcription complex within the context of a SARS-CoV replicon []. SUD consists of three globular domains separated by short linker peptide segments: SUD-N, SUD-M, and SUD-C []. Among these, SUD-N and SUD-M are macrodomains. The SUD-N domain is a related macrodomain which also binds G-quadruplexes []. While SUD-N is specific to the NSP3 of SARS and betacoronaviruses of the sarbecovirus subgenera (B lineage), SUD-M is present in most NSP3 proteins except the NSP3 from betacoronaviruses of the embecovirus subgenera (A lineage). SUD-M, despite its name, is not specific to SARS. SUD-C adopts a frataxin-like fold, has structural similarity to DNA-binding domains of DNA-modifying enzymes, binds single-stranded RNA, and regulates the RNA binding behavior of the SUD-M macrodomain. SARS-CoV Nsp3 contains a third macrodomain (the X-domain). The X-domain may function as a module binding poly(ADP-ribose); however, SUD-N and SUD-M do not bind ADP-ribose, as the triple glycine sequence involved in its binding is not conserved in these [].
This family contains the SARS coronavirus 3b protein which is predominantly localized in the nucleolus, and induces G0/G1 arrest and apoptosis in transfected cells [, ]. SARS-CoV mRNA 3 encodes the distinct proteins ORF3a and ORF3b proteins, which are translated in different reading frames. SARS-CoV accessory protein ORF3b antagonizes interferon (IFN) function by modulating the activity of IFN regulatory factor 3 (IRF3). The IFN system functions as the first line of defense against viral infection in mammalian cells. Viral infection triggers a series of cellular events that lead to the production of IFN and several downstream antiviral genes, helping to establish an antiviral state. Viruses encode IFN antagonists to counteract the antiviral effects of IFN. SARS-CoV ORF3b, ORF6, and N proteins function as IFN antagonists. ORF3b inhibits both IFN synthesis and signaling [, ]. It localizes to the nucleus in transfected cells [].
This entry corresponds to ORF7b (also known as accessory protein 7b, NS7B, and 7b) from human SARS coronavirus (SARS-CoV) and SARS-CoV-2, which belong to the betacoronavirus B lineage (Sarbecovirus) [].ORF7b/NS7b from betacoronavirus in the B lineage are not related to NS7b proteins from other betacoronavirus lineages. The SARS-CoV ORF7b protein is a highly hydrophobic 43 amino acid protein which is homologous to an accessory but structural component of SARS-CoV virion. While ORF7b is packaged into virions, it is not required for the virus budding process, as gene 7 deletion viruses replicate efficiently in vitro and in vivo. Moreover, ORF7b possesses a transmembrane helical domain (TMD), between 9-29 amino acid residues, is necessary for its Golgi complex localization, as replacing it with the TMD from the human endoprotease furin results in aberrant localization [, ].
This entry represents the EndoU-like endoribonucleases from a variety of organisms that range from viruses to humans. The founding member XendoU is a uridylate-specific, divalent cation-dependent enzyme that produces molecules with 2',3'-cyclic phosphate ends [, ]. The human EndoU, known as PP11, is a endoribonuclease with placental tissue specificity [].In human SARS coronavirus, EndoU (also known as NSP15) is part of the replicase-transcriptase complex that plays important roles in virus replication and transcription. NSP15 is a Uridylate-specific endoribonuclease that cleaves the 5'-polyuridines from negative-sense viral RNA, termed PUN RNA [], either upstream or downstream of uridylates, at GUU or GU to produce molecules with 2',3'-cyclic phosphate ends []. PUN RNA is a CoV MDA5-dependent pathogen-associated molecular pattern (PAMP) [].
All coronaviruses such as the SARS virus and the Middle East Respiratory Syndrome (MERS) virus encode in their genomes at least one papain-like protease (PLpro) enzyme which cleaves the viral replicase polyproteins at three sites releasing non-structural protein NSP1, NSP2 and NSP3. PLpro also possesses deubiquitinating and deISGylating activities. These have been suggested to suppress the host antiviral response by counteracting the post-translational modification of signalling molecules that activate the innate immune response. PLpro is made up of an N-terminal ubiquitin-like domain (found in many ubiquitin-specific proteases or USPs) and a C-terminal catalytic domain containing a right-handed fingers, palm, and thumb domain organisation [, , , ].This superfamily represents the N-terminal ubiquitin-like domain of the papain-like viral protease from alpha and betacoronavirus. It has a β-grasp fold [].
All coronaviruses such as the SARS virus and the Middle East Respiratory Syndrome (MERS) virus encode in their genomes at least one papain-like protease (PLpro) enzyme which cleaves the viral replicase polyproteins at three sites releasing non-structural protein NSP1, NSP2 and NSP3. PLpro also possesses deubiquitinating and deISGylating activities. These have been suggested to suppress the host antiviral response by counteracting the post-translational modification of signalling molecules that activate the innate immune response. PLpro is made up of an N-terminal ubiquitin-like domain (found in many ubiquitin-specific proteases or USPs) and a C-terminal catalytic domain containing a right-handed fingers, palm, and thumb domain organisation [, , , ].This entry represents the thumb domain of the papain-like viral protease from coronavirus. It consists of 6 α-helices in MERS-CoV []and 4 α-helices in SARS-CoV [].
This entry represents the ORF8 immunoglobulin (Ig) domain protein of Severe acute respiratory syndrome (SARS) coronavirus 2 (SARS-CoV-2, also known as a 2019 novel coronavirus, 2019-nCoV) and related Sarbecovirus ORF8 proteins.ORF8 is an accessory protein that is not shared by all members of subgenus sarbecovirus. The presence and location of ORF8 in the SARS-CoV-2 genome has led its classification with SARS-CoV [, ]. ORF8 is a potential pathogenicity factor which evolves rapidly to counter the immune response and facilitate the transmission between hosts []. ORF8 has been suggested to be one of the relevant genes in the study of human adaptation of the virus [, ].The ORF8 protein is a fast-evolving protein in SARS-related CoVs, with a tendency to recombine and undergo deletions. During the early phases of the SARS (SARS-CoV) epidemic in 2002, human isolates were found to possess a unique continuous ORF8 with 366 nucleotides and a predicted protein with 122 amino acids. During the middle and late phases of the SARS epidemic, two functional ORFs (ORF8a and ORF8b) were emerged; they are predicted to encode two small proteins, 8a with 39 amino acids and 8b with 84 amino acids. Interestingly, SARS-CoV-2 ORF8 has not undergone any significantly measurable deletion events, so its function as a full-length protein might be more important to its pathogenicity []. ORF8 plays a role in modulating host immune response []which may act by down-regulating major histocompatibility complex class I (MHC-I) []. It may inhibit expression of some members of the IFN-stimulated gene (ISG) family including hosts IGF2BP1/ZBP1, MX1 and MX2, and DHX58 []. ORF8 also binds to IL17RA receptor, leading to IL17 pathway activation and an increased secretion of pro-inflammatory factors, contributing to cytokine storm during COVID-19 infection [].
The CoV Spike (S) protein is an envelope glycoprotein that plays the most important role in viral attachment, fusion, and entry into host cells, and serves as a major target for the development of neutralizing antibodies, inhibitors of viral entry, and vaccines. It is synthesised as a precursor protein that is cleaved into an N-terminal S1 subunit (~700 amino acids) and a C-terminal S2 subunit (~600 amino acids) that mediates attachment and membrane fusion, respectively. Three S1/S2 heterodimers assemble to form a trimer spike protruding from the viral envelope. The S1 subunit contains a receptor-binding domain (RBD), while the S2 subunit contains a hydrophobic fusion peptide and two heptad repeat regions. S1 contains two structurally independent domains, the N-terminal domain (NTD) and the C-terminal domain (C-domain). Depending on the virus, either the NTD or the C-domain can serve as the receptor-binding domain (RBD). Most CoVs, including SARS-CoV-2, SARS-CoV, and MERS-CoV use the C-domain to bind their receptors. However, CoV such as mouse hepatitis virus (MHV) uses the NTD to bind its receptor, mouse carcinoembryonic antigen related cell adhesion molecule 1a (mCEACAM1a). The S1 NTD contributes to the Spike trimer interface [, , , , ].This entry represents the receptor-binding domain (RDB) of the Spike protein S1 subunit from the porcine hemagglutinating encephalomyelitis virus (HEV), which is related to SARS and MERS betacoronaviruses and is associated with acute outbreaks of wasting and encephalitis in nursing piglets from pig farms. Porcine HEV uses 9-O-acetyl-sialic acid (9-O-Ac-Sia) as a receptor, like the closely related HCoV-OC43 and HCoV-HKU1 viruses [].
The multi-domain non-structural protein NSP3 is the largest protein encoded by the coronavirus (CoV) genome, with an average molecular mass of about 200 kD. While some of the domains differ between CoV genera, eight domains of NSP3 exist in all known CoVs: the ubiquitin-like domain 1 (Ubl1), the Glu-rich acidic domain (also called "hypervariable region"), a macrodomain (also named "X domain"), the ubiquitin-like domain 2 (Ubl2), the papain-like protease 2 (PL2pro), the NSP3 ectodomain (3Ecto, also called "zinc-finger domain"), as well as the domains Y1 and CoV-Y of unknown functions. There are also two transmembrane regions, TM1 and TM2, which exist in all CoVs [].SUD consists of three globular domains separated by short linker peptide segments: SUD-N, SUD-M, and SUD-C []. Among these, SUD-N and SUD-M are macrodomains. The SUD-N domain is a related macrodomain which also binds G-quadruplexes []. While SUD-N is specific to the NSP3 of SARS and betacoronaviruses of the sarbecovirus subgenera (B lineage), SUD-M is present in most NSP3 proteins except the NSP3 from betacoronaviruses of the embecovirus subgenera (A lineage). SUD-M, despite its name, is not specific to SARS. SUD-C adopts a frataxin-like fold, has structural similarity to DNA-binding domains of DNA-modifying enzymes, binds single-stranded RNA, and regulates the RNA binding behavior of the SUD-M macrodomain. SARS-CoV Nsp3 contains a third macrodomain (the X-domain). The X-domain may function as a module binding poly(ADP-ribose); however, SUD-N and SUD-M do not bind ADP-ribose, as the triple glycine sequence involved in its binding is not conserved in these [].
Coronaviruses (CoVs) have a similar genomic structure and encodes four structural proteins (S, E, M and N) and a variable number of accessory proteins. Accessory proteins play an important role in virus-host interactions, especially in antagonizing or regulating host immunity and virus adaptation to the host. There are large variations in the number of accessory proteins (1-10) among coronaviruses. BetaCoVs have 3-5 accessory proteins, except for SARS-CoV and SARS-CoV-2, which possess the largest number of accessory proteins among all coronaviruses (10 and 9, respectively). ORF8 is the most variable accessory protein among those encoded by SARS related coronaviruses (SARSr-CoVs) and isnot shared by all members of subgenus Sarbecovirus. SARSr ORF8 accessory proteins are characterized by the presence of an N-terminal hydrophobic signal peptide, a conserved N-glycosylation site, and enough cysteine residues with the potential to form disulfide bonds, drawing their picture as structurally stable potential ER-resident proteins. There is functional overlap between these proteins with involvement in immune modulation, which is probably accomplished through involvement in protein quality control. When ORF8 is exogenously overexpressed in cells, it disrupts IFN-I signaling. Unlike ORF8a/b of SARS-CoV, the SARS-CoV-2 ORF8 downregulates MHC-I in cells.This entry represents the immunoglobulin (Ig)-like domain from SARSr ORF8 [, , , , ].
This entry represents a macrodomain referred to as SUD-N (N-terminal subdomain) of the SARS-unique domain (SUD) which binds G-quadruplexes (unusual nucleic-acid structures formed by consecutive guanosine nucleotides). It is found in the non-structural protein 3 (NSP3) of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and highly related coronaviruses [].SUD consists of three globular domains separated by short linker peptide segments: SUD-N, SUD-M, and SUD-C []. Among these, SUD-N and SUD-M are macrodomains. The SUD-N domain is a related macrodomain which also binds G-quadruplexes []. While SUD-N is specific to the NSP3 of SARS and betacoronaviruses of the sarbecovirus subgenera (B lineage), SUD-M is present in most NSP3 proteins except the NSP3 from betacoronaviruses of the embecovirus subgenera (A lineage). SUD-M, despite its name, is not specific to SARS. SUD-C adopts a frataxin-like fold, has structural similarity to DNA-binding domains of DNA-modifying enzymes, binds single-stranded RNA, and regulates the RNA binding behavior of the SUD-M macrodomain. SARS-CoV Nsp3 contains a third macrodomain (the X-domain). The X-domain may function as a module binding poly(ADP-ribose); however, SUD-N and SUD-M do not bind ADP-ribose, as the triple glycine sequence involved in its binding is not conserved in these [].
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains []. Both domains contribute to the binding of viral RNA genome [, ].This entry represents the C-terminal domain of the nucleocapsid proteins from Coronavirus. The C-terminal domain of the N protein (N-CTD) is involved in dimerization, and is thus, also called the dimerization domain [, ]. Structurally, the C-terminal domain forms a tightly intertwined dimer with an intermolecular four-stranded central β-sheet platform flanked by alpha helices, indicating that the basic building block for coronavirus nucleocapsid formation is a dimeric assembly of N protein [].
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains []. Both domains contribute to the binding of viral RNA genome [, ].This entry represents the N-terminal domain of the nucleocapsid (N) protein predominantly from Coronavirus. It exhibits a U-shaped structure, with two arms rich in basic residues, providing a module for specific interaction with RNA [, ]. The overall structure of the N-terminal domain found in SARS-CoV-2 is similar to other reported coronavirus nucleocapsid protein N-terminal domains, but the surface charge distribution patterns are different. This domain also interacts with the viral membrane protein during virion assembly and plays a critical role in enhancing the efficiency of virus transcription and assembly. It has been identified as an important drug target [].
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains []. Both domains contribute to the binding of viral RNA genome [, ].This entry represents the N-terminal domain of the nucleocapsid (N) protein predominantly from Coronavirus. It exhibits a U-shaped structure, with two arms rich in basic residues, providing a module for specific interaction with RNA [, ]. The overall structure of the N-terminal domain found in SARS-CoV-2 is similar to other reported coronavirus nucleocapsid protein N-terminal domains, but the surface charge distribution patterns are different. This domain also interacts with the viral membrane protein during virion assembly and plays a critical role in enhancing the efficiency of virus transcription and assembly. It has been identified as an important drug target [].
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains []. Both domains contribute to the binding of viral RNA genome [, ].This entry represents the C-terminal domain of the nucleocapsid proteins from Coronavirus and Arterivirus. The C-terminal domain of the N protein (N-CTD) is involved in dimerization, and is thus, also called the dimerization domain []. Structurally, the C-terminal domain forms a tightly intertwined dimer with an intermolecular four-stranded central β-sheet platform flanked by alpha helices, indicating that the basic building block for coronavirus nucleocapsid formation is a dimeric assembly of N protein [].
The CoV Spike (S) protein is an envelope glycoprotein that plays the most important role in viral attachment, fusion, and entry into host cells, and serves as a major target for the development of neutralizing antibodies, inhibitors of viral entry, and vaccines. It is synthesised as a precursor protein that is cleaved into an N-terminal S1 subunit (~700 amino acids) and a C-terminal S2 subunit (~600 amino acids) that mediates attachment and membrane fusion, respectively. Three S1/S2 heterodimers assemble to form a trimer spike protruding from the viral envelope. The S1 subunit contains a receptor-binding domain (RBD), while the S2 subunit contains a hydrophobic fusion peptide and two heptad repeat regions. S1 contains two structurally independent domains, the N-terminal domain (NTD) and the C-terminal domain (C-domain). Depending on the virus, either the NTD or the C-domain can serve as the receptor-binding domain (RBD). Most CoVs, including SARS-CoV-2, SARS-CoV, and MERS-CoV use the C-domain to bind their receptors. However, CoV such as mouse hepatitis virus (MHV) uses the NTD to bind its receptor, mouse carcinoembryonic antigen related cell adhesion molecule 1a (mCEACAM1a). The S1 NTD contributes to the Spike trimer interface [, , , , ].This domain corresponds to the receptor binding domain (RBD) of the Spike S1 subunit from human coronavirus (CoV) HKU1, isolates N5 and N2. HKU1 is a human lineage A betacoronavirus that causes mild yet prevalent respiratory disease, and is related to the zoonotic SARS and MERS betacoronaviruses. These viruses use 9-O-acetyl-sialic acid (9-O-Ac-Sia) as a receptor which is terminally linked to oligosaccharides decorating glycoproteins and gangliosides at the host cell surface [, ].
This entry represents the N-terminal region of the SUD domain (SUD-N or Mac2) found in non-structural protein NSP3, the product of ORF1a in group 2 (beta) coronaviruses. It is found in human SARS-CoV and SARS-CoV-2 polyprotein 1a and 1ab, and in related coronavirus polyproteins [].SUD consists of three globular domains separated by short linker peptide segments: SUD-N, SUD-M, and SUD-C []. Among these, SUD-N and SUD-M are macrodomains. The SUD-N domain is a related macrodomain which also binds G-quadruplexes []. While SUD-N is specific to the NSP3 of SARS and betacoronaviruses of the sarbecovirus subgenera (B lineage), SUD-M is present in most NSP3 proteins except the NSP3 from betacoronaviruses of the embecovirus subgenera (A lineage). SUD-M, despite its name, is not specific to SARS. SUD-C adopts a frataxin-like fold, has structural similarity to DNA-binding domains of DNA-modifying enzymes, binds single-stranded RNA, and regulates the RNA binding behavior of the SUD-M macrodomain. SARS-CoV Nsp3 contains a third macrodomain (the X-domain). The X-domain may function as a module binding poly(ADP-ribose); however, SUD-N and SUD-M do not bind ADP-ribose, as the triple glycine sequence involved in its binding is not conserved in these [].Nsp3c-N and Nsp3c-M each display a typical α/β/α Macro domain fold, in spite of the complete absence of sequence similarities. The central β-sheet with six β-strands in the order β1-β6-β5-β2-β4-β3 is flanked by two (or three) helices on either side. Only the last strand, β3, is antiparallel to the other strands. Currently, most known functions of Nsp3c-N/M are connected with RNA binding. All the residues important for binding ADP-ribose and for de-MARylation/de-PARylation activity are not conserved in Nsp3c-N/M; therefore Nsp3c-N/M cannot bind ADP-ribose. Both Nsp3c-N and Nsp3c-M domains bind unusual nucleic acid structures formed by consecutives guanosine nucleotides, where four strands of nucleic acid are forming a superhelix (so-called G-quadruplexes) [, , , , ].
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains.Domains N1b and N2b are linked by another domain N2a that contains an SR-rich region (rich in serine and arginine residues). A priming phosphorylation of specific serine residues by an as yet unknown kinase, triggers the subsequent phosphorylation by the host glycogen synthase kinase-3 (GSK-3) of several residues in the SR-rich region. This phosphorylation allows the N protein to associate with the RNA helicase DDX1 permitting template read-through, and enabling the transition from discontinuous transcription of subgenomic mRNAs (sgmRNAs) to continuous synthesis of longer sgmRNAs and genomic RNA (gRNA). Production of gRNA in the presence of N oligomers may promote the formation of ribonucleoprotein complexes, and the newly transcribed sgmRNA would guarantee efficient synthesis of structural proteins [, , ].It has been shown that N proteins interact with nonstructural protein 3 (NSP3) and thus are recruited to the replication-transcription complexes (RTCs). In MHV, the N1b and N2a domains mediate the binding to NSP3 in a gRNA-independent manner. At the RTCs, the N protein is required for the stimulation of gRNA replication and sgmRNA transcription. It remains unclear, however, how and why the N protein orchestrates viral RNA synthesis. The cytoplasmic N-terminal ubiquitin-like domain of NSP3 and the SR-rich region of the N2a domain of the N protein may be important for this interaction. The direct association of N protein with RTCs is a critical step for MHV infection [].This entry represents the nucleocapsid protein from Betacoronavirus.
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains.Domains N1b and N2b are linked by another domain N2a that contains an SR-rich region (rich in serine and arginine residues). A priming phosphorylation of specific serine residues by an as yet unknown kinase, triggers the subsequent phosphorylation by the host glycogen synthase kinase-3 (GSK-3) of several residues in the SR-rich region. This phosphorylation allows the N protein to associate with the RNA helicase DDX1 permitting template read-through, and enabling the transition from discontinuous transcription of subgenomic mRNAs (sgmRNAs) to continuous synthesis of longer sgmRNAs and genomic RNA (gRNA). Production of gRNA in the presence of N oligomers may promote the formation of ribonucleoprotein complexes, and the newly transcribed sgmRNA would guarantee efficient synthesis of structural proteins [, , ].It has been shown that N proteins interact with nonstructural protein 3 (NSP3) and thus are recruited to the replication-transcription complexes (RTCs). In MHV, the N1b and N2a domains mediate the binding to NSP3 in a gRNA-independent manner. At the RTCs, the N protein is required for the stimulation of gRNA replication and sgmRNA transcription. It remains unclear, however, how and why the N protein orchestrates viral RNA synthesis. The cytoplasmic N-terminal ubiquitin-like domain of NSP3 and the SR-rich region of the N2a domain of the N protein may be important for this interaction. The direct association of N protein with RTCs is a critical step for MHV infection [].The entry represents the Coronavirus nucleocapsid protein.
This entry contains coronavirus (CoV) cysteine endopeptidases that belong to MEROPS peptidase family C16 (subfamilies C16A and C16B, clan CA). These peptidases are involved in viral polyprotein processing, releasing NSP1, NSP2 and NSP3 proteins []and they also function as deubiquitinating and deISG15ylating (interferon-induced gene 15) enzymes, disrupting host viral immune response to facilitate viral proliferation and replication. Therefore, this is an important target to develop antiviral treatments [].All coronaviruses encode between one and two accessory cysteine proteinases that recognise and process one or three sites in the amino-terminal half of the replicase polyprotein during assembly of the viral replication complex. MHV, HCoV and TGEV encode two accessory proteinases, called coronavirus papain-like proteinase 1 and 2 (PL1-PRO and PL2-PRO) []. IBV and SARS encodes only one called PL-PRO (PL2-PRO, conserved in all CoVs) [, , ]. The structures of both PL-PROs are similar and they also have restricted specificities. The PL1-PRO of TGEV cleaves the polyprotein between Nsp2-Nsp3 recognising the Lys-Met-Gly-Gly motif, and recognises Leu-Arg-Gly-Gly in ubiquitin (ub) which shows that it is able to accommodate residues as different as Lys and Leu. In contrast, PL-PRO from SARS-CoV recognises Leu-Xaa-Gly-Gly (Xaa could be any amino acid) and cleaves peptide bonds between Nsp1-Nsp2, Nsp2-Nsp3 and between Nsp3-Nsp4 [, , ]. In Ub and ISG15 proteins, it recognises Leu-Arg-Gly-Gly motifs. SARS-CoV and SARS-CoV-2 are closely related but exhibit different host substrate preferences: SARS-CoV-2 PL-PRO preferentially cleaves the ubiquitin-like ISG15, whereas SARS-CoV PL-PRO predominantly targets ubiquitin chains [, ].The peptidase family C16 domain is about 260 amino acids in length and the solved structures determined that it consists of thumb, palm, and fingers subdomains. The thumb is comprised of six α-helices and a small β-hairpin; the fingers subdomain is made of six β-strands and two α-helices and includes a zinc binding site, in which the zinc ion is coordinated by four cysteine residues. Zinc binding is essential for structural integrity and protease activity, with a conformation that varies most between different PL-PRO structures. The palm subdomain is comprised of six β-strands and includes the catalytic residues Cys-His-Asp, located at the interface between the thumb and palm subdomains [].
This entry represents the macrodomain referred to as SUD-M (middle SUD subdomain) of the SARS-unique domain (SUD) which binds G-quadruplexes (unusual nucleic-acid structures formed by consecutive guanosine nucleotides) []. It can be found in non-structural protein 3 (NSP3) of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and related coronaviruses []. NSP3 binds to viral RNA, nucleocapsid protein, as well as other viral proteins, and participates in polyprotein processing. It is a multifunctional protein comprising up to 16 different domains and regions []. In SARS-CoV the SUD-M (527-651) domain has been shown to bind single-stranded poly(A). It has been shown through the contact area with this RNA on the protein surface, and the electrophoretic mobility shift assays, that SUD-M has higher affinity for purine bases than for pyrimidine bases [].SUD consists of three globular domains separated by short linker peptide segments: SUD-N, SUD-M, and SUD-C []. Among these, SUD-N and SUD-M are macrodomains. The SUD-N domain is a related macrodomain which also binds G-quadruplexes []. While SUD-N is specific to the NSP3 of SARS and betacoronaviruses of the sarbecovirus subgenera (B lineage), SUD-M is present in most NSP3 proteins except the NSP3 from betacoronaviruses of the embecovirus subgenera (A lineage). SUD-M, despite its name, is not specific to SARS. SUD-C adopts a frataxin-like fold, has structural similarity to DNA-binding domains of DNA-modifying enzymes, binds single-stranded RNA, and regulates the RNA binding behavior of the SUD-M macrodomain. SARS-CoV Nsp3 contains a third macrodomain (the X-domain). The X-domain may function as a module binding poly(ADP-ribose); however, SUD-N and SUD-M do not bind ADP-ribose, as the triple glycine sequence involved in its binding is not conserved in these [].Nsp3c-N and Nsp3c-M each display a typical α/β/α Macro domain fold, in spite of the complete absence of sequence similarities. The central β-sheet with six β-strands in the order β1-β6-β5-β2-β4-β3 is flanked by two (or three) helices on either side. Only the last strand, β3, is antiparallel to the other strands. Currently, most known functions of Nsp3c-N/M are connected with RNA binding. All the residues important for binding ADP-ribose and for de-MARylation/de-PARylation activity are not conserved in Nsp3c-N/M; therefore Nsp3c-N/M cannot bind ADP-ribose. Both Nsp3c-N and Nsp3c-M domains bind unusual nucleic acid structures formed by consecutives guanosine nucleotides, where four strands of nucleic acid are forming a superhelix (so-called G-quadruplexes) [, , , , ].
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains.Domains N1b and N2b are linked by another domain N2a that contains an SR-rich region (rich in serine and arginine residues). A priming phosphorylation of specific serine residues by an as yet unknown kinase, triggers the subsequent phosphorylation by the host glycogen synthase kinase-3 (GSK-3) of several residues in the SR-rich region. This phosphorylation allows the N protein to associate with the RNA helicase DDX1 permitting template read-through, and enabling the transition from discontinuous transcription of subgenomic mRNAs (sgmRNAs) to continuous synthesis of longer sgmRNAs and genomic RNA (gRNA). Production of gRNA in the presence of N oligomers may promote the formation of ribonucleoprotein complexes, and the newly transcribed sgmRNA would guarantee efficient synthesis of structural proteins [, , ].It has been shown that N proteins interact with nonstructural protein 3 (NSP3) and thus are recruited to the replication-transcription complexes (RTCs). In MHV, the N1b and N2a domains mediate the binding to NSP3 in a gRNA-independent manner. At the RTCs, the N protein is required for the stimulation of gRNA replication and sgmRNA transcription. It remains unclear, however, how and why the N protein orchestrates viral RNA synthesis. The cytoplasmic N-terminal ubiquitin-like domain of NSP3 and the SR-rich region of the N2a domain of the N protein may be important for this interaction. The direct association of N protein with RTCs is a critical step for MHV infection [].This entry represents the nucleocapsid protein from gammacoronavirus.
The Nucleocapsid (N) protein is a highly immunogenic phosphoprotein also implicated in viral genome replication and in modulating cell signalling pathways. The N protein interacts with genomic and subgenomic RNA molecules. Together with the envelope protein M, it participates in genome condensation and packaging. The N protein is a highly immunogenic and abundantly expressed protein during infection, capable of inducing protective immune responses against SARS-CoV and SARS-CoV-2 [, , , , ].Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains.DomainsN1b and N2b are linked by another domain N2a that contains an SR-rich region (rich in serine and arginine residues). A priming phosphorylation of specific serine residues by an as yet unknown kinase, triggers the subsequent phosphorylation by the host glycogen synthase kinase-3 (GSK-3) of several residues in the SR-rich region. This phosphorylation allows the N protein to associate with the RNA helicase DDX1 permitting template read-through, and enabling the transition from discontinuous transcription of subgenomic mRNAs (sgmRNAs) to continuous synthesis of longer sgmRNAs and genomic RNA (gRNA). Production of gRNA in the presence of N oligomers may promote the formation of ribonucleoprotein complexes, and the newly transcribed sgmRNA would guarantee efficient synthesis of structural proteins [, , ].It has been shown that N proteins interact with nonstructural protein 3 (NSP3) and thus are recruited to the replication-transcription complexes (RTCs). In MHV, the N1b and N2a domains mediate the binding to NSP3 in a gRNA-independent manner. At the RTCs, the N protein is required for the stimulation of gRNA replication and sgmRNA transcription. It remains unclear, however, how and why the N protein orchestrates viral RNA synthesis. The cytoplasmic N-terminal ubiquitin-like domain of NSP3 and the SR-rich region of the N2a domain of the N protein may be important for this interaction. The direct association of N protein with RTCs is a critical step for MHV infection [].This entry represents the nucleocapsid protein from alphacoronavirus.
This entry represents a domainfound in betacoronavirus cysteine endopeptidases that belong to MEROPS peptidase families C30 (clan PA) and C16 (subfamiles C16A and C16B, clan CA). These peptidase are involved in viral polyprotein processing. All coronaviruses encodes between one and two accessory cysteine proteinases that recognise and process one or two sites in the amino-terminal half of the replicase polyprotein during assembly of the viral replication complex. MHV, HCoV and TGEV encode two accesssory proteinases, called coronavirus papain-like proteinase 1 and 2 (PL1-PRO and PL2-PRO). IBV and SARS encodes only one called PL-PRO []. Coronavirus papain-like proteinases 1 and 2 have restricted specificities, cleaving respectively two and one bond(s)in the polyprotein. This restricted activity may be due to extended specificity sites: Arg or Lys at the cleavage site position P5 are required for PL1-PRO [], and Phe at the cleavage site position P6 is required for PL2-PRO []. PL1-PRO releases p28 and p65 from the N terminus of the polyprotein; PL2-PRO cleaves between p210 and p150. A cysteine peptidase is a proteolytic enzyme that hydrolyses a peptide bond using the thiol group of a cysteine residue as a nucleophile. Hydrolysis involves usually a catalytic triad consisting of the thiol group of the cysteine, the imidazolium ring of a histidine, and a third residue, usually asparagine or aspartic acid, to orientate and activate the imidazolium ring. In only one family of cysteine peptidases, is the role of the general base assigned to a residue other than a histidine: in peptidases from family C89 (acid ceramidase) an arginine is the general base. Cysteine peptidases can be grouped into fourteen different clans, with members of each clan possessing a tertiary fold unique to the clan. Four clans of cysteine peptidases share structural similarities with serine and threonine peptidases and asparagine lyases. From sequence similarities, cysteine peptidases can be clustered into over 80 different families []. Clans CF, CM, CN, CO, CP and PD contain only one family.Cysteine peptidases are often active at acidic pH and are therefore confined to acidic environments, such as the animal lysosome or plant vacuole. Cysteine peptidases can be endopeptidases, aminopeptidases, carboxypeptidases, dipeptidyl-peptidases or omega-peptidases. They are inhibited by thiol chelators such as iodoacetate, iodoacetic acid, N-ethylmaleimide or p-chloromercuribenzoate.Clan CA includes proteins with a papain-like fold. There is a catalytic triad which occurs in the order: Cys/His/Asn (or Asp). A fourth residue, usually Gln, is important for stabilising the acyl intermediate that forms during catalysis, and this precedes the active site Cys. The fold consists of two subdomains with the active site between them. One subdomain consists of a bundle of helices, with the catalytic Cys at the end of one of them, and the other subdomain is a β-barrel with the active site His and Asn (or Asp). There are over thirty families in the clan, and tertiary structures have been solved for members of most of these. Peptidases in clan CA are usually sensitive to the small molecule inhibitor E64, which is ineffective against peptidases from other clans of cysteine peptidases [].Clan CD includes proteins with a caspase-like fold. Proteins in the clan have an α/β/α sandwich structure. There is a catalytic dyad which occurs in the order His/Cys. The active site His occurs in a His-Gly motif and the active site Cys occurs in an Ala-Cys motif; both motifs are preceded by a block of hydrophobic residues []. Specificity is predominantly directed towards residues that occupy the S1 binding pocket, so that caspases cleave aspartyl bonds, legumains cleave asparaginyl bonds, and gingipains cleave lysyl or arginyl bonds.Clan CE includes proteins with an adenain-like fold. The fold consists of two subdomains with the active site between them. One domain is a bundle of helices, and the other a β-barrell. The subdomains are in the opposite order to those found in peptidases from clan CA, and this is reflected in the order of active site residues: His/Asn/Gln/Cys. This has prompted speculation that proteins in clans CA and CE are related, and that members of one clan are derived from a circular permutation of the structure of the other.Clan CL includes proteins with a sortase B-like fold. Peptidases in the clan hydrolyse and transfer bacterial cell wall peptides. The fold shows a closed β-barrel decorated with helices with the active site at one end of the barrel []. The active site consists of a His/Cys catalytic dyad.Cysteine peptidases with a chymotrypsin-like fold are included in clan PA, which also includes serine peptidases. Cysteine peptidases that are N-terminal nucleophile hydrolases are included in clan PB. Cysteine peptidases with a tertiary structure similar to that of the serine-type aspartyl dipeptidase are included in clan PC. Cysteine peptidases with an intein-like fold are included in clan PD, which also includes asparagine lyases.
