This entry represents the catalytic domain of kexin, furin and related proteins []. Protein convertases, whose members include furin (MEROPS identifier S08.071) and kexin (S08.070), are members of the peptidase S8 or subtilase family of peptidases []. Kexins are involved in the activation of peptide hormones, growth factors, and viral proteins. Furin cleaves cell surface vasoactive peptides and proteins involved in cardiovascular tissue remodeling in the TGN, at cell surface, or in endosomes but rarely in the ER. Furin also plays a key role in blood pressure regulation though the activation of transforming growth factor (TGF)-beta. High specificity is seen for cleavage after dibasic (Lys-Arg or Arg-Arg) or multiple basic residues in protein convertases [].The subtilisin family is one of the largest serine peptidase families characterised to date. Over 200 subtilises are presently known, more than 170 of which with their complete amino acid sequence []. It is widespread, being found in eubacteria, archaebacteria, eukaryotes and viruses []. The vast majority of the family are endopeptidases, although there is an exopeptidase, tripeptidyl peptidase [, ]. Structures have been determined for several members of the subtilisin family: they exploit the same catalytic triad as the chymotrypsins, although the residues occur in a different order (HDS in chymotrypsin and DHS in subtilisin), but the structures show no other similarity [, ]. Some subtilisins are mosaic proteins, while others contain N- and C-terminal extensions that show no sequence similarity to any other known protein [].
Coronovirus enter target cells through fusion of viral and cellular membranesmediated by the viral envelope glycoprotein S. Trimers of Coronovirus S glycoprotein constitute the typical viral spikes. The precursor S protein is processed by host cell furin or furin-like protease to yield the mature S1 and S2 proteins [].This entry represents the spike glycoprotein from Alphacoronavirus.
The envelope glycoprotein Gp160 of HIV1 and simian immunodeficiency virus (SIV) is cleaved into the surface protein Gp120 and the transmembrane protein Gp41 []. This entry represents the full length Gp160. Gp160 oligomerizes in the host endoplasmic reticulum into predominantly trimers. In a second time, Gp160 transits in the host Golgi, where glycosylation is completed. The precursor is then proteolytically cleaved in the trans-Golgi and thereby activated by cellular furin or furin-like proteases to produce Gp120 and Gp41 [].
This domain, found in Pseudomonas aeruginosa exotoxin A, is responsible for transmembrane targeting of the toxin, as well as transmembrane translocation of the catalytic domain into the cytoplasmic compartment. A furin cleavage site is present within the domain: cleavage generates a 37kDa carboxy-terminal fragment, which includes the enzymatic domain, which is then is translocated into the cytoplasm. Itadopts a helical structure, with six α-helices forming a bundle []. Proteins contain this domain include exotoxin A from Pseudomonas aeruginosa and cholix toxin from Vibrio cholerae []. They are NAD-dependent ADP-ribosyltransferases (ADPRT) that catalyses the transfer of the ADP-ribosyl moiety of oxidized NAD onto eukaryotic elongation factor 2 (eEF-2) thus arresting protein synthesis [].
Prohormone convertases (PCs) 1 and 2 are a family of eukaryotic subtilisins thought to mediate the proteolytic cleavage of many peptide precursors [].This family represents proSAAS, which belongs to MEROPS inhibitor family I49. It is a neuroendocrine secretory protein which is a potent endogenous PC1 inhibitor [, ]. ProSAAS consists of an N-terminal domain and a C-terminal domain separated by a furin cleavage site. The inhibitory region of proSAAS is located in the C-terminal domain and includes the LLRVKR hexapeptide sequence [, , ]. Furthermore, ProSAAS has been shown to function as an amyloid anti-aggregant chaperon in Alzheimer's disease []. ProSAAS-derived peptides are involved in several physiologically important processes [, , ].
This domain, found in Pseudomonas aeruginosa exotoxin A, is responsible for transmembrane targeting of the toxin, as well as transmembrane translocation of the catalytic domain into the cytoplasmic compartment. A furin cleavage site is present within the domain: cleavage generates a 37kDa carboxy-terminal fragment, which includes the enzymatic domain, which is then is translocated into the cytoplasm. It adopts a helical structure, with six α-helices forming a bundle []. Proteins contain this domain include exotoxin A from Pseudomonas aeruginosa and cholix toxin from Vibrio cholerae []. They are NAD-dependent ADP-ribosyltransferases (ADPRT) that catalyses the transfer of the ADP-ribosyl moiety of oxidized NAD onto eukaryotic elongation factor 2 (eEF-2) thus arresting protein synthesis [].
ADAMDEC1 peptidase (also known as decysin; MEROPS identifier M12.219) unusually for an ADAM peptidase lacks the disintegrin and ADAM-CR domains []. It is the only ADAM metallopeptidase in which the third zinc ligand is an aspartic acid rather than a histidine []. It is synthesized as a precursor and activated by furin []. The active enzyme has limited specificity for Leu in P1' []. ADAMDEC1 was initially thought to be involved in the immune response, because it was found in mature spleen follicular dendritic cells and germinal centres [], but it is also found in uterine stromal cells []and transcripts are highly expressed in pulmonary sarcoidosis [].
Yeast Vps10p is a receptor for sorting and transport of the soluble vacuolar hydrolase carboxypeptidase Y to the lysosome-like vacuole []. In mammalian cells, proteins containing this domain are involved in the transport of lipoproteins and sorting of endosomal proteins. They may also act as receptors for some neuropeptides.The N terminus of murine brain SorCS contains two putativecleavage sites for the convertase furin which mark the beginning of the VPS10domain, which is followed bya module of imperfect leucine-rich repeats and a transmembrane domain. Theshortintracellular C terminus contains consensus signals for rapidinternalization. The identifiedputative binding motifs for SH2 and SH3 domains are unique in the family ofVPS10 domainreceptors. SorCS is predominantly expressed in brain, but also in heart,liver, and kidney.SorCS transcripts detected by in situ hybridization in the murine centralnervous system pointto a neuronal expression [, ].
This entry contains the ORF7b, also called NS7b, of Severe Acute Respiratory Syndrome coronaviruses (SARS-CoVs) and related betacoronaviruses identified in Chinese horseshoe bats, including bat SARS-like-CoV WIV1 and HKU3.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 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 [, ].
Prohormone convertases (PCs) 1 and 2 are a family of eukaryotic subtilisins thought to mediate the proteolytic cleavage of many peptide precursors []. Protein 7B2 (secretogranin V) functions as a molecular chaperone for PC2, preventing its premature activation in the regulated secretory pathway []. 7B2 represents a potent inhibitor of PC2, and it is also required for the activation of PC2, which is synthesised as a zymogen. Protein 7B2 has an N-terminalactivation domain and a C-terminal (CT) inhibitory domain (MEROPS inhibitor family I21), separated by a furin cleavage site []. 7B2 is synthesised as a precursor protein that is cleaved into the N-terminal fragment and the C-terminal peptide [].7B2 has been linked to neurodegenerative diseases. It may act as an anti-aggregation chaperone []. However, in an APP model mouse 7B2 knockout has been shown to reduce rather than enhance plaque burden [].
The family of type 2 integral membrane protein (ITM2) consists of three members, ITM2A, ITM2B and ITM2C []. Members of this family contain a BRICHOS domain, which possesses chaperone activity [,].ITM2A is a target gene of GATA-3, a T cell-specific transcription factor [].ITM2B is proteolytically cleaved at three locations; cleavage by furin in the C-terminal region generates a 23-residue peptide (ABri23), processing by ADAM10 results in release of the BRICHOS domain from the membrane-bound N-terminal part and intramembrane cleavage by SPPL2a/2b liberates the intracellular domain []. ITM2B (BRI2) is a target of BCL6 transcriptional repression and a short form of the ITM2B protein, similarly to other targets of BCL6, induces apoptosis in hematopoietic cell lines []. ITM2B interacts with amyloid precursor protein (APP) and regulates amyloid beta (Abeta) production [, ]. ITM2C (BRI3) also inhibits amyloid precursor protein processing, but through a different mechanism to ITM2B [].
ADAM19 peptidase (also known as meltrin beta; MEROPS identifier M12.214) is located in the Golgi. It sheds the ectodomain of NRG1beta1 from the Golgi membranes []and also sheds other proteins, such as heparin-binding epidermal growth factor, tumour necrosis factor and the amyloid precursor protein (APP) []. Because the peptides released from APP do not form amyloid deposits, ADAM19 may have a protective role in Alzheimer's disease []. ADAM19 is a multidomain protein with a propeptide, a metallopeptidase, a disintegrin, and a cysteine-rich domain, followed by a transmembrane region and a cytoplasmic tail. The propeptide is removed by furin []. The cytoplasmic tail interacts with the SH3 domain of Tks5(I) []. The ADAM19 gene is expressed in muscle and nerve cells during embryonic development and a short isoform is found in dorsal root ganglia []. The gene is also over-expressed in several human cancers and cancer cell lines including astrocytomas and glioblastomas []and in the placenta, where ADAM19 may reduce cell invasion [].
ADAM15 peptidase (or metargidin; MEROPS identifier M12.215) is a sheddase-like metalloendopeptidase with a propeptide, a metallopeptidase domain, a disintegrin domain, an ADAM-CR domain, a transmembrane region and a proline-rich cytoplasmic tail. It probably exists naturally in a heterodimer with an unidentified, 56kDa protein with which it immunoprecipitates []. The precursor is correctly processed by furin []. ADAM15 sheds the ectodomain of fibroblast growth factor 2iiib []. In human (but not mouse) the disintegrin domain contains an RGD motif and binding to integrins has been demonstrated []. Because platelets bind to ADAM15 and are activated, clotting can occur, and ADAM15 is a therapeutic target for cardiovascular disease []. The gene is overexpressed in several human cancers and has roles in pathological neovascularization and prostate cancer metastasis []. A shorter spliced form with a different cytoplasmic tail containing an Src-binding site has been isolated from cancer cells [, ].
The type I glycoprotein S of Coronavirus, trimers of which constitute the typical viral spikes, is assembled into virions through noncovalent interactions with the M protein. The spike glycoprotein is translated as a large polypeptide that is subsequently cleaved to S1 () and S2 []. The cleavage of S can occur at two distinct sites: S2 or S2' []. The spike is present in two very different forms: pre-fusion (the form on mature virions) and post-fusion (the form after membrane fusion has been completed). The spike is cleaved sequentially by host proteases at two sites: first at the S1/S2 boundary (i.e. S1/S2 site) and second within S2 (i.e. S2' site). After the cleavages, S1 dissociates from S2, allowing S2 to transition to the post-fusion structure []. Both chimeric S proteins appeared to cause cell fusion when expressed individually, suggesting that they were biologically fully active []. The spike is a type I membrane glycoprotein that possesses a conserved transmembrane anchor and an unusual cysteine-rich (cys) domain that bridges the putative junction of the anchor and the cytoplasmic tail [].SARS-CoV S is largely uncleaved after biosynthesis. It can be later processed by endosomal cathepsin L, trypsin, thermolysin, and elastase, which are shown to induce syncytia formation and virus entry. Other proteases that are of potential biological relevance in potentiating SARS-CoV S include TMPRSS2, TMPRSS11a, and HAT which are localized on the cell surface and are highly expressed in the human airway []. The furin-like S2' cleavage site at KR/SF with P1 and P2 basic residues and a P2' hydrophobic Phe downstream of the IFP is identical between the SARS-CoV-2 and SARS-CoV. One or more furin-like enzymes would cleave the S2' site at KR/SF [, ]. Deletion of SARS-CoV-2 furin cleavage site suggests that it may not be required for viral entry but may affect replication kinetics and altered sites have been still seen proteolytically cleaved. Several substitutions within the S2' cleavage domain of SARS-COV-2 have been reported, including P812L/S/T, S813I/G, F817L, I818S/V, but further experimental study of their consequences and the replication properties of the altered viruses are required to understand the role of furin cleavage in SARS-CoV-2 infection and virulence []. The S2 subunit normally contains multiple key components, including one or more fusion peptides (FP), a second proteolytic site (S2') and two conserved heptad repeats (HRs), driving membrane penetration and virus-cell fusion. The HRs can trimerize into a coiled-coil structure built of three HR1-HR2 helical hairpins presenting as a canonical six-helix bundle and drag the virus envelope and the host cell bilayer into close proximity, preparing for fusion to occur []. The fusion core is composed of HR1 and HR2 and at least three membranotropic regions that are denoted as the fusion peptide (FP), internal fusion peptide (IFP), and pretransmembrane domain (PTM). The HR regions are further flanked by the three membranotropic components. Both FP and IFP are located upstream of HR1, while PTM is distally downstream of HR2 and directly precedes the transmembrane domain of SARS-CoV S. All of these three components are able to partition into the phospholipid bilayer to disturb membrane integrity. []. During the pandemic, many conservative amino acid changes in FP segment of SARS-CoV-2 have been reported (i.e., L821I, L822F, K825R, V826L, T827I, L828P, A829T, D830G/A, A831V/S/T, G832C/S, F833S, I834T), although their impact is not known as the active conformation and mode of insertion of SARS-CoV-2 fusion peptide have not been experimentally characterised. Differences in HR1 sequences between SARS-CoV and SARS-CoV-2 suggest that SARS-CoV-2 HR2 makes stronger interactions with HR1. However, the substitutions observed in the solvent accessible surface of the HR1 domain (e.g., D936Y, S943P, S939F) of SARS-CoV-2 do not seem to be involved in stabilizing interactions with HR2. Substitutions in HR2 (e.g., K1073N, V1176F) or the TM or cytoplasmic tail domains have also been observed, but further experimental work is required to determine the effects of these changes [].
These proteins contain a domain found in serine peptidases belonging to the MEROPS peptidase families S8 (subfamilies S8A (subtilisin) and S8B (kexin) and S53 (sedolisin), both of which are members of clan SB [].The subtilisin family is one of the largest serine peptidase families characterised to date. Over 200 subtilises are presently known, more than 170 of which with their complete amino acid sequence []. It is widespread, being found in eubacteria, archaebacteria, eukaryotes and viruses []. The vast majority of the family are endopeptidases, although there is an exopeptidase, tripeptidyl peptidase [, ]. Structures have been determined for several members of the subtilisin family: they exploit the same catalytic triad as the chymotrypsins, although the residues occur in a different order (HDS in chymotrypsin and DHS in subtilisin), but the structures show no other similarity [, ]. Some subtilisins are mosaic proteins, while others contain N- and C-terminal extensions that show no sequence similarity to any other known protein [].The proprotein-processing endopeptidases kexin, furin and related enzymesform a distinct subfamily known as the kexin subfamily (S8B). These preferentially cleave C-terminally to paired basic amino acids. Members of this subfamily can be identified by subtly different motifs around the active site [, ]. Members of the kexin subfamily, along with endopeptidases R, T and K from the yeast Tritirachium and cuticle-degrading peptidase from Metarhizium, require thiol activation. This can be attributed to the presence of a cysteine near to the active site histidine []. Only one viral member of the subtilisin family is known, a 56kDa protease from herpes virus 1, which infects the channel catfish []. Sedolisins (serine-carboxyl peptidases) are proteolytic enzymes whose fold resembles that of subtilisin; however, they are considerably larger, with the mature catalytic domains containing approximately 375 amino acids. The defining features of these enzymes are a unique catalytic triad, Ser-Glu-Asp, as well as the presence of an aspartic acid residue in the oxyanion hole. High-resolution crystal structures have now been solved for sedolisin from Pseudomonas sp. 101, as well as for kumamolisin from a thermophilic bacterium, Bacillus sp. MN-32. Mutations in the human gene leads to a fatal neurodegenerative disease [].
This entry represents the second carboxypeptidase (CP)-like domain of carboxypeptidase D (CPD; EC 3.4.17.22; MEROPS M14.016).Carboxypeptidase D (CPD) differs from all other metallocarboxypeptidases in that it contains multiple CP-like domains []. CPD belongs to the N/E-like subfamily (subfamily M14B) of the M14 family of metallocarboxypeptidases (MCPs) []. CPD is a single-chain protein containing a signal peptide, three tandem repeats of CP-like domains separated by short bridge regions, followed by a transmembrane domain, and a C-terminal cytosolic tail. The first two CP-like domains of CPD contain all of the essential active site and substrate-binding residues, while the third CP-like domain lacks critical residues necessary for enzymatic activity and is inactive towards standard CP substrates. Domain I is optimally active at pH 6.3-7.5 and prefers substrates with C-terminal Arg, whereas domain II is active at pH 5.0-6.5 and prefers substrates with C-terminal Lys [, , ]. CPD functions in the processing of proteins that transit the secretory pathway, and is present in all vertebrates as well as Drosophila[]. It is broadly distributed in all tissue types. Within cells, CPD is present in the trans-Golgi network and immature secretory vesicles, but is excluded from mature vesicles []. It is thought to play a role in the processing of proteins that are initially processed by furin or related endopeptidases present in the trans-Golgi network, such as growth factors and receptors []. CPD is implicated in the pathogenesis of lupus erythematosus (LE), it is regulated by TGF-beta in various cell types of murine and human origin and is significantly down-regulated in CD14 positive cells isolated from patients with LE. As down-regulation of CPD leads to down-modulation of TGF-beta, CPD may have a role in a positive feedback loop [].The carboxypeptidase A family can be divided into four subfamilies: M14A(carboxypeptidase A or digestive), M14B (carboxypeptidase H or regulatory), M14C (gamma-D-glutamyl-L-diamino acid peptidase I) and M14D (AGTPBP-1/Nna1-like proteins) [, ]. Members of subfamily M14B have longer C-termini than those of subfamily M14A [], and carboxypeptidase M (a member of the H family) is bound to the membrane by a glycosylphosphatidylinositol anchor, unlike the majority of the M14 family, which are soluble []. The zinc ligands have been determined as two histidines and a glutamate,and the catalytic residue has been identified as a C-terminal glutamate,but these do not form the characteristic metalloprotease HEXXH motif [, ]. Members of the carboxypeptidase A family are synthesised as inactive molecules with propeptides that must be cleaved to activate the enzyme. Structural studies of carboxypeptidases A and B reveal the propeptide to exist as a globular domain, followed by an extended α-helix; this shields the catalytic site, without specifically binding to it, while the substrate-binding site is blocked by making specific contacts [, ].
These proteins contain a domain superfamily found in serine peptidases belonging to the MEROPS peptidase families S8 (subfamilies S8A (subtilisin) and S8B (kexin) and S53 (sedolisin), both of which are members of clan SB [].The subtilisin family is one of the largest serine peptidase families characterised to date. Over 200 subtilises are presently known, more than 170 of which with their complete amino acid sequence []. It is widespread, being found in eubacteria, archaebacteria, eukaryotes and viruses []. The vast majority of the family are endopeptidases, although there is an exopeptidase, tripeptidyl peptidase [, ]. Structures have been determined for several members of the subtilisin family: they exploit the same catalytic triad as the chymotrypsins, although the residues occur in a different order (HDS in chymotrypsin and DHS in subtilisin), but the structures show no other similarity [, ]. Some subtilisins are mosaic proteins, while others contain N- and C-terminal extensions that show no sequence similarity to any other known protein [].The proprotein-processing endopeptidases kexin, furin and related enzymesform a distinct subfamily known as the kexin subfamily (S8B). These preferentially cleave C-terminally to paired basic amino acids. Members of this subfamily can be identified by subtly different motifs around the active site [, ]. Members of the kexin subfamily, along with endopeptidases R, T and K from the yeast Tritirachium and cuticle-degrading peptidase from Metarhizium, require thiol activation. This can be attributed to the presence of a cysteine near to the active site histidine []. Only one viral member of the subtilisin family is known, a 56kDa protease from herpes virus 1, which infects the channel catfish []. Sedolisins (serine-carboxyl peptidases) are proteolytic enzymes whose fold resembles that of subtilisin; however, they are considerably larger, with the mature catalytic domains containing approximately 375 amino acids. The defining features of these enzymes are a unique catalytic triad, Ser-Glu-Asp, as well as the presence of an aspartic acid residue in the oxyanion hole. High-resolution crystal structures have now been solved for sedolisin from Pseudomonas sp. 101, as well as for kumamolisin from a thermophilic bacterium, Bacillus sp. MN-32. Mutations in the human gene leads to a fatal neurodegenerative disease [].
The type I glycoprotein S of Coronavirus, trimers of which constitute the typical viral spikes, is assembled into virions through noncovalent interactions with the M protein. The spike glycoprotein is translated as a large polypeptide that is subsequently cleaved to S1 () and S2 []. The cleavage of S can occur at two distinct sites: S2 or S2' []. The spike is present in two very different forms: pre-fusion (the form on mature virions) and post-fusion (the form after membrane fusion has been completed). The spike is cleaved sequentially by host proteases at two sites: first at the S1/S2 boundary (i.e. S1/S2 site) and second within S2 (i.e. S2' site). After the cleavages, S1 dissociates from S2, allowing S2 to transition to the post-fusion structure []. Both chimeric S proteins appeared to cause cell fusion when expressed individually, suggesting that they were biologically fully active []. The spike is a type I membrane glycoprotein that possesses a conserved transmembrane anchor and an unusual cysteine-rich (cys) domain that bridges the putative junction of the anchor and the cytoplasmic tail [].SARS-CoV S is largely uncleaved after biosynthesis. It can be later processed by endosomal cathepsin L, trypsin, thermolysin, and elastase, which are shown to induce syncytia formation and virus entry. Other proteases that are of potential biological relevance in potentiating SARS-CoV S include TMPRSS2, TMPRSS11a, and HAT which are localized on the cell surface and are highly expressed in the human airway []. The furin-like S2' cleavage site at KR/SF with P1 and P2 basic residues and a P2' hydrophobic Phe downstream of the IFP is identical between the SARS-CoV-2 and SARS-CoV. One or more furin-like enzymes would cleave the S2' site at KR/SF [, ]. Deletion of SARS-CoV-2 furin cleavage site suggests that it may not be required for viral entry but may affect replication kinetics and altered sites have been still seen proteolytically cleaved. Several substitutions within the S2' cleavage domain of SARS-COV-2 have been reported, including P812L/S/T, S813I/G, F817L, I818S/V, but further experimental study of their consequences and the replication properties of the altered viruses are required to understand the role of furin cleavage in SARS-CoV-2 infection and virulence []. The S2 subunit normally contains multiple key components, including one or more fusion peptides (FP), a second proteolytic site (S2') and two conserved heptad repeats (HRs), driving membrane penetration and virus-cell fusion. The HRs can trimerize into a coiled-coil structure built of three HR1-HR2 helical hairpins presenting as a canonical six-helix bundle and drag the virus envelope and the host cell bilayer into close proximity, preparing for fusion to occur []. The fusion core is composed of HR1 and HR2 and at least three membranotropic regions that are denoted as the fusion peptide (FP), internal fusion peptide (IFP), and pretransmembrane domain (PTM). The HR regions are further flanked by the three membranotropic components. Both FP and IFP are located upstream of HR1, while PTM is distally downstream of HR2 and directly precedes the transmembrane domain of SARS-CoV S. All of these three components are able to partition into the phospholipid bilayer to disturb membrane integrity. []. During the pandemic, many conservative amino acid changes in FP segment of SARS-CoV-2 have been reported (i.e., L821I, L822F, K825R, V826L, T827I, L828P, A829T, D830G/A, A831V/S/T, G832C/S, F833S, I834T), although their impact is not known as the active conformation and mode of insertion of SARS-CoV-2 fusion peptide have not been experimentally characterised. Differences in HR1 sequences between SARS-CoV and SARS-CoV-2 suggest that SARS-CoV-2 HR2 makes stronger interactions with HR1. However, the substitutions observed in the solvent accessible surface of the HR1 domain (e.g., D936Y, S943P, S939F) of SARS-CoV-2 do not seem to be involved in stabilizing interactions with HR2. Substitutions in HR2 (e.g., K1073N, V1176F) or the TM or cytoplasmic tail domains have also been observed, but further experimental work is required to determine the effects of these changes [].This entry represents the cysteine rich intravirion region found at the C-terminal of coronavirus spike proteins (S) []. These cysteine residues are targets for palmitoylation, necessary for efficiently S incorporation into virions and S-mediated membrane fusions.
The type I glycoprotein S of Coronavirus, trimers of which constitute the typical viral spikes, is assembled into virions through noncovalent interactions with the M protein. The spike glycoprotein is translated as a large polypeptide that is subsequently cleaved to S1 () and S2 []. The cleavage of S can occur at two distinct sites: S2 or S2' []. The spike is present in two very different forms: pre-fusion (the form on mature virions) and post-fusion (the form after membrane fusion has been completed). The spike is cleaved sequentially by host proteases at two sites: first at the S1/S2 boundary (i.e. S1/S2 site) and second within S2 (i.e. S2' site). After the cleavages, S1 dissociates from S2, allowing S2 to transition to the post-fusion structure []. Both chimeric S proteins appeared to cause cell fusion when expressed individually, suggesting that they were biologically fully active []. The spike is a type I membrane glycoprotein that possesses a conserved transmembrane anchor and an unusual cysteine-rich (cys) domain that bridges the putative junction of the anchor and the cytoplasmic tail [].SARS-CoV S is largely uncleaved after biosynthesis. It can be later processed by endosomal cathepsin L, trypsin, thermolysin, and elastase, which are shown to induce syncytia formation and virus entry. Other proteases that are of potential biological relevance in potentiating SARS-CoV S include TMPRSS2, TMPRSS11a, and HAT which are localized on the cell surface and are highly expressed in the human airway []. The furin-like S2' cleavage site at KR/SF with P1 and P2 basic residues and a P2' hydrophobic Phe downstream of the IFP is identical between the SARS-CoV-2 and SARS-CoV. One or more furin-like enzymes would cleave the S2' site at KR/SF [, ]. Deletion of SARS-CoV-2 furin cleavage site suggests that it may not be required for viral entry but may affect replication kinetics and altered sites have been still seen proteolytically cleaved. Several substitutions within the S2' cleavage domain of SARS-COV-2 have been reported, including P812L/S/T, S813I/G, F817L, I818S/V, but further experimental study of their consequences and the replication properties of the altered viruses are required to understand the role of furin cleavage in SARS-CoV-2 infection and virulence []. The S2 subunit normally contains multiple key components, including one or more fusion peptides (FP), a second proteolytic site (S2') and two conserved heptad repeats (HRs), driving membrane penetration and virus-cell fusion. The HRs can trimerize into a coiled-coil structure built of three HR1-HR2 helical hairpins presenting as a canonical six-helix bundle and drag the virus envelope and the host cell bilayer into close proximity, preparing for fusion to occur []. The fusion core is composed of HR1 and HR2 and at least three membranotropic regions that are denoted as the fusion peptide (FP), internal fusion peptide (IFP), and pretransmembrane domain (PTM). The HR regions are further flanked by the three membranotropic components. Both FP and IFP are located upstream of HR1, while PTM is distally downstream of HR2 and directly precedes the transmembrane domain of SARS-CoV S. All of these three components are able to partition into the phospholipid bilayer to disturb membrane integrity. []. During the pandemic, many conservative amino acid changes in FP segment of SARS-CoV-2 have been reported (i.e., L821I, L822F, K825R, V826L, T827I, L828P, A829T, D830G/A, A831V/S/T, G832C/S, F833S, I834T), although their impact is not known as the active conformation and mode of insertion of SARS-CoV-2 fusion peptide have not been experimentally characterised. Differences in HR1 sequences between SARS-CoV and SARS-CoV-2 suggest that SARS-CoV-2 HR2 makes stronger interactions with HR1. However, the substitutions observed in the solvent accessible surface of the HR1 domain (e.g., D936Y, S943P, S939F) of SARS-CoV-2 do not seem to be involved in stabilizing interactions with HR2. Substitutions in HR2 (e.g., K1073N, V1176F) or the TM or cytoplasmic tail domains have also been observed, but further experimental work is required to determine the effects of these changes [].This entry represents the heptad repeat 1 (HR1) from coronavirus Spike glycoprotein, S2 subunit. This region forms a long trimeric helical coiled-coil structure with peptides from the HR2 region packing in an oblique antiparallel manner on the grooves of the HR1 trimer in a mixed extended and helical conformation. Packing of the helical parts of HR2 on the HR1 trimer grooves and formation of a six-helical bundle plays an important role in the formation of a stable post-fusion structure. In contrast to their extended helical conformations in the post-fusion state, the HR1 motifs within S2 form several shorter helices in their pre-fusion state [, ].
The type I glycoprotein S of Coronavirus, trimers of which constitute the typical viral spikes, is assembled into virions through noncovalent interactions with the M protein. The spike glycoprotein is translated as a large polypeptide that is subsequently cleaved to S1 () and S2 []. The cleavage of S can occur at two distinct sites: S2 or S2' []. The spike is present in two very different forms: pre-fusion (the form on mature virions) and post-fusion (the form after membrane fusion has been completed). The spike is cleaved sequentially by host proteases at two sites: first at the S1/S2 boundary (i.e. S1/S2 site) and second within S2 (i.e. S2' site). After the cleavages, S1 dissociates from S2, allowing S2 to transition to the post-fusion structure []. Both chimeric S proteins appeared to cause cell fusion when expressed individually, suggesting that they were biologically fully active []. The spike is a type I membrane glycoprotein that possesses a conserved transmembrane anchor and an unusual cysteine-rich (cys) domain that bridges the putative junction of the anchor and the cytoplasmic tail [].SARS-CoV S is largely uncleaved after biosynthesis. It can be later processed by endosomal cathepsin L, trypsin, thermolysin, and elastase, which are shown to induce syncytia formation and virus entry. Other proteases that are of potential biological relevance in potentiating SARS-CoV S include TMPRSS2, TMPRSS11a, and HAT which are localized on the cell surface and are highly expressed in the human airway []. The furin-like S2' cleavage site at KR/SF with P1 and P2 basic residues and a P2' hydrophobic Phe downstream of the IFP is identical between the SARS-CoV-2 and SARS-CoV. One or more furin-like enzymes would cleave the S2' site at KR/SF [, ]. Deletion of SARS-CoV-2 furin cleavage site suggests that it may not be required for viral entry but may affect replication kinetics and altered sites have been still seen proteolytically cleaved. Several substitutions within the S2' cleavage domain of SARS-COV-2 have been reported, including P812L/S/T, S813I/G, F817L, I818S/V, but further experimental study of their consequences and the replication properties of the altered viruses are required to understand the role of furin cleavage in SARS-CoV-2 infection and virulence []. The S2 subunit normally contains multiple key components, including one or more fusion peptides (FP), a second proteolytic site (S2') and two conserved heptad repeats (HRs), driving membrane penetration and virus-cell fusion. The HRs can trimerize into a coiled-coil structure built of three HR1-HR2 helical hairpins presenting as a canonical six-helix bundle and drag the virus envelope and the host cell bilayer into close proximity, preparing for fusion to occur []. The fusion core is composed of HR1 and HR2 and at least three membranotropic regions that are denoted as the fusion peptide (FP), internal fusion peptide (IFP), and pretransmembrane domain (PTM). The HR regions are further flanked by the three membranotropic components. Both FP and IFP are located upstream of HR1, while PTM is distally downstream of HR2 and directly precedes the transmembrane domain of SARS-CoV S. All of these three components are able to partition into the phospholipid bilayer to disturb membrane integrity. []. During the pandemic, many conservative amino acid changes in FP segment of SARS-CoV-2 have been reported (i.e., L821I, L822F, K825R, V826L, T827I, L828P, A829T, D830G/A, A831V/S/T, G832C/S, F833S, I834T), although their impact is not known as the active conformation and mode of insertion of SARS-CoV-2 fusion peptide have not been experimentally characterised. Differences in HR1 sequences between SARS-CoV and SARS-CoV-2 suggest that SARS-CoV-2 HR2 makes stronger interactions with HR1. However, the substitutions observed in the solvent accessible surface of the HR1 domain (e.g., D936Y, S943P, S939F) of SARS-CoV-2 do not seem to be involved in stabilizing interactions with HR2. Substitutions in HR2 (e.g., K1073N, V1176F) or the TM orcytoplasmic tail domains have also been observed, but further experimental work is required to determine the effects of these changes [].This entry represents the heptad repeat 2 (HR2) from coronavirus Spike glycoprotein, S2 subunit. It adopts a mixed conformation: the central part fold into a nine-turn α-helix, while the residues on either side of the helix adopt an extended conformation. Packing of the helical parts of HR2 on the HR1 trimer grooves and formation of a six-helical bundle plays an important role in the formation of a stable post-fusion structure [, ].
Bacillus subtilis produces and secretes proteases and other types of exoenzymes at the end of the exponential phase of growth []. The ones that make up this group are known as bacillopeptidase F (MEROPS identifier S08.017), encoded by bpr, a serine protease with high esterolytic activity which is inhibited by PMSF []. Bacillopeptidase F is fibrinolytic and is synthesized as a precursor, which is either activated autocatalytically or by another, unidentified B. subtilis peptidase []. Like other members of the peptidases S8 family these have a Asp/His/Ser catalytic triad similar to that found in trypsin-like proteases, but do not share their three-dimensional structure and are not homologous to trypsin. The stability of these enzymes may be enhanced by calcium, some members have been shown to bind up to 4 ions via binding sites with different affinity [].These proteins contain a domain found in serine peptidases belonging to the MEROPS peptidase families S8 (subfamilies S8A (subtilisin) and S8B (kexin) and S53 (sedolisin), both of which are members of clan SB [].The subtilisin family is one of the largest serine peptidase families characterised to date. Over 200 subtilises are presently known, more than 170 of which with their complete amino acid sequence []. It is widespread, being found in eubacteria, archaebacteria, eukaryotes and viruses []. The vast majority of the family are endopeptidases, although there is an exopeptidase, tripeptidyl peptidase [, ]. Structures have been determined for several members of the subtilisin family: they exploit the same catalytic triad as the chymotrypsins, although the residues occur in a different order (HDS in chymotrypsin and DHS in subtilisin), but the structures show no other similarity [, ]. Some subtilisins are mosaic proteins, while others contain N- and C-terminal extensions that show no sequence similarity to any other known protein [].The proprotein-processing endopeptidases kexin, furin and related enzymesform a distinct subfamily known as the kexin subfamily (S8B). These preferentially cleave C-terminally to paired basic amino acids. Members of this subfamily can be identified by subtly different motifs around the active site [, ]. Members of the kexin subfamily, along with endopeptidases R, T and K from the yeast Tritirachium and cuticle-degrading peptidase from Metarhizium, require thiol activation. This can be attributed to the presence of a cysteine near to the active site histidine []. Only one viral member of the subtilisin family is known, a 56kDa protease from herpes virus 1, which infects the channel catfish []. Sedolisins (serine-carboxyl peptidases) are proteolytic enzymes whose fold resembles that of subtilisin; however, they are considerably larger, with the mature catalytic domains containing approximately 375 amino acids. The defining features of these enzymes are a unique catalytic triad, Ser-Glu-Asp, as well as the presence of an aspartic acid residue in the oxyanion hole. High-resolution crystal structures have now been solved for sedolisin from Pseudomonas sp. 101, as well as for kumamolisin from a thermophilic bacterium, Bacillus sp. MN-32. Mutations in the human gene leads to a fatal neurodegenerative disease [].
This entry represents the third carboxypeptidase (CP)-like domain of Carboxypeptidase D (CPD; MEROPS identifier XM14.001; EC 3.4.17.22)(MEROPS identifier M14.950). Carboxypeptidase D (CPD) differs from all other metallocarboxypeptidases in that it contains multiple CP-like domains []. CPD belongs to the N/E-like subfamily (subfamily M14B) of the M14 family of metallocarboxypeptidases (MCPs) []. CPD is a single-chain protein containing a signal peptide, three tandem repeats of CP-like domains separated by short bridge regions, followed by a transmembrane domain, and a C-terminal cytosolic tail. The first two CP-like domains of CPD contain all of the essential active site and substrate-binding residues, while the third CP-like domain lacks critical residues necessary for enzymatic activity and is inactive towards standard CP substrates. Domain I is optimally active at pH 6.3-7.5 and prefers substrates with C-terminal Arg, whereas domain II is active at pH 5.0-6.5 and prefers substrates with C-terminal Lys [, , ]. CPD functions in the processing of proteins that transit the secretory pathway, and is present in all vertebrates as well as Drosophila[]. It is broadly distributed in all tissue types. Within cells, CPD is present in the trans-Golgi network and immature secretory vesicles, but is excluded from mature vesicles []. It is thought to play a role in the processing of proteins that are initially processed by furin or related endopeptidases present in the trans-Golgi network, such as growth factors and receptors []. CPD is implicated in the pathogenesis of lupus erythematosus (LE), it is regulated by TGF-beta in various cell types of murine and human origin and is significantly down-regulated in CD14 positive cells isolated from patients with LE. As down-regulation of CPD leads to down-modulation of TGF-beta, CPD may have a role in a positive feedback loop [].The carboxypeptidase A family can be divided into four subfamilies: M14A(carboxypeptidase A or digestive), M14B (carboxypeptidase H or regulatory), M14C (gamma-D-glutamyl-L-diamino acid peptidase I) and M14D (AGTPBP-1/Nna1-like proteins) [, ]. Members of subfamily M14B have longer C-termini than those of subfamily M14A [], and carboxypeptidase M (a member of the H family) is bound to the membrane by a glycosylphosphatidylinositol anchor, unlike the majority of the M14 family, which are soluble []. The zinc ligands have been determined as two histidines and a glutamate,and the catalytic residue has been identified as a C-terminal glutamate,but these do not form the characteristic metalloprotease HEXXH motif [, ]. Members of the carboxypeptidase A family are synthesised as inactive molecules with propeptides that must be cleaved to activate the enzyme. Structural studies of carboxypeptidases A and B reveal the propeptide to exist as a globular domain, followed by an extended α-helix; this shields the catalytic site, without specifically binding to it, while the substrate-binding site is blocked by making specific contacts [, ].
