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 [, ].
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 [, ].
Potyviruses form one of the most numerous groups of plant viruses and are a major cause of crop loss worldwide. The helper-component proteinase (HC-Pro) is an indispensable, multifunctional protein of members of the genus Potyvirus and other viruses of the family Potyviridae. It is directly involved in diverse steps of viral infection, such as aphid plant-to-plant transmission, polyprotein processing, and suppression of host antiviral RNA silencing. HC-Pro is generally divided into three functional domains: a N-terminal domain, a central region, and a cysteine protease domain (CPD) in the C-terminal region. The HC-Pro CPD domain has a protease activity that autocatalytically cleaves a Gly-Gly dipeptide at its own C terminus to release HC-Pro from the rest of the viral polyprotein. Cysteine and histidine residues form the catalytic dyad at the active site. The HC-Pro CPD domain constitutes the peptidase family C6 of the CA clan [].The HC-Pro CPD domain adopts a compact oval-shaped alpha/beta fold. The secondary structure elements include four α-helices (alpha1-alpha4) and two short β-strands (beta1 and beta2) arranged in the order alpha1-alpha2-alpha3-beta1-beta2-alpha4. In addition, two 3(10) helices are located between alpha3 and beta1 and downstream of alpha4. The four helices form a helix bundle packed against one face of a short β-hairpin formed by strands beta1 and beta2. The catalytic residue Cys is located at the N terminus of helix alpha1, and the other catalytic residue His is located on strand beta2. The substrate binding cleft is lined by the loop connecting helices alpha2 and alpha3 and the N-terminal region of helix alpha1 on one side and by strand beta2 on the other side [].This superfamily represents the CPD domain of the HC-Pro protein.
This entry represents the CPD domain of the HC-Pro protein. Potyviruses form one of the most numerous groups of plant viruses and are a major cause of crop loss worldwide. The helper-component proteinase (HC-Pro) is an indispensable, multifunctional protein of members of the genus Potyvirus and other viruses of the family Potyviridae. It is directly involved in diverse steps of viral infection, such as aphid plant-to-plant transmission, polyprotein processing, and suppression of host antiviral RNA silencing. HC-Pro is generally divided into three functional domains: a N-terminal domain, a central region, and a cysteine protease domain (CPD) in the C-terminal region. The HC-Pro CPD domain has a protease activity that autocatalytically cleaves a Gly-Gly dipeptide at its own C terminus to release HC-Pro from the rest of the viral polyprotein. Cysteine and histidine residues form the catalytic dyad at the active site. The HC-Pro CPD domain constitutes the peptidase family C6 of the CA clan [].The HC-Pro CPD domain adopts a compact oval-shaped alpha/beta fold. The secondary structure elements include four α-helices (alpha1-alpha4) and two short β-strands (beta1 and beta2) arranged in the order alpha1-alpha2-alpha3-beta1-beta2-alpha4. In addition, two 3(10) helices are located between alpha3 and beta1 and downstream of alpha4. The four helices form a helix bundle packed against one face of a short β-hairpin formed by strands beta1 and beta2. The catalytic residue Cys is located at the N terminus of helix alpha1, and the other catalytic residue His is located on strand beta2. The substrate binding cleft is lined by the loop connecting helices alpha2 and alpha3 and the N-terminal region of helix alpha1 on one side and by strand beta2 on the other side [].
Large bacterial protein toxins autotranslocate functional effector domains tothe eukaryotic cell cytosol, resulting in alterations to cellular functions that ultimately benefit the infecting pathogen. Among these toxins, the clostridial glucosylating toxins (CGTs) produced by Gram-positive bacteria and the multifunctional-autoprocessing RTX (MARTX) toxins of Gram-negative bacteria have distinct mechanisms of post-translocation, but a shared mechanism of post-translocation autoprocessing that releases these functional domains from the large holotoxins. These toxins carry an embedded cysteine protease domain (CPD) that is regulated by a unique allosteric activation mechanism. Binding of the eukaryotic-specific small molecule inositol hexakisphosphate (InsP(6)) to a basic cleft within the CPD induces a structural rearrangement that exposes the protease active site to its substrates. Proteins containing this domain belong to the peptidase family C80 of clan CD [, , , ].The CGT/MARTX CPD domain consists of a central β-sheet that is surrounded by α-helices. Additional β-strands at the C terminus form a subdomain known as the β-flap, that is loosely attached to the core protease. The CGT/MARTX CPD catalytic dyad is composed of one His and one Cys residue. The distance between the catalytic residues indicates that the Cys is not activated by protonation from His, but rather suggests that the Cys is substrate-activated by close alignment of the scissile bond, while the His functions solely to protonate the leaving group [, , ].
Large bacterial protein toxins autotranslocate functional effector domains tothe eukaryotic cell cytosol, resulting in alterations to cellular functions that ultimately benefit the infecting pathogen. Among these toxins, the clostridial glucosylating toxins (CGTs) produced by Gram-positive bacteria and the multifunctional-autoprocessing RTX (MARTX) toxins of Gram-negative bacteria have distinct mechanisms of post-translocation, but a shared mechanism of post-translocation autoprocessing that releases these functional domains from the large holotoxins. These toxins carry an embedded cysteine protease domain (CPD) that is regulated by a unique allosteric activation mechanism. Binding of the eukaryotic-specific small molecule inositol hexakisphosphate (InsP(6)) to a basic cleft within the CPD induces a structural rearrangement that exposes the protease active site to its substrates. Proteins containing this domain belong to the peptidase family C80 of clan CD [, , , ].The CGT/MARTX CPD domain consists of a central β-sheet that is surrounded by α-helices. Additional β-strands at the C terminus form a subdomain known as the β-flap, that is loosely attached to the core protease. The CGT/MARTX CPD catalytic dyad is composed of one His and one Cys residue. The distance between the catalytic residues indicates that the Cys is not activated by protonation from His, but rather suggests that the Cys is substrate-activated by close alignment of the scissile bond, while the His functions solely to protonate the leaving group [, , ].
