Arterivirus papain-like cysteine protease alpha (PCPalpha) domain (IPR008741)

Short name: AV_PCPalpha

Overlapping homologous superfamilies

Domain relationships



Arteriviruses are enveloped, positive-stranded RNA viruses and include pathogens of major economic concern to the swine- and horse-breeding industries:

  • Equine arteritis virus (EAV).
  • Porcine reproductive and respiratory syndrome virus (PRRSV).
  • Mice actate dehydrogenase-elevating virus.
  • Simian hemorrhagic fever virus.

The arterivirus replicase gene is composed of two open reading frames (ORFs). ORF1a is translated directly from the genomic RNA, whereas ORF1b can be expressed only by ribosomal frameshifting, yelding a 1ab fusion protein. Both replicase gene products are multidomain precursor proteins which are proteolytically processed into functional nonstructural proteins (nsps) by a complex proteolytic cascade that is directed by four (PRRSV/LDV) or three (EAV) proteinase domains encoded in ORF1a. The arterivirus replicase processing scheme involves the rapid autoproteolytic release of two or three N-terminal nsps (nsp1 (or nsp1alpha/1beta) and nsp2) and the subsequent processing of the remaining polyproteins by the "main protease" residing in nsp4, together resulting in a set of 13 or 14 individual nsps.

The arterivirus nsp1 region contains a tandem of papain-like cysteine autoprotease domains (PCPalpha and PCPbeta), but in EAV PCPalpha has lost its enzymatic activity, resulting in the 'merge' of nsp1alpha and nsp1beta into a single nsp1 subunit. Thus, instead of three self-cleaving N-terminal subunits, EAV has two: nsp1 and nsp2. The PCPalpha and PCPbeta domains mediate the nsp1alpha|1beta and nsp1beta|2 cleavages, respectively. The catalytic dyad of PCPalpha and PCPbeta domains is composed of Cys and His residues. In EAV, a Lys residue is found in place of the catalytic Cys residue, which explains the proteolytic deficiency of the EAV PCPalpha domain [PMID: 7769711, PMID: 10725411, PMID: 11172046, PMID: 20696193]. The PCPalpha and PCPbeta domains form respectively MEROPS peptidase families C31 and C32.

The PCPalpha and PCPbeta domains have a typical papain fold, which consists of a compact global region containing sequentially connected left (L) and right (R) parts in a so-called standard orientation. The L subdomain of PCPalpha consists of four alpha-helices, while the R subdomain is formed by three antiparallel beta strands [PMID: 19706710]. The L subdomain of the PCBbeta consists of three alpha-helices, while the R subdomain is formed by four antiparallel beta-strands [PMID: 20410261]. The Cys and His residues face each other at the L-R interface and form the catalytic centre of the PCPalpha and PCPbeta domains [PMID: 19706710, PMID: 20410261].

This entry represents the PCPalpha domain (peptidase C31).

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 [PMID: 11517925]. 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 beta-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 [PMID: 7044372].

Clan CD includes proteins with a caspase-like fold. Proteins in the clan have an alpha/beta/alpha 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 [PMID: 9891971]. 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 beta 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 beta barrel decorated with helices with the active site at one end of the barrel [PMID: 14725770]. 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.

GO terms

Biological Process

No terms assigned in this category.

Molecular Function

GO:0004197 cysteine-type endopeptidase activity

Cellular Component

No terms assigned in this category.

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.
PROSITE profiles