Peptidase S21 (IPR001847)

Short name: Peptidase_S21

Overlapping homologous superfamilies


Family relationships



This group of serine peptidases belong to MEROPS peptidase family S21 (assemblin family, clan 21). Assemblin is a serine protease with a unique fold and an active site that comprises the unusual triad Ser-His-His. It is found in herpesviruses, one of the groups of DNA viruses. Assemblin is involved in the late stages of assembly of the viral prohead, breaking down the scaffold protein upon which the prohead is assembled. In Epstein-Barr virus (EBV), a member of the Gammaherpesvirus family, it is also known as the maturational protease (BVRF2) [PMID: 19158247]. The structure of assemblin has been revealed [PMID: 26161660, PMID: 24977643]. The dimerization of assemblin may induce allosteric changes that activate this protease [PMID: 26161660].

Proteolytic enzymes that exploit serine in their catalytic activity are ubiquitous, being found in viruses, bacteria and eukaryotes [PMID: 7845208]. They include a wide range of peptidase activity, including exopeptidase, endopeptidase, oligopeptidase and omega-peptidase activity. Many families of serine protease have been identified, these being grouped into clans on the basis of structural similarity and other functional evidence [PMID: 7845208]. Structures are known for members of the clans and the structures indicate that some appear to be totally unrelated, suggesting different evolutionary origins for the serine peptidases [PMID: 7845208].

Not withstanding their different evolutionary origins, there are similarities in the reaction mechanisms of several peptidases. Chymotrypsin, subtilisin and carboxypeptidase C have a catalytic triad of serine, aspartate and histidine in common: serine acts as a nucleophile, aspartate as an electrophile, and histidine as a base [PMID: 7845208]. The geometric orientations of the catalytic residues are similar between families, despite different protein folds [PMID: 7845208]. The linear arrangements of the catalytic residues commonly reflect clan relationships. For example the catalytic triad in the chymotrypsin clan (PA) is ordered HDS, but is ordered DHS in the subtilisin clan (SB) and SDH in the carboxypeptidase clan (SC) [PMID: 7845208, PMID: 8439290].

GO terms

Biological Process

GO:0006508 proteolysis

Molecular Function

GO:0004252 serine-type endopeptidase activity

Cellular Component

No terms assigned in this category.

Contributing signatures

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