InterPro domain: IPR000243

The proteasome (or macropain) ( 3.4.25.1 ) [ 1 , 2 , 3 , 4 , 5 ] is a multicatalytic proteinase complex in eukaryotes and archaea, and in some bacteria, that seems to be involved in an ATP/ubiquitin-dependent nonlysosomal proteolytic pathway. In eukaryotes the proteasome is composed of 28 distinct subunits which form a highly ordered ring-shaped structure (20S ring) of about 700kDa. Most proteasome subunits can be classified, on the basis on sequence similarities into two groups, alpha (A) and beta (B). These are arranged in four rings of seven proteins, consisting of a ring of alpha subunits, two rings of beta subunits, and a ring of alpha subunits. In eukaryotes, each alpha and each beta ring consists of different proteins. Three of the beta subunits are peptidases in subfamily T1A, and each has a distinctive specificity (trypsin-like, chymotrypsin-like and glutamyl peptidase-like). The peptidases are N-terminal nucleophile hydrolases in which the N-terminal threonine is the nucleophile in the hydrolytic reaction [ 6 ]. In the immunoproteasome, the catalytic components are replaced by three specialist, catalytic beta subunits [ 7 ]. In bacteria and archaea there is only one alpha subunit and one beta subunit, and each ring is a homoseptamer.

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