1RYP

The 26S proteasome is the central enzyme of non-lysosomal protein degradation. It is involved in the removal of misfolded or incorrectly assembled proteins, and also in the degradation of short lived regulatory proteins including transcription factors and the cyclins of cell-cycle control. The catalytic core of the complex is formed by the 20S proteasome, which has the form of a barrel-shaped particle composed of four stacked seven-membered rings. In yeast and higher eukaryotes, the rings are made up of 14 different but related subunits, the overall complex containing two subunits of each type. These can be classified into two families, alpha-type and beta-type. The beta type subunits form the inner two rings of the complex and at least three of them (beta-1, beta-2 and beta-5) are catalytically active. The cleavage specificities of these sites are determined largely by their S1 pockets and the three major specificities of the proteasome - peptidylglutamil-hydrolysing, trypsin-like, and chymotrypsin-like - have been assigned respectively to beta-1, beta-2 and beta-5.
The proteasome from archaea such as Thermoplasma acidophilum is simpler, consisting of 14 copies each of only two different subunits, alpha and beta. The archaeal enzyme has only a single, chymotrypsin-like activity although it has been shown to hydrolyse almost any peptide bond in denatured substrates.

In all cases the beta subunits are synthesised as inactive precursors which undergo autocatalytic cleavage to expose the catalytically active N-terminal residue.