Fig. 1.

Subunit composition of immunoproteasomes and thymoproteasomes

Proteasomes have a cylindrical structure made up of four stacked rings: two outer α-rings composed up of α subunits and two inner β-rings composed of β subunits. In eukaryotes, α- and β-rings are made up of seven distinct subunits, respectively. Catalytic β subunits, β1, β2 and β5, are colored. Immunoproteasomes and thymoproteasomes, which occur only jawed vertebrates, differ from standard 20S proteasomes in the composition of catalytic β subunits.