PA28 is a ring-shaped 11S multimeric complex that can bind to the two ends of the 20S proteasome and dramatically stimulates its capacity to hydrolyze small peptides (but not ubiquitinated proteins or large substrates). It functions in immuno-proteasome assembly, is required for antigen processing, and enhances the generation of class I peptides. In mammals, PA28 is composed of two homologous subunits; PA28α and PA28β, both of which are interferon-induced. In vitro, PA28α forms a heptameric ring that can stimulate peptide hydrolysis by 20S particles to the same extent as the heteromeric (3α3β) complex. This protein has been purified from E.coli.