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. Three subunits (alpha, beta and gamma) of the 11S regulator have been identified. Six gamma subunits combine to form a homo-hexameric ring. This protein has been purified from E. Coli.