PA28 (also called the 11S regulatory complex) is another activator of the 20S proteasome, which assembles on either one or both ends of the 20S proteasome in an ATP-independent manner. PA28 can greatly enhance the peptidase activities of the 20S proteasome, but not for degradation of ubiquitinated proteins. The PA28 activator is a complex of two alternating homologous subunits, PA28alpha and PA28beta, which assembles as a hexameric ring with an alpha3beta3 stoichiometry. PA28alpha and PA28beta are induced by interferon. Functionally, PA28 enhances the generation of class I peptides for antigen presentation. The third subunit, PA28gamma, was recently identified in the nucleus. Recombinant PA28alpha and PA28beta form a heptomeric ring, whereas PA28gamma forms a hexameric ring. All of them can stimulate the peptidase activities of the 20S proteasome.
