The Proteasome Activator 28 (PA28) Regulatory Complex, also known as the 11S Regulatory (REG) Complex, is a ring-shaped, multimeric, ATP-independent regulatory complex that can bind to one or both ends of the 20S Proteasome or 20S Immunoproteasome (1,2). The PA28 Regulatory Complex stimulates the proteasome to hydrolyze small peptides, but not ubiquitinated proteins, for the production of peptides for MHC class I antigen presentation (3-7). Three PA28 Regulatory Complex subunits, alpha, beta, and gamma, have been identified and can oligomerize to form the PA28 Regulatory Complex (8-10). While the expression of the PA28 alpha and beta subunits are induced by IFN-gamma, the PA28 gamma subunit is not significantly affected (11,12).
The human PA28 Activator beta Subunit, also known as 11S REG beta, is a 239 amino acid (aa) member of the PA28 family of regulatory proteins with a predicted molecular weight of 27 kDa. The mouse and rat PA28 beta orthologs share 93% and 92% aa sequence identity with the human protein, respectively. In vivo, PA28 beta forms hetero-heptamers or -hexamers with PA28 alpha (13-15). PA28 beta cannot form active homo-oligomers in vitro and cannot activate the 20S Proteasome in the absence of PA28 alpha (8). In addition to regulating production of MHC class I antigens, the PA28 alpha/beta regulatory complex may also protect against oxidative stress (16). This protein has been purified from E. Coli.
