The 20S proteosome has 28 protein subunits arranged as an (α1–α7, β1–β7)2 complex in four stacked rings. The interior of the complex has the active sites. The β-type subunits are synthesized as pro-proteins and are proteolytically cleaved before assembly. The 26S proteosome is a 2000-kDa protein complex composed of the catalytic 20S proteosome and the regulatory PA700 subcomplex that consists of approximately 20 heterogeneous proteins. The proteosome activator complex (PA28 complex, 11S regulator) is an alternative proteosome activator that does not involve ubiquitin. The complex is composed of two homologous subunits called PA28α (Reg1a, 249 aa) and -β (Reg1b, 239 aa), which form a hexameric ring, and is expressed constitutively in antigen-presenting cells. A third subunit, PA28γ (254 aa), is the Ki antigen. Expression of the PA28 complex is up-regulated by interferon-γ and enhances the generation of MHC class I binding peptides by altering the cleavage pattern of the proteosome.
Subjects: Medicine and Health.