【背景资料】:The 11S proteasome regulator, also known as the PA28 proteasome activator, is a 200kDa protein complex composed of two different subunits, alpha and beta, that both have an apparent molecular weight of ~28kDa. In vivo, the 11S proteasome most probably exists as a hepta-heteromeric complex, the exact stoichiometry of which (α3/β4; α4/β3; stochastic composition) is still to be fully elucidated.Although the transcription of 11S α and β genes is stimulated by interferon-ϒ, the complex is constitutively present and broadly distributed in mammalian tissues. The 11S activator modulates the proteasome-catalysed production of antigenic peptides presented to the immune system on MHC class I molecules, which implies a cellular function in antigen processing.11S regulator(s) can bind to the 20S proteasome core and, in so doing, accelerate the hydrolysis of peptide substrates, however, this binding does not promote the degradation of protein substrates. Several reports show that in addition to increasing proteasomal activity, 11S binding also subtly changes the substrate specificity of the proteasome.A nuclear homo-heptameric form of the regulator built up by 11S gamma subunit (designated 11S ϒ) has been shown to facilitate the degradation of an intact protein substrate by 20S proteasomes. The 11S proteasome complex is also a component of the so-called ´hybrid-proteasome` that consists of 11S regulator bound to a 26S proteasome.