Properties of the Nuclear Proteasome Activator PA28γ (REGγ)

PA28 or 11S REG is a proteasome activator composed of homologous α- and β-subunits and predominantly found in the cytosol. A homologous protein originally known as the Ki antigen but now called PA28γ or REGγ is predominantly localized in the nucleus. To further characterize the biochemical properties of PA28γ, we expressed and purified homogenous recombinant human protein with and without an N-terminal 6-His extension. PA28γ is a heptamer based on the molecular masses of the native and monomeric proteins. The heptameric 6-His fusion protein can dimerize. Recombinant PA28γ stimulates the proteasome-mediated hydrolysis of synthetic substrates containing hydrophobic, basic, and acidic amino acids in the P1 position. Stimulation is dependent on substrate size. PA28γ only minimally stimulates degradation of the oxidized B chain of insulin. PA28γ may facilitate the later stages of protein metabolism in the nucleus and/or have a more specialized role in controlling the levels of biologically active peptides in the nucleus.