Proteasome activator (PA28) subunits, α, β and γ (Ki antigen) in NT2 neuronal precursor cells and HeLa S3 cells

The catalytic activity of the 20S proteasome can be modulated by endogenous proteins. A proteasome activator protein termed PA28 or 11S regulator, composed of two homologous subunits (α and β) and a separate but related protein termed Ki antigen or PA28γ have been characterized. To explore the functional relationship of these proteins, NT2 clone D1 human neuronal precursor cells, as well as HeLa S3 cells were labeled by immunofluorescence and immunoelectron microscopy with three different antisera directed against peptides derived from their sequences. It was found that both PA28α and PA28β antisera label the cytoplasm and the nucleoli. In contrast, the PA28γ antiserum labels the nucleus but not the nucleoli while in the cytoplasm it labels two different classes of structures identified as microtubular-like extensions and inclusion bodies that are most likely autophagosomes. The latter do not contain proteasome δ subunit antigen. The microtubular-like structures colocalize with β-tubulin, are dispersed by nocodazole and are not affected by brefeldin A treatment. PA28α and PA28β are co-localized in the cell whereas PA28γ has a different distribution. PA28γ complexed with the proteasome may serve a function other than or in addition to activation and may also have a proteasome-independent function.