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 (alpha and beta) and a separate but related protein termed Ki antigen or PA28gamma 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 PA28alpha and PA28beta antisera label the cytoplasm and the nucleoli. In contrast, the PA28gamma 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 delta subunit antigen. The microtubular-like structures colocalize with beta-tubulin, are dispersed by nocodazole and are not affected by brefeldin A treatment. PA28alpha and PA28beta are co-localized in the cell whereas PA28gamma has a different distribution. PA28gamma complexed with the proteasome may serve a function other than or in addition to activation and may also have a proteasome-independent function.