C. Wojcik et al., PROTEASOME ACTIVATOR (PA28) SUBUNITS, ALPHA-SUBUNIT, BETA-SUBUNIT ANDGAMMA-SUBUNIT (KI ANTIGEN) IN NT2 NEURONAL PRECURSOR CELLS AND HELA S3 CELLS, European journal of cell biology, 77(2), 1998, pp. 151-160
The catalytic activity of the 205 proteasome can be modulated by endog
enous proteins. A proteasome activator protein termed PA28 or 11S regu
lator, composed of two homologous subunits (alpha and beta) and a sepa
rate but related protein termed Ki antigen or PA28 gamma have been cha
racterized. 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 PA28 alpha and PA28 beta antisera la
bel the cytoplasm and the nucleoli. In contrast, the PA28 gamma antise
rum 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 autophagosom
es. The latter do not contain proteasome delta subunit antigen. The mi
crotubular-like structures colocalize with beta-tubulin, are dispersed
by nocodazole and are not affected by brefeldin A treatment. PA28 alp
ha and PA28 beta are co-localized in the cell whereas PA28 gamma has a
different distribution. PA28 gamma complexed with the proteasome may
serve a function other than or in addition to activation and may also
have a proteasome-independent function.