Cytokine Induced Changes in Proteasome Subunit Composition Are Concentration Dependent

In eukaryotes, 20S proteasome subunit composition is controlled by the cytokine interferon-gamma (IFN-gamma). IFN-gamma induces the synthesis of the beta-subunits LMP2, LMP7 and MECL-1, which in consequence replace their constitutive subunit homologs delta, MB1 and MC14/Z in the 20S complex. By pulse labeling mouse RMA cells and immunoprecipitation of proteasome complexes with the antibody MP3, we have analysed the effect of different IFN-gamma concentrations on proteasomal subunit composition. Our experiments show that IFN-gamma concentrations as low as 5 U/ml induce subunit substitutions and that overall proteasomal subunit composition is dependent on the cytokine concentration used. An IFN-gamma concentration of 50 U/ml is sufficient for complete replacement of subunit delta by LMP2. In contrast, IFN-gamma treatment never induces a complete replacement of subunit MC14 by MECL-1. These subunits are present at an approximate 1:1 molar ratio, suggesting that both subunits coexist in the same 20S proteasome complex. Furthermore, different regulatory mechanisms have to be postulated for the synthesis and incorporation of the three IFN-gamma inducible proteasome subunits. Both IFN-gamma as well as IL-2 also seem to influence the modification state of the alpha subunit C8. Since the subunit composition is dependent on the cytokine concentration used and strongly influences the proteolytic properties of the 20S proteasome complex, our experiments represent a caveat for experiments in which IFN-gamma dependent proteasomal enzyme characteristics have been analysed without monitoring the subunit composition.