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The proteasome is a 700-kD multisubunit enzyme complex with several proteolytically active sites. The enzyme complex is involved in both ubiquitin-dependent and -independent protein degradation and may contribute to the processing of antigens presented by major histocompatibility complex (MHC) class I molecules. Here we demonstrate that treatment of mouse fibroblast cells with 20 U interferon gamma (IFN-gamma) for 3 d induces a change in the proteasome subunit composition and that the beta-type subunit LMP2, which is encoded in the MHC class II region, is incorporated into the enzyme complex. This is paralleled by reduction of the homologous delta-subunit. IFN-gamma stimulation results in a downregulation of the chymotrypsin-like Suc-LLVY-MCA peptide hydrolyzing activity of 20S proteasomes whereas the trypsin-like activity remains unaffected. When tested as a substrate a synthetic 25-mer polypeptide whose sequence covers the antigenic nonapeptide YPHFMPTNL of the MCMV pp89, 20S proteasomes of IFN-gamma-induced cells exhibit altered chymotrypsin-like cleavage site preferences. In the absence of IFN-gamma induction, the naturally processed nonamer peptide that is presented by MHC class I molecules appears as a minor cleavage product. IFN-gamma activation does not result in an increase of the final peptide but results in a different set of peptides. We hypothesize that these peptides represent precursor peptides that can be trimmed to final peptide size.

Interferon gamma stimulation modulates the proteolytic activity and cleavage site preference of 20S mouse proteasomes.