Interferon y Stimulation Modulates the Proteolytic Activity and Cleavage Site Preference of 20S Mouse Proteasomes

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 qr (IFN-y) for 3 d induces a change in the proteasome
subunit composition and that the B-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
6-subunit. IFN-3' 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-3'-induced cells exhibit altered
chymotrypsin-like cleavage site preferences. In the absence of
IFN-qr induction, the naturally processed nonamer peptide that is
presented by MHC class.I molecules appears as a minor cleavage
product. IFN-'y 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.