INCORPORATION OF MAJOR HISTOCOMPATIBILITY COMPLEX - ENCODED SUBUNITS LMP2 AND LMP7 CHANGES THE QUALITY OF THE 20S PROTEASOME POLYPEPTIDE PROCESSING PRODUCTS INDEPENDENT OF INTERFERON-GAMMA

The 20S proteasome is the enzyme complex responsible for the processin g of antigens bound by major histocompatibility complex class I molecu les. The role of the interferon-gamma (IFN-gamma)-inducible proteasome subunits LMP2 and LMP7 in this process is, however, still controversi al. We have studied the effects of IFN-gamma-independent LMP incorpora tion on the quality of peptides processed from the murine cytomegalovi rus IE pp89 25-mer polypeptide substrate through dual cleavages by 20S proteasomes. The incorporation of a single LMP subunit or both LMP2 a nd LMP7 induces changes in 20S proteasome subunit stoichiometry, alter s its cleavage site preference and in consequence, the quality of the generated peptides. When the several hydrolytic activities are tested with short fluorogenic peptide substrates, the V-max, S-0.5(K-m), or b oth values of 20S proteasomes are altered, depending on the combinatio n of LMP. There exists, however, no obvious correlation between the ob served changes in hydrolytic activities against short fluorogenic pept ides and the changes in dual cleavage site usage within the 25-mer pol ypeptide substrate. As judged from the calculated Hill coefficients, t he presence of both LMP subunits induces a drastic increase in positiv e cooperativity between the proteasome subunits.