PROTEASOME SUBUNIT-X AND SUBUNIT-Y ALTER PEPTIDASE ACTIVITIES IN OPPOSITE WAYS TO THE INTERFERON-GAMMA-INDUCED SUBUNITS LMP2 AND LMP7

Most antigenic peptides presented on major histocompatibility complex class I molecules are generated by proteasomes, Interferon-gamma, whic h stimulates antigen presentation, induces new proteasome beta-subunit s LMP2 and LMP7, which replace the homologous beta-subunits Y (delta) and X (epsilon). As a result, the capacity of the proteasome to cleave model peptides increases after hydrophobic and basic residues and fal ls after acidic residues, To clarify the function of these subunits, a re examined the effects of overexpressing subunits X (delta) and Y (ep silon). Transfection of the Y gene into HeLa cells stimulated the prot easomal cleavage after acidic residues without altering other peptidas e activities. This effect was propertional to the amount of the Y subu nits and opposite to the effect of its homolog, LMP2. Y appears to pro mote cleavages after acidic residues, Furthermore, in mutants lacking the LMP genes (in contrast to wild-type cells), interferon-gamma treat ment increased the proteasome content of Y subunits and enhanced posta cidic cleavages. Transfection with cDNA for the X subunit reduced hydr olysis after hydrophobic and basic residues, an effect opposite to tra nsfection of LMP2 and LMP7, Surprisingly, transfection of X increased the amounts not only of X, but also of Y, while decreasing LMPB conten t. Thus, the loss of the Y subunit upon interferon-gamma treatment or LMPB transfection accounts for the suppression of postacidic cleavages , and the loss of X contributes to the increased hydrolysis after hydr ophobic and basic residues. These adaptations should favor the product ion of the kinds of peptides that are presented on major histocompatib ility complex class I molecules.