PEPTIDASE ACTIVITIES OF PROTEASOMES ARE DIFFERENTIALLY REGULATED BY THE MAJOR HISTOCOMPATIBILITY COMPLEX-ENCODED GENES FOR LMP2 AND LMP7

Recent studies have implicated proteasomes in the generation of the an tigenic peptides that are presented on major histocompatibility comple x class I molecules to T lymphocytes. Interferon gamma modifies the su bunit composition of proteasomes and causes changes in their peptidase activities that should favor the production of peptides with hydropho bic or basic carboxyl termini (i.e., the types found on major histocom patibility complex class I molecules). It has been proposed that these changes in peptidase activity are due to incorporation into proteasom es of the major histocompatibility complex-encoded subunits LMP2 and - 7, which are induced by interferon gamma. Here we show by gene transfe ction into lymphoblasts or HeLa cells that LMP7 increases the capacity (V-max) of 20S and 26S proteasomes to cleave peptides after hydrophob ic and basic residues without affecting hydrolysis after acidic residu es. These changes depended on the amount of LMP7 subunits incorporated into proteasomes. Transfection of LMP2 reduced cleavage of peptides a fter acidic residues, increased hydrolysis after basic residues, and d id not affect the hydrophobic activity. Since the activity of the tota l proteasome population changed after incorporation of only small amou nts of LMP2 or -7, these subunits must cause major alterations in pept idase activity. Thus, their expression can account for the changes in proteasome activity induced by interferon gamma, and these findings le nd further support to the proposed roles of LMPs in altering the natur e of the peptides generated for antigen presentation.