MHC-LINKED LMP GENE-PRODUCTS SPECIFICALLY ALTER PEPTIDASE ACTIVITIES OF THE PROTEASOME

PROTEASOMES are highly conserved macromolecular structures which funct ion as endopeptidases1-3. They are found in the cytoplasm and nucleus of eukaryotic tissues and consist of at least 14 non-identical subunit s with molecular masses ranging from approximately 20 to 32K. Proteaso mes are essential in the selective degradation of ubiquitinated and ce rtain non-ubiquitinated proteins, acting as the proteolytic core of an energy-dependent 26S (1,500K) proteolytic complex. Two proteasome sub units, LMP2 and LMP7 (refs 4-7), are encoded within the major histocom patibility complex (MHC), implicating proteasomes in antigen processin g8-9. Here we determine the function of these two MHC-linked subunits by comparing the proteolytic activities of purified proteasomes contai ning (LMP+) or lacking (LMP-) these components. We find that proteasom es of both types have endopeptidase activity against substrates bearin g hydrophobic, basic or acidic residues immediately preceding the clea vage site (the P1 position) and at sites following asparagine, glycine and proline residues. The activity of LMP+ proteasomes is much higher than that of LMP- proteasomes against substrates with hydrophobic, ba sic or asparagine residues at P1, whereas their activities are compara ble when acidic and glycine residues are present at P1. The MHC-linked LMP2 and LMP7 subunits therefore function to amplify specific endopep tidase activities of the proteasome.