Contribution of proteasomal β-subunits to the cleavage of peptide substrates analyzed with yeast mutants

Proteasomes generate peptides that can be presented by major histocompatibility complex (MHC) class I molecules in vertebrate cells. Using yeast 20 S proteasomes carrying different inactivated β-subunits, we investigated the specificities and contributions of the different β-subunits to the degradation of polypeptide substrates containing MHC class I ligands and addressed the question of additional proteolytically active sites apart from the active β-subunits. We found a clear correlation between the contribution of the different subunits to the cleavage of fluorogenic and long peptide substrates, with β5/Pre2 cleaving after hydrophobic, β2/Pup1 after basic, and β1/Pre3 after acidic residues, but with the exception that β2/Pup1 and β1/Pre3 can also cleave after some hydrophobic residues. All proteolytic activities including the 'branched chain amino acid-preferring' component are associated with β5/Pre2, β1/Pre3, or β2/Pup1, arguing against additional proteolytic sites. Because of the high homology between yeast and mammalian 20 S proteasomes in sequence and subunit topology and the conservation of cleavage specificity between mammalian and yeast proteasomes, our results can be expected to also describe most of the proteolytic activity of mammalian 20 S proteasomes leading to the generation of MHC class I ligands.