RATE OF ANTIGEN DEGRADATION BY THE UBIQUITIN-PROTEASOME PATHWAY INFLUENCES MHC CLASS-I PRESENTATION

The effect on MHC class I Ag presentation of enhancing a protein's rat e of degradation by the ubiquitin-proteasome pathway was investigated, In extracts of mouse B-lymphoblasts and reticulocytes, as in rabbit r eticulocytes, proteins with acidic or basic N-termini are conjugated t o ubiquitin and degraded by the 26S proteasome very rapidly, We found that the rate of MHC class I presentation of microinjected beta-galact osidase was enhanced when this antigenic protein was modified with suc h a destabilizing amino-terminal residue, This enhanced presentation w as inhibited by blocking potential ubiquitination sites on the protein through methylation of amino groups and by peptide aldehyde inhibitor s of the proteasome. Furthermore, in B lymphoblast cell extracts, the rapid degradation of these beta-galactosidase constructs required ATP and ubiquitin and was blocked by inhibitors of proteasomes. Their rate s of degradation in extracts correlated with their rates of class I Ag presentation in vivo. These results indicate that ubiquitin conjugati on is a key rate-limiting step in Ag presentation and provide further evidence for a critical role of ubiquitin and the 26S proteasome in ge nerating MHC class I-presented peptides.