The group II chaperonin TRiC protects proteolytic intermediates from degradation in the MHC class I antigen...

MHC class I molecules present precisely cleaved peptides of intracellular proteins on the cell surface. For most antigenic precursors, presentation requires transport of peptide fragments into the ER, but the nature of the cytoplasmic peptides and their chaperones is obscure. By tracking proteolytic intermediates in living cells, we show that intracellular proteolysis yields a mixture of antigenic peptides containing only N-terminal flanking residues for ER transport. Some of these peptides were bound to the group II chaperonin TRiC and were protected from degradation. Destabilization of TRiC by RNA interference inhibited the expression of peptide-loaded MHC I molecules on the cell surface. Thus, the TRiC chaperonin serves a function in protecting proteolytic intermediates in the MHC I antigen processing pathway.