Selection of Substrates for MHC Class I‐Restricted Antigen Processing

We have proposed a model for selection of cytosolic proteins for MHC class I‐restricted antigen processing (Nat Rev Immunol, 2007, 7: 403). In short, incomplete capture of nascent polypeptides by the protein folding machinery, resulting in uncompacted hydrophobic domains, triggers ubiquitin (Ub)‐independent degradation by the 20S proteasome. We have tested this model in three ways. First, we revisited the classic Townsend et al. finding (Nature, 1986, 324: 575) that directing influenza hemagglutinin to the cytosol through signal sequence ablation substantially increases presentation. We hypothesized that the unembeded transmembrane domain (TM), rather than misfolding, stimulates rapid degradation. Indeed, various manipulations of the human IL‐2 receptor alpha chain (Tac) and its TM support this idea. Second, using a dominant‐interfering Ub system, we have probed the role of polyUb’n in the presentation of various antigen processing substrates. To date, all stable cytosolic proteins tested are largely, if not exclusively, presented in a polyUb‐independent manner as opposed to other types of processing substrate. Finally, we have probed the role of the two ribosome‐associated chaperone complexes, RAC and NAC, in antigen processing. NAC, but not RAC silencing resulted in a ~2‐fold increase in peptide/class I complexes at the cell surface. Thus, our proposed pathway remains viable.