Premature Translational Termination Products Are Rapidly Degraded Substrates for MHC Class I Presentation

Nearly thirty percent of all newly synthesized polypeptides are targeted for rapid proteasome-mediated degradation. These rapidly degraded polypeptides (RDPs) are a source of antigenic substrates for the MHC class I presentation pathway, allowing for immunosurveillance of newly synthesized proteins by cytotoxic T lymphocytes. Despite the recognized role of RDPs in MHC I presentation, it remains unclear what molecular characteristics distinguish RDPs from their more stable counterparts. It has been proposed that premature translational termination products may constitute a form of RDP; indeed, in prokaryotes translational drop-off products are normal by-products of protein synthesis and are subsequently rapidly degraded. To study the cellular fate of premature termination products, we used the antibiotic puromycin as a means to experimentally manipulate prematurely terminated polypeptide production in human cells. At low concentrations, puromycin enhanced flux into rapidly degraded polypeptide pools, with small polypeptides being markedly more labile then high molecular weight puromycin adducts. Immunoprecipitation experiments using anti-puromycin antisera demonstrated that the majority of peptidyl-puromycins are rapidly degraded in a proteasome-dependent manner. Low concentrations of puromycin increased the recovery of cell surface MHC I-peptide complexes, indicating that prematurely terminated polypeptides can be processed for presentation via the MHC I pathway. In the continued presence of puromycin, however, MHC I export to the cell surface was inhibited, coincident with the accumulation of polyubiquitinated proteins. The time- and dose-dependent effects of puromycin suggest that the pool of peptidyl-puromycin adducts differ in their targeting to various proteolytic pathways that, in turn, differ in the efficiency with which they access the MHC I presentation machinery. These studies highlight the diversity of cellular proteolytic pathways necessary for the metabolism and immunosurveillance of prematurely terminated polypeptides that are, by their nature, highly heterogeneous.