MHC-I presentation of peptides derived from intact protein products of the pioneer round of translation.

Among the earliest protein products of most cellular genes are those synthesized during the pioneer round of translation (PRT), a key step in nonsense-mediated mRNA decay (NMD) that allows scanning of new transcripts for the presence of a premature termination codon (PTC). It has been demonstrated that at least some PRT degradation products can be targeted to major histocompatibility (MHC)-I presentation. To gain new insight into this putative PRT-to-MHC-I route, we have assembled 2 pairs of reporter genes so that the 2 genes in each pair encode an identical fusion protein between a model antigenic peptide and enhanced green fluorescent protein (EGFP), one of which harbors a PTC. We expressed these genes in different mouse and human cell lines and confirmed enhanced NMD activity for the PTC(+) gene in each pair by monitoring the effect of cycloheximide on the level of the respective mRNA. We then exploited several strategies for establishing the ratio between level of peptide presentation and total amount of protein product. We consistently observed significantly higher ratios for the PTC(+) mRNAs compared with the PTC(-) ones, pointing to correlation between the turnover of otherwise identical proteins and the fate of their template mRNA. Using confocal microscopy, we showed a clear link between NMD, the presence of misfolded EGFP polypeptides, and enhanced MHC-I peptide presentation. Altogether, these findings imply that identical full-length gene products differing only in 3' noncoding sequences can be differentially degraded and targeted to the MHC-I presentation pathway, suggesting a more general role for the PRT in establishing the MHC-I peptidome.-Weinstein-Marom, H., Hendel, L., Laron, E. A., Sharabi-Nov, A., Margalit, A., Gross, G. MHC-I presentation of peptides derived from intact protein products of the pioneer round of translation.