Tight Linkage between Translation and MHC Class I Peptide Ligand Generation Implies Specialized Antigen Processing for Defective Ribosomal Products

Abstract

There is mounting evidence that MHC class I peptide ligands are predominantly generated from defective ribosomal products and other classes of polypeptides degraded rapidly (t1/2 < 10 min) following their synthesis. The most direct evidence supporting this conclusion is the rapid inhibition of peptide ligand generation following cycloheximide-mediated inhibition of protein synthesis. In this study, we show that this linkage is due to depleting the pool of rapidly degraded proteins, and not to interference with other protein synthesis-dependent processes. Our findings indicate that in the model systems used in this study, MHC class I peptides are preferentially generated from rapidly degraded polypeptides relative to slowly degraded proteins. This conclusion is supported by the properties of peptide presentation from slowly degraded (t1/2 = 4 h) defective ribosomal products generated artificially by incorporation of the amino acid analog canavanine into a model viral Ag. We propose that specialized machinery exists to link protein synthesis with class I peptide ligand generation to enable the rapid detection of viral gene expression.

Autophagy inhibition promotes defective neosynthesized proteins storage in ALIS, and induces redirection toward proteasome processing and MHCI-restricted presentation

“…Interestingly, proteasome inhibition increased the accumulation of fluorescent, properly conformed GFP-SL8 by nearly 40%, suggesting that a subset of newly synthesized proteasomedegraded GFP might not be irreversibly misfolded and can become fluorescent if provided with enough time to fold. 35 Conversely, autophagy inhibition only weakly increased fluorescent GFP-SL8 levels, while it enhanced antigen presentation by almost 40% (Fig. 5C).…”

Compartmentalized MHC class I antigen processing enhances immunosurveillance by circumventing the law of mass action

“…The observation that peptides generated in the cytosol by different processes access the class I pathway differently helps explain the puzzling discrepancy between the ability of peptides from DRiPs versus "retirees" to access the class I pathway (29,30) and the ability of exogenous antigens to avoid competition from endogenous peptides in accessing the class I pathway. This is likely an example of a cellular process that enhances its efficiency through physical or physiochemical compartmentalization (23,31).…”

Translating DRiPs: MHC class I immunosurveillance of pathogens and tumors

“…This was implied by the dependence of overall peptide generation on protein synthesis [47,119], despite the unabated degradation of the proteome during acute CHXinduced protein synthesis blockade [57,120] (see Fig. 1 in ref. [120]).…”

“…1 in ref. [120]). Differential substrate access was observed when expressing rapidly versus relatively slowly degraded, full-length proteasome substrates (half-lives of 10 min vs. 70 min), the latter of which is presented at twice the efficiency per protein degraded [46], or with canavanine-misfolded proteins, where RDPs were preferentially presented over slowly degraded forms [120]. Furthermore, competition studies [121] revealed that DRiP-derived SIINFEKL is presented in a manner that completely avoids competition with high-affinity, K b -binding, 8-mer peptides, generated in excess amounts in the cytosol from direct synthesis of minigenes or by proteolytic liberation from ubiquitin-fusion proteins.…”