MLN4924 Inhibits Defective Ribosomal Product Antigen Presentation Independently of Direct NEDDylation of Protein Antigens

Abstract

Successful direct MHC class I Ag presentation is dependent on the protein degradation machinery of the cell to generate antigenic peptides that can be loaded onto MHC class I molecules for surveillance by CD8⁺ T cells of the immune system. Most often this process involves the ubiquitin (Ub)–proteasome system; however, other Ub-like proteins have also been implicated in protein degradation and direct Ag presentation. In this article, we examine the role of neuronal precursor cell–expressed developmentally downregulated protein 8 (NEDD8) in direct Ag presentation in mouse cells. NEDD8 is the Ub-like protein with highest similarity to Ub, and fusion of NEDD8 to the N terminus of a target protein can lead to the degradation of target proteins. We find that appending NEDD8 to the N terminus of the model Ag OVA resulted in degradation by both the proteasome and the autophagy protein degradation pathways, but only proteasomal degradation, involving the proteasomal subunit NEDD8 ultimate buster 1, resulted in peptide presentation. When directly compared with Ub, NEDD8 fusion was less efficient at generating peptides. However, inactivation of the NEDD8-conugation machinery by treating cells with MLN4924 inhibited the presentation of peptides from the defective ribosomal product–derived form of a model Ag. These results demonstrate that NEDD8 activity in the cell is important for direct Ag presentation, but not by directly targeting proteins for degradation.