NEDD8 activity is important for direct antigen MHC class I antigen presentation

Successful direct MHC class I antigen presentation is dependent on the protein degradation machinery of the cell to generate antigenic peptides which 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-proteasome system, however other ubiquitin-like (UBL) proteins have also been implicated in protein degradation and direct antigen presentation. Here, we examine the role of neuronal precursor cell-expressed developmentally down-regulated protein 8 (NEDD8) in direct antigen presentation. NEDD8 is the UBL with highest similarity to ubiquitin and fusion of NEDD8 to the amino-terminus of a target protein can lead to the target proteins degradation. We find that appending NEDD8 to the N-terminus of the model antigen ovalbumin resulted in degradation by both the proteasome and autophagy protein degradation pathways, but only proteasomal degradation, involving the proteasomal subunit NEDD8 ultimate buster 1 (NUB1), resulted in peptide presentation. When directly compare to ubiquitin, 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 Defective Ribosomal Products (DRiPs) derived from a model antigen. These results demonstrate that NEDD8 activity in the cell is important for direct antigen presentation, but not by directly targeting proteins for degradation.