Ubiquitinated proteins in exosomes secreted by myeloid-derived suppressor cells.

We provide evidence at the molecular level that ubiquitinated proteins are present in exosomes shed by myeloid-derived suppressor cells (MDSC). Ubiquitin was selected as a post-translational modification of interest because it is known to play a determinant role in the endosomal trafficking that culminates in exosome release. Enrichment was achieved by two immunoprecipitations, first at the protein level and subsequently at the peptide level. Fifty ubiquitinated proteins were identified by tandem mass spectrometry filtering at a 5% spectral false discovery rate and using the conservative requirement that glycinylglycine-modified lysine residues were observed in tryptic peptides. Thirty five of these proteins have not previously been reported to be ubiquitinated. The ubiquitinated cohort spans a range of protein sizes and favors basic pI values and hydrophobicity. Five proteins associated with endosomal trafficking were identified as ubiquitinated, along with pro-inflammatory high mobility group protein B1 and proinflammatory histones.