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.