Differences in the production of spliced antigenic peptides by the standard proteasome and the immunoproteasome

Peptide splicing allows the production of antigenic peptides composed of two fragments initially non-contiguous in the parental protein. The proposed mechanism of splicing is a transpeptidation occurring within the proteasome. Three spliced peptides, derived from FGF-5, melanoma protein gp100 and nuclear protein SP110, have been described. Here, we compared the production of these spliced peptides by the standard proteasome and the immunoproteasome. Differential isotope labelling was used to quantify ( by mass spectrometry) the fragments contained in digests obtained with precursor peptides and purified proteasomes. The results show that both the standard and the immunoproteasomes can produce spliced peptides although they differ in their efficiency of production of each peptide. The FGF-5 and gp100 peptides are more efficiently produced by the standard proteasome, whereas the SP110 peptide is more efficiently produced by the immunoproteasome. This seems to result from differences in the production of the two splicing partners, which depends on a balance between cleavages liberating or destroying those fragments. By showing that splicing depends on the efficiency of production of the splicing partners, these results also support the transpeptidation model of peptide splicing. Furthermore, given the presence of immunoproteasomes in dendritic cells and cells exposed to IFN-gamma, the findings may be relevant for vaccine design.