Using iTRAQ Combined with Tandem Affinity Purification to Enhance Low-Abundance Proteins Associated with Somatically Mutated EGFR Core Complexes in Lung Cancer

In this study, we report a novel use for the iTRAQ reagent combined with a peptide mass inclusion list to enhance the signal of low-abundance proteins during analysis by mass spectrometry. C-tagged-SH-EGFR was retrovirally transduced into two mutant lung cancer cell lines (HCC827 and PC9), and the core protein complexes were enriched by tandem affinity purification. Tryptically digested peptides were derivatized with iTRAQ and analyzed by higher-energy collision-induced dissociation mass spectrometry. The data revealed that UBS3B is a member of the EGFR core complex in the HCC827 cell line, which was not apparent by standard, unbiased one-dimensional shotgun analysis and collision-induced dissociation. The expression level of UBS3B, however, was 6-10 times lower than that observed in the PC9 cell line. Thus, using iTRAQ in this fashion allows the identification of low-abundance interactors when combined with samples where the same protein has a higher abundance. Ultimately, this approach may uncover proteins that were previously unknown or only suspected as members of core protein complexes.