We previously reported a simple method to analyze the interaction of cell-surface molecules in living cells. This method termed enzyme-mediated activation of radical sources (EMARS) is featured by radical formation of the labeling reagent by horseradish peroxidase (HRP). Herein, we propose an approach to the cell-surface molecular interactome by using combination of this EMARS reaction and MS-based proteomics techniques. In the current study, we employed a novel labeling reagent, fluorescein-conjugated arylazide. The fluorescein-tagged proteins resulting from the EMARS reaction were directly detected in the electrophoresis gels with a fluorescence image analyzer. These products were also purified and concentrated by immunoaffinity chromatography with anti-fluorescein antibody-immobilized resins. The purified fluorescein-tagged proteins were subsequently subjected to an MS-based proteomics analysis. Analysis using HRP-conjugated cholera toxin subunit B, which recognizes a lipid raft marker, ganglioside GM1, revealed 30 membrane and secreted proteins that were candidates for the cell-surface molecules coclustering with GM1. The proposed approach will provide a clue to study functional molecular interactions in a variety of biological events on the cell surface.
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