Abstract
Post-translational modifications (PTMs) of receptor tyrosine kinases (RTKs) at the plasma membrane (PM) determine the signal transduction efficacy alone and in combination. However, current approaches to identify PTMs provide ensemble results, inherently overlooking combinatorial PTMs in a single polypeptide molecule. Here, we describe a single-molecule blotting (SiMBlot) assay that combines biotinylation of cell surface receptors with single-molecule fluorescence microscopy. This method enables quantitative measurement of the phosphorylation status of individual membrane receptor molecules and colocalization analysis of multiple immunofluorescence signals to directly visualize pairwise site-specific phosphorylation patterns at the single-molecule level. Strikingly, application of SiMBlot to study ligand-dependent epidermal growth factor receptor (EGFR) phosphorylation, which is widely thought to be multi-phosphorylated, reveals that EGFR on cell membranes is hardly multi-phosphorylated, unlike in vitro autophosphorylated EGFR. Therefore, we expect SiMBlot to aid understanding of vast combinatorial PTM patterns, which are concealed in ensemble methods, and to broaden knowledge of RTK signaling.