The single molecule pull-down (SiMPull) assay allows for capture of individual proteins or macromolecular complexes that can then be interrogated via single molecule imaging. We describe several technical improvements over previous protocols that make it possible to directly detect and quantify the phosphorylation state of thousands of individual membrane receptors, and thereby estimate both the fraction of receptors phosphorylated at specific tyrosine residues and the frequency of multisite phosphorylation. These improvements include 1) the reduction of autofluorescence in the green spectral channel; 2) a simplified imaging chamber that accommodates higher sample number with lower sample volume; 3) corrections for membrane receptor surface expression; 4) three-color multiplex imaging; and 5) corrections for steric hindrance of dual antibody binding. These improvements enabled the first direct detection of multisite phosphorylation on full-length Epidermal Growth Factor Receptor (EGFR) and revealed that phosphorylation fraction varied by tyrosine residue. These SiMPull measurements provide a new level of detail in the status of receptor phosphorylation that was previously inaccessible with traditional biochemical techniques.
INTRODUCTIONThe ability of a cell to respond rapidly and specifically to changes in the surrounding environment is controlled by protein-protein interactions at the plasma membrane and along the signaling cascade. While much is known about the biochemical events that govern signaling pathways, this information has mostly been derived from population-based measurements that typically average over millions of cells and/or proteins. However, there is growing evidence that the heterogeneity of the system contributes to how cellular information is processed [1][2][3][4] . To better understand the role of protein phosphorylation heterogeneity in directing signaling outcomes, we have adapted the single molecule pull down (SiMPull) assay to identifying the phosphorylation state of individual receptors.SiMPull is a powerful technique that allows for interrogation of macromolecular complexes at the individual protein level. Jain et al. first demonstrated the ability of this technique to capture macromolecular complexes 5 . SiMPull samples are prepared in a manner similar to IP/Western Blot protocols, but the sample is interrogated using single molecule microscopy. Briefly, cells are lysed and the protein of interest is captured by antibodies bound to the coverglass. If the proteins are fluorescently tagged, either by fluorescent proteins or subsequent antibody labeling, their presence will be quantified by single molecule imaging (Fig. 1A).Here, we describe a modification of SiMPull for the study of phosphorylation patterns of transmembrane receptors. Traditionally, protein phosphorylation has been measured using ensemble techniques, such as Western Blot analysis or flow cytometry, which provide information on the relative changes of a protein phosphorylation amount. However, these techniques cannot p...