An integrated global strategy for cell lysis, fractionation, enrichment and mass spectrometric analysis of phosphorylated peptides

Recently, the field of phosphoproteomics has progressed to the point where thousands of protein phosphorylations can be analyzed simultaneously and used to address significant biological questions. However, several challenges still exist in current LC-MS/MS-based phosphoproteomics methods. Among these are the increased dynamic range of phosphoproteomics samples (due to low stoichiometry of most protein phosphorylations), insufficient inhibition of phosphatase activity, and neutral losses which occur during phosphopeptide fragmentation by MS(n). Here we present an improved method, free of conventional phosphatase inhibitors, for sample treatment to minimize phosphatase activity and improve the efficiency of phosphopeptide enrichment. We also present a solution-based IEF method for phosphopeptide fractionation and explore the utility of various fragmentation methods for identifying phosphopeptides and localizing phosphorylation sites.