Rapid and reproducible single-stage phosphopeptide enrichment of complex peptide mixtures: application to general and phosphotyrosine-specific phosphoproteomics experiments

Reversible protein phosphorylation is an essential regulatory component of virtually every cellular process and is frequently dysregulated in cancer. However, significant analytical barriers persist that hamper the routine application of phosphoproteomics in translational settings. Here, we present a straightforward and reproducible approach for the broadscale analysis of protein phosphorylation that relies on a single phosphopeptide enrichment step using titanium dioxide microspheres from whole cell lysate digests and compared it to the well-established SCX-TiO(2) workflow for phosphopeptide purification on a proteome-wide scale. We demonstrate the scaleabilty of our approach from 200 μg to 5 mg of total NCI-H23 non-small cell lung adenocarcinoma cell lysate digest and determine its quantitative reproducibility by label-free analysis of phosphopeptide peak areas from replicate purifications (median CV: 20% RSD). Finally, we combine this approach with immunoaffinity phosphotyrosine enrichment, enabling the identification of 3168 unique nonredundant phosphotyrosine peptides in two LC-MS/MS runs from 8 mg of HeLa peptides, each with 80% phosphotyrosine selectivity, at a peptide FDR of 0.2%. Taken together, we establish and validate a robust approach for proteome-wide phosphorylation analysis in a variety of scenarios that is easy to implement in biomedical research and translational settings.