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    Multiplexed Phosphoproteomic Profiling Using Titanium Dioxide and Immunoaffinity Enrichments Reveals Complementary Phosphorylation Events

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    A comprehensive view of protein phosphorylation remains an unmet challenge in the field of cell biology. Mass spectrometry-based proteomics is one of the most promising approaches for identifying thousands of phosphorylation events in a single experiment, yet the full breadth of the phosphoproteome has yet to be elucidated. In this article, we examined the complementarity of two methods for phosphopeptide enrichment based on either titanium dioxide (TiO<sub>2</sub>) enrichment or phosphorylation motif-specific immunoaffinity precipitation (IAP) with four different antibodies. Each method identified nearly 2000 phosphoproteins. However, distinct populations of phosphopeptides were observed. Despite quantifying over 10 000 unique phosphorylation events using TiO<sub>2</sub> and over 3900 with IAP, less than 5% of the sites were in common. Agreeing with published literature, the ratio of pS:pT:pY phosphorylation for the TiO<sub>2</sub>-enriched data set approximated 90:10:<1. In contrast, that ratio for the combined IAP data sets was 51:29:20. These differences not only suggest the complementarity between multiple enrichment methods but also emphasize their collective importance in obtaining a comprehensive view of the phosphoproteome