Increasing phosphoproteome coverage and identification of phosphorylation motifs through combination of different HPLC fractionation methods.

Protein phosphorylation activates or deactivates many other proteins especially protein enzymes, and plays a significant role in a wide range of cellular processes. Recent advances in phosphopeptide enrichment procedures and mass spectrometry-based peptide sequencing techniques have enabled us to identify large number of protein phosphorylation sites. In this study, we combined three different HPLC techniques in fractionating enriched phosphopeptides before RPLC-MS/MS analysis, and found that although between 4000-5000 unique phosphopeptides could be identified following any of the HPLC fraction method, different HPLC method yielded a considerable amount of non-overlapping unique phosphopeptides. Combining data from all the HPLC methods, we were able to identify 9069 unique phosphopeptides and 3260 phosphoproteins covering 9463 unique phosphorylation sites, indicating that different HPLC methods are complementary to each other, and can be used together in order to increase the phosphoproteome coverage. A number of new phosphorylation sites and novel phosphorylation motifs were also discovered from our study.