Phosphotyrosine Biased Enrichment of Tryptic Peptides from Cancer Cells by Combining pY-MIP and TiO2 Affinity Resins.

Protein phosphorylation at distinct tyrosine residues (pY) is essential for fast, specific, and accurate signal transduction in cells. Enrichment of pY-containing peptides derived from phosphoproteins is commonly facilitated by use of immobilized anti-pY antibodies prior to phosphoproteomics analysis by mass spectrometry. We here report on an alternative approach for pY-peptide enrichment using inexpensive pY-imprinted polymer (pY-MIP). We assessed by mass spectrometry the performance of pY-MIP for enrichment and sequencing of phosphopeptides obtained by tryptic digestion of protein extracts from HeLa cells. The combination of pY-MIP- and TiO2-based phosphopeptide enrichment provided more than 90% selectivity for phosphopeptides. Mass spectrometry signal intensities were enhanced for most pY-phosphopeptides (approximately 70%) when using the pY-MIP-TiO2 combination as compared to TiO2 alone. pY constituted up to 8% of the pY-MIP-TiO2-enriched phosphopeptide fractions. The pY-MIP-TiO2 and the TiO2 protocols yielded comparable numbers of distinct phosphopeptides, 1693 and 1842, respectively, from microgram levels of peptide samples. Detailed analysis of physicochemical properties of pY-MIP-TiO2-enriched phosphopeptides demonstrated that this protocol retrieved phosphopeptides that tend to be smaller (<24 residues), less acidic, and almost exclusively monophosphorylated, as compared to TiO2 alone. These unique properties render the pY-MIP-based phosphopeptide enrichment technique an attractive alternative for applications in phosphoproteomics research.