Phosphorylation site localization in peptides by MALDI MS/MS and the Mascot Delta Score

Owing to its broad biological significance, the large-scale analysis of protein phosphorylation is more and more getting into the focus of proteomic research. Thousands of phosphopeptides can nowadays be identified using state-of-the-art tandem mass spectrometers in conjunction with sequence database searching, but localizing the phosphate group to a particular amino acid in the peptide sequence is often still difficult. Using 180 individually synthesized phosphopeptides with precisely known phosphorylation sites (p-sites), we have assessed the merits of the Mascot Delta Score (MD score) for the assignment of phosphorylation sites from tandem mass spectra (MS/MS) generated on four different matrix-assisted laser desorption ionization (MALDI) mass spectrometers including tandem time-of-flight (TOF/TOF), quadrupole time-of-flight, and ion trap mass analyzers. The results show that phosphorylation site identification is generally possible with false localization rates of about 10%. However, a comparison to previous work also revealed that phosphorylation site determination by MALDI MS/MS is less accurate than by ESI-MS/MS particularly if several and/or adjacent possible phosphorylation acceptor sites exist in a peptide sequence. We are making the tandem MS spectra and phosphopeptide collection available to the community so that scientists may adapt the MD scores reported here to their analytical environment and so that informatics developers may integrate the MD score into proteomic data analysis pipelines.