Data analysis and interpretation remain major logistical challenges when attempting to identify large numbers of protein phosphorylation sites by nanoscale reverse-phase liquid chromatography/tandem mass spectrometry (LC-MS/MS) (Supplementary Figure 1 online). In this report we address challenges that are often only addressable by laborious manual validation, including data set error, data set sensitivity and phosphorylation site localization. We provide a large-scale phosphorylation data set with a measured error rate as determined by the target-decoy approach, we demonstrate an approach to maximize data set sensitivity by efficiently distracting incorrect peptide spectral matches (PSMs), and we present a probability-based score, the Ascore, that measures the probability of correct phosphorylation site localization based on the presence and intensity of site-determining ions in MS/MS spectra. We applied our methods in a fully automated fashion to nocodazole-arrested HeLa cell lysate where we identified 1,761 nonredundant phosphorylation sites from 491 proteins with a peptide false-positive rate of 1.3%.
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We thank David Chiang, James Candlin and the software developers at Sage-N-Research for early access to Sequest-Sorcerer and on-the-fly peptide reversal within the Sequest algorithm. We thank P. Everley, C. Bakalarski and B. Faherty for in-house software development and D. Schwartz for motif analysis. HeLa cell lysate was generously provided by M. Rape. This work was supported in part by grants from the National Institutes of Health (HG03456 and GM67945).
The authors declare no competing financial interests.
General Strategy for the large-scale analysis of protein phosphorylation. (PDF 757 kb)
Determining an effective search space using the target/decoy strategy. (PDF 863 kb)
Residue composition from data sets of known phosphorylation sites. (PDF 1768 kb)
Comparison of the Ascore versus Sequest scoring criteria for site localization. (PDF 3070 kb)
Comparison of the Ascore versus Mascot scoring criteria for site localization. (PDF 3256 kb)
Sequence logos (http://weblogo.berkeley.edu/) of the motifs identified in this large-scale data set shown in Supplementary table 1 with an Ascore > 15. (PDF 978 kb)
Biological processes of 362 of the 491 proteins identified in this experiment. (PDF 622 kb)
SDS-PAGE gel used in this experiment. (PDF 5115 kb)
Filtering criteria for the entire experiment. (PDF 1053 kb)
2,836 identified phosphopeptides from the entire experiment. (XLS 1649 kb)
Synthetic peptide libraries. (XLS 815 kb)
Immunoprecipitation experiments. (XLS 495 kb)
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