title: A mass-tolerant database search identifies a large proportion of unassigned spectra in shotgun proteomics as modified peptides.
abstract: Fewer than half of all tandem mass spectrometry (MS/MS) spectra acquired in shotgun proteomics experiments are typically matched to a peptide with high confidence. Here we determine the identity of unassigned peptides using an ultra-tolerant Sequest database search that allows peptide matching even with modifications of unknown masses up to ± 500 Da. In a proteome-wide data set on HEK293 cells (9,513 proteins and 396,736 peptides), this approach matched an additional 184,000 modified peptides, which were linked to biological and chemical modifications representing 523 distinct mass bins, including phosphorylation, glycosylation and methylation. We localized all unknown modification masses to specific regions within a peptide. Known modifications were assigned to the correct amino acids with frequencies >90%. We conclude that at least one-third of unassigned spectra arise from peptides with substoichiometric modifications.
authors.affiliation: Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Joel M
authors.initials: JM
authors.lastname: Chick
authors.affiliation: Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Deepak
authors.initials: D
authors.lastname: Kolippakkam
authors.affiliation: Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: David P
authors.identifier: 0000000278195261
authors.initials: DP
authors.lastname: Nusinow
authors.affiliation: Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Bo
authors.identifier: 0000000251203241
authors.initials: B
authors.lastname: Zhai
authors.affiliation: Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Ramin
authors.initials: R
authors.lastname: Rad
authors.affiliation: Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Edward L
authors.initials: EL
authors.lastname: Huttlin
authors.affiliation: Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Steven P
authors.initials: SP
authors.lastname: Gygi
mesh_terms: D000595:Amino Acid Sequence; D030562:Databases, Protein; D057809:HEK293 Cells; D006801:Humans; D008969:Molecular Sequence Data; D010449:Peptide Mapping / Q000379:methods*; D010455:Peptides / Q000032:analysis* / Q000737:chemistry; D011506:Proteins / Q000032:analysis / Q000737:chemistry; D040901:Proteomics / Q000379:methods*; D053719:Tandem Mass Spectrometry
chemical_list: D010455:Peptides; D011506:Proteins