PTMSearchPlus: Software tool for automated protein identification and post-translational modification characterization by integrating accurate intact protein mass and bottom-up mass spectrometric data searches

PTMSearchPlus is a software tool for the automated integration of accurate intact protein mass (AIPM) and bottom-up (BU) mass spectra searches/data in order to both confidently identify the intact proteins and to characterize their post-translational modifications (PTMs). The development of PTMSearchPlus was motivated by the desire to effectively integrate high-resolution intact protein molecular masses with bottom-up peptide MS/MS data. PTMSearchPlusrequires as input both intact protein and proteolytic peptide mass spectra collected from the same protein mixture, a FASTA protein database, and a selection of possible PTMs, the types and ranges of which can be specified. The output of PTMSearchPlus is a list of intact and modified proteins matching the AIPM data concomitant with their respective peptides found by the BU search. This list also contains protein and peptide sequence coverage information, scores, etc. that can be used for further evaluation or refiltering of the results. Corresponding and annotated AIPM and BU mass spectra are also displayed for visual inspection when a listed protein or a peptide is selected. These and other controls ensure that the user can manually evaluate, modify (e.g., remove obvious false positives, low quality spectra etc.), and save the results of the automated search if necessary. Driven by the exponential growth in the number of possible peptide candidates in a BU search when multiple PTMs are probed, the advantages on search speed by limiting the total number of possible PTMs on a peptide in the BU search or by performing an "AIPM predicted" BU search are also discussed in addition to the integration approach. The features of PTMSearchPlus are demonstrated using both a protein standard mixture and a complex protein mixture from Escherichia coli. Experimental data revealed a unique advantage of coupling AIPM and the BU data sets that is mutually beneficial for both approaches. Namely, AIPM data can confirm that no PTM peptides were missed in a BU search, while the BU search determines the location of the PTM. This information is not available using an AIPM search alone.