title: The APEX Quantitative Proteomics Tool: generating protein quantitation estimates from LC-MS/MS proteomics results.
abstract: BACKGROUND
  Mass spectrometry (MS) based label-free protein quantitation has mainly focused on analysis of ion peak heights and peptide spectral counts. Most analyses of tandem mass spectrometry (MS/MS) data begin with an enzymatic digestion of a complex protein mixture to generate smaller peptides that can be separated and identified by an MS/MS instrument. Peptide spectral counting techniques attempt to quantify protein abundance by counting the number of detected tryptic peptides and their corresponding MS spectra. However, spectral counting is confounded by the fact that peptide physicochemical properties severely affect MS detection resulting in each peptide having a different detection probability. Lu et al. (2007) described a modified spectral counting technique, Absolute Protein Expression (APEX), which improves on basic spectral counting methods by including a correction factor for each protein (called Oi value) that accounts for variable peptide detection by MS techniques. The technique uses machine learning classification to derive peptide detection probabilities that are used to predict the number of tryptic peptides expected to be detected for one molecule of a particular protein (Oi). This predicted spectral count is compared to the protein's observed MS total spectral count during APEX computation of protein abundances.
  
  
  RESULTS
  The APEX Quantitative Proteomics Tool, introduced here, is a free open source Java application that supports the APEX protein quantitation technique. The APEX tool uses data from standard tandem mass spectrometry proteomics experiments and provides computational support for APEX protein abundance quantitation through a set of graphical user interfaces that partition th parameter controls for the various processing tasks. The tool also provides a Z-score analysis for identification of significant differential protein expression, a utility to assess APEX classifier performance via cross validation, and a utility to merge multiple APEX results into a standardized format in preparation for further statistical analysis.
  
  
  CONCLUSIONS
  The APEX Quantitative Proteomics Tool provides a simple means to quickly derive hundreds to thousands of protein abundance values from standard liquid chromatography-tandem mass spectrometry proteomics datasets. The APEX tool provides a straightforward intuitive interface design overlaying a highly customizable computational workflow to produce protein abundance values from LC-MS/MS datasets.
authors.affiliation: Pathogen Functional Genomics Resource Center, J Craig Venter Institute, Rockville, MD 20850, USA. braisted@jcvi.org
authors.forename: John C
authors.initials: JC
authors.lastname: Braisted
authors.forename: Srilatha
authors.initials: S
authors.lastname: Kuntumalla
authors.forename: Christine
authors.initials: C
authors.lastname: Vogel
authors.forename: Edward M
authors.initials: EM
authors.lastname: Marcotte
authors.forename: Alan R
authors.initials: AR
authors.lastname: Rodrigues
authors.forename: Rong
authors.initials: R
authors.lastname: Wang
authors.forename: Shih-Ting
authors.initials: ST
authors.lastname: Huang
authors.forename: Erik S
authors.initials: ES
authors.lastname: Ferlanti
authors.forename: Alexander I
authors.initials: AI
authors.lastname: Saeed
authors.forename: Robert D
authors.initials: RD
authors.lastname: Fleischmann
authors.forename: Scott N
authors.initials: SN
authors.lastname: Peterson
authors.forename: Rembert
authors.initials: R
authors.lastname: Pieper
mesh_terms: D002853:Chromatography, Liquid; D030562:Databases, Protein; D020543:Proteome / Q000032:analysis*; D040901:Proteomics / Q000379:methods*; D020539:Sequence Analysis, Protein / Q000379:methods; D012984:Software; D053719:Tandem Mass Spectrometry; D014584:User-Computer Interface
chemical_list: D020543:Proteome