title: Stoichiometry and absolute quantification of proteins with mass spectrometry using fluorescent and isotope-labeled concatenated peptide standards.
abstract: We have explored a general approach for the determination of absolute amounts and the relative stoichiometry of proteins in a mixture using fluorescence and mass spectrometry. We engineered a gene to express green fluorescent protein (GFP) with a synthetic fusion protein (GAB-GFP) in Escherichia coli to function as a spectroscopic standard for the quantification of an analogous stable isotope-labeled, non-fluorescent fusion protein (GAB*) and for the quantification and stoichiometric analysis of purified transducin, a heterotrimeric G-protein complex. Both GAB-GFP and GAB* contain concatenated sequences of specific proteotypic peptides that are derived from the alpha, beta, and gamma protein subunits of transducin and that are each flanked by spacer regions that maintain the native proteolytic properties for these peptide fragments. Spectroscopic quantification of GAB-GFP provided a molar scale for mass spectrometric ratios from tryptic peptides of GAB* and defined molar responses for mass spectrometric signal intensities from a purified transducin complex. The stoichiometry of transducin subunits alpha, beta, and gamma was measured to be 1:1.1:1.15 over a 5-fold range of labeled internal standard with a relative standard deviation of 9%. Fusing a unique genetically coded spectroscopic signal element with concatenated proteotypic peptides provides a powerful method to accurately quantify and determine the relative stoichiometry of multiple proteins present in complexes or mixtures that cannot be readily assessed using classical gravimetric, enzymatic, or antibody-based technologies.
authors.affiliation: Laboratory of Neurotoxicology, National Institute of Mental Health, NCI, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: Dhaval
authors.initials: D
authors.lastname: Nanavati
authors.forename: Marjan
authors.initials: M
authors.lastname: Gucek
authors.forename: Jacqueline L S
authors.initials: JL
authors.lastname: Milne
authors.forename: Sriram
authors.initials: S
authors.lastname: Subramaniam
authors.forename: Sanford P
authors.initials: SP
authors.lastname: Markey
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D049452:Green Fluorescent Proteins / Q000737:chemistry / Q000378:metabolism*; D007553:Isotope Labeling; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D011506:Proteins / Q000032:analysis* / Q000737:chemistry; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D012015:Reference Standards; D015203:Reproducibility of Results; D013997:Time Factors; D015293:Transducin / Q000737:chemistry / Q000378:metabolism; D014357:Trypsin / Q000378:metabolism
chemical_list: D010455:Peptides; D021122:Protein Subunits; D011506:Proteins; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D014357:Trypsin; D015293:Transducin