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Stoichiometry and Absolute Quantification of Proteins with Mass Spectrometry Using Fluorescent and Isotope-labeled Concatenated Peptide Standards*

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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 α, β, and γ 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 α, β, and γ 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.

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Published, MCP Papers in Press, November 19, 2007, DOI 10.1074/mcp.M700345-MCP200

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This work was supported by the Intramural Research Programs of the National Institute of Mental Health (Grant 1 Z01 MH000274) and the NCI, National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.mcponline.org) contains supplemental material.