An important area of proteomics involves the need for quantification, whether relative or absolute. Many methods now exist for relative quantification, but to support biomarker proteomics and systems biology, absolute quantification rather than relative quantification is required. Absolute quantification usually involves the concomitant mass spectrometric determination of signature proteotypic peptides and stable isotope-labeled analogs. However, the availability of standard labeled signature peptides in accurately known amounts is a limitation to the widespread adoption of this approach. We describe the design and synthesis of artificial QconCAT proteins that are concatamers of tryptic peptides for several proteins. This protocol details the methods for the design, expression, labeling, purification, characterization and use of the QconCATs in the absolute quantification of complex protein mixtures. The total time required to complete this protocol (from the receipt of the QconCAT expression plasmid to the absolute quantification of the set of proteins encoded by the QconCAT protein in an analyte sample) is approximately 29 d.

Stable isotope-labeled proteotypic peptides are used as surrogate standards for absolute quantification of proteins in proteomics. However, a stable isotope-labeled peptide has to be synthesized, at relatively high cost, for each protein to be quantified. To multiplex protein quantification, we developed a method in which gene design de novo is used to create and express artificial proteins (QconCATs) comprising a concatenation of proteotypic peptides. This permits absolute quantification of multiple proteins in a single experiment. This complete study was constructed to define the nature, sources of error, and statistical behavior of a QconCAT analysis. The QconCAT protein was designed to contain one tryptic peptide from 20 proteins present in the soluble fraction of chicken skeletal muscle. Optimized DNA sequences encoding these peptides were concatenated and inserted into a vector for high level expression in Escherichia coli. The protein was expressed in a minimal medium containing amino acids selectively labeled with stable isotopes, creating an equimolar series of uniformly labeled proteotypic peptides. The labeled QconCAT protein, purified by affinity chromatography and quantified, was added to a homogenized muscle preparation in a known amount prior to proteolytic digestion with trypsin. As anticipated, the QconCAT was completely digested at a rate far higher than the analyte proteins, confirming the applicability of such artificial proteins for multiplexed quantification. The nature of the technical variance was assessed and compared with the biological variance in a complete study. Alternative ionization and mass spectrometric approaches were investigated, particularly LC-ESI-TOF MS and MALDI-TOF MS, for analysis of proteins and tryptic peptides. QconCATs offer a new and efficient approach to precise and simultaneous absolute quantification of multiple proteins, subproteomes, or even entire proteomes. Molecular & Cellular Proteomics 6:1416 -1427, 2007.As the field of proteomics matures as a discipline, there is an increasing realization of the importance of absolute as well as relative quantification, and considerable effort is being directed toward experimental strategies to achieve this goal. Most commonly, relative protein quantification by mass spectrometry has been based on differential stable isotope labeling implemented by metabolic incorporation (1, 2) or through derivatization strategies such as ICAT (3). The mass-coded abundance tagging method (4) avoids the use of stable isotopes but requires assumptions concerning mass spectrometric response factors. To achieve relative quantification of proteins without isotope labeling or chemical modification steps, quantitative comparisons have been made of equivalent sets of mass spectrometric data by considering peptide detectability in repetitively acquired spectra or by comparing integrated extracted ion chromatograms following liquid chromatography-mass spectrometry analyses (5).In principle, any of the approaches adopted for relative quantificatio...

The 'protein world' exhibits additional complexity caused by post-translational modifications. One such process is nonenzymic deamidation of asparagine which is controlled partly by primary sequence, but also higher order protein structure. We have studied the deamidation of an N-terminal peptide in muscle glyceraldehyde 3-phosphate dehydrogenase to relate three-dimensional structure, proteolysis, and deamidation. This work has significant consequences for identification of proteins using peptide mass fingerprinting.