Current chemical tagging strategies for proteome analysis by mass spectrometry.

Proteomics, the analysis of the protein complement of a cell or an organism, has grown rapidly as a subdiscipline of the life sciences. Mass spectrometry (MS) is one of the central detection techniques in proteome analysis, yet it has to rely on prior sample preparation steps that reduce the enormous complexity of the protein mixtures obtained from biological systems. For that reason, a number of so-called tagging (or labeling) strategies have been developed that target specific amino acid residues or post-translational modifications, enabling the enrichment of subfractions via affinity clean-up, resulting in the identification of an ever increasing number of proteins. In addition, the attachment of stable-isotope-labeled tags now allows the relative quantitation of protein levels of two samples, e.g. those representing different cell states, which is of great significance for drug discovery and molecular biology. Finally, tagging schemes also serve to facilitate interpretation of MS/MS spectra, therefore assisting in de novo elucidation of protein sequences and automated database searching. This review summarizes the different application fields for tagging strategies for today's MS-based proteome analysis. Advantages and drawbacks of the numerous strategies that have appeared in the literature in the last years are highlighted, and an outlook on emerging tagging techniques is given.