Global kinetic analysis of proteolysis via quantitative targeted proteomics

Abstract

Mass spectrometry-based proteomics is a powerful tool for identifying hundreds to thousands of posttranslational modifications in complex mixtures. However, it remains enormously challenging to simultaneously assess the intrinsic catalytic efficiencies (k cat ∕K M ) of these modifications in the context of their natural interactors. Such fundamental enzymological constants are key to determining substrate specificity and for establishing the timing and importance of cellular signaling. Here, we report the use of selected reaction monitoring (SRM) for tracking proteolysis induced by human apoptotic caspases-3, -7, -8, and -9 in lysates and living cells. By following the appearance of the cleaved peptides in lysate as a function of time, we were able to determine hundreds of catalytic efficiencies in parallel. Remarkably, we find the rates of substrate hydrolysis for individual caspases vary greater than 500-fold indicating a sequential process. Moreover, the rank-order of substrate cutting is similar in apoptotic cells, suggesting that cellular structures do not dramatically alter substrate accessibility. Comparisons of extrinsic (TRAIL) and intrinsic (staurosporine) inducers of apoptosis revealed similar substrate profiles, suggesting the final proteolytic demolitions proceed by similarly ordered plans. Certain biological processes were rapidly targeted by the caspases, including multiple components of the endocyotic pathway and miRNA processing machinery. We believe this massively parallel and quantitative label-free approach to obtaining basic enzymological constants will facilitate the study of proteolysis and other posttranslational modifications in complex mixtures.apoptosis | caspase | enzymology | mass spectrometry | selected reaction monitoring A poptosis is a form of programmed cell death that serves to eliminate unnecessary, infected, or tumorigenic cells from eukaryotic organisms. While many intrinsic and extrinsic stimuli can initiate apoptosis, these ultimately converge on the activation of a related family of aspartate-specific cysteine proteases, the caspases, that execute widespread proteolysis and induce noninflammatory death (1). We and others have surveyed N termini that occur in apoptotic cells and collectively reported more than 1,000 caspase-derived cleavages (2-5). This explosion of proteomic data has defined a vast array of caspase substrates proteolyzed during apoptosis. While these data identify caspase targets, and in some cases the sites of proteolysis, they fail to reveal the relative rates of cleavage, a parameter necessary to establish the order of proteolytic events and their importance in extracts and intact cells.The recent application of selected reaction monitoring (SRM) methods, traditionally used for metabolite identification, to proteomic studies has enabled the simultaneous label-free quantification of hundreds of peptides (6, 7). Our development of a N-terminal enrichment platform (3) is ideally suited to the application of SRM to apoptotic proteolysis. Using this platfor...

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