Chemical cross-linking/mass spectrometry targeting acidic residues in proteins and protein complexes

Abstract

The study of proteins and protein complexes using chemical crosslinking followed by the MS identification of the cross-linked peptides has found increasingly widespread use in recent years. Thus far, such analyses have used almost exclusively homobifunctional, amine-reactive cross-linking reagents. Here we report the development and application of an orthogonal cross-linking chemistry specific for carboxyl groups. Chemical cross-linking of acidic residues is achieved using homobifunctional dihydrazides as cross-linking reagents and a coupling chemistry at neutral pH that is compatible with the structural integrity of most protein complexes. In addition to cross-links formed through insertion of the dihydrazides with different spacer lengths, zero-length cross-link products are also obtained, thereby providing additional structural information. We demonstrate the application of the reaction and the MS identification of the resulting cross-linked peptides for the chaperonin TRiC/CCT and the 26S proteasome. The results indicate that the targeting of acidic residues for cross-linking provides distance restraints that are complementary and orthogonal to those obtained from lysine cross-linking, thereby expanding the yield of structural information that can be obtained from cross-linking studies and used in hybrid modeling approaches. P roteins exert the majority of their functions in the form of protein complexes to control cellular signaling, protein synthesis, folding and degradation, and many more essential processes. Therefore, elucidating the composition and structure of such complexes has been a longstanding goal of biological research.MS-based proteomics has emerged as one of the main techniques to identify and quantify proteins and their modifications in biological samples such as isolated complexes, proteome fractions, or whole proteomes. Various MS methods now provide structural information on protein assemblies (1-3). Among them, chemical cross-linking and identification of cross-linked peptides by MS (XL-MS) has been increasingly applied to determine the subunit arrangements of biologically relevant complexes (4-6). Such XL-MS experiments indicate the locations of cross-linking sites and thus the spatial proximity of reactive groups that are connected by a covalent bond. This information is then used to determine the positioning of subunits or locate interacting regions, alone or in combination with other techniques such as NMR spectroscopy, electron microscopy, and X-ray crystallography.In the last few years, optimized protocols and new computational tools for the reliable analysis of XL-MS datasets resulted in significant advances of the XL-MS technology (4-6). These advances have contributed to the emergence of a robust, integrated XL-MS method that has been successfully applied for structure determination of a number of large protein complexes (7-11) and the detection of direct, physical interactions in whole cells (12)(13)(14). To date, the cross-linking chemistries applied in these studies have targ...

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