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Finding and using Diagnostic Ions in Collisional Induced Crosslinked Peptide Fragmentation Spectra

ABSTRACT: Crosslinking mass spectrometry (XL-MS) has emerged as a powerful tool in its own right for the investigation of protein structures and interactions. Utilizing standard shotgun MS mass spectrometry equipment and specialized database search software, crosslinked peptide-pairs can be identified and directly translated into distance constraints for protein structure and protein-protein interaction investigations. Whereas the gas-phase dissociation behavior of linear peptides is well understood, less is however known about the gas-phase dissociation behavior of crosslinked peptides. In this work, we set out to expose the behavior of commonly used crosslinking reagents, both non- as well as gas-phase cleavable, using synthetic peptides to establish mechanistic insights. We describe that crosslinked peptide pairs generate specific fragmentation patterns under HCD and CID fragmentation conditions, distinct from mono-linked peptide and non-modified peptides. We discuss in detail the resulting diagnostic ions that can help distinguishing linear peptides from mono-linked and crosslinked peptide pairs and how that may be used to further increase the efficiency of XL-MS analysis.

INSTRUMENT(S): Q Exactive

ORGANISM(S): Bos Taurus (bovine)

SUBMITTER: Barbara Steigenberger

LAB HEAD: Albert A.J.R. Heck

PROVIDER: PXD013083 | Pride | 2019-07-16

REPOSITORIES: Pride

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Project description:Sample preparation: P-AMPK+AMP, P-AMPK+ATPγS, and AMPK+ATPγS protein samples were crosslinked in HBST buffer (25 mM HEPES pH 7.5 at room temperature, 200 mM NaCL, and 1 mM TCEP). Non-crosslinked negative controls were generated using the same procedure without the addition of DSSO. Reactions were initiated by spiking 1 uL of 75 mM DSSO (Thermo-Fisher) in DMSO into a 50 uL solution containing 10 uM protein and mixing. The final molar ratio of crosslinker:protein was 150:1. The reactions were incubated at 25°C for 45 minutes prior to being quenched by the addition of Tris pH 8.0 to a final concentration of 50 mM. Each crosslink reaction was done in triplicate and the reaction replicates were pooled after quenching. 10 μL samples of the pooled samples were loaded for SDS-PAGE analysis with Coomassie staining to confirm the presence of crosslinked protein. The remaining 140 μL of crosslinked and non-crosslinked samples were then acetone precipitated at -20°C overnight and the protein was pelleted by centrifugation at 16,000 rcf for 5 minutes at 4°C. After decanting, the pellets were dried for 15 minutes at room temperature before being resuspended in 12.5 uL of resuspension buffer (50 mM ammonium bicarbonate, 8M urea, pH 8.0). ProteaseMAX (Promega) was added to 0.02% and the solutions were mixed on an orbital shaker operating at 400 RPM for 5 minutes. After resuspension, 87.5 uL of digestion buffer (50 mM ammonium bicarbonate, pH 8.0) was added. Cysteines in the protein samples were reduced and akylated as follows. DTT was added to 5 mM final concentration and the protein solutions were incubated at 56°C in an orbital shaker operating at 400 RPM and for 20 minutes. After reduction, 2.7 uL of 550 mM iodoacetamide was added and the solutions were incubated in the dark at room temperature for 15 minutes. Reduced and alkylated protein solutions were digested using trypsin at a ratio of 1:200 (w/w trypsin:protein) and incubating at 37°C. After overnight incubation at 37°C, the samples were acidified by the addition of trifluoroacetic acid (TFA, Thermo-Fischer) to 1%. Samples were then frozen and stored at -20°C until analysis.

Project description:The HOP2/MND1 heterodimer is essential for meiotic homologous recombination in plants and other eukaryotes, and promotes the repair of DNA double strand breaks. The HOP2/MND1 dimer forms via the central, split coiled coils (CC1 and CC2) present in both proteins, and the resulting complex contains several flexible regions such as the hinge between the coils. To investigate the conformational flexibility of the heterodimer, important for understanding mechanistic details of HOP2/MND1 function, we studied the spatial relation of the complex partners and their domains in solution. We performed chemical cross-linking in combination with mass spectrometry (XL-MS) to generate distance restraints, which were subsequently used for molecular modeling. The final XL-MS workflow encompassed the use of cross-linkers with varying spacer arm lengths, quenching, digestion, size exclusion enrichment and HCD based LC-MS/MS detection prior to data evaluation. We applied and systematically tested two different homobifunctional amine-reactive crosslinkers (DSS-11.4 Å and BS2G 7.7 Å) and one zero-length heterobifunctional crosslinker (EDC). Crosslinked peptides of four biological replicates were analyzed prior to 3D structure prediction by protein threading and protein-protein docking for crosslink guided molecular modeling. Our miniaturized SEC approach reduced the required starting material and led to a high amount of crosslinked peptides allowing the analysis of replicates. The majority of the identified crosslinks was found in the coiled coil domains, indicating a parallel orientation of the interaction partners. Furthermore, flexibility of the C-terminal head domains of HOP2 and MND1 was observed. The experimentally derived distance constraints combined with an iterative comparative modeling approach not only confirm the elongated, open conformation predicted by the crystal structure of the Giardia lamblia Hop2-Mnd1 heterodimer, but further suggest the coexistence of a closed complex conformation in solution.

Project description:Over the past few decades cross-linking mass spectrometry (XLMS) has become a powerful tool for identification of protein-protein interactions and for gaining insight into the structures of proteins in living cells, tissues, and organelles. The development of new crosslinkers, enrichment strategies and data acquisition methods led to the establishment of numerous new software tools specifically for the analysis and interpretation of cross-linking data. We previously published one of these tools called MS Annika, a cross-linking search engine which can accurately identify cross-linked peptides in MS2 spectra from a variety of different MS-cleavable crosslinkers. In this publication we present an updated MS Annika and a new search algorithm that additionally supports processing of data from MS2-MS3-based approaches and identification of peptides from MS3 spectra. In the new MS2-MS3 search algorithm, MS3 spectra are matched to their corresponding precursor doublet peak in the MS2 scan to identify the crosslink modification and the monoisotopic peptide mass. This information is then used to adjust the MS3 spectra for search with MS Amanda, our in-house developed peptide search engine, to identify the cross-linked peptides. Peptides that are identified in the MS2 scan and one or more of the associated product MS3 scans are re-scored with a novel scoring function to reflect the increased confidence. Finally, the detected cross-links are validated by estimating the false discovery rate (FDR) using a target-decoy approach. We evaluated the MS3-search-capabilities of MS Annika on five different datasets covering a variety of experimental approaches and compared it to XlinkX and MaXLinker, two other cross-linking search engines that support MS3 crosslink identification. Three of the datasets were benchmark datasets of synthetic peptides that allow calculation of an experimentally validated FDR, and we show that MS Annika detects up to 4 times more true unique crosslinks than MaXLinker and up to 35% more than XlinkX while simultaneously yielding less false positive hits and therefore a more accurate FDR than the other two search engines. Additionally, for the other two datasets we could show that MS Annika finds between 74% to 2.5 times more crosslinks at 1% estimated FDR and reveals protein-protein interactions that are not detected by either XlinkX or MaXLinker.

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Finding and using Diagnostic Ions in Collisional Induced Crosslinked Peptide Fragmentation Spectra

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