LinX: A Software Tool for Uncommon Cross-Linking Chemistry.

Chemical cross-linking mass spectrometry has become a popular tool in structural biology. Although several algorithms exist that efficiently analyze data-dependent mass spectrometric data, the algorithm to identify and quantify intermolecular cross-links located at the interaction interface of homodimer molecules was missing. The algorithm in LinX utilizes high mass accuracy for ion identification. In contrast with standard data-dependent analysis, LinX enables the elucidation of cross-linked peptides originating from the interaction interface of homodimers labeled by 14N/15N, including their ratio or cross-links from protein-nucleic acid complexes. The software is written in Java language, and its source code and a detailed user's guide are freely available at https://github.com/KukackaZ/LinX or https://ms-utils.org/LinX. Data are accessible via the ProteomeXchange server with the data set identifier PXD023522.

Motif orientation matters: Structural characterization of TEAD1 recognition of genomic DNA.

TEAD transcription factors regulate gene expression through interactions with DNA and other proteins. They are crucial for the development of eukaryotic organisms and to control the expression of genes involved mostly in cell proliferation and differentiation; however, their deregulation can lead to tumorigenesis. To study the interactions of TEAD1 with M-CAT motifs and their inverted versions, the KD of each complex was determined, and H/D exchange, quantitative chemical cross-linking, molecular docking, and smFRET were utilized for structural characterization. ChIP-qPCR was employed to correlate the results with a cell line model. The results obtained showed that although the inverted motif has 10× higher KD, the same residues were affected by the presence of M-CAT in both orientations. Molecular docking and smFRET revealed that TEAD1 binds the inverted motif rotated 180°. In addition, the inverted motif was proven to be occupied by TEAD1 in Jurkat cells, suggesting that the low-affinity binding sites present in the human genome may possess biological relevance.

Natural Killer Cell Activation Receptor NKp30 Oligomerization Depends on Its N-Glycosylation.

NKp30 is one of the main human natural killer (NK) cell activating receptors used in directed immunotherapy. The oligomerization of the NKp30 ligand binding domain depends on the length of the C-terminal stalk region, but our structural knowledge of NKp30 oligomerization and its role in signal transduction remains limited. Moreover, ligand binding of NKp30 is affected by the presence and type of N-glycosylation. In this study, we assessed whether NKp30 oligomerization depends on its N-glycosylation. Our results show that NKp30 forms oligomers when expressed in HEK293S GnTI- cell lines with simple N-glycans. However, NKp30 was detected only as monomers after enzymatic deglycosylation. Furthermore, we characterized the interaction between NKp30 and its best-studied cognate ligand, B7-H6, with respect to glycosylation and oligomerization, and we solved the crystal structure of this complex with glycosylated NKp30, revealing a new glycosylation-induced mode of NKp30 dimerization. Overall, this study provides new insights into the structural basis of NKp30 oligomerization and explains how the stalk region and glycosylation of NKp30 affect its ligand affinity. This furthers our understanding of the molecular mechanisms involved in NK cell activation, which is crucial for the successful design of novel NK cell-based targeted immunotherapeutics.

MS-Based Approaches Enable the Structural Characterization of Transcription Factor/DNA Response Element Complex.

The limited information available on the structure of complexes involving transcription factors and cognate DNA response elements represents a major obstacle in the quest to understand their mechanism of action at the molecular level. We implemented a concerted structural proteomics approach, which combined hydrogen-deuterium exchange (HDX), quantitative protein-protein and protein-nucleic acid cross-linking (XL), and homology analysis, to model the structure of the complex between the full-length DNA binding domain (DBD) of Forkhead box protein O4 (FOXO4) and its DNA binding element (DBE). The results confirmed that FOXO4-DBD assumes the characteristic forkhead topology shared by these types of transcription factors, but its binding mode differs significantly from those of other members of the family. The results showed that the binding interaction stabilized regions that were rather flexible and disordered in the unbound form. Surprisingly, the conformational effects were not limited only to the interface between bound components, but extended also to distal regions that may be essential to recruiting additional factors to the transcription machinery. In addition to providing valuable new insights into the binding mechanism, this project provided an excellent evaluation of the merits of structural proteomics approaches in the investigation of systems that are not directly amenable to traditional high-resolution techniques.

Binding of eIF3 in complex with eIF5 and eIF1 to the 40S ribosomal subunit is accompanied by dramatic structural changes.

eIF3 is a large multiprotein complex serving as an essential scaffold promoting binding of other eIFs to the 40S subunit, where it coordinates their actions during translation initiation. Perhaps due to a high degree of flexibility of multiple eIF3 subunits, a high-resolution structure of free eIF3 from any organism has never been solved. Employing genetics and biochemistry, we previously built a 2D interaction map of all five yeast eIF3 subunits. Here we further improved the previously reported in vitro reconstitution protocol of yeast eIF3, which we cross-linked and trypsin-digested to determine its overall shape in 3D by advanced mass-spectrometry. The obtained cross-links support our 2D subunit interaction map and reveal that eIF3 is tightly packed with its WD40 and RRM domains exposed. This contrasts with reported cryo-EM structures depicting eIF3 as a molecular embracer of the 40S subunit. Since the binding of eIF1 and eIF5 further fortified the compact architecture of eIF3, we suggest that its initial contact with the 40S solvent-exposed side makes eIF3 to open up and wrap around the 40S head with its extended arms. In addition, we mapped the position of eIF5 to the region below the P- and E-sites of the 40S subunit.

The C-type lectin-like receptor Nkrp1b: Structural proteomics reveals features affecting protein conformation and interactions.

The cytotoxicity of mouse natural killer (NK) cells in response to pathological changes in target cells is regulated via the Nkrp1b receptor. Here, we characterized the Nkrp1b structure and structural features (stalk, loop, and oligomerization state) that affect its interactions. To study the Nkrp1b protein structure and the functional importance of its stalk, two Nkrp1b protein variants differing by the presence of the stalk were prepared. These variants were studied using a combination of structural mass spectrometry approaches with computational modeling to derive structural models. In addition, information about biological activity and localization in mammalian cells was acquired using scanning microscopy techniques and western blotting. Based on these methods, we obtained the structure of Nkrp1b ectodomain in its monomeric and dimeric conformations, identified the dimerization interface, and determined disulfide connections within the molecule. We found that Nkrp1b occurs as a mixture of monomers and homodimers, both in vitro and in vivo. SIGNIFICANCE: Despite the long-standing assumption that Nkrp1 proteins are homodimers connected by disulfide bonds in the stalk region, our data showed that both Nkrp1b protein variants form monomers and homodimers irrespective of the presence of the stalk. We demonstrated that the stalk is not crucial for protein dimerization or ligand binding and that Nkrp1b interacts with its natural ligands only in its monomeric conformation; therefore, dimers may have another regulatory function. Using a unique combination of computational, biochemical, and biological methods, we revealed the structural conformation and behavior of Nkrp1b in its native state. In addition, it is a first report utilizing the intermolecular chemical cross-linking of light- and heavy-labeled protein chains together with ion mobility-mass spectrometry to design the structural models of protein homodimers.

14-3-3 protein masks the nuclear localization sequence of caspase-2.

Caspase-2 is an apical protease responsible for the proteolysis of cellular substrates directly involved in mediating apoptotic signaling cascades. Caspase-2 activation is inhibited by phosphorylation followed by binding to the scaffolding protein 14-3-3, which recognizes two phosphoserines located in the linker between the caspase recruitment domain and the p19 domains of the caspase-2 zymogen. However, the structural details of this interaction and the exact role of 14-3-3 in the regulation of caspase-2 activation remain unclear. Moreover, the caspase-2 region with both 14-3-3-binding motifs also contains the nuclear localization sequence (NLS), thus suggesting that 14-3-3 binding may regulate the subcellular localization of caspase-2. Here, we report a structural analysis of the 14-3-3ζ:caspase-2 complex using a combined approach based on small angle X-ray scattering, NMR, chemical cross-linking, and fluorescence spectroscopy. The structural model proposed in this study suggests that phosphorylated caspase-2 and 14-3-3ζ form a compact and rigid complex in which the p19 and the p12 domains of caspase-2 are positioned within the central channel of the 14-3-3 dimer and stabilized through interactions with the C-terminal helices of both 14-3-3ζ protomers. In this conformation, the surface of the p12 domain, which is involved in caspase-2 activation by dimerization, is sterically occluded by the 14-3-3 dimer, thereby likely preventing caspase-2 activation. In addition, 14-3-3 protein binding to caspase-2 masks its NLS. Therefore, our results suggest that 14-3-3 protein binding to caspase-2 may play a key role in regulating caspase-2 activation. DATABASE: The atomic coordinates and structure factors have been deposited in the Protein Data Bank, www.ww pdb.org (PDB ID codes 6GKF and 6GKG).

Impact of Chemical Cross-Linking on Protein Structure and Function.

Chemical cross-linking coupled with mass spectrometry is a popular technique for deriving structural information on proteins and protein complexes. Also, cross-linking has become a powerful tool for stabilizing macromolecular complexes for single-particle cryo-electron microscopy. However, an effect of cross-linking on protein structure and function should not be forgotten, and surprisingly, it has not been investigated in detail so far. Here, we used kinetic studies, mass spectrometry, and NMR spectroscopy to systematically investigate an impact of cross-linking on structure and function of human carbonic anhydrase and alcohol dehydrogenase 1 from Saccharomyces cerevisiae. We found that cross-linking induces rather local structural disturbances and the overall fold is preserved even at a higher cross-linker concentration. The results establish general experimental conditions for chemical cross-linking with minimal effect on protein structure and function.

Coordination and redox state-dependent structural changes of the heme-based oxygen sensor AfGcHK associated with intraprotein signal transduction.

The heme-based oxygen sensor histidine kinase AfGcHK is part of a two-component signal transduction system in bacteria. O2 binding to the Fe(II) heme complex of its N-terminal globin domain strongly stimulates autophosphorylation at His183 in its C-terminal kinase domain. The 6-coordinate heme Fe(III)-OH- and -CN- complexes of AfGcHK are also active, but the 5-coordinate heme Fe(II) complex and the heme-free apo-form are inactive. Here, we determined the crystal structures of the isolated dimeric globin domains of the active Fe(III)-CN- and inactive 5-coordinate Fe(II) forms, revealing striking structural differences on the heme-proximal side of the globin domain. Using hydrogen/deuterium exchange coupled with mass spectrometry to characterize the conformations of the active and inactive forms of full-length AfGcHK in solution, we investigated the intramolecular signal transduction mechanisms. Major differences between the active and inactive forms were observed on the heme-proximal side (helix H5), at the dimerization interface (helices H6 and H7 and loop L7) of the globin domain and in the ATP-binding site (helices H9 and H11) of the kinase domain. Moreover, separation of the sensor and kinase domains, which deactivates catalysis, increased the solvent exposure of the globin domain-dimerization interface (helix H6) as well as the flexibility and solvent exposure of helix H11. Together, these results suggest that structural changes at the heme-proximal side, the globin domain-dimerization interface, and the ATP-binding site are important in the signal transduction mechanism of AfGcHK. We conclude that AfGcHK functions as an ensemble of molecules sampling at least two conformational states.

Protein Chips Compatible with MALDI Mass Spectrometry Prepared by Ambient Ion Landing.

We present a technology that allows the preparation of matrix-assisted laser desorption/ionization (MALDI)-compatible protein chips by ambient ion landing of proteins and successive utilization of the resulting protein chips for the development of bioanalytical assays. These assays are based on the interaction between the immobilized protein and the sampled analyte directly on the protein chip and subsequent in situ analysis by MALDI mass spectrometry. The electrosprayed proteins are immobilized on dry metal and metal oxide surfaces, which are nonreactive under normal conditions. The ion landing of electrosprayed protein molecules is performed under atmospheric pressure by an automated ion landing apparatus that can manufacture protein chips with a predefined array of sample positions or any other geometry of choice. The protein chips prepared by this technique are fully compatible with MALDI ionization because the metal-based substrates are conductive and durable enough to be used directly as MALDI plates. Compared to other materials, the nonreactive surfaces show minimal nonspecific interactions with chemical species in the investigated sample and are thus an ideal substrate for selective protein chips. Three types of protein chips were used in this report to demonstrate the bioanalytical applications of ambient ion landing. The protein chips with immobilized proteolytic enzymes showed the usefulness for fast in situ peptide MALDI sequencing; the lectin-based protein chips showed the ability to enrich glycopeptides from complex mixtures with subsequent MALDI analysis, and the protein chips with immobilized antibodies were used for a novel immunoMALDI workflow that allowed the enrichment of antigens from the serum followed by highly specific MALDI detection.

Mapping protein structural changes by quantitative cross-linking.

Chemical cross-linking is a promising technology for protein tertiary structure determination. Though the data has low spatial resolution, it is possible to obtain it at physiological conditions on proteins that are not amenable to standard high resolution techniques such as X-ray, NMR analysis and cryo-EM. Here we demonstrate the utilization of isotopically labeled chemical cross-linking to visualize protein conformation rearrangements. Since calmodulin exists in two distinct conformations (calcium-free and calcium-containing forms), we selected this protein for testing the potential and the limits of a new technique. After cross-linking of both calmodulin forms, the calcium-free and calcium-containing forms were mixed together and digested under different conditions and the products of proteolysis were monitored using high resolution mass spectrometry. Finally, the ratios of heavy/light cross-links were calculated by mMass open source platform.