Monitoring conformational changes in peroxisome proliferator-activated receptor α by a genetically encoded photoamino acid, cross-linking, and mass spectrometry

High-Performance Workflow for Identifying Site-Specific Crosslinks Originating from a Genetically Incorporated, Photoreactive Amino Acid

In conventional crosslinking mass spectrometry, proteins are crosslinked using a highly selective, bifunctional chemical reagent, which limits crosslinks to residues that are accessible and reactive to the reagent. Genetically incorporating a photoreactive amino acid offers two key advantages: any site can be targeted, including those that are inaccessible to conventional crosslinking reagents, and photoreactive amino acids can potentially react with a broad range of interaction partners. However, broad reactivity imposes additional challenges for crosslink identification. In this study, we incorporate benzoylphenylalanine (BPA), a photoreactive amino acid, at selected sites in an intrinsically disordered region of the human protein HSPB5. We report and characterize a workflow for identifying and visualizing residue-level interactions originating from BPA. We routinely identify 30 to 300 crosslinked peptide spectral matches with this workflow, which is up to ten times more than existing tools for residue-level BPA crosslink identification. Most identified crosslinks are assigned to a precision of one or two residues, which is supported by a high degree of overlap between replicate analyses. Based on these results, we anticipate that this workflow will support the more general use of genetically incorporated, photoreactive amino acids for characterizing the structures of proteins that have resisted high-resolution characterization.

Improved Cross-Linking Coverage for Protein Complexes Containing Low Levels of Lysine by Using an Enrichable Photo-Cross-Linker

Chemical cross-linking coupled with mass spectrometry (XL-MS) is an important technique for the structural analysis of protein complexes where the coverage of amino acids and the identification of cross-linked sites are crucial. Photo-cross-linking has multisite reactivity and is valuable for the structural analysis of chemical cross-linking. However, a high degree of heterogeneity results from this multisite reactivity, which results in samples with higher complexity and lower abundance. Additionally, the applicability of photo-cross-linking is limited to purified protein complexes. In this work, we demonstrate a photo-cross-linker, alkynyl-succinimidyl-diazirine (ASD) with the reactive groups of N-hydroxysuccinimide ester and diazirine, as well as the click-enrichable alkyne group. Photo-cross-linkers can provide higher site reactivity for proteins that contain only a small number of lysine residues, thereby complementing the more commonly used lysine-targeting cross-linkers. By systematically analyzing proteins with differing lysine contents and differing flexibilities, we demonstrated clear enhancement in structure elucidation for proteins containing less lysine and with high flexibility. In addition, enrichment approaches of alkynyl-azide click chemistry conjugated with biotin-streptavidin purification (coinciding with parallel orthogonal digestion) improved the identification coverage of cross-links. We show that this photo-cross-linking approach can be used for membrane proteome-wide complex analysis. This method led to the identification of a total of 14066 lysine-X cross-linked site pairs from a total of 2784 proteins. Thus, this cross-linker is a valuable addition to a photo-cross-linking toolkit and improves the identification coverage of XL-MS in functional structure analysis.

Cleavable Cross-Linkers and Mass Spectrometry for the Ultimate Task of Profiling Protein-Protein Interaction Networks in Vivo

Cross-linking mass spectrometry (XL-MS) has matured into a potent tool to identify protein-protein interactions or to uncover protein structures in living cells, tissues, or organelles. The unique ability to investigate the interplay of proteins within their native environment delivers valuable complementary information to other advanced structural biology techniques. This Review gives a comprehensive overview of the current possible applications as well as the remaining limitations of the technique, focusing on cross-linking in highly complex biological systems like cells, organelles, or tissues. Thanks to the commercial availability of most reagents and advances in user-friendly data analysis, validation, and visualization tools, studies using XL-MS can, in theory, now also be utilized by nonexpert laboratories.

Exploring GPCR-arrestin interfaces with genetically encoded crosslinkers

β-arrestins (βarr1 and βarr2) are ubiquitous regulators of G protein-coupled receptor (GPCR) signaling. Available data suggest that β-arrestins dock to different receptors in different ways. However, the structural characterization of GPCR-arrestin complexes is challenging and alternative approaches to study GPCR-arrestin complexes are needed. Here, starting from the finger loop as a major site for the interaction of arrestins with GPCRs, we genetically incorporate non-canonical amino acids for photo- and chemical crosslinking into βarr1 and βarr2 and explore binding topologies to GPCRs forming either stable or transient complexes with arrestins: the vasopressin receptor 2 (rhodopsin-like), the corticotropin-releasing factor receptor 1, and the parathyroid hormone receptor 1 (both secretin-like). We show that each receptor leaves a unique footprint on arrestins, whereas the two β-arrestins yield quite similar crosslinking patterns. Furthermore, we show that the method allows defining the orientation of arrestin with respect to the GPCR. Finally, we provide direct evidence for the formation of arrestin oligomers in the cell.

Critical Insight into the Design of PPAR-γ Agonists by Virtual Screening Techniques

BACKGROUND

Design of novel PPAR-γ modulators with better binding efficiency and fewer side effects to treat type 2 diabetes is still a challenge for medicinal chemists. Cost and time efficient computational methods have presently become an integral part of research in nuclear receptors and their ligands, enabling hit to lead identification and lead optimization. This review will focus on cutting-edge technologies used in most recent studies on the design of PPAR- γ agonists and will discuss the chemistry of few molecules which emerged successful.

METHODS

Literature review was carried out in google scholar using customized search from 2011- 2017. Computer-aided design methods presented in this article were used as search terms to retrieve corresponding literature.

RESULTS

Virtual screening of natural product libraries is an effective strategy to harness nature as the source of ligands for PPARs. Rigid and induced fit docking and core hopping approach in docking are rapid screening methods to predict the PPAR- γ and PPAR-α/ γ dual agonistic activity. Onedimensional drug profile matching is one of the recent virtual screening methods by which an antiprotozoal drug, Nitazoxanide was identified as a PPAR- γ agonist.

CONCLUSION

It is concluded that to achieve a convincing and reliable design of PPAR-γ agonist by virtual screening techniques, customized workflow comprising of appropriate models is essential in which methods may be applied either sequentially or simultaneously.

Conformational Shift of a β-Hairpin Peptide upon Complex Formation with an Oligo-proline Peptide Studied by Mass Spectrometry

So-called super-secondary structures such as the β-hairpin, studied here, form an intermediate hierarchy between secondary and tertiary structures of proteins. Their sequence-derived 'pure' peptide backbone conformation is combined with 'remote' interstrand or interresidue contacts reminiscent of the 3D-structure of full-length proteins. This renders them ideally suited for studying potential nucleation sites of protein folding reactions as well as intermolecular interactions. But β-hairpins do not merely serve as model systems; their unique structure characteristics warrant a central role in structural studies on their own. In this study we applied photo cross-linking in combination with high-resolution mass spectrometry and computational modeling as well as with ion mobility-mass spectrometry to elucidate these structural properties. Using variants of a known β-hairpin representative, the so-called trpzip peptide and its ligands, we found evidence for a conformational transition of the β-hairpin and its impact on ligand binding.

Monitoring Solution Structures of Peroxisome Proliferator-Activated Receptor β/δ upon Ligand Binding

Peroxisome proliferator-activated receptors (PPARs) have been intensively studied as drug targets to treat type 2 diabetes, lipid disorders, and metabolic syndrome. This study is part of our ongoing efforts to map conformational changes in PPARs in solution by a combination of chemical cross-linking and mass spectrometry (MS). To our best knowledge, we performed the first studies addressing solution structures of full-length PPAR-β/δ. We monitored the conformations of the ligand-binding domain (LBD) as well as full-length PPAR-β/δ upon binding of two agonists. (Photo-) cross-linking relied on (i) a variety of externally introduced amine- and carboxyl-reactive linkers and (ii) the incorporation of the photo-reactive amino acid p-benzoylphenylalanine (Bpa) into PPAR-β/δ by genetic engineering. The distances derived from cross-linking experiments allowed us to monitor conformational changes in PPAR-β/δ upon ligand binding. The cross-linking/MS approach proved highly advantageous to study nuclear receptors, such as PPARs, and revealed the interplay between DBD (DNA-binding domain) and LDB in PPAR-β/δ. Our results indicate the stabilization of a specific conformation through ligand binding in PPAR-β/δ LBD as well as full-length PPAR-β/δ. Moreover, our results suggest a close distance between the N- and C-terminal regions of full-length PPAR-β/δ in the presence of GW1516. Chemical cross-linking/MS allowed us gaining detailed insights into conformational changes that are induced in PPARs when activating ligands are present. Thus, cross-linking/MS should be added to the arsenal of structural methods available for studying nuclear receptors.

Chemical and protein structural basis for biological crosstalk between PPARα and COX enzymes

We have previously validated a probabilistic framework that combined computational approaches for predicting the biological activities of small molecule drugs. Molecule comparison methods included molecular structural similarity metrics and similarity computed from lexical analysis of text in drug package inserts. Here we present an analysis of novel drug/target predictions, focusing on those that were not obvious based on known pharmacological crosstalk. Considering those cases where the predicted target was an enzyme with known 3D structure allowed incorporation of information from molecular docking and protein binding pocket similarity in addition to ligand-based comparisons. Taken together, the combination of orthogonal information sources led to investigation of a surprising predicted relationship between a transcription factor and an enzyme, specifically, PPARα and the cyclooxygenase enzymes. These predictions were confirmed by direct biochemical experiments which validate the approach and show for the first time that PPARα agonists are cyclooxygenase inhibitors.

Reliable identification of cross-linked products in protein interaction studies by 13C-labeled p-benzoylphenylalanine

We describe the use of the (13)C-labeled artificial amino acid p-benzoyl-L-phenylalanine (Bpa) to improve the reliability of cross-linked product identification. Our strategy is exemplified for two protein-peptide complexes. These studies indicate that in many cases the identification of a cross-link without additional stable isotope labeling would result in an ambiguous assignment of cross-linked products. The use of a (13)C-labeled photoreactive amino acid is considered to be preferred over the use of deuterated cross-linkers as retention time shifts in reversed phase chromatography can be ruled out. The observation of characteristic fragment ions additionally increases the reliability of cross-linked product assignment. Bpa possesses a broad reactivity towards different amino acids and the derived distance information allows mapping of spatially close amino acids and thus provides more solid structural information of proteins and protein complexes compared to the longer deuterated amine-reactive cross-linkers, which are commonly used for protein 3D-structure analysis and protein-protein interaction studies.

A comparative cross-linking strategy to probe conformational changes in protein complexes

Chemical cross-linking, together with mass spectrometry (MS), is a powerful combination for probing subunit interactions within static protein assemblies. To probe conformational changes in response to stimuli, we have developed a comparative cross-linking strategy, using lysine-specific deuterated and nondeuterated bis(sulfosuccinimidyl)suberate cross-linking reagents (BS3). Here we describe the experimental procedures as well as the data analysis, validation and interpretation. The protocol involves first assigning cross-linked peptides in the complex without ligand binding, or with post-translational modifications (PTMs) at natural abundance, using a standard procedure with labeled cross-linkers, proteolysis and assignment of cross-linked peptides after liquid chromatography-tandem MS (LC-MS/MS) and database searching. An aliquot of the protein complex is then exposed to a stimulus: either ligand binding or incubation with a phosphatase or kinase to bring about changes in PTMs. Two solutions--one containing the apo/untreated complex and the other containing the enzymatically modified/ligand-bound complex--are then cross-linked independently. Typically, nondeuterated BS3-d0 is used for the untreated complex and deuterated BS3-d4 is used for the experiment. The two aliquots are then incubated at equal concentrations, digested and processed as before. The ratios of labeled and unlabeled cross-linked peptides provide a direct readout of the effect of the stimulus. We exemplify our method by quantifying changes in subunit interactions induced by dephosphorylation of an ATP synthase. The protocol can also be used to determine the conformational changes in protein complexes induced by various stimuli including ligand/drug binding, oligomerization and other PTMs. Application of the established protocol takes ~9 d, including protein complex purification.

Dynamic protein ligand interactions--insights from MS

Proteins undergo dynamic interactions with carbohydrates, lipids and nucleotides to form catalytic cores, fine‐tuned for different cellular actions. The study of dynamic interactions between proteins and their cognate ligands is therefore fundamental to the understanding of biological systems. During the last two decades MS, and its associated techniques, has become accepted as a method for the study of protein–ligand interactions, not only for covalent complexes, where the use of MS is well established, but also, and significantly for protein–ligand interactions, for noncovalent assemblies. In this review, we employ a broad definition of a ligand to encompass protein subunits, drug molecules, oligonucleotides, carbohydrates, and lipids. Under the appropriate conditions, MS can reveal the composition, heterogeneity and dynamics of these protein–ligand interactions, and in some cases their structural arrangements and binding affinities. Herein, we highlight MS approaches for studying protein–ligand complexes, including those containing integral membrane subunits, and showcase examples from recent literature. Specifically, we tabulate the myriad of methodologies, including hydrogen exchange, proteomics, hydroxyl radical footprinting, intact complexes, and crosslinking, which, when combined with MS, provide insights into conformational changes and subtle modifications in response to ligand‐binding interactions.

Catch me if you can: challenges and applications of cross-linking approaches

Biomolecular complexes are the groundwork of life and the basis for cell signaling, energy transfer, motion, stability and cellular metabolism. Understanding the underlying complex interactions on the molecular level is an essential step to obtain a comprehensive insight into cellular and systems biology. For the investigation of molecular interactions, various methods, including Förster resonance energy transfer, nuclear magnetic resonance spectroscopy, X-ray crystallography and yeast two-hybrid screening, can be utilized. Nevertheless, the most reliable approach for structural proteomics and the identification of novel protein-binding partners is chemical cross-linking. The rationale is that upon forming a covalent bond between a protein and its interaction partner (protein, lipid, RNA/DNA, carbohydrate) the native complex state is "frozen" and accessible for detailed mass spectrometric analysis. In this review we provide a synopsis on crosslinker design, chemistry, pitfalls, limitations and novel applications in the field, and feature an overview of current software applications.