Structural Diversity of Nucleosomes Characterized by Native Mass Spectrometry

Evaluation for Ion Heating of H2A-H2B Dimer in Ion Mobility Spectrometry-Mass Spectrometry

Ion mobility spectrometry-mass spectrometry (IMS-MS) provides m/z values and collision cross sections (CCSs) of gas-phase ions. In our previous study, an intrinsically disordered protein, the H2A-H2B dimer, was analyzed using IMS-MS, resulting in two conformational populations of CCS. Based on experimental and theoretical approaches, this resulted from a structural diversity of intrinsically disordered regions. We predicted that this phenomenon is related to ion heating in the IMS-MS instrument. In this study, to reveal the effect of ion heating from parameters in the IMS-MS instrument on the conformational population of the H2A-H2B dimer, we investigated the arrival time distributions of the H2A-H2B dimer by changing values of three instrumental parameters, namely, cone voltage located in the first vacuum chamber, trap collision energy (trap CE) for tandem mass spectrometry, and trap bias voltage for the entrance of IMS. These results revealed that the two populations observed for the H2A-H2B dimer were due to the trap bias voltage. Furthermore, to evaluate the internal energies of the analyte ions with respect to each parameter, benzylpyridinium derivatives were used as temperature-sensitive probes. The results showed that the trap CE voltage imparts greater internal energy to the ions than the trap bias voltage. In addition, this slight change in the internal energy caused by the trap bias voltage resulted in the structural diversity of the H2A-H2B dimer. Therefore, the trap bias voltage should be set with attention to the properties of the analytes, even if the effect of the trap bias voltage on the internal energy is negligible.

Dissecting the structural heterogeneity of proteins by native mass spectrometry

A single gene yields many forms of proteins via combinations of post-transcriptional/post-translational modifications. Proteins also fold into higher-order structures and interact with other molecules. The combined molecular diversity leads to the heterogeneity of proteins that manifests as distinct phenotypes. Structural biology has generated vast amounts of data, effectively enabling accurate structural prediction by computational methods. However, structures are often obtained heterologously under homogeneous states in vitro. The lack of native heterogeneity under cellular context creates challenges in precisely connecting the structural data to phenotypes. Mass spectrometry (MS) based proteomics methods can profile proteome composition of complex biological samples. Most MS methods follow the "bottom-up" approach, which denatures and digests proteins into short peptide fragments for ease of detection. Coupled with chemical biology approaches, higher-order structures can be probed via incorporation of covalent labels on native proteins that are maintained at the peptide level. Alternatively, native MS follows the "top-down" approach and directly analyzes intact proteins under nondenaturing conditions. Various tandem MS activation methods can dissect the intact proteins for in-depth structural elucidation. Herein, we review recent native MS applications for characterizing heterogeneous samples, including proteins binding to mixtures of ligands, homo/hetero-complexes with varying stoichiometry, intrinsically disordered proteins with dynamic conformations, glycoprotein complexes with mixed modification states, and active membrane protein complexes in near-native membrane environments. We summarize the benefits, challenges, and ongoing developments in native MS, with the hope to demonstrate an emerging technology that complements other tools by filling the knowledge gaps in understanding molecular heterogeneity of proteins. This article is protected by copyright. All rights reserved.

Native Mass Spectrometry: Recent Progress and Remaining Challenges

Native mass spectrometry (nMS) has emerged as an important tool in studying the structure and function of macromolecules and their complexes in the gas phase. In this review, we cover recent advances in nMS and related techniques including sample preparation, instrumentation, activation methods, and data analysis software. These advances have enabled nMS-based techniques to address a variety of challenging questions in structural biology. The second half of this review highlights recent applications of these technologies and surveys the classes of complexes that can be studied with nMS. Complementarity of nMS to existing structural biology techniques and current challenges in nMS are also addressed.

Biophysics is reshaping our perception of the epigenome: from DNA-level to high-throughput studies

Epigenetic research holds great promise to advance our understanding of biomarkers and regulatory processes in health and disease. An increasing number of new approaches, ranging from molecular to biophysical analyses, enable identifying epigenetic changes on the level of a single gene or the whole epigenome. The aim of this review is to highlight how the field is shifting from completely molecular-biology-driven solutions to multidisciplinary strategies including more reliance on biophysical analysis tools. Biophysics not only offers technical advancements in imaging or structure analysis but also helps to explore regulatory interactions. New computational methods are also being developed to meet the demand of growing data volumes and their processing. Therefore, it is important to capture these new directions in epigenetics from a biophysical perspective and discuss current challenges as well as multiple applications of biophysical methods and tools. Specifically, we gradually introduce different biophysical research methods by first considering the DNA-level information and eventually higher-order chromatin structures. Moreover, we aim to highlight that the incorporation of bioinformatics, machine learning, and artificial intelligence into biophysical analysis allows gaining new insights into complex epigenetic processes. The gained understanding has already proven useful in translational and clinical research providing better patient stratification options or new therapeutic insights. Together, this offers a better readiness to transform bench-top experiments into industrial high-throughput applications with a possibility to employ developed methods in clinical practice and diagnostics.

Mass Spectrometry of Nucleic Acid Noncovalent Complexes

Nucleic acids have been among the first targets for antitumor drugs and antibiotics. With the unveiling of new biological roles in regulation of gene expression, specific DNA and RNA structures have become very attractive targets, especially when the corresponding proteins are undruggable. Biophysical assays to assess target structure as well as ligand binding stoichiometry, affinity, specificity, and binding modes are part of the drug development process. Mass spectrometry offers unique advantages as a biophysical method owing to its ability to distinguish each stoichiometry present in a mixture. In addition, advanced mass spectrometry approaches (reactive probing, fragmentation techniques, ion mobility spectrometry, ion spectroscopy) provide more detailed information on the complexes. Here, we review the fundamentals of mass spectrometry and all its particularities when studying noncovalent nucleic acid structures, and then review what has been learned thanks to mass spectrometry on nucleic acid structures, self-assemblies (e.g., duplexes or G-quadruplexes), and their complexes with ligands.

Histone Tail Conformations: A Fuzzy Affair with DNA

The core histone tails are critical in chromatin structure and signaling. Studies over the past several decades have provided a wealth of information on the histone tails and their interaction with chromatin factors. However, the conformation of the histone tails in a chromatin relevant context has remained elusive. Only recently has enough evidence emerged to start to build a structural model of the tails in the context of nucleosomes and nucleosome arrays. Here, we review these studies and propose that the histone tails adopt a high-affinity fuzzy complex with DNA, characterized by robust but dynamic association. Furthermore, we discuss how these DNA-bound conformational ensembles promote distinct chromatin structure and signaling, and that their fuzzy nature is important in transitioning between functional states.

Mass Spectrometric Characterization of Histone H3 Isolated from in-Vitro Reconstituted and Acetylated Nucleosome Core Particle

Post-translational modifications (PTMs) of histone N-terminal tails in nucleosome core particle (NCP), such as acetylation, play crucial roles in regulating gene expression. To unveil the regulation mechanism, atomic-level structural analysis of in-vitro modified NCP is effective with verifying the PTMs of histones. So far, identification of PTMs of NCP originating from living cells has mainly been performed using mass spectrometry (MS) techniques, such as bottom-up approach. The bottom-up approach is the most established method for protein characterization, but it does not always provide sufficient information on the acetylated sites of lysine residues in the histone tails if trypsin digestion is carried out. For histone proteins, which have many basic amino acids, trypsin generates too many short fragments that cannot be perfectly analyzed by tandem MS. In this study, we investigated the in vitro acetylation sites in the histone H3 tail using a top-down sequence analysis, matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) experiment, in combination with aminopeptidase digestion. Aminopeptidase can cleave peptide bonds one-by-one from the N-terminus of peptides or proteins, generating N-terminally truncated peptides and/or proteins. As a result, it was identified that this method enables sequence characterization of the entire region of the H3 tail. Also, application of this method to H3 in in-vitro acetylated NCP enabled assigning acetylation sites of H3. Thus, this method was found to be effective for obtaining information on in-vitro acetylation of NCP for structural biology study.

Screening of protein-ligand interactions under crude conditions by native mass spectrometry

A convenient analytical system for protein-ligand interactions under crude conditions was developed using native mass spectrometry (MS). As a model protein, Escherichia coli (E. coli) dihydrofolate reductase (DHFR) with and without a histidine tag was used for the study. First, overexpressed DHFR with a His-tag was roughly purified with a Ni-sepharose resin and subjected to native mass spectrometry with or without incubation with an inhibitor, Methotrexate (MTX). Even only with the minimum cleanup by the Ni-sepharose resin, intact ions of DHFR-nicotinamide adenine dinucleotide phosphate (NADPH) and DHFR-NADPH-ligand complexes were successfully observed. By optimizing the preparation procedures of the crude sample for native MS, e.g., avoiding sonication for cell lysis, we successfully observed intact ions of the specific DHFR-NADPH-MTX ternary complex starting with cultivation of E. coli in ≤ 25 mL medium. When the crude DHFR sample was mixed with two, four, or eight candidate compounds, only ions of the specific protein-ligand complex were observed. This indicates that the present system can be used as a rapid and convenient method for the rough determination of binding of specific ligands to the target protein without the time-consuming purification of protein samples. Moreover, it is important to rapidly determine specific interactions with target proteins under conditions similar to those in "real" biological systems. Graphical abstract.

Native Mass Spectrometry of Protein and DNA Complexes Prepared in Nonvolatile Buffers

Inorganic salts and nonvolatile-buffer components affect the structure and stability of proteins, and some protein complexes are unable to maintain their function and structure without them. However, it is well-known that these components cause suppression of analyte ionization during the electrospray ionization process. Thus, to establish appropriate methods for observation of the intact ions of protein and DNA complexes by native mass spectrometry (native MS) in the presence of nonvolatile buffer components, we herein examined the effect of ammonium acetate addition to a model homotetramer protein, alcohol dehydrogenase (ADH), which was prepared in a range of nonvolatile buffers, including Tris-HCl, phosphate, and HEPES buffers. Furthermore, native MS of nucleosome core particle (NCP), a large protein-DNA complex, prepared in nonvolatile buffer, was also examined. Intact ADH and NCP ions could be observed upon the addition of ammonium acetate, but NCP does not require as high of a concentration of ammonium acetate as ADH. Well-resolved peaks with different charge numbers could be observed for NCP prepared in Tris-HCl by addition of a lower amount of ammonium acetate than for ADH. This suggests that the effects of additives on native MS of biomolecular complexes can vary depending on the intramolecular interactions present. More specifically, NCP is stabilized mainly by electrostatic interactions, whereas the ADH tetramer depends on the presence of hydrophobic interactions between the four subunits. The results presented herein therefore are expected to contribute to structural biology studies of unstable protein-DNA complexes that are formed transiently during the transcription process.

Rapid and Definitive Analysis of In Vitro DNA Methylation by Nano-electrospray Ionization Mass Spectrometry

CpG methylation of DNA is an epigenetic marker that is highly related to the regulation of transcription initiation. For analysis of CpG methylation in genomic DNA sequences, bisulfite-induced modification in combination with polymerase chain reaction (PCR) is usually utilized, but it cannot be straightforwardly applied to methylated short- and middle-sized DNAs, such as < 500 base pairs (bp), which are often utilized in structural biology studies. In the present study, we applied nano-electrospray ionization mass spectrometry (nano-ESI-MS) for the characterization of methylated DNA with < 400 bp prepared in vitro. First, double-stranded DNA oligomers were methylated with recombinant M.SssI DNA methylase, which has been reported to modify completely and exclusively CpG sites in the sequence. The fragments generated by the digestion with methylation-insensitive restriction nuclease were then analyzed to identify the methylation levels by nano-ESI-MS, without liquid chromatography (LC) separation. By methylation-insensitive nuclease digestion, we divided the DNA strands into several fragments, and nano-ESI-MS enabled the accurate analysis of methylation levels in the DNA fragments with a relatively small amount of DNA sample prepared under optimized conditions. Furthermore, it was revealed that M.SssI methylase hardly modifies the CpG sites closely positioned at the ends of linear DNA. The present method is similar to the strategy for post-translational modification analysis of proteins and is promising for the rapid and definitive characterization of methylated DNA that may be used in structural biology studies.

Structural visualization of key steps in nucleosome reorganization by human FACT

Facilitates chromatin transcription (FACT) is a histone chaperone, which accomplishes both nucleosome assembly and disassembly. Our combined cryo-electron microscopy (EM) and native mass spectrometry (MS) studies revealed novel key steps of nucleosome reorganization conducted by a Mid domain and its adjacent acidic AID segment of human FACT. We determined three cryo-EM structures of respective octasomes complexed with the Mid-AID and AID regions, and a hexasome alone. We discovered extensive contacts between a FACT region and histones H2A, H2B, and H3, suggesting that FACT is competent to direct functional replacement of a nucleosomal DNA end by its phosphorylated AID segment (pAID). Mutational assays revealed that the aromatic and phosphorylated residues within pAID are essential for octasome binding. The EM structure of the hexasome, generated by the addition of Mid-pAID or pAID, indicated that the dissociation of H2A-H2B dimer causes significant alteration from the canonical path of the nucleosomal DNA.