A methyl-TROSY approach for NMR studies of high-molecular-weight DNA with application to the nucleosome core particle

Reducing experimental time through spin-lattice relaxation enhancement via dissolved oxygen

Large proteins and dilute spin systems within a deuterated background are often characterized by long proton (1H) spin-lattice relaxation times (T1), which directly impacts the recycle delay and hence, the total experimental time. Dioxygen (O2) is a well-known paramagnetic species whose short electronic spin-lattice relaxation time (7.5 ps) contributes to effective spin-lattice relaxation of high gamma nuclei. Oxygen's chemical potential and high diffusivity also allows it to access both the protein exterior and much of the (hydrophobic) interior of the protein. Consequently, at O2 partial pressures of ~ 10 bar, 1H and 19F spin-lattice relaxation rates (R1) typically reach 3-5 Hz (versus rates of 0.7-1.0 Hz without oxygen) with comparable line-broadening in protein NMR spectra. Using fluoroacetate dehalogenase (FAcD) a soluble 35 kDa homodimeric enzyme, a nanodisc-stabilized G protein-coupled receptor (A2AR), and bovine serum albumin (BSA) as test cases, a 3-fold savings in time was achieved in acquiring 1H-15 N HSQC and 19F NMR spectra, after oxygenation at 9 bar for 24 h. Additional spin-diffusion effects are anticipated to contribute to uniform 1H spin-lattice relaxation for both solvent-exposed and buried protons, as demonstrated by T1 relaxation analysis of amides in 15N-labeled FAcD. Finally, we show that in protein samples dissolved oxygen pre-equilibrated at 9 bar (pO2) is largely retained in solution at 20° C or lower, using a standard NMR tube for a period of 3-4 days, thus avoiding the use of specialized apparatus or high-pressure NMR tubes in the spectrometer. The convenience of being able to add or remove the quenching species, while avoiding any complex apparatus in the NMR experiment, makes this a practical tool for both 19F, 1H-13 C, and 1H-15 N NMR studies of proteins.

NMR-Based Measurements of Site-Specific Electrostatic Potentials of Histone Tails in Nucleosome Core Particles

Electrostatics play a dominant role in guiding many biological processes. This is especially the case in the context of chromatin, where charge interactions modulate diverse activities such as DNA repair, transcription, replication, condensation, and phase separation. Using NMR experiments quantifying solvent paramagnetic relaxation enhancements of backbone amide and side chain methyl protons in the presence of paramagnetic cosolutes and focusing on the nucleosome core particle (NCP), we report near surface electrostatic potentials of tail residues of each of the four histone components of the NCP. These are all negative, despite sequences comprising a high density of positively charged amino acids, emphasizing the strong contribution of the negatively charged DNA with which the tails interact. Changes in electrostatic potentials of as much as 60 mV between isolated histone tails and tails in the context of the NCP are calculated. Notably, the tail potentials can vary significantly from each other, with enrichment in glycine residues correlating with less negative values, highlighting differences in the interactions with DNA.

Advances in Isotope Labeling for Solution Nucleic Acid Nuclear Magnetic Resonance Spectroscopy

The availability of structural biology methods for nucleic acid still lags behind that of proteins, as evidenced by the smaller number of structures (DNA: 2513, RNA: 1899, nucleic acid-protein complexes: 13 842, protein: 196 887) deposited in the protein database. The skewed ratio of nucleic acid structures, relative to proteins (≈1:50), is inverted with respect to the cellular output of RNA and proteins in higher organisms (≈50:1). While nuclear magnetic resonance (NMR) is an attractive biophysical tool capable of bridging this gap at the molecular level, the conformational flexibility, line broadening, and low chemical shift dispersion of nucleic acids have made the NMR method challenging, especially for structures larger than 35 nucleotides. The incorporation of NMR-active isotopes is a f strategy to combat these problems. Significant strides made to push the size limits of nucleic acid structures solved by NMR using chemoenzymatic 13C- methyl and aromatic 15N- and 19F-13C-labeling are reviewed and challenges and opportunities are evaluated. Combining these isotopic labeling patterns with superior NMR spectroscopic properties, and new DNA/RNA synthesis methods (palindrome-nicking-dependent amplification and segmental labeling and site-specific modifications by template-directed tension), may stimulate advances in NMR studies of large DNA/RNA and their complexes with important biological functions.

H2A.Z facilitates Sox2-nucleosome interaction by promoting DNA and histone H3 tail mobility

Epigenetic regulation of eukaryotic chromatin structure and function can be modulated by histone variants and post-translational modifications. The conserved variant H2A.Z has been functionally linked to pioneer factors Sox2 and Oct4 that open chromatin and initiate cell fate-specific expression programs. However, the molecular basis for their interaction remains unknown. Using biochemistry, nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations, we examine the role of H2A.Z nucleosome dynamics in pioneer factor binding. We find that H2A.Z facilitates Sox2 and Oct4 binding at distinct locations in 601 nucleosomes. We further link this to increased DNA accessibility and perturbed dynamics of the H3 N-terminal tail, which we show competes with Sox2 for DNA binding. Our simulations validate a coupling between H2A.Z-mediated DNA unwrapping and altered H3 N-tail conformations with fewer contacts to DNA and the H2A.Z C- terminal tail. This destabilizing effect of H2A.Z is DNA sequence dependent and enhanced with the less stable Lin28B nucleosome. Collectively, our findings suggest that H2A.Z promotes pioneer factor binding by increasing access to DNA and reducing competition with H3 tails. This could have broader implications for how epigenetic marks or oncogenic mutations tune pioneer factor engagement with chromatin and thus affect its structure and recognition.

The Internal Structural Dynamics of Elastin-Like Polypeptide Assemblies by 13C-Direct Detected NMR Spectroscopy

Elastin-like polypeptides (ELPs) are biocompatible polymers exhibiting lower critical solution temperature (LCST) behavior, making them valuable in various applications, including drug delivery and tissue engineering. This study addresses the atomistic-level understanding of ELP self-assembly, focusing on their internal structural dynamics. Conventional proton-detected nuclear magnetic resonance (NMR) spectroscopy faces limitations in studying ELP aggregates due to accelerated proton exchange processes, which cause significant resonance broadening. Herein, we show how to overcome this hurdle by using carbon-13-detected NMR. This method mitigates issues related to amide proton exchange, allowing for a residue-resolved view of the internal configuration of ELP aggregates. With this method, we record residue-resolved 15N relaxation rates, revealing three features. (i) Proline residues within the PGXGV pentapeptide repeats (X being any amino acid except proline) of ELP become motional restricted upon aggregation, indicating their role as interchain contacts. (ii) Pentapeptides with alanine guest residue X display particularly significantly reduced motional freedom upon aggregation. (iii) Even within large ELP aggregates, fast internal dynamics characterize the peptide chains in a way that is reminiscent of condensed liquid phases. The presented study is the first proof of concept that 13C-direct detection is a viable tool to delineate the internal structural dynamics of condensed ELP phases by NMR. It might, thus, help to foster new investigations of their aggregation mechanisms.

Structural and dynamic studies of chromatin by solid-state NMR spectroscopy

Chromatin is a complex of DNA with histone proteins organized into nucleosomes that regulates genome accessibility and controls transcription, replication and repair by dynamically switching between open and compact states as a function of different parameters including histone post-translational modifications and interactions with chromatin modulators. Continuing advances in structural biology techniques including X-ray crystallography, cryo-electron microscopy and nuclear magnetic resonance (NMR) spectroscopy have facilitated studies of chromatin systems, in spite of challenges posed by their large size and dynamic nature, yielding important functional and mechanistic insights. In this review we highlight recent applications of magic angle spinning solid-state NMR - an emerging technique that is uniquely-suited toward providing atomistic information for rigid and flexible regions within biomacromolecular assemblies - to detailed characterization of structure, conformational dynamics and interactions for histone core and tail domains in condensed nucleosomes and oligonucleosome arrays mimicking chromatin at high densities characteristic of the cellular environment.

Hyperpolarized nuclear Overhauser enhancement of alanine methyl groups by doubly relayed proton exchange

Hyperpolarized water in dissolution dynamic nuclear polarization (dDNP) experiments has emerged as a promising method for enhancing nuclear magnetic resonance (NMR) signals, particularly in studies of proteins and peptides. Herein, we focus on the application of "proton exchange-doubly relayed" nuclear Overhauser effects (NOE) from hyperpolarized water to achieve positive signal enhancement of methyl groups in the side chain of an alanine-glycine peptide. In particular, we show a cascade hyperpolarization transfer. Initial proton exchange between solvent and amide introduces hyperpolarization into the peptide. Subsequently, intermolecular NOE relays the hyperpolarization first to Ala-Hα and then in a second step to the Ala-CH3 moiety. Both NOEs have negative signs. Hence, the twice-relayed NOE pathway leads to a positive signal enhancement of the methyl group with respect to the thermal equilibrium magnetization. This effect might indicate a way towards hyperpolarized water-based signal enhancement for methyl groups, which are often used for NMR studies of large proteins in solution.

Structural dynamics in chromatin unraveling by pioneer transcription factors

Pioneer transcription factors are proteins with a dual function. First, they regulate transcription by binding to nucleosome-free DNA regulatory elements. Second, they bind to DNA while wrapped around histone proteins in the chromatin and mediate chromatin opening. The molecular mechanisms that connect the two functions are yet to be discovered. In recent years, pioneer factors received increased attention mainly because of their crucial role in promoting cell fate transitions that could be used for regenerative therapies. For example, the three factors required to induce pluripotency in somatic cells, Oct4, Sox2, and Klf4 were classified as pioneer factors and studied extensively. With this increased attention, several structures of complexes between pioneer factors and chromatin structural units (nucleosomes) have been resolved experimentally. Furthermore, experimental and computational approaches have been designed to study two unresolved, key scientific questions: First, do pioneer factors induce directly local opening of nucleosomes and chromatin fibers upon binding? And second, how do the unstructured tails of the histones impact the structural dynamics involved in such conformational transitions? Here we review the current knowledge about transcription factor-induced nucleosome dynamics and the role of the histone tails in this process. We discuss what is needed to bridge the gap between the static views obtained from the experimental structures and the key structural dynamic events in chromatin opening. Finally, we propose that integrating nuclear magnetic resonance spectroscopy with molecular dynamics simulations is a powerful approach to studying pioneer factor-mediated dynamics of nucleosomes and perhaps small chromatin fibers using native DNA sequences.

Monitoring Anthracycline Cancer Drug-Nucleosome Interaction by NMR Using a Specific Isotope Labeling Approach for Nucleosomal DNA

Chromatinized DNA is targeted by proteins and small molecules to regulate chromatin function. For example, anthracycline cancer drugs evict nucleosomes in a mechanism that is still poorly understood. We here developed a flexible method for specific isotope labeling of nucleosomal DNA enabling NMR studies of such nucleosome interactions. We describe the synthesis of segmental one-strand 13C-thymidine labeled 601-DNA, the assignment of the methyl signals, and demonstrate its use to observe site-specific binding to the nucleosome by aclarubicin, an anthracycline cancer drug that intercalates into the DNA minor grooves. Our results highlight intrinsic conformational heterogeneity in the 601 DNA sequence and show that aclarubicin binds an exposed AT-rich region near the DNA end. Overall, our data point to a model where the drug invades the nucleosome from the terminal ends inward, eventually resulting in histone eviction and nucleosome disruption.

A delayed decoupling methyl-TROSY pulse sequence for atomic resolution studies of folded proteins and RNAs in condensates

Solution NMR spectroscopy has tremendous potential for providing atomic resolution insights into the interactions between proteins and nucleic acids partitioned into condensed phases of phase-separated systems. However, the highly viscous nature of the condensed phase challenges applications, and in particular, the extraction of quantitative, site-specific information. Here, we present a delayed decoupling-based HMQC pulse sequence for methyl-TROSY studies of 'client' proteins and nucleic acids partitioned into 'scaffold' proteinaceous phase-separated solvents. High sensitivity and excellent quality spectra are recorded of a nascent form of superoxide dismutase and of a small RNA fragment partitioned into CAPRIN1 condensates.

Cryo-EM of the Nucleosome Core Particle Bound to Ran-RCC1 Reveals a Dynamic Complex

Ras-related nuclear protein (Ran) is a member of the Ras superfamily of small guanosine triphosphatases (GTPases) and a regulator of multiple cellular processes. In healthy cells, the GTP-bound form of Ran is concentrated at chromatin, creating a Ran•GTP gradient that provides the driving force for nucleocytoplasmic transport, mitotic spindle assembly, and nuclear envelope formation. The Ran•GTP gradient is maintained by the regulator of chromatin condensation 1 (RCC1), a guanine nucleotide exchange factor that accelerates GDP/GTP exchange in Ran. RCC1 interacts with nucleosomes, which are the fundamental repeating units of eukaryotic chromatin. Here, we present a cryo-EM analysis of a trimeric complex composed of the nucleosome core particle (NCP), RCC1, and Ran. While the contacts between RCC1 and Ran in the complex are preserved compared with a previously determined structure of RCC1-Ran, our study reveals that RCC1 and Ran interact dynamically with the NCP and undergo rocking motions on the nucleosome surface. Furthermore, the switch 1 region of Ran, which plays an important role in mediating conformational changes associated with the substitution of GDP and GTP nucleotides in Ras family members, appears to undergo disorder-order transitions and forms transient contacts with the C-terminal helix of histone H2B. Nucleotide exchange assays performed in the presence and absence of NCPs are not consistent with an active role for nucleosomes in nucleotide exchange, at least in vitro. Instead, the nucleosome stabilizes RCC1 and serves as a hub that concentrates RCC1 and Ran to promote efficient Ran•GDP to Ran•GTP conversion.

Poly(ADP-ribosyl)ation enhances nucleosome dynamics and organizes DNA damage repair components within biomolecular condensates

Nucleosomes, the basic structural units of chromatin, hinder recruitment and activity of various DNA repair proteins, necessitating modifications that enhance DNA accessibility. Poly(ADP-ribosyl)ation (PARylation) of proteins near damage sites is an essential initiation step in several DNA-repair pathways; however, its effects on nucleosome structural dynamics and organization are unclear. Using NMR, cryoelectron microscopy (cryo-EM), and biochemical assays, we show that PARylation enhances motions of the histone H3 tail and DNA, leaving the configuration of the core intact while also stimulating nuclease digestion and ligation of nicked nucleosomal DNA by LIG3. PARylation disrupted interactions between nucleosomes, preventing self-association. Addition of LIG3 and XRCC1 to PARylated nucleosomes generated condensates that selectively partition DNA repair-associated proteins in a PAR- and phosphorylation-dependent manner in vitro. Our results establish that PARylation influences nucleosomes across different length scales, extending from the atom-level motions of histone tails to the mesoscale formation of condensates with selective compositions.

Optimal control pulses for the 1.2-GHz (28.2-T) NMR spectrometers

The ability to measure nuclear magnetic resonance (NMR) spectra with a large sample volume is crucial for concentration-limited biological samples to attain adequate signal-to-noise (S/N) ratio. The possibility to measure with a 5-mm cryoprobe is currently absent at the 1.2-GHz NMR instruments due to the exceedingly high radio frequency power demands, which is four times compared to 600-MHz instruments. Here, we overcome the high-power demands by designing optimal control (OC) pulses with up to 20 times lower power requirements than currently necessary at a 1.2-GHz spectrometer. We show that multidimensional biomolecular NMR experiments constructed using these OC pulses can bestow improvement in the S/N ratio of up to 26%. With the expected power limitations of a 5-mm cryoprobe, we observe an enhancement in the S/N ratio of more than 240% using our OC sequences. This motivates the development of a cryoprobe with a larger volume than the current 3-mm cryoprobes.

Intrinsic structural dynamics dictate enzymatic activity and inhibition

Enzymes are known to sample various conformations, many of which are critical for their biological function. However, structural characterizations of enzymes predominantly focus on the most populated conformation. As a result, single-point mutations often produce structures that are similar or essentially identical to those of the wild-type enzyme despite large changes in enzymatic activity. Here, we show for mutants of a histone deacetylase enzyme (HDAC8) that reduced enzymatic activities, reduced inhibitor affinities, and reduced residence times are all captured by the rate constants between intrinsically sampled conformations that, in turn, can be obtained independently by solution NMR spectroscopy. Thus, for the HDAC8 enzyme, the dynamic sampling of conformations dictates both enzymatic activity and inhibitor potency. Our analysis also dissects the functional role of the conformations sampled, where specific conformations distinct from those in available structures are responsible for substrate and inhibitor binding, catalysis, and product dissociation. Precise structures alone often do not adequately explain the effect of missense mutations on enzymatic activity and drug potency. Our findings not only assign functional roles to several conformational states of HDAC8 but they also underscore the paramount role of dynamics, which will have general implications for characterizing missense mutations and designing inhibitors.

Mixture-Based Screening of Focused Combinatorial Libraries by NMR: Application to the Antiapoptotic Protein hMcl-1

We report on an innovative ligand discovery strategy based on protein NMR-based screening of a combinatorial library of ∼125,000 compounds that was arranged in 96 distinct mixtures. Using sensitive solution protein NMR spectroscopy and chemical perturbation-based screening followed by an iterative synthesis, deconvolutions, and optimization strategy, we demonstrate that the approach could be useful in the identification of initial binding molecules for difficult drug targets, such as those involved in protein-protein interactions. As an application, we will report novel agents targeting the Bcl-2 family protein hMcl-1. The approach is of general applicability and could be deployed as an effective screening strategy for de novo identification of ligands, particularly when tackling targets involved in protein-protein interactions.

Molecular Mechanism of Nucleosome Recognition by the Pioneer Transcription Factor Sox

Pioneer transcription factors (PTFs) have the remarkable ability to directly bind to chromatin to stimulate vital cellular processes. In this work, we dissect the universal binding mode of Sox PTF by combining extensive molecular simulations and physiochemistry approaches, along with DNA footprinting techniques. As a result, we show that when Sox consensus DNA is located at the solvent-facing DNA strand, Sox binds to the compact nucleosome without imposing any significant conformational changes. We also reveal that the base-specific Sox:DNA interactions (base reading) and Sox-induced DNA changes (shape reading) are concurrently required for sequence-specific nucleosomal DNA recognition. Among three different nucleosome positions located on the positive DNA arm, a sequence-specific reading mechanism is solely satisfied at the superhelical location 2 (SHL2). While SHL2 acts transparently for solvent-facing Sox binding, among the other two positions, SHL4 permits only shape reading. The final position, SHL0 (dyad), on the other hand, allows no reading mechanism. These findings demonstrate that Sox-based nucleosome recognition is essentially guided by intrinsic nucleosome properties, permitting varying degrees of DNA recognition.

Structural studies of protein-nucleic acid complexes: A brief overview of the selected techniques

Protein-nucleic acid complexes are involved in all vital processes, including replication, transcription, translation, regulation of gene expression and cell metabolism. Knowledge of the biological functions and molecular mechanisms beyond the activity of the macromolecular complexes can be determined from their tertiary structures. Undoubtably, performing structural studies of protein-nucleic acid complexes is challenging, mainly because these types of complexes are often unstable. In addition, their individual components may display extremely different surface charges, causing the complexes to precipitate at higher concentrations used in many structural studies. Due to the variety of protein-nucleic acid complexes and their different biophysical properties, no simple and universal guideline exists that helps scientists chose a method to successfully determine the structure of a specific protein-nucleic acid complex. In this review, we provide a summary of the following experimental methods, which can be applied to study the structures of protein-nucleic acid complexes: X-ray and neutron crystallography, nuclear magnetic resonance (NMR) spectroscopy, cryogenic electron microscopy (cryo-EM), atomic force microscopy (AFM), small angle scattering (SAS) methods, circular dichroism (CD) and infrared (IR) spectroscopy. Each method is discussed regarding its historical context, advancements over the past decades and recent years, and weaknesses and strengths. When a single method does not provide satisfactory data on the selected protein-nucleic acid complex, a combination of several methods should be considered as a hybrid approach; thus, specific structural problems can be solved when studying protein-nucleic acid complexes.

Correlating histone acetylation with nucleosome core particle dynamics and function

Epigenetic modifications of chromatin play a critical role in regulating the fidelity of the genetic code and in controlling the translation of genetic information into the protein components of the cell. One key posttranslational modification is acetylation of histone lysine residues. Molecular dynamics simulations, and to a smaller extent experiment, have established that lysine acetylation increases the dynamics of histone tails. However, a systematic, atomic resolution experimental investigation of how this epigenetic mark, focusing on one histone at a time, influences the structural dynamics of the nucleosome beyond the tails, and how this translates into accessibility of protein factors such as ligases and nucleases, has yet to be performed. Herein, using NMR spectroscopy of nucleosome core particles (NCPs), we evaluate the effects of acetylation of each histone on tail and core dynamics. We show that for histones H2B, H3, and H4, the histone core particle dynamics are little changed, even though the tails have increased amplitude motions. In contrast, significant increases to H2A dynamics are observed upon acetylation of this histone, with the docking domain and L1 loop particularly affected, correlating with increased susceptibility of NCPs to nuclease digestion and more robust ligation of nicked DNA. Dynamic light scattering experiments establish that acetylation decreases inter-NCP interactions in a histone-dependent manner and facilitates the development of a thermodynamic model for NCP stacking. Our data show that different acetylation patterns result in nuanced changes to NCP dynamics, modulating interactions with other protein factors, and ultimately controlling biological output.

Less is more: A simple methyl-TROSY based pulse scheme offers improved sensitivity in applications to high molecular weight complexes

The HMQC pulse sequence and variants thereof have been exploited in studies of high molecular weight protein complexes, taking advantage of the fact that fast and slow relaxing magnetization components are sequestered along two distinct magnetization transfer pathways. Despite the simplicity of the HMQC scheme an even shorter version can be designed, based on elimination of the terminal refocusing period, as a further means of increasing signal. Here we present such an experiment, and show that significant sensitivity gains, in some cases by factors of two or more, are realized in studies of proteins varying in molecular masses from 100 kDa to 1 MDa.

Nucleosome Core Particle Reconstitution with Recombinant Histones and Widom 601 DNA

Reconstitution of nucleosomes from recombinant histones and DNA is a widely used tool for studying nucleosome structure, dynamics, and interactions. Preparation of reconstituted nucleosomes allows for the study of nucleosomes with defined compositions. Here, we describe methods for refolding recombinant human histones, reconstituting nucleosome core particles with 147 bp Widom 601 DNA, and purification via sucrose gradient.

Nucleic acid-protein interfaces studied by MAS solid-state NMR spectroscopy

Solid-state NMR (ssNMR) has become a well-established technique to study large and insoluble protein assemblies. However, its application to nucleic acid-protein complexes has remained scarce, mainly due to the challenges presented by overlapping nucleic acid signals. In the past decade, several efforts have led to the first structure determination of an RNA molecule by ssNMR. With the establishment of these tools, it has become possible to address the problem of structure determination of nucleic acid-protein complexes by ssNMR. Here we review first and more recent ssNMR methodologies that study nucleic acid-protein interfaces by means of chemical shift and peak intensity perturbations, direct distance measurements and paramagnetic effects. At the end, we review the first structure of an RNA-protein complex that has been determined from ssNMR-derived intermolecular restraints.

FACT modulates the conformations of histone H2A and H2B N-terminal tails within nucleosomes

Gene expression is regulated by the modification and accessibility of histone tails within nucleosomes. The histone chaperone FACT (facilitate chromatin transcription), comprising SPT16 and SSRP1, interacts with nucleosomes through partial replacement of DNA with the phosphorylated acidic intrinsically disordered (pAID) segment of SPT16; pAID induces an accessible conformation of the proximal histone H3 N-terminal tail (N-tail) in the unwrapped nucleosome with FACT. Here, we use NMR to probe the histone H2A and H2B tails in the unwrapped nucleosome. Consequently, both the H2A and H2B N-tails on the pAID-proximal side bind to pAID with robust interactions, which are important for nucleosome assembly with FACT. Furthermore, the conformations of these N-tails on the distal DNA-contact site are altered from those in the canonical nucleosome. Our findings highlight that FACT both proximally and distally regulates the conformations of the H2A and H2B N-tails in the asymmetrically unwrapped nucleosome.

DNA methylation: Precise modulation of chromatin structure and dynamics

DNA methylation plays a vital role in epigenetic regulation in both plants and animals, and typically occurs at the 5-carbon position of the cytosine pyrimidine ring within the CpG dinucleotide steps. Cytosine methylation can alter DNA's geometry, mechanical and physico-chemical properties - thus influencing the molecular signaling events vital for transcription, replication and chromatin remodeling. Despite the profound effect cytosine methylation can have on DNA, the underlying atomistic mechanisms remain enigmatic. Many studies so far have produced controversial findings on how cytosine methylation dictates DNA flexibility and accessibility, nucleosome stability and dynamics. Here, we review the most recent experimental and computational studies that provide precise characterization of structure and function of cytosine methylation and its versatile roles in modulating DNA mechanics, nucleosome and chromatin structure, stability and dynamics. Moreover, the review briefly discusses the relationship between DNA methylation and nucleosome positioning, and the crosstalk between DNA methylation and histone tail modifications.

5mC and H3K9me3 of TRAF3IP2 promoter region accelerates the progression of translocation renal cell carcinoma

Background

In our previous study, we found that lncRNA TRAF3IP2 antisense RNA 1 (TRAF3IP2-AS1) could play a critical role in the progression of NONO-TFE3 translocation renal cell carcinoma (NONO-TFE3 tRCC). However, the function of TRAF3IP2 (TRAF3 interacting protein 2), encoded by the complementary strand of TRAF3IP2-AS1, remains poorly understood in NONO-TFE3 tRCC.

Methods

Immunohistochemistry, western blot, and qRT-PCR were undertaken to study the expression and clinical significance of TRAF3IP2 in Xp11.2 tRCC tissues and cells. The functions of TRAF3IP2 in tRCC were investigated by proliferation analysis, EdU staining, colony and sphere formation assay, Transwell assay, and apoptosis analysis. The regulatory mechanisms among TRAF3IP2, NOTCH1, and TRAF3IP2-AS1 were investigated by luciferase assay, RNA immunoprecipitation, western blot, methylated DNA Immunoprecipitation, and CRISPR/dCas9-based system.

Results

The results showed that TRAF3IP2 was highly expressed in NONO-TFE3 tRCC tissues and cells, and the silence of TRAF3IP2 inhibited the proliferation, migration, and invasion of UOK109 cells which were derived from cancer tissue of patient with NONO-TFE3 tRCC. Mechanistic studies revealed that TRAF3IP2 functioned as a co-activator of NOTCH1 to activate the NOTCH1 pathway. Meanwhile, HNRNPK, DNMT1 and SETDB1 could be recruited by TRAF3IP2-AS1 to the promoter region of TRAF3IP2, which mediated 5-hydroxymethylcytosine (5mC) on DNA and trimethylated lysine 9 of histone H3 (H3K9me3) at transcriptional level to repress the expression of TRAF3IP2.

Conclusions

TRAF3IP2 functions as an oncogene in NONO-TFE3 tRCC progression and might serve as a novel target for NONO-TFE3 tRCC therapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40364-022-00402-3.

Recent Advances in Investigating Functional Dynamics of Chromatin

Dynamics spanning the picosecond-minute time domain and the atomic-subcellular spatial window have been observed for chromatin in vitro and in vivo. The condensed organization of chromatin in eukaryotic cells prevents regulatory factors from accessing genomic DNA, which requires dynamic stabilization and destabilization of structure to initiate downstream DNA activities. Those processes are achieved through altering conformational and dynamic properties of nucleosomes and nucleosome–protein complexes, of which delineating the atomistic pictures is essential to understand the mechanisms of chromatin regulation. In this review, we summarize recent progress in determining chromatin dynamics and their modulations by a number of factors including post-translational modifications (PTMs), incorporation of histone variants, and binding of effector proteins. We focus on experimental observations obtained using high-resolution techniques, primarily including nuclear magnetic resonance (NMR) spectroscopy, Förster (or fluorescence) resonance energy transfer (FRET) microscopy, and molecular dynamics (MD) simulations, and discuss the elucidated dynamics in the context of functional response and relevance.

In-Cell Structural Biology by NMR: The Benefits of the Atomic Scale

In-cell structural biology aims at extracting structural information about proteins or nucleic acids in their native, cellular environment. This emerging field holds great promise and is already providing new facts and outlooks of interest at both fundamental and applied levels. NMR spectroscopy has important contributions on this stage: It brings information on a broad variety of nuclei at the atomic scale, which ensures its great versatility and uniqueness. Here, we detail the methods, the fundamental knowledge, and the applications in biomedical engineering related to in-cell structural biology by NMR. We finally propose a brief overview of the main other techniques in the field (EPR, smFRET, cryo-ET, etc.) to draw some advisable developments for in-cell NMR. In the era of large-scale screenings and deep learning, both accurate and qualitative experimental evidence are as essential as ever to understand the interior life of cells. In-cell structural biology by NMR spectroscopy can generate such a knowledge, and it does so at the atomic scale. This review is meant to deliver comprehensive but accessible information, with advanced technical details and reflections on the methods, the nature of the results, and the future of the field.

Visualizing Conformational Ensembles of the Nucleosome by NMR

The formation of chromatin not only compacts the eukaryotic genome into the nucleus but also provides a mechanism for the regulation of all DNA templated processes. Spatial and temporal modulation of the chromatin structure is critical in such regulation and involves fine-tuned functioning of the basic subunit of chromatin, the nucleosome. It has become apparent that the nucleosome is an inherently dynamic system, but characterization of these dynamics at the atomic level has remained challenging. NMR spectroscopy is a powerful tool for investigating the conformational ensemble and dynamics of proteins and protein complexes, and recent advances have made the study of large systems possible. Here, we review recent studies which utilize NMR spectroscopy to uncover the atomic level conformation and dynamics of the nucleosome and provide a better understanding of the importance of these dynamics in key regulatory events.

Synthesis of [7-15N]-GTPs for RNA structure and dynamics by NMR spectroscopy

Several isotope-labeling strategies have been developed for the study of RNA by nuclear magnetic resonance (NMR) spectroscopy. Here, we report a combined chemical and enzymatic synthesis of [7-15N]-guanosine-5'-triphosphates for incorporation into RNA via T7 RNA polymerase-based in vitro transcription. We showcase the utility of these labels to probe both structure and dynamics in two biologically important RNAs.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00706-022-02892-1.

Untargeted 2D NMR Metabolomics of [13C-methyl]Methionine-Labeled Tumor Models Reveals the Non-DNA Methylome and Provides Clues to Methyl Metabolism Shift during Tumor Progression

For more than a decade, DNA and histone methylations have been the focus of extensive work, although their relationship with methyl group metabolism was overlooked. Recently, it has emerged that epigenetic methylations are influenced by methyl donor nutrient availability, cellular levels of S-adenosyl-methionine (SAM), and cytoplasmic methyltransferase activities. SAM-dependent methyltransferases methylate a wide range of targets, from small molecules to proteins and nucleic acids. However, few investigations of the global methylome of tumors have been performed. Here, untargeted NMR metabolomics of two mouse tumor models labeled with [¹³C-methyl]methionine were used to search for the NMR-visible set of cellular methyl acceptors denoted the global methylome. Tumor models were B16 melanoma cell cultures and B16 melanoma tumors, which may be considered as two stages of B16 tumor development. Based on 2D ¹H-¹³C NMR spectra and orthogonal partial least squares discriminant analysis of spectra, our study revealed markedly different global methylomes for melanoma models. The methylome of B16 melanoma cell cultures was dominated by histone methylations, whereas that of B16 melanoma tumors was dominated by cytoplasmic small-molecule methylations. Overall, the technique gave access to the non-DNA methylome. Comparison of tumor models also exhibiting differential expression of aerobic glycolysis provided clues to a methyl metabolism shift during tumor progression.

Bacterial Transcriptional Regulators: A Road Map for Functional, Structural, and Biophysical Characterization

The different niches through which bacteria move during their life cycle require a fast response to the many environmental queues they encounter. The sensing of these stimuli and their correct response is driven primarily by transcriptional regulators. This kind of protein is involved in sensing a wide array of chemical species, a process that ultimately leads to the regulation of gene transcription. The allosteric-coupling mechanism of sensing and regulation is a central aspect of biological systems and has become an important field of research during the last decades. In this review, we summarize the state-of-the-art techniques applied to unravel these complex mechanisms. We introduce a roadmap that may serve for experimental design, depending on the answers we seek and the initial information we have about the system of study. We also provide information on databases containing available structural information on each family of transcriptional regulators. Finally, we discuss the recent results of research about the allosteric mechanisms of sensing and regulation involving many transcriptional regulators of interest, highlighting multipronged strategies and novel experimental techniques. The aim of the experiments discussed here was to provide a better understanding at a molecular level of how bacteria adapt to the different environmental threats they face.

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.

Analysis of conformational exchange processes using methyl-TROSY-based Hahn echo measurements of quadruple-quantum relaxation

Transverse nuclear spin relaxation is a sensitive probe of chemical exchange on timescales on the order of microseconds to milliseconds. Here we present an experiment for the simultaneous measurement of the relaxation rates of two quadruple-quantum transitions in 13 CH3 -labelled methyl groups. These coherences are protected against relaxation by intra-methyl dipolar interactions and so have unexpectedly long lifetimes within perdeuterated biomacromolecules. However, these coherences also have an order of magnitude higher sensitivity to chemical exchange broadening than lower order coherences and therefore provide ideal probes of dynamic processes. We show that analysis of the static magnetic field dependence of zero-, double- and quadruple-quantum Hahn echo relaxation rates provides a robust indication of chemical exchange and can determine the signed relative magnitudes of proton and carbon chemical shift differences between ground and excited states. We also demonstrate that this analysis can be combined with established Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion measurements, providing improved precision in parameter estimates, particularly in the determination of 1 H chemical shift differences.

Emerging Contributions of Solid-State NMR Spectroscopy to Chromatin Structural Biology

The eukaryotic genome is packaged into chromatin, a polymer of DNA and histone proteins that regulates gene expression and the spatial organization of nuclear content. The repetitive character of chromatin is diversified into rich layers of complexity that encompass DNA sequence, histone variants and post-translational modifications. Subtle molecular changes in these variables can often lead to global chromatin rearrangements that dictate entire gene programs with far reaching implications for development and disease. Decades of structural biology advances have revealed the complex relationship between chromatin structure, dynamics, interactions, and gene expression. Here, we focus on the emerging contributions of magic-angle spinning solid-state nuclear magnetic resonance spectroscopy (MAS NMR), a relative newcomer on the chromatin structural biology stage. Unique among structural biology techniques, MAS NMR is ideally suited to provide atomic level information regarding both the rigid and dynamic components of this complex and heterogenous biological polymer. In this review, we highlight the advantages MAS NMR can offer to chromatin structural biologists, discuss sample preparation strategies for structural analysis, summarize recent MAS NMR studies of chromatin structure and dynamics, and close by discussing how MAS NMR can be combined with state-of-the-art chemical biology tools to reconstitute and dissect complex chromatin environments.

A quantitative model predicts how m6A reshapes the kinetic landscape of nucleic acid hybridization and conformational transitions

N6-methyladenosine (m6A) is a post-transcriptional modification that controls gene expression by recruiting proteins to RNA sites. The modification also slows biochemical processes through mechanisms that are not understood. Using temperature-dependent (20°C-65°C) NMR relaxation dispersion, we show that m6A pairs with uridine with the methylamino group in the anti conformation to form a Watson-Crick base pair that transiently exchanges on the millisecond timescale with a singly hydrogen-bonded low-populated (1%) mismatch-like conformation in which the methylamino group is syn. This ability to rapidly interchange between Watson-Crick or mismatch-like forms, combined with different syn:anti isomer preferences when paired (~1:100) versus unpaired (~10:1), explains how m6A robustly slows duplex annealing without affecting melting at elevated temperatures via two pathways in which isomerization occurs before or after duplex annealing. Our model quantitatively predicts how m6A reshapes the kinetic landscape of nucleic acid hybridization and conformational transitions, and provides an explanation for why the modification robustly slows diverse cellular processes.

Protein intrinsic disorder on a dynamic nucleosomal landscape

The complex nucleoprotein landscape of the eukaryotic cell nucleus is rich in dynamic proteins that lack a stable three-dimensional structure. Many of these intrinsically disordered proteins operate directly on the first fundamental level of genome compaction: the nucleosome. Here we give an overview of how disordered interactions with and within nucleosomes shape the dynamics, architecture, and epigenetic regulation of the genetic material, controlling cellular transcription patterns. We highlight experimental and computational challenges in the study of protein disorder and illustrate how integrative approaches are increasingly unveiling the fine details of nuclear interaction networks. We finally dissect sequence properties encoded in disordered regions and assess common features of disordered nucleosome-binding proteins. As drivers of many critical biological processes, disordered proteins are integral to a comprehensive molecular view of the dynamic nuclear milieu.

Methodological advancements for characterising protein side chains by NMR spectroscopy

The surface of proteins is covered by side chains of polar amino acids that are imperative for modulating protein functionality through the formation of noncovalent intermolecular interactions. However, despite their tremendous importance, the unique structures of protein side chains require tailored approaches for investigation by nuclear magnetic resonance spectroscopy and so have traditionally been understudied compared with the protein backbone. Here, we review substantial recent methodological advancements within nuclear magnetic resonance spectroscopy to address this issue. Specifically, we consider advancements that provide new insight into methyl-bearing side chains, show the potential of using non-natural amino acids and reveal the actions of charged side chains. Combined, the new methods promise unprecedented characterisations of side chains that will further elucidate protein function.

Harnessing methylation and AdoMet-utilising enzymes for selective modification in cascade reactions

Enzyme-mediated methylation is a very important reaction in nature, yielding a wide range of modified natural products, diversifying small molecules and fine-tuning the activity of biomacromolecules. The field has attracted much attention over the recent years and interesting applications of the dedicated enzymes in biocatalysis and biomolecular labelling have emerged. In this review article, we summarise the concepts and recent advances in developing (chemo)-enzymatic cascades for selective methylation, alkylation and photocaging as tools to study biological methylation and as biotransformations to generate site-specifically alkylated products.

Developments in solution-state NMR yield broader and deeper views of the dynamic ensembles of nucleic acids

Nucleic acids do not fold into a single conformation, and dynamic ensembles are needed to describe their propensities to cycle between different conformations when performing cellular functions. We review recent advances in solution-state nuclear magnetic resonance (NMR) methods and their integration with computational techniques that are improving the ability to probe the dynamic ensembles of DNA and RNA. These include computational approaches for predicting chemical shifts from structure and generating conformational libraries from sequence, measurements of exact nuclear Overhauser effects, development of new probes to study chemical exchange using relaxation dispersion, faster and more sensitive real-time NMR techniques, and new NMR approaches to tackle large nucleic acid assemblies. We discuss how these advances are leading to new mechanistic insights into gene expression and regulation.

NMR spectroscopy captures the essential role of dynamics in regulating biomolecular function

Biomolecules are in constant motion. To understand how they function, and why malfunctions can cause disease, it is necessary to describe their three-dimensional structures in terms of dynamic conformational ensembles. Here, we demonstrate how nuclear magnetic resonance (NMR) spectroscopy provides an essential, dynamic view of structural biology that captures biomolecular motions at atomic resolution. We focus on examples that emphasize the diversity of biomolecules and biochemical applications that are amenable to NMR, such as elucidating functional dynamics in large molecular machines, characterizing transient conformations implicated in the onset of disease, and obtaining atomic-level descriptions of intrinsically disordered regions that make weak interactions involved in liquid-liquid phase separation. Finally, we discuss the pivotal role that NMR has played in driving forward our understanding of the biomolecular dynamics-function paradigm.

A suite of 19F based relaxation dispersion experiments to assess biomolecular motions

Proteins and nucleic acids are highly dynamic bio-molecules that can populate a variety of conformational states. NMR relaxation dispersion (RD) methods are uniquely suited to quantify the associated kinetic and thermodynamic parameters. Here, we present a consistent suite of ¹⁹F-based CPMG, on-resonance R_1ρ and off-resonance R_1ρ RD experiments. We validate these experiments by studying the unfolding transition of a 7.5 kDa cold shock protein. Furthermore we show that the ¹⁹F RD experiments are applicable to very large molecular machines by quantifying dynamics in the 360 kDa half-proteasome. Our approach significantly extends the timescale of chemical exchange that can be studied with ¹⁹F RD, adds robustness to the extraction of exchange parameters and can determine the absolute chemical shifts of excited states. Importantly, due to the simplicity of ¹⁹F NMR spectra, it is possible to record complete datasets within hours on samples that are of very low costs. This makes the presented experiments ideally suited to complement static structural information from cryo-EM and X-ray crystallography with insights into functionally relevant motions.

The precious fluorine on the ring: fluorine NMR for biological systems

The fluorine-19 nucleus was recognized early to harbor exceptional properties for NMR spectroscopy. With 100% natural abundance, a high gyromagnetic ratio (83% sensitivity compared to 1H), a chemical shift that is extremely sensitive to its surroundings and near total absence in biological systems, it was destined to become a favored NMR probe, decorating small and large molecules. However, after early excitement, where uptake of fluorinated aromatic amino acids was explored in a series of animal studies, 19F-NMR lost popularity, especially in large molecular weight systems, due to chemical shift anisotropy (CSA) induced line broadening at high magnetic fields. Recently, two orthogonal approaches, (i) CF3 labeling and (ii) aromatic 19F-13C labeling leveraging the TROSY (Transverse Relaxation Optimized Spectroscopy) effect have been successfully applied to study large biomolecular systems. In this perspective, we will discuss the fascinating early work with fluorinated aromatic amino acids, which reveals the enormous potential of these non-natural amino acids in biological NMR and the potential of 19F-NMR to characterize protein and nucleic acid structure, function and dynamics in the light of recent developments. Finally, we explore how fluorine NMR might be exploited to implement small molecule or fragment screens that resemble physiological conditions and discuss the opportunity to follow the fate of small molecules in living cells.