Gadolinium-Based NMR Spin Relaxation Measurements of Near-Surface Electrostatic Potentials of Biomolecules.

NMR spectroscopy is an important tool for the measurement of the electrostatic properties of biomolecules. To this point, paramagnetic relaxation enhancements (PREs) of 1H nuclei arising from nitroxide cosolutes in biomolecular solutions have been used to measure effective near-surface electrostatic potentials (ϕENS) of proteins and nucleic acids. Here, we present a gadolinium (Gd)-based NMR method, exploiting Gd chelates with different net charges, for measuring ϕENS values and demonstrate its utility through applications to a number of biomolecular systems. The use of Gd-based cosolutes offers several advantages over nitroxides for ϕENS measurements. First, unlike nitroxide compounds, Gd chelates enable electrostatic potential measurements on oxidation-sensitive proteins that require reducing agents. Second, the large electron spin quantum number of Gd (7/2) results in notably larger PREs for Gd chelates when used at the same concentrations as nitroxide radicals. Thus, it is possible to measure ϕENS values exclusively from + and - charged compounds even for highly charged biomolecules, avoiding the use of neutral cosolutes that, as we further establish here, limits the accuracy of the measured electrostatic potentials. In addition, the smaller concentrations of cosolutes required minimize potential binding to sites on macromolecules. Fourth, the closer proximity of the paramagnetic center and charged groups within Gd chelates, in comparison to the corresponding nitroxide compounds, enables more accurate predictions of ϕENS potentials for cross-validation of the experimental results. The Gd-based method described here, thus, broadens the applicability of studies of biomolecular electrostatics using solution NMR spectroscopy.

Phosphorylation by Protein Kinase C Weakens DNA-Binding Affinity and Folding Stability of the HMGB1 Protein.

The HMGB1 protein typically serves as a DNA chaperone that assists DNA-repair enzymes and transcription factors but can translocate from the nucleus to the cytoplasm or even to extracellular space upon some cellular stimuli. One of the factors that triggers the translocation of HMGB1 is its phosphorylation near a nuclear localization sequence by protein kinase C (PKC), although the exact modification sites on HMGB1 remain ambiguous. In this study, using spectroscopic methods, we investigated the HMGB1 phosphorylation and its impact on the molecular properties of the HMGB1 protein. Our nuclear magnetic resonance (NMR) data on the full-length HMGB1 protein showed that PKC specifically phosphorylates the A-box domain, one of the DNA binding domains of HMGB1. Phosphorylation of S46 and S53 was particularly efficient. Over a longer reaction time, PKC phosphorylated some additional residues within the HMGB1 A-box domain. Our fluorescence-based binding assays showed that the phosphorylation significantly reduces the binding affinity of HMGB1 for DNA. Based on the crystal structures of HMGB1-DNA complexes, this effect can be ascribed to electrostatic repulsion between the negatively charged phosphate groups at the S46 side chain and DNA backbone. Our data also showed that the phosphorylation destabilizes the folding of the A-box domain. Thus, phosphorylation by PKC weakens the DNA-binding affinity and folding stability of HMGB1.

Influence of an Intrinsically Disordered Region on Protein Domains Revealed by NMR-Based Electrostatic Potential Measurements.

Many human proteins possess intrinsically disordered regions containing consecutive aspartate or glutamate residues ("D/E repeats"). Approximately half of them are DNA/RNA-binding proteins. In this study, using nuclear magnetic resonance (NMR) spectroscopy, we investigated the electrostatic properties of D/E repeats and their influence on folded domains within the same protein. Local electrostatic potentials were directly measured for the HMGB1 protein, its isolated D/E repeats, and DNA-binding domains by NMR. The data provide quantitative information about the electrostatic interactions between distinct segments of HMGB1. Due to the interactions between the D/E repeats and the DNA-binding domains, local electrostatic potentials of the DNA-binding domains within the full-length HMGB1 protein were largely negative despite the presence of many positively charged residues. Our NMR data on counterions and electrostatic potentials show that the D/E repeats and DNA have similar electrostatic properties and compete for the DNA-binding domains. The competition promotes dissociation of the protein-DNA complex and influences the molecular behavior of the HMGB1 protein. These effects may be general among the DNA/RNA-binding proteins with D/E repeats.

Robust Enzymatic Production of DNA G-Quadruplex, Aptamer, DNAzyme, and Other Oligonucleotides: Applications for NMR.

Single-stranded DNA (ssDNA) oligonucleotides are widely used in biological research, therapeutics, biotechnology, and nanomachines. Large-scale enzymatic production of ssDNA oligonucleotides forming noncanonical structures has been difficult. Here, we present a simple and robust method named "palindrome-nicking-dependent amplification" (PaNDA) for enzymatic production of a large amount of ssDNA oligonucleotides. It utilizes a strand-displacing DNA polymerase and a nicking enzyme together with input DNA and deoxynucleotide triphosphates at 55 °C. Scaling up of PaNDA is straightforward due to its isothermal nature. The ssDNA products can easily be isolated through anion-exchange chromatography under nondenaturing conditions. We demonstrate applications of PaNDA to 13C/15N-labeling of various DNA strands, including a 22-nt telomere repeat G-quadruplex, a 26-nt therapeutic aptamer, and a 33-nt DNAzyme. The 13C/15N-labeling by PaNDA greatly facilitates the characterization of noncanonical DNA by nuclear magnetic resonance (NMR) spectroscopy. For example, the behavior of therapeutic DNA aptamers in human serum can be investigated.

Analyzing paramagnetic NMR data on target DNA search by proteins using a discrete-state kinetic model for translocation.

Before reaching their targets, sequence-specific DNA-binding proteins nonspecifically bind to DNA through electrostatic interactions and stochastically change their locations on DNA. Investigations into the dynamics of DNA-scanning by proteins are nontrivial due to the simultaneous presence of multiple translocation mechanisms and many sites for the protein to nonspecifically bind to DNA. Nuclear magnetic resonance (NMR) spectroscopy can provide information about the target DNA search processes at an atomic level. Paramagnetic relaxation enhancement (PRE) is particularly useful to study how the proteins scan DNA in the search process. Previously, relatively simple two-state or three-state exchange models were used to explain PRE data reflecting the target search process. In this work, using more realistic discrete-state stochastic kinetics models embedded into an NMR master equation, we analyzed the PRE data for the HoxD9 homeodomain interacting with DNA. The kinetic models that incorporate sliding, dissociation, association, and intersegment transfer can reproduce the PRE profiles observed at some different ionic strengths. The analysis confirms the previous interpretation of the PRE data and shows that the protein's probability distribution among nonspecific sites is nonuniform during the target DNA search process.

Direct measurements of biomolecular electrostatics through experiments.

Biomolecular electrostatics has been a subject of computational investigations based on 3D structures. This situation is changing because emerging experimental tools allow us to quantitatively investigate biomolecular electrostatics without any use of structure information. Now, electrostatic potentials around biomolecules can directly be measured for many residues simultaneously by nuclear magnetic resonance (NMR) spectroscopy. This NMR method can be used to study electrostatic aspects of various processes, including macromolecular association and liquid-liquid phase separation. Applications to structurally flexible biomolecules such as intrinsically disordered proteins are particularly useful. The new tools also facilitate examination of theoretical models and methods for biomolecular electrostatics.

Negatively charged, intrinsically disordered regions can accelerate target search by DNA-binding proteins.

In eukaryotes, many DNA/RNA-binding proteins possess intrinsically disordered regions (IDRs) with large negative charge, some of which involve a consecutive sequence of aspartate (D) or glutamate (E) residues. We refer to them as D/E repeats. The functional role of D/E repeats is not well understood, though some of them are known to cause autoinhibition through intramolecular electrostatic interaction with functional domains. In this work, we investigated the impacts of D/E repeats on the target DNA search kinetics for the high-mobility group box 1 (HMGB1) protein and the artificial protein constructs of the Antp homeodomain fused with D/E repeats of varied lengths. Our experimental data showed that D/E repeats of particular lengths can accelerate the target association in the overwhelming presence of non-functional high-affinity ligands ('decoys'). Our coarse-grained molecular dynamics (CGMD) simulations showed that the autoinhibited proteins can bind to DNA and transition into the uninhibited complex with DNA through an electrostatically driven induced-fit process. In conjunction with the CGMD simulations, our kinetic model can explain how D/E repeats can accelerate the target association process in the presence of decoys. This study illuminates an unprecedented role of the negatively charged IDRs in the target search process.

Many DNA/RNA-binding proteins possess intrinsically disordered regions (IDRs) with large negative charge, some of which involve a consecutive sequence of aspartate (D) or glutamate (E) residues. We refer to them as D/E repeats. The functional role of D/E repeats is not well understood, though some of them are known to cause autoinhibition. Here, using the HMGB1 protein and the artificial protein constructs of the Antp homeodomain fused with D/E repeats, we demonstrate that D/E repeats can accelerate the target search process in the presence of non-functional high-affinity ligands ('decoys'). Our coarse-grained molecular dynamics (CGMD) simulations and kinetic model provide mechanistic insight into this acceleration. Our current study illuminates an unprecedented role of the negatively charged IDRs.

DNA base order parameter determination without influence of chemical exchange.

Nuclear magnetic resonance (NMR) spectroscopy is a versatile tool used to investigate the dynamic properties of biological macromolecules and their complexes. NMR relaxation data can provide order parameters S2, which represent the mobility of bond vectors reorienting within a molecular frame. Determination of S2 parameters typically involves the use of transverse NMR relaxation rates. However, the accuracy in S2 determination can be diminished by elevation of the transverse relaxation rates through conformational or chemical exchange involving protonation/deprotonation or non-Watson-Crick base-pair states of nucleic acids. Here, we propose an approach for determination of S2 parameters without the influence of exchange processes. This approach utilizes transverse and longitudinal 13C chemical shift anisotropy (CSA) - dipole-dipole (DD) cross-correlation rates instead of 13C transverse relaxation rates. Anisotropy in rotational diffusion is taken into consideration. An application of this approach to nucleotide base CH groups of a uniformly 13C/15N-labeled DNA duplex is demonstrated.

Measuring Local Electrostatic Potentials Around Nucleic Acids by Paramagnetic NMR Spectroscopy.

Electrostatic potentials around macromolecules in the presence of mobile charges are difficult to assess especially for highly charged systems. Here, we report measurements of local electrostatic potentials around DNA by paramagnetic NMR. Through quantitative analysis of NMR paramagnetic relaxation enhancement arising from positively charged or neutral paramagnetic cosolutes, we were able to determine local electrostatic potentials around 1H nuclei at >100 sites in major and minor grooves of 13C,15N-labeled 15-bp DNA at 100 mM NaCl. Our experimental electrostatic potential data directly confirmed the Coulombic end effects of DNA. The effective near-surface electrostatic potentials from the NMR data were in good agreement with the theoretical predictions with the Poisson-Boltzmann equation. This NMR method allows for unprecedented experimental investigations into the electrostatic properties of nucleic acids.

Assessment of the Components of the Electrostatic Potential of Proteins in Solution: Comparing Experiment and Theory.

In this work, the components of the protein electrostatic potentials in solution are analyzed with NMR paramagnetic relaxation enhancement experiments and compared with continuum solution theory, and multiscale simulations. To determine the contributions of the solution components, we analyze them at different ionic strengths from 0 to 745 mM. A theoretical approximation allows the determination of the electrostatic potential at a given proton without reference to the protein structure given the ratio of paramagnetic relaxation enhancements rates between a cationic and an anionic probe. The results derived from simulations show good agreement with experiment and simple continuum solvent theory for many of the residues. A discrepancy including a switch of sign of the electrostatic potential was observed for particular residues. By considering the components of the potential, we found the discrepancy is mainly caused by angular correlations of the probe molecules with these residues. The correction for the correlations allows a more accurate analysis of the experiments determining the electrostatic potential of proteins in solution.

Diffusion NMR-based comparison of electrostatic influences of DNA on various monovalent cations.

Counterions are important constituents for the structure and function of nucleic acids. Using 7Li and 133Cs nuclear magnetic resonance (NMR) spectroscopy, we investigated how ionic radii affect the behavior of counterions around DNA through diffusion measurements of Li+ and Cs+ ions around a 15-bp DNA duplex. Together with our previous data on 23Na+ and 15NH4+ ions around the same DNA under the same conditions, we were able to compare the dynamics of four different monovalent ions around DNA. From the apparent diffusion coefficients at varied concentrations of DNA, we determined the diffusion coefficients of these cations inside and outside the ion atmosphere around DNA (Db and Df, respectively). We also analyzed ionic competition with K+ ions for the ion atmosphere and assessed the relative affinities of these cations for DNA. Interestingly, all cations (i.e., Li+, Na+, NH4+, and Cs+) analyzed by diffusion NMR spectroscopy exhibited nearly identical Db/Df ratios despite the differences in their ionic radii, relative affinities, and diffusion coefficients. These results, along with the theoretical relationship between diffusion and entropy, suggest that the entropy change due to the release of counterions from the ion atmosphere around DNA is also similar regardless of the monovalent ion types. These findings and the experimental diffusion data on the monovalent ions are useful for examination of computational models for electrostatic interactions or ion solvation.

Slow Rotational Dynamics of Cytosine NH2 Groups in Double-Stranded DNA.

Aromatic NH2 groups are essential as hydrogen-bond donors in secondary structures of DNA and RNA. Although rapid rotations of NH2 groups of adenine and guanine bases were previously characterized, there has been a lack of quantitative information about slow rotations of cytosine NH2 groups in Watson-Crick base pairs. In this study, using an NMR method we had recently developed, we determined the kinetic rate constants and energy barriers for cytosine NH2 rotations in a 15-base-pair DNA duplex. Our data show that the rotational dynamics of cytosine NH2 groups depend on local environments. Qualitative correlation between the ranges of 15N chemical shifts and rotational time scales for various NH2 groups of nucleic acids and proteins illuminates a relationship between the partial double-bond character of the C-N bond and the time scale for NH2 rotations.

Protein Electrostatics Investigated through Paramagnetic NMR for Nonpolar Groups.

Experimental validation of theoretical models for protein electrostatics remains rare. Recently, we have developed a paramagnetic NMR-based method for de novo determination of effective near-surface electrostatic potentials, which allows for straightforward examination of electrostatic models for biomolecules. In the current work, we expand this method and demonstrate that effective near-surface electrostatic potentials can readily be determined from 1H paramagnetic relaxation enhancement (PRE) data for protein CαH and CH3 groups. The experimental data were compared with those predicted from the Poisson-Boltzmann theory. The impact of structural dynamics on the effective near-surface electrostatic potentials was also assessed. The agreement between the experimental and theoretical data was particularly good for methyl 1H nuclei. Compared to the conventional pKa-based validation, our paramagnetic NMR-based approach can provide a far larger number of experimental data that can directly be used to examine the validity of theoretical electrostatic models for proteins.

Dynamics of Cations around DNA and Protein as Revealed by 23Na Diffusion NMR Spectroscopy.

Counterions are vital for the structure and function of biomolecules. However, the behavior of counterions remains elusive due to the difficulty in characterizing mobile ions. Here, we demonstrate that the dynamics of cations around biological macromolecules can be revealed by 23Na diffusion nuclear magnetic resonance (NMR) spectroscopy. NMR probe hardware capable of generating strong magnetic field gradients enables 23Na NMR-based diffusion measurements for Na+ ions in solutions of biological macromolecules and their complexes. The dynamic properties of Na+ ions interacting with the macromolecules can be investigated using apparent 23Na diffusion coefficients measured under various conditions. Our diffusion data clearly show that Na+ ions retain high mobility within the ion atmosphere around DNA. The 23Na diffusion NMR method also permits direct observation of the release of Na+ ions from nucleic acids upon protein-nucleic acid association. The entropy change due to the ion release can be estimated from the diffusion data.

Hindered Rotations of Protein Asparagine/Glutamine Side-Chain NH2 Groups: Impact of Hydrogen Bonding with DNA.

Hindered rotation about an sp2 C-N bond is known to occur in arginine (Arg), asparagine (Asn), and glutamine (Gln) side chains of proteins. However, very little is known about the rotational dynamics of Asn and Gln side-chain NH2 groups. Here, using a unique NMR method, we quantitatively characterized the hindered rotations of protein Asn/Gln side-chain NH2 groups. This NMR method yields simple NH2-selective spectra that allow for an accurate determination of the kinetic rate constants for the hindered rotations. Through the NMR measurements at different temperatures, we investigated the energy barriers that restrict the C-N bond rotations of protein side-chain NH2 groups. Through a comparison of the kinetic data for the free and DNA-bound states of the Antp homeodomain, we also examined the impact of hydrogen bonding on the hindered rotations of the side-chain NH2 groups. Our data suggest that the hydrogen bonding increases the energy barriers by 1-6 kJ/mol.

De novo determination of near-surface electrostatic potentials by NMR.

Electrostatic potentials computed from three-dimensional structures of biomolecules by solving the Poisson-Boltzmann equation are widely used in molecular biophysics, structural biology, and medicinal chemistry. Despite the approximate nature of the Poisson-Boltzmann theory, validation of the computed electrostatic potentials around biological macromolecules is rare and methodologically limited. Here, we present a unique and powerful NMR method that allows for straightforward and extensive comparison with electrostatic models for biomolecules and their complexes. This method utilizes paramagnetic relaxation enhancement arising from analogous cationic and anionic cosolutes whose spatial distributions around biological macromolecules reflect electrostatic potentials. We demonstrate that this NMR method enables de novo determination of near-surface electrostatic potentials for individual protein residues without using any structural information. We applied the method to ubiquitin and the Antp homeodomain-DNA complex. The experimental data agreed well with predictions from the Poisson-Boltzmann theory. Thus, our experimental results clearly support the validity of the theory for these systems. However, our experimental study also illuminates certain weaknesses of the Poisson-Boltzmann theory. For example, we found that the theory predicts stronger dependence of near-surface electrostatic potentials on ionic strength than observed in the experiments. Our data also suggest that conformational flexibility or structural uncertainties may cause large errors in theoretical predictions of electrostatic potentials, particularly for highly charged systems. This NMR-based method permits extensive assessment of near-surface electrostatic potentials for various regions around biological macromolecules and thereby may facilitate improvement of the computational approaches for electrostatic potentials.

Dynamic Autoinhibition of the HMGB1 Protein via Electrostatic Fuzzy Interactions of Intrinsically Disordered Regions.

Highly negatively charged segments containing only aspartate or glutamate residues ("D/E repeats") are found in many eukaryotic proteins. For example, the C-terminal 30 residues of the HMGB1 protein are entirely D/E repeats. Using nuclear magnetic resonance (NMR), fluorescence, and computational approaches, we investigated how the D/E repeats causes the autoinhibition of HMGB1 against its specific binding to cisplatin-modified DNA. By varying ionic strength in a wide range (40-900 mM), we were able to shift the conformational equilibrium between the autoinhibited and uninhibited states toward either of them to the full extent. This allowed us to determine the macroscopic and microscopic equilibrium constants for the HMGB1 autoinhibition at various ionic strengths. At a macroscopic level, a model involving the autoinhibited and uninhibited states can explain the salt concentration-dependent binding affinity data. Our data at a microscopic level show that the D/E repeats and other parts of HMGB1 undergo electrostatic fuzzy interactions, each of which is weaker than expected from the macroscopic autoinhibitory effect. This discrepancy suggests that the multivalent nature of the fuzzy interactions enables strong autoinhibition at a macroscopic level despite the relatively weak intramolecular interaction at each site. Both experimental and computational data suggest that the D/E repeats interact preferentially with other intrinsically disordered regions (IDRs) of HMGB1. We also found that mutations mimicking post-translational modifications relevant to nuclear export of HMGB1 can moderately modulate DNA-binding affinity, possibly by impacting the autoinhibition. This study illuminates a functional role of the fuzzy interactions of D/E repeats.

Experimental approaches for investigating ion atmospheres around nucleic acids and proteins.

Ionic interactions are crucial to biological functions of DNA, RNA, and proteins. Experimental research on how ions behave around biological macromolecules has lagged behind corresponding theoretical and computational research. In the 21st century, quantitative experimental approaches for investigating ionic interactions of biomolecules have become available and greatly facilitated examinations of theoretical electrostatic models. These approaches utilize anomalous small-angle X-ray scattering, atomic emission spectroscopy, mass spectrometry, or nuclear magnetic resonance (NMR) spectroscopy. We provide an overview on the experimental methodologies that can quantify and characterize ions within the ion atmospheres around nucleic acids, proteins, and their complexes.

Probing the Diacylglycerol Binding Site of Presynaptic Munc13-1.

Munc13-1 is a presynaptic active zone protein that acts as a master regulator of synaptic vesicle priming and neurotransmitter release in the brain. It has been implicated in the pathophysiology of several neurodegenerative diseases. Diacylglycerol and phorbol ester activate Munc13-1 by binding to its C1 domain. The objective of this study is to identify the structural determinants of ligand binding activity of the Munc13-1 C1 domain. Molecular docking suggested that residues Trp-588, Ile-590, and Arg-592 of Munc13-1 are involved in ligand interactions. To elucidate the role of these three residues in ligand binding, we generated W588A, I590A, and R592A mutants in full-length Munc13-1, expressed them as GFP-tagged proteins in HT22 cells, and measured their ligand-induced membrane translocation by confocal microscopy and immunoblotting. The extent of 1,2-dioctanoyl-sn-glycerol (DOG)- and phorbol ester-induced membrane translocation decreased in the following order: wild type > I590A > W588A > R592A and wild type > W588A > I590A > R592A, respectively. To understand the effect of the mutations on ligand binding, we also measured the DOG binding affinity of the isolated wild-type C1 domain and its mutants in membrane-mimicking micelles using nuclear magnetic resonance methods. The DOG binding affinity decreased in the following order: wild type > I590A > R592A. No binding was detected for W588A with DOG in micelles. This study shows that Trp-588, Ile-590, and Arg-592 are essential determinants for the activity of Munc13-1 and the effects of the three residues on the activity are ligand-dependent. This study bears significance for the development of selective modulators of Munc13-1.

Quantifying and visualizing weak interactions between anions and proteins.

The molecular properties of proteins are influenced by various ions present in the same solution. While site-specific strong interactions between multivalent metal ions and proteins are well characterized, the behavior of other ions that are only weakly interacting with proteins remains elusive. In the current study, using NMR spectroscopy, we have investigated anion-protein interactions for three proteins that are similar in size but differ in overall charge. Using a unique NMR-based approach, we quantified anions accumulated around the proteins. The determined numbers of anions that are electrostatically attracted to the charged proteins were notably smaller than the overall charge valences and were consistent with predictions from the Poisson-Boltzmann theory. This NMR-based approach also allowed us to measure ionic diffusion and characterize the anions interacting with the positively charged proteins. Our data show that these anions rapidly diffuse while bound to the proteins. Using the same experimental approach, we observed the release of the anions from the protein surface upon the formation of the Antp homeodomain-DNA complex. Using paramagnetic relaxation enhancement (PRE), we visualized the spatial distribution of anions around the free proteins and the Antp homeodomain-DNA complex. The obtained PRE data revealed the localization of anions in the vicinity of the highly positively charged regions of the free Antp homeodomain and provided further evidence of the release of anions from the protein surface upon the protein-DNA association. This study sheds light on the dynamic behavior of anions that electrostatically interact with proteins.