Direct Measurement of 15N Chemical Shift Anisotropy in Solution

Molecular dynamics as an efficient process to predict 15N chemical shift anisotropy at very high NMR magnetic fields

The emergence of very high NMR magnetic fields will certainly encourage the study of larger biological systems with their dynamics and interactions. NMR spin relaxation allows probing the dynamical properties of proteins where the 15N longitudinal (R1) and transverse (R2) relaxation rates in addition to the 1H-15N heteronuclear NOE describe the ps-ns time scale. Their analytical representation involves the chemical shift anisotropy (CSA) effect that represents the major contribution at a very high magnetic field above 18.8 T. An accurate analysis of the latter parameters in terms of model free (MF) requires considering its effect. Until now, a uniform value of -160 ppm for the CSA has been widely used to derive the backbone order parameters (S2), giving rise to a large fluctuation of its value at very high magnetic fields. Conversely, the use of a site-specific CSA improves the accurate analysis of protein dynamics but requires a cost-effective experimental multi-field approach. In the present paper, we show how the CSA mainly contributes to the relaxation parameters at 28.2 T compared to lower magnetic fields and may bias the determination of S2. We propose to replace the time-consuming measurement of spin relaxation at multiple fields by a combination of molecular dynamics (MD) and the measurement of spin relaxation at one very high magnetic field only. We applied this strategy to three well-folded proteins (ubiquitin, GB3 and ribonuclease H) to show that the determined order parameters are in good agreement with the ones obtained by means of experimental data only.

Ubiquitin's Conformational Heterogeneity as Discerned by Nuclear Magnetic Resonance Spectroscopy

Visualizing a protein's molecular motions has been a long standing topic of research in the biophysics community. Largely this has been done by exploiting nuclear magnetic resonance spectroscopy (NMR), and arguably no protein's molecular motions have been better characterized by NMR than that of ubiquitin (Ub), a 76 amino acid polypeptide essential in ubiquitination-a key regulatory system within cells. Herein, we discuss ubiquitin's conformational plasticity as visualized, at atomic resolution, by more than 35 years of NMR work. In our discussions we point out the differences between data acquired in vitro, ex vivo, as well as in vivo and stress the need to investigate Ub's conformational plasticity in more biologically representative backgrounds.

N-terminal domain of polypyrimidine-tract binding protein is a dynamic folding platform for adaptive RNA recognition

The N-terminal RNA recognition motif domain (RRM1) of polypyrimidine tract binding protein (PTB) forms an additional C-terminal helix α3, which docks to one edge of the β-sheet upon binding to a stem-loop RNA containing a UCUUU pentaloop. Importantly, α3 does not contact the RNA. The α3 helix therefore represents an allosteric means to regulate the conformation of adjacent domains in PTB upon binding structured RNAs. Here we investigate the process of dynamic adaptation by stem-loop RNA and RRM1 using NMR and MD in order to obtain mechanistic insights on how this allostery is achieved. Relaxation data and NMR structure determination of the free protein show that α3 is partially ordered and interacts with the domain transiently. Stem-loop RNA binding quenches fast time scale dynamics and α3 becomes ordered, however microsecond dynamics at the protein-RNA interface is observed. MD shows how RRM1 binding to the stem-loop RNA is coupled to the stabilization of the C-terminal helix and helps to transduce differences in RNA loop sequence into changes in α3 length and order. IRES assays of full length PTB and a mutant with altered dynamics in the α3 region show that this dynamic allostery influences PTB function in cultured HEK293T cells.

Studying micro to millisecond protein dynamics using simple amide 15N CEST experiments supplemented with major-state R2 and visible peak-position constraints

Over the last decade amide 15N CEST experiments have emerged as a popular tool to study protein dynamics that involves exchange between a 'visible' major state and sparsely populated 'invisible' minor states. Although initially introduced to study exchange between states that are in slow exchange with each other (typical exchange rates of, 10 to 400 s-1), they are now used to study interconversion between states on the intermediate to fast exchange timescale while still using low to moderate (5 to 350 Hz) 'saturating' B1 fields. The 15N CEST experiment is very sensitive to exchange as the exchange delay TEX can be quite long (~0.5 s) allowing for a large number of exchange events to occur making it a very powerful tool to detect minor sates populated ([Formula: see text]) to as low as 1%. When systems are in fast exchange and the 15N CEST data has to be described using a model that contains exchange, the exchange parameters are often poorly defined because the [Formula: see text] versus [Formula: see text] and [Formula: see text] versus exchange rate ([Formula: see text]) plots can be quite flat with shallow or no minima and the analysis of such 15N CEST data can lead to wrong estimates of the exchange parameters due to the presence of 'spurious' minima. Here we show that the inclusion of experimentally derived constraints on the intrinsic transverse relaxation rates and the inclusion of visible state peak-positions during the analysis of amide 15N CEST data acquired with moderate B1 values (~50 to ~350 Hz) results in convincing minima in the [Formula: see text] versus [Formula: see text] and the [Formula: see text] versus [Formula: see text] plots even when exchange occurs on the 100 μs timescale. The utility of this strategy is demonstrated on the fast-folding Bacillus stearothermophilus peripheral subunit binding domain that folds with a rate constant ~104 s-1. Here the analysis of 15N CEST data alone results in [Formula: see text] versus [Formula: see text] and [Formula: see text] versus [Formula: see text] plots that contain shallow minima, but the inclusion of visible-state peak positions and restraints on the intrinsic transverse relaxation rates of both states during the analysis of the 15N CEST data results in pronounced minima in the [Formula: see text] versus [Formula: see text] and [Formula: see text] versus [Formula: see text] plots and precise exchange parameters even in the fast exchange regime ([Formula: see text]~5). Using this strategy we find that the folding rate constant of PSBD is invariant (~10,500 s-1) from 33.2 to 42.9 °C while the unfolding rates (~70 to ~500 s-1) and unfolded state populations (~0.7 to ~4.3%) increase with temperature. The results presented here show that protein dynamics occurring on the 10 to 104 s-1 timescale can be studied using amide 15N CEST experiments.

Bromodomain Interactions with Acetylated Histone 4 Peptides in the BRD4 Tandem Domain: Effects on Domain Dynamics and Internal Flexibility

The bromodomain and extra-terminal (BET) protein BRD4 regulates gene expression via recruitment of transcriptional regulatory complexes to acetylated chromatin. Like other BET proteins, BRD4 contains two bromodomains, BD1 and BD2, that can interact cooperatively with target proteins and designed ligands, with important implications for drug discovery. Here, we used nuclear magnetic resonance (NMR) spectroscopy to study the dynamics and interactions of the isolated bromodomains, as well as the tandem construct including both domains and the intervening linker, and investigated the effects of binding a tetra-acetylated peptide corresponding to the tail of histone 4. The peptide affinity is lower for both domains in the tandem construct than for the isolated domains. Using ¹⁵N spin relaxation, we determined the global rotational correlation times and residue-specific order parameters for BD1 and BD2. Isolated BD1 is monomeric in the apo state but apparently dimerizes upon binding the tetra-acetylated peptide. Isolated BD2 partially dimerizes in both the apo and peptide-bound states. The backbone order parameters reveal marked differences between BD1 and BD2, primarily in the acetyl-lysine binding site where the ZA loop is more flexible in BD2. Peptide binding reduces the order parameters of the ZA loop in BD1 and the ZA and BC loops in BD2. The AB loop, located distally from the binding site, shows variable dynamics that reflect the different dimerization propensities of the domains. These results provide a basis for understanding target recognition by BRD4.

Isotope Labels Combined with Solution NMR Spectroscopy Make Visible the Invisible Conformations of Small-to-Large RNAs

RNA is central to the proper function of cellular processes important for life on earth and implicated in various medical dysfunctions. Yet, RNA structural biology lags significantly behind that of proteins, limiting mechanistic understanding of RNA chemical biology. Fortunately, solution NMR spectroscopy can probe the structural dynamics of RNA in solution at atomic resolution, opening the door to their functional understanding. However, NMR analysis of RNA, with only four unique ribonucleotide building blocks, suffers from spectral crowding and broad linewidths, especially as RNAs grow in size. One effective strategy to overcome these challenges is to introduce NMR-active stable isotopes into RNA. However, traditional uniform labeling methods introduce scalar and dipolar couplings that complicate the implementation and analysis of NMR measurements. This challenge can be circumvented with selective isotope labeling. In this review, we outline the development of labeling technologies and their application to study biologically relevant RNAs and their complexes ranging in size from 5 to 300 kDa by NMR spectroscopy.

TRACT revisited: an algebraic solution for determining overall rotational correlation times from cross-correlated relaxation rates

Accurate rotational correlation times ([Formula: see text]) are critical for quantitative analysis of fast timescale NMR dynamics. As molecular weights increase, the classic derivation of [Formula: see text] using transverse and longitudinal relaxation rates becomes increasingly unsuitable due to the non-trivial contribution of remote dipole-dipole interactions to longitudinal relaxation. Derivations using cross-correlated relaxation experiments, such as TRACT, overcome these limitations but are erroneously calculated in 65% of the citing literature. Herein, we developed an algebraic solutions to the Goldman relationship that facilitate rapid, point-by-point calculations for straightforward identification of appropriate spectral regions where global tumbling is likely to be dominant. The rigid-body approximation of the Goldman relationship has been previously shown to underestimate TRACT-based rotational correlation time estimates. This motivated us to develop a second algebraic solution that employs a simplified model-free spectral density function including an order parameter term that could, in principle, be set to an average backbone S² ≈ 0.9 to further improve the accuracy of [Formula: see text] estimation. These solutions enabled us to explore the boundaries of the Goldman relationship as a function of the H-N internuclear distance ([Formula: see text]), difference of the two principal components of the axially-symmetric ¹⁵N CSA tensor ([Formula: see text]), and angle of the CSA tensor relative to the N-H bond vector ([Formula: see text]). We hope our algebraic solutions and analytical strategies will increase the accuracy and application of the TRACT experiment.

Quantifying the effects of long-range 13C-13C dipolar coupling on measured relaxation rates in RNA

Selective stable isotope labeling has transformed structural and dynamics analysis of RNA by NMR spectroscopy. These methods can remove ¹³C-¹³C dipolar couplings that complicate ¹³C relaxation analyses. While these phenomena are well documented for sites with adjacent ¹³C nuclei (e.g. ribose C1'), less is known about so-called isolated sites (e.g. adenosine C2). To investigate and quantify the effects of long-range (> 2 Å) ¹³C-¹³C dipolar interactions on RNA dynamics, we simulated adenosine C2 relaxation rates in uniformly [U-¹³C/¹⁵N]-ATP or selectively [2-¹³C]-ATP labeled RNAs. Our simulations predict non-negligible ¹³C-¹³C dipolar contributions from adenosine C4, C5, and C6 to C2 longitudinal (R₁) relaxation rates in [U-¹³C/¹⁵N]-ATP labeled RNAs. Moreover, these contributions increase at higher magnetic fields and molecular weights to introduce discrepancies that exceed 50%. This will become increasingly important at GHz fields. Experimental R₁ measurements in the 61 nucleotide human hepatitis B virus encapsidation signal ε RNA labeled with [U-¹³C/¹⁵N]-ATP or [2-¹³C]-ATP corroborate these simulations. Thus, in the absence of selectively labeled samples, long-range ¹³C-¹³C dipolar contributions must be explicitly taken into account when interpreting adenosine C2 R₁ rates in terms of motional models for large RNAs.

Minute Additions of DMSO Affect Protein Dynamics Measurements by NMR Relaxation Experiments through Significant Changes in Solvent Viscosity

Studies of protein-ligand binding often rely on dissolving the ligand in dimethyl sulfoxide (DMSO) to achieve sufficient solubility, and then titrating the ligand solution into the protein solution. As a result, the final protein-ligand solution contains small amounts of DMSO in the buffer. Here we report how the addition of DMSO impacts studies of protein conformational dynamics. We used 15 N NMR relaxation to compare the rotational diffusion correlation time (τC ) of proteins in aqueous buffer with and without DMSO. We found that τC scales with the viscosity of the water-DMSO mixture, which depends sensitively on the amount of DMSO and varies by a factor of 2 across the relevant concentration range. NMR relaxation studies of side chains dynamics are commonly interpreted using τC as a fixed parameter, obtained from backbone 15 N relaxation data acquired on a separate sample. Model-free calculations show that errors in τC , arising from mismatched DMSO concentration between samples, lead to significant errors in order parameters. Our results highlight the importance of determining τC for each sample or carefully matching the DMSO concentrations between samples.

Theoretical investigations into the variability of the 15N solid-state NMR parameters within an antimicrobial peptide ampullosporin A

The solid-state NMR measurements play an indispensable role in studies of interactions between biological membranes and peptaibols, which are amphipathic oligopeptides with a high abundance of alpha-aminobutyric acid (Aib). The solid-state NMR investigations are important in establishing the molecular models of the pore forming and antimicrobial properties of peptaibols, but rely on certain simplifications. Some of the underlying assumptions concern the parameters describing the 15N NMR chemical shielding tensor (CST) of the amide nitrogens in Aib and in conventional amino acids. Here the density functional theory (DFT) based calculations were applied to the known crystal structure of one of peptaibols, Ampullosporin A, in order to explicitly describe the variation of the 15N NMR parameters within its backbone. Based on the DFT computational data it was possible to verify the validity of the assumptions previously made about the differences between Aib and other amino acids in the isotropic part of the CST. Also the trends in the magnitudes and orientations of the anisotropic components of the CST, as revealed by the DFT calculations of the full periodic structure of Ampullosporin A, were thoroughly analyzed, and may be employed in future studies of peptaibols.

Ab Initio Prediction of NMR Spin Relaxation Parameters from Molecular Dynamics Simulations

¹H-¹⁵N NMR spin relaxation and relaxation dispersion experiments can reveal the time scale and extent of protein motions across the ps-ms range, where the ps-ns dynamics revealed by fundamental quantities R₁, R₂, and heteronuclear NOE can be well-sampled by molecular dynamics simulations (MD). Although the principles of relaxation prediction from simulations are well-established, numerous NMR-MD comparisons have hitherto focused upon the aspect of order parameters S² due to common artifacts in the prediction of transient dynamics. We therefore summarize here all necessary components and highlight existing and proposed solutions, such as the inclusion of quantum mechanical zero-point vibrational corrections and separate MD convergence of global and local motions in coarse-grained and all-atom force fields, respectively. For the accuracy of the MD prediction to be tested, two model proteins GB3 and Ubiquitin are used to validate five atomistic force fields against published NMR data supplemented by the coarse-grained force field MARTINI+EN. In Amber and CHARMM-type force fields, quantitative agreement was achieved for structured elements with minimum adjustment of global parameters. Deviations from experiment occur in flexible loops and termini, indicating differences in both the extent and time scale of backbone motions. The lack of systematic patterns and water model dependence suggests that modeling of the local environment limits prediction accuracy. Nevertheless, qualitative accuracy in a 2 μs CHARMM36m Stam2 VHS domain simulation demonstrates the potential of MD-based interpretation in combination with NMR-measured dynamics, increasing the utility of spin relaxation in integrative structural biology.

Quantifying protein dynamics in the ps-ns time regime by NMR relaxation

Both ¹⁵N chemical shift anisotropy (CSA) and sufficiently rapid exchange linebroadening transitions exhibit relaxation contributions that are proportional to the square of the magnetic field. Deconvoluting these contributions is further complicated by residue-dependent variations in protein amide ¹⁵N CSA values which have proven difficult to accurately measure. Exploiting recently reported improvements for the implementation of T₁ and T_1ρ experiments, field strength-dependent studies have been carried out on the B3 domain of protein G (GB3) as well as on the immunophilin FKBP12 and a H87V variant of that protein in which the major conformational exchange linebroadening transition is suppressed. By applying a zero frequency spectral density rescaling analysis to the relaxation data collected at magnetic fields from 500 to 900 MHz ¹H, differential residue-specific ¹⁵N CSA values have been obtained for GB3 which correlate with those derived from solid state and liquid crystalline NMR measurements to a level similar to the correlation among those previously reported studies. Application of this analysis protocol to FKBP12 demonstrated an efficient quantitation of both weak exchange linebroadening contributions and differential residue-specific ¹⁵N CSA values. Experimental access to such differential residue-specific ¹⁵N CSA values should significantly facilitate more accurate comparisons with molecular dynamics simulations of protein motion that occurs within the timeframe of global molecular tumbling.

The Quest for Simplicity: Remarks on the Free-Approach Models

Nuclear magnetic relaxation provides a powerful method giving insight into molecular motions at atomic resolution on a broad time scale. Dynamics of biological macromolecules has been widely exploited by measuring (15)N and (13)C relaxation data. Interpretation of these data relies almost exclusively on the use of the model-free approach (MFA) and its extended version (EMFA) which requires no particular physical model of motion and a small number of parameters. It is shown that EMFA is often unable to cope with three different time scales and fails to describe slow internal motions properly. In contrast to EMFA, genuine MFA with two time scales can reproduce internal motions slower than the overall tumbling. It is also shown that MFA and simplified EMFA are equivalent with respect to the values of the N-H bond length and chemical shift anisotropy. Therefore, the vast majority of (15)N relaxation data for proteins can be satisfactorily interpreted solely with MFA.

Access to Cα backbone dynamics of biological solids by 13C T1 relaxation and molecular dynamics simulation

We introduce a labeling scheme for magic angle spinning (MAS) solid-state NMR that is based on deuteration in combination with dilution of the carbon spin system. The labeling strategy achieves spectral editing by simplification of the HαCα and aliphatic side chain spectral region. A reduction in both proton and carbon spin density in combination with fast spinning (≥50 kHz) is essential to retrieve artifact-free (13)C-R1 relaxation data for aliphatic carbons. We obtain good agreement between the NMR experimental data and order parameters extracted from a molecular dynamics (MD) trajectory, which indicates that carbon based relaxation parameters can yield complementary information on protein backbone as well as side chain dynamics.

Cytochrome-P450-cytochrome-b5 interaction in a membrane environment changes 15N chemical shift anisotropy tensors

It has been well realized that the dependence of chemical shift anisotropy (CSA) tensors on the amino acid sequence, secondary structure, dynamics, and electrostatic interactions can be utilized in the structural and dynamic studies of proteins by NMR spectroscopy. In addition, CSA tensors could also be utilized to measure the structural interactions between proteins in a protein-protein complex. To this end, we report the experimentally measured backbone amide-(15)N CSA tensors for a membrane-bound 16.7 kDa full-length rabbit cytochrome-b5 (cytb5), in complexation with a 55.8 kDa microsomal rabbit cytochrome P450 2B4 (cytP4502B4). The (15)N-CSAs, determined using the (15)N CSA/(15)N-(1)H dipolar coupling transverse cross-correlated rates, for free cytb5 are compared with those for the cytb5 bound to cytP4502B4. An overall increase in backbone amide-(15)N transverse cross-correlated rates for the cytb5 residues in the cytb5-cytP450 complex is observed as compared to the free cytb5 residues. Due to fast spin-spin relaxation (T2) and subsequent broadening of the signals in the complex, we could measure amide-(15)N CSAs only for 48 residues of cytb5 as compared to 84 residues of free cytb5. We observed a change in (15)N CSA for most residues of cytb5 in the complex, as compared to free cytb5, suggesting a dynamic interaction between the oppositely charged surfaces of anionic cytb5 and cationic cytP450. The mean values of (15)N CSA determined for residues in helical, sheet, and turn regions of cytb5 in the complex are -184.5, -146.8, and -146.2 ppm, respectively, with an overall average value of -165.5 ppm (excluding the values from residues in more flexible termini). The measured CSA value for residues in helical conformation is slightly larger as compared to previously reported values. This may be attributed to the paramagnetic effect from Fe(III) of the heme in cytb5, which is similar to our previously reported values for the free cytb5.

Quantum chemical calculations of amide-15N chemical shift anisotropy tensors for a membrane-bound cytochrome-b5

There is considerable interest in determining amide-(15)N chemical shift anisotropy (CSA) tensors from biomolecules and understanding their variation for structural and dynamics studies using solution and solid-state NMR spectroscopy and also by quantum chemical calculations. Due to the difficulties associated with the measurement of CSA tensors from membrane proteins, NMR-based structural studies heavily relied on the CSA tensors determined from model systems, typically single crystals of model peptides. In the present study, the principal components of backbone amide-(15)N CSA tensors have been determined using density functional theory for a 16.7 kDa membrane-bound paramagnetic heme containing protein, cytochrome-b(5) (cytb(5)). All the calculations were performed by taking residues within 5 Å distance from the backbone amide-(15)N nucleus of interest. The calculated amide-(15)N CSA spans agree less well with our solution NMR data determined for an effective internuclear distance r(N-H) = 1.023 Å and a constant angle β = 18° that the least shielded component (δ(11)) makes with the N-H bond. The variation of amide-(15)N CSA span obtained using quantum chemical calculations is found to be smaller than that obtained from solution NMR measurements, whereas the trends of the variations are found to be in close agreement. We believe that the results reported in this study will be useful in studying the structure and dynamics of membrane proteins and heme-containing proteins, and also membrane-bound protein-protein complexes such as cytochromes-b5-P450.

Parameterization of solvent-protein interaction and its use on NMR protein structure determination

NMR structure determination is frequently hindered by an insufficient amount of distance information for determining the correct fold of the protein in its early stages. In response we introduce a simple and general structure-based metric that can be used to incorporate NMR-based restraints on protein surface accessibility. This metric is inversely proportional to the sum of the inverse square distances to neighboring heavy atoms. We demonstrate the use of this restraint using a dataset from the water to protein magnetization transfer experiment on the protein Bax and the solvent paramagnetic relaxation enhancement experiment on the protein ubiquitin and Qua1 homodimer. The calculated solvent accessibility values using the new empirical function are well correlated with the experimental data. By incorporating an associated energy term into Xplor-NIH, we show that structure calculation with a limited number of additional experimental restraints, improves both the precision and accuracy of the resulting structures. This new empirical energy term will have general applicability to other types of solvent accessibility data.

Measurement of ¹⁵N relaxation rates in perdeuterated proteins by TROSY-based methods

While extracting dynamics parameters from backbone (15)N relaxation measurements in proteins has become routine over the past two decades, it is increasingly recognized that accurate quantitative analysis can remain limited by the potential presence of systematic errors associated with the measurement of (15)N R(1) and R(2) or R(1ρ) relaxation rates as well as heteronuclear (15)N-{(1)H} NOE values. We show that systematic errors in such measurements can be far larger than the statistical error derived from either the observed signal-to-noise ratio, or from the reproducibility of the measurement. Unless special precautions are taken, the problem of systematic errors is shown to be particularly acute in perdeuterated systems, and even more so when TROSY instead of HSQC elements are used to read out the (15)N magnetization through the NMR-sensitive (1)H nucleus. A discussion of the most common sources of systematic errors is presented, as well as TROSY-based pulse schemes that appear free of systematic errors to the level of <1 %. Application to the small perdeuterated protein GB3, which yields exceptionally high S/N and therefore is an ideal test molecule for detection of systematic errors, yields relaxation rates that show considerably less residue by residue variation than previous measurements. Measured R(2)'/R(1)' ratios fit an axially symmetric diffusion tensor with a Pearson's correlation coefficient of 0.97, comparable to fits obtained for backbone amide RDCs to the Saupe matrix.

Determination of 15N chemical shift anisotropy from a membrane-bound protein by NMR spectroscopy

Chemical shift anisotropy (CSA) tensors are essential in the structural and dynamic studies of proteins using NMR spectroscopy. Results from relaxation studies in biomolecular solution and solid-state NMR experiments on aligned samples are routinely interpreted using well-characterized CSA tensors determined from model compounds. Since CSA tensors, particularly the (15)N CSA, highly depend on a number of parameters including secondary structure, electrostatic interaction, and the amino acid sequence, there is a need for accurately determined CSA tensors from proteins. In this study, we report the backbone amide-(15)N CSA tensors for a 16.7-kDa membrane-bound and paramagnetic-heme containing protein, rabbit Cytochrome b(5) (cytb(5)), determined using the (15)N CSA/(15)N-(1)H dipolar transverse cross-correlation rates. The mean values of (15)N CSA determined for residues in helical, sheet, and turn regions are -187.9, -166.0, and -161.1 ppm, respectively, with an overall average value of -171.7 ppm. While the average CSA value determined from this study is in good agreement with previous solution NMR experiments on small globular proteins, the CSA value determined for residues in helical conformation is slightly larger, which may be attributed to the paramagnetic effect from Fe(III) of the heme unit in cytb(5). However, like in previous solution NMR studies, the CSA values reported in this study are larger than the values measured from solid-state NMR experiments. We believe that the CSA parameters reported in this study will be useful in determining the structure, dynamics, and orientation of proteins, including membrane proteins, using NMR spectroscopy.

Calculation of chemical shift anisotropy in proteins

Individual peptide groups in proteins must exhibit some variation in the chemical shift anisotropy (CSA) of their constituent atoms, but not much is known about the extent or origins of this dispersion. Direct spectroscopic measurement of CSA remains technically challenging, and theoretical methods can help to overcome these limitations by estimating shielding tensors for arbitrary structures. Here we use an automated fragmentation quantum mechanics/molecular mechanics (AF-QM/MM) approach to compute (15)N, (13)C' and (1)H chemical shift tensors for human ubiquitin and the GB1 and GB3 fragments of staphylococcal protein G. The average and range of variation of the anisotropies is in good agreement with experimental estimates from solid-state NMR, and the variation among residues is somewhat smaller than that estimated from solution-state measurements. Hydrogen-bond effects account for much of the variation, both between helix and sheet regions, and within elements of secondary structure, but other effects (including variations in torsion angles) may play a role as well.

Ultrahigh resolution protein structures using NMR chemical shift tensors

NMR chemical shift tensors (CSTs) in proteins, as well as their orientations, represent an important new restraint class for protein structure refinement and determination. Here, we present the first determination of both CST magnitudes and orientations for (13)Cα and (15)N (peptide backbone) groups in a protein, the β1 IgG binding domain of protein G from Streptococcus spp., GB1. Site-specific (13)Cα and (15)N CSTs were measured using synchronously evolved recoupling experiments in which (13)C and (15)N tensors were projected onto the (1)H-(13)C and (1)H-(15)N vectors, respectively, and onto the (15)N-(13)C vector in the case of (13)Cα. The orientations of the (13)Cα CSTs to the (1)H-(13)C and (13)C-(15)N vectors agreed well with the results of ab initio calculations, with an rmsd of approximately 8°. In addition, the measured (15)N tensors exhibited larger reduced anisotropies in α-helical versus β-sheet regions, with very limited variation (18 ± 4°) in the orientation of the z-axis of the (15)N CST with respect to the (1)H-(15)N vector. Incorporation of the (13)Cα CST restraints into structure calculations, in combination with isotropic chemical shifts, transferred echo double resonance (13)C-(15)N distances and vector angle restraints, improved the backbone rmsd to 0.16 Å (PDB ID code 2LGI) and is consistent with existing X-ray structures (0.51 Å agreement with PDB ID code 2QMT). These results demonstrate that chemical shift tensors have considerable utility in protein structure refinement, with the best structures comparable to 1.0-Å crystal structures, based upon empirical metrics such as Ramachandran geometries and χ(1)/χ(2) distributions, providing solid-state NMR with a powerful tool for de novo structure determination.

NMR cross-correlated relaxation rates reveal ion coordination sites in DNA

In this work, a novel NMR method for the identification of preferential coordination sites between physiologically relevant counterions and nucleic acid bases is demonstrated. In this approach, the NMR cross-correlated relaxation rates between the aromatic carbon chemical shift anisotropy and the proton-carbon dipolar interaction are monitored as a function of increasing Na(+), K(+), and Mg(2+) concentrations. Increasing the counterion concentration modulates the residence times of the counterions at specific sites around the nucleic acid bases. It is demonstrated that the modulation of the counterion concentration leads to sizable variations of the cross-correlated relaxation rates, which can be used to probe the site-specific counterion coordination. In parallel, the very same measurements report on the rotational tumbling of DNA, which, as shown here, depends on the nature of the ion and its concentration. This methodology is highly sensitive and easily implemented. The method can be used to cross-validate and/or complement direct but artifact-prone experimental techniques such as X-ray diffraction, NMR analysis with substitutionary ions, and molecular dynamics simulations. The feasibility of this technique is demonstrated on the extraordinarily stable DNA mini-hairpin d(GCGAAGC).

Density functional calculations of backbone 15N shielding tensors in beta-sheet and turn residues of protein G

We performed density functional calculations of backbone (15)N shielding tensors in the regions of beta-sheet and turns of protein G. The calculations were carried out for all twenty-four beta-sheet residues and eight beta-turn residues in the protein GB3 and the results were compared with the available experimental data from solid-state and solution NMR measurements. Together with the alpha-helix data, our calculations cover 39 out of the 55 residues (or 71%) in GB3. The applicability of several computational models developed previously (Cai et al. in J Biomol NMR 45:245-253, 2009) to compute (15)N shielding tensors of alpha-helical residues is assessed. We show that the proposed quantum chemical computational model is capable of predicting isotropic (15)N chemical shifts for an entire protein that are in good correlation with experimental data. However, the individual components of the predicted (15)N shielding tensor agree with experiment less well: the computed values show much larger spread than the experimental data, and there is a profound difference in the behavior of the tensor components for alpha-helix/turns and beta-sheet residues. We discuss possible reasons for this.

Integrated computational approach to the analysis of NMR relaxation in proteins: application to ps-ns main chain 15N-1H and global dynamics of the Rho GTPase binding domain of plexin-B1

An integrated computational methodology for interpreting NMR spin relaxation in proteins has been developed. It combines a two-body coupled-rotator stochastic model with a hydrodynamics-based approach for protein diffusion, together with molecular dynamics based calculations for the evaluation of the coupling potential of mean force. The method is applied to ¹⁵N relaxation of N-H bonds in the Rho GTPase binding (RBD) domain of plexin-B1, which exhibits intricate internal mobility. Bond vector dynamics are characterized by a rhombic local ordering tensor, S, with principal values S₀² and S₂², and an axial local diffusion tensor, D₂, with principal values D(2,||) and D(2,⊥). For α-helices and β-sheets we find that S₀² ~ -0.5 (strong local ordering), -1.2 < S₂² < -0.8 (large S tensor anisotropy), D(2,⊥) ~ D₁ = 1.93 × 10⁷ s⁻¹ (D₁ is the global diffusion rate), and log(D(2,||)/D₁) ~ 4. For α-helices the z-axis of the local ordering frame is parallel to the C(α)-C(α) axis. For β-sheets the z-axes of the S and D₂ tensors are parallel to the N-H bond. For loops and terminal chain segments the local ordering is generally weaker and more isotropic. On average, D(2,⊥) ~ D₁ also, but log(D(2,||)/D₁) is on the order of 1-2. The tensor orientations are diversified. This study sets forth an integrated computational approach for treating NMR relaxation in proteins by combining stochastic modeling and molecular dynamics. The approach developed provides new insights by its application to a protein that experiences complex dynamics.

Backbone dynamics of deoxy and carbonmonoxy hemoglobin by NMR/SRLS

The slowly relaxing local structure (SRLS) approach, developed for NMR spin relaxation analysis in proteins, is applied herein to amide ¹⁵N relaxation in deoxy and carbonmonoxy hemoglobin. Experimental data including ¹⁵N T₁, T₂ and ¹⁵N-{¹H} NOE, acquired at 11.7 and 14.1 T, and 29 and 34 °C, are analyzed. The restricted local motion of the N-H bond is described in terms of the principal value (S(0)(2)) and orientation (β(D)) of an axial local ordering tensor, S, and the principal values (R(||)(L) and R(⊥)(L)) and orientation (β(O)) of an axial local diffusion tensor, R(L). The parameters c₀² (the potential coefficient in terms of which S(0)(2) is defined), R(||)(L), β(D), and β(O) are determined by data fitting; R(⊥)(L) is set equal to the global motional rate, R(C), found previously to be (5.2-5.8) × 10⁶ 1/s in the temperature range investigated. The principal axis of S is (nearly) parallel to the C(i-1)(α)-C(i)(α) axis; when the two axes are parallel, β(D) = -101.3° (in the frame used). The principal axis of R(L) is (nearly) parallel to the N-H bond; when the two axes are parallel, β(O) = -101.3°. For "rigid" N-H bonds located in secondary structure elements the best-fit parameters are S(0)(2) = 0.88-0.95 (corresponding to local potentials of 8.6-19.9 k(B)T), R(||)(L) = 10⁹-10¹⁰ 1/s, β(D) = -101.3° ± 2.0°, and β(O) = -101.3° ± 4°. For flexible N-H bonds located in loops the best-fit values are S(0)(2) = 0.75-0.80 (corresponding to local potentials of 4.5-5.5 k(B)T), R(||)(L) = (1.0-6.3) × 10⁸ 1/s, β(D) = -101.3° ± 4.0°, and β(O) = -101.3° ± 10°. These results are important in view of their physical clarity, inherent potential for further interpretation, consistency, and new qualitative insights provided (vide infra).

Recent developments in (15)N NMR relaxation studies that probe protein backbone dynamics

Nuclear Magnetic Resonance (NMR) relaxation is a powerful technique that provides information about internal dynamics associated with configurational energetics in proteins, as well as site-specific information involved in conformational equilibria. In particular, (15)N relaxation is a useful probe to characterize overall and internal backbone dynamics of proteins because the relaxation mainly reflects reorientational motion of the N-H bond vector. Over the past 20 years, experiments and protocols for analysis of (15)N R (1), R 2, and the heteronuclear (15)N-{(1)H} NOE data have been well established. The development of these methods has kept pace with the increase in the available static-magnetic field strength, providing dynamic parameters optimized from data fitting at multiple field strengths. Using these methodological advances, correlation times for global tumbling and order parameters and correlation times for internal motions of many proteins have been determined. More recently, transverse relaxation dispersion experiments have extended the range of NMR relaxation studies to the milli- to microsecond time scale, and have provided quantitative information about functional conformational exchange in proteins. Here, we present an overview of recent advances in (15)N relaxation experiments to characterize protein backbone dynamics.

The impact of hydrogen bonding on amide 1H chemical shift anisotropy studied by cross-correlated relaxation and liquid crystal NMR spectroscopy

Site-specific ¹H chemical shift anisotropy (CSA) tensors have been derived for the well-ordered backbone amide moieties in the B3 domain of protein G (GB3). Experimental input data include residual chemical shift anisotropy (RCSA), measured in six mutants that align differently relative to the static magnetic field when dissolved in a liquid crystalline Pf1 suspension, and cross-correlated relaxation rates between the ¹H^N CSA tensor and either the ¹H−¹⁵N, the ¹H−¹³C′, or the ¹H−¹³C^α dipolar interactions. Analyses with the assumption that the ¹H^N CSA tensor is symmetric with respect to the peptide plane (three-parameter fit) or without this premise (five-parameter fit) yield very similar results, confirming the robustness of the experimental input data, and that, to a good approximation, one of the principal components orients orthogonal to the peptide plane. ¹H^N CSA tensors are found to deviate strongly from axial symmetry, with the most shielded tensor component roughly parallel to the N−H vector, and the least shielded component orthogonal to the peptide plane. DFT calculations on pairs of N-methyl acetamide and acetamide in H-bonded geometries taken from the GB3 X-ray structure correlate with experimental data and indicate that H-bonding effects dominate variations in the ¹H^N CSA. Using experimentally derived ¹H^N CSA tensors, the optimal relaxation interference effect needed for narrowest ¹H^N TROSY line widths is found at ∼1200 MHz.

Site-specific backbone amide (15)N chemical shift anisotropy tensors in a small protein from liquid crystal and cross-correlated relaxation measurements

Site-specific (15)N chemical shift anisotropy (CSA) tensors have been derived for the well-ordered backbone amide (15)N nuclei in the B3 domain of protein G (GB3) from residual chemical shift anisotropy (RCSA) measured in six different mutants that retain the native structure but align differently relative to the static magnetic field when dissolved in a liquid crystalline Pf1 suspension. This information is complemented by measurement of cross-correlated relaxation rates between the (15)N CSA tensor and either the (15)N-(1)H or (15)N-(13)C' dipolar interaction. In agreement with recent solid state NMR measurements, the (15)N CSA tensors exhibit only a moderate degree of variation from averaged values, but have larger magnitudes in alpha-helical (-173 +/- 7 ppm) than in beta-sheet (-162 +/- 6 ppm) residues, a finding also confirmed by quantum computations. The orientations of the least shielded tensor component cluster tightly around an in-peptide-plane vector that makes an angle of 19.6 +/- 2.5 degrees with the N-H bond, with the asymmetry of the (15)N CSA tensor being slightly smaller in alpha-helix (eta = 0.23 +/- 0.17) than in beta-sheet (eta = 0.31 +/- 0.11). The residue-specific (15)N CSA values are validated by improved agreement between computed and experimental (15)N R(1rho) relaxation rates measured for (15)N-{(2)H} sites in GB3, which are dominated by the CSA mechanism. Use of residue-specific (15)N CSA values also results in more uniform generalized order parameters, S(2), and predicts considerable residue-by-residue variations in the magnetic field strengths where TROSY line narrowing is most effective.

Increase in backbone mobility of the VTS1p-SAM domain on binding to SRE-RNA

The sterile alpha motif (SAM) domain of VTS1p, a posttranscriptional gene regulator, belongs to a family of SAM domains conserved from yeast to humans. Even though SAM domains were originally classified as protein-protein interaction domains, recently, it was shown that the yeast VTS1p-SAM and the SAM domain of its Drosophila homolog Smaug can specifically recognize RNA hairpins termed Smaug recognition element (SRE). Structural studies of the SRE-RNA complex of VTS1p-SAM revealed that the SAM domain primarily recognizes the shape of the RNA fold induced by the Watson-Crick base-pairing in the RNA pentaloop. Only the central G nucleotide is specifically recognized. The VTS1p-SAM domain recognizes SRE-RNAs with a CNGGN pentaloop where N is any nucleotide. The C1-G4 base pair in the wild type can be replaced by any pair of nucleotides that can form base pairs even though the binding affinity is greatest with a pyrimidine in position 1 and a purine in position 4. The interaction thus combines elements of sequence-specific and non-sequence-specific recognitions. The lack of structural rearrangements in either partner following binding is rather intriguing, suggesting that molecular dynamics may play an important role in imparting relaxed specificity with respect to the exact combination of nucleotides in the loop, except for the central nucleotide. In this work, we extend our previous studies of SRE-RNA interaction with VTS1p, by comparing the dynamics of the VTS1p-SAM domain both in its free form and when bound to SRE-RNA. The 15N relaxation studies of backbone dynamics suggest the presence of a dynamic interaction interface, with residues associated with specific G3 recognition becoming more rigid on RNA binding while other regions attain increased flexibility. The results parallel the observations from our studies of dynamics changes in SRE-RNA upon binding to VTS1p-SAM and shows that molecular dynamics could play a crucial role in modulating binding affinity and possibly contribute to the free energy of the interaction through an entropy-driven mechanism.

Simple tests for the validation of multiple field spin relaxation data

(15)N spin relaxation data is widely used to extract detailed dynamic information regarding bond vectors such as the amide N-H bond of the protein backbone. Analysis is typically carried using the Lipari-Szabo model-free approach. Even though the original model-free equation can be determined from single field R (1), R (2) and NOE, over-determination of more complex motional models is dependent on the recording of multiple field datasets. This is especially important for the characterization of conformational exchange which affects R (2) in a field dependent manner. However, severe artifacts can be introduced if inconsistencies arise between experimental setups with different magnets (or samples). Here, we propose the use of simple tests as validation tools for the assessment of consistency between different datasets recorded at multiple magnetic fields. Synthetic data are used to show the effects of inconsistencies on the proposed tests. Moreover, an analysis of data currently deposited in the BMRB is performed. Finally, two cases from our laboratory are presented. These tests are implemented in the open-source program relax, and we propose their use as a routine check-up for assessment of multiple field dataset consistency prior to any analysis such as model-free calculations. We believe this will aid in the extraction of higher quality dynamics information from (15)N spin relaxation data.

Selective characterization of microsecond motions in proteins by NMR relaxation

The three-dimensional structures of macromolecules fluctuate over a wide range of time-scales. Separating the individual dynamic processes according to frequency is of importance in relating protein motions to biological function and stability. We present here a general NMR method for the specific characterization of microsecond motions at backbone positions in proteins even in the presence of other dynamics such as large-amplitude nanosecond motions and millisecond chemical exchange processes. The method is based on measurement of relaxation rates of four bilinear coherences and relies on the ability of strong continuous radio frequency fields to quench millisecond chemical exchange. The utility of the methodology is demonstrated and validated through two specific examples focusing on the thermo-stable proteins, ubiquitin and protein L, where it is found that small-amplitude microsecond dynamics are more pervasive than previously thought. Specifically, these motions are localized to alpha helices, loop regions, and regions along the rim of beta sheets in both of the proteins examined. A third example focuses on a 28 kDa ternary complex of the chaperone Chz1 and the histones H2A.Z/H2B, where it is established that pervasive microsecond motions are localized to a region of the chaperone that is important for stabilizing the complex. It is further shown that these motions can be well separated from extensive millisecond dynamics that are also present and that derive from exchange of Chz1 between bound and free states. The methodology is straightforward to implement, and data recorded at only a single static magnetic field are required.

Density functional calculations of chemical shielding of backbone 15N in helical residues of protein G

We performed density functional calculations of backbone (15)N chemical shielding tensors in selected helical residues of protein G. Here we describe a computationally efficient methodology to include most of the important effects in the calculation of chemical shieldings of backbone (15)N. We analyzed the role of long-range intra-protein electrostatic interactions by comparing models with different complexity in vacuum and in charge field. Our results show that the dipole moment of the alpha-helix can cause significant deshielding of (15)N; therefore, it needs to be considered when calculating (15)N chemical shielding. We found that it is important to include interactions with the side chains that are close in space when the charged form for ionizable side chains is adopted in the calculation. We also illustrate how the ionization state of these side chains can affect the chemical shielding tensor elements. Chemical shielding calculations using a 8-residue fragment model in vacuum and adopting the charged form of ionizable side chains yield a generally good agreement with experimental data.

Chemical shift anisotropy of imino 15N nuclei in Watson-Crick base pairs from magic angle spinning liquid crystal NMR and nuclear spin relaxation

Knowledge of (15)N chemical shift anisotropy is prerequisite both for quantitative interpretation of nuclear spin relaxation rates in terms of local dynamics and for the use of residual chemical shift anisotropy (RCSA) as a constraint in structure determination. Accurate measurement of the very small RCSA from the difference in (15)N chemical shift under isotropic and weakly aligning liquid crystalline conditions is very sensitive to minute differences in sample conditions, such as pH or ionic strength. For this reason, chemical shifts were measured for the same solution, under static liquid crystalline alignment, and under magic angle spinning conditions where alignment relative to the magnetic field is removed. Measurements were made for 14 well-resolved G-N(1) and 6 U-N(3) (15)N nuclei in a sample of tRNA(Val). Fitting these RCSA data together with (15)N-(1)H dipole-CSA cross-correlated relaxation measurements to the recently refined structural model of tRNA(Val) yields the magnitude, asymmetry, and orientation of the (15)N CSA tensors.

RNA phosphodiester backbone dynamics of a perdeuterated cUUCGg tetraloop RNA from phosphorus-31 NMR relaxation analysis

We have analyzed the relaxation properties of all (31)P nuclei in an RNA cUUCGg tetraloop model hairpin at proton magnetic field strengths of 300, 600 and 900 MHz in solution. Significant H, P dipolar contributions to R (1) and R (2) relaxation are observed in a protonated RNA sample at 600 MHz. These contributions can be suppressed using a perdeuterated RNA sample. In order to interpret the (31)P relaxation data (R (1), R (2)), we measured the (31)P chemical shift anisotropy (CSA) by solid-state NMR spectroscopy under various salt and hydration conditions. A value of 178.5 ppm for the (31)P CSA in the static state (S (2) = 1) could be determined. In order to obtain information about fast time scale dynamics we performed a modelfree analysis on the basis of our relaxation data. The results show that subnanosecond dynamics detected around the phosphodiester backbone are more pronounced than the dynamics detected for the ribofuranosyl and nucleobase moieties of the individual nucleotides (Duchardt and Schwalbe, J Biomol NMR 32:295-308, 2005; Ferner et al., Nucleic Acids Res 36:1928-1940, 2008). Furthermore, the dynamics of the individual phosphate groups seem to be correlated to the 5' neighbouring nucleobases.

Slow motions in chicken villin headpiece subdomain probed by cross-correlated NMR relaxation of amide NH bonds in successive residues

The villin headpiece subdomain (HP36) is a widely used system for protein-folding studies. Nuclear magnetic resonance cross-correlated relaxation rates arising from correlated fluctuations of two N-H(N) dipole-dipole interactions involving successive residues were measured at two temperatures at which HP36 is at least 99% folded. The experiment revealed the presence of motions slower than overall tumbling of the molecule. Based on the theoretical analysis of the spectral densities we show that the structural and dynamic contributions to the experimental cross-correlated relaxation rate can be separated under certain conditions. As a result, dynamic cross-correlated order parameters describing slow microsecond-to-millisecond motions of N-H bonds in neighboring residues can be introduced for any extent of correlations in the fluctuations of the two bond vectors. These dynamic cross-correlated order parameters have been extracted for HP36. The comparison of their values at two different temperatures indicates that when the temperature is raised, slow motions increase in amplitude. The increased amplitude of these fluctuations may reflect the presence of processes directly preceding the unfolding of the protein.

15N relaxation studies of Apo-Mts1: a dynamic S100 protein

Mts1 is a member of the S100 family of EF-hand calcium-binding proteins. Like most S100 proteins, Mts1 exists as a dimer in solution and contains one canonical and one pseudo-EF-hand motif per monomer, each of which consists of two alpha helices connected by a loop capable of coordinating a calcium ion. The backbone dynamics of murine apo-Mts1 homodimer have been examined by nuclear magnetic resonance spectroscopy. Longitudinal and transverse relaxation data and steady-state (1)H- (15)N nuclear Overhauser effects were analyzed using model-free formalism. The extracted global correlation time is 9.94 ns. Results indicate that the protein backbone is most rigid at the dimer interface, made up of helices 1 and 4 from each monomer with mean S (2) ( S avg (2)) values approximately 0.9, flanked by helices 2 and 3 with lower S avg (2) values of 0.84 and 0.77, respectively. Each calcium-binding site along with the hinge joining the two EF-hands and the N- and C-termini are considerably more flexible than the dimer interface on a range of time scales and more flexible than the corresponding regions of other S100 proteins studied to date. As the hinge and the C-terminal tail are believed to interact with target proteins, these dynamic characteristics may have implications for Mts1 activity.

Structure and dynamics of the P7 protein from the bacteriophage phi 12

Cystoviruses are a class of enveloped double-stranded RNA viruses that use a multiprotein polymerase complex (PX) to replicate and transcribe the viral genome. Although the structures of the polymerase and ATPase components of the cystoviral PX are known and their functional behavior is understood to a large extent, no atomic-resolution structural information is available for the major capsid protein P1 that defines the overall structure and symmetry of the viral capsid and the essential protein P7. Toward obtaining a complete structural and functional understanding of the cystoviral PX, we have obtained the structure of P7 from the cystovirus phi 12 at a resolution of 1.8 A. The N-terminal core region (1-129) of P7 forms a novel homodimeric alpha/beta-fold having structural similarities with BRCT domains implicated in multiple protein-protein interactions in DNA repair proteins. Our results, combined with the known role of P7 in stabilizing the nucleation complex during capsid assembly, hint toward its participation in key protein-protein interactions within the cystoviral PX. Additionally, we have found through solution NMR studies that the C-terminal tail of P7 (130-169) that is essential for virus viability, although highly disordered, contains a nascent helix. We demonstrate for the first time, through NMR titrations, that P7 is capable of interacting with RNA. We find that both the N-terminal core and the dynamic C-terminal tail of P7 play a role in RNA recognition. This interaction leads to a significant reduction of the degree of disorder in the C-terminal tail. Given the requirement of P7 in maintaining genome packaging efficiency and transcriptional fidelity, our data suggest a central biological role for P7-RNA interactions.

Density functional calculations of 15N chemical shifts in solvated dipeptides

We performed density functional calculations to examine the effects of solvation, hydrogen bonding, backbone conformation, and the side chain on 15N chemical shielding in proteins. We used N-methylacetamide (NMA) and N-formyl-alanyl-X (with X being one of the 19 naturally occurring amino acids excluding proline) as model systems. In addition, calculations were performed for selected fragments from protein GB3. The conducting polarizable continuum model was employed to include the effect of solvent in the density functional calculations. Our calculations for NMA show that the augmentation of the polarizable continuum model with the explicit water molecules in the first solvation shell has a significant influence on isotropic 15N chemical shift but not as much on the chemical shift anisotropy. The difference in the isotropic chemical shift between the standard beta-sheet and alpha-helical conformations ranges from 0.8 to 6.2 ppm depending on the residue type, with the mean of 2.7 ppm. This is in good agreement with the experimental chemical shifts averaged over a database of 36 proteins containing >6100 amino acid residues. The orientation of the 15N chemical shielding tensor as well as its anisotropy and asymmetry are also in the range of values experimentally observed for peptides and proteins.

Effector-induced structural fluctuation regulates the ligand affinity of an allosteric protein: binding of inositol hexaphosphate has distinct dynamic consequences for the T and R states of hemoglobin

The present study reports distinct dynamic consequences for the T- and R-states of human normal adult hemoglobin (Hb A) due to the binding of a heterotropic allosteric effector, inositol hexaphosphate (IHP). A nuclear magnetic resonance (NMR) technique based on modified transverse relaxation optimized spectroscopy (TROSY) has been used to investigate the effect of conformational exchange of Hb A in both deoxy and CO forms, in the absence and presence of IHP, at 14.1 and 21.1 T, and at 37 degrees C. Our results show that the majority of the polypeptide backbone amino acid residues of deoxy- and carbonmonoxy-forms of Hb A in the absence of IHP is not mobile on the micros-ms time scale, with the exception of several amino acid residues, that is, beta109Val and beta132Lys in deoxy-Hb A, and alpha40Lys in HbCO A. The mobility of alpha40Lys in HbCO A can be explained by the crystallographic data showing that the H-bond between alpha40Lys and beta146His in deoxy-Hb A is absent in HbCO A. However, the conformational exchange of beta109Val, which is located in the intradimer (alpha 1beta 1 or alpha 2beta 2) interface, is not consistent with the crystallographic observations that show rigid packing at this site. IHP binding appears to rigidify alpha40Lys in HbCO A, but does not significantly affect the flexibility of beta109Val in deoxy-Hb A. In the presence of IHP, several amino acid residues, especially those at the interdimer (alpha 1beta 2 or alpha 2beta 1) interface of HbCO A, exhibit significant conformational exchange. The affected residues include the proximal beta92His in the beta-heme pocket, as well as some other residues located in the flexible joint (betaC helix-alphaFG corner) and switch (alphaC helix-betaFG corner) regions that play an important role in the dimer-dimer rotation of Hb during the oxygenation process. These findings suggest that, upon IHP binding, HbCO A undergoes a conformational fluctuation near the R-state but biased toward the T-state, apparently along the trajectory of its allosteric transition, accompanied by structural fluctuations in the heme pocket of the beta-chain. In contrast, no significant perturbation of the dynamic features on the ms-micros time scale has been observed upon IHP binding to deoxy-Hb A. We propose that the allosteric effector-induced quaternary structural fluctuation may contribute to the reduced ligand affinity of ligated hemoglobin. Conformational exchange mapping of the beta-chain of HbCO A observed at 21.1 T shows significantly increased scatter in the chemical exchange contribution to the transverse relaxation rate ( R ex) values, relative to those at lower fields, due to the enhanced effect of the local chemical shift anisotropy (CSA) fluctuation. A spring-on-scissors model is proposed to interpret the dynamic phenomena induced by the heterotropic effector, IHP.

Multidimensional solid state NMR of anisotropic interactions in peptides and proteins

Accurate determinations of chemical shift anisotropy (CSA) tensors are valuable for NMR of biological systems. In this review we describe recent developments in CSA measurement techniques and applications, particularly in the context of peptides and proteins. These techniques include goniometeric measurements of single crystals, slow magic-angle spinning studies of powder samples, and CSA recoupling under moderate to fast MAS. Experimental CSA data can be analyzed by comparison with ab initio calculations for structure determination and refinement. This approach has particularly high potential for aliphatic (13)C analysis, especially Calpha tensors which are directly related to structure. Carbonyl and (15)N CSA tensors demonstrate a more complex dependence upon hydrogen bonding and electrostatics, in addition to conformational dependence. The improved understanding of these tensors and the ability to measure them quantitatively provide additional opportunities for structure determination, as well as insights into dynamics.

Tom20 recognizes mitochondrial presequences through dynamic equilibrium among multiple bound states

Most mitochondrial proteins are synthesized in the cytosol and imported into mitochondria. The N-terminal presequences of mitochondrial-precursor proteins contain a diverse consensus motif (phi chi chi phi phi, phi is hydrophobic and chi is any amino acid), which is recognized by the Tom20 protein on the mitochondrial surface. To reveal the structural basis of the broad selectivity of Tom20, the Tom20-presequence complex was crystallized. Tethering a presequence peptide to Tom20 through a disulfide bond was essential for crystallization. Unexpectedly, the two crystals with different linker designs provided unique relative orientations of the presequence with respect to Tom20, and neither configuration could fully account for the hydrophobic preference at the three hydrophobic positions of the consensus motif. We propose the existence of a dynamic equilibrium in solution among multiple states including the two bound states. In accordance, NMR 15N relaxation analyses suggested motion on a sub-millisecond timescale at the Tom20-presequence interface. We suggest that the dynamic, multiple-mode interaction is the molecular mechanism facilitating the broadly selective specificity of the Tom20 receptor toward diverse mitochondrial presequences.

Unraveling protein dynamics through fast spectral density mapping

Spectral density mapping at multiple NMR field strengths is probably the best method to describe the dynamical behavior of a protein in solution through the analysis of 15N heteronuclear relaxation parameters. Nevertheless, such analyses are scarcely reported in the literature, probably because this method is excessively demanding in spectrometer measuring time. Indeed, when using n different magnetic fields and assuming the validity of the high frequency approximation, the discrete sampling of the spectral density function with 2n + 1 points needs the measurement of 3n 15N heteronuclear relaxation measurements (n R1, n R2, and n15N{1H}NOEs). Based on further approximations, we proposed a new strategy that allows us to describe the spectral density with n + 2 points, with the measurement of a total of n + 2 heteronuclear relaxation parameters. Applied to the dynamics analysis of the protein p13( MTCP1) at three different NMR fields, this approach allowed us to divide by nearly a factor of two the total measuring time, without altering further results obtained by the "model free" analysis of the resulting spectral densities. Furthermore, simulations have shown that this strategy remains applicable to any low isotropically tumbling protein (tauc>3 ns), and is valid for the types of motion generally envisaged for proteins.

Functional dynamics of response regulators using NMR relaxation techniques

A fundamental concept of phosphorylation-mediated signaling is the precise switching between discrete functional conformations. According to the traditional view, phosphorylation induces a new, active conformation. In this chapter, a series of NMR experiments performed on a response regulator are described that challenge this traditional notion. The combination of NMR relaxation experiments with chemical shift data and the linkage to structure/function reveals a fundamentally different activation mechanism. The NMR data for the response regulator NtrC provide kinetic (rates of interconversion), thermodynamic (relative populations), and structural (chemical shift) information for the conformational exchange process. The results demonstrate that both the inactive and active states are present before phosphorylation, and activation occurs via a shift of this preexisting equilibrium. This concept is in accordance with the energy landscape view of proteins that embraces the existence of conformational substates. We conjecture that this population-shift mechanism is a general paradigm for response regulator activation and possibly more universal for phosphorylation-mediated signaling.

On the accurate measurement of amide one-bond 15N-1H couplings in proteins: effects of cross-correlated relaxation, selective pulses and dynamic frequency shifts

Amide one-bond 15N-1H scalar couplings of 15N- and [15N,2H]-isotopically enriched ubiquitin have been measured with the Quantitative J approach by monitoring NMR signal intensity modulation. Scalar couplings of the non-deuterated protein are in average approximately 0.6 Hz larger than values of deuterated ubiquitin. This deviation is 30 times the error derived from experiment reproducibility. Refocusing dipole/dipole cross-correlated relaxation decreases the discrepancy to approximately 0.1 Hz, suggesting that it likely originates from relaxation interference. Alternatively, the subtraction of J values obtained at different magnetic fields largely reduces the relaxation effects. In contrast, the dynamic frequency shift whose main contribution to 1J(15N-1H) arises from 15N chemical shielding anisotropy/NH dipole cross-correlation, is not eliminated by refocusing spin evolution under this interaction. Furthermore, the average difference of 1J(15N-1H) values at two magnetic fields closely agrees with the theoretical expected difference in the dynamic frequency shift.