Structure, dynamics and function of the outer membrane protein A (OmpA) and influenza hemagglutinin fusion domain in detergent micelles by solution NMR

Recent progress from our laboratories to determine structures of small membrane proteins (up to 20 kDa) in detergent micelles by solution nuclear magnetic resonance (NMR) is reviewed. NMR opens a new window to also study, for the first time, the dynamics of membrane proteins. We report on recent attempts to correlate dynamic measurements on OmpA with the ion channel function of this protein. We also summarize how NMR and spin-label electron paramagnetic resonance spectroscopy and selective mutagenesis can be combined to provide a structural basis towards understanding the mechanism of influenza hemagglutinin-mediated membrane fusion.     

Solution conformation and dynamics of a tetrasaccharide related to the LewisX antigen deduced by NMR relaxation measurements

¹H-NMR cross-relaxation rates and nonselectivelongitudinal relaxation times have been obtained at two magnetic fields (7.0and 11.8 T) and at a variety of temperatures for the branchedtetrasaccharide methyl3-O--N-acetyl-galactosaminyl--galactopyranosyl-(14)[3-O--fucosyl]-glucopyranoside (1), an inhibitor of astrocyte growth. Inaddition, ¹³C-NMR relaxation data have also been recorded atboth fields. The ¹H-NMR relaxation data have been interpretedusing different motional models to obtain proton–proton correlationtimes. The results indicate that the GalNAc and Fuc rings display moreextensive local motion than the two inner Glc and Gal moieties, since thosepresent significantly shorter local correlation times. The¹³C-NMR relaxation parameters have been interpreted in termsof the Lipari–Szabo model-free approach. Thus, order parameters andinternal motion correlation times have been deduced. As obtained for the¹H-NMR relaxation data, the two outer residues possess smallerorder parameters than the two inner rings. Internal correlation times are inthe order of 100 ps. The hydroxymethyl groups have also different behaviour,with the exocyclic carbon on the glucopyranoside unit showing the highestS². Molecular dynamics simulations using a solvated systemhave also been performed and internal motion correlation functions have beendeduced from these calculations. Order parameters and interproton distanceshave been compared to those inferred from the NMR measurements. The obtainedresults are in fair agreement with the experimental data.     

Internal mobility of cyclic RGD hexapeptides studied by 13C NMR relaxation and the model-free approach

Summary The internal mobility of three isomeric cyclic RGD hexapeptides designed to contain two -turns in defined positions, cyclo(Arg-Gly-Asp-Gly-d-Pro-Pro) (I), cyclo(Arg-Gly-Asp-d-Pro-Gly-Pro) (II) and cyclo(Arg-Gly-Asp-d-Pro-Pro-Gly) (III), have been studied by ¹³C NMR longitudinal and transverse relaxation experiments and measurements of steady-state heteronuclear {¹H}-¹³C NOE enhancement with ¹³C at natural abundance. The data were interpreted according to the model-free formalism of Lipari and Szabo, which is usually applied to data from macromolecules or larger sized peptides with overall rotational correlation times exceeding 1 ns, to yield information about internal motions on the 10–100 ps time scale. The applicability of the model-free analysis with acceptable uncertainties to these small peptides, with overall rotational correlation times slightly below 0.3 ns, was demonstrated for this specific instance. Chemical exchange contributions to T₂ from slower motions were also identified in the process. According to the order parameters obtained for its backbone -carbon atoms, II has the most rigid backbone conformation on the 10–100 ps time scale, and I the most flexible. This result coincides with the results of earlier NMR-constrained conformational searches, which indicated greatest uncertainty in the structure of I and least in II.     

Model-free model elimination: A new step in the model-free dynamic analysis of NMR relaxation data

Model-free analysis is a technique commonly used within the field of NMR spectroscopy to extract atomic resolution, interpretable dynamic information on multiple timescales from the R ₁, R ₂, and steady state NOE. Model-free approaches employ two disparate areas of data analysis, the discipline of mathematical optimisation, specifically the minimisation of a χ² function, and the statistical field of model selection. By searching through a large number of model-free minimisations, which were setup using synthetic relaxation data whereby the true underlying dynamics is known, certain model-free models have been identified to, at times, fail. This has been characterised as either the internal correlation times, τ _e , τ _f , or τ _s , or the global correlation time parameter, local τ _m , heading towards infinity, the result being that the final parameter values are far from the true values. In a number of cases the minimised χ² value of the failed model is significantly lower than that of all other models and, hence, will be the model which is chosen by model selection techniques. If these models are not removed prior to model selection the final model-free results could be far from the truth. By implementing a series of empirical rules involving inequalities these models can be specifically isolated and removed. Model-free analysis should therefore consist of three distinct steps: model-free minimisation, model-free model elimination, and finally model-free model selection. Failure has also been identified to affect the individual Monte Carlo simulations used within error analysis. Each simulation involves an independent randomised relaxation data set and model-free minimisation, thus simulations suffer from exactly the same types of failure as model-free models. Therefore, to prevent these outliers from causing a significant overestimation of the errors the failed Monte Carlo simulations need to be culled prior to calculating the parameter standard deviations.     

The major outer membrane protein of Fusobacterium nucleatum (FomA) folds and inserts into lipid bilayers via parallel folding pathways

Membrane protein insertion and folding was studied for the major outer membrane protein of Fusobacterium nucleatum (FomA), which is a voltage-dependent general diffusion porin. The transmembrane domain of FomA forms a beta-barrel that is predicted to consist of 14 beta-strands. Here, unfolded FomA is shown to insert and fold spontaneously and quantitatively into phospholipid bilayers upon dilution of the denaturant urea, which was shown previously only for outer membrane protein A (OmpA) of Escherichia coli. Folding of FomA is demonstrated by circular dichroism and fluorescence spectroscopy, by SDS-polyacrylamide gel electrophoresis, and by single-channel recordings. Refolded FomA had a single-channel conductance of 1.1 nS at 1 M KCl, in agreement with the conductance of FomA isolated from membranes in native form. In contrast to OmpA, which forms a smaller eight-stranded beta-barrel domain, folding kinetics of the larger FomA were slower and provided evidence for parallel folding pathways of FomA into lipid bilayers. Two pathways were observed independent of membrane thickness with two different lipid bilayers, which were either composed of dicapryl phosphatidylcholine or dioleoyl phosphatidylcholine. This is the first observation of parallel membrane insertion and folding pathways of a beta-barrel membrane protein from an unfolded state in urea into lipid bilayers. The kinetics of both folding pathways depended on the chain length of the lipid and on temperature with estimated activation energies of 19 kJ/mol (dicapryl phosphatidylcholine) and 70 kJ/mol (dioleoyl phosphatidylcholine) for the faster pathways.     

Propagation of experimental uncertainties using the Lipari-Szabo model-free analysis of protein dynamics

In this paper we make use of the graphical procedure previously described [Jin, D. et al. (1997) J. Am. Chem. Soc., 119, 6923–6924] to analyze NMR relaxation data using the Lipari-Szabo model-free formalism. The graphical approach is advantageous in that it allows the direct visualization of the experimental uncertainties in the motional parameter space. Some general rules describing the relationship between the precision of the relaxation measurements and the precision of the model-free parameters and how this relationship changes with the overall tumbling time (m) are summarized. The effect of the precision in the relaxation measurements on the detection of internal motions not close to the extreme narrowing limit is analyzed. We also show that multiple timescale internal motions may be obscured by experimental uncertainty, and that the collection of relaxation data at very high field strength can improve the ability to detect such deviations from the simple Lipari-Szabo model.     

Model-independent interpretation of NMR relaxation data for unfolded proteins: the acid-denatured state of ACBP

We have investigated the acid-unfolded state of acyl-coenzyme A binding protein (ACBP) using 15N laboratory frame nuclear magnetic resonance (NMR) relaxation experiments at three magnetic field strengths. The data have been analyzed using standard model-free fitting and models involving distribution of correlation times. In particular, a model-independent method of analysis that does not assume any analytical form for the correlation time distribution is proposed. This method explains correlations between model-free parameters and the analytical distribution parameters found by other authors. The analysis also shows that the relaxation data are consistent with and complementary to information obtained from other parameters, especially secondary chemical shifts and residual dipolar couplings, and strengthens the conclusions of previous observations that three out of the four regions that form helices in the native structure appear to contain residual secondary structure also in the acid-denatured state.     

A suite of Mathematica notebooks for the analysis of protein main chain 15N NMR relaxation data

A suite of Mathematica notebooks has been designed to ease the analysis of protein main chain ¹⁵N NMR relaxation data collected at a single magnetic field strength. Individual notebooks were developed to perform the following tasks: nonlinear fitting of ¹⁵N-T ₁ and -T ₂ relaxation decays to a two parameter exponential decay, calculation of the principal components of the inertia tensor from protein structural coordinates, nonlinear optimization of the principal components and orientation of the axially symmetric rotational diffusion tensor, model-free analysis of ¹⁵N-T ₁, -T ₂, and {¹H}–¹⁵N NOE data, and reduced spectral density analysis of the relaxation data. The principle features of the notebooks include use of a minimal number of input files, integrated notebook data management, ease of use, cross-platform compatibility, automatic visualization of results and generation of high-quality graphics, and output of analyses in text format.L. Spyracopoulos is an AHFMR Medical Research Senior Scholar     

Insights into outer membrane protein crystallization

Outer membrane proteins are structurally distinct from those that reside in the inner membrane and play important roles in bacterial pathogenicity and human metabolism. X-ray crystallography studies on >40 different outer membrane proteins have revealed that the transmembrane portion of these proteins can be constructed from either β-sheets or less commonly from α-helices. The most common architecture is the β-barrel, which can be formed from either a single anti-parallel sheet, fused at both ends to form a barrel or from multiple peptide chains. Outer membrane proteins exhibit considerable rigidity and stability, making their study through x-ray crystallography particularly tractable. As the number of structures of outer membrane proteins increases a more rational approach to their crystallization can be made. Herein we analyse the crystallization data from 53 outer membrane proteins and compare the results to those obtained for inner membrane proteins. A targeted sparse matrix screen for outer membrane protein crystallization is presented based on the present analysis.     

Protein dynamics detected in a membrane-embedded potassium channel using two-dimensional solid-state NMR spectroscopy

We report longitudinal ¹⁵N relaxation rates derived from two-dimensional (¹⁵N, ¹³C) chemical shift correlation experiments obtained under magic angle spinning for the potassium channel KcsA-Kv1.3 reconstituted in multilamellar vesicles. Thus, we demonstrate that solid-state NMR can be used to probe residue-specific backbone dynamics in a membrane-embedded protein. Enhanced backbone mobility was detected for two glycine residues within the selectivity filter that are highly conserved in potassium channels and that are of core relevance to the filter structure and ion selectivity.     

Accuracy and precision of NMR relaxation experiments and MD simulations for characterizing protein dynamics

Model-free parameters obtained from nuclear magnetic resonance (NMR) relaxation experiments and molecular dynamics (MD) simulations commonly are used to describe the intramolecular dynamical properties of proteins. To assess the relative accuracy and precision of experimental and simulated model-free parameters, three independent data sets derived from backbone ¹⁵N NMR relaxation experiments and two independent data sets derived from MD simulations of Escherichia coli ribonuclease HI are compared. The widths of the distributions of the differences between the order parameters for pairs of NMR data sets are congruent with the uncertainties derived from statistical analyses of individual data sets; thus, current protocols for analyzing NMR data encapsulate random uncertainties appropriately. Large differences in order parameters for certain residues are attributed to systematic differences between samples for intralaboratory comparisons and unknown, possibly magnetic field-dependent, experimental effects for interlaboratory comparisons. The widths of distributions of the differences between the order parameters for two NMR sets are similar to widths of distributions for an NMR and an MD set or for two MD sets. The linear correlations between the order parameters for an MD set and an NMR set are within the range of correlations observed between pairs of NMR sets. These comparisons suggest that the NMR and MD generalized order parameters for the backbone amide N—H bond vectors are of comparable accuracy for residues exhibiting motions on a fast time scale (<100 ps). Large discrepancies between NMR and MD order parameters for certain residues are attributed to the occurrence of “rare” motional events over the simulation trajectories, the disruption of an element of secondary structure in one of the simulations, and lack of consensus among the experimental data sets. Consequently, (easily detectable) severe distortions of local protein structure and infrequent motional events in MD simulations appear to be the most serious artifacts affecting the accuracy and precision, respectively, of MD order parameters relative to NMR values. In addition, MD order parameters for motions on a fast (<100 ps) timescale are more precisely determined than their NMR counterparts, thereby permitting more detailed dynamic characterization of biologically important residues by MD simulation than is sometimes possible by experimental methods. Proteins 28:481–493, 1997. © 1997 Wiley-Liss, Inc.     

Determination of protein rotational correlation time from NMR relaxation data at various solvent viscosities

An accurate determination of the overall rotation of a protein plays a crucial role in the investigation of its internal motions by NMR. In the present work, an innovative approach to the determination of the protein rotational correlation time _R from the heteronuclear relaxation data is proposed. The approach is based on a joint fit of relaxation data acquired at several viscosities of a protein solution. The method has been tested on computer simulated relaxation data as compared to the traditional _R determination method from T₁/T₂ ratio. The approach has been applied to ribonuclease barnase from Bacillus amyloliquefaciens dissolved in an aqueous solution and deuterated glycerol as a viscous component. The resulting rotational correlation time of 5.56 ± 0.01 ns and other rotational diffusion tensor parameters are in good agreement with those determined from T₁/T₂ ratio.     

NMR structural studies of the bacterial outer membrane protein OmpX in oriented lipid bilayer membranes

The β-barrels found in the outer membranes of prokaryotic and eukaryotic organisms constitute an important functional class of proteins. Here we present solid-state NMR spectra of the bacterial outer membrane protein OmpX in oriented lipid bilayer membranes. We show that OmpX is folded in both glass-supported oriented lipid bilayers and in lipid bicelles that can be magnetically oriented with the membrane plane parallel or perpendicular to the direction of the magnetic field. The presence of resolved peaks in these spectra demonstrates that OmpX undergoes rotational diffusion around an axis perpendicular to the membrane surface. A tightly hydrogen-bonded domain of OmpX resists exchange with D₂O for days and is assigned to the transmembrane β-barrel, while peaks at isotropic resonance frequencies that disappear rapidly in D₂O are assigned to the extracellular and periplasmic loops. The two-dimensional ¹H/¹⁵N separated local field spectra of OmpX have several resolved peaks, and agree well with the spectra calculated from the crystal structure of OmpX rotated with the barrel axis nearly parallel (5° tilt) to the direction of the magnetic field. The data indicate that it will be possible to obtain site-specific resonance assignments and to determine the structure, tilt, and rotation of OmpX in membranes using the solid-state NMR methods that are currently being applied to α-helical membrane proteins.     

The three domains of the mitochondrial outer membrane protein Mim1 have discrete functions in assembly of the TOM complex

The assembly of mitochondrial outer membrane proteins is an essential process, mediated by the SAM complex and a set of additional protein modules. We show that one of these, Mim1, is anchored in the outer membrane with its N-terminus exposed to the cytosol and its C-terminus in the mitochondrial intermembrane space. Using an in vitro assay to measure the multi-step pathway for assembly of Tom40 into the TOM complex, we find that an “early reaction” mediated by the SAM complex is regulated by the N-terminal domain of Mim1. In addition, a “late reaction” catalysed by the Sam37 subunit of the SAM complex is also influenced by Mim1. Thus, Mim1 participates at multiple stages in the assembly of the TOM complex.     

Secondary and tertiary structure formation of the beta-barrel membrane protein OmpA is synchronized and depends on membrane thickness

The mechanism of membrane insertion and folding of a beta-barrel membrane protein has been studied using the outer membrane protein A (OmpA) as an example. OmpA forms an eight-stranded beta-barrel that functions as a structural protein and perhaps as an ion channel in the outer membrane of Escherichia coli. OmpA folds spontaneously from a urea-denatured state into lipid bilayers of small unilamellar vesicles. We have used fluorescence spectroscopy, circular dichroism spectroscopy, and gel electrophoresis to investigate basic mechanistic principles of structure formation in OmpA. Folding kinetics followed a second-order rate law and is strongly depended on the hydrophobic thickness of the lipid bilayer. When OmpA was refolded into model membranes of dilaurylphosphatidylcholine, fluorescence kinetics were characterized by a rate constant that was about fivefold higher than the rate constants of formation of secondary and tertiary structure, which were determined by circular dichroism spectroscopy and gel electrophoresis, respectively. The formation of beta-sheet secondary structure and closure of the beta-barrel of OmpA were correlated with the same rate constant and coupled to the insertion of the protein into the lipid bilayer. OmpA, and presumably other beta-barrel membrane proteins therefore do not follow a mechanism according to the two-stage model that has been proposed for the folding of alpha-helical bundle membrane proteins. These different folding mechanisms are likely a consequence of the very different intramolecular hydrogen bonding and hydrophobicity patterns in these two classes of membrane proteins.     

The measurement of immersion depth and topology of membrane proteins by solution state NMR

An important component of the study of membrane proteins involves the determination of details associated with protein topology — for example, the location of transmembrane residues, specifics of immersion depth, orientation of the protein in the membrane, and extent of solvent exposure for each residue. Solution state NMR is well suited to the determination of immersion depth with the use of paramagnetic additives designed to give rise to depth-specific relaxation effects or chemical shift perturbations. Such additives include spin labels designed to be “anchored” within a given region of the membrane or small freely diffusing paramagnetic species, whose partitioning properties across the water membrane interface create a gradient of paramagnetic effects which correlate with depth. This review highlights the use of oxygen and other small paramagnetic additives in studies of immersion depth and topology of membrane proteins in lipid bilayers and micelles.     

Accessibility of tobacco lipid transfer protein cavity revealed by 15N NMR relaxation studies and molecular dynamics simulations

Plant LTP1 are small helical proteins stabilized by four disulfide bridges and are characterized by the presence of an internal cavity, in which various hydrophobic ligands can be inserted. Recently, we have determined the solution structure of the recombinant tobacco LTP1_1. Unexpectedly, despite a global fold very similar to the structures already known for cereal seed LTP1, its binding properties are different: Tobacco LTP1_1 is able to bind only one monoacylated lipid, whereas cereal LTP1 can bind either one or two. The 3D structure of tobacco LTP1_1 revealed the presence of a hydrophobic cluster, not observed on cereal LTP1 structures, which may hinder one of the two entrances of the cavity defined for wheat LTP1. To better understand the mechanism of lipid entrance for tobacco LTP1_1 and to define the regions of the protein monitoring the accessibility of the cavity, we have complemented our structural data by the study of the internal dynamics of tobacco LTP1_1, using (15)N magnetic relaxation rate data and MD simulations at room and high temperatures. This work allowed us to define two regions of the protein experiencing the largest motions. These two regions delineate a portal that opens up during the simulation constituting a unique entrance of the hydrophobic cavity, in contrast with wheat LTP1 where two routes were detected. The hydrophobic interactions resulting from a few point mutations are strong enough to completely block the second portal so that the accessibility of the cavity is restricted to one entrance, explaining why this particular LTP1 binds only one lipid molecule.     

Optimisation of NMR dynamic models II. A new methodology for the dual optimisation of the model-free parameters and the Brownian rotational diffusion tensor

Finding the dynamics of an entire macromolecule is a complex problem as the model-free parameter values are intricately linked to the Brownian rotational diffusion of the molecule, mathematically through the autocorrelation function of the motion and statistically through model selection. The solution to this problem was formulated using set theory as an element of the universal set —the union of all model-free spaces (d’Auvergne EJ and Gooley PR (2007) Mol BioSyst 3(7), 483–494). The current procedure commonly used to find the universal solution is to initially estimate the diffusion tensor parameters, to optimise the model-free parameters of numerous models, and then to choose the best model via model selection. The global model is then optimised and the procedure repeated until convergence. In this paper a new methodology is presented which takes a different approach to this diffusion seeded model-free paradigm. Rather than starting with the diffusion tensor this iterative protocol begins by optimising the model-free parameters in the absence of any global model parameters, selecting between all the model-free models, and finally optimising the diffusion tensor. The new model-free optimisation protocol will be validated using synthetic data from Schurr JM et al. (1994) J Magn Reson B 105(3), 211–224 and the relaxation data of the bacteriorhodopsin (1–36)BR fragment from Orekhov VY (1999) J Biomol NMR 14(4), 345–356. To demonstrate the importance of this new procedure the NMR relaxation data of the Olfactory Marker Protein (OMP) of Gitti R et al. (2005) Biochem 44(28), 9673–9679 is reanalysed. The result is that the dynamics for certain secondary structural elements is very different from those originally reported. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.