Secondary Structure, Dynamics, and Topology of a Seven-Helix Receptor in Native Membranes, Studied by Solid-State NMR Spectroscopy

Selective correlations between aliphatic 13C nuclei in protein solid-state NMR

Solid-state nuclear magnetic resonance (NMR) is a potent tool for studying the structures and dynamics of insoluble proteins. It starts with signal assignment through multi-dimensional correlation experiments, where the aliphatic 13Cα-13Cβ correlation is indispensable for identifying specific residues. However, developing efficient methods for achieving this correlation is a challenge in solid-state NMR. We present a simple band-selective zero-quantum (ZQ) recoupling method, named POST-C4161 (PC4), which enhances 13Cα-13Cβ correlations under moderate magic-angle spinning (MAS) conditions. PC4 requires minimal 13C radio-frequency (RF) field and proton decoupling, exhibits high stability against RF variations, and achieves superior efficiency. Comparative tests on various samples, including the formyl-Met-Leu-Phe (fMLF) tripeptide, microcrystalline β1 immunoglobulin binding domain of protein G (GB1), and membrane protein of mechanosensitive channel of large conductance from Methanosarcina acetivorans (MaMscL), demonstrate that PC4 selectively enhances 13Cα-13Cβ correlations by up to 50 % while suppressing unwanted correlations, as compared to the popular dipolar-assisted rotational resonance (DARR). It has addressed the long-standing need for selective 13C-13C correlation methods. We anticipate that this simple but efficient PC4 method will have immediate applications in structural biology by solid-state NMR.

Stability of the nitroxide biradical AMUPol in intact and lysed mammalian cells

Dynamic Nuclear Polarization (DNP) enhanced solid state NMR increases experimental sensitivity, potentially enabling detection of biomolecules at their physiological concentrations. The sensitivity of DNP experiments is due to the transfer of polarization from electron spins of free radicals to the nuclear spins of interest. Here, we investigate the reduction of AMUPol in both lysed and intact HEK293 cells. We find that nitroxide radicals are reduced with first order reduction kinetics by cell lysates at a rate of ∼ 12% of the added nitroxide radical concentration per hour. We also found that electroporation delivered a consistent amount of AMUPol to intact cells and that nitroxide radicals are reduced just slightly more rapidly (∼15% per hour) by intact cells than by cell lysates. The two nitroxide radicals of AMUPol are reduced independently and this leads to considerable accumulation of the DNP-silent monoradical form of AMUPol, particularly in preparations of intact cells where nearly half of the AMUPol is already reduced to the DNP silent monoradical form at the earliest experimental time points. This confirms that the loss of the DNP-active biradical form of AMUPol is faster than the nitroxide reduction rate. Finally, we investigate the effect of adding N-ethyl maleimide, a well-known inhibitor of thiol (-SH) group-based reduction of nitroxide biradicals in cells, on AMUPol reduction, cellular viability, and DNP performance. Although pre-treatment of cells with NEM effectively inhibited the reduction of AMUPol, exposure to NEM compromised cellular viability and, surprisingly, did not improve DNP performance. Collectively, these results indicate that, currently, the most effective strategy to obtain high DNP enhancements for DNP-assisted in-cell NMR is to minimize room temperature contact times with cellular constituents and suggest that the development of bio-resistant polarization agents for DNP could considerably increase the sensitivity of DNP-assisted in-cell NMR experiments.

Dynamics properties of membrane proteins in native cell membranes revealed by solid-state NMR spectroscopy

Cell membranes provide an environment that is essential to the functions of membrane proteins. Cell membranes are mainly composed of proteins and highly diverse phospholipids. The influence of diverse lipid compositions of native cell membranes on the dynamics of the embedded membrane proteins has not been examined. Here we employ solid-state NMR to investigate the dynamics of E. coli Aquaporin Z (AqpZ) in its native inner cell membranes, and reveal the influence of diverse lipid compositions on the dynamics of AqpZ by comparing it in native cell membranes to that in POPC/POPG bilayers. We demonstrate that the dynamic rigidity of AqpZ generally conserves in both native cell membranes and POPC/POPG bilayers, due to its tightly packed tetrameric structure. In the gel and the liquid crystal phases of lipids, our experimental results show that AqpZ is more dynamic in native cell membranes than that in POPC/POPG bilayers. In addition, we observe that phase transitions of lipids in native membranes are less sensitive to temperature variations compared with that in POPC/POPG bilayers, which results in that the dynamics of AqpZ is less affected by the phase transitions of lipids in native cell membranes than that in POPC/POPG bilayers. This study provides new insights into the dynamics of membrane proteins in native cell membranes.

Solid-State NMR Dipolar and Chemical Shift Anisotropy Recoupling Techniques for Structural and Dynamical Studies in Biological Systems

With the development of NMR methodology and technology during the past decades, solid-state NMR (ssNMR) has become a particularly important tool for investigating structure and dynamics at atomic scale in biological systems, where the recoupling techniques play pivotal roles in modern high-resolution MAS NMR. In this review, following a brief introduction on the basic theory of recoupling in ssNMR, we highlight the recent advances in dipolar and chemical shift anisotropy recoupling methods, as well as their applications in structural determination and dynamical characterization at multiple time scales (i.e., fast-, intermediate-, and slow-motion). The performances of these prevalent recoupling techniques are compared and discussed in multiple aspects, together with the representative applications in biomolecules. Given the recent emerging advances in NMR technology, new challenges for recoupling methodology development and potential opportunities for biological systems are also discussed.

Solid-State NMR Spectroscopy on Microbial Rhodopsins

Microbial rhodopsins represent the most abundant phototrophic systems known today. A similar molecular architecture with seven transmembrane helices and a retinal cofactor linked to a lysine in helix 7 enables a wide range of functions including ion pumping, light-controlled ion channel gating, or sensing. Deciphering their molecular mechanisms therefore requires a combined consideration of structural, functional, and spectroscopic data in order to identify key factors determining their function. Important insight can be gained by solid-state NMR spectroscopy by which the large homo-oligomeric rhodopsin complexes can be studied directly within lipid bilayers. This chapter describes the methodological background and the necessary sample preparation requirements for the study of photointermediates, for the analysis of protonation states, H-bonding and chromophore conformations, for 3D structure determination, and for probing oligomer interfaces of microbial rhodopsins. The use of data extracted from these NMR experiments is discussed in the context of complementary biophysical methods.

A molecular vision of fungal cell wall organization by functional genomics and solid-state NMR

Vast efforts have been devoted to the development of antifungal drugs targeting the cell wall, but the supramolecular architecture of this carbohydrate-rich composite remains insufficiently understood. Here we compare the cell wall structure of a fungal pathogen Aspergillus fumigatus and four mutants depleted of major structural polysaccharides. High-resolution solid-state NMR spectroscopy of intact cells reveals a rigid core formed by chitin, β-1,3-glucan, and α-1,3-glucan, with galactosaminogalactan and galactomannan present in the mobile phase. Gene deletion reshuffles the composition and spatial organization of polysaccharides, with significant changes in their dynamics and water accessibility. The distribution of α-1,3-glucan in chemically isolated and dynamically distinct domains supports its functional diversity. Identification of valines in the alkali-insoluble carbohydrate core suggests a putative function in stabilizing macromolecular complexes. We propose a revised model of cell wall architecture which will improve our understanding of the structural response of fungal pathogens to stresses.

The fungal cell wall is a complex structure composed mainly of glucans, chitin and glycoproteins. Here, the authors use solid-state NMR spectroscopy to assess the cell wall architecture of Aspergillus fumigatus, comparing wild-type cells and mutants lacking major structural polysaccharides, with insights into the distinct functions of these components.

Extensively sparse 13C labeling to simplify solid-state NMR 13C spectra of membrane proteins

Solid-state Nuclear Magnetic Resonance (ssNMR) is an emerging technique to investigate the structures and dynamics of membrane proteins in an artificial or native membrane environment. However, the structural studies of proteins by ssNMR are usually prolonged or impeded by signal assignments, especially the assignments of signals for collection of distance restraints, because of serious overlapping of signals in 2D ¹³C-¹³C spectra. Sparse labeling of ¹³C spins is an effective approach to simplify the ¹³C spectra and facilitate the extractions of distance restraints. Here, we propose a new reverse labeling combination of six types of amino acid residues (Ile, Leu, Phe, Trp, Tyr and Lys), and show a clean reverse labeling effect on a model membrane protein E. coli aquaporin Z (AqpZ). We further combine this reverse labeling combination and alternate ¹³C-¹²C labeling, and demonstrate an enhanced dilution effect in ¹³C-¹³C spectra. In addition, the influences of reverse labeling on the labeling of the other types of residues are quantitatively analyzed in the two strategies (1, reverse labeling and 2, reverse labeling combining alternate ¹³C-¹²C labeling). The signal intensities of some other types of residues in 2D ¹³C-¹³C spectra are observed to be 20-50% weaker because of the unwanted reverse labeling. The extensively sparse ¹³C labeling proposed in this study is expected to be useful in the collection of distance restraints using 2D ¹³C-¹³C spectra of membrane proteins.

Proton-Detected Solid-State NMR of the Cell-Free Synthesized α-Helical Transmembrane Protein NS4B from Hepatitis C Virus

Proton-detected 100 kHz magic-angle-spinning (MAS) solid-state NMR is an emerging analysis method for proteins with only hundreds of microgram quantities, and thus allows structural investigation of eukaryotic membrane proteins. This is the case for the cell-free synthesized hepatitis C virus (HCV) nonstructural membrane protein 4B (NS4B). We demonstrate NS4B sample optimization using fast reconstitution schemes that enable lipid-environment screening directly by NMR. 2D spectra and relaxation properties guide the choice of the best sample preparation to record 2D 1 H-detected 1 H,15 N and 3D 1 H,13 C,15 N correlation experiments with linewidths and sensitivity suitable to initiate sequential assignments. Amino-acid-selectively labeled NS4B can be readily obtained using cell-free synthesis, opening the door to combinatorial labeling approaches which should enable structural studies.

A solid-state NMR tool box for the investigation of ATP-fueled protein engines

Motor proteins are involved in a variety of cellular processes. Their main purpose is to convert the chemical energy released during adenosine triphosphate (ATP) hydrolysis into mechanical work. In this review, solid-state Nuclear Magnetic Resonance (NMR) approaches are discussed allowing studies of structures, conformational events and dynamic features of motor proteins during a variety of enzymatic reactions. Solid-state NMR benefits from straightforward sample preparation based on sedimentation of the proteins directly into the Magic-Angle Spinning (MAS) rotor. Protein resonance assignment is the crucial and often time-limiting step in interpreting the wealth of information encoded in the NMR spectra. Herein, potentials, challenges and limitations in resonance assignment for large motor proteins are presented, focussing on both biochemical and spectroscopic approaches. This work highlights NMR tools available to study the action of the motor domain and its coupling to functional processes, as well as to identify protein-nucleotide interactions during events such as DNA replication. Arrested protein states of reaction coordinates such as ATP hydrolysis can be trapped for NMR studies by using stable, non-hydrolysable ATP analogues that mimic the physiological relevant states as accurately as possible. Recent advances in solid-state NMR techniques ranging from Dynamic Nuclear Polarization (DNP), ³¹P-based heteronuclear correlation experiments, ¹H-detected spectra at fast MAS frequencies >100 kHz to paramagnetic NMR are summarized and their applications to the bacterial DnaB helicase from Helicobacter pylori are discussed.

Structural characterization of the human membrane protein VDAC2 in lipid bilayers by MAS NMR

The second isoform of the human voltage dependent anion channel (VDAC2) is a mitochondrial porin that translocates calcium and other metabolites across the outer mitochondrial membrane. VDAC2 has been implicated in cardioprotection and plays a critical role in a unique apoptotic pathway in tumor cells. Despite its medical importance, there have been few biophysical studies of VDAC2 in large part due to the difficulty of obtaining homogeneous preparations of the protein for spectroscopic characterization. Here we present high resolution magic angle spinning nuclear magnetic resonance (NMR) data obtained from homogeneous preparation of human VDAC2 in 2D crystalline lipid bilayers. The excellent resolution in the spectra permit several sequence-specific assignments of the signals for a large portion of the VDAC2 N-terminus and several other residues in two- and three-dimensional heteronuclear correlation experiments. The first 12 residues appear to be dynamic, are not visible in cross polarization experiments, and they are not sufficiently mobile on very fast timescales to be visible in 13C INEPT experiments. A comparison of the NMR spectra of VDAC2 and VDAC1 obtained from highly similar preparations demonstrates that the spectral quality, line shapes and peak dispersion exhibited by the two proteins are nearly identical. This suggests an overall similar dynamic behavior and conformational homogeneity, which is in contrast to two earlier reports that suggested an inherent conformational heterogeneity of VDAC2 in membranes. The current data suggest that the sample preparation and spectroscopic methods are likely applicable to studying other human membrane porins, including human VDAC3, which has not yet been structurally characterized.

Global response of diacylglycerol kinase towards substrate binding observed by 2D and 3D MAS NMR

Escherichia coli diacylglycerol kinase (DGK) is an integral membrane protein, which catalyses the ATP-dependent phosphorylation of diacylglycerol (DAG) to phosphatic acid (PA). It is a unique trimeric enzyme, which does not share sequence homology with typical kinases. It exhibits a notable complexity in structure and function despite of its small size. Here, chemical shift assignment of wild-type DGK within lipid bilayers was carried out based on 3D MAS NMR, utilizing manual and automatic analysis protocols. Upon nucleotide binding, extensive chemical shift perturbations could be observed. These data provide evidence for a symmetric DGK trimer with all of its three active sites concurrently occupied. Additionally, we could detect that the nucleotide substrate induces a substantial conformational change, most likely directing DGK into its catalytic active form. Furthermore, functionally relevant interprotomer interactions are identified by DNP-enhanced MAS NMR in combination with site-directed mutagenesis and functional assays.

Lignin-polysaccharide interactions in plant secondary cell walls revealed by solid-state NMR

Lignin is a complex aromatic biopolymer that strengthens and waterproofs plant secondary cell walls, enabling mechanical stability in trees and long-distance water transport in xylem. Lignin removal is a key step in paper production and biomass conversion to biofuels, motivating efforts to re-engineer lignin biosynthesis. However, the physical nature of lignin's interactions with wall polysaccharides is not well understood. Here we show that lignin self-aggregates to form highly hydrophobic and dynamically unique nanodomains, with extensive surface contacts to xylan. Solid-state NMR spectroscopy of intact maize stems, supported by dynamic nuclear polarization, reveals that lignin has abundant electrostatic interactions with the polar motifs of xylan. Lignin preferentially binds xylans with 3-fold or distorted 2-fold helical screw conformations, indicative of xylans not closely associated with cellulose. These findings advance our knowledge of the molecular-level organization of lignocellulosic biomass, providing the structural foundation for optimization of post-harvest processing for biofuels and biomaterials.

Automated assignment of NMR spectra of macroscopically oriented proteins using simulated annealing

An automated technique for the sequential assignment of NMR backbone resonances of oriented protein samples has been developed and tested based on ¹⁵N-¹⁵N homonuclear exchange and spin-exchanged separated local-field spectra. By treating the experimental spectral intensity as a pseudopotential, the Monte-Carlo Simulated Annealing algorithm has been employed to seek lowest-energy assignment solutions over a large sampling space where direct enumeration would be unfeasible. The determined sequential assignments have been scored based on the positions of the crosspeaks resulting from the possible orders for the main peaks. This approach is versatile in terms of the parameters that can be specified to achieve the best-fit result. At a minimum the algorithm requires a continuous segment of the main-peak chemical shifts obtained from a uniformly labeled sample and a spin-exchanged experimental spectrum represented as a 2D matrix array. With selective labeling experiments, groups of chemical shifts corresponding to specific locations in the protein backbone can be fixed, thereby decreasing the sampling space. The output from the program consists of a list of top-score main peak assignments, which can be subjected to further scoring criteria until a consensus solution is found. The algorithm has first been tested on a synthetic spectrum with randomly generated chemical shifts and dipolar couplings for the main peaks. The original assignments have been successfully recovered for as many as 100 main peaks when residue-type information was used even in the presence of substantial spectral peak overlap. The algorithm was then applied to assigning two sets of experimental spectra to recover and confirm the previously established assignments in an automated fashion. For the 20-residue transmembrane domain of Pf1 coat protein reconstituted in magnetically aligned bicelles, the original assignment by Park et al. (2010) was recovered by the automated algorithm with additional input from 5 selectively labeled amino acid spectra. The second case considered was the 46 residue Pf1 bacteriophage from Thiriot et al. (2005) and Knox et al. (2010), of which 38 residues were fit. Automated fitting resulted in several possible assignments but not exactly the original assignment. By using a post-fitting filtering procedure based on the number of missed cross peaks and Pf1 helical structure, a consensus spectroscopic assignment is proposed covering 84% of the original assignment. While the automated assignment works best in spectra with well-resolved crosspeaks, it also tolerates substantial spectral crowding to yield reasonable assignments in the cases where ambiguity and degeneracy of possible assignment solutions are inevitable.

Molecular architecture of fungal cell walls revealed by solid-state NMR

The high mortality of invasive fungal infections, and the limited number and inefficacy of antifungals necessitate the development of new agents with novel mechanisms and targets. The fungal cell wall is a promising target as it contains polysaccharides absent in humans, however, its molecular structure remains elusive. Here we report the architecture of the cell walls in the pathogenic fungus Aspergillus fumigatus. Solid-state NMR spectroscopy, assisted by dynamic nuclear polarization and glycosyl linkage analysis, reveals that chitin and α-1,3-glucan build a hydrophobic scaffold that is surrounded by a hydrated matrix of diversely linked β-glucans and capped by a dynamic layer of glycoproteins and α-1,3-glucan. The two-domain distribution of α-1,3-glucans signifies the dual functions of this molecule: contributing to cell wall rigidity and fungal virulence. This study provides a high-resolution model of fungal cell walls and serves as the basis for assessing drug response to promote the development of wall-targeted antifungals.

Selective labeling and unlabeling strategies in protein solid-state NMR spectroscopy

Selective isotope labeling is central in NMR experiments and often allows to push the limits on the systems investigated. It has the advantage to supply additional resolution by diminishing the number of signals in the spectra. This is particularly interesting when dealing with the large protein systems which are currently becoming accessible to solid-state NMR studies. Isotope labeled proteins for NMR experiments are most often expressed in E. coli systems, where bacteria are grown in minimal media supplemented with ¹⁵NH₄Cl and ¹³C-glucose as sole source of nitrogen and carbon. For amino acids selective labeling or unlabeling, specific amino acids are supplemented in the minimal medium. The aim is that they will be incorporated in the protein by the bacteria. However, E. coli amino-acid anabolism and catabolism tend to interconnect different pathways, remnant of a subway system. These connections lead to inter conversion between amino acids, called scrambling. A thorough understanding of the involved pathways is thus important to obtain the desired labeling schemes, as not all combinations of amino acids are adapted. We present here a detailed overview of amino-acid metabolism in this context. Each amino-acid pathway is described in order to define accessible combinations for ¹³C or ¹⁵N specific labeling or unlabeling. Using as example the ABC transporter BmrA, a membrane protein of 600 residues, we demonstrate how these strategies can be applied. Indeed, even though there is no size limit in solid-state NMR, large (membrane) proteins are still a challenge due to heavy signal overlap. To initiate resonance assignment in these large systems, we describe how selectively labeled samples can be obtained with the addition of labeled or unlabeled amino acids in the medium. The reduced spectral overlap enabled us to identify typical spectral fingerprints and to initiate sequential assignment using the more sensitive 2D DARR experiments with long mixing time showing inter-residue correlations.

Solid-state MAS NMR resonance assignment methods for proteins

The prerequisite to structural or functional studies of proteins by NMR is generally the assignment of resonances. Since the first assignment of proteins by solid-state MAS NMR was conducted almost two decades ago, a wide variety of different pulse sequences and methods have been proposed and continue to be developed. Traditionally, a variety of 2D and 3D ¹³C-detected experiments have been used for the assignment of backbone and side-chain ¹³C and ¹⁵N resonances. These methods have found widespread use across the field. But as the hardware has changed and higher spinning frequencies and magnetic fields are becoming available, the ability to use direct proton detection is opening up a new set of assignment methods based on triple-resonance experiments. This review describes solid-state MAS NMR assignment methods using carbon detection and proton detection at different deuteration levels. The use of different isotopic labelling schemes as an aid to assignment in difficult cases is discussed as well as the increasing number of software packages that support manual and automated resonance assignment.

Hidden motions and motion-induced invisibility: Dynamics-based spectral editing in solid-state NMR

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy enables the structural characterization of a diverse array of biological assemblies that include amyloid fibrils, non-amyloid aggregates, membrane-associated proteins and viral capsids. Such biological samples feature functionally relevant molecular dynamics, which often affect different parts of the sample in different ways. Solid-state NMR experiments' sensitivity to dynamics represents a double-edged sword. On the one hand, it offers a chance to measure dynamics in great detail. On the other hand, certain types of motion lead to signal loss and experimental inefficiencies that at first glance interfere with the application of ssNMR to overly dynamic proteins. Dynamics-based spectral editing (DYSE) ssNMR methods leverage motion-dependent signal losses to simplify spectra and enable the study of sub-structures with particular motional properties.

Direct assignment of 13C solid-state NMR signals of TFoF1 ATP synthase subunit c-ring in lipid membranes and its implication for the ring structure

F_oF₁-ATP synthase catalyzes ATP hydrolysis/synthesis coupled with a transmembrane H⁺ translocation in membranes. The F_o c-subunit ring plays a major role in this reaction. We have developed an assignment strategy for solid-state ¹³C NMR (ssNMR) signals of the F_o c-subunit ring of thermophilic Bacillus PS3 (TF_o c-ring, 72 residues), carrying one of the basic folds of membrane proteins. In a ssNMR spectrum of uniformly ¹³C-labeled sample, the signal overlap has been a major bottleneck because most amino acid residues are hydrophobic. To overcome signal overlapping, we developed a method designated as COmplementary Sequential assignment with MInimum Labeling Ensemble (COSMILE). According to this method, we generated three kinds of reverse-labeled samples to suppress signal overlapping. To assign the carbon signals sequentially, two-dimensional C^α(i+1)-C'C^α(i) correlation and dipolar assisted rotational resonance (DARR) experiments were performed under magic-angle sample spinning. On the basis of inter- and intra-residue ¹³C-¹³C chemical shift correlations, 97% of C^α, 97% of C^β and 92% of C' signals were assigned directly from the spectra. Secondary structure analysis predicted a hairpin fold of two helices with a central loop. The effects of saturated and unsaturated phosphatidylcholines on TF_o c-ring structure were examined. The DARR spectra at 15 ms mixing time are essentially similar to each other in saturated and unsaturated lipid membranes, suggesting that TF_o c-rings have similar structures under the different environments. The spectrum of the sample in saturated lipid membranes showed better resolution and structural stability in the gel state. The C-terminal helix was suggested to locate in the outer layer of the c-ring.

Microbial Rhodopsins

Microbial rhodopsins (MRs) are a large family of photoactive membrane proteins, found in microorganisms belonging to all kingdoms of life, with new members being constantly discovered. Among the MRs are light-driven proton, cation and anion pumps, light-gated cation and anion channels, and various photoreceptors. Due to their abundance and amenability to studies, MRs served as model systems for a great variety of biophysical techniques, and recently found a great application as optogenetic tools. While the basic aspects of microbial rhodopsins functioning have been known for some time, there is still a plenty of unanswered questions. This chapter presents and summarizes the available knowledge, focusing on the functional and structural studies.

Combined 1H-Detected Solid-State NMR Spectroscopy and Electron Cryotomography to Study Membrane Proteins across Resolutions in Native Environments

Membrane proteins remain challenging targets for structural biology, despite much effort, as their native environment is heterogeneous and complex. Most methods rely on detergents to extract membrane proteins from their native environment, but this removal can significantly alter the structure and function of these proteins. Here, we overcome these challenges with a hybrid method to study membrane proteins in their native membranes, combining high-resolution solid-state nuclear magnetic resonance spectroscopy and electron cryotomography using the same sample. Our method allows the structure and function of membrane proteins to be studied in their native environments, across different spatial and temporal resolutions, and the combination is more powerful than each technique individually. We use the method to demonstrate that the bacterial membrane protein YidC adopts a different conformation in native membranes and that substrate binding to YidC in these native membranes differs from purified and reconstituted systems.

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CryoET and ssNMR give complementary information about proteins in native membranes

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One sample can be prepared for both methods without the use of detergents

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Hybrid method shows differences between purified and native preparations of YidC

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Sample preparation reduces costs and time and suggests new strategy for assignment

1H-detected MAS solid-state NMR experiments enable the simultaneous mapping of rigid and dynamic domains of membrane proteins

Magic angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy is emerging as a unique method for the atomic resolution structure determination of native membrane proteins in lipid bilayers. Although 13C-detected ssNMR experiments continue to play a major role, recent technological developments have made it possible to carry out 1H-detected experiments, boosting both sensitivity and resolution. Here, we describe a new set of 1H-detected hybrid pulse sequences that combine through-bond and through-space correlation elements into single experiments, enabling the simultaneous detection of rigid and dynamic domains of membrane proteins. As proof-of-principle, we applied these new pulse sequences to the membrane protein phospholamban (PLN) reconstituted in lipid bilayers under moderate MAS conditions. The cross-polarization (CP) based elements enabled the detection of the relatively immobile residues of PLN in the transmembrane domain using through-space correlations; whereas the most dynamic region, which is in equilibrium between folded and unfolded states, was mapped by through-bond INEPT-based elements. These new 1H-detected experiments will enable one to detect not only the most populated (ground) states of biomacromolecules, but also sparsely populated high-energy (excited) states for a complete characterization of protein free energy landscapes.

Direct observation of structure and dynamics during phase separation of an elastomeric protein

Despite its growing importance in biology and in biomaterials development, liquid-liquid phase separation of proteins remains poorly understood. In particular, the molecular mechanisms underlying simple coacervation of proteins, such as the extracellular matrix protein elastin, have not been reported. Coacervation of the elastin monomer, tropoelastin, in response to heat and salt is a critical step in the assembly of elastic fibers in vivo, preceding chemical cross-linking. Elastin-like polypeptides (ELPs) derived from the tropoelastin sequence have been shown to undergo a similar phase separation, allowing formation of biomaterials that closely mimic the material properties of native elastin. We have used NMR spectroscopy to obtain site-specific structure and dynamics of a self-assembling elastin-like polypeptide along its entire self-assembly pathway, from monomer through coacervation and into a cross-linked elastic material. Our data reveal that elastin-like hydrophobic domains are composed of transient β-turns in a highly dynamic and disordered chain, and that this disorder is retained both after phase separation and in elastic materials. Cross-linking domains are also highly disordered in monomeric and coacervated ELP3 and form stable helices only after chemical cross-linking. Detailed structural analysis combined with dynamic measurements from NMR relaxation and diffusion data provides direct evidence for an entropy-driven mechanism of simple coacervation of a protein in which transient and nonspecific intermolecular hydrophobic contacts are formed by disordered chains, whereas bulk water and salt are excluded.

Water Distribution, Dynamics, and Interactions with Alzheimer's β-Amyloid Fibrils Investigated by Solid-State NMR

Water is essential for protein folding and assembly of amyloid fibrils. Internal water cavities have been proposed for several amyloid fibrils, but no direct structural and dynamical data have been reported on the water dynamics and site-specific interactions of water with the fibrils. Here we use solid-state NMR spectroscopy to investigate the water interactions of several Aβ40 fibrils. 1H spectral lineshapes, T2 relaxation times, and two-dimensional (2D) 1H-13C correlation spectra show that there are five distinct water pools: three are peptide-bound water, while two are highly dynamic water that can be assigned to interfibrillar water and bulk-like matrix water. All these water pools are associated with the fibrils on the nanometer scale. Water-transferred 2D correlation spectra allow us to map out residue-specific hydration and give evidence for the presence of a water pore in the center of the three-fold symmetric wild-type Aβ40 fibril. In comparison, the loop residues and the intramolecular strand-strand interface have low hydration, excluding the presence of significant water cavities in these regions. The Osaka Aβ40 mutant shows lower hydration and more immobilized water than wild-type Aβ40, indicating the influence of peptide structure on the dynamics and distribution of hydration water. Finally, the highly mobile interfibrillar and matrix water exchange with each other on the time scale of seconds, suggesting that fibril bundling separates these two water pools, and water molecules must diffuse along the fibril axis before exchanging between these two environments. These results provide insights and experimental constraints on the spatial distribution and dynamics of water pools in these amyloid fibrils.

Probing Residue-Specific Water-Protein Interactions in Oriented Lipid Membranes via Solid-State NMR Spectroscopy

Water plays a central role in membrane protein folding and function. It not only catalyzes lipid membrane self-assembly but also affects the structural integrity and conformational dynamics of membrane proteins. Magic angle spinning (MAS) solid-state NMR (ssNMR) is the technique of choice for measuring water accessibility of membrane proteins, providing a measure for membrane protein topology and insertion within lipid bilayers. However, the sensitivity and resolution of membrane protein samples for MAS experiments are often dictated by hydration levels, which affect the structural dynamics of membrane proteins. Oriented-sample ssNMR (OS-ssNMR) is a complementary technique to determine both structure and topology of membrane proteins in liquid crystalline bilayers. Recent advancements in OS-ssNMR involve the use of oriented bicellar phases that have improved both sensitivity and resolution. Importantly, for bicelle formation and orientation, lipid bilayers must be well organized and hydrated, resulting in the protein's topology being similar to that found in native membranes. Under these conditions, the NMR resonances become relatively narrow, enabling a better separation of ¹H-¹⁵N dipolar couplings and anisotropic ¹⁵N chemical shifts with separated local field (SLF) experiments. Here, we report a residue-specific water accessibility experiment for a small membrane protein, sarcolipin (SLN), embedded in oriented lipid bicelles as probed by new water-edited SLF (WE-SLF) experiments. We show that SLN's residues belonging to the juxtamembrane region are more exposed to the water-lipid interface than the corresponding membrane-embedded residues. The information that can be obtained from the WE-SLF experiments can be interpreted using a simple theoretical model based on spin-diffusion theory and offers a complete characterization of membrane proteins in realistic membrane bilayer systems.

Nuclear magnetic resonance (NMR) applied to membrane–protein complexes

Increasing evidence suggests that most proteins occur and function in complexes rather than as isolated entities when embedded in cellular membranes. Nuclear magnetic resonance (NMR) provides increasing possibilities to study structure, dynamics and assembly of such systems. In our review, we discuss recent methodological progress to study membrane–protein complexes (MPCs) by NMR, starting with expression, isotope-labeling and reconstitution protocols. We review approaches to deal with spectral complexity and limited spectral spectroscopic sensitivity that are usually encountered in NMR-based studies of MPCs. We highlight NMR applications in various classes of MPCs, including G-protein-coupled receptors, ion channels and retinal proteins and extend our discussion to protein–protein complexes that span entire cellular compartments or orchestrate processes such as protein transport across or within membranes. These examples demonstrate the growing potential of NMR-based studies of MPCs to provide critical insight into the energetics of protein–ligand and protein–protein interactions that underlie essential biological functions in cellular membranes.

Structure determination of uniformly (13)C, (15)N labeled protein using qualitative distance restraints from MAS solid-state (13)C-NMR observed paramagnetic relaxation enhancement

Magic angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) is a powerful method for structure determination of insoluble biomolecules. However, structure determination by MAS solid-state NMR remains challenging because it is difficult to obtain a sufficient amount of distance restraints owing to spectral complexity. Collection of distance restraints from paramagnetic relaxation enhancement (PRE) is a promising approach to alleviate this barrier. However, the precision of distance restraints provided by PRE is limited in solid-state NMR because of incomplete averaged interactions and intermolecular PREs. In this report, the backbone structure of the B1 domain of streptococcal protein G (GB1) has been successfully determined by combining the CS-Rosetta protocol and qualitative PRE restraints. The derived structure has a Cα RMSD of 1.49 Å relative to the X-ray structure. It is noteworthy that our protocol can determine the correct structure from only three cysteine-EDTA-Mn(2+) mutants because this number of PRE sites is insufficient when using a conventional structure calculation method based on restrained molecular dynamics and simulated annealing. This study shows that qualitative PRE restraints can be employed effectively for protein structure determination from a limited conformational sampling space using a protein fragment library.

In situ structural studies of Anabaena sensory rhodopsin in the E. coli membrane

Magic-angle spinning nuclear magnetic resonance is well suited for the study of membrane proteins in the nativelike lipid environment. However, the natural cellular membrane is invariably more complex than the proteoliposomes most often used for solid-state NMR (SSNMR) studies, and differences may affect the structure and dynamics of the proteins under examination. In this work we use SSNMR and other biochemical and biophysical methods to probe the structure of a seven-transmembrane helical photoreceptor, Anabaena sensory rhodopsin (ASR), prepared in the Escherichia coli inner membrane, and compare it to that in a bilayer formed by DMPC/DMPA lipids. We find that ASR is organized into trimers in both environments but forms two-dimensional crystal lattices of different symmetries. It favors hexagonal packing in liposomes, but may form a square lattice in the E. coli membrane. To examine possible changes in structure site-specifically, we perform two- and three-dimensional SSNMR experiments and analyze the differences in chemical shifts and peak intensities. Overall, this analysis reveals that the structure of ASR is largely conserved in the inner membrane of E. coli, with many of the important structural features of rhodopsins previously observed in ASR in proteoliposomes being preserved. Small, site-specific perturbations in protein structure that occur as a result of the membrane changes indicate that the protein can subtly adapt to its environment without large structural rearrangement.

Proton detection for signal enhancement in solid-state NMR experiments on mobile species in membrane proteins

Direct proton detection is becoming an increasingly popular method for enhancing sensitivity in solid-state nuclear magnetic resonance spectroscopy. Generally, these experiments require extensive deuteration of the protein, fast magic angle spinning (MAS), or a combination of both. Here, we implement direct proton detection to selectively observe the mobile entities in fully-protonated membrane proteins at moderate MAS frequencies. We demonstrate this method on two proteins that exhibit different motional regimes. Myelin basic protein is an intrinsically-disordered, peripherally membrane-associated protein that is highly flexible, whereas Anabaena sensory rhodopsin is composed of seven rigid transmembrane α-helices connected by mobile loop regions. In both cases, we observe narrow proton linewidths and, on average, a 10× increase in sensitivity in 2D insensitive nuclear enhancement of polarization transfer-based HSQC experiments when proton detection is compared to carbon detection. We further show that our proton-detected experiments can be easily extended to three dimensions and used to build complete amino acid systems, including sidechain protons, and obtain inter-residue correlations. Additionally, we detect signals which do not correspond to amino acids, but rather to lipids and/or carbohydrates which interact strongly with membrane proteins.

Pf1 bacteriophage hydration by magic angle spinning solid-state NMR

High resolution two- and three-dimensional heteronuclear correlation spectroscopy ((1)H-(13)C, (1)H-(15)N, and (1)H-(13)C-(13)C HETCOR) has provided a detailed characterization of the internal and external hydration water of the Pf1 virion. This long and slender virion (2000 nm × 7 nm) contains highly stretched DNA within a capsid of small protein subunits, each only 46 amino acid residues. HETCOR cross-peaks have been unambiguously assigned to 25 amino acids, including most external residues 1-21 as well as residues 39-40 and 43-46 deep inside the virion. In addition, the deoxyribose rings of the DNA near the virion axis are in contact with water. The sets of cross-peaks to the DNA and to all 25 amino acid residues were from the same hydration water (1)H resonance; some of the assigned residues do not have exchangeable side-chain protons. A mapping of the contacts onto structural models indicates the presence of water "tunnels" through a highly hydrophobic region of the capsid. The present results significantly extend and modify results from a lower resolution study, and yield a comprehensive hydration surface map of Pf1. In addition, the internal water could be distinguished from external hydration water by means of paramagnetic relaxation enhancement. The internal water population may serve as a conveniently localized magnetization reservoir for structural studies.

Efficient cellular solid-state NMR of membrane proteins by targeted protein labeling

Solid-state NMR spectroscopy (ssNMR) has made significant progress towards the study of membrane proteins in their native cellular membranes. However, reduced spectroscopic sensitivity and high background signal levels can complicate these experiments. Here, we describe a method for ssNMR to specifically label a single protein by repressing endogenous protein expression with rifampicin. Our results demonstrate that treatment of E. coli with rifampicin during induction of recombinant membrane protein expression reduces background signals for different expression levels and improves sensitivity in cellular membrane samples. Further, the method reduces the amount of time and resources needed to produce membrane protein samples, enabling new strategies for studying challenging membrane proteins by ssNMR.

Membrane proteins in their native habitat as seen by solid-state NMR spectroscopy

Membrane proteins play many critical roles in cells, mediating flow of material and information across cell membranes. They have evolved to perform these functions in the environment of a cell membrane, whose physicochemical properties are often different from those of common cell membrane mimetics used for structure determination. As a result, membrane proteins are difficult to study by traditional methods of structural biology, and they are significantly underrepresented in the protein structure databank. Solid-state Nuclear Magnetic Resonance (SSNMR) has long been considered as an attractive alternative because it allows for studies of membrane proteins in both native-like membranes composed of synthetic lipids and in cell membranes. Over the past decade, SSNMR has been rapidly developing into a major structural method, and a growing number of membrane protein structures obtained by this technique highlights its potential. Here we discuss membrane protein sample requirements, review recent progress in SSNMR methodologies, and describe recent advances in characterizing membrane proteins in the environment of a cellular membrane.

Probing a cell-embedded megadalton protein complex by DNP-supported solid-state NMR

Studying biomolecules at atomic resolution in their native environment is the ultimate aim of structural biology. We investigated the bacterial type IV secretion system core complex (T4SScc) by cellular dynamic nuclear polarization-based solid-state nuclear magnetic resonance spectroscopy to validate a structural model previously generated by combining in vitro and in silico data. Our results indicate that T4SScc is well folded in the cellular setting, revealing protein regions that had been elusive when studied in vitro.

Lipid bilayer-bound conformation of an integral membrane beta barrel protein by multidimensional MAS NMR

The human voltage dependent anion channel 1 (VDAC) is a 32 kDa β-barrel integral membrane protein that controls the transport of ions across the outer mitochondrial membrane. Despite the determination of VDAC solution and diffraction structures, a structural basis for the mechanism of its function is not yet fully understood. Biophysical studies suggest VDAC requires a lipid bilayer to achieve full function, motivating the need for atomic resolution structural information of VDAC in a membrane environment. Here we report an essential step toward that goal: extensive assignments of backbone and side chain resonances for VDAC in DMPC lipid bilayers via magic angle spinning nuclear magnetic resonance (MAS NMR). VDAC reconstituted into DMPC lipid bilayers spontaneously forms two-dimensional lipid crystals, showing remarkable spectral resolution (0.5-0.3 ppm for (13)C line widths and <0.5 ppm (15)N line widths at 750 MHz). In addition to the benefits of working in a lipid bilayer, several distinct advantages are observed with the lipid crystalline preparation. First, the strong signals and sharp line widths facilitated extensive NMR resonance assignments for an integral membrane β-barrel protein in lipid bilayers by MAS NMR. Second, a large number of residues in loop regions were readily observed and assigned, which can be challenging in detergent-solubilized membrane proteins where loop regions are often not detected due to line broadening from conformational exchange. Third, complete backbone and side chain chemical shift assignments could be obtained for the first 25 residues, which comprise the functionally important N-terminus. The reported assignments allow us to compare predicted torsion angles for VDAC prepared in DMPC 2D lipid crystals, DMPC liposomes, and LDAO-solubilized samples to address the possible effects of the membrane mimetic environment on the conformation of the protein. Concluding, we discuss the strengths and weaknesses of the reported assignment approach and the great potential for even more complete assignment studies and de novo structure determination via (1)H detected MAS NMR.

Insight into the conformational stability of membrane-embedded BamA using a combined solution and solid-state NMR approach

The β-barrel assembly machinery (BAM) is involved in folding and insertion of outer membrane proteins in Gram-negative bacteria, a process that is still poorly understood. With its 790 residues, BamA presents a challenge to current NMR methods. We utilized a "divide and conquer" approach in which we first obtained resonance assignments for BamA's periplasmic POTRA domains 4 and 5 by solution NMR. Comparison of these assignments to solid-state NMR (ssNMR) data obtained on two BamA constructs including the transmembrane domain and one or two soluble POTRA domains suggested that the fold of POTRA domain 5 critically depends on the interface with POTRA 4. Using specific labeling schemes we furthermore obtained ssNMR resonance assignments for residues in the extracellular loop 6 that is known to be crucial for BamA-mediated substrate folding and insertion. Taken together, our data provide novel insights into the conformational stability of membrane-embedded, non-crystalline BamA.

Advanced solid-state NMR techniques for characterization of membrane protein structure and dynamics: application to Anabaena Sensory Rhodopsin

Studies of the structure, dynamics, and function of membrane proteins (MPs) have long been considered one of the main applications of solid-state NMR (SSNMR). Advances in instrumentation, and the plethora of new SSNMR methodologies developed over the past decade have resulted in a number of high-resolution structures and structural models of both bitopic and polytopic α-helical MPs. The necessity to retain lipids in the sample, the high proportion of one type of secondary structure, differential dynamics, and the possibility of local disorder in the loop regions all create challenges for structure determination. In this Perspective article we describe our recent efforts directed at determining the structure and functional dynamics of Anabaena Sensory Rhodopsin, a heptahelical transmembrane (7TM) protein. We review some of the established and emerging methods which can be utilized for SSNMR-based structure determination, with a particular focus on those used for ASR, a bacterial protein which shares its 7TM architecture with G-protein coupled receptors.

Structural characterization of triple transmembrane domain containing fragments of a yeast G protein-coupled receptor in an organic : aqueous environment by solution-state NMR spectroscopy

This report summarizes recent biophysical and protein expression experiments on polypeptides containing the N-terminus, the first, second, and third transmembrane (TM) domains and the contiguous loops of the α-factor receptor Ste2p, a G protein-coupled receptor. The 131-residue polypeptide Ste2p(G31-R161), TM1-TM3, was investigated by solution NMR in trifluoroethanol/water. TM1-TM3 contains helical TM domains at the predicted locations, supported by continuous sets of medium-range NOEs. In addition, a short helix N-terminal to TM1 was detected, as well as a short helical stretch in the first extracellular loop. Two 161-residue polypeptides, [Ste2p(M1-R161), NT-TM1-TM3], that contain the entire N-terminal sequence, one with a single mutation, were directly expressed and isolated from Escherichia coli in yields as high as 30 mg/L. Based on its increased stability, the L11P mutant will be used in future experiments to determine long-range interactions. The study demonstrated that 3-TM domains of a yeast G protein-coupled receptor can be produced in isotopically labeled form suitable for solution NMR studies. The quality of spectra is superior to data recorded in micelles and allows more rapid data analysis. No tertiary contacts have been determined, and if present, they are likely transient. This observation supports earlier studies by us that secondary structure was retained in smaller fragments, both in organic solvents and in detergent micelles, but that stable tertiary contacts may only be present when the protein is imbedded in lipids.

Structural basis of the green-blue color switching in proteorhodopsin as determined by NMR spectroscopy

Proteorhodopsins (PRs) found in marine microbes are the most abundant retinal-based photoreceptors on this planet. PR variants show high levels of environmental adaptation, as their colors are tuned to the optimal wavelength of available light. The two major green and blue subfamilies can be interconverted through a L/Q point mutation at position 105. Here we reveal the structural basis behind this intriguing color-tuning effect. High-field solid-state NMR spectroscopy was used to visualize structural changes within green PR directly within the lipid bilayer upon introduction of the green-blue L105Q mutation. The observed effects are localized within the binding pocket and close to retinal carbons C14 and C15. Subsequently, magic-angle spinning (MAS) NMR spectroscopy with sensitivity enhancement by dynamic nuclear polarization (DNP) was applied to determine precisely the retinal structure around C14-C15. Upon mutation, a significantly stretched C14-C15 bond, deshielding of C15, and a slight alteration of the retinal chain's out-of-plane twist was observed. The L105Q blue switch therefore acts locally on the retinal itself and induces a conjugation defect between the isomerization region and the imine linkage. Consequently, the S0-S1 energy gap increases, resulting in the observed blue shift. The distortion of the chromophore structure also offers an explanation for the elongated primary reaction detected by pump-probe spectroscopy, while chemical shift perturbations within the protein can be linked to the elongation of late-photocycle intermediates studied by flash photolysis. Besides resolving a long-standing problem, this study also demonstrates that the combination of data obtained from high-field and DNP-enhanced MAS NMR spectroscopy together with time-resolved optical spectroscopy enables powerful synergies for in-depth functional studies of membrane proteins.

Recent advances in magic angle spinning solid state NMR of membrane proteins

Membrane proteins mediate many critical functions in cells. Determining their three-dimensional structures in the native lipid environment has been one of the main objectives in structural biology. There are two major NMR methodologies that allow this objective to be accomplished. Oriented sample NMR, which can be applied to membrane proteins that are uniformly aligned in the magnetic field, has been successful in determining the backbone structures of a handful of membrane proteins. Owing to methodological and technological developments, Magic Angle Spinning (MAS) solid-state NMR (ssNMR) spectroscopy has emerged as another major technique for the complete characterization of the structure and dynamics of membrane proteins. First developed on peptides and small microcrystalline proteins, MAS ssNMR has recently been successfully applied to large membrane proteins. In this review we describe recent progress in MAS ssNMR methodologies, which are now available for studies of membrane protein structure determination, and outline a few examples, which highlight the broad capability of ssNMR spectroscopy.

Probing membrane protein structure using water polarization transfer solid-state NMR

Water plays an essential role in the structure and function of proteins, lipid membranes and other biological macromolecules. Solid-state NMR heteronuclear-detected (1)H polarization transfer from water to biomolecules is a versatile approach for studying water-protein, water-membrane, and water-carbohydrate interactions in biology. We review radiofrequency pulse sequences for measuring water polarization transfer to biomolecules, the mechanisms of polarization transfer, and the application of this method to various biological systems. Three polarization transfer mechanisms, chemical exchange, spin diffusion and NOE, manifest themselves at different temperatures, magic-angle-spinning frequencies, and pulse irradiations. Chemical exchange is ubiquitous in all systems examined so far, and spin diffusion plays the key role in polarization transfer within the macromolecule. Tightly bound water molecules with long residence times are rare in proteins at ambient temperature. The water polarization-transfer technique has been used to study the hydration of microcrystalline proteins, lipid membranes, and plant cell wall polysaccharides, and to derive atomic-resolution details of the kinetics and mechanism of ion conduction in channels and pumps. Using this approach, we have measured the water polarization transfer to the transmembrane domain of the influenza M2 protein to obtain information on the structure of this tetrameric proton channel. At short mixing times, the polarization transfer rates are site-specific and depend on the pH, labile protons, sidechain conformation, as well as the radial position of the residues in this four-helix bundle. Despite the multiple dependences, the initial transfer rates reflect the periodic nature of the residue positions from the water-filled pore, thus this technique provides a way of gleaning secondary structure information, helix tilt angle, and the oligomeric structure of membrane proteins.

Solid-state NMR shows that dynamically different domains of membrane proteins have different hydration dependence

Hydration has a profound influence on the structure, dynamics, and functions of membrane and membrane-embedded proteins. So far the hydration response of molecular dynamics of membrane proteins in lipid bilayers is poorly understood. Here, we reveal different hydration dependence of the dynamics in dynamically different domains of membrane proteins by multidimensional magic angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy using 121-residue integral diacylglycerol kinase (DAGK) in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG) lipid bilayers as a model system. The highly mobile and immobile domains of DAGK and their water accessibilities are identified site-specifically by scalar- and dipolar-coupling based MAS ssNMR experiments, respectively. Our experiments reveal different hydration dependence of the dynamics in highly mobile and immobile domains of membrane proteins. We demonstrate that the fast, large-amplitude motions in highly mobile domains are not triggered until 20% hydration, enhanced at 20-50% hydration and unchanged at above 50% hydration. In contrast, motions on submicrosecond time scale of immobile residues are observed to be independent of the hydration levels in gel phase of lipids, and at the temperature near gel-liquid crystalline phase transition, amplitude of whole-molecule rotations around the bilayer normal is dominated by the fluidity of lipid bilayers, which is strongly hydration dependent. The hydration dependence of the dynamics of DAGK revealed by this study provides new insights into the correlations of hydration to dynamics and function of membrane proteins in lipid bilayers.

NMR structures of membrane proteins in phospholipid bilayers

Membrane proteins have always presented technical challenges for structural studies because of their requirement for a lipid environment. Multiple approaches exist including X-ray crystallography and electron microscopy that can give significant insights into their structure and function. However, nuclear magnetic resonance (NMR) is unique in that it offers the possibility of determining the structures of unmodified membrane proteins in their native environment of phospholipid bilayers under physiological conditions. Furthermore, NMR enables the characterization of the structure and dynamics of backbone and side chain sites of the proteins alone and in complexes with both small molecules and other biopolymers. The learning curve has been steep for the field as most initial studies were performed under non-native environments using modified proteins until ultimately progress in both techniques and instrumentation led to the possibility of examining unmodified membrane proteins in phospholipid bilayers under physiological conditions. This review aims to provide an overview of the development and application of NMR to membrane proteins. It highlights some of the most significant structural milestones that have been reached by NMR spectroscopy of membrane proteins, especially those accomplished with the proteins in phospholipid bilayer environments where they function.