Mapping protein conformational heterogeneity under pressure with site-directed spin labeling and double electron-electron resonance

A pressure-jump EPR system to monitor millisecond conformational exchange rates of spin-labeled proteins

Site-directed spin labeling electron paramagnetic resonance (SDSL-EPR) using nitroxide spin labels is a well-established technology for mapping site-specific secondary and tertiary structure and for monitoring conformational changes in proteins of any degree of complexity, including membrane proteins, with high sensitivity. SDSL-EPR also provides information on protein dynamics in the timescale of ps-μs using continuous wave lineshape analysis and spin lattice relaxation time methods. However, the functionally important time domain of μs-ms, corresponding to large-scale protein motions, is inaccessible to those methods. To extend SDSL-EPR to the longer time domain, the perturbation method of pressure-jump relaxation is implemented. Here, we describe a complete high-pressure EPR system at Q-band for both static pressure and ms-timescale pressure-jump measurements on spin-labeled proteins. The instrument enables pressure jumps both up and down from any holding pressure, ranging from atmospheric pressure to the maximum pressure capacity of the system components (~3500 bar). To demonstrate the utility of the system, we characterize a local folding-unfolding equilibrium of T4 lysozyme. The results illustrate the ability of the system to measure thermodynamic and kinetic parameters of protein conformational exchange on the ms timescale.

A pressure-jump EPR system to monitor millisecond conformational exchange rates of spin-labeled proteins

Site-directed spin labeling electron paramagnetic resonance (SDSL-EPR) using nitroxide spin labels is a well-established technology for mapping site-specific secondary and tertiary structure and for monitoring conformational changes in proteins of any degree of complexity, including membrane proteins, with high sensitivity. SDSL-EPR also provides information on protein dynamics in the time scale of ps-µs using continuous wave lineshape analysis and spin lattice relaxation time methods. However, the functionally important time domain of µs-ms, corresponding to large-scale protein motions, is inaccessible to those methods. To extend SDSL-EPR to the longer time domain, the perturbation method of pressure-jump relaxation is implemented. Here, we describe a complete high-pressure EPR system at Q-band for both static pressure and millisecond-timescale pressure-jump measurements on spin-labeled proteins. The instrument enables pressure jumps both up and down from any holding pressure, ranging from atmospheric pressure to the maximum pressure capacity of the system components (~3500 bar). To demonstrate the utility of the system, we characterize a local folding-unfolding equilibrium of T4 lysozyme. The results illustrate the ability of the system to measure thermodynamic and kinetic parameters of protein conformational exchange on the millisecond timescale.

Connexin Gap Junction Channels and Hemichannels: Insights from High-Resolution Structures

Connexins (Cxs) are a family of integral membrane proteins, which function as both hexameric hemichannels (HCs) and dodecameric gap junction channels (GJCs), behaving as conduits for the electrical and molecular communication between cells and between cells and the extracellular environment, respectively. Their proper functioning is crucial for many processes, including development, physiology, and response to disease and trauma. Abnormal GJC and HC communication can lead to numerous pathological states including inflammation, skin diseases, deafness, nervous system disorders, and cardiac arrhythmias. Over the last 15 years, high-resolution X-ray and electron cryomicroscopy (cryoEM) structures for seven Cx isoforms have revealed conservation in the four-helix transmembrane (TM) bundle of each subunit; an αβ fold in the disulfide-bonded extracellular loops and inter-subunit hydrogen bonding across the extracellular gap that mediates end-to-end docking to form a tight seal between hexamers in the GJC. Tissue injury is associated with cellular Ca2+ overload. Surprisingly, the binding of 12 Ca2+ ions in the Cx26 GJC results in a novel electrostatic gating mechanism that blocks cation permeation. In contrast, acidic pH during tissue injury elicits association of the N-terminal (NT) domains that sterically blocks the pore in a "ball-and-chain" fashion. The NT domains under physiologic conditions display multiple conformational states, stabilized by protein-protein and protein-lipid interactions, which may relate to gating mechanisms. The cryoEM maps also revealed putative lipid densities within the pore, intercalated among transmembrane α-helices and between protomers, the functions of which are unknown. For the future, time-resolved cryoEM of isolated Cx channels as well as cryotomography of GJCs and HCs in cells and tissues will yield a deeper insight into the mechanisms for channel regulation. The cytoplasmic loop (CL) and C-terminal (CT) domains are divergent in sequence and length, are likely involved in channel regulation, but are not visualized in the high-resolution X-ray and cryoEM maps presumably due to conformational flexibility. We expect that the integrated use of synergistic physicochemical, spectroscopic, biophysical, and computational methods will reveal conformational dynamics relevant to functional states. We anticipate that such a wealth of results under different pathologic conditions will accelerate drug discovery related to Cx channel modulation.

DEERefiner-assisted structural refinement using pulsed dipolar spectroscopy: a study on multidrug transporter LmrP

Pulsed dipolar spectroscopy, such as double electron-electron resonance (DEER), has been underutilized in protein structure determination, despite its ability to provide valuable spatial information. In this study, we present DEERefiner, a user-friendly MATLAB-based GUI program that enables the modeling of protein structures by combining an initial structure and DEER distance restraints. We illustrate the effectiveness of DEERefiner by successfully modeling the ligand-dependent conformational changes of the proton-drug antiporter LmrP to an extracellular-open-like conformation with an impressive precision of 0.76 Å. Additionally, DEERefiner was able to uncover a previously hypothesized but experimentally unresolved proton-dependent conformation of LmrP, characterized as an extracellular-closed/partially intracellular-open conformation, with a precision of 1.16 Å. Our work not only highlights the ability of DEER spectroscopy to model protein structures but also reveals the potential of DEERefiner to advance the field by providing an accessible and applicable tool for precise protein structure modeling, thereby paving the way for deeper insights into protein function.

chiLife: An open-source Python package for in silico spin labeling and integrative protein modeling

Here we introduce chiLife, a Python package for site-directed spin label (SDSL) modeling for electron paramagnetic resonance (EPR) spectroscopy, in particular double electron-electron resonance (DEER). It is based on in silico attachment of rotamer ensemble representations of spin labels to protein structures. chiLife enables the development of custom protein analysis and modeling pipelines using SDSL EPR experimental data. It allows the user to add custom spin labels, scoring functions and spin label modeling methods. chiLife is designed with integration into third-party software in mind, to take advantage of the diverse and rapidly expanding set of molecular modeling tools available with a Python interface. This article describes the main design principles of chiLife and presents a series of examples.

Illuminating the Structural Basis of Tau Aggregation by Intramolecular Distance Tracking: A Perspective on Methods

The aggregation of the tau protein is central to several neurodegenerative diseases, collectively known as tauopathies. High-resolution views of tau tangles accumulated under pathological conditions in post-mortem brains have been revealed recently by cryogenic electron microscopy. One of the striking discoveries was that fibril folds are unique to and homogeneous within one disease family, but typically different between different tauopathies. It is widely believed that seeded aggregation can achieve structural propagation of tau fibrils and generate pathological fibril structures. However, direct molecular level measurement of structural evolution during aggregation is missing. Here, we discuss our perspective on the biophysical approaches that can contribute to the ongoing debate regarding the prion-like propagation of tau and the role of cofactors. We discuss the unique potential of double electron-electron resonance (DEER)-based intramolecular distance measurement, sensitive to two to several nanometers distances. DEER can track the structural evolution of tau along the course of aggregation from the completely disordered state, to partially ordered and highly ordered fibril states, and has the potential to be a key tool to elucidate the disease-specific tau aggregation pathways.

Viewing rare conformations of the β2 adrenergic receptor with pressure-resolved DEER spectroscopy

The β₂ adrenergic receptor (β₂AR) is an archetypal G protein coupled receptor (GPCR). One structural signature of GPCR activation is a large-scale movement (ca. 6 to 14 Å) of transmembrane helix 6 (TM6) to a conformation which binds and activates a cognate G protein. The β₂AR exhibits a low level of agonist-independent G protein activation. The structural origin of this basal activity and its suppression by inverse agonists is unknown but could involve a unique receptor conformation that promotes G protein activation. Alternatively, a conformational selection model proposes that a minor population of the canonical active receptor conformation exists in equilibrium with inactive forms, thus giving rise to basal activity of the ligand-free receptor. Previous spin-labeling and fluorescence resonance energy transfer experiments designed to monitor the positional distribution of TM6 did not detect the presence of the active conformation of ligand-free β₂AR. Here we employ spin-labeling and pressure-resolved double electron-electron resonance spectroscopy to reveal the presence of a minor population of unliganded receptor, with the signature outward TM6 displacement, in equilibrium with inactive conformations. Binding of inverse agonists suppresses this population. These results provide direct structural evidence in favor of a conformational selection model for basal activity in β₂AR and provide a mechanism for inverse agonism. In addition, they emphasize 1) the importance of minor populations in GPCR catalytic function; 2) the use of spin-labeling and variable-pressure electron paramagnetic resonance to reveal them in a membrane protein; and 3) the quantitative evaluation of their thermodynamic properties relative to the inactive forms, including free energy, partial molar volume, and compressibility.

Activation of the G-Protein-Coupled Receptor Rhodopsin by Water

Visual rhodopsin is an important archetype for G-protein-coupled receptors, which are membrane proteins implicated in cellular signal transduction. Herein, we show experimentally that approximately 80 water molecules flood rhodopsin upon light absorption to form a solvent-swollen active state. An influx of mobile water is necessary for activating the photoreceptor, and this finding is supported by molecular dynamics (MD) simulations. Combined force-based measurements involving osmotic and hydrostatic pressure indicate the expansion occurs by changes in cavity volumes, together with greater hydration in the active metarhodopsin-II state. Moreover, we discovered that binding and release of the C-terminal helix of transducin is coupled to hydration changes as may occur in visual signal amplification. Hydration-dehydration explains signaling by a dynamic allosteric mechanism, in which the soft membrane matter (lipids and water) has a pivotal role in the catalytic G-protein cycle.

Pressure Effect on Zero-Field Splitting Parameter of Hemin: Model Case of Hemoproteins under Pressure

We experimentally studied the pressure dependence of the zero-field splitting (ZFS) parameter of hemin (iron(III) protoporphyrin IX chloride), which is a model complex of hemoproteins, via high-frequency and high-field electron paramagnetic resonance (HFEPR) under pressure. Owing to the large ZFS, the pressure effect on the electronic structure of iron-porphyrin complexes has not yet been explored using EPR. Therefore, we systematically studied this effect using our newly developed sub-terahertz EPR spectroscopy system in the frequency range of 80-515 GHz, under magnetic fields up to 10 T and pressure up to 2 GPa. We observed a systematic shift of the resonance fields of hemin upon pressure application, from which the axial component of the ZFS parameter was found to increase from D = 6.9 to 7.9 cm^-1 at 2 GPa. In contrast to the previous methods used to study proteins under pressure, which mainly focused on conformational changes, our HFEPR technique can obtain more microscopic insights into the electronic structures of metal ions under pressure. In this sense, our technique provides novel opportunities to study the pressure effects on biofunctional active centers of versatile metalloproteins.

Gi- and Gs-coupled GPCRs show different modes of G-protein binding

Activation of G protein-coupled receptors (GPCRs) initiates conformational shifts that trigger interaction with a specific G-protein subtype from a structurally homologous set. A major unsolved problem is the mechanism by which this selectivity is achieved. Structures of GPCR–G protein complexes so far fail to reveal the origin of selectivity because they all involve one G-protein subtype (G_s). In this work, we report a structural model of the activated GPCR rhodopsin in complex with another G-protein subtype (G_i) derived from intermolecular distance mapping with DEER-EPR and refinement with modeling. Comparison of the model with structures of complexes involving G_s reveals distinct GPCR–G protein-binding modes, the differences of which suggest key features of the structural selectivity filter.

More than two decades ago, the activation mechanism for the membrane-bound photoreceptor and prototypical G protein-coupled receptor (GPCR) rhodopsin was uncovered. Upon light-induced changes in ligand–receptor interaction, movement of specific transmembrane helices within the receptor opens a crevice at the cytoplasmic surface, allowing for coupling of heterotrimeric guanine nucleotide-binding proteins (G proteins). The general features of this activation mechanism are conserved across the GPCR superfamily. Nevertheless, GPCRs have selectivity for distinct G-protein family members, but the mechanism of selectivity remains elusive. Structures of GPCRs in complex with the stimulatory G protein, G_s, and an accessory nanobody to stabilize the complex have been reported, providing information on the intermolecular interactions. However, to reveal the structural selectivity filters, it will be necessary to determine GPCR–G protein structures involving other G-protein subtypes. In addition, it is important to obtain structures in the absence of a nanobody that may influence the structure. Here, we present a model for a rhodopsin–G protein complex derived from intermolecular distance constraints between the activated receptor and the inhibitory G protein, G_i, using electron paramagnetic resonance spectroscopy and spin-labeling methodologies. Molecular dynamics simulations demonstrated the overall stability of the modeled complex. In the rhodopsin–G_i complex, G_i engages rhodopsin in a manner distinct from previous GPCR–G_s structures, providing insight into specificity determinants.

The effects of glycine, TMAO and osmolyte mixtures on the pressure dependent enzymatic activity of α-chymotrypsin

Different natural osmolytes modulate the pressure dependent enzyme kinetics in different ways.

High-pressure EPR spectroscopy studies of the E. coli lipopolysaccharide transport proteins LptA and LptC

The use of pressure is an advantageous approach to the study of protein structure and dynamics because it can shift the equilibrium populations of protein conformations toward higher energy states that are not of sufficient population to be observable at atmospheric pressure. Recently, the Hubbell group at the University of California, Los Angeles, reintroduced the application of high pressure to the study of proteins by electron paramagnetic resonance (EPR) spectroscopy. This methodology is possible using X-band EPR spectroscopy due to advances in pressure intensifiers, sample cells, and resonators. In addition to the commercial availability of the pressure generation and sample cells by Pressure Biosciences Inc., a five-loop-four-gap resonator required for the initial high pressure EPR spectroscopy experiments by the Hubbell group, and those reported here, was designed by James S. Hyde and built and modified at the National Biomedical EPR Center. With these technological advances, we determined the effect of pressure on the essential periplasmic lipopolysaccharide (LPS) transport protein from Escherichia coli, LptA, and one of its binding partners, LptC. LptA unfolds from the N-terminus to the C-terminus, binding of LPS does not appreciably stabilize the protein under pressure, and monomeric LptA unfolds somewhat more readily than oligomeric LptA upon pressurization to 2 kbar. LptC exhibits a fold and relative lack of stability upon LPS binding similar to LptA, yet adopts an altered, likely monomeric, folded conformation under pressure with only its C-terminus unraveling. The pressure-induced changes likely correlate with functional changes associated with binding and transport of LPS.

A new approach to distance measurements between two spin labels in the >10 nm range

ESR spectroscopy can be efficiently used to acquire the distance between two spin labels placed on a macromolecule by measuring their mutual dipolar interaction frequency, as long as the distance is not greater than ∼10 nm. Any hope to significantly increase this figure is hampered by the fact that all available spin labels have a phase memory time (T_m), restricted to the microseconds range, which provides a limited window during which the dipolar interaction frequency can be measured. Thus, due to the inverse cubic dependence of the dipolar frequency over the labels' separation distance, evaluating much larger distances, e.g. 20 nm, would require to have a T_m that is ∼200 microsecond, clearly beyond any hope. Here we propose a new approach to greatly enhancing the maximum measured distance available by relying on another type of dipole interaction-mediated mechanism called spin diffusion. This mechanism operates and can be evaluated during the spin lattice relaxation time, T₁ (commonly in the milliseconds range), rather than only during T_m. Up until recently, the observation of spin diffusion in solid electron spin systems was considered experimentally impractical. However, recent developments have enabled its direct measurement by means of high sensitivity pulsed ESR that employs intense short magnetic field gradients, thus opening the door to the subsequent utilization of these capabilities. The manuscript presents the subject of spin diffusion, the ways it can be directly measured, and a theoretical discussion on how intramolecular spin-pair distance, even in the range of 20-30 nm, could be accurately extracted from spin diffusion measurements.

The role of conformational heterogeneity in regulating the apoptotic activity of BAX protein

Inactive BAX exists in two states. A shift in the equilibrium would initiate apoptosis.

Biophysical and Biochemical Characterization of Avian Secretory Component Provides Structural Insights into the Evolution of the Polymeric Ig Receptor

The polymeric Ig receptor (pIgR) transports polymeric Abs across epithelia to the mucosa, where proteolytic cleavage releases the ectodomain (secretory component [SC]) as an integral component of secretory Abs, or as an unliganded protein that can mediate interactions with bacteria. SC is conserved among vertebrates, but domain organization is variable: mammalian SC has five domains (D1-D5), whereas avian, amphibian, and reptilian SC lack the D2 domain, and fish SC lacks domains D2-D4. In this study, we used double electron-electron resonance spectroscopy and surface plasmon resonance binding studies to characterize the structure, dynamics, and ligand binding properties of avian SC, avian SC domain variants, and a human SC (hSC) variant lacking the D2 domain. These experiments demonstrated that, unlike hSC, which adopts a compact or "closed" domain arrangement, unliganded avian SC is flexible and exists in both closed and open states, suggesting that the mammalian SC D2 domain stabilizes the closed conformation observed for hSC D1-D5. Experiments also demonstrated that avian and mammalian pIgR share related, but distinct, mechanisms of ligand binding. Together, our data reveal differences in the molecular recognition mechanisms associated with evolutionary changes in the pIgR protein.

Effector Roles of Putidaredoxin on Cytochrome P450cam Conformational States

In this study, the effector role of Pdx (putidaredoxin) on cytochrome P450cam conformation is refined by attaching two different spin labels, MTSL or BSL (bifunctional spin-label) onto the F or G helices and using DEER (double electron-electron resonance) to measure the distance between labels. Recent EPR and crystallographic studies have observed that oxidized Pdx induces substrate-bound P450cam to change from the closed to the open state. However, this change was not observed by DEER in the reduced Pdx complex with carbon-monoxide-bound P450cam (Fe(2+)CO). In addition, recent NMR studies have failed to observe a change in P450cam conformation upon binding Pdx. Hence, resolving these issues is important for a full understanding the effector role of Pdx. Here we show that oxidized Pdx induces camphor-bound P450cam to shift from the closed to the open conformation when labeled on either the F or G helices with MTSL. BSL at these sites can either narrow the distance distribution widths dramatically or alter the extent of the conformational change. In addition, we report DEER spectra on a mixed oxidation state containing oxidized Pdx and ferrous CO-bound P450cam, showing that P450cam remains closed. This indicates that CO binding to the heme prevents P450cam from opening, overriding the influence exerted by Pdx binding. Finally, we report the open form P450cam crystal structure with substrate bound, which suggests that crystal packing effects may prevent conformational conversion. Using multiple labeling approaches, DEER provides a unique perspective to resolve how the conformation of P450cam depends on Pdx and ligand states.

Coherent pump pulses in Double Electron Electron Resonance spectroscopy

The recent introduction of shaped pulses to Double Electron Electron Resonance (DEER) spectroscopy has led to significant enhancements in sensitivity through increased excitation bandwidths and improved control over spin dynamics. The application of DEER has so far relied on the presence of an incoherent pump channel to average out most undesired coherent effects of the pump pulse(s) on the observer spins. However, in fully coherent EPR spectrometers that are increasingly used to generate shaped pulses, the presence of coherent pump pulses means that these effects need to be explicitly considered. In this paper, we examine the effects of coherent rectangular and sech/tanh pump pulses in DEER experiments with up to three pump pulses. We show that, even in the absence of significant overlap of the observer and pump pulse excitation bandwidths, coherence transfer pathways involving both types of pulses generate spin echoes of considerable intensity. These echoes introduce artefacts, which, if not identified and removed, can easily lead to misinterpretation. We demonstrate that the observed echoes can be quantitatively modelled using a simple spin quantum dynamics approach that includes instrumental transfer functions. Based on an analysis of the echo crossing artefacts, we propose efficient phase cycling schemes for their suppression. This enables the use of advanced DEER experiments, characterized by high sensitivity and increased accuracy for long-distance measurements, on novel fully coherent EPR spectrometers.

The structure and dynamics of secretory component and its interactions with polymeric immunoglobulins

As a first-line vertebrate immune defense, the polymeric immunoglobulin receptor (pIgR) transports polymeric IgA and IgM across epithelia to mucosal secretions, where the cleaved ectodomain (secretory component; SC) becomes a component of secretory antibodies, or when unliganded, binds and excludes bacteria. Here we report the 2.6Å crystal structure of unliganded human SC (hSC) and comparisons with a 1.7Å structure of teleost fish SC (tSC), an early pIgR ancestor. The hSC structure comprises five immunoglobulin-like domains (D1-D5) arranged as a triangle, with an interface between ligand-binding domains D1 and D5. Electron paramagnetic resonance measurements confirmed the D1-D5 interface in solution and revealed that it breaks upon ligand binding. Together with binding studies of mutant and chimeric SCs, which revealed domain contributions to secretory antibody formation, these results provide detailed models for SC structure, address pIgR evolution, and demonstrate that SC uses multiple conformations to protect mammals from pathogens.

DOI:http://dx.doi.org/10.7554/eLife.10640.001

A sticky substance called mucus lines our airways and gut, where it acts as a physical barrier to prevent bacteria and other microbes from entering the body. Mucus also contains proteins called antibodies that can bind to and neutralize molecules from microbes (known as antigens).

The primary antibody found in mucus is called Immunoglobulin A. This antibody is produced by immune cells within the body and must pass through the “epithelial” cells that line the airway or gut to reach the layer of mucus. These epithelial cells have a receptor protein called the polymeric immunoglobulin receptor (plgR) that binds to Immunoglobulin A molecules, transports them across the cell, and then releases them into the mucus layer. The pIgR also releases Immunoglobulin A into breast milk, which protects nursing infants until their own immune system has developed.

When released into the mucus layer, the Immunoglobulin A antibodies remain attached to a portion of pIgR known as the secretory component. This part of the receptor serves to stabilize and protect the antibodies from being degraded and helps the antibodies to bind to other host and bacterial proteins. Researchers have noted that the secretory component can be released into the mucus even when it is not attached to an antibody. These “free” secretory components have been shown to help prevent bacteria and the toxins they produce from entering the body. Despite the importance of secretory component in immune responses, the three-dimensional structure of the secretory component and how it interacts with antibodies and bacteria remained unknown.

Here, Stadtmueller et al. use a technique called X-ray crystallography to determine a three-dimensional model of the free form of a secretory component from humans, and compare it to an ancestral secretory component protein found in fish. Further experiments on the human protein revealed how the structure of the secretory component changes when antibodies bind to it. Stadtmueller et al. propose a model for how both forms of the secretory component can protect the body from microbes and other external agents. The next challenge is to develop a three-dimensional model of the secretory component when it is bound to Immunoglobulin A.

DOI:http://dx.doi.org/10.7554/eLife.10640.002

High-Pressure EPR and Site-Directed Spin Labeling for Mapping Molecular Flexibility in Proteins

High hydrostatic pressure is a powerful probe of protein conformational flexibility. Pressurization reveals regions of elevated compressibility, and thus flexibility, within individual conformational states, but also shifts conformational equilibria such that "invisible" excited states become accessible for spectroscopic characterization. The central aim of this chapter is to describe recently developed instrumentation and methodologies that enable high-pressure site-directed spin labeling electron paramagnetic resonance (SDSL-EPR) experiments on proteins and to demonstrate the information content of these experiments by highlighting specific recent applications. A brief introduction to the thermodynamics of proteins under pressure is presented first, followed by a discussion of the principles underlying SDSL-EPR detection of pressure effects in proteins, and the suitability of SDSL-EPR for this purpose in terms of timescale and ability to characterize conformational heterogeneity. Instrumentation and practical considerations for variable-pressure continuous wave EPR and pressure-resolved double electron-electron resonance (PR DEER) experiments are reviewed, and finally illustrations of data analysis using recent applications are presented. Although high-pressure SDSL-EPR is in its infancy, the recent applications presented highlight the considerable potential of the method to (1) identify compressible (flexible) regions in a folded protein; (2) determine thermodynamic parameters that relate conformational states in equilibrium; (3) populate and characterize excited states of proteins undetected at atmospheric pressure; (4) reveal the structural heterogeneity of conformational ensembles and provide distance constraints on the global structure of pressure-populated states with PR DEER.

Development of electron spin echo envelope modulation spectroscopy to probe the secondary structure of recombinant membrane proteins in a lipid bilayer

Membrane proteins conduct many important biological functions essential to the survival of organisms. However, due to their inherent hydrophobic nature, it is very difficult to obtain structural information on membrane-bound proteins using traditional biophysical techniques. We are developing a new approach to probe the secondary structure of membrane proteins using the pulsed EPR technique of Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy. This method has been successfully applied to model peptides made synthetically. However, in order for this ESEEM technique to be widely applicable to larger membrane protein systems with no size limitations, protein samples with deuterated residues need to be prepared via protein expression methods. For the first time, this study shows that the ESEEM approach can be used to probe the local secondary structure of a (2) H-labeled d8 -Val overexpressed membrane protein in a membrane mimetic environment. The membrane-bound human KCNE1 protein was used with a known solution NMR structure to demonstrate the applicability of this methodology. Three different α-helical regions of KCNE1 were probed: the extracellular domain (Val21), transmembrane domain (Val50), and cytoplasmic domain (Val95). These results indicated α-helical structures in all three segments, consistent with the micelle structure of KCNE1. Furthermore, KCNE1 was incorporated into a lipid bilayer and the secondary structure of the transmembrane domain (Val50) was shown to be α-helical in a more native-like environment. This study extends the application of this ESEEM approach to much larger membrane protein systems that are difficult to study with X-ray crystallography and/or NMR spectroscopy.

SIGNAL TRANSDUCTION. Structural basis for nucleotide exchange in heterotrimeric G proteins

G protein-coupled receptors (GPCRs) relay diverse extracellular signals into cells by catalyzing nucleotide release from heterotrimeric G proteins, but the mechanism underlying this quintessential molecular signaling event has remained unclear. Here we use atomic-level simulations to elucidate the nucleotide-release mechanism. We find that the G protein α subunit Ras and helical domains-previously observed to separate widely upon receptor binding to expose the nucleotide-binding site-separate spontaneously and frequently even in the absence of a receptor. Domain separation is necessary but not sufficient for rapid nucleotide release. Rather, receptors catalyze nucleotide release by favoring an internal structural rearrangement of the Ras domain that weakens its nucleotide affinity. We use double electron-electron resonance spectroscopy and protein engineering to confirm predictions of our computationally determined mechanism.

Structure-relaxation mechanism for the response of T4 lysozyme cavity mutants to hydrostatic pressure

Application of hydrostatic pressure shifts protein conformational equilibria in a direction to reduce the volume of the system. A current view is that the volume reduction is dominated by elimination of voids or cavities in the protein interior via cavity hydration, although an alternative mechanism wherein cavities are filled with protein side chains resulting from a structure relaxation has been suggested [López CJ, Yang Z, Altenbach C, Hubbell WL (2013) Proc Natl Acad Sci USA 110(46):E4306-E4315]. In the present study, mechanisms for elimination of cavities under high pressure are investigated in the L99A cavity mutant of T4 lysozyme and derivatives thereof using site-directed spin labeling, pressure-resolved double electron-electron resonance, and high-pressure circular dichroism spectroscopy. In the L99A mutant, the ground state is in equilibrium with an excited state of only ∼ 3% of the population in which the cavity is filled by a protein side chain [Bouvignies et al. (2011) Nature 477(7362):111-114]. The results of the present study show that in L99A the native ground state is the dominant conformation to pressures of 3 kbar, with cavity hydration apparently taking place in the range of 2-3 kbar. However, in the presence of additional mutations that lower the free energy of the excited state, pressure strongly populates the excited state, thereby eliminating the cavity with a native side chain rather than solvent. Thus, both cavity hydration and structure relaxation are mechanisms for cavity elimination under pressure, and which is dominant is determined by details of the energy landscape.

Towards the free energy landscape for catalysis in mammalian nitric oxide synthases

The general requirement for conformational sampling in biological electron transfer reactions catalysed by multi-domain redox systems has been emphasized in recent years. Crucially, we lack insight into the extent of the conformational space explored and the nature of the energy landscapes associated with these reactions. The nitric oxide synthases (NOS) produce the signalling molecule NO through a series of complex electron transfer reactions. There is accumulating evidence that protein domain dynamics and calmodulin binding are implicated in regulating electron flow from NADPH, through the FAD and FMN cofactors, to the haem oxygenase domain, where NO is generated. Simple models based on static crystal structures of the isolated reductase domain have suggested a role for large-scale motions of the FMN-binding domain in shuttling electrons from the reductase domain to the oxygenase domain. However, detailed insight into the higher-order domain architecture and dynamic structural transitions in NOS enzymes during enzyme turnover is lacking. In this review, we discuss the recent advances made towards mapping the catalytic free energy landscapes of NOS enzymes through integration of both structural techniques (e.g. cryo-electron microscopy) and biophysical techniques (e.g. pulsed-electron paramagnetic resonance). The general picture that emerges from these experiments is that NOS enzymes exist in an equilibrium of conformations, comprising a 'rugged' or 'frustrated' energy landscape, with a key regulatory role for calmodulin in driving vectorial electron transfer by altering the conformational equilibrium. A detailed understanding of these landscapes may provide new opportunities for discovery of isoform-specific inhibitors that bind at the dynamic interfaces of these multi-dimensional energy landscapes.

Stationary-phase EPR for exploring protein structure, conformation, and dynamics in spin-labeled proteins

Proteins tethered to solid supports are of increasing interest in bioanalytical chemistry and protein science in general. However, the extent to which tethering modifies the energy landscape and dynamics of the protein is most often unknown because there are few biophysical methods that can determine secondary and tertiary structures and explore conformational equilibria and dynamics of a tethered protein with site-specific resolution. Site-directed spin labeling (SDSL) combined with electron paramagnetic resonance (EPR) offers a unique opportunity for this purpose. Here, we employ a general strategy using unnatural amino acids that enables efficient and site-specific tethering of a spin-labeled protein to a Sepharose solid support. Remarkably, EPR spectra of spin-labeled T4 lysozyme (T4L) reveal that a single site-specific attachment suppresses rotational motion of the protein sufficiently to allow interpretation of the spectral line shape in terms of protein internal dynamics. Importantly, line shape analysis and distance mapping using double electron–electron resonance reveal that internal dynamics, the tertiary fold, conformational equilibria, and ligand binding of the tethered proteins were similar to those in solution, in contrast to random attachment via native lysine residues. The results of this study set the stage for the development of an EPR-based flow system that will house soluble and membrane proteins immobilized site-specifically, thereby enabling facile screening of structural and dynamical effects of binding partners.

Long-range distance measurements in proteins at physiological temperatures using saturation recovery EPR spectroscopy

Site-directed spin labeling in combination with EPR is a powerful method for providing distances on the nm scale in biological systems. The most popular strategy, double electron-electron resonance (DEER), is carried out at cryogenic temperatures (50-80 K) to increase the short spin-spin relaxation time (T2) upon which the technique relies. A challenge is to measure long-range distances (20-60 Å) in proteins near physiological temperatures. Toward this goal we are investigating an alternative approach based on the distance-dependent enhancement of spin-lattice relaxation rate (T1(-1)) of a nitroxide spin label by a paramagnetic metal. With a commonly used nitroxide side chain (R1) and Cu(2+), it has been found that interspin distances ≤25 Å can be determined in this way (Jun et al. Biochemistry 2006, 45, 11666). Here, the upper limit of the accessible distance is extended to ≈40 Å using spin labels with long T1, a high-affinity 5-residue Cu(2+) binding loop inserted into the protein sequence, and pulsed saturation recovery to measure relaxation enhancement. Time-domain Cu(2+) electron paramagnetic resonance, quantum mechanical calculations, and molecular dynamics simulations provide information on the structure and geometry of the Cu(2+) loop and indicate that the metal ion is well-localized in the protein. An important aspect of these studies is that both Cu(2+)/nitroxide DEER at cryogenic temperatures and T1 relaxation measurements at room temperature can be carried out on the same sample, allowing both validation of the relaxation method and assessment of the effect of freezing on protein structure.

Identifying and quantitating conformational exchange in membrane proteins using site-directed spin labeling

Protein structures are not static but sample different conformations over a range of amplitudes and time scales. These fluctuations may involve relatively small changes in bond angles or quite large rearrangements in secondary structure and tertiary fold. The equilibrium between discrete structural substates on the microsecond to millisecond time scale is sometimes termed conformational exchange. Protein dynamics and conformational exchange are believed to provide the basis for many important activities, such as protein–protein and protein–ligand interactions, enzymatic activity and protein allostery; however, for many proteins, the dynamics and conformational exchange that lead to function are poorly defined.

Spectroscopic methods, such as NMR, are among the most important methods to explore protein dynamics and conformational exchange; however, they are difficult to implement in some systems and with some types of exchange events. Site-directed spin labeling (SDSL) is an EPR based approach that is particularly well-suited to high molecular-weight systems such as membrane proteins. Because of the relatively fast time scale for EPR spectroscopy, it is an excellent method to examine exchange. Conformations that are in exchange are captured as distinct populations in the EPR spectrum, and this feature when combined with the use of methods that can shift the free energy of conformational substates allows one to identify regions of proteins that are in dynamic exchange. In addition, modern pulse EPR methods have the ability to examine conformational heterogeneity, resolve discrete protein states, and identify the substates in exchange.

Protein crystallography has provided high-resolution models for a number of membrane proteins; but because of conformational exchange, these models do not always reflect the structures that are present when the protein is in a native bilayer environment. In the case of the Escherichia coli vitamin B₁₂ transporter, BtuB, the energy coupling segment of this protein undergoes a substrate-dependent unfolding, which acts to couple this outer-membrane protein to the inner-membrane protein TonB. EPR spectroscopy demonstrates that the energy coupling segment is in equilibrium between ordered and disordered states, and that substrate binding shifts this equilibrium to favor an unfolded state. However, in crystal structures of BtuB, this segment is resolved and folded within the protein, and neither the presence of this equilibrium nor the substrate-induced change is revealed. This is a result of the solute environment and the crystal lattice, both of which act to stabilize one conformational substate of the transporter.

Using SDSL, it can be shown that conformational exchange is present in other regions of BtuB and in other members of this transporter family. Conformational exchange has also been examined in systems such as the plasma membrane SNARE protein, syntaxin 1A, where dynamics are controlled by regulatory proteins such as munc18. Regulating the microsecond to millisecond time scale dynamics in the neuronal SNAREs is likely to be a key feature that regulates assembly of the SNAREs and neurotransmitter release.

Probing the non-native H helix translocation in apomyoglobin folding intermediates

Apomyoglobin folds via sequential helical intermediates that are formed by rapid collapse of the A, B, G, and H helix regions. An equilibrium molten globule with a similar structure is formed near pH 4. Previous studies suggested that the folding intermediates are kinetically trapped states in which folding is impeded by non-native packing of the G and H helices. Fluorescence spectra of mutant proteins in which cysteine residues were introduced at several positions in the G and H helices show differential quenching of W14 fluorescence, providing direct evidence of translocation of the H helix relative to helices A and G in both the kinetic and equilibrium intermediates. Förster resonance energy transfer measurements show that a 5-({2-[(acetyl)amino]ethyl}amino)naphthalene-1-sulfonic acid acceptor coupled to K140C (helix H) is closer to Trp14 (helix A) in the equilibrium molten globule than in the native state, by a distance that is consistent with sliding of the H helix in an N-terminal direction by approximately one helical turn. Formation of an S108C–L135C disulfide prevents H helix translocation in the equilibrium molten globule by locking the G and H helices into their native register. By enforcing nativelike packing of the A, G, and H helices, the disulfide resolves local energetic frustration and facilitates transient docking of the E helix region onto the hydrophobic core but has only a small effect on the refolding rate. The apomyoglobin folding landscape is highly rugged, with several energetic bottlenecks that frustrate folding; relief of any one of the major identified bottlenecks is insufficient to speed progression to the transition state.