Uncovering a membrane-distal conformation of KRAS available to recruit RAF to the plasma membrane.

The small GTPase KRAS is localized at the plasma membrane where it functions as a molecular switch, coupling extracellular growth factor stimulation to intracellular signaling networks. In this process, KRAS recruits effectors, such as RAF kinase, to the plasma membrane where they are activated by a series of complex molecular steps. Defining the membrane-bound state of KRAS is fundamental to understanding the activation of RAF kinase and in evaluating novel therapeutic opportunities for the inhibition of oncogenic KRAS-mediated signaling. We combined multiple biophysical measurements and computational methodologies to generate a consensus model for authentically processed, membrane-anchored KRAS. In contrast to the two membrane-proximal conformations previously reported, we identify a third significantly populated state using a combination of neutron reflectivity, fast photochemical oxidation of proteins (FPOP), and NMR. In this highly populated state, which we refer to as "membrane-distal" and estimate to comprise ∼90% of the ensemble, the G-domain does not directly contact the membrane but is tethered via its C-terminal hypervariable region and carboxymethylated farnesyl moiety, as shown by FPOP. Subsequent interaction of the RAF1 RAS binding domain with KRAS does not significantly change G-domain configurations on the membrane but affects their relative populations. Overall, our results are consistent with a directional fly-casting mechanism for KRAS, in which the membrane-distal state of the G-domain can effectively recruit RAF kinase from the cytoplasm for activation at the membrane.

Site-Specific Labeling and 19F NMR Provide Direct Evidence for Dynamic Behavior of the Anthrax Toxin Pore ϕ-Clamp Structure.

The anthrax toxin protective antigen (PA), the membrane binding and pore-forming component of the anthrax toxin, was studied using 19F NMR. We site-specifically labeled PA with p-fluorophenylalanine (pF-Phe) at Phe427, a critically important residue that comprises the ϕ-clamp that is required for translocation of edema factor (EF) and lethal factor (LF) into the host cell cytosol. We utilized 19F NMR to follow low-pH-induced structural changes in the prepore, alone and bound to the N-terminal PA binding domain of LF, LFN. Our studies indicate that pF-Phe427 is dynamic in the prepore state and then becomes more dynamic in the transition to the pore. An increase in dynamic behavior at the ϕ-clamp may provide the necessary room for movement needed in translocating EF and LF into the cell cytosol.

Templated Collagen "Double Helices" Maintain Their Structure.

The self-assembly of collagen-mimetic peptides (CMPs) that form sticky-ended triple helices has allowed the production of surprisingly stable artificial collagen fibers and hydrogels. Assembly through sticky ends requires the recognition of a single strand by a templated strand dimer. Although CMPs and their triple helices have been studied extensively, the structure of a strand dimer is unknown. Here, we evaluate the physical characteristics of such dimers, using disulfide-templated (PPG)10 dimers as a model. Such "linked-dimers" retain their collagen-like structure even in the absence of a third strand, but only when their strands are capable of adopting a triple-helical fold. The intrinsic collagen-like structure of templated CMP pairs helps to explain the success of sticky-ended CMP association and changes the conception of new synthetic collagen designs.

I-PINE web server: an integrative probabilistic NMR assignment system for proteins.

Various methods for understanding the structural and dynamic properties of proteins rely on the analysis of their NMR chemical shifts. These methods require the initial assignment of NMR signals to particular atoms in the sequence of the protein, a step that can be very time-consuming. The probabilistic interaction network of evidence (PINE) algorithm for automated assignment of backbone and side chain chemical shifts utilizes a Bayesian probabilistic network model that analyzes sequence data and peak lists from multiple NMR experiments. PINE, which is one of the most popular and reliable automated chemical shift assignment algorithms, has been available to the protein NMR community for longer than a decade. We announce here a new web server version of PINE, called Integrative PINE (I-PINE), which supports more types of NMR experiments than PINE (including three-dimensional nuclear Overhauser enhancement and four-dimensional J-coupling experiments) along with more comprehensive visualization of chemical shift based analysis of protein structure and dynamics. The I-PINE server is freely accessible at http://i-pine.nmrfam.wisc.edu . Help pages and tutorial including browser capability are available at: http://i-pine.nmrfam.wisc.edu/instruction.html . Sample data that can be used for testing the web server are available at: http://i-pine.nmrfam.wisc.edu/examples.html .

Applications of Parametrized NMR Spin Systems of Small Molecules.

We have developed technology for producing accurate spectral fingerprints of small molecules through modeling of NMR spin system matrices to encapsulate their chemical shifts and scalar couplings. We describe here how libraries of these spin systems utilizing unique and reproducible atom numbering can be used to improve NMR-based ligand screening and metabolomics studies. We introduce new Web services that facilitate the analysis of NMR spectra of mixtures of small molecules to yield their identification and quantification. The library of parametrized compounds has been expanded to cover simulations of 1H NMR spectra at a variety of magnetic fields of more than 1100 compounds, included are many common metabolites and a library of drug-like molecular fragments used in ligand screening. The compound library and related Web services are freely available from http://gissmo.nmrfam.wisc.edu/ .

Spin System Modeling of Nuclear Magnetic Resonance Spectra for Applications in Metabolomics and Small Molecule Screening.

The exceptionally rich information content of nuclear magnetic resonance (NMR) spectra is routinely used to identify and characterize molecules and molecular interactions in a wide range of applications, including clinical biomarker discovery, drug discovery, environmental chemistry, and metabolomics. The set of peak positions and intensities from a reference NMR spectrum generally serves as the identifying signature for a compound. Reference spectra normally are collected under specific conditions of pH, temperature, and magnetic field strength, because changes in conditions can distort the identifying signatures of compounds. A spin system matrix that parametrizes chemical shifts and coupling constants among spins provides a much richer feature set for a compound than a spectral signature based on peak positions and intensities. Spin system matrices expand the applicability of NMR spectral libraries beyond the specific conditions under which data were collected. In addition to being able to simulate spectra at any field strength, spin parameters can be adjusted to systematically explore alterations in chemical shift patterns due to variations in other experimental conditions, such as compound concentration, pH, or temperature. We present methodology and software for efficient interactive optimization of spin parameters against experimental 1D-1H NMR spectra of small molecules. We have used the software to generate spin system matrices for a set of key mammalian metabolites and are also using the software to parametrize spectra of small molecules used in NMR-based ligand screening. The software, along with optimized spin system matrix data for a growing number of compounds, is available from http://gissmo.nmrfam.wisc.edu/ .

NMRmix: A Tool for the Optimization of Compound Mixtures in 1D (1)H NMR Ligand Affinity Screens.

NMR ligand affinity screening is a powerful technique that is routinely used in drug discovery or functional genomics to directly detect protein-ligand binding events. Binding events can be identified by monitoring differences in the 1D (1)H NMR spectrum of a compound with and without protein. Although a single NMR spectrum can be collected within a short period (2-10 min per sample), one-by-one screening of a protein against a library of hundreds or thousands of compounds requires a large amount of spectrometer time and a large quantity of protein. Therefore, compounds are usually evaluated in mixtures ranging in size from 3 to 20 compounds to improve the efficiency of these screens in both time and material. Ideally, the NMR signals from individual compounds in the mixture should not overlap so that spectral changes can be associated with a particular compound. We have developed a software tool, NMRmix, to assist in creating ideal mixtures from a large panel of compounds with known chemical shifts. Input to NMRmix consists of an (1)H NMR peak list for each compound, a user-defined overlap threshold, and additional user-defined parameters if default settings are not used. NMRmix utilizes a simulated annealing algorithm to optimize the composition of the mixtures to minimize spectral peak overlaps so that each compound in the mixture is represented by a maximum number of nonoverlapping chemical shifts. A built-in graphical user interface simplifies data import and visual evaluation of the results.

Probabilistic validation of protein NMR chemical shift assignments.

Data validation plays an important role in ensuring the reliability and reproducibility of studies. NMR investigations of the functional properties, dynamics, chemical kinetics, and structures of proteins depend critically on the correctness of chemical shift assignments. We present a novel probabilistic method named ARECA for validating chemical shift assignments that relies on the nuclear Overhauser effect data . ARECA has been evaluated through its application to 26 case studies and has been shown to be complementary to, and usually more reliable than, approaches based on chemical shift databases. ARECA is available online at http://areca.nmrfam.wisc.edu/.

Integrative NMR for biomolecular research.

NMR spectroscopy is a powerful technique for determining structural and functional features of biomolecules in physiological solution as well as for observing their intermolecular interactions in real-time. However, complex steps associated with its practice have made the approach daunting for non-specialists. We introduce an NMR platform that makes biomolecular NMR spectroscopy much more accessible by integrating tools, databases, web services, and video tutorials that can be launched by simple installation of NMRFAM software packages or using a cross-platform virtual machine that can be run on any standard laptop or desktop computer. The software package can be downloaded freely from the NMRFAM software download page ( http://pine.nmrfam.wisc.edu/download_packages.html ), and detailed instructions are available from the Integrative NMR Video Tutorial page ( http://pine.nmrfam.wisc.edu/integrative.html ).

The Complex Energy Landscape of the Protein IscU.

IscU, the scaffold protein for iron-sulfur (Fe-S) cluster biosynthesis in Escherichia coli, traverses a complex energy landscape during Fe-S cluster synthesis and transfer. Our previous studies showed that IscU populates two interconverting conformational states: one structured (S) and one largely disordered (D). Both states appear to be functionally important because proteins involved in the assembly or transfer of Fe-S clusters have been shown to interact preferentially with either the S or D state of IscU. To characterize the complex structure-energy landscape of IscU, we employed NMR spectroscopy, small-angle x-ray scattering (SAXS), and differential scanning calorimetry. Results obtained for IscU at pH 8.0 show that its S state is maximally populated at 25°C and that heating or cooling converts the protein toward the D state. Results from NMR and DSC indicate that both the heat- and cold-induced S→D transitions are cooperative and two-state. Low-resolution structural information from NMR and SAXS suggests that the structures of the cold-induced and heat-induced D states are similar. Both states exhibit similar (1)H-(15)N HSQC spectra and the same pattern of peptidyl-prolyl peptide bond configurations by NMR, and both appear to be similarly expanded compared with the S state based on analysis of SAXS data. Whereas in other proteins the cold-denatured states have been found to be slightly more compact than the heat-denatured states, these two states occupy similar volumes in IscU.

NMRFAM-SDF: a protein structure determination framework.

The computationally demanding nature of automated NMR structure determination necessitates a delicate balancing of factors that include the time complexity of data collection, the computational complexity of chemical shift assignments, and selection of proper optimization steps. During the past two decades the computational and algorithmic aspects of several discrete steps of the process have been addressed. Although no single comprehensive solution has emerged, the incorporation of a validation protocol has gained recognition as a necessary step for a robust automated approach. The need for validation becomes even more pronounced in cases of proteins with higher structural complexity, where potentially larger errors generated at each step can propagate and accumulate in the process of structure calculation, thereby significantly degrading the efficacy of any software framework. This paper introduces a complete framework for protein structure determination with NMR--from data acquisition to the structure determination. The aim is twofold: to simplify the structure determination process for non-NMR experts whenever feasible, while maintaining flexibility by providing a set of modules that validate each step, and to enable the assessment of error propagations. This framework, called NMRFAM-SDF (NMRFAM-Structure Determination Framework), and its various components are available for download from the NMRFAM website (http://nmrfam.wisc.edu/software.htm).

Protein conformational dynamics in the mechanism of HIV-1 protease catalysis.

We have used chemical protein synthesis and advanced physical methods to probe dynamics-function correlations for the HIV-1 protease, an enzyme that has received considerable attention as a target for the treatment of AIDS. Chemical synthesis was used to prepare a series of unique analogues of the HIV-1 protease in which the flexibility of the "flap" structures (residues 37-61 in each monomer of the homodimeric protein molecule) was systematically varied. These analogue enzymes were further studied by X-ray crystallography, NMR relaxation, and pulse-EPR methods, in conjunction with molecular dynamics simulations. We show that conformational isomerization in the flaps is correlated with structural reorganization of residues in the active site, and that it is preorganization of the active site that is a rate-limiting factor in catalysis.

Forazoline†A: marine-derived polyketide with antifungal in†vivo efficacy.

Forazoline A, a novel antifungal polyketide with in vivo efficacy against Candida albicans, was discovered using LCMS-based metabolomics to investigate marine-invertebrate-associated bacteria. Forazoline A had a highly unusual and unprecedented skeleton. Acquisition of (13)C-(13)C gCOSY and (13)C-(15)N HMQC NMR data provided the direct carbon-carbon and carbon-nitrogen connectivity, respectively. This approach represents the first example of determining direct (13)C-(15)N connectivity for a natural product. Using yeast chemical genomics, we propose that forazoline A operated through a new mechanism of action with a phenotypic outcome of disrupting membrane integrity.

Electron transfer mechanism of the Rieske protein from Thermus thermophilus from solution nuclear magnetic resonance investigations.

We report nuclear magnetic resonance (NMR) data indicating that the Rieske protein from the cytochrome bc complex of Thermus thermophilus (TtRp) undergoes modest redox-state-dependent and ligand-dependent conformational changes. To test models concerning the mechanism by which TtRp transfers between different sites on the complex, we monitored (1)H, (15)N, and (13)C NMR signals as a function of the redox state and molar ratio of added ligand. Our studies of full-length TtRp were conducted in the presence of dodecyl phosphocholine micelles to solvate the membrane anchor of the protein and the hydrophobic tail of the ligand (hydroubiquinone). NMR data indicated that hydroubiquinone binds to TtRp and stabilizes an altered protein conformation. We utilized a truncated form of the Rieske protein lacking the membrane anchor (trunc-TtRp) to investigate redox-state-dependent conformational changes. Local chemical shift perturbations suggested possible conformational changes at prolyl residues. Detailed investigations showed that all observable prolyl residues of oxidized trunc-TtRp have trans peptide bond configurations but that two of these peptide bonds (Cys151-Pro152 and Gly169-Pro170 located near the iron-sulfur cluster) become cis in the reduced protein. Changes in the chemical shifts of backbone signals provided evidence of redox-state- and ligand-dependent conformational changes localized near the iron-sulfur cluster. These structural changes may alter interactions between the Rieske protein and the cytochrome b and c sites and provide part of the driving force for movement of the Rieske protein between these two sites.

Structural characterization of Hsp12, the heat shock protein from Saccharomyces cerevisiae, in aqueous solution where it is intrinsically disordered and in detergent micelles where it is locally α-helical.

Hsp12 (heat shock protein 12) belongs to the small heat shock protein family, partially characterized as a stress response, stationary phase entry, late embryonic abundant-like protein located at the plasma membrane to protect membrane from desiccation. Here, we report the structural characterization of Hsp12 by NMR and biophysical techniques. The protein was labeled uniformly with nitrogen-15 and carbon-13 so that its conformation could be determined in detail both in aqueous solution and in two membrane-mimetic environments, SDS and dodecylphosphocholine (DPC) micelles. Secondary structural elements determined from assigned chemical shifts indicated that Hsp12 is dynamically disordered in aqueous solution, whereas it gains four helical stretches in the presence of SDS micelles and a single helix in presence of DPC. These conclusions were reinforced by circular dichroism spectra of the protein in all three environments. The lack of long range interactions in NOESY spectra indicated that the helices present in SDS micelles do not pack together. R(1) and R(2), relaxation and heteronuclear NOE measurements showed that the protein is disordered in aqueous solution but becomes more ordered in presence of detergent micelles. NMR spectra collected in presence of paramagnetic spin relaxation agents (5DSA, 16DSA, and Gd(DTPA-BMA)) indicated that the amphipathic α-helices of Hsp12 in SDS micelles lie on the membrane surface. These observations are in agreement with studies suggesting that Hsp12 functions to protect the membrane from desiccation.

DCAP: a broad-spectrum antibiotic that targets the cytoplasmic membrane of bacteria.

Persistent infections are frequently caused by dormant and biofilm-associated bacteria, which often display characteristically slow growth. Antibiotics that require rapid cell growth may be ineffective against these organisms and thus fail to prevent reoccurring infections. In contrast to growth-based antimicrobial agents, membrane-targeting drugs effectively kill slow-growing bacteria. Herein we introduce 2-((3-(3,6-dichloro-9H-carbazol-9-yl)-2-hydroxypropyl)amino)-2-(hydroxymethyl)propane-1,3-diol (DCAP), a potent broad-spectrum antibiotic that reduces the transmembrane potential of Gram-positive and Gram-negative bacteria and causes mislocalization of essential membrane-associated proteins, including MinD and FtsA. Importantly, DCAP kills nutrient-deprived microbes and sterilizes bacterial biofilms. DCAP is lethal against bacterial cells, has no effect on red blood cell membranes, and only decreases the viability of mammalian cells after ≥6 h. We conclude that membrane-active compounds are a promising solution for treating persistent infections. DCAP expands the limited number of compounds in this class of therapeutic small molecules and provides new opportunities for the development of potent broad-spectrum antimicrobial agents.

Accurate structure and dynamics of the metal-site of paramagnetic metalloproteins from NMR parameters using natural bond orbitals.

A natural bond orbital (NBO) analysis of unpaired electron spin density in metalloproteins is presented, which allows a fast and robust calculation of paramagnetic NMR parameters. Approximately 90% of the unpaired electron spin density occupies metal-ligand NBOs, allowing the majority of the density to be modeled by only a few NBOs that reflect the chemical bonding environment. We show that the paramagnetic relaxation rate of protons can be calculated accurately using only the metal-ligand NBOs and that these rates are in good agreement with corresponding rates measured experimentally. This holds, in particular, for protons of ligand residues where the point-dipole approximation breaks down. To describe the paramagnetic relaxation of heavy nuclei, also the electron spin density in the local orbitals must be taken into account. Geometric distance restraints for (15)N can be derived from the paramagnetic relaxation enhancement and the Fermi contact shift when local NBOs are included in the analysis. Thus, the NBO approach allows us to include experimental paramagnetic NMR parameters of (15)N nuclei as restraints in a structure optimization protocol. We performed a molecular dynamics simulation and structure determination of oxidized rubredoxin using the experimentally obtained paramagnetic NMR parameters of (15)N. The corresponding structures obtained are in good agreement with the crystal structure of rubredoxin. Thus, the NBO approach allows an accurate description of the geometric structure and the dynamics of metalloproteins, when NMR parameters are available of nuclei in the immediate vicinity of the metal-site.

PONDEROSA, an automated 3D-NOESY peak picking program, enables automated protein structure determination.

SUMMARY: PONDEROSA (Peak-picking Of Noe Data Enabled by Restriction of Shift Assignments) accepts input information consisting of a protein sequence, backbone and sidechain NMR resonance assignments, and 3D-NOESY ((13)C-edited and/or (15)N-edited) spectra, and returns assignments of NOESY crosspeaks, distance and angle constraints, and a reliable NMR structure represented by a family of conformers. PONDEROSA incorporates and integrates external software packages (TALOS+, STRIDE and CYANA) to carry out different steps in the structure determination. PONDEROSA implements internal functions that identify and validate NOESY peak assignments and assess the quality of the calculated three-dimensional structure of the protein. The robustness of the analysis results from PONDEROSA's hierarchical processing steps that involve iterative interaction among the internal and external modules. PONDEROSA supports a variety of input formats: SPARKY assignment table (.shifts) and spectrum file formats (.ucsf), XEASY proton file format (.prot), and NMR-STAR format (.star). To demonstrate the utility of PONDEROSA, we used the package to determine 3D structures of two proteins: human ubiquitin and Escherichia coli iron-sulfur scaffold protein variant IscU(D39A). The automatically generated structural constraints and ensembles of conformers were as good as or better than those determined previously by much less automated means. AVAILABILITY: The program, in the form of binary code along with tutorials and reference manuals, is available at http://ponderosa.nmrfam.wisc.edu/.

Efficient stable isotope labeling and purification of vitamin D receptor from inclusion bodies.

Vitamin D receptor (VDR) plays a crucial role in many cellular processes including calcium and phosphate homeostasis. Previous purification methods from prokaryotic and eukaryotic expression systems were challenged by low protein solubility accompanied by multi purification steps resulting in poor protein recovery. The full-length VDR and its ligand binding domain (LBD) were mostly (>90%) insoluble even when expressed at low temperatures in the bacterial system. We describe a one-step procedure that results in the purification of rat VDR and LBD proteins in high-yield from Escherichia coli inclusion bodies. The heterologously expressed protein constructs retained full function as demonstrated by ligand binding and DNA binding assays. Furthermore, we describe an efficient strategy for labeling these proteins with (2)H, (13)C, and (15)N for structural and functional studies by nuclear magnetic resonance (NMR) spectroscopy. This efficient production system will facilitate future studies on the mechanism of vitamin D action including characterization of the large number of synthetic vitamin D analogs that have been developed.