Solid-state NMR chemical shift analysis for determining the conformation of ATP bound to Na,K-ATPase in its native membrane

Structures of membrane proteins determined by X-ray crystallography and, increasingly, by cryo-electron microscopy often fail to resolve the structural details of unstable or reactive small molecular ligands in their physiological sites. This work demonstrates that 13C chemical shifts measured by magic-angle spinning (MAS) solid-state NMR (SSNMR) provide unique information on the conformation of a labile ligand in the physiological site of a functional protein in its native membrane, by exploiting freeze-trapping to stabilise the complex. We examine the ribose conformation of ATP in a high affinity complex with Na,K-ATPase (NKA), an enzyme that rapidly hydrolyses ATP to ADP and inorganic phosphate under physiological conditions. The 13C SSNMR spectrum of the frozen complex exhibits peaks from all ATP ribose carbon sites and some adenine base carbons. Comparison of experimental chemical shifts with density functional theory (DFT) calculations of ATP in different conformations and protein environments reveals that the ATP ribose ring adopts an C3'-endo (N) conformation when bound with high affinity to NKA in the E1Na state, in contrast to the C2'-endo (S) ribose conformations of ATP bound to the E2P state and AMPPCP in the E1 complex. Additional dipolar coupling-mediated measurements of H-C-C-H torsional angles are used to eliminate possible relative orientations of the ribose and adenine rings. The utilization of chemical shifts to determine membrane protein ligand conformations has been underexploited to date and here we demonstrate this approach to be a powerful tool for resolving the fine details of ligand-protein interactions.

Analysis of the orientation of cholesterol in high-density lipoprotein nanodiscs using solid-state NMR

Cholesterol is an essential component of eukaryotic cellular membranes that regulates the order and phase behaviour of dynamic lipid bilayers. Although cholesterol performs many vital physiological roles, hypercholesterolaemia and the accumulation of cholesterol in atherosclerotic plaques can increase the risk of coronary heart disease morbidity. The risk is mitigated by the transportation of cholesterol from peripheral tissue to the liver by high-density lipoprotein (HDL), 6-20 nm-diameter particles of lipid bilayers constrained by an annular belt of the protein apolipoprotein A-I (apoA-I). Information on the dynamics and orientation of cholesterol in HDL is pertinent to the essential role of HDL in cholesterol cycling. This work investigates whether the molecular orientation of cholesterol in HDL differs from that in the unconstrained lipid bilayers of multilamellar vesicles (MLVs). Solid-state NMR (ssNMR) measurements of dynamically-averaged ¹³C-¹³C and ¹³C-¹H dipolar couplings were used to determine the average orientation of triple ¹³C-labelled cholesterol in palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers in reconstituted HDL (rHDL) nanodiscs and in MLVs. Individual ¹³C-¹³C dipolar couplings were measured from [2,3,4-¹³C₃]cholesterol in a one-dimensional NMR experiment, by using a novel application of a method to excite double quantum coherence at rotational resonance. The measured dipolar couplings were compared with average values calculated from orientational distributions of cholesterol generated using a Gaussian probability density function. The data were consistent with small differences in the average orientation of cholesterol in rHDL and MLVs, which may reflect the effects of the constrained and unconstrained lipid bilayers in the two environments. The calculated distributions of cholesterol in rHDL and MLVs that were consistent with the NMR data also agreed well with orientational distributions extracted from previous molecular dynamics simulations of HDL nanodiscs and planar POPC bilayers.

19 F Solid-State NMR and Vibrational Raman Characterization of Corticosteroid Drug-Lipid Membrane Interactions

Drug interactions with phospholipid bilayers underpin their behaviour in cell membranes and in liposomal delivery formulations. Liposomal drug delivery in ocular medicine can overcome the physical barriers of the eye and better enable the active molecule to reach its target. Here, Raman and ¹⁹F solid‐state NMR spectroscopy are used to characterise the interactions of two ocular corticosteroid drugs, difluprednate (DFP) and fluorometholone (FML), with multilamellar vesicles of phosphatidylcholine (PC). ³¹P NMR confirms that the lipid bilayer tolerates a high drug concentration (a drug: lipid molar ratio of 1 : 10). The ¹⁹F NMR spectra of the drugs in lipid bilayers reveal that FML and DFP have different average orientations within the lipid bilayer. Raman spectra of dried lipid films reveal that PC separates from DFP but not from FML, the less lipophilic of the two drugs. This combined approach will assist the design of, and inform the development of, improved liposomal preparations.

Aggregation Kinetics and Filament Structure of a Tau Fragment Are Influenced by the Sulfation Pattern of the Cofactor Heparin

A pathological signature of Alzheimer's disease (AD) is the formation of neurofibrillary tangles comprising filamentous aggregates of the microtubule associated protein tau. Tau self-assembly is accelerated by polyanions including heparin, an analogue of heparan sulfate. Tau filaments colocalize with heparan sulfate proteoglycans (HSPGs) in vivo, and HSPGs may also assist the transcellular propagation of tau aggregates. Here, we investigate the role of the sulfate moieties of heparin in the aggregation of a recombinant tau fragment Δtau187, comprising residues 255-441 of the C-terminal microtubule-binding domain. The effects that the selective removal of the N-, 2-O-, and 6-O-sulfate groups from heparin have on the kinetics of tau aggregation, aggregate morphology, and protein structure and dynamics were examined. Aggregation kinetics monitored by thioflavin T (ThT) fluorescence revealed that aggregation is considerably slower in the presence of 2-O-desulfated heparin than with N- or 6-O-desulfated heparin. Transmission electron microscopy revealed that tau filaments induced by 2-O-desulfated heparin were more slender than filaments formed in the presence of intact heparin or 6-O-desulfated heparin. The 2-O-desulfated heparin-induced filaments had more extensive regions of flexibility than the other filaments, according to circular dichroism and solid-state NMR spectroscopy. These results indicate that the sulfation pattern of heparin regulates tau aggregation, not purely though electrostatic forces but also through conformational perturbations of heparin when the 2-O-sulfate is removed. These findings may have implications for the progression of AD, as the sulfation pattern of GAGs is known to change during the aging process, which is the main risk factor for the disease.

Raman Spectroscopy with 2D Perturbation Correlation Moving Windows for the Characterization of Heparin-Amyloid Interactions

It has been shown extensively that glycosaminoglycan (GAG)-protein interactions can induce, accelerate, and impede the clearance of amyloid fibrils associated with systemic and localized amyloidosis. Obtaining molecular details of these interactions is fundamental to our understanding of amyloid disease. Consequently, there is a need for analytical approaches that can identify protein conformational transitions and simultaneously characterize heparin interactions. By combining Raman spectroscopy with two-dimensional (2D) perturbation correlation moving window (2DPCMW) analysis, we have successfully identified changes in protein secondary structure during pH- and heparin-induced fibril formation of apolipoprotein A-I (apoA-I) associated with atherosclerosis. Furthermore, from the 2DPCMW, we have identified peak shifts and intensity variations in Raman peaks arising from different heparan sulfate moieties, indicating that protein-heparin interactions vary at different heparin concentrations. Raman spectroscopy thus reveals new mechanistic insights into the role of GAGs during amyloid fibril formation.

Sensitive Morphological Characterization of Oriented High-Density Lipoprotein Nanoparticles Using 31 P NMR Spectroscopy

The biological function of high-density lipoprotein (HDL) nanoparticles, the so-called good cholesterol that is associated with a low risk of heart disease, depends on their composition, morphology, and size. The morphology of HDL particles composed of apolipoproteins, lipids and cholesterol is routinely visualised by transmission electron microscopy (TEM), but higher-resolution tools are needed to observe more subtle structural differences between particles of different composition. Here, reconstituted HDL formulations are oriented on glass substrates and solid-state 31 P NMR spectroscopy is shown to be highly sensitive to the surface curvature of the lipid headgroups. The spectra report potentially functionally important differences in the morphology of different HDL preparations that are not detected by TEM. This method provides new morphological insights into HDL comprising a naturally occurring apolipoprotein A-I mutant, which may be linked to its atheroprotective properties, and holds promise as a future research tool in the clinical analysis of plasma HDL.

Circular Dichroism Spectroscopy Identifies the β-Adrenoceptor Agonist Salbutamol As a Direct Inhibitor of Tau Filament Formation in Vitro

Potential drug treatments for Alzheimer's disease (AD) may be found by identifying compounds that block the assembly of the microtubule-associated protein tau into neurofibrillar tangles associated with neuron destabilization and cell death. Here, a small library of structurally diverse compounds was screened in vitro for the ability to inhibit tau aggregation, using high-throughput synchrotron radiation circular dichroism as a novel tool to monitor the structural changes in the protein as it assembles into filaments. The catecholamine epinephrine was found to be the most effective tau aggregation inhibitor of all 88 screened compounds. Subsequently, we tested chemically similar phenolamine drugs from the β-adrenergic receptor agonist class, using conventional circular dichroism spectroscopy, thioflavin T fluorescence, and transmission electron microscopy. Two compounds, salbutamol and dobutamine, used widely in the treatment of respiratory and cardiovascular disease, impede the aggregation of tau in vitro. Dobutamine reduces both the rate and yield of tau filament formation over 24 h; however, it has little effect on the structural transition of tau into β-sheet structures over 24 h. Salbutamol also reduces the yield and rate of filament formation and additionally inhibits tau's structural change into β-sheet-rich aggregates. Salbutamol has a good safety profile and a half-life that facilitates permeation through the blood-brain barrier and could represent an expediated approach to developing AD therapeutics. These results provide the motivation for the in vivo evaluation of pre-existing β-adrenergic receptor agonists as a potential therapy for AD through the reduction of tau deposition.

Drug orientations within statin-loaded lipoprotein nanoparticles by 19F solid-state NMR

NMR measurements of 19F chemical shift anisotropy and 1H-19F dipolar couplings provide unprecedented information on the molecular orientations of two fluorine-containing statin drugs within the heterogeneous environment of reconstituted high-density lipoprotein (rHDL) nanoparticles, a drug delivery system under clinical investigation.

Orientation of a Diagnostic Ligand Bound to Macroscopically Aligned Amyloid-β Fibrils Determined by Solid-State NMR

With amyloid diseases poised to become a major health burden in countries with aging populations, diagnostic molecules that aid the detection of amyloid in vitro and in vivo are of considerable clinical value. Understanding how such ligands recognize their amyloid targets would help to design diagnostics that target specific amyloid types associated with a particular disease, but methods to provide comprehensive information are underdeveloped. Here, solid-state NMR is used to determine the molecular orientation of the amyloid diagnostic 1-fluoro-2,5-bis[( E)-3-carboxy-4-hydroxystyryl]-benzene (FSB) when bound to fibrils of the Alzheimer's amyloid-β polypeptide aligned on a planar substrate. The 19F NMR spectrum of the aligned complex reveals that FSB is oriented approximately parallel with the fibril long axis and bridges four hydrogen-bonded β-sheets. In addition to providing atomic details to aid the design of amyloid-specific diagnostics, this approach will also illuminate the molecular mechanisms of accessory molecules in amyloid disease.

Average orientation of a fluoroaromatic molecule in lipid bilayers from DFT-informed NMR measurements of 1H-19F dipolar couplings

Fluorine is often incorporated into the aromatic moieties of synthetic bioactive molecules such as pharmaceuticals and disease diagnostics in order to alter their physicochemical properties. Fluorine substitution may increase a molecule's lipophilicity, thereby enabling its diffusion across cell membranes to enhance bioavailability or to exert a direct physiological effect from within the lipid bilayer. Understanding the structure, dynamics and orientation of fluoroaromatic molecules in lipid bilayers can provide useful insight into the effect of fluorine on their mode of action, and their interactions with membrane-embedded targets or efflux proteins. Here we demonstrate that NMR measurements of 19F chemical shift anisotropy combined with 1H-19F dipolar coupling measurements together report on the average orientation of a lipophilic fluoroaromatic molecule, 4-(6-fluorobenzo[d]thiazol-2-yl)aniline (FBTA), rapidly rotating within a lipid bilayer. The 19F chemical shift tensor orientation in the molecular frame was calculated by density functional theory and corroborated by 1H-19F PISEMA NMR. It was then possible to analyse the line shapes of proton-coupled and proton-decoupled 19F spectra of FBTA in chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) bilayers to restrict the average axis of molecular reorientation of FBTA in the bilayer to a limited range orientations. This approach, which exploits the high sensitivity and gyromagnetic ratios of 19F and 1H, will be useful for comparing the membrane properties of related bioactive fluoroaromatic compounds.

Epigallocatechin-3-gallate remodels apolipoprotein A-I amyloid fibrils into soluble oligomers in the presence of heparin

Amyloid deposits of WT apolipoprotein A-I (apoA-I), the main protein component of high-density lipoprotein, accumulate in atherosclerotic plaques where they may contribute to coronary artery disease by increasing plaque burden and instability. Using CD analysis, solid-state NMR spectroscopy, and transmission EM, we report here a surprising cooperative effect of heparin and the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG), a known inhibitor and modulator of amyloid formation, on apoA-I fibrils. We found that heparin, a proxy for glycosaminoglycan (GAG) polysaccharides that co-localize ubiquitously with amyloid in vivo, accelerates the rate of apoA-I formation from monomeric protein and associates with insoluble fibrils. Mature, insoluble apoA-I fibrils bound EGCG (K_D = 30 ± 3 μm; B_max = 40 ± 3 μm), but EGCG did not alter the kinetics of apoA-I amyloid assembly from monomer in the presence or absence of heparin. EGCG selectively increased the mobility of specific backbone and side-chain sites of apoA-I fibrils formed in the absence of heparin, but the fibrils largely retained their original morphology and remained insoluble. By contrast, fibrils formed in the presence of heparin were mobilized extensively by the addition of equimolar EGCG, and the fibrils were remodeled into soluble 20-nm-diameter oligomers with a largely α-helical structure that were nontoxic to human umbilical artery endothelial cells. These results argue for a protective effect of EGCG on apoA-I amyloid associated with atherosclerosis and suggest that EGCG-induced remodeling of amyloid may be tightly regulated by GAGs and other amyloid co-factors in vivo, depending on EGCG bioavailability.

Molecular Origins of the Compatibility between Glycosaminoglycans and Aβ40 Amyloid Fibrils

The Aβ peptide forms extracellular plaques associated with Alzheimer's disease. In addition to protein fibrils, amyloid plaques also contain non-proteinaceous components, including glycosaminoglycans (GAGs). We have shown previously that the GAG low-molecular-weight heparin (LMWH) binds to Aβ40 fibrils with a three-fold-symmetric (3Q) morphology with higher affinity than Aβ40 fibrils in alternative structures, Aβ42 fibrils, or amyloid fibrils formed from other sequences. Solid-state NMR analysis of the GAG-3Q fibril complex revealed an interaction site at the corners of the 3Q fibril structure, but the origin of the binding specificity remained obscure. Here, using a library of short heparin polysaccharides modified at specific sites, we show that the N-sulfate or 6-O-sulfate of glucosamine, but not the 2-O-sulfate of iduronate within heparin is required for 3Q binding, indicating selectivity in the interactions of the GAG with the fibril that extends beyond general electrostatic complementarity. By creating 3Q fibrils containing point substitutions in the amino acid sequence, we also show that charged residues at the fibril three-fold apices provide the majority of the binding free energy, while charged residues elsewhere are less critical for binding. The results indicate, therefore, that LMWH binding to 3Q fibrils requires a precise molecular complementarity of the sulfate moieties on the GAG and charged residues displayed on the fibril surface. Differences in GAG binding to fibrils with distinct sequence and/or structure may thus contribute to the diverse etiology and progression of amyloid diseases.

Novel stereoselective bufadienolides reveal new insights into the requirements for Na(+), K(+)-ATPase inhibition by cardiotonic steroids

Cardiotonic steroids (CTS) are clinically important drugs for the treatment of heart failure owing to their potent inhibition of cardiac Na(+), K(+)-ATPase (NKA). Bufadienolides constitute one of the two major classes of CTS, but little is known about how they interact with NKA. We report a remarkable stereoselectivity of NKA inhibition by native 3β-hydroxy bufalin over the 3α-isomer, yet replacing the 3β-hydroxy group with larger polar groups in the same configuration enhances inhibitory potency. Binding of the two (13)C-labelled glycosyl diastereomers to NKA were studied by solid-state NMR (SSNMR), which revealed interactions of the glucose group of the 3β- derivative with the inhibitory site, but much weaker interactions of the 3α- derivative with the enzyme. Molecular docking simulations suggest that the polar 3β-groups are closer to the hydrophilic amino acid residues in the entrance of the ligand-binding pocket than those with α-configuration. These first insights into the stereoselective inhibition of NKA by bufadienolides highlight the important role of the hydrophilic moieties at C3 for binding, and may explain why only 3β-hydroxylated bufadienolides are present as a toxic chemical defence in toad venom.

Long-Range Effects of Na(+) Binding in Na,K-ATPase Reported by ATP

This paper addresses the question of long-range interactions between the intramembranous cation binding sites and the cytoplasmic nucleotide binding site of the ubiquitous ion-transporting Na,K-ATPase using (13)C cross-polarization magic-angle spinning (CP-MAS) solid-state nuclear magnetic resonance. High-affinity ATP binding is induced by the presence of Na(+) as well as of Na-like substances such as Tris(+), and these ions are equally efficient promoters of nucleotide binding. CP-MAS analysis of bound ATP with Na,K-ATPase purified from pig kidney membranes reveals subtle differences in the nucleotide interactions within the nucleotide site depending on whether Na(+) or Tris(+) is used to induce binding. Differences in chemical shifts for ATP atoms C1' and C5' observed in the presence of Na(+) or Tris(+) suggest alterations in the residues surrounding the bound nucleotide, hydrogen bonding, and/or conformation of the ribose ring. This is taken as evidence of a long-distance communication between the Na(+)-filled ion sites in the membrane interior and the nucleotide binding site in the cytoplasmic domain and reflects the first conformational change ultimately leading to phosphorylation of the enzyme. Stopped-flow fluorescence measurements with the nucleotide analogue eosin show that the dissociation rate constant for eosin is larger in Tris(+) than in Na(+), giving kinetic evidence of the difference in structural effects of Na(+) and Tris(+). According to the recent crystal structure of the E1·AlF4(-)·ADP·3Na(+) form, the coupling between the ion binding sites and the nucleotide side is mediated by, among others, the M5 helix.

Comparisons with amyloid-β reveal an aspartate residue that stabilizes fibrils of the aortic amyloid peptide medin

Aortic medial amyloid (AMA) is the most common localized human amyloid, occurring in virtually all of the Caucasian population over the age of 50. The main protein component of AMA, medin, readily assembles into amyloid-like fibrils in vitro. Despite the prevalence of AMA, little is known about the self-assembly mechanism of medin or the molecular architecture of the fibrils. The amino acid sequence of medin is strikingly similar to the sequence of the Alzheimer disease (AD) amyloid-β (Aβ) polypeptides around the structural turn region of Aβ, where mutations associated with familial, early onset AD, have been identified. Asp(25) and Lys(30) of medin align with residues Asp(23) and Lys(28) of Aβ, which are known to form a stabilizing salt bridge in some fibril morphologies. Here we show that substituting Asp(25) of medin with asparagine (D25N) impedes assembly into fibrils and stabilizes non-cytotoxic oligomers. Wild-type medin, by contrast, aggregates into β-sheet-rich amyloid-like fibrils within 50 h. A structural analysis of wild-type fibrils by solid-state NMR suggests a molecular repeat unit comprising at least two extended β-strands, separated by a turn stabilized by a Asp(25)-Lys(30) salt bridge. We propose that Asp(25) drives the assembly of medin by stabilizing the fibrillar conformation of the peptide and is thus reminiscent of the influence of Asp(23) on the aggregation of Aβ. Pharmacological comparisons of wild-type medin and D25N will help to ascertain the pathological significance of this poorly understood protein.

Ligand orientation in a membrane-embedded receptor site revealed by solid-state NMR with paramagnetic relaxation enhancement

Paramagnetic relaxation-enhanced solid-state NMR reveals a ouabain analogue with an inverted orientation in the Na,K-ATPase inhibitory site.

Comparison of the structure and function of phospholamban and the arginine-14 deficient mutant associated with dilated cardiomyopathy

Phospholamban (PLB) is a pentameric protein that plays an important role in regulating cardiac contractility via a reversible inhibitory association with the sarcoplasmic reticulum Ca²⁺ATPase (SERCA), the enzyme responsible for maintaining correct calcium homeostasis. Here we study the functional and biophysical characteristics of a PLB mutant associated with human dilated cardiomyopathy (DCM), with a deletion of arginine at position 14 (PLBR14Δ). In agreement with recent findings, we find that PLBR14Δ has a reduced inhibitory effect on SERCA compared to wild type PLB (PLBWT) when reconstituted into lipid membranes. The mutation also leads to a large reduction in the protein kinase A-catalysed phosphorylation of Ser-16 in the cytoplasmic domain of PLBR14Δ. Measurements on SERCA co-reconstituted with an equimolar mixture of PLBWT and PLBR14Δ (representing the lethal heterozygous state associated with DCM) indicates that the loss-of-function mutation has a dominant effect on PLBWT functionality and phosphorylation capacity, suggesting that mixed PLBWT/PLBR14Δ pentamers are formed that have characteristics typical of the mutant protein. Structural and biophysical analysis of PLBR14Δ indicates that the mutation perturbs slightly the helical structure of the PLB cytoplasmic domain and reduces its affinity for the phospholipid bilayer surface, thereby altering the orientation of the cytoplasmic domain relative to the wild-type protein. These results indicate that the structure and function consequences of the R14 deletion have profound effects on the regulation of SERCA which may contribute to the aetiology of DCM.

Synthesis of bufalin derivatives with inhibitory activity against prostate cancer cells

Bufalin possesses a strong anti-cancer effect, but the cardiac toxicity targeting the Na(+), K(+)-ATPase limits its application. Here, two bufalin derivatives, bufadienolactam (1) and secobufalinamide (2), were synthesised by treating bufalin with ammonium acetate in dimethylformamide solution. Their structures were elucidated by extensive spectroscopic methods. The structure of compound 2 was further confirmed by single-crystal X-ray diffraction analysis. Compounds 1 and 2 expressed strong inhibitory activities against androgen-dependent prostate cancer cells (IC₅₀ values about 10 μM), but only weak inhibition on Na(+), K(+)-ATPase (Ki about 70 μM), indicating that they might be potential anti-prostate cancer agents without severe cardiac toxicity.

Structures, chemotaxonomic significance, cytotoxic and Na+,K+-ATPase inhibitory activities of new cardenolides from Asclepias curassavica

New cardenolides with potent cytotoxic and Na⁺,K⁺-ATPase inhibitory activities were isolated from the ornamental milkweed Asclepias curassavica.

Heparin promotes the rapid fibrillization of a peptide with low intrinsic amyloidogenicity

Amyloid deposits in vivo are complex mixtures composed of protein fibrils and nonfibrillar components, including polysaccharides of the glycosaminoglycan (GAG) class. It has been widely documented that GAGs influence the initiation and progress of self-assembly by several disease-associated amyloidogenic proteins and peptides in vitro. Here we investigated whether the GAG heparin can serve as a cofactor to induce amyloid-like fibril formation in a peptide predicted to have a weak propensity to aggregate and not associated with amyloid disorders. We selected the 23-residue peptide PLB(1-23), which corresponds to the acetylated cytoplasmic domain of the phospholamban transmembrane protein. PLB(1-23) remains unfolded in aqueous solution for >24 h and does not bind thioflavin T over this time period, in agreement with computer predictions that the peptide has a very low intrinsic amyloidogenicity. In the presence of low-molecular mass (5 kDa) heparin, which binds PLB(1-23) with micromolar affinity, the peptide undergoes spontaneous and rapid assembly into amyloid-like fibrils, the effect being more pronounced at pH 5.5 than at pH 7.4. At the lower pH, peptide aggregation is accompanied by a transition to a β-sheet rich structure. These results are consistent with the polyanionic heparin serving as a scaffold to enhance aggregation by aligning the peptide molecules in the correct orientation and with the appropriate periodicity. PLB(1-23) is toxic to cells when added in isolation, and promotion of fibril formation by heparin can reduce the toxicity of this peptide, consistent with the notion that amyloid-like fibrils represent a benign end stage of fibrillization. This work provides insight into the role that heparin and other glycosaminoglycans may play in amyloid formation and provides therapeutic avenues targeting the reduction of cytotoxicity of species along the amyloid formation pathway.

Insights into the molecular architecture of a peptide nanotube using FTIR and solid-state NMR spectroscopic measurements on an aligned sample

Queuing up: Molecular orientation within macroscopically aligned nanotubes of the peptide AAAAAAK can be studied by solid-state NMR and IR spectroscopy. Line shape analysis of the NMR spectra indicates that the peptide N-H bonds are tilted 65-70° relative to the nanotube long axis. Re-evaluation of earlier X-ray fiber diffraction data suggests that the peptide molecules are hydrogen-bonded in a helical arrangement along the nanotube axis.

Amyloid-derived peptide forms self-assembled monolayers on gold nanoparticle with a curvature-dependent β-sheet structure

Using a combination of Fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance (SSNMR) techniques, the secondary structure of peptides anchored on gold nanoparticles of different sizes is investigated. The structure of the well-studied CALNN-capped nanoparticles is compared to the structure of nanoparticles capped with a new cysteine-terminated peptide, CFGAILSS. The design of that peptide is derived from the minimal amyloidogenic sequence FGAIL of the human islet polypeptide amylin. We demonstrate that CFGAILSS forms extended fibrils in solution. When constrained at a nanoparticle surface, CFGAILSS adopts a secondary structure markedly different from CALNN. Taking into account the surface selection rules, the FTIR spectra of CFGAILSS-capped gold nanoparticles indicate the formation of β-sheets which are more prominent for 25 nm diameter nanoparticles than for 5 nm nanoparticles. No intermolecular (13)C-(13)C dipolar coupling is detected with rotational resonance SSNMR for CALNN-capped nanoparticles, while CALNN is in a random coil configuration. Coupling is detected for CFGAILSS-capped gold nanoparticles, however, consistent with an intermolecular (13)C-(13)C distance of 5.0 ± 0.3 Å, in agreement with intermolecular hydrogen bonding in a parallel β-sheet structure.

Solid-state NMR reveals differences in the packing arrangements of peptide aggregates derived from the aortic amyloid polypeptide medin

Several polypeptides aggregate into insoluble amyloid fibrils associated with pathologies such as Alzheimer's disease, Parkinson's disease and type 2 diabetes. Understanding the structural and sequential motifs that drive fibrillisation may assist in the discovery and refinement of effective therapies. Here we investigate the effects of three predicted amyloidogenic regions on the structure of aggregates formed by medin, a poorly characterised polypeptide associated with aortic medial amyloidosis. Solid-state NMR is used to compare the dynamics and sheet packing arrangement of the C-terminal region encompassing residues F(43) GSV within full-length medin (Med(1-50) ) and two shorter peptide fragments, Med(30-50) and Med(42-49) , lacking specific sequences predicted to be amyloidogenic.(.) Results show that all three peptides have different aggregate morphologies, and Med(30-50) and Med(1-50) have different sheet packing arrangements and dynamics to Med(42-49) . These results imply that at least two of the three predicted amyloidogenic regions are required for the formation and elongation of medin fibres observed in the disease state.

Solid-state NMR detection of 14N-13C dipolar couplings between amino acid side groups provides constraints on amyloid fibril architecture

Solid-state nuclear magnetic resonance (SSNMR) is a powerful technique for the structural analysis of amyloid fibrils. With suitable isotope labelling patterns, SSNMR can provide constraints on the secondary structure, alignment and registration of β-strands within amyloid fibrils and identify the tertiary and quaternary contacts defining the packing of the β-sheet layers. Detection of (14)N-(13)C dipolar couplings may provide potentially useful additional structural constraints on β-sheet packing within amyloid fibrils but has not until now been exploited for this purpose. Here a frequency-selective, transfer of population in double resonance SSNMR experiment is used to detect a weak (14)N-(13)C dipolar coupling in amyloid-like fibrils of the peptide H(2)N-SNNFGAILSS-COOH, which was uniformly (13)C and (15)N labelled across the four C-terminal amino acids. The (14)N-(13)C interatomic distance between leucine and asparagine side groups is constrained between 2.4 and 3.8 Å, which allows current structural models of the β-spine arrangement within the fibrils to be refined. This procedure could be useful for the general structural analysis of other proteins in condensed phases and environments, such as biological membranes.

A study of the membrane association and regulatory effect of the phospholemman cytoplasmic domain

Phospholemman (PLM) is a single-span transmembrane protein belonging to the FXYD family of proteins. PLM (or FXYD1) regulates the Na,K-ATPase (NKA) ion pump by altering its affinity for K(+) and Na(+) and by reducing its hydrolytic activity. Structural studies of PLM in anionic detergent micelles have suggested that the cytoplasmic domain, which alone can regulate NKA, forms a partial helix which is stabilized by interactions with the charged membrane surface. This work examines the membrane affinity and regulatory function of a 35-amino acid peptide (PLM(38-72)) representing the PLM cytoplasmic domain. Isothermal titration calorimetry and solid-state NMR measurements confirm that PLM(38-72) associates strongly with highly anionic phospholipid membranes, but the association is weakened substantially when the negative surface charge is reduced to a more physiologically relevant environment. Membrane interactions are also weakened when the peptide is phosphorylated at S68, one of the substrate sites for protein kinases. PLM(38-72) also lowers the maximal velocity of ATP hydrolysis (V(max)) by NKA, and phosphorylation of the peptide at S68 gives rise to a partial recovery of V(max). These results suggest that the PLM cytoplasmic domain populates NKA-associated and membrane-associated states in dynamic equilibrium and that phosphorylation may alter the position of the equilibrium. Interestingly, peptides representing the cytoplasmic domains of two other FXYD proteins, Mat-8 (FXYD3) and CHIF (FXYD4), have little or no interaction with highly anionic phospholipid membranes and have no effect on NKA function. This suggests that the functional and physical properties of PLM are not conserved across the entire FXYD family.

Structural analysis of uniformly (13)C-labelled solids from selective angle measurements at rotational resonance

We demonstrate that individual H-C-C-H torsional angles in uniformly labelled organic solids can be estimated by selective excitation of (13)C double-quantum coherences under magic-angle spinning at rotational resonance. By adapting a straightforward one-dimensional experiment described earlier [T. Karlsson, M. Eden, H. Luhman, M.H. Levitt, J. Magn. Reson. 145 (2000) 95-107], a double-quantum filtered spectrum selective for Calpha and Cbeta of uniformly labelled L-[(13)C,(15)N]valine is obtained with 25% efficiency. The evolution of Calpha-Cbeta double-quantum coherence under the influence of the dipolar fields of bonded protons is monitored to provide a value of the Halpha-Calpha-Cbeta-Hbeta torsional angle that is consistent with the crystal structure. In addition, double-quantum filtration selective for C6 and C1' of uniformly labelled [(13)C,(15)N]uridine is achieved with 12% efficiency for a (13)C-(13)C distance of 2.5A, yielding a reliable estimate of the C6-H and C1'-H projection angle defining the relative orientations of the nucleoside pyrimidine and ribose rings. This procedure will be useful, in favourable cases, for structural analysis of fully labelled small molecules such as receptor ligands that are not readily synthesised with labels placed selectively at structurally diagnostic sites.

Solid-state NMR measurements of the kinetics of the interaction between phospholamban and Ca2 + -ATPase in lipid bilayers

Phospholamban (PLB) is a small transmembrane protein that regulates calcium transport across the sarcoplasmic reticulum (SR) of cardiac cells via a reversible inhibitory interaction with Ca2+-ATPase. In this work solid-state NMR methods have been used to investigate the dynamics of the inhibitory association between PLB and Ca2+-ATPase. Skeletal muscle Ca2+-ATPase was incorporated into phosphatidylcholine membranes together with a ten-fold excess of a null-cysteine mutant of PLB labelled with 13C at Leu-44 in the transmembrane domain ([alpha-13C-L44]AAA-PLB). In these membranes the PLB variant was found to partially inhibit Ca2+-ATPase by reducing the affinity of the enzyme for calcium. Cross-polarization magic angle spinning (CP-MAS) 13C NMR spectra of the membranes exhibited a signature peak from [alpha-13C-L44]AAA-PLB at 56 ppm. Changes in the intensity of the peak were observed at different temperatures, which was diagnostic of direct interaction between [alpha-13C-L44]AAA-PLB and Ca2+-ATPase. Measurements of dipolar couplings between the 13C label and neighbouring protons were analysed to show that the mean residency time for the association of AAA-PLB with Ca2+-ATPase was on the order of 2.5 ms at temperatures between 0 degrees C and 30 degrees C. This new NMR approach will be useful for examining how the association of the two proteins is affected by physiological stimuli such as kinases and the elevation of calcium concentration.

Relative substrate affinities of wild-type and mutant forms of the Escherichia coli sugar transporter GalP determined by solid-state NMR

Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is used for the first time to examine the relative substrate-binding affinities of mutant forms of the Escherichia coli sugar transporter GalP in membrane preparations. The SSNMR method of (13)C cross-polarization magic-angle spinning (CP-MAS) is applied to five site-specific mutants (W56F, W239F, R316W, T336Y and W434F), which have a range of different sugar-transport activities compared to the wild-type protein. It is shown that binding of the substrate D-glucose can be detected independently of sugar transport activity using SSNMR, and that the NMR peak intensities for uniformly (13)C-labelled glucose are consistent with wild-type GalP and the mutants having different affinities for the substrate. The W239F and W434F mutants showed binding affinities similar to that of the wild-type protein, whereas the affinity of glucose-binding to the W56F mutant was reduced. The R316W mutant showed no detectable binding; this position corresponds to the second basic residue in the highly conserved (R/K)XGR(R/K) motif in the major facilitator superfamily of transport proteins and to a mutation in human GLUT1 found in individuals with GLUT1-deficiency syndrome. The T336Y mutant also showed no detectable binding; this mutation is likely to have perturbed helix structure or packing to an extent that conformational changes in the protein are hindered. The effects of the mutations on substrate-binding are discussed with reference to the putative positions of the residues in a 3D homology model of GalP based on the X-ray crystal structure of the E. coli glycerol-3-phosphate transporter GlpT.

Cytoplasmic residues of phospholamban interact with membrane surfaces in the presence of SERCA: a new role for phospholipids in the regulation of cardiac calcium cycling?

The 52-amino acid transmembrane protein phospholamban (PLB) regulates calcium cycling in cardiac cells by forming a complex with the sarco(endo)plasmic reticulum calcium ATPase (SERCA) and reversibly diminishing the rate of calcium uptake by the sarcoplasmic reticulum. The N-terminal cytoplasmic domain of PLB interacts with the cytoplasmic domain of SERCA, but, in the absence of the enzyme, can also associate with the surface of anionic phospholipid membranes. This work investigates whether the cytoplasmic domain of PLB can also associate with membrane surfaces in the presence of SERCA, and whether such interactions could influence the regulation of the enzyme. It is shown using solid-state NMR and isothermal titration calorimetry (ITC) that an N-terminally acetylated peptide representing the first 23 N-terminal amino acids of PLB (PLB1-23) interacts with membranes composed of zwitterionic phosphatidylcholine (PC) and anionic phosphatidylglycerol (PG) lipids in the absence and presence of SERCA. Functional measurements of SERCA in sarcoplasmic reticulum (SR) vesicles, planar SR membranes and reconstituted into PC/PG membranes indicate that PLB1-23 lowers the maximal rate of ATP hydrolysis by acting at the cytoplasmic face of the enzyme. A small, but statistically significant, reduction in the inhibitory effect of the peptide is observed for SERCA reconstituted into PC/PG membranes compared to SERCA in membranes of PC alone. It is suggested that interactions between the cytoplasmic domain of PLB and negatively charged phospholipids might play a role in moderating the regulation of SERCA, with implications for cardiac muscle contractility.

Design of an N-methylated peptide inhibitor of alpha-synuclein aggregation guided by solid-state NMR

Many neurodegenerative diseases are associated with the aggregation of misfolded proteins into amyloid oligomers or fibrils that are deposited as pathological lesions within areas of the brain. An attractive therapeutic strategy for preventing or ameliorating amyloid formation is to identify agents that inhibit the onset or propagation of protein aggregation. Here we demonstrate how solid-state nuclear magnetic resonance (ssNMR) may be used to identify key residues within amyloidogenic protein sequences that may be targeted to inhibit the aggregation of the host protein. For alpha-synuclein, the major protein component of Lewy bodies associated with Parkinson's disease, we have used a combination of ssNMR and biochemical data to identify the key region for self-aggregation of the protein as residues 77-82 (VAQKTV). We used our new structural information to design a peptide derived from residues 77 to 82 of alpha-synuclein with an N-methyl group at the C-terminal residue, which was able to disrupt the aggregation of alpha-synuclein. Thus, we have shown how structural data obtained from ssNMR can guide the design of modified peptides for use as amyloid inhibitors, as a primary step toward developing therapeutic compounds for prevention and/or treatment of amyloid diseases.

Membrane interactions of peptides representing the polybasic regions of three Rho GTPases are sensitive to the distribution of arginine and lysine residues

Rho GTPases are a multifunctional family of proteins that are localized at cellular membranes via an isoprenyl group covalently linked to a C-terminal cysteine. Close to this primary site of membrane anchoring there is often found an additional polybasic region (PBR), which plays a secondary role in membrane binding and targeting of the complex. Here, peptides derived from the PBRs of the Rho family proteins Rac1 (K(183)KRKRK), TCL (K(198)KKKKR) and Cdc42 (P(182)KKSRR) were prepared with hexalysine (K(6)) and hexaarginine (R(6)) to study their interactions with multilamellar vesicles of phosphatidylglycerol (DOPG) and headgroup-deuterated dimyristoylphosphatidylcholine (DMPC-d(4)) using (2)H and (31)P NMR. The membranes retained their lamellar architecture after peptide binding, but the (2)H NMR line shapes for DMPC-d(4) indicated that the bound peptides altered the orientation of the choline headgroups, consistent with a change in membrane surface charge. Rac1 and TCL peptides appeared to affect the headgroup orientation similarly to K(6), although the perturbations were weaker and unlike those induced by the Cdc42 peptide and R(6). Magic-angle spinning (31)P NMR spectra of the membranes showed significant and selective broadening of the peak for DMPC after addition of the peptides, with R(6) and the Cdc42 peptide having the greatest effect. The selective broadening may be a consequence of the lipids separating into short-lived domains enriched in peptide-bound DOPG and peptide-free DMPC. These results illustrate that a complex relationship exists between the sequence of PBRs and their behaviour at membrane surfaces, which may have implications for the cellular functions and localization of Rho GTPases.