Histidine Pair Research Articles

Metal-Induced Fluorescence Quenching of Photoconvertible Fluorescent Protein DendFP.

Sensitive and accurate detection of specific metal ions is important for sensor development and can advance analytical science and support environmental and human medical examinations. Fluorescent proteins (FPs) can be quenched by specific metal ions and spectroscopically show a unique fluorescence-quenching sensitivity, suggesting their potential application as FP-based metal biosensors. Since the characteristics of the fluorescence quenching are difficult to predict, spectroscopic analysis of new FPs is important for the development of FP-based biosensors. Here we reported the spectroscopic and structural analysis of metal-induced fluorescence quenching of the photoconvertible fluorescent protein DendFP. The spectroscopic analysis showed that Fe2+, Fe3+, and Cu2+ significantly reduced the fluorescence emission of DendFP. The metal titration experiments showed that the dissociation constants (Kd) of Fe2+, Fe3+, and Cu2+ for DendFP were 24.59, 41.66, and 137.18 μM, respectively. The tetrameric interface of DendFP, which the metal ions cannot bind to, was analyzed. Structural comparison of the metal-binding sites of DendFP with those of iq-mEmerald and Dronpa suggested that quenchable DendFP has a unique metal-binding site on the β-barrel that does not utilize the histidine pair for metal binding.

Insights into the Evolution of Non‐shivering Thermogenesis from UCP1 of the Naked Mole Rat (Heterocephalus glaber)

Recognized as the primary location of non‐shivering thermogenesis (NST) in eutherian mammals, brown adipose tissue (BAT) functions to accelerate rewarming in hibernators and defend body temperature among newborns and small‐bodied species enduring cold stress. Uncoupling protein 1 (UCP1) is highly expressed in BAT and underlies the molecular mechanism of NST. UCP1 resides in the inner mitochondrial membrane where, upon functional induction by free fatty acids, it catalyzes mitochondrial proton leak, collapsing the proton gradient that stores oxidation energy. The ensuing futile cycling of protons increases substrate oxidation and thus heat production. Central questions remain regarding the exact mechanism by which UCP1 facilitates proton leak, stemming from the lack of a crystal structure of the protein. It is also unknown whether UCP1 function has been shaped by evolution to meet differing thermoregulatory demands of various lineages. We adopt a comparative approach, targeting species with extreme ecophysiological characteristics expected to culminate in drastic differences in thermoregulatory demands and thus UCP1 function, to provide insights into protein structure‐function relationships. For instance, the naked mole rat (NMR; Heterocephalus glaber) is well‐suited to survive in hypoxic burrows that exhibit high year‐round temperatures (30‐34°C). Hinting at a reduced NST capacity, NMRs also display poikilothermy, failing to maintain constant body temperatures when artificially cold challenged. Moreover, NMR UCP1 displays several unique amino acid substitutions, one of which, H145Q (histidine to glutamine), occurs at the well‐known “histidine pair motif”. This motif, typically comprised of histidines at positions 145 and 147, has been hypothesized to be a critical final proton acceptor/donor site shuttling protons into the mitochondrial matrix. The exact mutation that occurs in the NMR, H145Q, was previously shown to diminish hamster UCP1‐mediated proton conductance in proteoliposomes by 90%, calling the functional performance of NMR UCP1 into question. We functionally tested wildtype NMR UCP1 as well as histidine pair mutant combinations at positions 145 and 147, not in the highly artificial system of proteoliposomes, which may promote artefactual protein behaviour, but instead in the more native mammalian environment of HEK293 cells. Using plate‐based respirometry techniques, we demonstrate that wildtype NMR UCP1 uncouples, thus NMRs retain this mechanism of NST. While we hypothesized that reversion of the H145Q mutation would increase UCP1‐mediated proton leak, surprisingly both variants function similarly when activated with free fatty acids and inhibited with guanosine diphosphate (GDP). Likewise, mutation of the histidine at position 147, the other half of the “histidine pair motif”, does not drastically alter UCP1 function. Overall, these data suggest that the “histidine pair motif” may not be crucial for protein function, contrary to previous claims. Comparative functional assessments of UCP1 variants provide a deeper understanding of structure‐function relationships of this enigmatic protein and the evolution of mammalian NST in response to ecophysiological constraints.

Catalytic Dyad Residues His41 and Cys145 Impact the Catalytic Activity and Overall Conformational Fold of the Main SARS-CoV-2 Protease 3-Chymotrypsin-Like Protease.

Coronaviruses are responsible for multiple pandemics and millions of deaths globally, including the current pandemic of coronavirus disease 2019 (COVID-19). Development of antivirals against coronaviruses, including the severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) responsible for COVID-19, is essential for containing the current and future coronavirus outbreaks. SARS-CoV-2 proteases represent important targets for the development of antivirals because of their role in the processing of viral polyproteins. 3-Chymotrypsin-like protease (3CLpro) is one such protease. The cleavage of SARS-CoV-2 polyproteins by 3CLpro is facilitated by a Cys145–His41 catalytic dyad. We here characterized the catalytic roles of the cysteine–histidine pair for improved understanding of the 3CLpro reaction mechanism, to inform the development of more effective antivirals against Sars-CoV-2. The catalytic dyad residues were substituted by site-directed mutagenesis. All substitutions tested (H41A, H41D, H41E, C145A, and C145S) resulted in a complete inactivation of 3CLpro, even when amino acids with a similar catalytic function to that of the original residues were used. The integrity of the structural fold of enzyme variants was investigated by circular dichroism spectroscopy to test if the catalytic inactivation of 3CLpro was caused by gross changes in the enzyme secondary structure. C145A, but not the other substitutions, shifted the oligomeric state of the enzyme from dimeric to a higher oligomeric state. Finally, the thermodynamic stability of 3CLpro H41A, H41D, and C145S variants was reduced relative the wild-type enzyme, with a similar stability of the H41E and C145A variants. Collectively, the above observations confirm the roles of His41 and Cys145 in the catalytic activity and the overall conformational fold of 3CLpro SARS-CoV-2. We conclude that the cysteine–histidine pair should be targeted for inhibition of 3CLpro and development of antiviral against COVID-19 and coronaviruses.

Paired Carboxylic Acids in Enzymes and Their Role in Selective Substrate Binding, Catalysis, and Unusually Shifted pKa Values.

Cathepsin A (CatA, EC 3.4.16.5, UniProtKB P10619 ) is a human lysosomal carboxypeptidase. Counterintuitively, crystal structures of CatA and its homologues show a cluster of Glu and Asp residues binding the C-terminal carboxylic acid of the product or inhibitor. Each of these enzymes functions in an acidic environment and contains a highly conserved pair of Glu residues with side chain carboxyl group oxygens that are approximately 2.3-2.6 Å apart. In small molecules, carboxyl groups separated by ∼3 Å can overcome the repulsive interaction by protonation of one of the two groups. The pKa of one group increases (pKa ∼ 11) and can be as much as ∼6 pH units higher than the paired group. Consequently, at low and neutral pH, one carboxylate can carry a net negative charge while the other can remain protonated and neutral. In CatA, E69 and E149 form a Glu pair that is important to catalysis as evidenced by the 56-fold decrease in kcat/Km in the E69Q/E149Q variant. Here, we have measured the pH dependencies of log(kcat), log(Km), and log(kcat/Km) for wild type CatA and its variants and have compared the measured pKa with calculated values. We propose a substrate-assisted mechanism in which the high pKa of E149 (>8.5) favors the binding of the carboxylate form of the substrate and promotes the abstraction of the proton from H429 of the catalytic triad effectively decreasing its pKa in a low-pH environment. We also identify a similar motif consisting of a pair of histidines in S-formylglutathione hydrolase.

Molecular Basis for the Selective Inhibition of Histone Deacetylase 6 by a Mercaptoacetamide Inhibitor.

Mercaptoacetamide histone deacetylase inhibitors are neuroprotective agents that do not exhibit the genotoxicity associated with more commonly used hydroxamate inhibitors. Here, we present the crystal structure of a selective mercaptoacetamide complexed with the C-terminal catalytic domain of HDAC6. When compared with the structure of a mercaptoacetamide bound to the class I isozyme HDAC8, different interactions are observed with the conserved tandem histidine pair in the active site. These differences likely contribute to the selectivity for inhibition of HDAC6, an important target for cancer chemotherapy and the treatment of neurodegenerative disease.

Rotamer Modelling of Cu(II) Spin Labels Based on the Double-Histidine Motif

Spin labels attached to two residues of a protein chain have less conformational flexibility than those attached to a single residue and thus lead to a narrower spatial distribution of the unpaired electron. The case of Cu2+ labels based on the double-histidine (dHis) motif is of particular interest, as it combines the advantage of precise localization of the unpaired electron with a labelling scheme orthogonal to the more common cysteine-based labelling. Here, we introduce an approach for in silico spin labelling of a protein by dHis motifs and Cu2+ complexes of iminodiacetic acid or nitrilotriacetic acid. We discuss a computerized scan for native histidine pairs that might be prone to bind such Cu2+ complexes and spin-labelling site pair scans that can identify suitable double mutants for labelling. Predicted distance distributions between two Cu2+ labels are compared to experimental distance distributions. We also test the hypothesis that elastic network modelling of conformational transitions with Cu2+-dHis labels can provide more accurate structural models than with nitroxide labels.

A bacterial flavin‐dependent oxidoreductase that captures carbon dioxide into biomass

Atmospheric carbon dioxide is used as a carbon source for building biomass in plants and photosynthetic microbes. Non‐photosynthetic processes that also fix carbon dioxide have more recently been discovered. This research focuses on a microbial mechanism for coupling acetone to CO2 to make a central metabolite, acetoacetate. The key reaction is catalyzed by NADPH‐2‐ketopropyl‐coenzyme M oxidoreductase/carboxylase (2‐KPCC), a bacterial enzyme that is part of the flavin and cysteine‐disulfide containing oxidoreductase family (DSORs) which are best known for reducing metallic or disulfide substrates. Our research asks: how has nature repurposed a DSOR to, uniquely, break a C‐S bond and then trap and fix CO2? 2‐KPCC lacks a conserved, catalytically essential acid‐base histidine possessed by all other DSOR enzymes, having instead a phenylalanine (F501) at the same position. Mutagenesis showed that a F501H mutant has a similar rate of catalytic turnover; however, the product is acetone instead of acetoacetate (Figure 1). We hypothesized that F501 is important for both the reductive half reaction – which generates the reactive, C‐S bond breaking form of the active site – and for the oxidative half, in which an enolacetone intermediate reacts with either the correct (CO2) or incorrect (H+) electrophile. In this study, we used real‐time and spectroscopic methods to examine the reductive half reaction. In typical DSORs, this reaction generates a Cys/FAD charge transfer species. However, we showed that 2‐KPCC generates an electronically unique form of the active site, in which the flavin is oxidized and a pair of active site histidines are reduced and protonated. We hypothesize that this form of the active site generates the substrate‐reactive Cys in a more nucleophilic form, where it is capable of cleaving relatively strong C‐S bonds. The resulting enolacetone carbanion is an extremely potent nucleophile that is capable of directly attacking CO2. Research on 2‐KPCC and other biological CO2 fixation methods adds to our arsenal of strategies for carbon dioxide capture and use.Support or Funding InformationDOE:DE‐FG02‐04ER15563This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Imidazole Nitrogens of Two Histidine Residues Participating in N-H···N Hydrogen Bonds in Protein Structures: Structural Bioinformatics Approach Combined with Quantum Chemical Calculations.

Protein structures are stabilized by different types of hydrogen bonds. However, unlike the DNA double helical structure, the N-H···N type of hydrogen bonds is relatively rare in proteins. N-H···N hydrogen bonds formed by imidazole groups of two histidine residues have not been investigated. We have systematically analyzed 5333 high-resolution protein structures with resolution 1.8 Å or better and identified 285 histidine pairs in which the nitrogen atoms of the imidazole side chains can potentially participate in N-H···N hydrogen bonds. The histidine pairs were further divided into two groups, neutral-neutral and protonated-neutral, depending on the protonation state of the donor histidine. Quantum chemical calculations were performed on imidazole groups adopting the same geometry observed in the protein structures. Average interaction energies between the interacting imidazole groups are -6.45 and -22.5 kcal/mol for neutral-neutral and protonated-neutral, respectively. Hydrogen bond interaction between the imidazole moieties is further confirmed by natural bond orbital analyses of the model compounds. Histidine residues involved in N-H···N hydrogen bonds are relatively more buried and have low B-factor values in the protein structures. N-H···N hydrogen bond formed by a pair of buried histidine residues can significantly contribute to the structural stability of proteins.

Bohr effect of native and chemically modified hemoglobins: Quantitative analyses based on the Wyman equation

Thirteen histidines and the α-chain terminal amino group (ACTA) make all of the contributions to the Bohr effect of human hemoglobin. The pKas of the 13 histidines in carbonmonoxy- and deoxyhemoglobin are known from 1H NMR studies. Those of ACTA are not so precisely known. We employed the Wyman equation and the 13 histidine pairs of pKas to determine the pKas of ACTA by curve-fitting to hemoglobin Bohr effect data. Using all 14 pairs of pKas as preliminary data, we employed the Wyman equation to fit the Bohr data for hemoglobin chemically modified at Cys93β with cystamine, cystine and iodoacetamide. We demonstrate quantitatively that the reduction of the Bohr effect upon chemical modification is due to three negatively contributing Bohr groups: His2β, His77β and His143β. These make twice their normal contributions to the Bohr effect in unmodified hemoglobin. We also find that the ACTA pKas increase with increasing ionic strength.

Harvesting the photoexcited holes on a photocatalytic proton reduction metal–organic framework

The highly porous titanium based metal-organic framework NH2-MIL-125(Ti) has recently attracted significant attention in the field of photocatalysis as a promising material for H+ reduction. This work reveals charge transfer upon visible light illumination from this MOF to two different charge acceptors, as an alternative to sacrificial electron donors. Charge transfer is demonstrated through a combined spectroscopic study between this MOF and: (1) 2-(1H-pyrazol-3-yl)phenol, a molecule that functionally mimics the tyrosine-histidine pair, responsible for shuttling the holes to the oxygen evolving centre in natural photosynthesis, and (2) TEMPO, a well known and stable radical. Charge transfer of the holes from the MOF to these occluded molecules takes place on the picosecond time scale. This work suggests that, by coupling a stable and recyclable charge acceptor to the photogenerated holes, the charges can be utilised for oxidation reactions and, thus, link the reduction to the oxidation reactions in water splitting.

The pH-dependent Client Release from the Collagen-specific Chaperone HSP47 Is Triggered by a Tandem Histidine Pair

Heat shock protein 47 (HSP47) is an endoplasmic reticulum (ER)-resident collagen-specific chaperone and essential for proper formation of the characteristic collagen triple helix. It preferentially binds to the folded conformation of its clients and accompanies them from the ER to the Golgi compartment, where it releases them and is recycled back to the ER. Unlike other chaperones, the binding and release cycles are not governed by nucleotide exchange and hydrolysis, but presumably the dissociation of the HSP47-procollagen complex is triggered by the lower pH in the Golgi (pH 6.3) compared with the ER (pH 7.4). Histidine residues have been suggested as triggers due to their approximate textbook pKa value of 6.1 for their side chains. We present here an extensive theoretical and experimental study of the 14 histidine residues present in canine HSP47, where we have mutated all histidine residues in the collagen binding interface and additionally all of those that were predicted to undergo a significant change in protonation state between pH 7 and 6. These mutants were characterized by biolayer interferometry for their pH-dependent binding to a collagen model. One mutant (H238N) loses binding, which can be explained by a rearrangement of the Arg(222) and Asp(385) residues, which are crucial for specific collagen recognition. Most of the other mutants were remarkably silent, but a double mutant with His(273) and His(274) exchanged for asparagines exhibits a much less pronounced pH dependence of collagen binding. This effect is mainly caused by a lower koff at the low pH values.

Purification, Refolding, and Crystallization of the Outer Membrane Protein OmpG from Escherichia coli.

OmpG is a pore-forming protein from E. coli outer membranes. Unlike the classical outer membrane porins, which are trimers, the OmpG channel is a monomeric β-barrel made of 14 antiparallel β-strands with short periplasmic turns and longer extracellular loops. The channel activity of OmpG is pH dependent and the channel is gated by the extracellular loop L6. At neutral/high pH, the channel is open and permeable for substrate molecules with a size up to 900 Da. At acidic pH, loop L6 folds across the channel and blocks the pore. The channel blockage at acidic pH appears to be triggered by the protonation of a histidine pair on neighboring β-strands, which repel one another, resulting in the rearrangement of loop L6 and channel closure. OmpG was purified by refolding from inclusion bodies and crystallized in two and three dimensions. Crystallization and analysis by electron microscopy and X-ray crystallography revealed the fundamental mechanisms essential for the channel activity.

Molecular dynamics simulations of histidine-containing cod antimicrobial peptide paralogs in self-assembled bilayers

Gaduscidin-1 and -2 (GAD-1 and GAD-2) are antimicrobial peptides (AMPs) that contain several histidine residues and are thus expected to exhibit pH-dependent activity. In order to help elucidate their mechanism of membrane disruption, we have performed molecular dynamics simulations with the peptides in both histidine-charged and histidine-neutral forms, along with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid molecules. The simulations employed GROMACS software and an OPLS-AA force field. Initially, the peptide and lipids were placed randomly in the simulation box and then were allowed to self-assemble. The results demonstrated a marked preference for the regions of the peptides that contain sequential pairs of histidine residues to associate closely with bilayer pores. This preference is observed even when the histidines are in their uncharged form. It appears that the relative compactness and rigidity of histidine pairs require the more aqueous and disordered environment of the pores to satisfy hydrophilic interactions. The final peptide structures exhibited a wide variety of structures and topologies, with the most helical structures positioning most parallel to the bilayer surface and the less ordered structures interacting more closely with the pore. Thus, the results give atomistic insight into those models of AMP mechanism that promote the importance of structural heterogeneity and imperfect amphipathicity to AMP activity and selectivity.

A bioinspired redox relay that mimics radical interactions of the Tyr-His pairs of photosystem II.

In water-oxidizing photosynthetic organisms, light absorption generates a powerfully oxidizing chlorophyll complex (P680(•+)) in the photosystem II reaction centre. This is reduced via an electron transfer pathway from the manganese-containing water-oxidizing catalyst, which includes an electron transfer relay comprising a tyrosine (Tyr)-histidine (His) pair that features a hydrogen bond between a phenol group and an imidazole group. By rapidly reducing P680(•+), the relay is thought to mitigate recombination reactions, thereby ensuring a high quantum yield of water oxidation. Here, we show that an artificial reaction centre that features a benzimidazole-phenol model of the Tyr-His pair mimics both the short-internal hydrogen bond in photosystem II and, using electron paramagnetic resonance spectroscopy, the thermal relaxation that accompanies proton-coupled electron transfer. Although this artificial system is much less complex than the natural one, theory suggests that it captures the essential features that are important in the function of the relay.

A Secondary Structural Transition in the C-helix Promotes Gating of Cyclic Nucleotide-regulated Ion Channels

Cyclic nucleotide-regulated ion channels bind second messengers like cAMP to a C-terminal domain, consisting of a β-roll, followed by two α-helices (B- and C-helices). We monitored the cAMP-dependent changes in the structure of the C-helix of a C-terminal fragment of HCN2 channels using transition metal ion FRET between fluorophores on the C-helix and metal ions bound between histidine pairs on the same helix. cAMP induced a change in the dimensions of the C-helix and an increase in the metal binding affinity of the histidine pair. cAMP also caused an increase in the distance between a fluorophore on the C-helix and metal ions bound to the B-helix. Stabilizing the C-helix of intact CNGA1 channels by metal binding to a pair of histidines promoted channel opening. These data suggest that ordering of the C-helix is part of the gating conformational change in cyclic nucleotide-regulated channels.

Improving the study of proton transfers between amino acid side chains in solution: choosing appropriate DFT functionals and avoiding hidden pitfalls

We have studied the influence of implicit solvent models, inclusion of explicit water molecules, inclusion of vibrational effects, and density functionals on the quality of the predicted pK a of small amino acid side chain models. We found that the inclusion of vibrational effects and explicit water molecules is crucial to improve the correlation between the computed and the experimental values. In these micro-solvated systems, the best agreement between DFT-computed electronic energies and benchmark values is afforded by BHHLYP and B97-2. However, approaching experimental results requires the addition of more than three explicit water molecules, which generates new problems related to the presence of multiple minima in the potential energy surface. It thus appears that a satisfactory ab initio prediction of amino acid side chain pK a will require methods that sample the configurational space in the presence of large solvation shells, while at the same time computing vibrational contributions to the enthalpy and entropy of the system under study in all points of that surface. Pending development of efficient algorithms for those computations, we strongly suggest that whenever counterintuitive protonation states are found in a computational study (e.g., the presence of a neutral aspartate/neutral histidine dyad instead of a deprotonated aspartate/protonated histidine pair), the reaction profile should be computed under each of the different protonation micro-states by constraining the relevant N–H or O–H bonds, in order to avoid artifacts inherent to the complex nature of the factors contributing to the pK a.

Evidence regarding the hypothesis that the histidine–histidine contact pairs may affect protein stability

It has been lately proposed that the interaction between like-charged residues stabilizes the native state of proteins. To explore this, we created a histidine–histidine pair in the Ca-III binding site of the Bacillus amyloliquefaciens α-amylase (BAA) and then examined the impact of this pairing on the BAA. For this purpose, we used site-directed mutagenesis (SDM) to substitute Pro407 with His, Ala, Gln, Arg, and Glu in the BAA. Subsequently, thermostability, kinetic parameters and structural properties of these variants were measured. Moreover, His–His pairing effect on the BAA thermostability was examined by simultaneous mutation of two residues (P407H/H406A and P407H/H406N). The data exhibited a significant improve in thermostability and structural features of enzyme by His replacement instead of Pro407. Other substitutions in this site did not have a significant effect on the enzyme properties, except for P407R, which yielded a partial improvement. The results also showed that the thermostabilities of double mutants significantly decreased compared with that of the P407H mutant. Moreover, the thermostability of P407H remarkably increased compared with that of other variants even in the absence of Ca2+. Our data clearly demonstrated that His406–His407 pairing was the major cause for improved thermal stability.

Agonist-Induced Conformational Changes in the Ligand Binding Domain of Cyclic Nucleotide-Regulated Channels

Cyclic nucleotide-gated (CNG) channels and hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channels are opened by the direct binding of cyclic nucleotides (e.g. adenosine 3’,5’-cyclic monophosphate, cAMP). The ligand binding domain is primarily formed by an eight-stranded β-roll followed by two helices (B and C helices). Following agonist binding to residues in the β-roll, the C helix is thought to initiate the opening conformational change by moving towards the binding pocket and transitioning from a less ordered coil to a stable helix. To further investigate this hypothesis, we expressed the soluble C-terminal domain of the HCN2 channel, which contains the cyclic nucleotide binding domain, and introduced cysteines into the C helix, which were labeled with the fluorophore bimane. Into the same helix, we placed pairs of histidines in helical register. Colored transition metal ions can bind to these histidines and quench the fluorophore by a FRET mechanism. The amount of quenching was used to estimate the distance between the fluorophore and the metal binding site in the presence and absence of cAMP. For all of the constructs tested, cAMP binding induced a large increase in metal affinity at the di-histidine binding sites, indicating a stabilization of the helical structure. We also observed a change in the total amount of quenching in some constructs indicating a change in distance between the fluorophore and metal binding site. These data suggest a conformational change within the C helix induced by ligand binding. When a similar di-histidine binding site was introduced into the C helix of intact CNG channels, binding of Ni2+ increased the efficacy of the partial agonist inosine 3’,5’-cyclic monophosphate (cIMP), consistent with the hypothesis that stabilization of the secondary structure of the C helix is part of the gating conformational change.

The pivotal twin histidines and aromatic triad of the Escherichia coli ammonium channel AmtB can be replaced

In Escherichia coli, each subunit of the trimeric channel protein AmtB carries a hydrophobic pore for transport of NH(4)(+) across the cytoplasmic membrane. Positioned along this substrate conduction pathway are two conserved elements--a pair of hydrogen-bonded histidines (H168/H318) located within the pore itself and a set of aromatic residues (F107/W148/F215) at its periplasmic entrance--thought to be critical to AmtB function. Using site-directed mutagenesis and suppressor genetics, we examined the requirement for these elements in NH(4)(+) transport. This analysis shows that AmtB can accommodate, by either direct substitution or suppressor generation, acidic residues at one or both positions of the H168/H318 twin-histidine site while retaining near wild-type activity. Similarly, study of the F107/W148/F215 triad indicates that good-to-excellent AmtB function is preserved upon individual and simultaneous replacement of these aromatic amino acids with aliphatic residues. Our findings lead us to conclude that these elements and their component parts are not required for AmtB function, but instead serve to optimize its performance.

Influenza A/M2 Proton Channel: Structure in a Lipid Bilayer Provides Insights into the Conductance Mechanism

The M2 protein of influenza A virus forms an acid-activated proton selective channel involved in modifying virion and trans-Golgi pH for virus infection. The transmembrane domain of M2 protein has been extensively studied by structural, functional, and computational methods. However, structural details about the functional core of this tetrameric protein, a His37-Trp41 cluster, remain unclear. We have now determined the structure of this channel reconstituted in a native-like liquid crystalline lipid bilayer environment by solid state NMR [1]. The structure of the M2 conductance domain (residues 22-62) in DOPC:DOPE(4:1) lipid bilayer presents the defining features of the native protein that have not been attainable from structures solubilized by detergents. The structure shows a left-handed helical bundle stabilized by interactions with four membrane-surface bound amphipathic helices. The structure also sheds light on the central role of the HxxxW quartet that regulates proton conductance through the channel by unique hydrogen bonding interactions between pairs of histidines and specific interactions with the tryptophan gate. The proposed transport mechanism explains observations on M2 proton conductance. A brief overview of the structure and mechanism will be presented along with experimental data in support of the transport mechanism.[1] M. Sharma, M. Yi, H. Dong, H. Qin, E. Peterson, D. D. Busath, H.-X. Zhou, and T. A. Cross (2010). Insight into the mechanism of the influenza A proton channel from a structure in a lipid bilayer. Science (in press).