RESUMO
The dopamine transporter (DAT) is crucial for regulating dopamine signalling and is the prime mediator for the rewarding and addictive effects of cocaine1. As part of the neurotransmitter sodium symporter family, DAT uses the Na+ gradient across cell membranes to transport dopamine against its chemical gradient2. The transport mechanism involves both intra- and extracellular gates that control substrate access to a central site. However, the molecular intricacies of this process and the inhibitory mechanism of cocaine have remained unclear. Here, we present the molecular structure of human DAT in complex with cocaine at a resolution of 2.66 Å. Our findings reveal that DAT adopts the expected LeuT-fold, posing in an outward-open conformation with cocaine bound at the central (S1) site. Notably, while an Na+ occupies the second Na+ site (Na2), the Na1 site seems to be vacant, with the side chain of Asn82 occupying the presumed Na+ space. This structural insight elucidates the mechanism for the cocaine inhibition of human DAT and deepens our understanding of neurotransmitter transport. By shedding light on the molecular underpinnings of how cocaine acts, our study lays a foundation for the development of targeted medications to combat addiction.
Assuntos
Cocaína , Proteínas da Membrana Plasmática de Transporte de Dopamina , Humanos , Sítios de Ligação , Cocaína/metabolismo , Cocaína/química , Cocaína/farmacologia , Microscopia Crioeletrônica , Dopamina/metabolismo , Dopamina/química , Proteínas da Membrana Plasmática de Transporte de Dopamina/química , Proteínas da Membrana Plasmática de Transporte de Dopamina/metabolismo , Proteínas da Membrana Plasmática de Transporte de Dopamina/ultraestrutura , Modelos Moleculares , Neurotransmissores/metabolismo , Ligação Proteica , Conformação Proteica/efeitos dos fármacos , Sódio/química , Sódio/metabolismoRESUMO
The regulation of dopamine (DA) removal from the synaptic cleft is a crucial process in neurotransmission and is facilitated by the sodium- and chloride-coupled dopamine transporter DAT. Psychostimulant drugs, cocaine, and amphetamine, both block the uptake of DA, while amphetamine also triggers the release of DA. As a result, they prolong or even amplify neurotransmitter signaling. Atypical inhibitors of DAT lack cocaine-like rewarding effects and offer a promising strategy for the treatment of drug use disorders. Here, we present the 3.2 Å resolution cryo-electron microscopy structure of the Drosophila melanogaster dopamine transporter (dDAT) in complex with the atypical non-competitive inhibitor AC-4-248. The inhibitor partially binds at the central binding site, extending into the extracellular vestibule, and locks the transporter in an outward open conformation. Our findings propose mechanisms for the non-competitive inhibition of DAT and attenuation of cocaine potency by AC-4-248 and provide a basis for the rational design of more efficacious atypical inhibitors.
RESUMO
The Na+,K+-ATPase generates electrochemical gradients of Na+ and K+ across the plasma membrane via a functional cycle that includes various phosphoenzyme intermediates. However, the structure and function of these intermediates and how metal fluorides mimick them require further investigation. Here, we describe a 4.0 Å resolution crystal structure and functional properties of the pig kidney Na+,K+-ATPase stabilized by the inhibitor beryllium fluoride (denoted E2-BeFx). E2-BeFx is expected to mimic properties of the E2P phosphoenzyme, yet with unknown characteristics of ion and ligand binding. The structure resembles the E2P form obtained by phosphorylation from inorganic phosphate (Pi) and stabilized by cardiotonic steroids, including a low-affinity Mg2+ site near ion binding site II. Our anomalous Fourier analysis of the crystals soaked in Rb+ (a K+ congener) followed by a low-resolution rigid-body refinement (6.9-7.5 Å) revealed preocclusion transitions leading to activation of the dephosphorylation reaction. We show that the Mg2+ location indicates a site of initial K+ recognition and acceptance upon binding to the outward-open E2P state after Na+ release. Furthermore, using binding and activity studies, we find that the BeFx-inhibited enzyme is also able to bind ADP/ATP and Na+. These results relate the E2-BeFx complex to a transient K+- and ADP-sensitive E∗P intermediate of the functional cycle of the Na+,K+-ATPase, prior to E2P.
Assuntos
Berílio , Glicosídeos Cardíacos , Fluoretos , Rim , ATPase Trocadora de Sódio-Potássio , Difosfato de Adenosina/metabolismo , Trifosfato de Adenosina/metabolismo , Animais , Berílio/química , Glicosídeos Cardíacos/química , Fluoretos/química , Rim/enzimologia , Cinética , Fosfatos/metabolismo , Fosforilação , Domínios Proteicos , ATPase Trocadora de Sódio-Potássio/antagonistas & inibidores , ATPase Trocadora de Sódio-Potássio/química , SuínosRESUMO
The human glycine transporter 1 (GlyT1) regulates glycine-mediated neuronal excitation and inhibition through the sodium- and chloride-dependent reuptake of glycine1-3. Inhibition of GlyT1 prolongs neurotransmitter signalling, and has long been a key strategy in the development of therapies for a broad range of disorders of the central nervous system, including schizophrenia and cognitive impairments4. Here, using a synthetic single-domain antibody (sybody) and serial synchrotron crystallography, we have determined the structure of GlyT1 in complex with a benzoylpiperazine chemotype inhibitor at 3.4 Å resolution. We find that the inhibitor locks GlyT1 in an inward-open conformation and binds at the intracellular gate of the release pathway, overlapping with the glycine-release site. The inhibitor is likely to reach GlyT1 from the cytoplasmic leaflet of the plasma membrane. Our results define the mechanism of inhibition and enable the rational design of new, clinically efficacious GlyT1 inhibitors.
Assuntos
Proteínas da Membrana Plasmática de Transporte de Glicina/antagonistas & inibidores , Proteínas da Membrana Plasmática de Transporte de Glicina/química , Glicina/metabolismo , Sítios de Ligação , Transporte Biológico/efeitos dos fármacos , Cristalografia , Humanos , Modelos Moleculares , Piperazinas/química , Piperazinas/farmacologia , Ligação Proteica , Conformação Proteica , Estabilidade Proteica , Anticorpos de Domínio Único , Sulfonas/química , Sulfonas/farmacologia , SíncrotronsRESUMO
Integral membrane proteins in eukaryotes are central to various cellular processes and key targets in structural biology, biotechnology and drug development. However, the number of available structures for eukaryotic membrane protein belies their physiological importance. Recently, the number of available eukaryotic membrane protein structures has been steadily increasing due to the development of novel strategies in construct design, expression and structure determination. Here, we examine the major expression systems exploited for eukaryotic membrane proteins. Additionally we strive to tabulate and describe the recent expression strategies in eukaryotic membrane protein structural biology. We find that a majority of targets have been expressed in advanced host systems and modified from their wild-type form with distinct focus on conformation and thermostabilisation. However, strategies for native protein purification should also be considered where possible, particularly in light of the recent advances in single particle cryo electron microscopy.
Assuntos
Eucariotos/genética , Engenharia Genética/métodos , Proteínas de Membrana/química , Proteínas de Membrana/genética , Animais , Cristalização , Detergentes/farmacologia , Expressão Gênica/efeitos dos fármacos , HumanosRESUMO
Bacterial members of the neurotransmitter:sodium symporter (NSS) family perform Na(+)-dependent amino-acid uptake and extrude H(+) in return. Previous NSS structures represent intermediates of Na(+)/substrate binding or intracellular release, but not the inward-to-outward return transition. Here we report crystal structures of Aquifex aeolicus LeuT in an outward-oriented, Na(+)- and substrate-free state likely to be H(+)-occluded. We find a remarkable rotation of the conserved Leu25 into the empty substrate-binding pocket and rearrangements of the empty Na(+) sites. Mutational studies of the equivalent Leu99 in the human serotonin transporter show a critical role of this residue on the transport rate. Molecular dynamics simulations show that extracellular Na(+) is blocked unless Leu25 is rotated out of the substrate-binding pocket. We propose that Leu25 facilitates the inward-to-outward transition by compensating a Na(+)- and substrate-free state and acts as the gatekeeper for Na(+) binding that prevents leak in inward-outward return transitions.
Assuntos
Sistemas de Transporte de Aminoácidos/metabolismo , Proteínas da Membrana Plasmática de Transporte de Neurotransmissores/metabolismo , Sequência de Aminoácidos , Sistemas de Transporte de Aminoácidos/química , Sistemas de Transporte de Aminoácidos/isolamento & purificação , Escherichia coli , Células HEK293 , Humanos , Leucina/metabolismo , Simulação de Dinâmica Molecular , Conformação Proteica , Sódio/metabolismoRESUMO
Pentameric ligand-gated ion channels have been identified as the principal target of general anesthetics (GA), whose molecular mechanism of action remains poorly understood. Bacterial homologs, such as the Gloeobacter violaceus receptor (GLIC), have been shown to be valid functional models of GA action. The GA bromoform inhibits GLIC at submillimolar concentration. We characterize bromoform binding by crystallography and molecular dynamics (MD) simulations. GLIC's open form structure identified three intra-subunit binding sites. We crystallized the locally closed form with an additional bromoform molecule in the channel pore. We systematically compare binding with the multiple potential sites of allosteric channel regulation in the open, locally closed, and resting forms. MD simulations reveal differential exchange pathways between sites from one form to the other. GAs predominantly access the receptor from the lipid bilayer in all cases. Differential binding affinity among the channel forms is observed; the pore site markedly stabilizes the inactive versus active state.
Assuntos
Canais Iônicos de Abertura Ativada por Ligante/química , Canais Iônicos de Abertura Ativada por Ligante/metabolismo , Regulação Alostérica , Anestésicos Gerais/farmacologia , Proteínas de Bactérias/química , Proteínas de Bactérias/metabolismo , Sítios de Ligação , Domínio Catalítico , Cristalografia por Raios X , Humanos , Modelos Biológicos , Simulação de Dinâmica Molecular , Conformação Proteica , Trialometanos/metabolismoRESUMO
Nicotinic acetylcholine receptors (nAChRs) are members of the pentameric ligand-gated ion channel superfamily that play important roles in the control of neurotransmitter release in the central and peripheral nervous system. These receptors are important therapeutic targets for the development of drugs against a number of mental health disorders and for marketed smoking cessation aids. Unfortunately, drug discovery has been hampered by difficulties in obtaining sufficiently selective compounds. Together with functional complexity of the receptors, this has made it difficult to obtain drugs with sufficiently high-target to off-target affinity ratios. The recent and ongoing progress in structural studies holds promise to help understand structure-function relationships of nAChR drugs at the atomic level. This will undoubtedly lead to the design of more efficient drugs with fewer side effects. As a high-resolution structure of a nAChR is yet to be determined, structural studies are to a large extent based on acetylcholine-binding proteins (AChBPs) that despite low overall sequence identity display a high degree of conservation of overall structure and amino acids at the ligand-binding site. Further, AChBPs reproduce relative binding affinities of ligands at nAChRs. Over the past decade, AChBPs have been used extensively as models for nAChRs and have aided the understanding of drug receptor interactions at nAChRs significantly.
Assuntos
Acetilcolina/metabolismo , Sítios de Ligação , Proteínas de Transporte/metabolismo , Receptores Nicotínicos/química , Animais , Humanos , Ligantes , Microscopia Eletrônica , Conformação Proteica , Receptores Nicotínicos/metabolismoRESUMO
Neuronal α4ß2 nicotinic acetylcholine receptors are attractive drug targets for psychiatric and neurodegenerative disorders and smoking cessation aids. Recently, a third agonist binding site between two α4 subunits in the (α4)(3)(ß2)(2) receptor subpopulation was discovered. In particular, three residues, H142, Q150, and T152, were demonstrated to be involved in the distinct pharmacology of the α4-α4 versus α4-ß2 binding sites. To obtain insight into the three-dimensional structure of the α4-α4 binding site, a surrogate protein reproducing α4-α4 binding characteristics was constructed by introduction of three point mutations, R104H, L112Q, and M114T, into the binding pocket of Lymnaea stagnalis acetylcholine-binding protein (Ls-AChBP). Cocrystallization with two agonists possessing distinct pharmacologic profiles, NS3920 [1-(6-bromopyridin-3-yl)-1,4-diazepane] and NS3573 [1-(5-ethoxypyridin-3-yl)-1,4-diazepane], highlights the roles of the three residues in determining binding affinities and functional properties of ligands at the α4-α4 interface. Confirmed by mutational studies, our structures suggest a unique ligand-specific role of residue H142 on the α4 subunit. In the cocrystal structure of the mutated Ls-AChBP with the high-efficacy ligand NS3920, the corresponding histidine forms an intersubunit bridge that reinforces the ligand-mediated interactions between subunits. The structures further reveal that the binding site residues gain different and ligand-dependent interactions that could not be predicted based on wild-type Ls-AChBP structures in complex with the same agonists. The results show that an unprecedented correlation between binding in engineered AChBPs and functional receptors can be obtained and provide new opportunities for structure-based design of drugs targeting specific nicotinic acetylcholine receptor interfaces.
Assuntos
Proteínas de Transporte/química , Proteínas de Transporte/metabolismo , Mimetismo Molecular/fisiologia , Engenharia de Proteínas/métodos , Receptores Nicotínicos/química , Receptores Nicotínicos/metabolismo , Animais , Sítios de Ligação/fisiologia , Cristalografia por Raios X , Relação Dose-Resposta a Droga , Feminino , Humanos , Insetos , Ligação Proteica/fisiologia , Estrutura Secundária de Proteína , Xenopus laevisRESUMO
In this issue of Structure, Bourne et al. (2015) report X-ray structures of acetylcholine binding protein with two fast-acting phycotoxins from the pinnatoxin family. The results may pave the way for development of new CNS-penetrant and subtype-selective nAChR antagonists.
Assuntos
Alcaloides/química , Iminas/química , Compostos Macrocíclicos/química , Toxinas Marinhas/química , Modelos Moleculares , Compostos de Espiro/químicaRESUMO
The nicotinic acetylcholine receptor α4ß2 is important for normal mammalian brain function and is known to express in two different stoichiometries, (α4)2(ß2)3 and (α4)3(ß2)2. While these are similar in many aspects, the (α4)3(ß2)2 stoichiometry differs by harboring a third orthosteric acetylcholine binding site located at the α4-α4 interface. Interestingly, the third binding site has, so far, only been documented using electrophysiological assays, actual binding affinities of nicotinic receptor ligands to this site are not known. The present study was therefore aimed at determining binding affinities of nicotinic ligands to the α4-α4 interface. Given that epibatidine shows large functional potency differences at α4-ß2 vs. α4-α4 interfaces, biphasic binding properties would be expected at (α4)3(ß2)2 receptors. However, standard saturation binding experiments with [(3)H]epibatidine did not reveal biphasic binding under the conditions utilized. Therefore, an engineered ß2 construct (ß2(HQT)), which converts the ß(-) face to resemble that of an α4(-) face, was utilized to create (α4)3(ß2(HQT))2 receptors harboring three α4-α4 interfaces. With this receptor, low affinity binding of epibatidine with a Kd of â¼5 nM was observed in sharp contrast to a Kd value of â¼10 pM observed for wild-type receptors. A strong correlation between binding affinities at the (α4)3(ß2(HQT))2 receptor and functional potencies at the wild-type receptor of a range of nicotinic ligands highlighted the validity of using the mutational approach. Finally, large differences in activities at α4-ß2 vs. α4-α4 interfaces were observed for structurally related agonists underscoring the need for establishing all binding parameters of compounds at α4ß2 receptors.
Assuntos
Acetilcolina/metabolismo , Modelos Moleculares , Agonistas Nicotínicos/farmacologia , Subunidades Proteicas/metabolismo , Receptores Nicotínicos/metabolismo , Acetilcolina/farmacologia , Animais , Azepinas/farmacocinética , Sítios de Ligação/efeitos dos fármacos , Sítios de Ligação/genética , Compostos Bicíclicos Heterocíclicos com Pontes/farmacocinética , Relação Dose-Resposta a Droga , Estimulação Elétrica , Células HEK293 , Humanos , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/genética , Nicotina/farmacologia , Oócitos , Ligação Proteica/efeitos dos fármacos , Subunidades Proteicas/genética , Piridinas/farmacocinética , Receptores Nicotínicos/genética , Transfecção , Trítio/farmacocinética , Xenopus laevisRESUMO
BACKGROUND: Pentameric ligand-gated ion channels (pLGICs) mediate fast chemical transmission of nerve signals in the central and peripheral nervous system. On the functional side, these molecules respond to the binding of a neurotransmitter (glycine, GABA, acetylcholine or 5HT3) in the extracellular domain (ECD) by opening their ionotropic pore in the transmembrane domain (TMD). The response to the neurotransmitter binding can be modulated by several chemical compounds acting at topographically distinct sites, as documented by a large body of literature. Notably, these receptors are the target of several classes of world-wide prescribed drugs, including general anesthetics, smoking cessation aids, anxiolytics, anticonvulsants, muscle relaxants, hypnotics and anti-emetics. On the structural side recent progress has been made on the crystallization of pLGICs in its different allosteric states, especially pLGICs of bacterial origin. Therefore, structure-function relationships can now be discussed at the atomic level for pLGICs. SCOPE OF REVIEW: This review focuses on the crystallographic structure of complexes of pLGICs with a number of ligands of pharmacological interest. First, we review structural data on two key functional aspects of these receptors: the agonist-induced activation and ion transport itself. The molecular understanding of both these functional aspects is important, as they are those that most pharmacological compounds target. Next, we describe modulation sites that have recently been documented by X-ray crystallography. Finally, we propose a simple geometric classification of all these pharmacological sites in pLGICs, based on icosahedrons. MAJOR CONCLUSIONS: This review illustrates the wealth of structural insight gained by comparing all available structures of members of the pLGIC family to rationalize the pharmacology of structurally diverse drugs acting at topographically distinct sites. It will be highlighted how sites that had been described earlier using biochemical techniques can be rationalized using structural data. Surprisingly, the use of icosahedral symmetry allows to link together several modulation sites, in a way that was totally unanticipated. GENERAL SIGNIFICANCE: Overall, understanding the interplay between the different modulation sites at the structural level should help the design of future drugs targeting pLGICs. This article is part of a Special Issue entitled structural biochemistry and biophysics of membrane proteins.
Assuntos
Canais Iônicos de Abertura Ativada por Ligante/química , Multimerização Proteica , Estrutura Quaternária de Proteína , Estrutura Terciária de Proteína , Animais , Sítios de Ligação , Cristalografia por Raios X , Humanos , Canais Iônicos de Abertura Ativada por Ligante/metabolismo , Ligantes , Modelos MolecularesRESUMO
Pentameric ligand-gated ion channels mediate fast chemical transmission of nerve signals. The structure of a bacterial proton-gated homolog has been established in its open and locally closed conformations at acidic pH. Here we report its crystal structure at neutral pH, thereby providing the X-ray structures of the two end-points of the gating mechanism in the same pentameric ligand-gated ion channel. The large structural variability in the neutral pH structure observed in the four copies of the pentamer present in the asymmetric unit has been used to analyze the intrinsic fluctuations in this state, which are found to prefigure the transition to the open state. In the extracellular domain (ECD), a marked quaternary change is observed, involving both a twist and a blooming motion, and the pore in the transmembrane domain (TMD) is closed by an upper bend of helix M2 (as in locally closed form) and a kink of helix M1, both helices no longer interacting across adjacent subunits. On the tertiary level, detachment of inner and outer ß sheets in the ECD reshapes two essential cavities at the ECD-ECD and ECD-TMD interfaces. The first one is the ligand-binding cavity; the other is close to a known divalent cation binding site in other pentameric ligand-gated ion channels. In addition, a different crystal form reveals that the locally closed and open conformations coexist as discrete ones at acidic pH. These structural results, together with site-directed mutagenesis, physiological recordings, and coarse-grained modeling, have been integrated to propose a model of the gating transition pathway.
Assuntos
Cristalografia por Raios X , Cianobactérias/metabolismo , Canais Iônicos de Abertura Ativada por Ligante/química , Sítio Alostérico , Animais , Sítios de Ligação , Cátions , Escherichia coli/metabolismo , Concentração de Íons de Hidrogênio , Canais Iônicos/química , Ligantes , Modelos Moleculares , Oócitos/metabolismo , Estrutura Quaternária de Proteína , Estrutura Terciária de Proteína , Transdução de Sinais , Xenopus laevisRESUMO
Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels that belong to the Cys-loop receptor superfamily. These receptors are allosteric proteins that exist in different conformational states, including resting (closed), activated (open), and desensitized (closed) states. The acetylcholine binding protein (AChBP) is a structural homologue of the extracellular ligand-binding domain of nAChRs. In previous studies, the degree of the C-loop radial extension of AChBP has been assigned to different conformational states of nAChRs. It has been suggested that a closed C-loop is preferred for the active conformation of nAChRs in complex with agonists whereas an open C-loop reflects an antagonist-bound (closed) state. In this work, we have determined the crystal structure of AChBP from the water snail Lymnaea stagnalis (Ls) in complex with dihydro-ß-erythroidine (DHßE), which is a potent competitive antagonist of nAChRs. The structure reveals that binding of DHßE to AChBP imposes closure of the C-loop as agonists, but also a shift perpendicular to previously observed C-loop movements. These observations suggest that DHßE may antagonize the receptor via a different mechanism compared to prototypical antagonists and toxins.