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The serotonin type 3A (5-HT3A) receptor is a homopentameric cation-selective member of the pentameric ligand-gated ion channel (pLGIC) superfamily. Members of this superfamily assemble from five subunits, each of which consists of three domains: extracellular (ECD), transmembrane (TMD), and intracellular domain (ICD). Previously, we have demonstrated that the 5-HT3A-ICD is required for the interaction between 5-HT3A and the chaperone protein resistance to inhibitors of choline esterase (RIC-3). Additionally, we have shown that 5-HT3A-ICD fused to maltose-binding protein (MBP) directly interacts with RIC-3, without the involvement of other protein(s). To elucidate the molecular determinants of this interaction, we developed different MBP-fused 5-HT3A-ICD constructs by deleting large segments of its amino acid sequence. We expressed seven engineered ICDs in Escherichia coli and purified them to homogeneity. Using a RIC-3 affinity pull-down assay, the interaction between MBP-5HT3A-ICD constructs and RIC-3 was investigated. In summary, we identify a 24-amino-acid-long segment of the 5-HT3A-ICD as a molecular determinant for the interaction between the 5-HT3A-ICD and RIC-3.

The Proton-Coupled Folate Transporter (PCFT) is a transmembrane transport protein that controls the absorption of dietary folates in the small intestine. PCFT also mediates uptake of chemotherapeutically used antifolates into tumor cells. PCFT has been identified within lipid rafts observed in phospholipid bilayers of plasma membranes, a micro environment that is altered in tumor cells. The present study aimed at investigating the impact of different lipids within Lipid-protein nanodiscs (LPNs), discoidal lipid structures stabilized by membrane scaffold proteins, to yield soluble PCFT expression in an E. coli lysate-based cell-free transcription/translation system. In the absence of detergents or lipids, we observed PCFT quantitatively as precipitate in this system. We then explored the ability of LPNs to support solubilized PCFT expression when present during in-vitro translation. LPNs consisted of either dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), or dimyristoyl phosphatidylglycerol (DMPG). While POPC did not lead to soluble PCFT expression, both DMPG and DMPC supported PCFT translation directly into LPNs, the latter in a concentration dependent manner. The results obtained through this study provide insights into the lipid preferences of PCFT. Membrane-embedded or solubilized PCFT will enable further studies with diverse biophysical approaches to enhance the understanding of the structure and molecular mechanism of folate transport through PCFT.

The Proton-Coupled Folate Transporter (PCFT) is a transmembrane transport protein that controls the absorption of dietary folates in the small intestine. PCFT also mediates uptake of chemotherapeutically used antifolates into tumor cells. PCFT has been identified within lipid rafts observed in phospholipid bilayers of plasma membranes, a micro environment that is altered in tumor cells. The present study aimed at investigating the impact of different lipids within Lipid-protein nanodiscs (LPNs), discoidal lipid structures stabilized by membrane scaffold proteins, to yield soluble PCFT expression in an E. coli lysate-based cell-free transcription/translation system. In the absence of detergents or lipids, we observed PCFT quantitatively as precipitate in this system. We then explored the ability of LPNs to support solubilized PCFT expression when present during in-vitro translation. LPNs consisted of either dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), or dimyristoyl phosphatidylglycerol (DMPG). While POPC did not lead to soluble PCFT expression, both DMPG and DMPC supported PCFT translation directly into LPNs, the latter in a concentration dependent manner. The results obtained through this study provide insights into the lipid preferences of PCFT. Membrane-embedded or solubilized PCFT will enable further studies with diverse biophysical approaches to enhance the understanding of the structure and molecular mechanism of folate transport through PCFT.HighlightsCell free expression of PCFT without any lipids or detergents resulted in quantitative precipitation of in-situ synthesized PCFT.Additives for expression of PCFT in the soluble fraction were identified.

The main principles of higher-order protein oligomerization are elucidated by many structural and biophysical studies. An astonishing number of proteins self-associate to form dimers or higher-order quaternary structures which further interact with other biomolecules to elicit complex cellular responses. In this study, we describe a simple and convenient approach to determine the oligomeric state of purified protein complexes that combines implementation of a novel form of clear-native gel electrophoresis and size exclusion chromatography in line with multi-angle light scattering. Here, we demonstrate the accuracy of this ensemble approach by characterizing the previously established pentameric state of the intracellular domain of serotonin type 3A (5-HT(3A)) receptors.

Serotonin type 3A receptors (5-HT 3A Rs) are pentameric ligand-gated ion channels, pharmacologically targeted for the treatment of severe nausea and vomiting. The intracellular domain of 5-HT 3A Rs has been shown to be a crucial determinant for limiting conductance. Structurally, it consists of a short L1-loop following the third transmembrane segment M3, a short α-helical MX-segment, a large unstructured L2-loop, and the membrane-associated MA-helix that continues into the last transmembrane segment M4. During gating, conformational changes occur in all three domains. Extracellular and transmembrane domains rotate counterclockwise and clockwise, respectively. Within the intracellular domain the MA-M4 helix breaks at Gly430, leading to pore widening above and below the helix break, and the MX-helix moves outward and upward towards the membrane. At their bottom, the MA-helices come into close apposition to shape a narrow hydrophobic constriction below the cytosolic channel chalice that, in its upper section, is framed by lateral windows through which the L1-loops thread. In the present study, we used disulfide bond formation between pairs of engineered cysteines to probe the proximity and mobility of the bottom segments of the MA-helices. Repeated agonist application or oxidation induced a current run down for channels with Cys pairs at I409C/R410C. For Cys pairs at L402C/L403C these conditions did not lead to altered currents. On the contrary, cross-linked subunits for both L402C/L403C and I409C/R410C were observed for both conditions using gel electrophoresis. Our results indicate that the proximity and orientation for Cys in both pairs is conducive for disulfide bond formation. While conformational changes associated with gating promote cross-linking for I409C/R410C that in turn inhibits gating, cross-linking of L402C/L403C is functionally silent. We infer that conformational changes associated with gating are more pronounced for the upper I409C/R410C pair in close proximity to the lateral windows as compared to the L402C/L403C pair at the apex of the inverted pentagonal pyramid contributed by the MA-helices. Statement of Significance The intracellular domain (ICD) of pentameric ligand-gated ion channels (pLGICs) is the most diverse domain within receptors of the Cys-loop superfamily. Despite being the least understood domain of pLGICs, we know that it influences channel function in multiple ways and shapes the cytosolic exit pathway of the channel. X-ray and cryo-EM structures have captured the structured segments of the ICD of 5-HT 3A receptors in different conformational states with lower resolution of the ICD as compared to the other domains. Here, we provide experimentally-derived evidence for the importance of the mobility of the MA-helices and functionally confirm ion-conduction through lateral portals as opposed to a vertical pathway for 5-HT 3A receptors.

The intracellular domain of the serotonin type 3A receptor, a pentameric ligand-gated ion channel, is crucial for regulating conductance. However, the specific ion conduction pathway through this domain is less clear. The intracellular domain starts with a short loop after the third transmembrane segment, followed by a short α-helical segment, a large unstructured loop, and finally the membrane-associated MA-helix that continues into the last transmembrane segment. The MA-helices from all five subunits form the extension of the transmembrane ion channel and shape what has been described as a “closed vestibule”, with their lateral portals obstructed by loops and their cytosolic ends forming a tight hydrophobic constriction. The question remains whether the lateral portals or cytosolic constriction conduct ions upon channel opening. In the present study, we used disulfide bond formation between pairs of engineered cysteines to probe the proximity and mobility of segments of the MA-helices most distal to the membrane bilayer. Our results indicate that the proximity and orientation for cysteine pairs at I409C/R410C, in close proximity to the lateral windows, and L402C/L403C, at the cytosolic ends of the MA-helices, are conducive for disulfide bond formation. While conformational changes associated with gating promote crosslinking for I409C/R410C, which in turn decreases channel currents, crosslinking of L402C/L403C is functionally silent in macroscopic currents. These results support the hypothesis that concerted conformational changes open the lateral portals for ion conduction, rendering ion conduction through the vertical portal unlikely.SignificanceThe intracellular domain (ICD) of pentameric ligand-gated ion channels (pLGICs) is the most diverse domain within receptors of the Cys-loop superfamily. Despite being the least understood domain of pLGICs, its impact on ion-channel function and contribution to the cytosolic exit pathway of the channel have been investigated. X-ray and cryo-EM structures have captured the structured segments of the ICD of 5-HT3A receptors in different conformational states with lower resolution of the ICD as compared to the other domains. Here, we provide experimentally derived evidence for the importance of the differential mobility of the cytosolic segment of the MA-helices, which supports the existence of lateral portals as opposed to a vertical pathway for 5-HT3A receptors.

The proton-coupled folate transporter (PCFT) provides an essential uptake route for the vitamin folic acid (B9) in mammals. In addition, it is currently of high interest for targeting chemotherapeutic agents to tumors due to the increased folic acid requirement of rapidly dividing tumor cells as well as the upregulated PCFT expression in several tumors. To understand its function, determination of its atomic structure and molecular mechanism of transport are essential goals that require large amounts of functional PCFT. Here, we present a high-level heterologous expression system for human PCFT using a recombinant baculovirus and Spodoptera frugiperda (Sf9) insect cells. We demonstrate folate transport functionality along the PCFT expression, isolation, and purification process. Importantly, purified PCFT transports folic acid after reconstitution. We thus succeeded in overcoming heterologous expression as a major bottleneck of PCFT research. The availability of an overexpression system for human PCFT provides the basis for future biochemical, biophysical and structural studies.

Nicotinic acetylcholine receptors (nAChR) are members of the Cys-loop ligand-gated ion channel superfamily. Muscle nAChR are heteropentamers that assemble from two α, and one each of β, γ, and δ subunits. Each subunit is composed of three domains, extracellular, transmembrane and intracellular. The transmembrane domain consists of four α-helical segments (M1–M4). Pioneering structural information was obtained using electronmicroscopy of Torpedo nAChR. The recently-solved X-ray structure of the first eukaryotic Cys-loop receptor, a truncated (intracellular domain missing) glutamate-gated chloride channel α (GluClα)showed the same overall architecture . However, a significant difference with regard to the vertical alignment between the channel-lining segment M2 and segment M3 was observed. Here we used functional studies utilizing disulfide trapping experiments in muscle nAChR to determine the spatial orientation between M2 and M3. Our results are in agreement with the vertical alignment as obtained when using the GluClα structure as a template to homology model muscle nAChR, however, they cannot be reconciled with the current Torpedo nAChR model. The vertical M2–M3 alignments as observed in X-ray structures of prokaryotic Gloeobacter violaceus ligand-gated ion channel (GLIC) and GluClα are in agreement. Our results further confirm that this alignment in Cys-loop receptors is conserved between prokaryotes and eukaryotes.

Folate cofactors play important roles in hundreds of metabolic reactions in a cell such as synthesis of protein and DNA precursors. The human proton-coupled folate transporter (PCFT) is the only means for absorption of dietary folates. PCFT also transports anti-cancer agents such as Pemetrexed and Methotrexate. Inadequate supply of folates and impaired PCFT function is associated with many disorders such as cancer, hereditary folate malabsorption, hypercysteinemia, heart diseases, obesity, anemia, neural tube defects and Alzheimer's disease. It is therefore important to study structural and functional characteristics of PCFT, to understand folate-homeostasis mechanisms and to develop better folate-based therapies. The primary requirement in structural and functional characterization of mammalian membrane proteins such as PCFT is the availability of good expression systems and methods to obtain large quantities of purified protein. To address this primary concern, we are establishing an efficient system to over-express PCFT and developing methods to obtain purified PCFT in good yields. We have expressed human PCFT in insect, yeast, and bacterial cells. We have successfully purified PCFT to homogeneity. Importantly, we also demonstrate that the heterologously-expressed PCFT is functional. Our findings are significant because 12-transmembrane helical, mammalian membrane proteins such as PCFT are notoriously difficult to express in versatile prokaryotic and single-celled eukaryotic systems. Furthermore, due to their predominant hydrophobic character and requirement of detergent and/or lipid environment for their stability, such membrane proteins are difficult to purify. Each of the three PCFT expression systems that we have developed offers unique advantages over another depending on the intended use of PCFT. Our findings will be significant for the advancement of knowledge concerning both the structure and function of PCFT through basic science research as well as for industrial applications including design and screening of PCFT-targeted agents.

Nicotinic acetylcholine (nACh), γ-aminobutyric acid type A (GABAA), glycine, and 5-HT3 (serotonin) receptors are members of the pentameric ligand-gated ion channel (pLGIC) superfamily. Each channel consists of five homologous subunits consisting of three domains, namely, extracellular (ECD), transmembrane (TMD), and intracellular domain (ICD). The ECD and TMD that display high sequence-similarity between different subunits are well-studied and their structures have been solved. The ICD on the contrary is extremely diverse between subunits and therefore may provide a specific target for future drug development. For both nAChR and 5-HT3A receptors the chaperone protein resistance to inhibitors of cholinesterase (RIC-3) modulates functional maturation. We have shown previously, that the replacement of the ICD in 5-HT3A by a heptapeptide abrogates this modulation by RIC-3. Additionally, we have shown, that adding either the 5-HT3A or nAChRα7 ICD to the prokaryotic Gloeobacter violaceus pLGIC (GLIC) confers RIC-3 sensitivity to otherwise RIC-3-insensitive GLIC. Together, this demonstrated that the ICD is required for the RIC-3 modulation. The present study is aimed at identifying the molecular identity within the 5-HT3A-ICD that interacts with RIC-3. To this end we have genetically engineered chimeras containing the 5-HT3A-ICD and expressed them in Escherichia coli (E. coli). These chimeras are then purified to homogeneity and their interaction with RIC-3, which is separately expressed and purified from E. coli, assessed. It is thought that RIC-3 associates with monomers and promotes pentamerization. Therefore, we investigate both 5-HT3A-ICD chimera constructs that form pentamers, but also those that form monomers. Additionally, we are using 5-HT3A-ICD chimera constructs to further identify interacting cytosolic proteins.

Pentameric ligand-gated ion channels (pLGICs) are abundant players in fast chemical signal transduction both in and outside the nervous system. During recent years, EM and X-ray structures of prokaryotic homologues and more recently also of eukaryotic pLGICs have been published in several functional states. Despite these atomic-level insights pertaining to the extracellular and transmembrane domains that are highly conserved, the diverse intracellular domain remains enigmatic. Ion channel chimeras as well as further optimized soluble chimeras provide novel insights into the intracellular domain of this superfamily of ion channels.

The GABAAρ1 pentameric ligand-gated ion channel (pLGIC) is an anion-conducting receptor of the Cys-loop superfamily gated by the neurotransmitter γ-aminobutyric acid (GABA). GABAAρ1 and other anionic pLGICs are the targets of pharmacotherapy including general anesthetics, anti-epileptics (anti-convulsants), sedatives, anxiolytics, and muscle relaxants. In an effort to improve current treatments and develop new drugs, further studies characterizing the pLGICs and more specifically their structural elements that contribute to their function are essential. Each subunit of eukaryotic Cys-loop receptors is comprised of three domains: extracellular domain (ECD), transmembrane domain (TMD), and intracellular domain (ICD). Several structural studies including X-ray crystallography of prokaryotic as well as eukaryotic pLGICs have elucidated the three-dimensional structures of both the ECD and TMD. However, the complete picture on the three-dimensional structure of the ICD is not available. This is partly due to the lack of the ICD in prokaryotic pLGICs, such as Gleoeobacter violaceus ligand-gated ion channel (GLIC). Furthermore, in order to obtain higher resolution X-ray crystallographs of mammalian Cys-loop receptors, the ICD is frequently modified. Of the three domains, the ICD of Cys-loop receptors in the animal kingdom has the most diversity in terms of amino acid sequence and length. Therefore, drugs selective for the ICD would be an ideal alternative to current pharmacotherapy ridden with many side effects that result from targeting the highly-conserved extracellular or transmembrane domains.Our project focused on the expression and large-scale purification of GABAAρ1 chimeras in amounts that would allow future structural studies. A soluble chimera containing the ICD was over-expressed in E. coli cells and purified to homogeneity using a two-step purification process. We were successful in producing approximately 3-4 mg of purified protein from a liter of bacterial culture.

The domain topology of eukaryotic pentameric ligand-gated ion channels (pLGICs) from the Cys-loop receptors superfamily exclusively contains three domains: the most diverse and at the same time least studied intracellular domain (ICD), the extracellular domain (ECD) and the transmembrane domain (TMD). The recently published report from our laboratory has clearly established that the engineered chimera comprised of the ICD of the 5-HT3A receptor downstream of a maltose binding protein (MBP) assembles into stable pentamers in solution (Pandhare A. et al., Sci. Rep. 6, 23921(2016)). Notably, the ordered assembly behavior assigns a novel role for the ICD in receptor oligomerization along with the other two domains, and further enriches its functional repertoire. Therefore, in order to fully characterize ICD oligomerization, current studies are directed at identifying the molecular determinants of pentameric organization by employing site-directed mutagenesis as well as heterologous expression in E. coli and protein purification followed by size exclusion chromatography in line with multi-angle light scattering (SEC-MALS). Additionally, we are studying the intrinsic assembly kinetics of pentamer formation to determine self-association affinities by utilizing isothermal titration calorimetry.

The molecular basis of general anesthetic interactions with GABA(A) receptors is uncertain. An accurate homology model would facilitate studies of anesthetic action. Construction of a GABA(A) model based on the 4 A resolution acetylcholine receptor structure is complicated by alignment uncertainty between the acetylcholine and GABA(A) receptor M3 and M4 transmembrane segments. Using disulfide crosslinking we previously established the orientation of M2 and M3 within a single GABA(A) subunit. The resultant model predicts that the betaM3 residue beta2M286, implicated in anesthetic binding, faces the adjacent alpha1-M1 segment and not into the beta2 subunit interior as some models have suggested. To assess the proximity of beta2M286 to the alpha1-M1 segment we expressed beta2M286C and gamma2 with 10 consecutive alpha1-M1 cysteine (Cys) mutants, alpha1I223C to alpha1L232C, in and flanking the extracellular end of alpha1-M1. In activated states, beta2M286C formed disulfide bonds with alpha1Y225C and alpha1Q229C based on electrophysiological assays and dimers on Western blots, but not with other alpha1-M1 mutants. beta2F289, one helical turn below beta2M286, formed disulfide bonds with alpha1I228C, alpha1Q229C and alpha1L232C in activated states. The intervening residues, beta2G287C and beta2C288, did not form disulfide bonds with alpha1-M1 Cys mutants. We conclude that the beta2-M3 residues beta2M286 and beta2F289 face the intersubunit interface in close proximity to alpha1-M1 and that channel gating induces a structural rearrangement in the transmembrane subunit interface that reduces the betaM3 to alphaM1 separation by approximately 7 A. This supports the hypothesis that some intravenous anesthetics bind in the betaM3-alphaM1 subunit interface consistent with azi-etomidate photoaffinity labeling.

Nicotinic acetylcholine receptors (nAChR) are cation-selective, ligand-gated ion channels of the cysteine (Cys)-loop gene superfamily. The recent crystal structure of a bacterial homolog fromErwinia chrysanthemi(ELIC) agrees with previous structures of the N-terminal domain of AChBP (acetylcholine-binding protein) and of the electron-microscopy-derivedTorpedonAChR structure. However, the ELIC transmembrane domain is significantly more tightly packed than the corresponding region of theTorpedonAChR. We investigated the tightness of protein packing surrounding the extracellular end of the M2 transmembrane segment and around the loop connecting the M2 and M3 segments using the substituted cysteine accessibility method. The M2 20′ to 27′ residues were highly water accessible and the variation in reaction rates were consistent with this region being α-helical. At all positions tested, the presence of ACh changed methanethiosulfonate ethylammonium (MTSEA) modification rates by TorpedonAChR transmembrane domain structure is a better model than the ELIC structure for eukaryotic Cys-loop receptors.

Cys-loop receptor neurotransmitter-gated ion channels are pentameric assemblies of subunits that contain three domains: extracellular, transmembrane, and intracellular. The extracellular domain forms the agonist binding site. The transmembrane domain forms the ion channel. The cytoplasmic domain is involved in trafficking, localization, and modulation by cytoplasmic second messenger systems but its role in channel assembly and function is poorly understood and little is known about its structure. The intracellular domain is formed by the large (>100 residues) loop between the α-helical M3 and M4 transmembrane segments. Putative prokaryotic Cys-loop homologues lack a large M3M4 loop. We replaced the complete M3M4 loop (115 amino acids) in the 5-hydroxytryptamine type 3A (5-HT3A) subunit with a heptapeptide from the prokaryotic homologue from Gloeobacter violaceus. The macroscopic electrophysiological and pharmacological characteristics of the homomeric 5-HT3A-glvM3M4 receptors were comparable to 5-HT3A wild type. The channels remained cation-selective but the 5-HT3A-glvM3M4 single channel conductance was 43.5 pS as compared with the subpicosiemens wild-type conductance. Coexpression of hRIC-3, a protein that modulates expression of 5-HT3 and acetylcholine receptors, significantly attenuated 5-HT–induced currents with wild-type 5-HT3A but not 5-HT3A-glvM3M4 receptors. A similar deletion of the M3M4 loop in the anion-selective GABA-ρ1 receptor yielded functional, GABA-activated, anion-selective channels. These results imply that the M3M4 loop is not essential for receptor assembly and function and suggest that the cytoplasmic domain may fold as an independent module from the transmembrane and extracellular domains.

For more than two decades, bupropion has been clinically prescribed for the treatment of depression (Wellbutrin®), and more recently for smoking cessation (Zyban®). Bupropion is conventionally described as a dual norepinephrine dopamine reuptake inhibitor, therefore effective as an antidepressant. However, we and others have shown that it also interacts with receptors of the Cys-loop superfamily, namely neuronal nicotinic acetylcholine receptors. In the continuum of exploring the molecular interactions of bupropion with other prominent members of the Cys-loop superfamily, we report here that bupropion additionally inhibits 5-hydroxytryptamine (serotonin) type 3A receptors (5-HT3ARs).5-HT3ARs are cation-conducting, homo-pentameric ligand-gated ion channels of the Cys-loop superfamily which are structurally and functionally distinct from other serotonin receptors that are G-proteins. 5-HT3 receptors are current targets for anti-emetics mainly used in cancer chemotherapy, as well as potential future targets for disorders including anxiety, schizophrenia, and Alzheimer's disease.Here we characterized our newly-identified interaction of bupropion with 5-HT3ARs with electrophysiological and radioligand binding studies. In oocytes, bupropion inhibited 5-HT3AR-mediated currents with an IC50 value of 87 µM (nH = 1.2). That 300 µM bupropion inhibited [3H]-5-HT or [3H]-granisetron (a competitive antagonist) binding by ∼ 20% indicated at best weak interaction with the agonist binding site. In contrast, the effect of different concentrations of bupropion on serotonin concentration-response curves suggested a non-competitive nature of interaction. Along these lines, importantly, the inhibition of serotonin-evoked currents recorded from oocytes was proportional to the length of preincubation time with bupropion.In summary, our results are indicative of a negative allosteric interaction of bupropion with the 5-HT3AR. Currently, experiments are underway to identify the site(s) of interaction for bupropion within the 5- HT3AR.

Construction of a GABAAreceptor homology model based on the acetylcholine (ACh) receptor structure is complicated by the low sequence similarity between GABAAand ACh M3 transmembrane segments that creates significant uncertainty in their alignment. We determined the orientation of the GABAAM2 and M3 transmembrane segments using disulfide cross-linking. The M2 residues α1M266 (11′) and α1T267 (12′) were mutated to cysteine in either wild type or single M3 cysteine mutant (α1V297C, α1A300C to α1A305C) backgrounds. We assayed spontaneous and induced disulfide bond formation. Reduction with DTT significantly potentiated GABA-induced currents in α1T267C-L301C and α1T267C-F304C. Copper phenanthroline-induced oxidation inhibited GABA-induced currents in these mutants and in α1T267C-A305C. Intrasubunit disulfide bonds formed between these Cys pairs, implying that the α-carbon separation was at most 5.6 Å. The reactive α1M3 residues (L301, F304, A305) lie on the same face of an α-helix. The unresponsive ones (A300, I302, E303) lie on the opposite face. In the resting state, the reactive side of α1M3 faces M2-α1T267. In conjunction with the ACh structure, our data indicate that alignment of GABAAand ACh M3 requires a single gap in the GABAAM2–M3 loop. In the presence of GABA, oxidation of α1T267C-L301C and α1T267C-F304C had no effect, but oxidation of α1T267C-A305C caused a significant increase in spontaneous channel opening. We infer that, as the channel opens, the distance and/or orientation between M2-α1T267 and M3-α1A305 changes such that the disulfide bond stabilizes the open state. This begins to define the conformational motion that M2 undergoes during channel opening.

Based on theTorpedoacetylcholine receptor structure, Unwin and colleagues (Miyazawa et al., 2003; Unwin, 2005) hypothesized that the transduction of agonist binding to channel gate opening involves a “pin-into-socket” interaction between αV46 at the tip of the extracellular β1-β2loop and the transmembrane M2 segment and M2-M3 loop. We mutated to cysteine the aligned positions in the 5-HT3Aand 5-HT3Bsubunit β1-β2loops K81 and Q70, respectively. The maximal 5-HT-activated currents in receptors containing 5-HT3A/K81C or 5-HT3B/Q70C were markedly reduced compared with wild type. Desensitization of wild-type currents involved fast and slow components. Mutant currents desensitized with only the fast time constant. Reaction with several methanethiosulfonate reagents potentiated currents to wild-type levels, but reaction with other more rigid thiol-reactive reagents caused inhibition. Single-channel conductances of wild type, K81C, and K81C after modification were similar. We tested the proximity of K81C to the M2-M3 loop by mutating M2-M3 loop residues to cysteine in the K81C background. Disulfide bonds formed in 5-HT3A/K81C/A304C and 5-HT3A/K81C/I305C when coexpressed with 5-HT3B. We conclude that in the resting state, K81 is not in a hydrophobic pocket as suggested by the pin-into-socket hypothesis. K81 interacts with the extracellular end of M2 and plays a critical role in channel opening and in the return from fast desensitization. We suggest that during channel activation, β1-β2loop movement moves M2 and the M2-M3 loop so that the M2 segments rotate/translate away from the channel axis, thereby opening the lumen. Recovery from fast desensitization requires the interaction between K81 and the extracellular end of M2.

The pentameric ligand-gated ion channel (pLGIC) superfamily includes the nicotinic acetylcholine (nACh), 5-HT3, ɣ-aminobutyric acid type A (GABAA), and glycine receptor. These channels are made up of five homologous subunits surrounding a central ion channel pore and can be divided into three pentameric domains. The extracellular domain (ECD) and transmembrane domain (TMD) of the channels show high levels of homology and have been well studied and structures have been published. In contrast, the intracellular domain (ICD) shows sequence and length divergence in the different receptors. Mutations in the ICD have been shown to alter receptor conductance. Gloeobacter violaceus LGIC (GLIC) is a prokaryotic pLGIC without a native ICD, which can be expressed and studied in Xenopus oocytes. To elucidate the mechanisms by which the ICD affects the channel pathway and conductance we have created chimeras of GLIC ECD and TMD with ICDs of both anion- and cation-conducting pLGICs (GABAρ1, Glyα1, nAChRα7, 5-HT3A). Functional expression of chimeras was first confirmed by applying low pH (pH 5) pulses to oocytes under two-electrode voltage clamp. We have begun to study the single channel conductance and gating kinetics of chimeric channels and confirmed that the GLIC wildtype has a single-channel conductance of 10pS. For the GLIC-5-HT3A-ICD chimera, we expect to see a decrease in conductance as we observed an increase in a construct of 5-HT3A upon removing the ICD. Amino acid engineering in both the chimeras and eukaryotic parent channels will then be used to further understanding of how the ICDs contribute to ion conducting properties.

Folate cofactors, Vitamin B9, play crucial roles in more than a hundred one-carbon metabolism reactions in mammalian cells. Humans cannot synthesize folates de novo and absorption through the diet is the only source of this vitamin. The proton-coupled folate transporter (PCFT) mediates this uptake in the upper small intestine by a pH-dependent process. PCFT also transports folates into the central nervous system. Point mutations in the PCFT gene cause Hereditary Folate Malabsorption (HFM), with associated hematological and neurological defects due to impaired folate transport. Certain solid tumor cell lines express high levels of PCFT mRNA. Importantly, the overall expression of PCFT is limited to only certain tissues in humans. Therefore, PCFT is a molecular target for specific delivery of anti-folate chemotherapeutic agents to tumor cells. Unfortunately, the success rate of anti-folate agents to reach clinical use is very low due to their side-effects. A structural model of PCFT can aid the development of specific anti-folate agents for their PCFT-targeted delivery and thus minimize their side-effects. To optimize and verify an initial structural model of PCFT, we applied a wide array of experimental approaches: solvent accessibility profiling through substituted cysteine accessibility scanning, studying the helix packing, and assaying the functionality of PCFT mutants. We complemented the experimental data with theoretical approaches for structure prediction such as homology modeling/threading, ligand docking and an extensive review of other secondary transporters and folate transport proteins. With these combined approaches, we have developed a structural model of PCFT that accurately reflects our experimental observations. This model forms a basis to understand the impacts of HFM mutations, and to develop folate analogues for folate deficiency intervention. Additionally, it will aid the design of PCFT structure-based anti-folate agents for the treatment of cancer.

International audience; Although the activity of the nicotinic acetylcholine receptor (nAChR) is exquisitely sensitive to its membrane environment, the underlying mechanisms remain poorly defined. The homologous prokaryotic pentameric ligand-gated ion channel, Gloebacter ligand-gated ion channel (GLIC), represents an excellent model for probing the molecular basis of nAChR sensitivity because of its high structural homology, relative ease of expression, and amenability to crystallographic analysis. We show here that membrane-reconstituted GLIC exhibits structural and biophysical properties similar to those of the membrane-reconstituted nAChR, although GLIC is substantially more thermally stable. GLIC, however, does not possess the same exquisite lipid sensitivity. In particular, GLIC does not exhibit the same propensity to adopt an uncoupled conformation where agonist binding is uncoupled from channel gating. Structural comparisons provide insight into the chemical features that may predispose the nAChR to the formation of an uncoupled state.

Cationic pentameric ligand-gated ion channels (pLGICs) of the Cys-loop superfamily function as receptors for endogenous (e.g. acetylcholine, serotonin) and/or exogenous (e.g. nicotine) ligands. These receptors are the targets for a number of current clinical drugs used to treat nicotine addiction, Alzheimer's disease, Parkinson's disease, epilepsy, pain perception, anxiety, and depression. In order to develop new, more effective drugs and improve current treatments, research is imperative to identify and characterize the different structural elements that mediate each aspect of receptor function. From an early electron density map of a pLGIC of the Cys-loop superfamily to the most recent high-resolution crystal structures of mammalian Cys-loop receptors, their structural information has revealed that the overall architecture of these pLGICs is highly-conserved. The intracellular domain (ICD) presents the notable exception in that the functions, lengths, amino acids sequences, and likely also structural folds are vastly different between subunits. Based on its diversity, the ICD presents a target for developing subtype-selective drugs with the hope of fewer side effects than current drugs, all of which target the highly-conserved extracellular or transmembrane domains.We developed a protein expression and purification strategy to produce the ICD of a cationic pLGIC in sufficient quantity and quality. Chimeras were generated by adding the ICD of the α7 nicotinic acetylcholine receptor to a soluble protein, and conditions for optimal expression in E. coli were identified. A two-step purification process consisting of affinity chromatography followed by size-exclusion chromatography yielded 5 mg of purified protein per liter of bacterial culture. Thus, our results are consistent with the successful large-scale expression and purification of the α7-ICD chimera that is now amenable to structural studies.

The 115 amino acids spanning intracellular domain (ICD) links the third (M3) and fourth (M4) transmembrane segments within each subunit of the 5-HT3A receptor, a cationic homo-pentameric ligand-gated ion channel (pLGIC). The advent of increasingly sophisticated structural studies has unraveled, in part, the three-dimensional structure of eukaryotic as well as prokaryotic pLGICs at atomic resolution. Intriguingly, within all these structures, the ICD has commonly not been completely resolved. To illustrate the lack of completeness, a recent high-resolution 5-HT3A structure, albeit providing novel structural insights, is deprived of 61 amino acids from its ICD resulting in channel activity differing remarkably from wild-type channels. Prokaryotic pLGICs lack the ICD, and instead contain a very short M3-M4 linker. In eukaryotic pLGICs, the ICD exemplifies the diversity of domain architecture, unlike the other two domains - extracellular and transmembrane - which are highly-conserved. Given the expanding functional implications of ICDs in subunit folding, assembly, targeting, posttranslational modifications, reverse allosteric conductance, and protein-protein interactions, it is of great importance to resolve the structures of full-length ICDs in order to bridge this knowledge gap.To attain this set goal, we produced and purified a soluble chimera containing the ICD of the mouse 5-HT3A receptor. Molecular weight determination by both size exclusion chromatography and dynamic light scattering indicates stable pentameric assembly of the chimera in solution. This surprising finding was further corroborated when the chimera was cross-linked with glutaraldehyde vapors and then resolved either by SDS-PAGE or ‘on-chip electrophoresis’, revealing a ladder of discrete covalently-linked oligomers. The unexpected observation that the ICD alone assembles into stable pentamers in solution and its extreme diversity with regard to both length and amino acid-composition is further suggestive of its orphaned role in receptor oligomerization and potentially the specificity thereof.

Folate vitamins are essential for DNA replication and cellular proliferation. However, mammalian organisms are devoid of de novo folate biosynthesis and thus rely on dietary sources to meet their metabolic needs. The proton coupled folate transporter (PCFT/SLC46A1) has been recently identified as the molecular entity of the carrier mediated intestinal folate uptake pathway for folic acids from food sources. PCFT is also involved in the absorption of chemotherapeutically used antifolates. Currently, there is limited information about the structure and function of PCFT. Hydropathy analysis suggests that there are 10-12 transmembrane segments. Further, using the Substituted Cysteine Accessibility Method (SCAM) evidence was provided for a 12 transmembrane segment topology. Blue Native Polyacrylamide Gel Electrophoresis (BN-PAGE) is a technique for separation of protein complexes in a native state with high resolution. We expressed PCFT in Xenopus laevis oocytes. Oocyte plasma membranes were polymerized to the vitelline membrane using ludox colloidal silica solution and polyacrylic acid, isolated by centrifugation, and plasma membrane proteins subsequently solubilized with digitonin and separated by BN-PAGE. The separation characteristics of native PCFT were compared to a molecular ruler produced by partial dissociation of homopentameric 5-hydroxytryptamine type 3A (5HT3A) receptors. Under native conditions, 5HT3A subunits largely migrated as a pentamer and PCFT only as a monomer. Treatment with denaturing agents generated a ladder of five bands for 5HT3A subunits, which consisted of monomer, dimer, trimer, tetramer and pentamer. Addition of crosslinking agents resulted in migration of 5HT3A subunits as a pentamer, even in the presence of denaturing agents. In contrast, crosslinking agents did not induce oligomeric assemblies of PCFT. These results indicate that functional plasma-membrane bound PCFT is a monomeric protein.

Folate cofactors play crucial roles in hundreds of reactions in cells including DNA and protein synthesis. The human proton-coupled folate transporter (PCFT) is the only means of absorption of dietary folates in humans. PCFT expression and function is associated with many disorders including hereditary folate malabsorption, neural tube defects, Down syndrome, cancer, heart diseases, Alzheimer's and Parkinson's disease. Upregulation of PCFT expression in tumor cells is of significant consideration for development of PCFT-targeted chemotherapeutic agents. However, not much is known about the structure and function of PCFT, which contributes to the low clinical success rate of folate-based agents. To address this gap in the knowledge we performed extensive Cys-mutagenesis studies. We analyzed 40 residues towards the extracellular face of PCFT, 35 positions towards the cytoplasmic face of PCFT and 28 positions along the proposed folate-binding pocket of PCFT. Based on the accessibility studies of the extracellular face of PCFT we determined loop-helix boundaries of this face and identified the glycerol-3-phosphate transporter (PDB#1PW4) and tripeptide-proton symporter (PDB#4APS) as the best templates for modeling PCFT. Based on the accessibility studies of the cytoplasmic face of PCFT, we identified loop-helix boundaries of this face. Here, we also show that our accessibility studies support the hypothesis that PCFT is present predominantly in an inward-open conformation in the absence of substrate (pH 7.5 and no folic acid). We also show that the folate-binding pocket of PCFT is formed by residues present in PCFT transmembrane helices I, IV, V, X and XI. Our results are of high significance in understanding the details of folate-homeostasis mechanisms and in design of PCFT-targeted therapeutic and diagnostic agents.

Bupropion is one of the most common drugs prescribed for treating depression and aiding smoking cessation, and is generally well tolerated with the exception of a rare dose-dependent incidence of seizures. It inhibits norepinephrine and dopamine reuptake, as well as noncompetitively antagonizes neuronal nicotinic acetylcholine receptors, which are members of the Cys-loop superfamily. While investigating its effects on other key members of this superfamily, bupropion showed complex modulation of the γ-aminobutyric acid type A (GABAA) α1β2γ2 receptor function.In the brain, GABAA α1β2γ2 receptors are one of the most abundant chloride-conducting hetero-pentameric ligand-gated ion channels. They are targets for a number of drugs including anesthetics, anticonvulsants, anxiolytics, hypnotics, and muscle relaxants.To the best of our knowledge, the interaction of bupropion with the GABAA receptor has not yet been described. Therefore, the primary goal of this study was to characterize the functional interactions of bupropion with the GABAA α1β2γ2 receptor. In oocytes, bupropion-alone (0.1 mM - 3 mM) directly activated GABAA receptor-mediated currents. The comparison of dose-response curves showed that bupropion-activation was lower in potency (EC50=1.5 mM vs. 1.3 μM) and efficacy (49.5% vs. 100%) than that for GABA, consistent with partial agonism of the GABAA receptor. Interestingly, at lower concentrations (0.001 µM - 100 μM) bupropion inhibited 0.5 μM GABA-induced currents by ∼ 8 - 20%; in contrast, at higher concentrations (1 mM - 3 mM) it potentiated GABA-induced currents by ∼ 50 - 70%. Notably, in many ways bupropion mimics the actions of some of the general anesthetics (e.g. propofol) at the GABAA receptor. This warrants exploring the bupropion binding site(s) within these receptors.

Cys-loop receptors are pentameric ligand-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that forms the ion channel pore and an intracellular domain (ICD). All prokaryotic homologs of Cys-loop receptors, for example, GLIC and ELIC, lack an ICD that consists of a 50 to 270 amino acid long loop in eukaryotes. Very little is known about the structure of the ICD since none of the recently published X-ray structures of prokaryotic homologs contained an ICD, and the Torpedo nAChR cryo-EM structure resolved only 1/3rd of the ICD. We have recently constructed chimeras in which the ICD from eukaryotic serotonin (5-HT3A) receptors was inserted into GLIC (1).To extend our studies we generated GLIC chimeras with ICDs from other Cys-loop receptors (GlyR-α1, GABA-ρ1, nAChR-α7). All chimeras were expressed heterologously in Xenopus laevis oocytes to investigate the ion channel functionality by using two-electrode voltage clamp experiments. All functional chimeras were also expressed in E. coli membranes to enable overexpression and protein purification. Chimeric proteins were successfully purified by using Ni-NTA affinity and size exclusion chromatography (SEC). Limited proteolysis and spectroscopic methods (circular dichroism and fourier transform infrared spectroscopy) will be used to study conformational features and secondary structure elements of eukaryotic ICDs of the engineered GLIC-ICD chimeras. Our data will shed light on the structure and function of the intracellular domain of Cys-loop receptors, which will enable identification of potentially important drug targeting sites in intracellular domains for the treatment of neurodegenerative diseases.1. Goyal R. et al, (2011). J Biol Chem 286, 34635-42

Receptors belonging to the Cys-loop gene superfamily of neurotransmitter-gated ion channels (e.g. acetylcholine, serotonin, GABA, and glycine receptors) mediate fast synaptic transmission. These receptors are targeted by a number of clinically used drugs (e.g. antiepileptics, antipsychotics, anesthetics). Cys-loop receptors contain three domains: extracellular (ECD), transmembrane (TMD), intracellular (ICD). The ECD and TMD have been studied in great detail and their 3-D structures have been determined. The recent identification of bacterial Cys-loop receptor homologues has propelled the structural knowledge to atomic resolution. However, all prokaryotic members lack the ICD. The ICD of metazoan Cys-loop receptors is the most diverse domain with respect to both length and amino-acid composition. The ICD therefore represents an attractive target for developing subtype-selective drugs with the promise of fewer side effects than drugs that target the highly-conserved ECD and TMD. We have engineered functional chimeras with ICDs from different eukaryotic receptors in the two-domain prokaryotic homologue GLIC (one chimera set published, Goyal, Salahudeen, Jansen, JBC 2011). Based on structure predictions others have hypothesized that the ICD is mainly unstructured, but contains an α-helical segment pre-TM4. We hypothesize based on computational results that cationic and anionic receptors have different ICD secondary structures, i.e., only cationic receptors contain an α-helical portion pre-TM4. For each eukaryotic ICD we generated 12 chimeras that differ by the linkers both N- and C-terminal to the inserted ICD. Interestingly, the number of functional chimeras in each set varies drastically between different ICDs. We are testing the functional consequences of inserting various ICDs using electrophysiological studies, and investigating the structure of the ICDs tethered to the TMD and at the lipid bilayer with spectroscopic and biochemical methods.

The Proton-Coupled Folate Transporter (PCFT) encoded by the gene SLC46A1 is the major pathway for uptake of folic acid in the intestine and into the brain. Mutations in the PCFT gene lead to hereditary folate malabsorption (HFM), a folate deficiency disease characterized by impaired intestinal folate absorption and transport of folate into the central nervous system. PCFT is also involved in the uptake of antifolates such as Pemetrexed by tumor cells. At the present time our knowledge about the structure/function relationships of PCFT are very limited. Our initial goal is to identify the lipid-protein interface of PCFT by identifying transmembrane segments labeled by hydrophobic photoreactive probes like 1-Azidopyrene (1-AP) and 2-[3H]diazofluorene ([3H]DAF). These probes partition efficiently (>95%) into the lipid bilayer and upon UV-irradiation form covalent linkages with close-by amino acids. 1-AP and [3H]DAF labeled peptides can then be identified by mass spectrometry or by automated Edman degradation coupled with radio-sequencing. To carry out these experiments requires large amounts (several mg) of PCFT protein. Out of three expression systems we have evaluated (mammalian, cell-free, and bacterial), expression in BL21 E. coli is so far the most promising. From preliminary labeling experiments we have confirmed the membrane insertion of PCFT: 1) PCFT membranes were washed with NaHCO3 and urea, and 2) With 1-AP labeled PCFT, we identified peptide fragments of PCFT using mass spectrometry. Future experiments will involve: 1) screening of additional bacterial expression host strains, 2) labeling of PCFT with [3H]DAF and identification of labeled peptides as well as individual labeled amino-acids using radio-sequencing. Our results will define the PCFT lipid-protein interface and therefore will help verify 1) the α-helical secondary structure of transmembrane segments and 2) the arrangement of transmembrane segments within the membrane-spanning domain.

The Cys-Loop gene super-family of ligand gated ion channels includes nicotinic acetylcholine (nACh), gamma aminobutyric acid (GABAA), glycine (Gly), and serotonin (5-HT3A) receptors. Each receptor is either a homo- or heteropentamer made up of 5 identical or homologous subunits. Each subunit has an extracellular N-terminal domain, which houses the ligand binding site; a transmembrane domain which spans the membrane four times as α-helical segments (M1-M4); and a long loop between M3 and M4 that constitutes the intracellular domain. M2 lines the ion channel, whereas M1, M3 and M4 are abluminal.Muscle nAChRs consist of four different subunits with the clockwise arrangement αγαβδ when viewed from the extracellular side. We used disulphide trapping between individually engineered Cys in the αM2 segments to investigate arrangement and flexibility of the upper part of αM2. Disulfide bond formation was monitored in α2βδγ nAChR expressed in Xenopus laevis oocytes by two electrode voltage clamp experiments and Western blotting. In properly arranged Cys pairs disulfide bond formation can either occur spontaneously or it can be induced by oxidizing with copper phenanthroline (CuPhen). Cystine bond formation is reversible by reducing with dithiothreitol (DTT). To eliminate interference from the native vicinal disulphide present in the ligand binding site of the α-subunit, we utilized the background mutations αC192S-C193S. Position αE262C that was previously shown to face the channel formed DTT reducible dimers both spontaneously and upon application of the oxidizing agent CuPhen. Based on Unwin's 4-A resolution model, the formation of disulfide bonds at this channel level would require substantial movement of the channel-lining M2 segments. No dimer formation was observed in αL263C. We are investigating a series of αM2 Cys mutants to determine which positions in M2 can form disulfide bonds.

Bupropion is clinically prescribed for the treatment of depression (Wellbutrin) and for smoking cessation (Zyban). While there is consensus that the primary mechanism of action involves increased synaptic concentrations of dopamine and norepinephrine via inhibition of the respective reuptake transporters, DAT and NET, there is growing evidence that some of the therapeutic benefits of bupropion may result from non-competitive antagonism of neuronal nicotinic acetylcholine receptors (nAChRs), in particular α4β2 and α3β4 subtypes. To aid in elucidating the mechanism of bupropion action and the development of new therapeutic agents for smoking cessation and depression, our goal is to determine the specific molecular interactions between bupropion and several nAChR subtypes (Torpedo, α4β2, α3β4). We first established that bupropion dose-dependently and reversibly inhibits ACh-induced currents in Xenopus laevis oocytes injected with affinity-purified and lipid-reconstituted Torpedo nAChRs (IC50 0.34 μM). Using a photoreactive analog of bupropion ([125I]-SADU-3-72), we next established that bupropion inhibits the binding of [125I]-SADU-3-72 to the Torpedo nAChR (IC50 closed state, 3.6 μM; desensitized state, 1.2 μM) and that binding to the closed nAChR is fully inhibited by tetracaine (IC50 0.42 μM) consistent with a bupropion/[125I]-SADU-3-72 binding site within the nAChR channel. [125I]-SADU-3-72 photolabeled Torpedo nAChR subunits in an agonist-sensitive and bupropion-inhibitable (specific) manner. Finally for the closed nAChR, we identified sites of specific [125I]-SADU-3-72 labeling within δM2 (δLeu265/δM2-9) and βM2 (βLeu257/βM2-9). In the desensitized state, TCP-inhibitable (specific) labeling was identified within δM2 (δLeu265, δM2-9 with minor labeling of δSer258, δM2-2). Our results establish that bupropion binds in the middle (M2-9) of the Torpedo nAChR channel, with a slightly broader binding locus in the desensitized channel. Currently, studies are underway to identify the site(s) of [125I]-SADU-3-72 labeling in affinity-purified α4β2 and α3β4 nAChRs.

Folate vitamins are essential for DNA replication and cellular proliferation. However, mammalian organisms are devoid of folate biosynthesis and rely on dietary sources to meet their metabolic requirement for folate cofactors. The proton coupled folate transporter (PCFT/SLC46A1) has been recently identified as the molecular entity responsible for intestinal folate uptake displaying optimal transport activity at acidic pH. PCFT is also involved in transport of chemotherapeutically used antifolates. Currently, there is limited experimentally derived information about the structure and transmembrane topology of PCFT. Hydropathy analysis suggests 10 to 12 transmembrane segments for PCFT. The aim of our study is to construct a reliable homology model and study the structure function interplay in more detail experimentally. We have made various DNA constructs with different N- and C-terminal tags for the heterologous expression and purification of PCFT: 1) for bacterial expression of the gene, 2) for expression in oocytes and mammalian cells, 3) for cell free expression using the Membrane Max cell free expression kit. We will screen the expression systems to determine which is optimal for generating suitable quantity / quality protein that will be used for structural studies. We will engineer individual Cysteines to study the topology with the Substituted Cysteine Accessibility Model (SCAM). Accessibility will be assessed by Western Blotting (PEGylation, biotinylation). The lipid-protein interface will be investigated by hydrophobic photoaffinity labeling. Both SCAM and photoaffinity labeling will be used to study substrate pathways. We will complement these studies with functional assays (uptake and two electrode voltage clamping). Results from our structure function studies of PCFT will be used in exploring therapeutic strategies for folate malabsorption and in optimizing antifolate drug therapies.