Chain-Like Semiconductive Fillers for Dielectric Enhancement and Loss Reduction of Polymer Composites

Dielectric loss is a crucial factor in determining the long-term endurance for security and energy loss of dielectric composites. Here, chain-like semiconductive fibers of titanium oxide, indium, and niobium-doped titanium oxide are used for enhancing the complex dielectric properties of a polymer through composite construction, which involves significant interface enhancements. The chain-like fibers significantly enhance the dielectric constant owing to the special morphology of the fillers and their interfacial polarization, especially at higher temperatures. The dielectric loss and electrical conductivity of the composites are substantially reduced across the entire investigated temperature range, achieved by passivating the fiber surface with an alumina shell using atomic layer deposition. The as-deposited alumina shell transformed from an amorphous to a crystalline phase through thermal annealing results in a porous shell and more effective suppression of the loss tangent and electrical conductivity. A plausible mechanism for loss suppression is associated with carrier movement along the surface of the fibers and bulk, leading to a higher loss tangent. The alumina shell blocks the carrier transport path, particularly at the interfaces, resulting in a reduced interfacial polarization contribution and energy storage loss. This study provides a method for inhibiting dielectric loss by fabricating fillers with special surfaces.

First Measurement of the Total Neutron Cross Section on Argon between 100 and 800 MeV

We report the first measurement of the neutron cross section on argon in the energy range of 100-800 MeV. The measurement was obtained with a 4.3-h exposure of the Mini-CAPTAIN detector to the WNR/LANSCE beam at LANL. The total cross section is measured from the attenuation coefficient of the neutron flux as it traverses the liquid argon volume. A set of 2631 candidate interactions is divided in bins of the neutron kinetic energy calculated from time-of-flight measurements. These interactions are reconstructed with custom-made algorithms specifically designed for the data in a time projection chamber the size of the Mini-CAPTAIN detector. The energy averaged cross section is 0.91±0.10(stat)±0.09(syst)  b. A comparison of the measured cross section is made to the GEANT4 and FLUKA event generator packages, where the energy averaged cross sections in this range are 0.60 and 0.68 b, respectively.

Chemical modification of the active site of the acetylcholine receptor

The receptor for acetylcholine in the subsynaptic membrane of the electroplax of Electrophorus electricus is a protein with a disulfide bond in the vicinity of the active site. This disulfide can be reduced and reoxidized with concomitant inhibition and restoration of the response to acetylcholine and other monoquaternary ammonium-depolarizing agents. Conversely, the bisquaternary hexamethonium, normally a competitive inhibitor, causes depolarization, and the activity of decamethonium is increased following reduction of the disulfide. The reduced receptor can be alkylated by various maleimide derivatives and is then no longer reoxidizable. Some quaternary ammonium maleimide derivatives act as affinity labels of the reduced receptor, alkylating it at a rate three orders of magnitude faster then do uncharged maleimide derivatives. Other types of potential affinity labels also react only with the reduced receptor and the resulting covalently attached quaternary ammonium moieties interact with the active site, strongly activating the receptor. These results suggest a model for the active site and its transitions in which an activator such as acetylcholine bridges between a negative subsite and a hydrophobic subsite in the vicinity of the disulfide, causing an altered conformation around the negative subsite and a decrasee of a few angstroms in the distance between the two subsites.

THE ASSOCIATION OF ACETYLCHOLINESTERASE AND MEMBRANE IN SUBCELLULAR FRACTIONS OF THE ELECTRIC TISSUE OF ELECTROPHORUS

Subcellular fractions of the electric tissue of the main organ of the eel Electrophorus electricus were prepared in sucrose media by differential centrifugation and differential discontinuous gradient centrifugation. The distributions of acetylcholinesterase, cytochrome oxidase, DNA, and protein were determined. The appearance of the fractions was determined by phase contrast microscopy and by electron microscopy. A fraction prepared by differectial centrifugation at 30,000 g for 20 minutes in 0.89 M sucrose contained 63 per cent of the total acetylcholinesterase activity at 4 times the specific activity of that of the tissue homogenate. A subfraction prepared by centrifugation in a discontinuous density gradient showed a peak of total and relative specific acetylcholinesterase activity of 35 per cent and 1.9, respectively. The average over-all purification was 7 times. The acetylcholinesterase peak was below the cytochrome oxidase peak and above the DNA peak in the density gradient. The presence of acetylcholinesterase in the fractions was correlated with the presence of large fragments of the cell membrane; however, the presence of other tissue components was noted. The acetylcholinesterase associated with membrane was found to be activated by incubation with sodium deoxycholate. The possible use of the peak fraction containing membranes rich in acetylcholinesterase for the investigation of other components of the acetylcholine system and of other properties of the membrane is discussed.

Of snakes, snails, and surrogates

The high-resolution structure of a synthetic 13-residue peptide in a tight complex with alpha-bungarotoxin conforms to the beta hairpin structure of a closely related segment in the ACh binding protein and reveals how the ACh binding protein and the homologous nicotinic ACh receptors bind alpha-bungarotoxin at their ACh binding sites.

Symmetrical dimer of the human dopamine transporter revealed by cross-linking Cys-306 at the extracellular end of the sixth transmembrane segment

There is evidence both for and against Na(+)- and Cl(-)-dependent neurotransmitter transporters forming oligomers. We found that cross-linking the human dopamine transporter (DAT), which is heterologously expressed in human embryonic kidney 293 cells, either with copper phenanthroline (CuP) or the bifunctional reagent bis-(2-methanethiosulfonatoethyl)amine hydrochloride (bis-EA) increased the apparent molecular mass determined with nonreducing SDS/PAGE from approximately 85 to approximately 195 kDa. After cross-linking, but not before, coexpressed, differentially epitope-tagged DAT molecules, solubilized in Triton X-100, were coimmunoprecipitated. Thus, the 195-kDa complex was a homodimer. Cross-linking of DAT did not affect tyramine uptake. Replacement of Cys-306 with Ala prevented cross-linking. Replacement of all of the non-disulfide-bonded cysteines in the extracellular and membrane domains, except for Cys-306, did not prevent cross-linking. We conclude that the cross-link is between Cys-306 at the extracellular end of TM6 in each of the two DATs. The motif GVXXGVXXA occurs at the intracellular end of TM6 in DAT and is found in a number of other neurotransmitter transporters. This sequence was originally found at the dimerization interface in glycophorin A, and it promotes dimerization in model systems. Mutation of either glycine disrupted DAT expression and function. The intracellular end of TM6, like the extracellular end, is likely to be part of the dimerization interface.

Acetylcholine receptor channel structure in the resting, open, and desensitized states probed with the substituted-cysteine-accessibility method

The nicotinic acetylcholine (ACh) receptors cycle among classes of nonconducting resting states, conducting open states, and nonconducting desensitized states. We previously probed the structure of the mouse-muscle ACh receptor channel in the resting state obtained in the absence of agonist and in the open states obtained after brief exposure to ACh. We now have probed the structure in the stable desensitized state obtained after many minutes of exposure to ACh. Muscle-type receptor has the subunit composition alpha(2)betagammadelta. Each subunit has four membrane-spanning segments, M1-M4. The channel lumen in the membrane domain is lined largely by M2 and to a lesser extent by M1 from each of the subunits. We determined the rates of reaction of a small, sulfhydryl-specific, charged reagent, 2-aminoethyl methanethiosulfonate with cysteines substituted for residues in alphaM2 and the alphaM1-M2 loop in the desensitized state and compared these rates to rates previously obtained in the resting and open states. The reaction rates of the substituted cysteines are different in the three functional states of the receptor, indicating significant structural differences. By comparing the rates of reaction of extracellularly and intracellularly added 2-aminoethyl methanethiosulfonate, we previously located the closed gate in the resting state between alphaG240 and alphaT244, in the predicted M1-M2 loop at the intracellular end of M2. Now, we have located the closed gate in the stable desensitized state between alphaG240 and alphaL251. The gate in the desensitized state includes the resting state gate and an extension further into M2.

Unraveling the mechanism of the lactose permease of Escherichia coli

We studied the effect of pH on ligand binding in wild-type lactose permease or mutants in the four residues-Glu-269, Arg-302, His-322, and Glu-325-that are the key participants in H(+) translocation and coupling between sugar and H(+) translocation. Although wild-type permease or mutants in Glu-325 and Arg-302 exhibit marked decreases in affinity at alkaline pH, mutants in either His-322 or Glu-269 do not titrate. The results offer a mechanistic model for lactose/H(+) symport. In the ground state, the permease is protonated, the H(+) is shared between His-322 and Glu-269, Glu-325 is charge-paired with Arg-302, and substrate is bound with high affinity at the outside surface. Substrate binding induces a conformational change that leads to transfer of the H(+) from His-322/Glu-269 to Glu-325 and reorientation of the binding site to the inner surface with a decrease in affinity. Glu-325 then is deprotonated on the inside because of rejuxtaposition with Arg-302. The His-322/Glu-269 complex then is reprotonated from the outside surface to reinitiate the cycle.

Delimiting the binding site for quaternary ammonium lidocaine derivatives in the acetylcholine receptor channel

The triethylammonium QX-314 and the trimethylammonium QX-222 are lidocaine derivatives that act as open-channel blockers of the acetylcholine (ACh) receptor. When bound, these blockers should occlude some of the residues lining the channel. Eight residues in the second membrane-spanning segment (M2) of the mouse-muscle alpha subunit were mutated one at a time to cysteine and expressed together with wild-type beta, gamma, and delta subunits in Xenopus oocytes. The rate constant for the reaction of each substituted cysteine with 2-aminoethyl methanethiosulfonate (MTSEA) was determined from the time course of the irreversible effect of MTSEA on the ACh-induced current. The reactions were carried out in the presence and absence of ACh and in the presence and absence of QX-314 and QX-222. These blockers had no effect on the reactions in the absence of ACh. In the presence of ACh, both blockers retarded the reaction of extracellularly applied MTSEA with cysteine substituted for residues from alphaVal255, one third of the distance in from the extracellular end of M2, to alphaGlu241, flanking the intracellular end of M2, but not with cysteine substituted for alphaLeu258 or alphaGlu262, at the extracellular end of M2. The reactions of MTSEA with cysteines substituted for alphaLeu258 and alphaGlu262 were considerably faster in the presence of ACh than in its absence. That QX-314 and QX-222 did not protect alphaL258C and alphaE262C against reaction with MTSEA in the presence of ACh implies that protection of the other residues was due to occlusion of the channel and not to the promotion of a less reactive state from a remote site. Given the 12-A overall length of the blockers and the alpha-helical conformation of M2 in the open state, the binding site for both blockers extends from alphaVal255 down to alphaSer248.

Contribution of the beta subunit M2 segment to the ion-conducting pathway of the acetylcholine receptor

We have applied the substituted-cysteine-accessibility method (SCAM) to the M2 segment and the M1-M2 loop of the acetylcholine (ACh) receptor beta subunit. Each residue from beta P248 to beta D273 was mutated one at a time to Cys, and the mutant beta subunits were expressed together with wild-type alpha, beta, and delta subunits in Xenopus oocytes. For each of the mutants, the ACh-induced current was near wild-type. The accessibility of the substituted Cys was inferred from the irreversible inhibition or potentiation of ACh-induced current by methanethiosulfonate (MTS) derivatives added extracellularly. Inhibition by MTSethylammonium of beta G255C, in the narrow part of the channel, was mainly due to a reduction in the single-channel conductance. Conversely, potentiation by MTSethylammonium of beta V266C, in a wider part of the channel, was mainly due to an increase in channel open-time. Two substituted Cys at the intracellular end of M2 and three at the extracellular end were accessible to MTSethylammonium in the absence of ACh. Three additional Cys in the middle of M2 and three in the M1-M2 loop were accessible in the presence of ACh. In the presence of ACh, the secondary structure of beta M2 is alpha-helical from beta G255 to beta V266 and extended from beta L268 to beta D273. The accessible residues in beta M2 are remarkably hydrophobic, while the accessible residues in the M1-M2 loop are charged. beta M2, like alpha M2, alpha M1, and beta M1, undergoes widespread structural changes concomitant with gating, but the gate itself is close to the intracellular end of the channel. Many aligned residues in the M2 segments of alpha and beta are not identically accessible, indicating that the two subunits contribute differently to the channel lining.

The location of the gate in the acetylcholine receptor channel

The cation-conducting channel of the nicotinic acetylcholine (ACh) receptor is lined by the first (M1) and second (M2) membrane-spanning segments of each of its five subunits. Six consecutive residues, alphaS239 to alphaT244, in the alpha subunit M1-M2 loop and at the intracellular end of M2 were mutated to cysteine. The accessibility of the substituted cysteines were probed with small, cationic, sulfhydryl-specific reagents added extracellularly and intracellularly. In the closed state of the channel, there is a barrier to these reagents added from either side between alphaG240 and alphaT244. ACh induces the removal of this barrier, which acts as an activation gate. The residues alphaG240, alphaE241, alphaK242, and alphaT244 line a narrow part of the channel, in which this gate is located.

State-dependent accessibility and electrostatic potential in the channel of the acetylcholine receptor. Inferences from rates of reaction of thiosulfonates with substituted cysteines in the M2 segment of the alpha subunit

Ion channel function depends on the chemical and physical properties and spatial arrangement of the residues that line the channel lumen and on the electrostatic potential within the lumen. We have used small, sulfhydryl-specific thiosulfonate reagents, both positively charged and neutral, to probe the environment within the acetylcholine (ACh) receptor channel. Rate constants were determined for their reactions with cysteines substituted for nine exposed residues in the second membrane-spanning segment (M2) of the alpha subunit. The largest rate constants, both in the presence and absence of ACh, were for the reactions with the cysteine substituted for alpha Thr244, near the intracellular end of the channel. In the open state of the channel, but not in the closed state, the rate constants for the reactions of the charged reagents with several substituted cysteines depended on the transmembrane electrostatic potential, and the electrical distance of these cysteines increased from the extracellular to the intracellular end of M2. Even at zero transmembrane potential, the ratios of the rate constants for the reactions of three positively charged reagents with alpha T244C, alpha L251C, and alpha L258C to the rate constant for the reaction of an uncharged reagent were much greater in the open than in the closed state. This dependence of the rate constants on reagent charge is consistent with an intrinsic electrostatic potential in the channel that is considerably more negative in the open state than in the closed state. The effects of ACh on the rate constants for the reactions of substituted Cys along the length of alpha M2, on the dependence of the rate constants on the transmembrane potential, and on the intrinsic potential support a location of a gate more intracellular than alpha Thr244.

Identification of acetylcholine receptor channel-lining residues in the M1 segment of the beta-subunit

The substituted cysteine accessibility method (SCAM) was applied to the first membrane-spanning segment (M1) of the mouse-muscle acetylcholine (ACh) receptor beta subunit. One at a time, each residue from betaR219 to betaP247, except betaC233, was mutated to Cys, and the mutant beta subunits were expressed together with wild-type alpha, gamma, and delta in Xenopus oocytes. All 28 mutants yielded functional receptors. The accessibility of the substituted Cys to the methanethiosulfonate (MTS) derivatives, MTS ethylammonium (MTSEA), MTS ethyltrimethylammonium (MTSET), and MTS ethylsulfonate (MTSES), added extracellularly in the absence or the presence of ACh, was inferred from their irreversible effects on ACh-induced current. Three consecutive residues close to the extracellular end of M1, betaF224C, betaY225C, and betaL226C, reacted both in the absence and presence of ACh, and one deeper residue, betaV229C reacted only in the presence of ACh. betaV229C also reacted with 2-aminoethyl-2-aminoethanethiosulfonate (AEAETS) and with 2-hydroxyethyl MTS (MTSEH). The rate constants for the reactions of betaV229C with MTSEA, which permeates the open channel, and with MTSEH, which is uncharged, were independent of membrane potential. The rate constant for the reaction of the doubly positively charged AEAETS, however, was dependent on membrane potential, consistent with the exposure of betaV229C in the open channel. The N-terminal third of betaM1, like that of alphaM1, contributes to the lining of the channel and undergoes structural changes during gating.

Functional effects on the acetylcholine receptor of multiple mutations of gamma Asp174 and delta Asp180

Residues gamma Asp174 and delta Asp180, previously implicated in the binding of acetylcholine (ACh) by the mouse muscle ACh receptor, were each mutated to nine other residues, Asn, Glu, Thr, Ala, Cys, His, Val, Tyr, and Lys. The effects of the mutations on ACh-induced current was determined on surface receptors containing wild-type alpha and beta subunits and mutant gamma and delta subunits. The mutations increased the concentration of ACh eliciting half-maximal current (EC50) by factors from 22 for the Glu mutant to 660 for the Lys mutant. Analysis of the effects in terms of the difference in the accessible surface areas of the mutant and wild-type side chains and the difference in side-chain charges indicated that, per binding site, Delta DeltaG0 for activation was a sum of 10 cal mol-1 A-2 of change in side-chain accessible surface area and of 0.95 kcal mol-1 positive step-1 in side-chain charge, equivalent to 1 mol of charge falling through 42 mV. The effects on the concentration of ACh (IC50, ACh) and of d-tubocurarine (IC50,dTC) causing half-maximal retardation of alpha-bungarotoxin binding were determined on complexes containing wild-type alpha and beta subunits and either mutant gamma or mutant delta subunit. The effects on IC50,ACh correlated well with the effects on EC50, with a similar magnitude for the influence of side-chain charge on the free energy of binding (in this case to the desensitized state) and on the electrostatic potential at the binding site. The effects on IC50,dTC were in all cases less than the effects on IC50,ACh, and the two sets of effects were poorly correlated. In line with the higher ACh affinity and lower d-tubocurarine affinity of the alpha-delta binding site compared to the alpha-gamma binding site, mutations of delta Asp180 had a greater effect on IC50,ACh than did the same mutations of gamma Asp174, and vice versa for effects on IC50,dTC. Consequently, all mutations decreased the asymmetry in the binding properties of the two types of sites.

Structure of the NMDA receptor channel M2 segment inferred from the accessibility of substituted cysteines

The structure of the NMDA receptor channel M2 segment was investigated by probing the extracellular and cytoplasmic faces of cysteine-substituted NR1-NR2C channels with charged sulfhydryl-specific reagents. The pattern of accessible positions suggests that the M2 segment forms a channel-lining loop originating and ending on the cytoplasmic side of the channel, with the ascending limb in an alpha-helical structure and the descending limb in an extended structure. A functionally critical asparagine (N-site) is positioned at the tip of the loop, and a cluster of hydrophilic residues of the descending limb, adjacent to the tip, forms the narrow constriction of the channel. An apparent asymmetric positioning of the NR1- and NR2-subunit N-site asparagines may account for their unequal role in Ca2+ permeability and Mg2+ block.

The contributions of aspartyl residues in the acetylcholine receptor gamma and delta subunits to the binding of agonists and competitive antagonists

The acetylcholine (ACh) receptors in muscle have the composition alpha2betagammadelta and contain two ACh binding sites. One is formed between an alpha subunit and the gamma subunit, and the other is formed between an alpha subunit and the delta subunit. Among the residues in the ACh binding sites are alphaCys-192 and alphaCys-193. The negatively charged deltaAsp-180 is at an appropriate distance from alphaCys-192/193 also to be in the ACh binding site and to interact electrostatically with the positively charged ammonium group common to agonists and competitive antagonists. Mutation to Asn of either deltaAsp-180 or the aligned residue in the gamma subunit, gammaAsp-174, decreased the affinities of three agonists, acetylcholine, tetramethylammonium, and succinyldicholine 170-560-fold. By contrast, these mutations decreased the affinities of three competitive antagonists, (+)-tubocurarine, hexamethonium, and dihydro-beta-erythroidine, only 2-15-fold. Agonists, but not antagonists, promote the transitions of the receptor from the resting state to the higher affinity active and desensitized states, and the greater effects of the mutations of gammaAsp-174 and deltaAsp-180 on the apparent affinities of agonists could reflect the involvement of these residues in the conformational changes of the receptor corresponding to its transitions to higher affinity states. In these transitions, one possibility is that gammaAsp-174 and deltaAsp-180 move closer to bound agonist.

Functional contributions of alpha5 subunit to neuronal acetylcholine receptor channels

Ligand-gated ion channels are multi-subunit complexes where each subunit-type is encoded by several related genes. Heterologous expression of any one of the neuronal nicotinic acetylcholine receptors (nAChR) alpha-type subunits, either alone or with any beta-type subunit, typically yields functional nAChR channels. A striking exception is the nAChR alpha5 subunit: although apparently complexed with beta2 and beta4 nAChR subunits in neurons, and expressed in a subset of neurons within the central and peripheral nervous systems, heterologous expression of alpha5, either alone or with any beta-type subunit has failed to yield functional channels. We demonstrate here that alpha5 does participate in nAChRs expressed in hetrologous systems and in primary neurons, and further that alpha5 contributes to the lining of functionally unique nAChR channels, but only if coexpressed with both another alpha- and beta-type subunit. Furthermore, channels containing the alpha5 subunit are potently activated and desensitized by nanomolar concentrations of nicotine.

Exposure of residues in the cyclic nucleotide-gated channel pore: P region structure and function in gating

In voltage-gated ion channels and in the homologous cyclic nucleotide-gated (CNG) channels, the loop between the S5 and S6 transmembrane segments (P region) is thought to form the lining of the pore. To investigate the structure and the role in gating of the P region of the bovine retinal CNG channel, we determined the accessibility of 11 cysteine-substituted P region residues to small, charged sulfhydryl reagents applied to the inside and outside of membrane patches in the open and closed states of the channel. The results suggest that the P region forms a loop that extends toward the central axis of the channel, analogous to the L3 loop of bacterial porin channels. Furthermore, the P region, in addition to forming the ion selectivity filter, functions as the channel gate, the structure of which changes when the channel opens.

Identification of acetylcholine receptor channel-lining residues in the M1 segment of the alpha-subunit

The muscle-type acetylcholine (ACh) receptor has the composition alpha 2 beta gamma delta. The subunits are arranged quasisymmetrically around a central, ion-conducting, water-filled channel. Each subunit has four membrane-spanning segments, M1-M4, and the channel through the membrane is formed among these segments. Substituting cysteine for each of the residues in and flanking the alpha M2 segment, we previously found that, at 10 of the 21 mutated positions, the cysteine was accessible to a small, positively charged, sulfhydryl-specific reagent, methanethiosulfonate ethylammonium (MTSEA), and inferred that the residues at these positions are exposed in the channel lumen. We have now applied the substituted-cysteine-accessibility method to alpha M1. We analyzed 15 consecutive residues, starting at alpha Pro211 at the extracellular end of M1. Wild-type alpha contains Cys222, which is inaccessible to MTSEA. We mutated each of the other 14 residues to cysteine and expressed the mutant alpha subunits, together with wild-type beta, gamma, and delta subunits, in Xenopus oocytes. Thirteen of the fourteen mutants gave robust ACh-induced currents. MTSEA irreversibly altered the ACh-induced response of seven cysteine-substitution mutants: alpha Y213C was susceptible to MSTEA added in the presence or the absence of ACh, alpha P211C, alpha I215C, alpha V216C, alpha N217C, and alpha I220C were susceptible in the absence of ACh, and alpha V218C was susceptible in the presence of ACh. These results imply that M1 is exposed in the channel, and its exposure changes during gating or desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping the binding-site crevice of the dopamine D2 receptor by the substituted-cysteine accessibility method

The binding site of the dopamine D2 receptor, like that of other homologous G protein-coupled receptors, is contained within a water-accessible crevice formed among its seven membrane-spanning segments. We have developed a method to map systematically all the residues forming the surface of this binding-site crevice, and we have applied this method to the third membrane-spanning segment (M3). We mutated, one at a time, 23 residues in and flanking M3 to cysteine and expressed the mutant receptors heterologously. Ten of these mutants reacted with charged, hydrophilic, lipophobic, sulfhydryl-specific reagents, added extracellularly, and were protected from reaction by a reversible dopamine antagonist. Thus, the side chains of these residues are exposed in the binding-site crevice, which like M3 extends from the extracellular to the intracellular side of the membrane. The pattern of exposure is consistent with a short loop followed by six turns of an alpha helix.

Structure of the nicotinic receptor acetylcholine-binding site. Identification of acidic residues in the delta subunit within 0.9 nm of the 5 alpha subunit-binding

In the nicotinic receptor, the quaternary ammonium group of acetylcholine (ACh) binds to a negative subsite at most 1 nm from a readily reducible disulfide formed between alpha-subunit residues Cys192 and Cys193. The cross-linker S-(2-[3H]glycylamidoethyl)dithio-2-pyridine formed a disulfide bond with reduced alpha Cys192/Cys193 and an amide bond with an acidic residue in the delta subunit (Czajkoswski, C., and Karlin, A. (1991) J. Biol. Chem. 266, 22603-22612). The fully extended cross-linking moiety -NHCH2CONHCH2CH2S- is is 0.9 nm long. After the disulfide bond linking -NHCH2CONHCH2CH2S- to the alpha subunit was reduced, -NHCH2CONHCH2CH2SH remained linked to the delta subunit by an amide bond. The delta subunit was cleaved at Met residues, and the radioactive fragments were isolated and sequenced by automated Edman degradation. Additionally, the isolated radioactive fragments were further cleaved at Trp residues and sequenced. Peaks of release of radioactivity were obtained in the sequencing cycles corresponding to delta Asp165, delta Asp180, and delta Glu182. The mutation of delta Asp180 to Asn decreased the affinity of the receptor for ACh 100-fold, whereas the mutation of either delta Asp165, delta Glu182, or 8 other acidic residues in the same region of delta decreased the affinity by 3-fold or less (Czajkowski, C., Kaufmann, C., and Karlin, A. (1993) Proc. Natl. Acad. Sci. U.S.A 90, 6285-6289). Because delta Asp180 both contributes to ACh binding and is suitably close to the binding site disulfide, it is likely to be part of the ACh-binding site formed in the interface between the alpha and the delta subunits.

A cysteine residue in the third membrane-spanning segment of the human D2 dopamine receptor is exposed in the binding-site crevice

The binding site in G-protein-linked neurotransmitter receptors is formed among their membrane-spanning segments. Because the binding site is in the plane of the bilayer and is accessible to charged, water-soluble agonists, it must lie in a crevice open to the extracellular, aqueous medium. Information about the structure of these receptors can be obtained by identifying the residues in the membrane-spanning segments which face this water-filled crevice. Human D2 dopamine receptor was expressed in human embryonic kidney 293 cells. Small, charged, sulfhydryl-specific methanethiosulfonate (MTS) derivatives irreversibly inhibited the binding of the D2-specific antagonist [3H]YM-09151-2 to these cells. The highly polar MTS derivatives should react with cysteine sulfhydryl groups only at the water-accessible surface of the receptor, which includes the surface of the binding-site crevice. In contrast, these reagents will have little access to sulfhydryls facing the lipid bilayer or buried in the protein interior. Positively charged MTS reagents irreversibly inhibited binding several hundredfold faster than a negatively charged MTS reagent, consistent with the affinity of the binding site for positively charged dopamine agonists and antagonists. Furthermore, both agonists and antagonists of the D2 receptor protected against irreversible inhibition by the MTS reagents. To identify the susceptible cysteine, we mutated, one at a time, five transmembrane and two extracellular cysteine residues to serine. Only the mutation of Cys118 to serine decreased the susceptibility of antagonist binding to irreversible inhibition by the MTS reagents. Thus, Cys118, a residue in the middle of the third membrane-spanning segment, is exposed in the D2 receptor binding-site crevice.

Identification of acetylcholine receptor channel-lining residues in the entire M2 segment of the alpha subunit

Each residue in and flanking the M2 membrane-spanning segment of the alpha subunit, from Glu-241 to Glu-262, was mutated to cysteine, and the mutant subunits were expressed together with wild-type beta, gamma, and delta subunits in Xenopus oocytes. Cysteines substituted for Glu-262, Leu-258, Val-255, Ser-252, Leu-251, Leu-250, Ser-248, Leu-245, Thr-244, and Glu-241 reacted with the positively charged, hydrophilic, sulfhydryl-specific reagent methanethiosulfonate ethylammonium (MTSEA), added extracellularly. These 10 residues, therefore, are exposed in the channel lumen. The pattern of exposure is compatible with an alpha helix, interrupted by an extended structure from Leu-250 to Ser-252. Acetylcholine caused subtle changes in the accessibilities of some of the engineered cysteines. Since all 10 residues are accessible to MTSEA in the closed state of the channel, the channel gate is at least as cytoplasmic as Glu-241, the most cytoplasmic of the residues tested.

Electrostatic potential of the acetylcholine binding sites in the nicotinic receptor probed by reactions of binding-site cysteines with charged methanethiosulfonates

All of the potent agonists and competitive antagonists of the acetylcholine receptors are positively charged, onium compounds. Among the interactions involved in the binding of these compounds, electrostatic forces undoubtedly make an important contribution. There is evidence that the acetylcholine binding site contains both acidic and aromatic amino acids. The acidic side chains could provide long-range charge-charge interactions with acetylcholine, while the aromatic side chains could provide short-range cation-pi-electron and hydrophobic interactions. To probe the long-range electrostatic interactions in the binding site, the rate constants for the reactions of sulfhydryl-specific reagents with cysteines in the binding site have been determined as a function of ionic strength. The reagents are the positively charged methanethiosulfonate ethylammonium and methanethiosulfonate ethyltrimethylammonium, the negatively charged methanethiosulfonate ethylsulfonate, and the neutral methyl methanethiosulfonate. In addition, the rate constants of the reactions of these methanethiosulfonates with positively charged, negatively charged, and uncharged simple thiol compounds have been similarly determined. An analysis of these rate constants in terms of absolute rate theory and Debye-Hückel theory is consistent with the acetylcholine binding site containing two to three negative charges and an electrostatic potential at zero ionic strength of about -80 mV relative to bulk solution.

Expression and characterization of human dopamine D2 receptor in baculovirus-infected insect cells

Multiple types of dopamine D2-like receptors (D2, D3, D4) have been identified. Differences in pharmacology among these receptors may have profound clinical ramifications for the treatment of psychosis. Analysis of the structure and function of their binding sites requires a source of large amounts of receptor, uncontaminated by the other types of D2-like receptor. We engineered a recombinant baculovirus containing the human D2 receptor cDNA (DRD2) to express this receptor in insect cells. Spodoptera frugiperda cells (Sf9 and Sf21) and Trichoplusia ni cells (TN-5) were infected with the recombinant baculovirus. Binding of the D2 antagonist [3H]YM-09151-2 to membranes fractions of these cells peaked at a specific activity of 5-8 pmol/mg protein, approximately 40 times that of membranes from bovine striatum. The receptor expressed in Sf9 cells was similar to that of striatum in its affinities for D2 agonists and antagonists. Sodium ion stimulated [3H]YM-09151-2 binding to D2 receptor in infected Sf9 cell membranes. This effect was fit by an allosteric model which predicted the apparent affinity of [3H]YM-09151-2. The D2 receptor expressed in Sf9 and TN-5 cells was photolabeled with N-(p-azido-m-[125I]iodophenylethyl)spiperone. The specifically labeled component(s) ran as a broad band of apparent molecular weight between 54,000 and 60,000. Deglycosylation of the labeled component(s) reduced its molecular weight to 46,000.

Negatively charged amino acid residues in the nicotinic receptor delta subunit that contribute to the binding of acetylcholine

In nicotinic receptors, the binding sites for acetylcholine are likely to contain negatively charged amino acid side chains that interact with the positively charged quaternary ammonium group of acetylcholine and of other potent agonists. We previously found that a 61-residue segment of the delta subunit contains aspartate or glutamate residues within 1 nm of cysteines in the acetylcholine binding site on the alpha subunit. We have now mutated, one at a time, the 12 aspartates and glutamates in this segment of the mouse muscle delta subunit and have expressed the mutant receptors in Xenopus oocytes. Both the concentration of acetylcholine eliciting half-maximal current (Kapp) and the Ki for the inhibition by acetylcholine of alpha-bungarotoxin binding were increased 100-fold by the mutation of delta Asp180 to Asn and 10-fold by the mutation of delta Glu189 to Gln. These two residues, and their homologs in the gamma and epsilon subunits, are likely to contribute to the acetylcholine binding sites.

Structure of nicotinic acetylcholine receptors

Nicotinic acetylcholine (ACh) receptors convert the binding of ACh into the opening of a cation-conducting channel. New information about the regions of the receptor most immediately involved in its function, namely the ACh-binding sites, the gate and the channel, has come from two approaches. One is the identification by labelling and by mutagenesis of residues contributing to these regions. Another is the determination of the three-dimensional structure of the receptor by electron microscopy. Although the identification of functionally relevant residues is incomplete and residues cannot yet be resolved in the three-dimensional structure, the two approaches are converging. There is still room in the gap for speculation.

Acetylcholine receptor channel structure probed in cysteine-substitution mutants

In order to understand the structural bases of ion conduction, ion selectivity, and gating in the nicotinic acetylcholine receptor, mutagenesis and covalent modification were combined to identify the amino acid residues that line the channel. The side chains of alternate residues--Ser248, Leu250, Ser252, and Thr254--in M2, a membrane-spanning segment of the alpha subunit, are exposed in the closed channel. Thus alpha 248-254 probably forms a beta strand, and the gate is closer to the cytoplasmic end of the channel than any of these residues. On channel opening, Leu251 is also exposed. These results lead to a revised view of the closed and open channel structures.

Agonist binding site of Torpedo electric tissue nicotinic acetylcholine receptor. A negatively charged region of the delta subunit within 0.9 nm of the alpha subunit binding site disulfide

The positively charged quaternary ammonium group of agonists of the nicotinic acetylcholine (ACh) receptor binds to a negative subsite at most about 1 nm from a readily reducible disulfide. This disulfide is formed by alpha Cys192 and Cys193 (Kao and Karlin, 1986). In order to identify Asp or Glu residues that may contribute to the negative subsite, we synthesized S-(2-[3H]glycylamidoethyl)dithio-2-pyridine. Purified ACh receptor from Torpedo californica was mildly reduced and reacted with S-(2-[3H]glycylamidoethyl)dithio-2-pyridine. The predominant product was a mixed disulfide between the 3H-N-glycylcysteamine moiety and alpha Cys192 or Cys193. In the extended conformation of [3H] N-glycylcysteamine, the distance from the glycyl amino group to the cysteamine thio group is 0.9 nm. Thus, the amino group of disulfide-linked [3H]N-glycylcysteamine could react with carboxyls within 0.9 nm of Cys192/Cys193. To promote amide bond formation between the tethered amino group and receptor carboxyls, we added 1-ethyl-3-(3'-dimethylaminopropyl)-carbodiimide. The predominant sites of amide coupling were on the delta subunit, in CNBr fragment 4 (delta 164-257). This reaction was inhibited by ACh. Only the first 61 residues of delta CNBr 4 are predicted to be extracellular, and there are 11 Asp or Gly residues in this region. One or more of these residues is likely to contribute to the binding of ACh.

Paraben preservatives do not increase intracranial pressure in cats

It has been hypothesized recently that succinylcholine-associated increases in intracranial pressure (ICP) are caused by the paraben preservatives contained in multidose vials. We tested that hypothesis in a standard feline model to determine the effects on ICP of equal-volume injections of preservative-free succinylcholine, succinylcholine with preservatives from multi-dose vials that contain both propylparaben and methylparaben, these preservatives alone at five times the dose contained in the succinylcholine, and normal saline. The preservatives alone increased ICP by 0.08 +/- 0.08 mmHg (+/- standard error; not significant). Normal saline had no effect on ICP. Preservative-free succinylcholine and succinylcholine with preservatives increased ICP by 4.2 +/- 0.10 and 3.8 +/- 0.07 mmHg respectively (P less than 0.01 compared to the preservatives alone and normal saline). The 99% upper confidence limit for the increase in ICP induced by the preservatives alone was 0.42 mmHg. This result suggests that parabens do not cause or substantially augment the ICP increase associated with succinylcholine administration.

Mapping the alpha-subunit site photolabeled by the noncompetitive inhibitor [3H]quinacrine azide in the active state of the nicotinic acetylcholine receptor

We have characterized the time-resolved labeling of a site on the Torpedo californica electrocyte acetylcholine receptor (ACHR) by the photoreactive noncompetitive inhibitor derivative quinacrine azide (QA). The dependence of [3H]QA labeling on acetylcholine (ACH) concentration and on time is consistent with the preferential labeling by [3H]QA of ACHR in the open state. The ACH-dependent [3H]QA labeling, which was associated predominantly with the alpha-subunit, was blocked by other noncompetitive inhibitors including quinacrine, chlorpromazine, proadifen, histrionicotoxin, and bupivacaine. alpha-Subunit from ACHR labeled with [3H]QA 20 ms after the addition of ACH was cleaved with CNBr, and the fragments were separated by high pressure liquid chromatography. A peptide containing a major site of specific labeling was purified on two different reverse-phase columns. By N-terminal sequencing, amino acid composition, binding to mercurial-agarose, and apparent molecular weight, this [3H]QA-labeled peptide was identified as alpha-208-243, a CNBr fragment containing the putative membrane-spanning helix M1.

The sidedness of the COOH terminus of the acetylcholine receptor delta subunit

The nicotinic acetylcholine receptor from Torpedo sp. occurs as a dimer, disulfide-cross-linked between delta subunits. We determined the sidedness of the COOH terminus of the acetylcholine receptor delta subunit by locating the delta-delta disulfide relative to the membrane and by identifying the Cys residue forming the disulfide. We used receptor-rich native membrane vesicles isolated from Torpedo californica electric tissue and characterized as to orientation and intactness. These vesicles had not been extracted and retained v ("43-kDa protein") as a marker of the cytoplasmic surface. Using the reduction of v as an assay of permeability, we showed that two reductants, 2-mercaptoethanesulfonate and reduced glutathione, were relatively impermeant. Both of these reductants reduced the delta-delta disulfide in sealed right-side-out vesicles equally in the presence and absence of saponin, and 2-mercaptoethanesulfonate reduced this disulfide equally in the presence and absence of Triton X-100. By contrast, surfactants enhanced the reduction of dimer in inside-out and sequestered vesicles. We conclude that the disulfide is extracellular. To identify the Cys residue forming the disulfide, we labeled the sulfhydryls both in receptor dimer and in monomer generated by mild reduction of dimer. By high performance liquid chromatography and NH2-terminal sequencing of cyanogen bromide fragments of labeled delta-delta dimer and delta monomer, we found that the penultimate residue, delta-Cys-500, uniquely formed an intersubunit disulfide and that this disulfide was uniquely reduced when receptor dimer was reduced to monomer. Therefore, the delta COOH terminus is extracellular.

The intactness and orientation of acetylcholine receptor-rich membrane from Torpedo californica electric tissue

By a mild and highly reproducible fractionation of Torpedo californica electric tissue, we prepared membrane which was 30 times enriched in nicotinic acetylcholine receptor (AChR). This preparation was neither alkali-stripped nor reconstituted and consequently contained nu (43-kDa protein), which is associated with the cytoplasmic aspect of the receptor. We tested this membrane for the presence of sealed vesicles and determined the orientation of these vesicles by combining three methods. Two of these methods were based on the accessibilities, in the presence and absence of detergent, of the extracellular acetylcholine binding site to alpha-bungarotoxin and of the intracellular nu to trypsin. These two methods are specific for AChR-containing membrane. The third method was morphometry of electron micrographs, by which we estimated the proportion of sequestered membrane. These methods taken together indicated that approximately 45% of the AChR-containing membrane was in the form of leaky vesicles or sheets, 33% was sealed right-side-out vesicles, 11% was sealed inside-out vesicles, and 11% was sequestered within multilamellar or multivesicular vesicles. The complexity of this membrane needs to be taken into account in sidedness studies of the AChR.

Rate of induction of hypotension with trimetaphan modifies the intracranial pressure response in cats

An infusion of 0.1% trimetaphan was administered to eight cats with artificially increased intracranial pressure (ICP) in order to decrease their mean arterial pressure (MAP) from 121 +/- 9.5 (SEM) to 58 +/- 4.6 mm Hg in less than 1 min. All cats developed an increase in intracranial pressure (ICP) (from 16 +/- 1.4 to 23 +/- 3.2 mm Hg) accompanied by a partial rebound in MAP. Eight additional cats received 0.1% trimetaphan to decrease their MAP from 128 +/- 13.4 to 52 +/- 8.1 mm Hg over more than 2 min. Four of these cats followed the same pattern, with ICP increases from 19 +/- 1.1 to 31 +/- 3.9 mm Hg, while in the other four ICP did not change. In nine of the 12 cats with an ICP increase, that increase was initiated before the partial MAP rebound. We conclude that trimetaphan causes clinically significant ICP increases in cats with increased ICP, that partial rebound in MAP frequently exacerbates these increases in ICP, and that rapid induction of hypotension tends to increase the frequency with which trimetaphan increases ICP.

Acetylcholine receptor binding site contains a disulfide cross-link between adjacent half-cystinyl residues

A conserved feature of all nicotinic receptors is the presence of a readily reducible disulfide bond adjacent to the acetylcholine binding site. Previously we showed that in intact receptor from Torpedo californica electric tissue reduction of this disulfide followed by affinity alkylation with 4-(N-maleimido)benzyltri[3H] methylammonium iodide specifically and uniquely labels the alpha subunit residues Cys-192 and Cys-193. To identify all of the half-cystinyl residues contributing to the binding site disulfide(s), we have now reduced receptor under mild conditions and alkylated with a mixture of 4-(N-maleimido)benzyltri[3H]methylammonium iodide and N-[1-14C]ethylmaleimide and find that Cys-192 and Cys-193 are labeled exclusively. Furthermore, from unreduced receptor we have isolated two cyanogen bromide peptides of alpha, one containing Cys-192 and Cys-193, and the other containing Cys-128 and Cys-142 (which are the other potential contributors to the binding site disulfide(s]. These isolated peptides incorporate iodo[1-14C]acetamide only following reduction by dithiothreitol. Our results demonstrate that: 1) the binding site disulfide is between Cys-192 and Cys-193; 2) Cys-128 is disulfide-cross-linked to Cys-142; and 3) under conditions that reduce Cys-192 and Cys-193 completely, Cys-128 and Cys-142 remain cross-linked. At the acetylcholine binding site, agonists induce a local conformational change that stabilizes the binding site disulfide against reduction. We suggest that a transition between two stable conformations of the vicinal disulfide, both involving a nonplanar cis peptide bond between Cys-192 and Cys-193, is associated with receptor activation by agonists.

Functional domains of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is a multisubunit, membrane-spanning protein that contains a gated, cation-conducting channel. Our approach to the understanding of the function of this receptor in molecular terms has been to locate its functionally significant sites in the sequences of its subunits and in its three-dimensional structure. In addition, we have tried to correlate transitions in the properties of these sites with functional transitions of the receptor. On binding acetylcholine, the nicotinic acetylcholine receptor enters at least two transient states, the open state and the rapid-onset desensitized state, and, in the continued presence of agonist, finally subsides into the slow-onset desensitized state. The transitions of the receptor between these various states are susceptible to regulation by acetylcholine and its congeners acting at one type of site and by a broad class of noncompetitive inhibitors (NCIs), including local anesthetics, acting at other sites. The chain composition of the receptor is alpha 2 beta gamma delta. The two acetylcholine binding sites are on the alpha chains, and two residues contributing to these sites, Cys-192 and Cys-193, have been identified. Furthermore, these adjacent Cys residues are cross-linked by a disulfide bond. In the quaternary structure of the receptor, the chains appear to be arranged in the order alpha gamma alpha beta delta around a central channel. Both the alpha and beta chains contribute to functionally significant NCI binding sites. The addition to receptor-rich membrane from Torpedo electric tissue of agonists (but not competitive antagonists) renders these NCI sites susceptible to photolabeling by the NCI quinacrine azide (QA). Furthermore, this susceptibility is transient, arising in milliseconds and subsiding in hundreds of milliseconds. These transiently susceptible sites are protected by other NCIs against photolabeling by QA. The time-course of the susceptibility and its dependence on agonist-concentration suggest that it might be the transient, rapid-onset desensitized state of the receptor that is most susceptible to photolabeling by QA.