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.

Binding of antibodies to acetylcholine receptors in Electrophorus and Torpedo electroplax membranes.

Antisera against purified acetylcholine receptors from the electric tissues of Torpedo californica and of Electrophorus electricus were raised in rabbits. The antisera contain antibodies which bind to both autologous and heterologous receptors in solution as shown by an immunoprecipitation assay. Antibodies in both types of antisera bind specifically to the postjunctional membrane on the innervated surface of the intact electroplax from Electrophorus electric tissue as demonstrated by an indirect immunohistochemical procedure using horseradish peroxidase conjugated to anti-rabbit IgG. Only anti-Electrophorus receptor antisera, however, cause inhibition of the receptor-mediated depolarization of the intact Electrophorus electroplax. The lack of inhibition by anti-Torpedo receptor antibodies, which do bind, suggests that the receptor does not undergo extensive movement during activity. The binding of anti-Torpedo antibodies to receptor-rich vesicles prepared by subcellular fractionation of Torpedo electric tissue was demonstrated by both direct and indirect immunohistochemical methods using ferritin conjugates. These vesicles can be conveniently collected and prepared for electron microscopy on Millipore filters, a procedure requiring only 25 micrograms of membrane protein per filter. In addition, it was possible to visualize the binding of anti-Torpedo receptor antibodies directly, without ferritin. These anti-Torpedo receptor antibodies, however, do not inhibit the binding of acetylcholine or of alpha-neurotoxin to receptor in Torpedo microsacs but do inhibit binding of alpha-neurotoxin to Torpedo receptor in Triton X-100 solution. It is likely that the principal antigenic determinants on receptor are at sites other than the acetylcholine-binding sites and that inhibition of receptor function, when it occurs, may be due to a stabilization by antibody binding of an inactive conformational state.

Acetylcholine receptor: covalent attachment of depolarizing groups at the active site.

Following reduction of the acetylcholine receptor in the electroplax with dithiothreitol, the quaternary ammonium compounds bromoacetylcholine bromide and the p-nitrophenyl ester of (p-carboxyphenyl) trimethylammonium iodide react near the active site probably with a sulfhydryl group. The covalently attached quaternary ammonium moieties additionally interact with the active site noncovalently to activate the receptor and cause depolarization of the cell.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The intrinsic electrostatic potential and the intermediate ring of charge in the acetylcholine receptor channel.

A ring of aligned glutamate residues named the intermediate ring of charge surrounds the intracellular end of the acetylcholine receptor channel and dominates cation conduction (Imoto et al. 1988). Four of the five subunits in mouse-muscle acetylcholine receptor contribute a glutamate to the ring. These glutamates were mutated to glutamine or lysine, and combinations of mutant and native subunits, yielding net ring charges of -1 to -4, were expressed in Xenopus laevis oocytes. In all complexes, the alpha subunit contained a Cys substituted for alphaThr244, three residues away from the ring glutamate alphaGlu241. The rate constants for the reactions of alphaThr244Cys with the neutral 2-hydroxyethyl-methanethiosulfonate, the positively charged 2-ammonioethyl-methanethiosulfonate, and the doubly positively charged 2-ammonioethyl-2'-ammonioethanethiosulfonate were determined from the rates of irreversible inhibition of the responses to acetylcholine. The reagents were added in the presence and absence of acetylcholine and at various transmembrane potentials, and the rate constants were extrapolated to zero transmembrane potential. The intrinsic electrostatic potential in the channel in the vicinity of the ring of charge was estimated from the ratios of the rate constants of differently charged reagents. In the acetylcholine-induced open state, this potential was -230 mV with four glutamates in the ring and increased linearly towards 0 mV by +57 mV for each negative charge removed from the ring. Thus, the intrinsic electrostatic potential in the narrow, intracellular end of the open channel is almost entirely due to the intermediate ring of charge and is strongly correlated with alkali-metal-ion conductance through the channel. The intrinsic electrostatic potential in the closed state of the channel was more positive than in the open state at all values of the ring charge. These electrostatic properties were simulated by theoretical calculations based on a simplified model of the channel.

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.