Selective labeling of the delta subunit of the acetylcholine receptor by a covalent local anesthetic

Picosecond dynamics of a peptide from the acetylcholine receptor interacting with a neurotoxin probed by tailored tryptophan fluorescence

A tryptophan analog, dehydro-N-acetyl-L-tryptophanamide (delta-NATA), which is produced enzymatically via L-tryptophan 2',3'-oxidase from Chromobacterium violaceum, is newly used for time-resolved fluorescence. The absorption and emission maxima of delta-NATA at 332 and 417 nm, respectively, in 20% dimethylformamide-water are significantly shifted to the red with respect to those of tryptophan in water, permitting us to measure its fluorescence in the presence of tryptophan residues. We demonstrate that the steady-state spectra and the fluorescence decay of delta-NATA are very sensitive to environment, changing dramatically with solvent as the chromophore is localized within a protein and when this tagged protein binds to a peptide. The tryptophan oxidase was also used to modify the single Trp of a neurotoxin from snake (Naja nigricollis) venom. Modification of the toxin alpha (dehydrotryptophan-toxin alpha) permitted its investigation in complex with a synthetic 15-amino acid peptide corresponding to a loop of the agonist-binding site of acetylcholine receptor (AchR) from Torpedo marmorata species. The peptide alpha-185 possesses a single Trp at the third position (Trp187 of AchR) and a disulfide bridge between Cys192 and Cys193. A single-exponential rotational diffusion time with a constant of 1.65 ns is measured for the isolated 15-amino acid peptide. This suggests that Trp motion in the peptide in solution is strongly correlated with the residues downstream the peptide sequence, which may in part be attributed to long-range order imposed by the disulfide bond. The dynamics of the bound peptide are very different: the presence of two correlation times indicates that the Trp187 of the peptide has a fast motion (taur1 = 140 ps and r(0)1 = 0.14) relative to the overall rotation of the complex (taur2 = 3.4 ns and r(0)2 = 0.04). The correlation of the Trp residue with its neighboring amino acid residues and with the overall motion of the peptide is lost, giving rise to its rapid restricted motion. Thus, the internal dynamics of interacting peptides change on binding.

How do short neurotoxins bind to a muscular-type nicotinic acetylcholine receptor?

We investigated the interacting surface between a short curarimimetic toxin and a muscular-type nicotinic acetylcholine receptor, looking for the ability of various biotinylated Naja nigricollis alpha-neurotoxin analogues to bind simultaneously the receptor and streptavidin. All these derivatives, modified at positions 10 (loop I), 27, 30, 33, 35 (loop II), 46, and 47 (loop III) or the N-terminal (erabutoxin numbering), still shared high affinity for the receptor, and in the absence of receptor they all bound soluble streptavidin. However, the proportion of the toxin-receptor complex that bound to streptavidin-coated beads, varied both with the location of the modification and with the length of the linker between biotin and the toxin. In the receptor-toxin complex, the concave side of loops II and III was not accessible to streptavidin, unlike the N terminus of the toxin and, to a certain extent, loop I. On the convex face, loop III was the most accessible, whereas the tip of loop II, especially Arg-30, seemed to be closer to the receptor. The present data demonstrate that short toxins neither penetrate deeply into a crevice as proposed earlier nor lie parallel to the receptor extracellular wall. These data also suggest that they may not lie strictly perpendicular to the cylindrical wall of the receptor. These results fit nicely with three-dimensional models of interaction between long neurotoxins and their receptors and support the idea that short and long curarimimetic toxins share a similar overall topology of interaction when bound to nicotinic receptors.

Role of local anesthetics on both cholinergic and serotonergic ionotropic receptors

A great body of experimental evidence indicates that the main target for the pharmacological action of local anesthetics (LAs) is the voltage-gated Na+ channel. However, the epidural and spinal anesthesia as well as the behavioral effects of LAs cannot be explained exclusively by its inhibitory effect on the voltage-gated Na+ channel. Thus, the involvement of other ion channel receptors has been suggested. Particularly, two members of the neurotransmitter-gated ion channel receptor superfamily, the nicotinic acetylcholine receptor (AChR) and the 5-hydroxytryptamine receptor (5-HT3R type). In this regard, the aim of this review is to explain and delineate the mechanism by which LAs inhibit both ionotropic receptors from peripheral and central nervous systems. Local anesthetics inhibit the ion channel activity of both muscle- and neuronal-type AChRs in a noncompetitive fashion. Additionally, LAs inhibit the 5-HT3R by competing with the serotonergic agonist binding sites. The noncompetitive inhibitory action of LAs on the AChR is ascribed to two possible blocking mechanisms. An open-channel-blocking mechanism where the drug binds to the open channel and/or an allosteric mechanism where LAs bind to closed channels. The open-channel-blocking mechanism is in accord with the existence of high-affinity LA binding sites located in the ion channel. The allosteric mechanism seems to be physiologically more relevant than the open-channel-blocking mechanism. The inhibitory property of LAs is also elicited by binding to several low-affinity sites positioned at the lipid-AChR interface. However, there is no clearcut evidence indicating whether these sites are located at either the annular or the nonannular lipid domain. Both tertiary (protonated) and quaternary LAs gain the interior of the channel through the hydrophilic pathway formed by the extracellular ion channel's mouth with the concomitant ion flux blockade. Nevertheless, an alternative mode of action is proposed for both deprotonated tertiary and permanently-uncharged LAs: they may pass from the lipid membrane core to the lumen of the ion channel through a hydrophobic pathway. Perhaps this hydrophobic pathway is structurally related to the nonannular lipid domain. Regarding the LA binding site location on the 5-HT3R, at least two amino acids have been involved. Glutamic acid at position 106 which is located in a residue sequence homologous to loop A from the principal component of the binding site for cholinergic agonists and competitive antagonists, and Trp67 which is positioned in a stretch of amino acids homologous to loop F from the complementary component of the cholinergic ligand binding site.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Macromolecular sites for specific neurotoxins and drugs on chemosensitive synapses and electrical excitation in biological membranes

The present review deals with the molecular mechanisms and elementary phenomena underlying the activation of the voltage- and chemo-sensitive membrane macromolecules: sodium- and potassium-ion channels and nicotinic ACh receptors and their associated ion channel. To achieve an understanding of their various kinetics and conformational states, a number of novel alkaloids, BTX, HTXs, gephyrotoxins, and certain psychotomimetic drugs such as phencyclidine, and many other pharmacologically active agents have been used. Biochemical assays and various electrophysiological techniques have been used in a number of biological preparations--e.g., Torpedo membranes, brain synaptosomes, amphibian and mammalian neuromuscular preparations--to describe the action of such agents. The availability of BTX and scorpion toxins together with aconitine and veratridine as activators and TTX and STX as antagonists of the voltage-sensitive sodium channels, made possible the identification and the physiological and pharmacological characterization of these channels. These studies provided the basis for understanding the mechanisms underlying electrical excitability and culminated, more recently, in the purification and reconstitution of sodium channels from rat brain and in the successful cloning of these channels with the elucidation of their primary structure. We now know that the sodium channel has a molecular mass of 316,000 daltons, consists of five subunits, and has multiple sites for various ligands. In contrast to sodium channels, various classes of potassium channels (inward and outward rectifier potassium channels and Ca(2+)-activated potassium channels) have been described. Unlike the sodium channels, there are no known specific activators for potassium channels. However, a number of potassium channel blockers such as 4-aminopyridine, HTX, histamine, and norepinephrine have been identified which complement the varying types of potassium channels in different neurons. One class of potassium channel blockers with profound medical and social implications comprises PCP and its analogues. The blockade of the potassium-induced 86Rb+ efflux from brain cells, the resulting prolongation of muscle and nerve action potentials, and the increase in transmitter release observed with PCP and some analogues are all highly suggestive of a role for the potassium channel in the behavioral effects of these drugs and its potential involvement in schizophrenia. A number of toxic principles of both plant and animal origin played a significant role in the development of our knowledge about the nAChR.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of azidophencyclidine-binding site on the nicotinic acetylcholine receptor alpha-subunit

Nicotinic acetylcholine receptors in receptor-rich membranes from Torpedo californica and from T. marmorata electric tissue were photolabeled with the non-competitive inhibitor [3H]azidophencyclidine. The receptor subunits were separated on SDS-polyacrylamide gels and the alpha-subunits recovered from the gel, were subjected to Staphylococcus aureus V8 protease cleavage. The proteolytic fragments were resolved by SDS-polyacrylamide gel electrophoresis and were identified on protein blots by 125I-labeled alpha-bungarotoxin binding and by staining with concanavalin A. The site of specific azidophencyclidine labeling has been localized to the V8-18 kDa fragment which binds toxin. Labeling of the V8-18 kDa fragment was observed in the absence and in the presence of carbamylcholine. This was found for both the species of Torpedo used here.

Characterization of the transient agonist-triggered state of the acetylcholine receptor rapidly labeled by the noncompetitive blocker [3H]chlorpromazine: additional evidence for the open channel conformation

The kinetics of covalent labeling of the alpha, beta, gamma, and delta chains of the acetylcholine receptor (AcChR) from Torpedo marmorata by the noncompetitive blocker [3H]chlorpromazine ([3H]CPZ) are investigated by using rapid mixing photolabeling techniques. In an initial study [Heidmann, T., & Changeux, J. P. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 1897-1901], it was shown that the rate of [3H]CPZ labeling increases 100-1000-fold upon simultaneous addition of nicotinic agonists to the AcChR and that prior addition of these agonists abolishes the effect. The data were interpreted in terms of the rapid labeling of the transient active state of the AcChR where the ion channel is in its open configuration. This interpretation was recently challenged [Cox, R. N., Kaldany, R. R. J., Di Paola, M., & Karlin, A. (1985) J. Biol. Chem. 260, 7186-7193] on the ground of studies with a different noncompetitive blocker, [3H]quinacrine azide, and the suggestion was made that this compound labels the rapidly desensitized closed channel conformation of the AcChR. In this paper it is shown that the rate of rapid labeling of the AcChR by [3H]CPZ decreases to negligible values upon exposure of the AcChR to nicotinic agonists, in the 100-500-ms time range. The absolute values of the rate constants of this decrease (10-15 s-1 for saturating concentrations of acetylcholine and carbamoylcholine) and their variation with agonist concentration (apparent dissociation constants of 40 microM and 0.4 mM for acetylcholine and carbamoylcholine, respectively) are those expected for the rapid desensitization of the AcChR.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Cocaine, phencyclidine, and procaine inhibition of the acetylcholine receptor: characterization of the binding site by stopped-flow measurements of receptor-controlled ion flux in membrane vesicles

Noncompetitive inhibition of acetylcholine receptor-controlled ion translocation was studied in membrane vesicles prepared from both Torpedo californica and Electrophorus electricus electroplax. Ion flux was measured in the millisecond time region by using a spectrophotometric stopped-flow method, based on fluorescence quenching of entrapped anthracene-1,5-disulfonic acid by Cs+, and a quench-flow technique using 86Rb+. The rate coefficient of ion flux prior to receptor inactivation (desensitization), JA, was measured at different acetylcholine and inhibitor concentrations, in order to assess which active (nondesensitized) receptor forms bind noncompetitive inhibitors. The degree of inhibition of JA by the inhibitors studied (cocaine, procaine, and phencyclidine) was found to be independent of acetylcholine concentration. The results are consistent with a mechanism in which each compound inhibits by binding to a single site that exists with equal affinity on all active receptor forms. Mechanisms in which the inhibitors bind exclusively to the open-channel form of the receptor are excluded by the data. The same conclusions were reached in cocaine experiments at 0-mV and procaine experiments at -25-mV transmembrane voltage in T. californica vesicles. It had been previously shown that phencyclidine, in addition to decreasing JA (by binding to active receptors), also increases the rate of rapid receptor inactivation (desensitization) and changes the equilibrium between active and inactive receptors (by binding better to inactivated receptor than to active receptor in the closed or open conformations). These effects were not observed with cocaine or procaine. Here it is shown that despite these differential effects on inactivation, cocaine and phencyclidine bind to the same inhibitory site on active receptors (in E. electricus vesicles).(ABSTRACT TRUNCATED AT 250 WORDS)

Structure of the high-affinity binding site for noncompetitive blockers of the acetylcholine receptor: serine-262 of the delta subunit is labeled by [3H]chlorpromazine

The membrane-bound acetylcholine receptor from Torpedo marmorata was photolabeled by the noncompetitive channel blocker [3H]chlorpromazine under equilibrium conditions in the presence of agonist. Incorporation of radioactivity into all subunits occurred and was reduced by addition of phencyclidine, a specific ligand for the high-affinity site for noncompetitive blockers. The delta subunit was purified and digested with trypsin, and the resulting fragments were fractionated by reversed-phase HPLC. The labeled peptide could not be purified to homogeneity because of its marked hydrophobic character, but a combination of differential CNBr subcleavage and cosequencing of partially purified fragments enabled us to identify Ser-262 as being labeled by [3H]chlorpromazine. The labeling of this particular residue was prevented by phencyclidine and thus took place at the level of, or in proximity to, the high-affinity site for noncompetitive blockers. Ser-262 is located in a hydrophobic and potentially transmembrane segment termed MII.

Allosteric effects of diprobutine on acetylcholine receptors

The nicotinic effects of a novel antiparkinsonian compound, diprobutine were investigated on the acetylcholine receptor (AChR) from Torpedo marmorata electric organ and on rat brain membranes by a variety of techniques including stopped flow measurements. On the nicotinic AChR from Torpedo, diprobutine behaved as a typical noncompetitive blocker: it inhibited the agonist-regulated 22Na+ efflux from excitable microsacs; it shifted in the ms-s time-range the conformation of the AChR towards a high affinity state for agonists; it competed with [3H]PCP bound to its high affinity 'allosteric' site. On rat brain membrane, it displaced [3H]PCP bound to its high affinity site. The pharmacological properties of diprobutine are discussed in the context of its biochemical effects.

Covalent labeling of functional states of the acetylcholine receptor. Effects of antagonists on the receptor conformation

Photoaffinity labeling of membrane-bound nicotinic acetylcholine receptor from Torpedo marmorata electric tissue with the ion-channel blocker [3H]TPMP+ reveals various functional states of the receptor protein if labeling is performed with ms time resolution. In the resting and in the activated state most of the label is incorporated into the alpha-polypeptide chains of the receptor complex. When equilibrated with agonists and antagonists, predominantly the delta-polypeptide chain (and to a lesser extent the beta-chain) reacts with the photolabel. Reactivity of the delta-chain increases after exposure to cholinergic effectors with a half-life slower than the kinetics of receptor activation or rapid desensitization. Agonists and antagonists stimulate photolabelling of the delta-chain with different kinetics. For acetylcholine, carbamoylcholine and suberyldicholine the half-life of the reactivity increases is 400 - 500 ms; for the antagonists hexamethonium, d-tubocurarine and flaxedil it is about 10 s. The latter slow kinetics are also observed when the receptor is preequilibrated with agonists or antagonists prior to mixing with [3H]TPMP+ and starting the photoreaction. We conclude that time-resolved photoaffinity labeling can convalently mark protein structures involved in receptor functions. Of special interest is the observation that antagonists also induce a conformational change in the receptor protein.

Demonstration and affinity labeling of a stereoselective binding site for a benzomorphan opiate on acetylcholine receptor-rich membranes from Torpedo electroplaque

The interaction of an optically pure benzomorphan opiate, (-)-N-allyl-N-normetazocine [(-)-ANMC], with the nicotinic acetylcholine receptor from Torpedo electroplaque was studied by using radioligand binding and affinity labeling. The binding was complex with at least two specific components having equilibrium dissociation constants of 0.3 microM and 2 microM. The affinity of the higher affinity component was decreased by carbamoylcholine but not by alpha-bungarotoxin. The effect of carbamoylcholine was not blocked by alpha-bungarotoxin. In comparison, the affinity of [3H]phencyclidine, a well-characterized ligand for a high-affinity site for noncompetitive blockers on the acetylcholine receptor, is increased by carbamoylcholine and the increase is blocked by alpha-bungarotoxin. The binding of (-)-[3H]ANMC was inhibited by a number of other benzomorphans, with (-) isomers being 4- to 5-fold more potent than (+) isomers. Phencyclidine inhibits the binding of (-)-[3H]ANMC to its high-affinity site by a mechanism that is not competitive. UV-catalyzed affinity labeling indicated that the high-affinity-binding site for (-)-[3H]ANMC is at least partially associated with the delta subunit. Tryptic degradation of the Torpedo marmorata delta chain suggested that (-)-ANMC labeled a 16,000-dalton COOH-terminal portion of the subunit. In contrast, 5-azido-[3H]trimethisoquin, a photoaffinity label of the high-affinity site for noncompetitive blockers, labels a 47,000-dalton NH2-terminal fragment of the delta subunit. These results suggest that (-)-[3H]ANMC binds to sites completely distinct from the binding sites for acetylcholine. The high-affinity-binding site for (-)-ANMC and that for phencyclidine and 5-azidotrimethisoquin are allosterically coupled but are regulated differently and are probably physically distinct.

Image analysis of the heavy form of the acetylcholine receptor from Torpedo marmorata

The structure of the heavy (H) form of the acetylcholine receptor, which comprises two covalently linked 250,000 Mr oligomers, has been investigated by numerical analysis of electron microscope images. Na-cholate solubilized Torpedo marmorata H-form receptor was reintegrated into artificial lipid vesicles and negatively stained with uranyl acetate prior to imaging in a conventional transmission microscope. The reconstituted preparations exhibited the standard polypeptide composition of the purified receptor (alpha 2 beta gamma delta) and the same transmembrane arrangement as in the native subsynaptic membrane. Covalent disulfide linkage between the two oligomers took place exclusively through the delta chains. In agreement with previous work (Cartaud et al., 1980) the H-form appeared as "doublets" of two coplanar 9 nm rosettes at a center-to-center distance of 9.2 +/- 1.1 nm. The relative angular orientation of the two rosettes in a doublet was examined by correlation analysis in the real space. It exhibited a marked variability, few of the doublets featuring any kind of symmetry, suggesting that the two oligomers of a doublets are connected via an extended and flexible chain or loop. The area of contact between the two rosettes of a doublet therefore does not necessarily represent a reliable clue as to the location of the delta chain within the structure. Averaged images obtained after reorientation and summation of up to 132 rosettes revealed the three major peaks and the two grooves already observed in previous studies. Two additional smaller peaks were identified. Tentative assignment of structural details to individual subunits was deduced from an examination of alpha-bungarotoxin-labeled doublets. The alpha subunits, which carry part or all of the acetylcholine binding sites, are probably located in nonadjacent positions in the vicinity of the newly found peaks. This assignment is consistent with the image analysis of receptor-toxin complexes recently reported by Zingsheim et al. (1982b).

Multiple binding sites for phencyclidine on the nicotinic acetylcholine receptor from Torpedo ocellata electric organ

Binding of [3H]-phencyclidine ( [3H]-PCP) to acetylcholine-receptor enriched membrane from Torpedo ocellata electric organ was studied over a ligand concentration range of 1 to 200 microM. The results indicate that [3H]-PCP is bound to two classes of sites: high affinity (Kd = 6-9 microM) and low affinity (Kd = 85 microM) binding sites. In the absence of cholinergic drugs the ratio of high affinity [3H]-PCP binding sites to 125I-alpha-bungarotoxin (alpha-Bgt) binding sites is 0.37, and that of low affinity [3H]-PCP binding sites to 125I-alpha-Bgt is 1.06. Low affinity [3H]-PCP binding can be completely inhibited by alpha-bungarotoxin (alpha-Bgt), carbamylcholine and d-tubocurarine. This inhibition, together with the one to one stoichiometry with 125I-alpha-Bgt, suggests that the sites to which [3H]-PCP binds with low affinity are the acetylcholine (AcCho) binding sites. In the presence of 1 microM alpha-Bgt which blocks binding of [3H]-PCP to the AcCho binding sites, the ratio of high affinity [3H]-PCP sites to 125I-alpha-Bgt sites is 0.5, indicating the existence of one high affinity PCP site per receptor molecule, The toxin, however, decreases the apparent affinity of [3H]-PCP towards the AcCho receptor as well as the potency of tetracaine or dibucaine in inhibiting [3H]-PCP binding to that receptor. In the latter case the effect involves changes from a biphasic to a simple inhibition curve. The results suggest that non-competitive blockers to the AcCho receptors may affect their own sites as well, and that they do this also by binding to the AcCho binding sites. This is also inferred from the accelerated dissociation of [3H]-PCP from its high affinity binding sites by unlabeled PCP in the concentration range of 10(-3) to 10(-4) M, at which the drug occupies AcCho binding sites as well.

Time-resolved photolabeling by the noncompetitive blocker chlorpromazine of the acetylcholine receptor in its transiently open and closed ion channel conformations

A rapid-mixing photolabeling apparatus is developed to resolve the kinetics of association of the noncompetitive channel blocker [3H]chlorpromazine (CPZ) with the membrane-bound acetylcholine (AcCho) receptor from Torpedo marmorata and to photolabel its subunits in the 100-milli-seconds to seconds time range. Rapid mixing of AcCho and [3H]CPZ with the receptor followed by brief (less than 20 msec) UV irradiation results in the selective labeling of the four chains of the AcCho receptor, according to a rapid bimolecular association process close to diffusion-controlled. Rapid association is not observed with the competitive antagonists d-tubocurarine or flaxedil or the snake venom alpha-toxins. Its initial rate increases with agonist concentration, with maxima of 0.6 for carbamoylcholine and 0.2 for phenyltrimethylammonium taking 1 for AcCho, with apparent dissociation constants of 30 microM, 400 microM, and 300 microM for AcCho, carbamoylcholine, and phenyltrimethylammonium, respectively, and with sigmoid shape (Hill coefficients of 1.1-1.3). Under conditions in which the receptor "desensitizes" and the ionic channel closes (preincubation with AcCho), rapid [3H]CPZ association decreases in parallel. It is concluded that the agonist-dependent rapid association of [3H]CPZ takes place at the level of a site common to all five subunits, which lies within the ion channel and becomes accessible when the channel opens.

A continuous-flow, rapid-mixing, photolabeling technique applied to the acetylcholine receptor

A continuous-flow technique is described in which a photoaffinity label, membrane rich in acetylcholine receptor, and various effectors are rapidly mixed, passed through a delay tube, through a tube in which they are irradiated, and are collected in a tube containing quencher. Delay times as short as 20 ms between mixing and photolysis are achievable. Because the flow is continuous, milliliter volumes of membrane can be labeled in a single run, which is convenient for the analysis of both the functional effects and sites of photolabeling. Using this technique, we have found that receptor in its transitory, active state, in which the channel is open, is more susceptible to photolabeling by the noncompetitive inhibitor analog [3H] quinacrine azide than is receptor in either its resting or desensitized states, in which the channel is closed. This technique should prove generally useful for the photolabeling of transient conformational states of macromolecules.

Effects of calcium on the binding of phencyclidine to acetylcholine receptor-rich membrane fragments from Torpedo californica electroplaque

The influence of calcium on the binding of phencyclidine (PCP) to acetylcholine (ACh) receptor-rich membrane fragments was investigated. Calcium decreased the equilibrium affinity for PCP in the presence, but not in the absence, of the cholinergic agonist carbamylcholine. The effect of calcium was rapidly reversible by EGTA, indicating that it was not attributable to a calcium-activated protease or a phospholipase. Following detergent solubilization of the nicotinic ACh receptor, the calcium effect on PCP remained, suggesting that calcium may interact directly with the receptor to exert its effect. Other divalent cations (Mn2+, La2+, Co2+, Mg2+) had similar effects. A correlate of "desensitization" of the ACh receptor can be observed using PCP binding, and a two-step "desensitization" process can be observed. Calcium seemed to increase the amplitude of a rapid component of receptor "desensitization." The results presented in this paper suggest that calcium may play a role in the modulation of the nicotinic ACh receptor.

Species differences determine azido phencyclidine labeling pattern in desensitized nicotinic acetylcholine receptors

Acetylcholine receptor enriched membranes from Torpedo ocellata, Torpedo marmorata and Torpedo californica were studied using [3H] azido-phencyclidine (AZ-PCP). [3H]-PCP binding to receptors from all three species revealed marked similarities. Photoaffinity labeling by [3H]-AZ-PCP resulted in the tagging of mainly alpha, beta and delta subunits in all species. When carbamylcholine was added, it enhanced the labeling of beta subunits in T. ocellata, delta in T. marmorata and alpha in T. californica, suggesting species differences in the photolabeling pattern. Multiple homologous binding sites for PCP between the receptor subunits would allow small variations in receptor structure to be manifested in labeling by AZ-PCP, with no differences in binding and functional properties of the receptors.

Binding of phencyclidine to the detergent solubilized acetylcholine receptor from Torpedo marmorata

The binding of phencyclidine to the acetylcholine receptor from Torpedo marmorata electroplaque was measured following solubilization of the receptor in sodium cholate followed by the exchange of cholate for Tween 80. In both the membrane-bound and solubilized AChR, the addition of cholinergic agonists simultaneously with the addition of PCP results in a 100 to 1000 fold increase in the PCP association rate and a 5 to 10 fold increase in the dissociation rate as compared to the unliganded AChR or AChR equilibrated with agonist prior to PCP addition. In addition, the number of binding sites and the pharmacological properties of the binding are not markedly changed in the soluble receptor. These results suggest that the acetylcholine receptor can undergo similar conformational transitions in the membrane-bound and the Tween 80 solubilized form and that phencyclidine can monitor these transitions in both cases.