Topographic studies of Torpedo acetylcholine receptor subunits as a transmembrane complex

Structural and functional crosstalk between acetylcholine receptor and its membrane environment

Nicotinic acetylcholine receptor (AChR) is a transmembrane protein belonging to the superfamily of rapid, ligand-operated channels. Theoretical models based on thermodynamic criteria assign portions of the polypeptide chains to the lipid bilayer region. From an experimental point of view, however, the relationship between the two moieties remains largely unexplored. Current studies from our laboratory are aimed at defining the structural, dynamic, and functional relationship between membrane lipids and AChR. We are particularly interested in establishing the characteristics of and differences between the lipids in each leaflet of the bilayer and the belt or "annular" lipids immediately surrounding AChR and the bulk bilayer lipids. We are also interested in determining the possible implications of lipid modifications on AChR channel properties. Toward these ends, fluorescence and other spectroscopic techniques, together with biochemical analyses and patch-clamp studies, are currently being undertaken. Correlations can be established between structural aspects of phospholipid packing in the immediate perimeter of AChR and other properties of these annular lipids revealed by dynamic spectroscopic and molecular modeling techniques. Lipid compositional analyses of the clonal muscle cell line BC3H-1 and chemical modification studies have been carried out by incubation of intact cells in culture and of membrane patches excised therefrom with liposomes of different lipid composition. These studies have been combined with electrophysiological measurements using the patch-clamp technique, with the aim of determining the possible effects of lipids on the channel properties of muscle-type AChR. A variety of experimental conditions, involving polar head and fatty acyl chain substitution of phospholipids and cholesterol incorporation, are being assayed in the BC3H-1 cells.

Nonequivalence of alpha-bungarotoxin binding sites in the native nicotinic receptor molecule

In the native, membrane-bound form of the nicotinic acetylcholine receptor (M-AcChR) the two sites for the cholinergic antagonist alpha-bungarotoxin (alpha-BGT) have different binding properties. One site has high affinity, and the M-AcChR/alpha-BGT complexes thus formed dissociate very slowly, similar to the complexes formed with detergent-solubilized AcChR (S-AcChR). The second site has much lower affinity (KD approximately 59 +/- 35 nM) and forms quickly reversible complexes. The nondenaturing detergent Triton X-100 is known to solubilize the AcChR in a form unable, upon binding of cholinergic ligands, to open the ion channel and to become desensitized. Solubilization of the AcChR in Triton X-100 affects the binding properties of this second site and converts it to a high-affinity, slowly reversible site. Prolonged incubation of M-AcChR at 4 degrees C converts the low-affinity site to a high-affinity site similar to those observed in the presence of Triton X-100. Although the two sites have similar properties when the AcChR is solubilized in Triton X-100, their nonequivalence can be demonstrated by the effect on alpha-BGT binding of concanavalin A, which strongly reduces the association rate of one site only. The Bmax of alpha-BGT to either Triton-solubilized AcChR or M-AcChR is not affected by the presence of concanavalin A. Occupancy of the high-affinity, slowly reversible site in M-AcChR inhibits the Triton X-100 induced conversion to irreversibility of the second site. At difference with alpha-BGT, the long alpha-neurotoxin from Naja naja siamensis venom (alpha-NTX) binds with high affinity and in a very slowly reversible fashion to two sites in the M-AcChR (Conti-Tronconi & Raftery, 1986). We confirm here that Triton-solubilized AcChR or M-AcChR binds in a very slowly reversible fashion the same amount of alpha-NTX.(ABSTRACT TRUNCATED AT 250 WORDS)

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

Identification of subunits of acetylcholine receptor that interact with a cholesterol photoaffinity probe

All four subunits of the acetylcholine receptor in membrane vesicles isolated from Torpedo californica have been labeled with [3H]cholesteryl diazoacetate. As this probe incorporates into lipid bilayers analogously to cholesterol, this result indicates that acetylcholine receptor interacts with cholesterol. This investigation also demonstrates that this probe is a useful reagent for studying the interaction of cholesterol with membrane proteins.

Transmembrane topology of acetylcholine receptor subunits probed with photoreactive phospholipids

The domains of the acetylcholine receptor subunits that contact the lipid phase were investigated by hydrophobic photolabeling of receptor-rich membrane fragments prepared from Torpedo marmorata and Torpedo californica electric organs. The radioactive arylazido phospholipids used carry a photoreactive group, either at the level of the lipid polar head group (PCI) or at the tip of the aliphatic chain (PCII), and thus probe respectively the "superficial" and "deep" regions of the lipid bilayer. The four subunits of T. marmorata and T. californica acetylcholine receptor reacted with both the PCI and PCII probes and thus are all exposed to the lipid phase. Ligands known to stabilize different conformations of the acetylcholine receptor (nicotinic agonists, snake alpha-toxin, and noncompetitive blockers) did not cause any significant change in the labeling pattern. The acetylcholine receptor associated 43 000-dalton v1 protein did not react with any of the probes. A striking difference in labeling between T. marmorata and T. californica acetylcholine receptors occurred at the level of the alpha-subunit when the superficial PCI probe was used. An approximately 5-fold higher labeling of the alpha-subunit as compared to the beta-, gamma-, and delta-subunits was observed by using receptor-rich membranes from T. marmorata but not from T. californica. The same difference persisted after purification of the labeled receptors from the two species and was restricted to an 8000-dalton C-terminal tryptic peptide. The only mutation observed in this region of the complete alpha-subunit sequence of the two species is the substitution of cysteine-424 in T. marmorata by serine-424 in T. californica.(ABSTRACT TRUNCATED AT 250 WORDS)

Myasthenia gravis and myasthenic syndromes

More than a decade ago myasthenic symptoms were observed in rabbits immunized with acetylcholine receptor (AChR) [119] and AChR deficiency was found at the neuromuscular junction in human myasthenia gravis (MG) [36]. By 1977 the autoimmune character of MG and the pathogenic role of AChR antibodies had been established by several measures. These included the demonstration of circulating AChR antibodies in nearly 90% of patients with MG [87], passive transfer with IgG of several features of the disease from human to mouse [149], localization of immune complexes (IgG and complement) on the postsynaptic membrane [30], and the beneficial effects of plasmapheresis [20, 123]. Substantial subsequent progress has occurred in understanding the structure and function of AChR and its interaction with AChR antibodies. The relationships of the concentration, specificities, and functional properties of the antibodies to the clinical state in MG have been carefully analyzed, and the mechanisms by which AChR antibodies impair neuromuscular transmission have been further investigated. The clinical classification of MG has been refined, the role of the thymus gland in the disease has been further clarified, and new information has become available on transient neonatal MG. The prognosis for generalized MG is improving, but there is still no consensus on its optimal management. Novel therapeutic approaches to MG are now being explored in animal models. Recognition of the autoimmune origin of acquired MG also implied that myasthenic disorders occurring in a genetic or congenital setting had a different cause. As a result, a number of congenital myasthenic syndromes have come to be recognized and investigated. Finally, an acquired disorder of neuromuscular transmission different from MG, the Lambert-Eaton myasthenic syndrome, has also been shown to have an autoimmune basis. In this syndrome, active zone particles of the presynaptic membrane are direct or indirect targets of the pathogenic autoantibodies.

Structure and transmembrane nature of the acetylcholine receptor in amphibian skeletal muscle as revealed by cross-reacting monoclonal antibodies

A collection of 126 monoclonal antibodies (mAbs) made against acetylcholine receptors (AChRs) from the electric organs of Torpedo californica or Electrophorus electricus was tested for cross-reactivity with AChRs in cryostat sections of skeletal muscle from Rana pipiens and Xenopus laevis by indirect immunofluorescence. 49 mAbs (39%) cross-reacted with AChRs from Rana, and 25 mAbs (20%) cross-reacted with AChRs from Xenopus. mAbs specific for each of the four subunits of electric organ AChR (alpha, beta, gamma, delta) cross-reacted with AChRs from each amphibian species. mAbs cross-reacting with Xenopus AChRs were, with one exception, a subset of the mAbs cross-reacting with Rana AChRs. The major difference detected between the two species was in binding by mAbs specific for the main immunogenic region (MIR) of the alpha-subunit. Whereas 22 of 33 anti-MIR mAbs tested cross-reacted with Rana AChRs, only one of these mAbs cross-reacted with Xenopus AChRs. Some (32) of the cross-reacting mAbs were tested for binding to AChRs in intact muscle. 21 of these mAbs bound to AChRs only when membranes were made permeable with saponin. Electron microscopy using immunoperoxidase or colloidal gold techniques revealed that these mAbs recognize cytoplasmic determinants and that mAbs that do not require saponin in order to bind AChRs in intact muscle recognize extracellular determinants. These results suggest that AChRs in skeletal muscle of Rana and Xenopus are composed of subunits corresponding to the alpha-, beta-, gamma-, and delta-subunits of AChRs from fish electric organs. The subunit specificity of mAbs whose binding was examined by electron microscopy suggests that parts of each subunit (alpha, beta, gamma, delta) are exposed on the cytoplasmic surface and that, as in AChRs from fish electric organs and mammalian muscle, the MIR on alpha-subunits of Rana AChRs is exposed on the extracellular surface.

Arrangement of MP26 in lens junctional membranes: analysis with proteases and antibodies

The major membrane protein of the bovine lens fiber cell is a 26-kilodalton (kD) protein (MP26), which appears to be a component of the extensive junctional specializations found in these cells. To examine the arrangement of MP26 within the junctional membranes, various proteases were incubated with fiber cell membranes that had been isolated with or without urea and/or detergents. These membranes were analyzed with electron microscopy and SDS-PAGE to determine whether the junctional specializations or the proteins were altered by proteolysis. Microscopy revealed no obvious structural changes. Electrophoresis showed that chymotrypsin, papain, and trypsin degraded MP26 to 21-22 kD species. A variety of protease treatments, including overnight digestions, failed to generate additional proteolysis. Regions on MP26 which were sensitive to these three proteases overlapped. Smaller peptides were cleaved from MP26 with V8 protease and carboxypeptidases A and B. Protein domains cleaved by these proteases also overlapped with regions sensitive to chymotrypsin, papain, and trypsin. Specific inhibition of the carboxypeptidases suggested that cleavage obtained with these preparations was not likely due to contaminating endoproteases. Since antibodies are not thought to readily penetrate the 2-3 nm extracellular gap in the fiber cell junctions, antibodies to MP26 were used to analyze the location of the protease-sensitive domains. Antisera were applied to control (26 kD) and proteolyzed (22 kD) membranes, with binding being evaluated by means of ELISA reactions on intact membranes. Antibody labeling was also done following SDS-PAGE and transfer to derivatized paper. Both assays showed a significant decrease in binding following proteolysis, with the 22 kD product showing no reaction with the anti-MP26 sera. These investigations suggest that MP26 is arranged with approximately four-fifths of the primary sequence "protected" by the lipid bilayer and the narrow extracellular gap. One-fifth of the molecule, including the C-terminus, appears to be exposed on the cytoplasmic side of the membrane.

(Na+ + K+)-ATPase correlated with a major group of intramembrane particles in freeze-fracture replicas of cultured chick myotubes

Immunofluorescence microscopy with a fluorescein-labeled monoclonal antibody was used to map the distribution of sodium- and potassium-ion stimulated ATPase [( Na,K]-ATPase) on the surface of tissue-cultured chick skeletal muscle. At this level of resolution it appeared that the (Na,K)-ATPase molecules were distributed nearly uniformly over the plasma membrane. These molecules could be cross-linked by use of the monoclonal antibody followed by a second antibody directed against the monoclonal antibody; the resulting fluorescent pattern was a set of small dots (patches) on the muscle surface. This pattern was stable over several hours, and there was little evidence of interiorization or of coalescence of the patches. Myotubes labeled with immunofluorescence were fixed in glutaraldehyde, cryoprotected with glycerin, then fractured and replicated by standard methods. Replicas of the immunofluorescence-labeled myotubes revealed clusters of intramembrane particles (IMP) only when the immunofluorescent images indicated a patching of the (Na,K)-ATPase molecules. Double antibody cross-linking of antigenic sites on myotubes with each of three other monoclonal antibodies to plasma membrane antigens likewise resulted in patched patterns of immunofluorescence, but in none of these cases were clusters of intramembrane particles found in freeze-fracture replicas. In each case it was shown that the (Na,K)-ATPase molecules were not patched. Other control experiments showed that patching of (Na,K)-ATPase molecules did not cause co-patching of one of the other plasma membrane proteins defined by a monoclonal antibody and did not cause detectable co-clustering of acetylcholine receptors. Detailed mapping showed that there was a one-to-one correspondence between immunofluorescent patches related to the (Na,K)-ATPase and clusters of IMP in a freeze-fracture replica of the same cell. We conclude that the intramembrane particles patched by double antibody cross-linkage of the (Na,K)-ATPase are caused by (Na,K)-ATPase molecules in the fracture plane. Quantification of the IMP indicated that the (Na,K)-ATPase-related particles account for up to 50% of particles evident in the replicas, or up to about 400 particles/micrometers2 of plasma membrane. Particles related to the (Na,K)-ATPase were similar to the average particle size and were as heterodisperse in size as the total population of IMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural homology of Torpedo californica acetylcholine receptor subunits

The nicotinic acetylcholine receptor (AChR) from the electroplax of the ray Torpedo californica is composed of five subunits present in a molar stoichiometry of alpha 2 beta gamma delta (refs 1-3) and contains both the binding site for the neurotransmitter and the cation gating unit (reviewed in refs 4-6). We have recently elucidated the complete primary structures of the alpha-, beta- and delta-subunit precursors of the T. californica AChR by cloning and sequencing cDNAs for these polypeptides. Here, we report the whole primary structure of the gamma-subunit precursor of the AChR deduced from the nucleotide sequence of the cloned cDNA. Comparison of the amino acid sequences of the four subunits reveals marked homology among them. The close resemblance among the hydrophilicity profiles and predicted secondary structures of all the subunits suggests that these polypeptides are oriented in a pseudosymmetric fashion across the membrane. Each subunit contains four putative transmembrane segments that may be involved in the ionic channel. The transmembrane topology of the subunit molecules has also been inferred.

Nucleotide and deduced amino acid sequences of Torpedo californica acetylcholine receptor gamma subunit

The nucleotide sequence has been determined of a cDNA clone that codes for the 60,000-dalton gamma subunit of Torpedo californica acetylcholine receptor. The length of the cDNA clone is 2,010 base pairs. The 5' and 3' untranslated regions have respective lengths of 31 and 461 base pairs. Data suggest that the putative polyadenylylation consensus sequence A-A-T-A-A-A may not be required for polyadenylylation of the mRNA corresponding to the cDNA clone described in this study. From the DNA sequence data, the amino acid sequence of the gamma subunit was deduced. The subunit is composed of 489 amino acids giving a molecular mass of 56,600 daltons. The deduced amino acid sequence data also indicate the presence of a 17-amino acid extension or signal peptide on this subunit. From these data, structural predictions for the gamma subunit are made such as potential membrane-spanning regions, possible asparagine-linked glycosylation sites, and the assignment of regions of the protein to the extracellular, internal, and cytoplasmic domains of the lipid bilayer.

Subunit structure of the acetylcholine receptor from Electrophorus electricus

The amino-terminal amino acid sequences of the four major peptides (Mr 41,000, 50,000, 55,000, and 62,000) present in purified preparations of Electrophorus electricus nicotinic acetylcholine receptor (AcChoR) have been determined for 24 cycles by automated sequence analysis procedures yielding four unique polypeptide sequences. The sequences showed a high degree of similarity, having identical residues in a number of positions ranging between 37% and 50% for specific pairs of subunits. Comparison of the sequences obtained with those of the subunits of similar molecular weight from Torpedo californica AcChoR revealed an even higher degree of homology (from 46% to 71%) for these two highly diverged species. Simultaneous sequence analysis of the amino termini present in native, purified Electrophorus AcChoR showed that these four related sequences were the only ones present and that they occur in a ratio of 2:1:1:1, with the smallest subunit ("alpha 1") being present in two copies. Genealogical analysis suggests that the subunits of both Torpedo and Electrophorus AcChoRs derive from a common ancestral gene, the divergence having occurred early in the evolution of the receptor. This shared ancestry and the very early divergence of the four subunits, as well as the highly conserved structure of the AcChoR complex along animal evolution, suggest that each of the subunits evolved to perform discrete crucial roles in the physiological function of the AcChoR.

Functional stability of Torpedo acetylcholine receptor. Effects of protease treatment

The effect of tryptic degradation on structural and functional properties of the membrane-bound acetylcholine receptor from Torpedo californica has been investigated. Under conditions of proteolysis which resulted in extensive degradation of receptor subunits, the membrane preparations retained their full capability of mediating agonist-induced cation flux as measured in rapid kinetic experiments. Low concentrations on trypsin also cleaved receptor dimers to monomers, and this effect was paralleled by degradation of the Mr 65 000 subunits which are known to contain sulfhydryl group(s) involved in receptor dimerization through an interchain disulfide bond(s). This conversion to monomers occurred at lower trypsin concentrations when the enzyme was added to the outside of the vesicles compared with the effects observed when the enzyme was present inside the vesicles. Similarly Mr 43 000 protein consistently found in preparations of the membrane-bound acetylcholine receptor, which can readily be removed without apparent effect on receptor function, displayed greater susceptibility to proteolysis when trypsin was added to the exterior medium rather than inside the vesicles. The results emphasize the full functionality of the monomeric form of the acetylcholine receptor comprised of four polypeptides.

Structure and function of an acetylcholine receptor

Structural analysis of an acetylcholine receptor from Torpedo californica leads to a three-dimensional model in which a "monomeric" receptor is shown to contain subunits arranged around a central ionophoretic channel, which in turn traverses the entire 110 A length of the molecule. The receptor extends approximately 15 A on the cytoplasmic side, 55 A on the synaptic side of the membrane. The alpha-bungarotoxin/agonist binding site is found to be approximately 55 A from the entrance to the central gated ion channel. A hypothesis for the mechanism of AcChR is presented which takes into account the structural and kinetic data, which is testable, and which serves as a focus for future studies on the agonist-induced structure change in AcChR.

Consequences of alkaline treatment for the ultrastructure of the acetylcholine-receptor-rich membranes from Torpedo marmorata electric organ

After fixation with glutaraldehyde and impregnation with tannic acid, the membrane that underlies the nerve terminals in Torpedo marmorata electroplaque presents a typical asymmetric triple-layered structure with an unusual thickness; in addition, it is coated with electron-dense material on its inner, cytoplasmic face. Filamentous structures are frequently found attached to these "subsynaptic densities." The organization of the subsynaptic membrane is partly preserved after homogenization of the electric organ and purification of acetylcholine-receptor (AchR)-rich membrane fragments. In vitro treatment at pH 11 and 4 degrees C of these AchR-rich membranes releases an extrinsic protein of 43,000 mol wt and at the same time causes the complete disappearance of the cytoplasmic condensations. Freeze-etching of native membrane fragments discloses remnants of the ribbonlike organization of the AchR rosettes. This organization disappears ater alkaline treatment and is replaced by a network which is not observed after rapid freezing and, therefore, most likely results from the lateral redistribution of the AchR rosettes during condition of slow freezing. A dispersion of the AchR rosettes in the plane of the membrane also occurs after fusion of the pH 11-treated fragments with phospholipid vesicles. These results are interpreted in terms of a structural stabilization and immobilization of the AchR by the 43,000-Mr protein binding to the inner face of the subsynaptic membrane.

Immunofluorescence localization at the mammalian neuromuscular junction of the Mr 43,000 protein of Torpedo postsynaptic membranes

Highly purified cholinergic postsynaptic membranes from Torpedo electric tissue contain, in addition to the acetylcholine receptor (AcChoR), major proteins of Mr 43,000 and Mr approximately 90,000 and minor proteins that can be removed from the membranes by alkaline treatment. We have prepared an antiserum to these alkaline-extractable proteins that reacts with the Mr 43,000 protein but not with any of the other major membrane proteins, including the AcChoR subunits. Immunofluorescent staining of sections of Torpedo electric tissue shows that this antiserum binds to the innervated but not the uninnervated surface of the electrocytes. In rat diaphragm muscle, the antigens recognized by this antiserum are highly concentrated at the synapse. Synaptic staining of muscle is eliminated by prior incubation of the antiserum with the Mr 43,000 protein but not by incubation with affinity-purified AcChoR. This antiserum stains end plates of muscles denervated for 7 days. Antiserum to AcChoR binds to the subsynaptic membranes of electrocytes and muscle but does not react with the Mr 43,000 protein. Purified AcChoR blocks staining of synapses by anti-AcChoR but the Mr 43,000 protein does not. These results indicate that the Mr 43,000 protein is located in the innervated membrane of Torpedo electrocytes and that an immunologically similar component is highly concentrated in the postsynaptic membrane of mammalian muscle.

Crystalline arrays of membrane-bound acetylcholine receptor

Electron micrographs of tubular structures with a crystalline arrangement of membrane-bound acetylcholine receptor oligomers have been analyzed by digital image reconstruction. The receptor molecules are oriented synaptic side out, and in projection they appear to be asymmetric and have a defined orientation. All four subunits are contained in the oligomers as demonstrated by immunoelectron microscopy; these structures therefore appear to be suitable for subunit localization in the oligomer.

Dissection of the 66 000-dalton subunit of the acetylcholine receptor

The 66 000-dalton or delta subunit of the acetylcholine receptor from Torpedo marmorata was covalently labeled in the presence of carbamoylcholine by 5-azido [3H]trimethisoquin (5-A[3H]T), a photoaffinity derivative of the local anesthetic trimethisoquin. After the attack of purified receptor with increasing concentrations of trypsin, the delta chain successively yielded fragments with apparent molecular weights of 50 000 (distinct from the beta subunit and referred to as the 50 000-bis (fragment), 49 000, and 47 000. With nondenatured (sodium cholate solubilized or membrane-bound) receptor, the 47 000-dalton fragment was not sensitive to trypsin and contained all of the covalent 5-A[3H]T label. This fragment was still glycosylated and had the same amino acid N terminus, valine, as the intact delta chain. A specific in vitro phosphorylation site of the delta subunit was located between the 49 000- and 50 000-dalton trypsin cleavage fragment and most likely is exposed to the cytoplasmic side of the membrane. A 16 000-dalton fragment of the delta chain was identified, which carriers a disulfide bond (or bonds) capable of cross-linking nonreduced receptor 9S monomerse into 12S dimers. The fragment did not remain associated with the receptor molecule after trypsin treatment.