Composition of acetylcholine receptor protein from skeletal muscle

Purification of acetylcholine receptor-like protein from fetal calf thymus

A cobrotoxin binding protein from the fetal calf thymus was isolated by affinity chromatography after solubilization with sodium cholate. The specific activity as a nicotinic acetylcholine receptor (AChR) was determined by assessing the binding to [3H]-alpha-bungarotoxin (BuTx), using the high-pressure liquid chromatography. An AChR-like protein was detected in the amount of 1.39-2.14 nmol per g protein. The first peak of 420k-protein from gel filtration of the eluate of affinity chromatography on a Sephacryl column showed one major polypeptide band with an Mr of 40k, by polyacrylamide gel electrophoresis in sodium dodecylsulfate, two major protein bands with pI 5.4-5.6 and 9.2 by isoelectric focusing, and reacted with sera from patients with myasthenia gravis.

Evidence for structural dissimilarity in the neurotransmitter binding region of purified acetylcholine receptors from human muscle and Torpedo electric organ

Acetylcholine receptor (AChR) purified from human skeletal muscle affinity-alkylated with bromoacetyl[methyl-3H]choline bromide ([3H]BAC) in mildly reducing conditions to yield a specifically radiolabeled polypeptide, Mr 44,000, the alpha-subunit. The binding of [125I]alpha-bungarotoxin to AChR was completely inhibited by affinity-alkylation, indicating that the human AChR's binding site for alpha-bungarotoxin is closely associated with the alpha-subunit's acetylcholine binding site. Structures in the vicinity of the alpha-bungarotoxin binding sites of AChRs from human muscle and Torpedo electric organ were compared by varying the conditions of alkylation. Under optimal conditions of reduction and alkylation, both human and Torpedo AChR incorporated BAC in equivalence to the number of alpha-bungarotoxin binding sites. However, with limited conditions of reduction but sufficient BAC to alkylate 100% of the alpha-bungarotoxin binding sites of human AChR, only 71% of the Torpedo AChR's binding sites were alkylated. In optimal conditions of reduction but with the minimal concentration of BAC that permitted 100% alkylation of the human AChR's alpha-bungarotoxin sites, only 74% of the Torpedo AChR's binding sites were alkylated. These data suggest that the neurotransmitter binding region of human muscle AChR is structurally dissimilar from that of Torpedo electric organ, having a higher binding affinity for BAC and an adjacent disulfide bond that is more readily accessible to reducing agents.

Acetylcholine receptors and myasthenia gravis

Recent years have seen considerable progress in understanding the nature of the molecular events involved in neuromuscular transmission. The acetylcholine receptor (AChR) has been purified to homogeneity and acetylcholine-induced ion transport has been reconstituted by incorporation of pure AChR into artificial membranes. Immunization against purified AChR induces a condition, clinically and physiologically similar to the human disease myasthenia gravis, which is due to circulating anti-AChR antibodies. This model, experimental autoimmune myasthenia gravis, is proving useful for investigating the role of genetic factors in determining the immune response to AChRs and for testing various experimental approaches to specific treatment. Myasthenia gravis is an autoimmune disease in which there is loss of acetylcholine receptors at the neuromuscular junction. Anti-AChR antibodies can be detected in the majority of patients and they cause loss of AChR by a variety of mechanisms. Anti-AChR antibody is heterogeneous and not restricted in idiotype. The role of the thymus in MG is still uncertain, but recent experiments implicate the presence of a cell type in MG thymus which may be involved in autosensitization to AChR.

Similarity of acetylcholine receptors of denervated, innervated and embryonic chicken muscles. 1. Molecular species and their purification

Acetylcholine receptors were purified to homogeneity from chicken embryonic, adult innervated and denervated muscles, by bio-specific chromatographies using immobilised alpha-neurotoxin and lentil lectin. A minimum specific activity for the pure receptor was estimated to be 6000 nmol alpha-toxin binding sites/g protein. For analysis, the receptors were radio-iodinated or tritiated to high specific radioactivity with succinimidyl-[2,3-3H]propionate. All of the iodinated protein present in the purified receptor preparation reacted with antibody against the pure acetylcholine receptor from Torpedo marmorata electric organ. In the case of all three muscle types used the same oligomeric forms were obtained. The principal form has a sedimentation coefficient of about 9 S, while a minor species (approximately 5S) was also appreciable in crude preparations of embryonic and denervated muscles. Immunization of rabbits with the homogenous receptor from chicken denervated muscle produced muscle weakness characteristic of experimental autoimmune myasthenia gravis. These antisera were equally reactive towards the receptor --125I-alpha-bungarotoxin complexes from chick innervated and denervated muscles. Likewise, the electrophoretic mobilities of the receptors (9-S form) from all three muscle types were identical, as were the isoelectric points of their complexes with 125I-alpha-bungarotoxin. Collectively, these findings and associated ones on subunit structure denote that the 9-S receptor molecules from junctional and extra-junctional area and embryonic stage of chicken muscle are indistinguishable by all criteria yet applied to them.

Mammalian muscle acetylcholine receptor purification and characterization

Nicotinic acetylcholine receptor (AcChR) was purified from fetal calf muscle by an affinity chromatographic method utilizing alpha-neurotoxin from Naja naja siamensis as an immobilized ligand. Preparations of AcChR with an average specific activity of 5 nmol of alpha-toxin bound/mg of protein were obtained, i.e., 75% of the theoretical specific activity assuming identity with Torpedo AcChR. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified AcChR consistently showed the presence of five polypeptides, having apparent Mr's of 42 000, 44 000, 49 000, 55 000, and 58 000, respectively. The peptide of Mr 44K was demonstrated to be actin. The amino acid composition of fetal calf AcChR was shown to be similar to that of Torpedo AcChR. In addition, calf AcChR contained large amounts of amino sugars. The sedimentation coefficient of the purified calf AcChR was found to be 9.25 +/- 0.25, i.e., similar to the monomeric form of electric organ AcChR. Determination of the isoelectric point of alpha-bungarotoxin/calf AcChR complexes revealed the presence of two charged forms, having pI values of 5.16 +/- 0.13 and 6.05 +/- 0.18, respectively.

Immunochemical properties of junctional and extrajunctional acetylcholine receptor

Immunological assays were performed to compare two distinct forms of the nicotinic acetylcholine receptor (AChR): junctional (JR) and extrajunctional receptor (EJR). Antibodies from myasthenia gravis patients' sera inhibited the binding of [125I]alpha-bungarotoxin (BGT), to EJR more effectively than binding to JR. Immunological differences between JR and EJR were confirmed by other assay methods. In all cases, EJR appeared to have antigenic determinants not found on JR. It was established that enzymatic removal of carbohydrates from EJR caused it to more closely resemble JR. Thus differences between JR and EJR may be due, in part, to carbohydrate residues found on EJR that are absent on JR. The extent of antibody binding to EJR was examined by gel filtration methods. Immunochemical studies of bands from SDS gels showed that antibodies are present in myasthenic serum which react with the 3 subunits (42, 53, 64 kdaltons) of AChR to varying degrees.

Acetylcholine receptor from mammalian skeletal muscle. Oligomeric forms and their subunit structures

1. The acetylcholine receptor of cat denervated skeletal muscle was solubilised with Triton X-100 in the presence of protease inhibitors and was shown to have a sedimentation coefficient of about 9 S. This oligomer can be converted to a smaller, active 4-S species. 2. This 9-S glycoprotein was purified to homogeneity (showing pI = 5.0) by improved biospecific chromatography on alpha-neurotoxin and lectin affinity gels, and shown to bind specifically 10--11.5 mumoles [2,3-3H]propionyl-alpha-bungarotoxin/g protein. The association rate constant (3 x 10(5) M-1 s-1 at 25 degrees C) for this reaction was similar to that observed with membrane-bound or unpurified receptor; affinity constants for nicotinic ligands were also similar in all these cases. 3. By a variety of techniques, a major polypeptide of Mr about 43,000 was detected in the pure protein. Likewise, both 9-S and 4-S oligomers isolated in a pure state at high yield (approximately equal to 80%) by a novel technique using anti-toxin immunoglobulin, contained the same size of subunit. 4. Sub-synaptic and extra-synaptic forms of the receptor were alkylated specifically in the membrane-bound state with the affinity reagent bromo[3H]acetylcholine. As in the case of the pure receptor from denervated muscle, the same size polypeptide (Mr 43,000) was labelled. This was true, also, for both the 9-S and the 4-S oligomer of the denervated muscle receptor. 5. Proposed oligomeric structures of acetylcholine receptors containing single and multiple-size subunits are discussed.

Molecular forms of the acetylcholine receptor from vertebrate muscles and Torpedo electric organ. Interactions with specific ligands

Multiple forms of the acetylcholine receptor solubilised from cat denervated muscle were separated by velocity sedimentation centrifugation. The kinetic properties of the two main forms (with sedimentation coefficients of 9 S and 4 S) were investigated using a pure preparation of a suitable probe, [3H]propionyl-alpha-bungaro-toxin. The binding of this toxin to each of these forms of the muscle receptor was consistent with a simple bimolecular reaction with a homogeneous class of binding sites. Negligible dissociation of the receptor-toxin complex was observed. This behaviour was also found for the different forms of the acetylcholine receptor of chick embryo muscle and of Torpedo marmorata electric organ. Association rate constants for binding of the 3H-labelled alpha-toxin to receptor from chick embryo muscle and the 9-S and 4-S forms from cat denervated muscle were 0.54 X 10(5), 1.76 X 10(5) and 2.69 X 10(5) M--1 S--1 respectively, at 25 degrees C. The values obtained for the 9-S and 13-S forms of receptor from T. marmorata were 4.51 X 10(5) and 9.93 X 10(5) M--1 S--1 respectively. The reaction of the 3H-labelled alpha-toxin with the receptor was second-order and linear in the presence of an antagonist, as in its absence, for the 4-S and 9-S forms of the cat denervated muscle receptor. This reaction of the receptor was inhibited by cholinergic ligands, with Ki values for two antagonists tested being greater with the 4-S form than with the 9-S form. Apparent negative interaction is observed for antagonists with this receptor, with Hill coefficients of about 0.78 and 0.64 for the 4-S and 9-S forms respectively. A ligand-induced affinity increase, produced by the agonists but not by the antagonists, was observed in this reaction for both forms of the muscle receptor. Two agonists tested showed no difference between these forms in their high-affinity states in either their binding affinities or Hill coefficients.

Reconstitution of a functional acetylcholine receptor. Polypeptide chains, ultrastructure, and binding sites for acetylcholine and local anesthetics

The 'reconstitution cycle' is composed of the following sequence of operations. Highly purified receptor-rich membranes prepared from Torpedo marmorata electric organ are exposed to pH 11 to remove the 43,000-Mr protein and dispersed into solution by sodium cholate under conditions where more than 85% of the receptor protein is in its 9-S form. Elimination of the detergent by filtration on a Sephadex column (or dialysis) yields a 'reconstituted receptor' fraction, under conditions which conserve part of the endogenous lipids, or 'reconstituted vesicles' in the presence of an excess of exogenous lipids. The polypeptide composition of these fractions was analysed by sodium dodecylsulfate gel electrophoresis. Conditions are defined for quantitative measurements of the various polypeptide chains. The 40,000-Mr chain, which is labelled by the affinity reagent 4-(N-maleimido)phenyl [3H]trimethylammonium and therefore carries the acetylcholine receptor site, is the dominant polypeptide in the alkaline-treated membranes and the reconstituted acetylcholine receptor. Electron microscopy discloses that many of the alkaline-treated membranes no longer form closed vesicles and do not show the transverse asymmetry of the native membranes observed after tannic acid fixation. In the reconstituted receptor fractions, the receptor molecules reaggregate into discs and may be exposed on both faces of the discs. In the reconstituted vesicles, receptor rosettes are integrated to the lipid vesicles. With native membranes, the radioactive local anesthetic [3H]trimethisoquin binds to three classes of sites: non-specific, low-affinity and high-affinity. Carbamylcholine causes an increase in the number of high-affinity sites up to approximately 0.7 times the number of alpha-125I-bungarotoxin sites. This ratio, the three classes of binding sites, and their regulation by carbamylcholine are conserved through the reconstitution cycle.

Stoichiometry of the ligand-binding sites in the acetylcholine-receptor oligomer from muscle and from electric organ. Measurement by affinity alkylation with bromoacetylcholine

1. The affinity alkylation reaction of the cholinergic, depolarising ligand, bromoacetylcholine with reduced acetylcholine receptor in the membrane fragments of Torpedo marmorata and in Triton-solubilised receptor from cat denervated muscle has been studied. 2. Brief pretreatment with 100 microM bromoacetylcholine abolishes all [3H]alpha-neurotoxin binding in both cases. 3. In the receptor from each of these sources, the number of sites of specific alpha-neurotoxin binding is exactly equal to the number of sites that can be specifically alkylated by bromol[3H]acetytlcholine, at saturation of either ligand. 4. The concentration-dependence of specific bromo[3H]acetylcholine binding is found to be biphasic. A first phase can be clearly discerned in which one-half of the total specific ligand-binding sites are alkylated readily, and a second phase in which the remainder react at higher reagent concentrations. The same discrimination of two equal sets of ligand sites can be obtained by pre-blockade using low concentrations of unlabelled bromoacetylcholine followed by reaction with [3H]alpha-neurotoxin or bromo[3H]acetylcholine. 5. In both phases, a single subunit of Mr about 43000 is the sole site of specific alkylation in both Torpedo and muscle. The reasons for the appearance of two equal but distinct populations in the ligand binding sites in the receptors are discussed.

Purification and characterization of nicotinic acetylcholine receptors from muscle

The nicotinic acetylcholine receptor was purified from normal and denervated rat skeletal muscle. The purification protocol included alpha-cobratoxin biospecific adsorption, ion exchange chromatography, and gel filtration steps. The highest specific activity achieved was 7.5 pmol of 125I-alpha-bungarotoxin binding sites per microgram protein. Sodium dodecyl sulfate gel electrophoresis of purified AChR revealed subunits with molecular weights of 42,000 and 66,000 daltons and a minor component with a molecular weight of 52,000 daltons. Normal muscle AChR is comprised of one toxin binding component. Upon denervation a second component appears, but both components are increased as a consequence of denervation. A dissociation constant of 1.5 x 10(-8)M was determined for d-tubocurarine from receptor from both normal and denervated muscle. A dissociation constant of 1 x 10(-7)M for acetylcholine, perhaps analogous to the high affinity acetylcholine binding observed in electric fish receptor, was determined.

Immunochemical similarities between subunits of acetylcholine receptors from Torpedo, Electrophorus, and mammalian muscle

Polypeptide chains composing acetylcholine receptors from the electric organs of Torpedo californica and Electrophorus electricus were purified and labeled with 125I. Immunochemical studies with these labeled chains showed that receptor from Electrophorus is composed of three chains corresponding to the alpha, beta, and gamma chains of receptor from Torpedo but lacks a chain corresponding to the delta chain of Torpedo. Experiments suggest that receptor from mammalian muscle contains four groups of antigenic determinants corresponding to all four of the Torpedo chains. Binding of 125I-labeled chains was measured by quantitative immune precipitation and electrophoresis. Antisera to the following immunogens were used: denatured alpha, beta, gamma, and delta chains of Torpedo receptor, native receptor from Torpedo and Electrophorus electric organs and from rat and fetal calf muscle, and human muscle receptor (from autoantisera of patients with myasthenia gravis). The four chains of Torpedo receptor were immunologically distinct from one another and from higher molecular weight chains found in electric organ membranes. Antibodies to these chains reacted very efficiently with native Torpedo receptor, but the reverse was not true. Antibodies to native receptor from Torpedo and Electrophorus reacted slightly with each of the chains of the corresponding receptor. However, cross-reaction between chains and antibodies to any native receptor was most obviuos with the alpha chain of Torpedo or the corresponding alpha' chain of Electrophorus. Antiserum to alpha chains exhibited higher titer aginst receptor from denervated rat muscle. Antibodies from myasthenia gravis patients did not cross-react detectably with 125I-labeled chains from electric organ receptors. Most interspecies cross-reaction occurred at conformationally dependent determinants whose subunit localization could not be determined by reaction with the denatured chains.