Protease effects on the structure of acetylcholine receptor membranes from Torpedo californica

Alpha-bungarotoxin binding to acetylcholine receptor membranes studied by low angle X-ray diffraction

The nicotinic acetylcholine receptor (nAChR) carries two binding sites for snake venom neurotoxins. alpha-Bungarotoxin from the Southeast Asian banded krait, Bungarus multicinctus, is a long neurotoxin which competitively blocks the nAChR at the acetylcholine binding sites in a relatively irreversible manner. Low angle x-ray diffraction was used to generate electron density profile structures at 14-A resolution for Torpedo californica nAChR membranes in the absence and presence of alpha-bungarotoxin. Analysis of the lamellar diffraction data indicated a 452-A lattice spacing between stacked nAChR membrane pairs. In the presence of alpha-bungarotoxin, the quality of the diffraction data and the lamellar lattice spacing were unchanged. In the plane of the membrane, the nAChRs packed together with a nearest neighbor distance of 80 A, and this distance increased to 85 A in the presence of toxin. Electron density profile structures were calculated in the absence and presence of alpha-bungarotoxin, revealing a location for the toxin binding sites. In native, fully-hydrated nAChR membranes, alpha-bungarotoxin binds to the nAChR outer vestibule and contacts the surface of the membrane bilayer.

Mapping the mAb 383C epitope to alpha 2(187-199) of the Torpedo acetylcholine receptor on the three-dimensional model

Monoclonal antibody 383C is an anti-acetylcholine receptor antibody whose binding to the receptor is blocked by alpha-bungarotoxin and by carbamylcholine. Monoclonal antibody 383C binds to the alpha subunit of the Torpedo acetylcholine (ACh) receptor as well as to its V8-protease 20 kDa fragment that possesses the affinity alkylatable Cys192/193. In an epitope scanning experiment spanning the N-terminal 211 amino acid residues of the alpha subunit, 383C binds uniquely to three overlapping peptides; alpha(184-196), alpha(187-199) and alpha(190-202). These peptides span a cluster of amino acid residues implicated in the binding of acetylcholine, including Cys192/193. To map the location of these residues on the three-dimensional model of the ACh receptor, we have employed a combination of X-ray diffraction from oriented complexes of 383C with ACh receptor-enriched membrane vesicles and electron microscopy of negatively stained tubular arrays of 383C/receptor complexes. The X-ray diffraction study finds extra electron density in the presence of 383C centered 35 A above the synaptic side phosphate head groups. The electron micrographic images display extra stain exclusion from the antibody at a site adjacent to the alpha2 subunit on the periphery of the rosette clockwise to the alpha2 vertex. This mapping localizes several residues of the ACh receptor alpha subunit involved in the binding of acetylcholine. Despite these residues being present in both alpha subunits, only the alpha2 subunit is decorated with this monoclonal antibody.

IL-12 is involved in the induction of experimental autoimmune myasthenia gravis, an antibody-mediated disease

IL-12 has been shown to be involved in the pathogenesis of Th1-mediated autoimmune diseases, but its role in antibody-mediated autoimmune pathologies is still unclear. We investigated the effects of exogenous and endogenous IL-12 in experimental autoimmune myasthenia gravis (EAMG). EAMG is an animal model for myasthenia gravis, a T cell-dependent, autoantibody-mediated disorder of neuromuscular transmission caused by antibodies to the muscle nicotinic acetylcholine receptor (AChR). Administration of IL-12 with Torpedo AChR (ToAChR) to C57BL/6 (B6) mice resulted in increased ToAChR-specific IFN-gamma production and increased anti-ToAChR IgG2a serum antibodies compared with B6 mice primed with ToAChR alone. These changes were associated with earlier and greater neurophysiological evidence of EAMG in the IL-12-treated mice, and reduced numbers of AChR. By contrast, when IL-12-deficient mice were immunized with ToAChR, ToAChR-specific Th1 cells and anti-ToAChR IgG2a serum antibodies were reduced compared to ToAChR-primed normal B6 mice, and the IL-12-deficient mice showed almost no neurophysiological evidence of EAMG and less reduction in AChR. These results indicate an important role of IL-12 in the induction of an antibody-mediated autoimmune disease, suggest that Th1-dependent complement-fixing IgG2a anti-AChR antibodies are involved in the pathogenesis of EAMG, and help to account for the lack of correlation between anti-AChR levels and clinical disease seen in many earlier studies.

The human-severe combined immunodeficiency myasthenic mouse model: a new approach for the study of myasthenia gravis

We have established a new chimeric human-mouse model of myasthenia gravis in severe combined immunodeficiency mice, using human peripheral blood lymphocytes that survive in the mouse and produce specific antibodies that mediate pathological changes typical of human myasthenia gravis. Mice given peripheral blood lymphocytes from both anti-acetylcholine receptor (AChR) antibody-positive and -negative patients with myasthenia gravis showed circulating anti-acetylcholine receptor antibodies, deposition of human IgG at muscle end-plates, and simplification of the postsynaptic membrane, findings characteristic of human myasthenia gravis. Mice given human peripheral blood lymphocytes from healthy donors and simultaneously immunized with Torpedo acetylcholine receptor showed the same changes. This chimeric model, utilizing human cells to reproduce the immunopathological findings of human myasthenia gravis in a nonhuman environment, offers new opportunities to study immune regulation in autoimmunity.

The beta 2-adrenergic agonist terbutaline suppresses acute passive transfer experimental autoimmune myasthenia gravis (EAMG)

Treatment of Lewis rats with the beta 2-adrenergic agonist terbutaline suppressed clinical symptoms of acute passive transfer EAMG induced with monoclonal anti-acetylcholine receptor antibody and accelerated clinical recovery in affected animals. Electrophysiological studies showed that the amplitude of the first compound muscle action potential was significantly larger in terbutaline-treated rats as compared to controls. In both groups, a comparable number of inflammatory cells at the muscle endplates was seen.

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Three-dimensional structure of the nicotinic acetylcholine receptor and location of the major associated 43-kD cytoskeletal protein, determined at 22 A by low dose electron microscopy and x-ray diffraction to 12.5 A

The three-dimensional structure of the nicotinic acetylcholine receptor (AChR) from Torpedo californica, crystallized both before and after removal of associated proteins, most notably the main 43-kD cytoskeletal protein that interacts both with AChR and actin, is determined to a resolution of 22 A. This is the first structural analysis where the 43-kD protein has been removed from the sample before crystallization. Thus, it provides the most reliable assessment of what constitutes the structure of the minimal five subunit AChR complex, and, by comparison with the native membrane, of the location of the 43-kD cytoskeletal protein. Image reconstruction of two-dimensional crystals includes information from electron images of up to +/- 52 degrees tilted specimens of latticed AChR. Hybrid density maps that include x-ray diffraction perpendicular to the membrane to 12.5 A resolution were used and eliminate some of the distortions introduced in maps based only on electron microscopic analyses. Comparison of the difference Fourier density maps between AChR with its normal complement of associated proteins, and without them shows that the main density, assigned to the actin-binding 43-kD component is closely associated with the lipid bilayer as well as with the cytoplasmic domain of the AChR. It binds beside the AChR, not beneath it as suggested by others (C. Toyoshima and N. Unwin 1988. Nature [Lond.]. 336:237-240). There is good agreement between the volumes of density for structural components and expected volumes based on their molecular weight. Acetylcholine receptors aggregate in the absence of any cytoskeletal proteins, suggesting that the AChR alone is sufficient to encode and stabilize clustering, and perhaps to do so during synaptogenesis. The main 43-kD component may play a role in location and rate of association of AChR. We show that the disulfide bond that cross-links delta-delta chains of adjacent pentamers in about 80% of AChR, is not required to stabilize the lattice of AChR. Latticed tube structures are stable indefinitely. The lattices described here have 20% less volume of lipid than those originally obtained and characterized by J. Kistler and R. M. Stroud (1981. Proc. Natl. Acad. Sci. USA. 78:3678-3682), or those subsequently characterized by A. Brisson and P. N. T. Unwin (1984. J. Cell Biol. 99:1202-1211) and A. Brisson and P. N. T. Unwin (1985. Nature (Lond.). 315:474-477).

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Disruption of active zones in frog neuromuscular junctions following treatment with proteolytic enzymes

Frog neuromuscular junction treated with proteolytic enzymes to remove the basal lamina were studied with freeze-fracture techniques in order to examine the influence of the basal lamina in the maintenance of active zone ultrastructure. The active zone is believed to be the site of transmitter release and has a unique membrane organization and location in the neuromuscular junction. After removal of the basal lamina by successive treatment of 0.01% collagenase and 0.1% protease for 1 h each, active zone disruption was observed. Some active zones became segmented, and some were also randomly located and oriented, but they still had normal double-row particle organization. Others contained only clusters of large intramembrane particles. These disorganized active zones were still functional as indicated by the presence of vesicle openings. Some enzyme-treated junctions were also exposed to the membrane cholesterol probe, filipin, to examine the expression of membrane lipid heterogeneity in disrupted active zones. As in normal active zones, filipin-sterol complexes were absent. The densities of background particles in the presynaptic membranes and of large particles thought to be acetylcholine receptors were not significantly altered by the enzyme treatment. Although a direct effect of the enzymes on active zone ultrastructure can not be totally excluded, the present work is consistent with a maintenance role of the basal lamina in active zone organization and location.

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)

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

(1-Pyrene)sulfonyl azide: a fluorescent probe for measuring the transmembrane topology of acetylcholine receptor subunits

(1-Pyrene)sulfonyl azide (PySA), a fluorescent, lipophilic photolabel, was used as a probe for the transmembrane topology of the acetylcholine receptor (AchR) subunits. Photolabeling of native, alkaline-extracted, and reconstituted AchR membrane preparations resulted in the labeling of all the AchR subunits. However the reconstituted AchR membrane preparations incorporated twice as much PySA into each mole of the AchR complex. Photolabeling of all subunits of the AchR does not appear to alter the agonist concentration response of AchR-mediated cation translocation.

Quantitative freeze-fracture and filipin-binding study of retinal pigment epithelial-cell basal membranes in diabetic rats

Breakdown of the blood-retinal barrier in diabetes may be related to alterations in the retinal pigment epithelial (RPE) cell layer. Morphological studies suggest increased permeability of diabetic RPE plasma membranes, and proliferation and flattening of the RPE basal infoldings have been observed in diabetic animals. In order to determine whether these phenomena are associated with changes in membrane protein or sterol composition, we have used quantitative electron-microscope freeze-fracture and filipin-binding techniques to study the RPE basal membrane in streptozotocin diabetic and 3-O-methyl glucose control rats. Perfusion-fixed retinas were processed for freeze-fracture and filipin-binding analysis. Filipin, a polyene antibiotic, binds specifically to 3-beta-hydroxy-sterols to produce membrane deformations recognizable by freeze-fracture. These analyses revealed an 11% increase in the density of intramembrane particles within the cytoplasmic (P-face) leaflet in diabetic rats as compared with the controls (P less than 0.01, t test). The increase occurred primarily in 6-9-nm particles, while smaller particles were decreased (P less than 0.001, chi-square test). Filipin binding was the same in both groups. These results suggest that alterations in intrinsic membrane proteins may contribute to permeability and surface area changes in the diabetic RPE but that RPE membrane sterols are not affected by diabetes.

Location of terbium binding sites on acetylcholine receptor-enriched membranes

Acetylcholine receptor-enriched membranes bind 45 terbium cations per receptor. The Tb(III) X-ray scattering factor changes by as much as 30% over a 50 eV range about the L3 absorption edge. We exploit these changes to modulate the contribution of these ions to the X-ray diffraction pattern of oriented receptor-enriched membranes by varying the incident X-ray energy. Difference Fourier analysis of the meridional diffraction amplitudes at two X-ray energies revealed six localized regions of Tb(III) density across the membrane. Most significant is the finding of 18 Tb(III) ions near the entrance and 11 ions near the exit of the ion channel as well as 4 or 5 Tb(III) ions localized in the channel itself. This evidence strongly suggests the presence of anionic carboxylate side-chains on the channel lining.

Evidence for a polarity in the distribution of proteins from the cytoskeleton in Torpedo marmorata electrocytes

The subcellular distribution of the 43,000-D protein (43 kD or v1) and of some major cytoskeletal proteins was investigated in Torpedo marmorata electrocytes by immunocytochemical methods (immunofluorescence and immunogold at the electron microscope level) on frozen-fixed sections and homogenates of electric tissue. A monoclonal antibody directed against the 43-kD protein (Nghiêm, H. O., J. Cartaud, C. Dubreuil, C. Kordeli, G. Buttin, and J. P. Changeux, 1983, Proc. Natl. Acad. Sci. USA, 80:6403-6407), selectively labeled the postsynaptic membrane on its cytoplasmic face. Staining by anti-actin and anti-desmin antibodies appeared evenly distributed within the cytoplasm: anti-desmin antibodies being associated with the network of intermediate-sized filaments that spans the electrocyte, and anti-actin antibodies making scattered clusters throughout the cytoplasm without preferential labeling of the postsynaptic membrane. On the other hand, a dense coating by anti-actin antibodies became apparent on the postsynaptic membrane in homogenates of electric tissue pointing to the possible artifactual redistribution of a soluble cytoplasmic actin pool. Anti-fodrin and anti-ankyrin antibodies selectively labeled the non-innervated membrane of the cell. F actin was also detected in this membrane. Filamin and vinculin, two actin-binding proteins recently localized at the rat neuromuscular junction (Bloch, R. J., and Z. W. Hall, 1983, J. Cell Biol., 97:217-223), were detected in the electrocyte by the immunoblot technique but not by immunocytochemistry. The data are interpreted in terms of the functional polarity of the electrocyte and of the selective interaction of the cytoskeleton with the innervated and non-innervated domains of the plasma membrane.

Freeze-fracture quantitative comparison of rabbit corneal epithelial and endothelial membranes

The intramembrane structure of epithelial apical, lateral and basal membranes was compared with that of the endothelium in normal rabbit corneas. The cytoplasmic membrane leaflet (P-face) of apical epithelial membrane displays randomly scattered intramembrane particles over the apical microvilli. Apical endothelial P-face membranes, by comparison, are covered by densely-packed particles. The lateral membranes of superficial epithelial cells display delicate tight junctions, punctate particle aggregates characteristic of desmosomes, and small gap junctions. Background intramembrane particles are randomly distributed and not densely packed. Large gap junctions and fewer desmosomes are present on deeper epithelial cells, including basal cells. By comparison, the endothelial lateral membranes display densely-packed, background particles and numerous triangular-shaped, gap junctional particle aggregates bordered by strands of particles resembling tight junctions. At the apical border, long anastomosing tight junctional strands are present. The basal E-face membranes of both cell layers adjacent to the basement membrane are similar in that they have sparse particles and vesicle fusion sites. Small particle aggregates are present on basal epithelial membranes at sites of hemidesmosomes. Quantitation of particle size and density on P-face apical and lateral membranes of each cell layer reveals that particle density on endothelial membranes is more than twice that on epithelial membranes. Particle sizes are similar on both cell layers and mean diameter ranges from 8.7 to 9.2 nm. The differences in intramembrane particle density and junctional aggregates on epithelial and endothelial membranes reflect membrane constituents associated with functional differences of these two cell layers.

Acetylcholine receptor: an allosteric protein

The nicotine receptor for the neurotransmitter acetylcholine is an allosteric protein composed of four different subunits assembled in a transmembrane pentamer alpha 2 beta gamma delta. The protein carries two acetylcholine sites at the level of the alpha subunits and contains the ion channel. The complete sequence of the four subunits is known. The membrane-bound protein undergoes conformational transitions that regulate the opening of the ion channel and are affected by various categories of pharmacologically active ligands.

Postsynaptic acetylcholine receptor efficacy is similarly increased by detergents and acetylcholinesterase inhibitors at an Aplysia synapse

At Aplysia H- and D-type cholinergic neuro-neuronal synapses, application of high concentrations of detergents (Triton X-100 and sodium deoxycholate) depressed synaptic transmission and the postsynaptic response to ionophoretic application of acetylcholine (ACh) or carbachol. However, when very low concentrations of detergents (of the order of 10(-9) M for sodium deoxycholate) were used, the nerve-evoked response as well as the ACh and carbachol ionophoretic responses were facilitated (by at least 200%), but only in H-type cells. This facilitation was similar to that previously observed in the same receptor type when acetylcholinesterase (AChE) was inhibited by various organophosphate or carbamate acetylcholinesterase inhibitors (AChEIs)3. Indeed, the effects of AChEI and detergents were not cumulative. We propose that on H-type synapses detergents may perturb a hypothetical molecular interaction between AChE and the acetylcholine receptor (AChR) by which AChE modulates the ability of the AChR to be activated by ACh or carbachol.

Peripheral proteins of postsynaptic membranes from Torpedo electric organ identified with monoclonal antibodies

Highly purified postsynaptic membranes from Torpedo electric organ contain the acetylcholine receptor as well as other proteins. To identify synapse-specific components, we prepared monoclonal antibodies (mabs) to proteins extracted from the membranes with either lithium diiodosalicylate or alkaline treatment. 10 mabs specific for three different proteins were obtained. Seven mabs reacted with a major 43,000-mol-wt protein (43K protein). This protein is composed of isoelectric variants (pl = 7.2-7.8) and each of the mabs reacted with all of the variants. Analysis of these mabs by competition for binding to 43K protein and by reaction with proteolytic fragments of 43K protein in immunoblots showed that they recognize at least five different epitopes. Two mabs reacted with a protein of 90,000 mol wt (90K protein) and one with a protein of 58,000 mol wt composed of isoelectric variants (pl = 6.4-6.7) (58K protein). The 43K and 58K proteins appeared to co-purify with the receptor-containing membranes while the 90K protein did not. Immunofluorescence experiments indicated that the anti-43K mabs bind to the innervated face of Torpedo electrocytes and that a component related to the 43K protein is found at the rat neuromuscular junction. The anti-58K mab stained the innervated face, although rather weakly, while the anti-90K mabs reacted intensely with the non-innervated membrane. Thus, the 43K protein and possibly also the 58K protein are synaptic components while the 90K protein is predominantly nonsynaptic.

Ultrastructural localization of the Mr 43,000 protein and the acetylcholine receptor in Torpedo postsynaptic membranes using monoclonal antibodies

Four mouse monoclonal antibodies (mabs) were shown by immunoblotting procedures to recognize the major, basic, membrane-bound Mr 43,000 protein (43K protein) of acetylcholine receptor-rich postsynaptic membranes from Torpedo nobiliana . These mabs and a mab against an extracellular determinant on the acetylcholine receptor were used to localize the two proteins in electroplax (Torpedo californica) and on unsealed postsynaptic membrane fragments at the ultrastructural level. Bound mabs were revealed with a rabbit anti-mouse Ig serum and protein A-colloidal gold. The anti-43K mabs bound only to the cytoplasmic surface of the postsynaptic membrane. The distributions of the receptor and the 43K protein along the membrane were found to be coextensive. Distances between the membrane center and gold particles were very similar for anti-receptor and anti-43K mabs (29 +/- 7 nm and 26 to 29 +/- 7 to 10 nm, respectively). These results show that the 43K protein is a receptor-specific protein having a restricted spatial relationship to the membrane. They thus support models in which the 43K protein is associated with the cytoplasmic domains of the receptor molecule.

Production and characterization of a monoclonal antibody directed against the 43,000-dalton v1 polypeptide from Torpedo marmorata electric organ

Subsynaptic membrane fragments prepared from Torpedo marmorata electric organ contain, in addition to the acetylcholine receptor polypeptides, a major protein band of apparent molecular mass 43,000 daltons. On two-dimensional gels, this band yields three spots referred to as v1, v2, and v3. Monoclonal antibodies against the 43,000-dalton proteins were developed in CBA mice. One of them reacted exclusively with the v1 polypeptide but not with v2 and v3. Staining by the "immunogold" reaction followed by observation by electron microscopy showed that this antibody exclusively labeled the innervated membrane of T. marmorata electroplaque on its cytoplasmic face. Electroblots of one-dimensional gels of membrane preparations from 80-mm embryo electric organ were prepared. After reaction with the anti-v1 monoclonal antibody, a strongly stained 43,000-dalton band was revealed.

(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)

Creatine kinase activity in the Torpedo electrocyte and in the nonreceptor, peripheral v proteins from acetylcholine receptor-rich membranes

The nonreceptor, peripheral v proteins (Mr 43,000 proteins) are conspicuous components of the acetylcholine receptor-rich membranes and the Torpedo electrocyte, so far devoid of any known enzymatic function. Creatine kinase (adenosine 5'-triphosphate:creatine N-phosphotransferase, EC 2.7.3.2) is identified in distinct polypeptides belonging to the family of v proteins. Embryonic (70- to 90-mm embryos), neonatal, and adult electric organs of Torpedo marmorata contain two isoenzymes of creatine kinase: the BB (brain) and the MM (muscle) forms. The proportion of the two isoenzymes does not appear to change in the course of ontogenic and postnatal development. Only the BB isoenzyme appears to be associated with the acetylcholine-rich membranes in adult Torpedo. The creatine kinase can be purified to homogeneity by chromatographic procedures that exploit the richness in free sulfhydryl groups of the enzyme. Specific activities of 150 units/mg are obtained from electric tissue. The enzyme subunits identified by two-dimensional gel electrophoresis and immunoblotting techniques have pI values in the 6.0-6.5 region and apparent molecular weights in the 40,000-43,000 range, the latter values depending on redox conditions.

Identification of homo-oligomers as potential intermediates in acetylcholine receptor subunit assembly

We have examined the sedimentation behavior, on sucrose density gradients, of acetylcholine receptor (AcChoR) subunits synthesized in vitro and integrated into heterologous rough microsomal membranes. In media containing nondenaturing detergents such as Triton X-100 or deoxycholate, the subunits appear to self-associate although, as previously reported, no heterologous interactions were detected. The sedimentation profiles assume a broad distribution in the region of 7-13 S. However, the peak fractions occupy the same region of the gradient as does native AcChoR, run in parallel. Such large homo-oligomers were not observed for another membrane protein, opsin, studied in the same way. This indicated that the associations are indeed between the AcChoR subunits and not simply between all newly synthesized membrane proteins. The homologous associations are interpreted to suggest a mechanism for maintaining the ionophore surfaces of the subunits in an energetically preferred, but metastable, configuration during the lengthy period of post-translational assembly.

Equilibrium binding of acetylcholine to the membrane-bound acetylcholine receptor

We have studied the binding of acetylcholine to membrane-bound acetylcholine receptor from Torpedo marmorata employing a highly accurate airfuge assay procedure. At equilibrium the receptor displays two classes of acetylcholine binding sites; these interact with only weak positive cooperativity. As a further difference to binding data deduced from electrophysiological dose/response curves, the equilibrium constants for the two classes of sites (Kd1 = 25 nM, Kd2 = 8 nM) are orders of magnitude lower than the concentration required for half-maximal response. Both the weaker-than-expected cooperativity of sites and the high binding affinities are likely to be due to desensitisation of the receptor during the period of incubation. The positively cooperative interaction of acetylcholine binding sites is only observed with membrane preparations obtained in the presence of appropriate chelating, sulfhydryl-blocking and active-serine-blocking agents. Aged membrane preparations loose the ability of site interactions while only small changes in the total number of binding sites are observed. In the absence of divalent ions, the affinity of binding of acetylcholine to the receptor is reduced. To assess the significance of the binding data obtained, several alternative reaction schemes for non-random binding to two sites at the receptor are considered. In addition, the effects of possible sources of experimental error on the shape of Scatchard plots are analysed.

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.

Cytoplasmic surface structure in postsynaptic membranes from electric tissue visualized by tannic-acid-mediated negative contrasting

In this study, acetylcholine receptor-rich postsynaptic membranes from electric tissues of the electric rays Narcine brasiliensis and Torpedo californica are negatively contrasted for thin-section electron microscopy through the use of tannic acid. Both outer (extracellular) and inner (cytoplasmic) membrane surfaces are negatively contrasted, and can be studied together in transverse sections. The hydrophobic portion of the membrane appears as a thin (approximately 2 nm), strongly contrasted band. This band is the only image given by membrane regions which are devoid of acetylcholine receptor. In regions of high receptor density, however, both surfaces of the membrane are seen to bear or be associated with material which extends approximately 6.5 nm beyond the center of the bilayer. The material on the outer surface can be identified with the well-known extracellular portion of the receptor molecule. A major portion of the inner surface image is eliminated by extraction of the membranes at pH 11 to remove peripheral membrane proteins, principally the 43,000 Mr (43K) protein. The images thus suggest a cytoplasmic localization of the 43K protein, with its distribution being coextensive with that of the receptor. They also suggest that the 43K protein extends farther from the cytoplasmic surface than does the receptor.

Nicotinic postsynaptic membranes from Torpedo: sidedness, permeability to macromolecules, and topography of major polypeptides

Experiments were conducted to examine the topographic arrangement of the polypeptides of the acetylcholine receptor (AcChR) and the nonreceptor Mr 43,000 protein in postsynaptic membranes isolated from Torpedo electric organ. When examined by electron microscopy, greater than 85% of vesicles were not permeable to ferritin or lactoperoxidase (LPO). Exposure to saponin was identified as a suitable procedure to permeabilize the vesicles to macromolecules with minimal alteration of vesicle size or ultrastructure. The sidedness of vesicles was examined morphologically and biochemically. Comparison of the distribution of intramembrane particles on freeze-fractured vesicles and the distribution found in situ indicated that greater than 85% of the vesicles were extracellular-side out. Vesicles labeled with alpha-bungarotoxin (alpha-Bgtx) were reacted with antibodies against alpha-BgTx or against purified AcChR of Torpedo. Bound antibodies were detected by the use of ferritin-conjugated goat anti-rabbit antibody and were located on the outside of greater than 99% of labeled vesicles. Similar results were obtained for normal vesicles or vesicles exposed to saponin. Quantification of the amount of [3H]-alpha-BgTx bound to vesicles before and after they were made permeable with saponin indicated that less than 5% of alpha-BgTx binding sites were cryptic in normal vesicles. It was concluded that greater than 95% of postsynaptic membranes were oriented extracellular-side out. LPO-catalyzed radioiodinations were performed on normal and saponin-treated vesicles and on vesicles from which the Mr (relative molecular mass) 43,000 protein had been removed by alkaline extraction. In normal vesicles, polypeptides of the AcChR were iodinated while the Mr 43,000 protein was not. In vesicles made permeable with saponin, the pattern of labeling of AcChR polypeptides was unchanged, but the Mr 43,000 protein was heavily iodinated. The relative iodination of AcChR polypeptides was unchanged in membranes equilibrated with agonist or with alpha-BgTx or after alkaline-extraction. It was concluded that the Mr 43,000 protein is present on the intracellular surface of the postsynaptic membrane and that AcChR polypeptides are exposed on the extracellular surface.

Oligomeric forms of the membrane-bound acetylcholine receptor disclosed upon extraction of the Mr 43,000 nonreceptor peptide

Oligomeric forms of the acetylcholine receptor are directly visualized by electron microscopy in receptor-rich membranes from torpedo marmorata. The receptor structures are quantitatively correlated with the molecular species so far identified only after detergent solubilization, and further related to the polypeptide composition of the membranes and changes thereof. The structural identification is made possibly by the increased fragility of the membranes after extraction of nonreceptor peptides and their subsequent disruption upon drying onto hydrophilic carbon supports. Receptor particles in native membranes depleted of nonreceptor peptides appear as single units of 7-8 nm, and double and multiple aggregates thereof. Particle doublets having a main-axis diameter of 19 +/- 3 nm predominate in these membranes. Linear aggregates of particles similar to those observed in rotary replicas of quick-frozen fresh electrolytes (Heuser, J.E. and S. R. Salpeter. 1979, J. Cell Biol. 82: 150-173) are also present in the alkaline-extracted membranes. Chemical modifications of the thiol groups shift the distribution of structural species. Dithiothreitol reduction, which renders almost exclusively the 9S, monomeric receptor form, results in the observation of the 7-8 nm particle in isolated form. The proportion of doublets increases in membranes alkylated with N-ethylmaleimide. Treatment with 5,5'-dithiobis-(nitrobenzoic acid) increases the proportion of higher oligomeric species, and particle aggregates (n=oligo) predominate. The nonreceptor v-peptide (doublet of M(r) 43,000) appears to play a role in the receptor monomer-polymer equilibria. Receptor protein and v-peptide co-aggregate upon reduction and reoxidation of native membranes. In membranes protected ab initio with N- ethylmaleimide, only the receptor appears to self-aggregate. The v-peptide cannot be extracted from these alkylated membranes, though it is easily released from normal, subsequently alkylated or reduced membranes. A stabilization of the dimeric species by the nonreceptor v-peptide is suggested by these experiments. Monospecific antibodies against the v-peptide are used in conjunction with rhodamine- labeled anti-bodies in an indirect immunoflourescence assay to map the vectorial exposure of the v-peptide. When intact membranes, v-peptide depleted and "holey" native membranes (treated with 0.3 percent saponin) are compared, maximal labeling is obtained with the latter type of membranes, suggesting a predominantly cytoplasmic exposure of the antigenic determinants of the v-peptide in the membrane. The influence of the v-peptide in the thiol-dependent interconversions of the receptor protein and the putative topography of the peptide are analyzed in the light of the present results.

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.

Effects of temperature, lipid modification and pH on the mobility of the major proteins of the receptor-rich membranes from Torpedo marmarata

The factors influencing the overall mobility of the major proteins of the acetylcholine receptor-rich membranes from Torpedo marmorata have been investigated by saturation transfer ESR spectroscopy and the lateral distribution of these proteins has been studied by electron microscopy. A spin-labelled derivative of maleimide, 3-maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl (MSL), was used under various conditions of incubation, enabling us to attach it mainly to either an extrinsic protein of 43 kdaltons, or an intrinsic protein (40 kdaltons) bearing the alpha-toxin-binding site. (1) The direct reaction of MSL with the membrane fragments resulted in almost exclusive labelling of the 43 kdalton protein, an extrinsic protein located on the inner face of the receptor-rich membranes. (2) After the free SH groups were blocked with N-ethylmaleimide and the disulfide bridges opened with the reducing agent dithiothreitol, MSL reacted with both the 40 and 43 kdalton proteins (6.0 +/- 0.6 MSL molecules per alpha-toxin-binding site). (3) After the latter labelling procedure membranes were exposed to pH 11, resulting in extraction of the 43 kdalton protein and leaving 2.2 +/- 0.4 MSL molecules per alpha-toxin-binding site; sodium dodecyl sulfate polyacrylamide gel electrophoresis performed with N-[14C] ethylmaleimide suggested that MSL was bound mainly to the 40 kdalton polypeptide chain of the acetylcholine receptor. The following conclusions were made with the native and alkaline-treated membranes: In the native membranes, saturation transfer ESR does not reveal any significant protein rotational diffusion (rotational correlation time tau C greater than 1 ms). Temperature variations and/or lipid modifications obtained by fusion of exogenous lipids and/or cholesterol exchange have little influence on the saturation transfer ESR spectra. Electron microscopy reveals that upon lipid addition, proteins remain in the form of clusters while areas depleted of proteins appear. On the other hand, alkaline treatment strikingly enhances the motion of the MSL-labelled proteins in the membrane (100 less than or equal to tau c less than or equal to 120 microseconds). Furthermore, the rotational diffusion of the MSL-labelled proteins (mainly the 40 kdalton protein) becomes sensitive to temperature, lipid composition and the lipid-to-proteins ratio. Electron microscopy shows that alkaline extraction does not cause large reorganization of the acetylcholine receptor in the plane of the membrane. However, when phospholipids are added to pH 11 treated membranes, a dispersion of the receptor and rosettes is observed. In contrast, cholesterol enrichment of the latter membranes induces clustering of the receptor immobilization as judged by saturation transfer ESR. Upon reassociation of the pH 11 soluble proteins with the alkaline-treated membranes, the restriction of the acetylcholine receptor rotational mobility is also restored (tau c greater than or equal to 1 ms).

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.

Functional reassembly of membrane proteins in planar lipid bilayers

Recent progress in membrane biology has brought us to a stage where it is possible to associate complex biological processes to identifiable membrane proteins. Technical advances in the biochemical characterization and purification of membrane proteins have contributed a wealth of structural information. The reconstitution approach has proved to be valuable in our efforts to understand the molecular mechanisms of membrane transport and energy transduction.