Immunochemical demonstration that amino acids 360-377 of the acetylcholine receptor gamma-subunit are cytoplasmic

Nuclear immunostaining in rat neuronal cells using two anti-Kir2.2 ion channel polyclonal antibodies

The inwardly rectifying potassium ion channel Kir2.2 has recently been demonstrated to have nuclear and plasma membrane subcellular localization. Nuclear expression of Kir2.2 is controversial, as a functional role for Kir2.0 potassium channels in the nucleus has not been investigated. However, in this report we have demonstrated Kir2.2 nuclear localization in sections of rat hindbrain and dorsal root ganglia tissue, using two anti- Kir2.2 polyclonal antisera with different epitope specificities. These data confirm nuclear localization and are suggestive of new functions of Kir2.0 potassium ion channels in the nucleus.

Cyclic AMP-dependent protein kinase phosphorylates residues in the C-terminal domain of the cardiac L-type calcium channel alpha1 subunit

The molecular basis of the regulation of cardiac L-type calcium channel activity by cAMP-dependent protein kinase (cA-PK) remains unclear. Direct cA-PK-dependent phosphorylation of the bovine ventricular alpha1 subunit in vitro has been demonstrated in microsomal membranes, detergent extracts and partially purified (+)-[3H]PN 200-110 receptor preparations. Two 32P-labeled phosphopeptides, derived from cyanogen bromide cleavage, of 4.7 and 9.5 kDa were immunoprecipitated specifically by site-directed antibodies against the rabbit cardiac alpha1 subunit amino acid sequences 1602-1616 and 1681-1694, respectively, consistent with phosphorylation at the cA-PK consensus sites at Ser(1627) and Ser(1700). No phosphopeptide products consistent with phosphorylation at three other C-terminal cA-PK consensus phosphorylation sites (Ser(1575), Ser(1848) and Ser(1928)) were identified using similar procedures suggesting that these sites are poor substrates for this kinase. Ser(1627) and Ser(1700) may represent sites of cA-PK phosphorylation involved in the physiological regulation of cardiac L-type calcium channel function.

Binding of monoclonal antibodies against the carboxyl terminal segment of the nicotinic receptor delta subunit suggests an unusual transmembrane disposition of this sequence region

Monoclonal antibodies (mAbs) specific for the carboxyl terminal region of the delta subunit of Torpedo nicotinic acetylcholine receptor (AChR), derived from mice immunized with AChR or a synthetic carboxyl terminal sequence of the delta subunit (C delta-mAbs), were used to determine the transmembrane disposition of their epitope(s) by immunoelectron microscopy, using AChR-rich postsynaptic membrane fragments from Torpedo electroplax. Some C delta-mAbs recognized only the cytoplasmic side of the membranes, some both sides to a similar extent, and others bound mostly, but not exclusively, to the cytoplasmic side. Binding of C delta-mAbs to the membranes was specifically blocked by synthetic peptides containing the carboxyl terminal region of the delta subunit. Control anti-AChR mAbs specific for the alpha or the delta subunits, whose epitopes have known transmembrane topology, uniquely recognized the expected side of the postsynaptic membrane. Residues involved in C delta-mAb binding were identified using single residue substituted peptide analogues of the sequence delta 481-501. All C delta-mAbs recognized epitopes within the same sequence segment, delta 485-493, at the carboxyl terminal of the AChR delta subunit. These results suggest that the delta subunit of the AChR might have alternative conformations, leading to exposure of the same sequence region on the extracellular or the cytoplasmic surface. Several Pro residues are present in this region. The alternative cis or trans conformation of one or more of them might result in different folding patterns of the carboxyl terminal sequence of the delta subunit, as described for a viral protein [Liddington, R. C., Yan, Y., Moulai, J., Sahli, R., Benjamin, T. L., & Harrison, S. C. (1991) Nature 354, 278-284.

Clustering of B and T epitopes within short sequence regions of the nicotinic acetylcholine receptor

The epitope repertoire of B cells, due to their selective ability to process their specific antigen and the potential bias imposed on the resulting peptides by the surface immunoglobulins bound to the antigen, may influence the T-helper repertoire. Immunization of C57B1/6 mice with Torpedo acetylcholine receptor (TAChR) causes experimental autoimmune myasthenia gravis (EAMG). Anti-TAChR CD4+ cells recognize epitopes within three sequence regions of the TAChR alpha subunit ('dominant epitopes'). Immunization of mice with denatured or synthetic TAChR antigens sensitizes CD4+ cells to other TAChR sequence regions ('cryptic epitopes'). We investigated here whether clustering of B and T epitopes within the same short sequence segments occurs during the anti-TAChR response, as previously described for the response to hexogenous antigens unrelated to homologous self proteins. Twelve 19-20 residue synthetic sequences of the TAChR alpha, gamma and delta subunits, containing dominant or cryptic CD4+ epitopes for C57B1/6 mice, were tested for ability to induce anti-peptide antibody production. C57B1/6 mice were immunized with the individual peptides. Ten peptides stimulated antibody production. Therefore > 80% of these short TAChR sequences also contain B epitopes. Therefore also in the anti-TAChR response leading to EAMG T and B cell epitopes frequently reside within the same short sequence segment.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Clustering of the acetylcholine receptor by the 43-kD protein: involvement of the zinc finger domain

A postsynaptic membrane-associated protein of M(r) 43,000 (43-kD protein) is involved in clustering of the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Previous studies have shown that recombinant mouse 43-kD protein forms membrane-associated clusters when expressed in Xenopus oocytes. Coexpression with the AChR results in colocalization of the receptor with the 43-kD protein clusters (Froehner, S. C., C. W. Luetje, P. B. Scotland, and J. Patrick, 1990. Neuron. 5:403-410). To understand the mechanism of this clustering, we have studied the role of the carboxy-terminal region of the 43-kD protein. The amino acid sequence of this region predicts two tandem zinc finger structures followed by a serine phosphorylation site. Both Torpedo 43-kD protein and the carboxy-terminal region of the mouse 43-kD protein bind radioisotopic zinc. Mutation of two histidine residues in this predicted domain greatly attenuates zinc binding, lending support to the proposal that this region forms zinc fingers. When expressed in oocytes, the ability of this mutant 43-kD protein to form clusters is greatly reduced. Its ability to interact with AChR, however, is retained. In contrast, a mutation that eliminates the potential serine phosphorylation site has no effect on clustering of the 43-kD protein or on interaction with the AChR. These findings suggest that protein interactions via the zinc finger domain of the 43-kD protein may be important for AChR clustering at the synapse.

Reporter epitopes: a novel approach to examine transmembrane topology of integral membrane proteins applied to the alpha 1 subunit of the nicotinic acetylcholine receptor

The development of a novel immunological method called the "reporter epitope" technique to probe the transmembrane topology of integral membrane proteins is described. Using this method, synthetic oligonucleotides encoding epitopes (reporter epitopes) for well characterized monoclonal antibodies (reporter mAbs) were inserted at various locations within the human acetylcholine receptor (AChR) alpha 1 subunit cDNA. The engineered subunits were then expressed along with Torpedo beta 1, gamma, and delta subunits in Xenopus oocytes, and the transmembrane location of the site of insertion was determined by the binding of the 125I-labeled reporter mAbs to whole oocytes. Control reporter epitope insertions at alpha 347 exhibited the expected cytoplasmic location. Reporter epitopes inserted at alpha 429 are located on the extracellular surface. Reporter epitopes that are 16-48 amino acids long do not disrupt assembly or function of hybrid AChRs when inserted near the carboxy terminus (at alpha 429) or in the large cytoplasmic domain (at alpha 347). However, because two reporter epitopes inserted at alpha 157 obliterated subunit assembly and a third reporter epitope when tolerated at this position was inaccessible from the extracellular surface and only marginally accessible after detergent solubilization of the AChRs, a definitive transmembrane location for this region was not possible. Nonetheless, the use of this approach has been successfully demonstrated, and it may be generally applicable to the study of other integral membrane proteins.

Lack of the carboxyl terminal sequence of tau in ghost tangles of Alzheimer's disease

Using seven independent antibodies against the amino terminal to the carboxyl terminal sequence of tau, we biochemically analyzed and compared the neuropathogenesis of two Alzheimer's disease brains from the viewpoint of abnormal processing on tau, the major constituent of paired helical filaments. One showed typical Alzheimer's disease with senile plaques and intracellular neurofibrillary tangles. The other showed advanced Alzheimer's disease with senile plaques and virtually the sole of ghost tangles without intracellular neurofibrillary tangles. We confirmed the previous observation that the carboxyl thirds of tau are tightly associated with paired helical filaments isolated in the presence of SDS. We found that biochemically, ghost tangles were abnormally phosphorylated and lacked the final carboxyl terminal sequence as well as the amino half of tau, unlike intracellular tangles. From these biochemical results taken together with the current evidence for ubiquitin in ghost tangles, we concluded that ghost tangles were extensively processed and irreversibly transformed into highly insoluble extracellular deposits.

Site-directed antibodies as topographical probes of the gastric H,K-ATPase alpha-subunit

Gastric acid is secreted by an ATP-driven H+ and K+ exchanger (H,K-ATPase), an integral apical membrane protein of parietal cells. Although the primary structure of the enzyme is known, its higher order structure is uncertain. In order to acquire topographical probes of native, microsomal H,K-ATPase, synthetic peptides corresponding to the 17 amino-terminal (N-peptide) and 16 carboxyl-terminal (C-peptide) residues of pig gastric H,K-ATPase alpha-subunit were coupled to keyhole limpet hemocyanin (KLH). Rabbits were immunized with peptide-KLH conjugates and their sera were tested for specificity by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and immunocytochemistry. All sera showed high ELISA reactivities with synthetic peptides, peptide-BSA conjugates, and microsomal H,K-ATPase adsorbed to microtiter wells (some titers greater than 1:10(4)). Immunoblots of H,K-ATPase resolved by SDS-PAGE showed both N-peptide and C-peptide antibodies reacting with a single 94 kDa band. All sera selectively stained parietal cells in pig gastric mucosal sections. Preimmune sera gave negative or weak signals in all assays. In competition ELISAs, N-peptide antibodies, but not C-peptide antibodies, were displaced from the corresponding bound synthetic peptides by added microsomal H,K-ATPase. One of the N-peptide antibodies inhibited H,K-ATPase activity by more than 50%; binding of this antibody was decreased when ATP or K+ were bound to the enzyme. These results indicate a cytoplasmically-oriented alpha-subunit N-terminus which may participate conformationally in the H,K-ATPase catalytic cycle, and suggest that antibodies against synthetic H,K-ATPase peptides are potentially useful probes of native microsomal H,K-ATPase topography.

Epitope mapping of polyclonal and monoclonal antibodies against two alpha-bungarotoxin-binding alpha subunits from neuronal nicotinic receptors

Recently, cDNAs for alpha subunits of two different neuronal alpha-bungarotoxin-binding proteins (alpha BgtBP) were isolated from chick brain, designated alpha BgtBP alpha 1 and alpha BgtBP alpha 2. These are now also referred to as subunits alpha 7 and alpha 8, respectively. Expression studies in Xenopus oocytes have indicated that alpha 7 subunits are able to form cation channels that are sensitive to nicotinic ligands, and therefore represent bona fide nicotinic acetylcholine receptor subunits. Polyclonal and monoclonal antibodies (mAbs) have been produced against: (i) affinity-purified chick brain alpha BgtBP; and (ii) fusion proteins containing the unique cytoplasmic sequences alpha 7(327-412) and alpha 8(293-435). Here, synthetic overlapping peptides corresponding to their deduced amino acid sequences are used to map the epitopes recognized by the different antibodies. The polyclonal response to affinity-purified alpha BgtBPs and the fusion proteins indicates that sequence segments 290-420 of both subunits contain several major and minor epitopes. mAbs selected for their ability to bind both native and denatured alpha BgtBPs isolated from chick brain also recognize subunit-specific sequential epitopes within the sequence segment 290-420. The epitopes recognized by the mAbs correspond to the minor epitopes defined using antisera. The mAbs characterized in these studies will provide useful probes for further studies of alpha BgtBP structure and histological localization.

Expression of fusion proteins of the nicotinic acetylcholine receptor from mammalian muscle identifies the membrane-spanning regions in the alpha and delta subunits

We have investigated the topology of the alpha and delta subunits of the nicotinic acetylcholine receptor (AChR) from mammalian muscle synthesized in an in vitro translation system supplemented with dog pancreatic microsomes. Fusion proteins were expressed in which a carboxy-terminal fragment of bovine prolactin was attached downstream of each of the major putative transmembrane domains, M1-M4 and MA, in the AChR subunits. The orientation of the prolactin domain relative to the microsomal membrane was then determined for each protein by a proteolysis protection assay. Since the prolactin domain contains no information which either directs or prevents its translocation, its transmembrane orientation depends solely on sequences within the AChR subunit portion of the fusion protein. When subunit-prolactin fusion proteins with the prolactin domain fused after either M2 or M4 were tested, prolactin-immunoreactive peptides that were larger than the prolactin domain itself were recovered. No prolactin-immunoreactive peptides were recovered after proteolysis of fusion proteins containing prolactin fused after M1, M3, or MA. These results support a model of AChR subunit topology in which M1-M4, but not MA, are transmembrane domains and the carboxy terminus is extracellular.

Differences between embryonic and adult Torpedo acetylcholine receptor gamma subunit

Antibodies to a synthetic peptide corresponding to residues 346-359 of the Torpedo acetylcholine receptor (AChR) gamma subunit, were employed to compare the adult and embryonic receptor. This peptide contains a consensus phosphorylation site for cAMP-dependent protein kinase (PKA). The anti-peptide antibodies discriminated between adult and embryonic AChRs, and reacted preferentially with the adult gamma form. These observed immunological differences did not seem to stem from different phosphorylation states. Our results suggest that the embryonic Torpedo AChR may have a gamma-like subunit that differs from the known adult form of this subunit, at least in the specific region that contains the phosphorylation site for PKA.

Topological dispositions of lysine alpha 380 and lysine gamma 486 in the acetylcholine receptor from Torpedo californica

The locations have been determined, with respect to the plasma membrane, of lysine alpha 380 and lysine gamma 486 in the alpha subunit and the gamma subunit, respectively, of the nicotinic acetylcholine receptor from Torpedo californica. Immunoadsorbents were constructed that recognize the carboxy terminus of the peptide GVKYIAE released by proteolytic digestion from positions 378-384 in the amino acid sequence of the alpha subunit of the acetylcholine receptor and the carboxy terminus of the peptide KYVP released by proteolytic digestion from positions 486-489 in the amino acid sequence of the gamma subunit. They were used to isolate these peptides from proteolytic digests of polypeptides from the acetylcholine receptor. Sealed vesicles containing the native acetylcholine receptor were labeled with pyridoxal phosphate and sodium [3H]-borohydride. Saponin was added to a portion of the vesicles prior to labeling to render them permeable to pyridoxal phosphate. The effect of saponin on the incorporation of pyridoxamine phosphate into lysine alpha 380 and lysine gamma 486 from the acetylcholine receptor in these vesicles was assessed with the immunoadsorbents. The peptides bound and released by the immunoadsorbents were positively identified and quantified by high-pressure liquid chromatography. Modification of lysine alpha 380 in the native acetylcholine receptor in sealed vesicles increased 5-fold in the presence of saponin, while modification of lysine gamma 486 was unaffected by the presence of saponin. The conclusions that follow from these results are that lysine alpha 380 is on the inside surface of a vesicle and lysine gamma 486 is on the outside surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunochemical localization of the epitope for a monoclonal antibody that neutralizes human platelet-derived growth factor mitogenic activity

A monoclonal antibody (mAb), sis 1, generated against human c-sis-encoded platelet-derived growth factor (PDGF) BB, was shown by enzyme-linked immunosorbent assay and Western blot (immunoblot) analysis to recognize human PDGF BB and human platelet PDGF AB but not the human PDGF AA. This monoclonal antibody potently inhibited PDGF receptor-binding and mitogenic activities of both human PDGF BB and PDGF AB but had no effect on PDGF AA. Finally, we demonstrated that an immunoaffinity-purified anti-c-sis peptide antibody (anti-V4) which also blocked binding of PDGF BB to its cognate receptor and competed with mAb sis 1 for binding to PDGF BB. All of these results suggest that mAb sis 1 recognizes an epitope of the c-sis gene product, PDGF BB, that spatially overlaps the V4 surface domain of PDGF BB, immunochemically localizing a region of PDGF BB critical for PDGF receptor binding and activation.

Immunochemical localization of the epitope for a monoclonal antibody that neutralizes human platelet-derived growth factor mitogenic activity

A monoclonal antibody (mAb), sis 1, generated against human c-sis-encoded platelet-derived growth factor (PDGF) BB, was shown by enzyme-linked immunosorbent assay and Western blot (immunoblot) analysis to recognize human PDGF BB and human platelet PDGF AB but not the human PDGF AA. This monoclonal antibody potently inhibited PDGF receptor-binding and mitogenic activities of both human PDGF BB and PDGF AB but had no effect on PDGF AA. Finally, we demonstrated that an immunoaffinity-purified anti-c-sis peptide antibody (anti-V4) which also blocked binding of PDGF BB to its cognate receptor and competed with mAb sis 1 for binding to PDGF BB. All of these results suggest that mAb sis 1 recognizes an epitope of the c-sis gene product, PDGF BB, that spatially overlaps the V4 surface domain of PDGF BB, immunochemically localizing a region of PDGF BB critical for PDGF receptor binding and activation.

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.

Structure-function relationships in nicotinic acetylcholine receptors

1. A combination of molecular, biochemical, electrophysiological and immunological approaches has begun to resolve some of the questions about structure-function relationships of nicotinic acetylcholine receptors (AchRs). 2. Current structural studies suggest that models of the subunits which propose four transmembrane domains are correct. 3. It is also probable that the carboxy termini of the subunits are extracellular, while the putative amphpathic helix is intracellular. 4. Electrophysiological and ligand-binding experiments suggest that the M2 region forms the wall of the ion channel. 5. We have isolated clones from PC12 and rat brain cDNA libraries which we have shown, by functional expression, code for members of a gene family of nicotinic acetylcholine receptor subunits. 6. In situ hybridization studies have shown that the neuronal receptor subunit mRNAs are expressed in the mammalian central nervous system. 7. The muscle and neuronal nicotinic AchR subtypes we have expressed show differences in their pharmacological properties. 8. The isolation and identification of clones which code for receptors and voltage-activated ion channels will help in the understanding of a variety of disease states and assist in the design of drugs which are specific for unique molecular targets.

The carboxyl third of tau is tightly bound to paired helical filaments

To obtain definitive evidence that tau is a component of paired helical filaments (PHF) in Alzheimer's disease, we fractionated and sequenced PHF-derived peptides according to a previously described procedure. In the PHF digest, we found four independent tau peptides that were located in the carboxyl third of tau. Subsequent extensive analysis of the PHF digest did not provide any other tau peptides. The conventional PHF antiserum and a new antiserum directed toward formic acid-denatured PHF reacted with the distinct CNBr fragments of tau localized on the carboxy-terminal portion of tau by protein sequencing. From these observations, we conclude that the carboxyl third of tau is tightly bound to PHF.

Evidence for the extramembranous location of the putative amphipathic helix of acetylcholine receptor

Evidence has been obtained demonstrating that the peptides GVKYIAE and AIKYIAE found in the potential amphipathic helices of the alpha and beta subunits, respectively, of acetylcholine receptor are not buried in the membrane. The peptide KYIAE was synthesized, and polyclonal antibodies were prepared against a conjugate of bovine serum albumin and synthetic peptide. An immunoadsorbent capable of binding and subsequently releasing peptides ending with the sequence-YIAE was produced by attaching these specific antibodies to agarose. Native acetylcholine receptor was labeled with pyridoxal phosphate and Na[3H]BH4. The labeled protein was stripped of phospholipid and digested with the protease from Staphylococcus aureus strain V8. The digest was submitted to immunoadsorption to isolate the labeled indigenous peptides. As a control, alpha and beta polypeptides prepared by gel filtration of a solution of acetylcholine receptor in detergent were stripped of detergent and labeled with pyridoxal phosphate and Na[3H]BH4 in the presence of 8 M urea. The labeled alpha and beta polypeptides were digested and submitted to immunoadsorption. The specific radioactivities of the indigenous peptides from the alpha and beta subunits labeled under native and denaturing conditions were nearly equal. In similar experiments using isethionyl (2', 4'-dinitrophenyl)-3-amino-propionimidate as the labeling agent, the indigenous peptides from native and denatured receptor were also labeled to the same extent. Since these peptides are labeled to the same extent whether or not the protein is denatured, they cannot be buried in the membrane.

Genetic reconstitution of functional acetylcholine receptor channels in mouse fibroblasts

Foreign genes can be stably integrated into the genome of a cell by means of DNA-mediated gene transfer techniques, and large quantities of homogenous cells that continuously express these gene products can then be isolated. Such an expression system can be used to study the functional consequences of introducing specific mutations into genes and to study the expressed protein in the absence of cellular components with which it is normally in contact. All four Torpedo acetylcholine receptor (AChR) subunit complementary DNA's were introduced into the genome of a mouse fibroblast cell by DNA-mediated gene transfer. A clonal cell line that stably produced high concentrations of correctly assembled cell surface AChR's and formed proper ligand-gated ion channels was isolated. With this new expression system, recombinant DNA, biochemical, pharmacological, and electrophysiological techniques were combined to study Torpedo AChR's in a single intact system. The physiological and pharmacological profiles of Torpedo AChR's expressed in mouse fibroblast cells differ in some details from those described earlier, and may provide a more accurate reflection of the properties of this receptor in its natural environment.

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.

A structural and dynamic model for the nicotinic acetylcholine receptor

Folding of the five polypeptide subunits (alpha 2 beta gamma delta) of the nicotinic acetylcholine receptor (AChR) into a functional structural model is described. The principles used to arrange the sequences into a structure include: (1) hydrophobicity----membrane-crossing segments; (2) amphipathic character----ion-carrying segments (ion channel with single group rotations); (3) molecular shape (elongated, pentagonal cylinder)----folding dimensions of exobilayer portion; (4) choice of acetylcholine binding sites----specific folding of exobilayer segments; (5) location of reducible disulfides (near agonist binding site)----additional specification of exobilayer arrangement; (6) genetic homology----consistency of functional group choices; (7) noncompetitive antagonist labeling----arrangement of bilayer helices. The AChR model is divided into three parts: (a) exobilayer consisting of 11 antiparallel beta-strands from each subunit; (b) bilayer consisting of four hydrophobic and one amphiphilic alpha-helix from each subunit; (c) cytoplasmic consisting of one (folded) loop from each subunit. The exobilayer strands can form a closed 'flower' (the 'resting state') which is opened ('activated') by agonists bound perpendicular to the strands. Rearrangement of the agonists to a strand-parallel position and partial closing of the 'flower' leads to a desensitized receptor. The actions of acetylcholine and succinoyl and suberoyl bis-cholines are clarified by the model. The opening and closing of the exobilayer 'flower' controls access to the ion channel which is composed of the five amphiphilic bilayer helices. A molecular mechanism for ion flow in the channel is given. Openings interrupted by short duration closings (50 microseconds) depend upon channel group motions. The unusual photolabeling of intrabilayer serines in alpha, beta and delta subunits but not in gamma subunits near the binding site for non-competitive antagonists is explained along with a mechanism for the action of these antagonists such as phencyclidine. The unusual alpha 192Cys-193Cys disulfide may have a special peptide arrangement, such as a cis-peptide bond to a following proline (G.A. Petsko and E.M. Kosower, unpublished results). The position of phosphorylatable sites and proline-rich segments are noted for the cytoplasmic loops. The dynamic behavior of the AChR channel and many different experimental results can be interpreted in terms of the model. An example is the lowering of ionic conductivity on substitution of bovine for Torpedo delta M2 segment. The model represents a useful construct for the design of experiments on AChR.

Definition of myasthenogenic sites of the human acetylcholine receptor using synthetic peptides

Experimental autoimmune myasthenia gravis (EAMG) and antibodies that modulate AChRs from cultured human muscle are induced by a disulfide-looped peptide comprising the human acetylcholine receptor (AChR) alpha-subunit residues 125-147 (H alpha 125-147). To delineate the essential antigenic requirements for induction of EAMG by this peptide, a series of peptides was synthesized: (a) a nonlooped analog (Cys 128 replaced by Ser) stimulated modulating autoantibodies, induced EAMG, and bound antibodies induced by native AChR; (b) H alpha 131-147, a heptadecylpeptide shorter by 6 N-terminal residues, induced modulating antibodies, EAMG, and T cells that responded to H alpha 125-147, but not to H alpha 137-147; (c) H alpha 137-147, an undecapeptide shorter than H alpha 125-147 by 12 N-terminal residues, did not stimulate T cells or induce antibody production, but it bound antibodies generated by the longer peptides. Thus, when coupled to an epitope that could stimulate helper T cells, the region 137-147 was able to stimulate B cells. This study has defined a myasthenogenic region of the human AChR's alpha-subunit, 17 amino acids long, that contains several distinct epitopes, including at least one N-terminal site inducing T-cell responses (region 131-136) and two possibly overlapping sites that induce antibodies (regions 131-136 and 137-147). Further definition of antigenic sites inducing helper and suppressor T-cell responses and stimulating production of autoantibodies to AChR is essential to the goal of antigen-specific immunotherapy for myasthenia gravis.

Evidence that the acetylcholine binding site is not formed by the sequence alpha 127-143 of the acetylcholine receptor

The sequence alpha 127-143 of the alpha subunit of the acetylcholine receptor has been proposed to contain several important features: (1) the acetylcholine binding site, (2) the only N-glycosylation site of the alpha subunit, at asparagine-alpha 141, and (3) two cysteine residues, at alpha 128 and alpha 142, that may participate in a disulfide bond known to be near the binding site. We tested these hypotheses by using antisera to receptor and its subunits and monoclonal antibodies to the synthetic peptide alpha 127-143 cyclized by a disulfide bond between alpha 128 and alpha 142. Antisera to receptor and its alpha subunit were able to immunoprecipitate the iodinated peptide, and this reaction was inhibited by soluble receptor, but not by membrane-bound receptor. alpha-Bungarotoxin did not inhibit antiserum binding to solubilized receptor. Similarly, cholinergic ligands had little or no effect on binding to immobilized receptors of anti-peptide monoclonal antibodies. In addition, these monoclonal antibodies, when bound to the receptor, did not affect toxin binding kinetics. By contrast, preincubation with concanavalin A did inhibit monoclonal antibody binding. Reduction of the receptor significantly decreased the binding of three of the monoclonal antibodies, but subsequent alkylation with N-ethylmaleimide or the affinity labeling reagent bromoacetylcholine had no additional effect on binding. A dithiothreitol concentration about 100-fold higher that the one needed to reduce the disulfide near the acetylcholine binding site was necessary to inhibit monoclonal antibody binding. We conclude that the sequence alpha 127-143 is not fully exposed on the surface when the receptor is in the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Location of antigenic determinants on primary sequences of subunits of nicotinic acetylcholine receptor by peptide mapping

The binding domains of 28 monoclonal antibodies (mAbs) against the alpha, beta, and delta subunits of the Torpedo acetylcholine receptor were mapped on the primary sequences of these subunits. Small peptide fragments (2000-20,000 daltons) of the purified subunits were obtained by digestion with staphylococcal V8 protease and papain, separated on a discontinuous polyacrylamide gel electrophoretic system, and electroblotted onto diaminophenyl thioether paper. The blots were probed with the various monoclonal antibodies and also with antibodies against carboxy-terminal decapeptides of the alpha, beta, and delta subunits to identify the carboxy-terminal fragments. From inspection of the binding patterns of the various antibodies to the subunits fragments and the molecular weights of these fragments, and by using the carboxy termini of the subunits as reference points, it was possible to deduce the regions on the primary sequence of each subunit in which the antibodies bound and in some cases to order the binding sites within these sequences. mAb 148, which inhibits receptor function by cross-linking receptor molecules on the cytoplasmic side, was mapped to the sequence beta 368-406. The main immunogenic region of the native receptor, which is of pathological importance in the autoimmune disease myasthenia gravis, was mapped by using mAb 210 to within 80 amino acid residues (alpha 46-127). The overall antigenic structure of alpha subunits was examined. Synthetic peptides have been used to locate determinants responsible for 83% of the antibodies in antisera to denatured alpha subunits and 46% of the antibodies to denatured alpha subunits in antisera to intact receptor. Theoretical models of the transmembrane orientation of the subunit polypeptide chains were tested by determining whether mapped monoclonal antibodies bound to the extracellular or intracellular surface of receptor-rich membranes. Our results confirm previous reports that the carboxy termini of the subunits are exposed on the intracellular surface, as is part of the region between a putative channel-forming domain (M5) and a putative membrane-spanning region (M3). However, contrary to current theoretical models, the region between M5 and the putative membrane-spanning sequence M4 also appears to be on the intracellular surface, implying that M4 and M5 are not membrane-spanning domains.(ABSTRACT TRUNCATED AT 400 WORDS)

Transmembrane topography of nicotinic acetylcholine receptor: immunochemical tests contradict theoretical predictions based on hydrophobicity profiles

In our preceding paper [Ratnam, M., Sargent, P. B., Sarin, V., Fox, J. L., Le Nguyen, D., Rivier, J., Criado, M., & Lindstrom, J. (1986) Biochemistry (preceding paper in this issue)], we presented results from peptide mapping studies of purified subunits of the Torpedo acetylcholine receptor which suggested that the sequence beta 429-441 is on the cytoplasmic surface of the receptor. Since this finding contradicts earlier theoretical models of the transmembrane structure of the receptor, which placed this sequence of the beta subunit on the extracellular surface, we investigated the location of the corresponding sequence (389-408) and adjacent sequences of the alpha subunit by a more direct approach. We synthesized peptides including the sequences alpha 330-346, alpha 349-364, alpha 360-378, alpha 379-385, and alpha 389-408 and shorter parts of these peptides. These peptides corresponded to a highly immunogenic region, and by using 125I-labeled peptides as antigens, we were able to detect in our library of monoclonal antibodies to alpha subunits between two and six which bound specifically to each of these peptides, except alpha 389-408. We obtained antibodies specific for alpha 389-408 both from antisera against the denatured alpha subunit and from antisera made against the peptide. These antibodies were specific to alpha 389-396. In binding assays, antibodies specific for all of these five peptides bound to receptor-rich membrane vesicles only after permeabilization of the vesicles to permit access of the antibodies to the cytoplasmic surface of the receptors, suggesting that the receptor sequences which bound these antibodies were located on the intracellular side of the membrane. Electron microscopy using colloidal gold to visualize the bound antibodies was used to conclusively demonstrate that all of these sequences are exposed on the cytoplasmic surface of the receptor. These results, along with our previous demonstration that the C-terminal 10 amino acids of each subunit are exposed on the cytoplasmic surface, show that the hydrophobic domain M4 (alpha 409-426), previously predicted from hydropathy profiles to be transmembranous, does not, in fact, cross the membrane. Further, these results show that the putative amphipathic transmembrane domain M5 (alpha 364-399) also does not cross the membrane. Our results thus indicate that the transmembrane topology of a membrane protein cannot be deduced strictly from the hydropathy profile of its primary amino acid sequence. We present a model for the transmembrane orientation of receptor subunit polypeptide chains which is consistent with current data.

Localization of a highly immunogenic region on the acetylcholine receptor alpha-subunit

Antibodies to synthetic peptides were employed in order to map domains on the alpha-subunit of the acetylcholine receptor to which several monoclonal antibodies are directed. Five peptides corresponding to residues 1-20, 126-143, 169-181, 330-340 and 351-368 of the receptor alpha-subunit were synthesized and antibodies against them were elicited. The anti-peptide antibodies were employed along with the monoclonal antibodies to identify fragments of S. aureus V8 protease digested- alpha-subunit in immunoblotting experiments. Our results demonstrate that a highly immunogenic region of the alpha-subunit is located on a carboxy-terminal 14 kDa portion of the alpha-subunit. This region also seems to undergo antigenic changes during muscle development. A monoclonal antibody directed against the cholinergic binding site of the acetylcholine receptor reacted with an 18 kDa segment of the alpha-subunit which bound alpha-bungarotoxin as well as antibodies directed against peptide 169-181.

Region of peptide 125-147 of acetylcholine receptor alpha subunit is exposed at neuromuscular junction and induces experimental autoimmune myasthenia gravis, T-cell immunity, and modulating autoantibodies

A major antigenic region of native nicotinic acetylcholine receptors (AcChoR) has been identified by using a synthetic disulfide-looped peptide corresponding to alpha-subunit residues 125-147 of Torpedo electric organ AcChoR: Lys-Ser-Tyr-Cys-Glu-Ile-Ile-Val-Thr-His-Phe- Pro-Phe-Asp-Gln-Gln-Asn-Cys-Thr-Met-Lys-Leu-Gly. The peptide bound 26-56% of polyclonal antibodies induced in rat, rabbit, and dog by immunization with native AcChoR. Rats inoculated with 50 micrograms of unconjugated peptide developed helper T-cell responses, delayed hypersensitivity, and antibodies to native AcChoR. Anti-peptide antibodies were more reactive with native than denatured AcChoR and bound to the alpha subunit. Some reacted exclusively with mammalian muscle AcChoR, some induced modulation of AcChoR on cultured myotubes, but none inhibited binding of alpha-bungarotoxin to solubilized or membrane-associated AcChoR. Repeated immunization induced experimental autoimmune myasthenia gravis: clinical signs in one rat and electrophysiologic and/or biochemical signs in 10 of 11 rats. Thus, at least part of the corresponding region of the mammalian AcChoR alpha subunit is extracellular at the neuromuscular junction and a potential target for pathogenic autoantibodies in patients with acquired myasthenia gravis.

Structural localization of the sequence alpha 235-242 of the nicotinic acetylcholine receptor

Two monoclonal antibodies (mAb 254 and 255) were obtained against a synthetic peptide corresponding to the sequence 235-242 of the alpha-subunit of Torpedo acetylcholine receptor. These mAbs could bind to receptor in native membrane vesicles only when these vesicles were permeabilized, suggesting that the sequence alpha 235-242 is exposed on the cytoplasmic surface of the receptor. Further evidence for the cytoplasmic localization of this sequence was partial competition for binding between these mAbs and mAbs previously demonstrated to bind to the cytoplasmic part of the receptor. A model is proposed which accounts for all the experimental data obtained thus far on the transmembrane orientation of the subunit polypeptide chains.