Monoclonal antibodies to cytoplasmic domains of the acetylcholine receptor

Rapsyn-mediated clustering of acetylcholine receptor subunits requires the major cytoplasmic loop of the receptor subunits

During synaptogenesis at the neuromuscular junction, nicotinic acetylcholine receptors (AChRs) are organized into high-density postsynaptic clusters that are critical for efficient synaptic transmission. Rapsyn, an AChR associated cytoplasmic protein, is essential for the aggregation and immobilization of AChRs at the neuromuscular junction. Previous studies have shown that when expressed in nonmuscle cells, both assembled and unassembled AChR subunits are clustered by rapsyn, and the clustering of the alpha subunit is dependent on its major cytoplasmic loop. In the present study, we investigated the mechanism of rapsyn-induced clustering of the AChR beta, gamma, and delta subunits by testing mutant subunits for the ability to cocluster with rapsyn in transfected QT6 cells. For each subunit, deletion of the major cytoplasmic loop, between the third and fourth transmembrane domains, dramatically reduced coclustering with rapsyn. Furthermore, each major cytoplasmic loop was sufficient to mediate clustering of an unrelated transmembrane protein. The AChR subunit mutants lacking the major cytoplasmic loops could assemble into alphadelta dimers, but these were poorly clustered by rapsyn unless at least one mutant was replaced with its wild-type counterpart. These results demonstrate that the major cytoplasmic loop of each AChR subunit is both necessary and sufficient for mediating efficient clustering by rapsyn, and that only one such domain is required for rapsyn-mediated clustering of an assembly intermediate, the alphadelta dimer.

A transmembrane motif governs the surface trafficking of nicotinic acetylcholine receptors

Surface expression of the nicotinic acetylcholine receptor (AChR) requires the assembly of multiple subunits in the endoplasmic reticulum (ER). Little is known, however, about the mechanism by which assembled receptor pentamers are transported to the cell membrane while unassembled subunits are retained in the ER. Here we report that a motif conserved in the transmembrane domain of AChR subunits is critically involved in this process. In COS cells, mutation within this signal allowed surface expression of unassembled subunits. Conversely, insertion of the sequence to unrelated proteins that are normally transported to the surface resulted in ER retention. The signal is buried in AChR pentamers, but is exposed on unassembled subunits in the ER, where it promotes protein degradation. We therefore conclude that this signal ensures surface trafficking of only functional AChRs.

Functional immobilisation of the nicotinic acetylcholine receptor in tethered lipid membranes

The nicotinic acetylcholine receptor from Torpedo was immobilised in tethered membranes. Surface plasmon resonance was used to quantify the binding of ligands and antibodies to the receptor. The orientation and structural integrity of the surface-reconstituted receptor was probed using monoclonal antibodies, demonstrating that approximately 65% of the receptors present their ligand-binding site towards the lumen of the flow cell and that at least 85% of these receptors are structurally intact. The conformation of the receptor in tethered membranes was investigated with Fourier transform infrared spectroscopy and found to be practically identical to that of receptors reconstituted in lipid vesicles. The affinity of small receptor ligands was determined in a competition assay against a monoclonal antibody directed against the ligand-binding site which yielded dissociation constants in agreement with radioligand binding assays. The presented method for the functional immobilisation of the nicotinic acetylcholine receptor in tethered membranes might be generally applicable to other membrane proteins.

Phosphorylation and cytoskeletal anchoring of the acetylcholine receptor by Src class protein-tyrosine kinases. Activation by rapsyn

Src class protein-tyrosine kinases bind to and phosphorylate the nicotinic acetylcholine receptor of skeletal muscle. This study provided evidence for the functional importance of Src kinases in regulating the nicotinic acetylcholine receptor at the neuromuscular junction. Three Src class kinases, Fyn, Fyk, and Src, each formed a complex with the endplate-specific cytoskeletal protein rapsyn. In addition, cellular phosphorylation by each kinase was stimulated by rapsyn in heterologous transfected cells. Several lines of evidence supported rapsyn as a substrate for Src kinases. Most importantly, rapsyn regulation of Fyn, Fyk, and Src resulted in phosphorylation of the nicotinic acetylcholine receptor beta and delta subunits and anchoring of the receptor to the cytoskeleton. Both nicotinic acetylcholine receptor phosphorylation and cytoskeletal anchoring were blocked by the Src kinase-selective inhibitor herbimycin A. Rapsyn alone also induced a modest increase in nicotinic acetylcholine receptor phosphorylation and cytoskeletal translocation. However, inhibition by herbimycin A and a catalytically inactive dominant negative Src demonstrated that the effects of rapsyn were mediated by endogenous Src kinases. These data support the importance of Src class kinases for stabilization of the nicotinic acetylcholine receptor at the endplate during synaptic differentiation at the neuromuscular junction.

Altered glycosylation sites of the delta subunit of the acetylcholine receptor (AChR) reduce alpha delta association and receptor assembly

We have used mutagenesis to investigate the potential N-glycosylation sites in the delta subunit of the mouse muscle acetylcholine receptor (AChR). Of the three sites, Asn76, Asn143, and Asn169, only the first two were glycosylated when the delta subunit was expressed in COS cells. Because the heterologously expressed delta subunit was similar in its properties to that expressed in C2 muscle cells, the sites of glycosylation are likely to be the same in both cases. In COS cells, mutations of the delta subunit that prevented glycosylation at either of the sites did not change its metabolic stability nor its steady-state level. These results are in contrast to those found previously for the alpha subunit, in which glycosylation at a single site metabolically stabilized the polypeptide (Blount, P., and Merlie, J. P. (1990) J. Cell Biol. 111, 2613-2622). Mutations of the delta subunit that prevented glycosylation, however, decreased its ability to form an alpha delta heterodimer when the alpha and delta subunit were expressed together. When all four subunits of the AChR (alpha, beta, delta, and epsilon) were coexpressed, mutation of the delta subunit to prevent glycosylation resulted in a reduced amount of fully assembled AChR and reduced surface AChR levels, consistent with the role of the heterodimer in the assembly reaction. These results suggest that glycosylation of the delta subunit at both Asn76 and Asn143 is needed for its efficient folding and/or its subsequent interaction with the alpha subunit.

The zinc finger domain of the 43-kDa receptor-associated protein, rapsyn: role in acetylcholine receptor clustering

We injected rat myotubes with proteins and antibodies to assess the importance of the zinc finger (ZnF) domain of the 43-kDa receptor-associated protein, rapsyn, in clustering acetylcholine receptors (AChR). Injection of rat myotubes with a fusion protein containing the ZnF domain of rapsyn disrupted AChR clusters. Clusters were unaffected by a fusion protein containing a double mutant that does not bind zinc. Similar results were obtained with the purified wild type and mutant ZnF domains. The ZnF of HIV-1 nucleocapsid protein had no effect. AChR clusters were also disrupted in myotubes injected with antibodies to the ZnF domain, followed by injection of anti-antibodies. Injection of antibodies directed against a different rapsyn epitope or against the cytoplasmic domain of the AChR had no effect. In transfection experiments with HEK 293 cells, the ZnF domain failed to associate with membrane aggregates containing full-length rapsyn, AChR, or rapsyn and AChR together. We conclude that the ZnF domain of rapsyn provides a binding site essential for AChR clustering, but that this site is unlikely to be involved in high affinity binding of rapsyn to itself or to AChR.

Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Deficient development and maintenance of postsynaptic specializations in mutant mice lacking an ‘adult’ acetylcholine receptor subunit

At many synapses, ‘fetal’ neurotransmitter receptor subunits are replaced by ‘adult’ subunits as development proceeds. To assess the significance of such transitions, we deleted the gene encoding the adult acetylcholine receptor (AChR) epsilon subunit, which replaces its fetal counterpart, the gamma subunit, at the skeletal neuromuscular junction during early postnatal life. Several aspects of postnatal maturation, including synapse elimination, proceeded normally in the absence of the adult AChR, but structural development of the endplate was compromised. Later, inadequate compensation by the gamma subunit led to severely reduced AChR density in mutant endplates relative to controls. This decreased density led to a profound reorganization of AChR-associated components of the postsynaptic membrane and cytoskeleton. Together, these results suggest novel roles for AChRs in assembly of the postsynaptic apparatus.

Assembly of the nicotinic acetylcholine receptor. The first transmembrane domains of truncated alpha and delta subunits are required for heterodimer formation in vivo

To investigate the mechanism of assembly of the mouse muscle acetylcholine receptor, we have expressed truncated N-terminal fragments of the alpha and delta subunits in COS cells and have examined their ability to fold, to associate into heterodimers, and to form a ligand-binding site. Truncated fragments of the alpha subunit that include all, part, or none of the first transmembrane domain (M1) folded to acquire alpha-bungarotoxin binding activity. Neither the full-length alpha subunit nor any of the fragments were expressed on the cell surface, although the shortest folded fragment lacking a transmembrane domain was secreted into the medium. When coexpressed with the delta subunit, the alpha subunit fragment possessing M1 formed a heterodimer containing a ligand-binding site, but shorter fragments, which lack transmembrane segments, did not associate with the delta subunit. N-terminal delta subunit fragments gave similar results. An N-terminal delta subunit fragment that contains M1 associated with the alpha subunit to form a heterodimer, while a fragment lacking M1 did not. These results show that a complete M1 domain is necessary for association of truncated N-terminal alpha and delta subunits into a heterodimer with high affinity ligand binding activity.

Formation of a ligand-binding site for the acetylcholine receptor in vitro

Investigation of the mechanisms by which the subunits of ligand-gated ion channels fold and associate to form oligomers has been hampered by the lack of an in vitro system in which these reactions occur. We have established conditions in a rabbit reticulocyte translation system supplemented with canine pancreatic microsomes under which the alpha and delta subunits of the nicotinic acetylcholine receptor (AChR) fold and assemble to form a heterodimer with a cholinergic binding site comparable with that found in the intact AChR. Folding of the alpha subunit was followed by its ability to bind alpha-bungarotoxin. Folding efficiency was highly sensitive to changes in the redox potential of the translation medium and was favored by an oxidizing environment. Acquisition of the toxin binding conformation required N-linked core glycosylation but not oligosaccharide trimming, suggesting that oligosaccharide-dependent interaction of chaperones with the alpha subunit is not essential for correct subunit folding. The conformationally mature alpha subunit specifically associated with the delta subunit but not the beta subunit to form a heterodimer with a high affinity ligand-binding site. These data demonstrate, for the first time, correct folding and assembly of the AChR subunits in an in vitro system.

Rapsyn clusters and activates the synapse-specific receptor tyrosine kinase MuSK

Nerve-induced clustering of the nicotinic acetylcholine receptor (AChR) requires rapsyn, a synaptic peripheral membrane protein, as well as protein-tyrosine kinase activity. Here, we show that rapsyn induces the clustering of the synapse-specific receptor-tyrosine kinase MuSK in transfected QT-6 fibroblasts. Furthermore, rapsyn stimulates the autophosphorylation of MuSK, leading to a subsequent MuSK-dependent increase in cellular tyrosine phosphorylation. Moreover, rapsyn-activated MuSK specifically phosphorylated the AChR beta subunit, the same subunit that is tyrosine phosphorylated during innervation or agrin treatment of muscle. These results suggest rapsyn may mediate the synaptic localization of MuSK in muscle and that MuSK may play an important role in the agrin-induced clustering of the AChR.

Selection of stably transfected cells expressing a high level of fetal muscle nicotinic receptors

We had earlier found that the numbers of mouse muscle nicotinic receptors expressed on the surface of individual cells of a stably transfected clonal line of quail fibroblasts varied from cell to cell (Kopta and Steinbach: J Neurosci 14:3922-3933, 1994). We have now used repeated selective passages of these clonal cells to produce a population of cells which expresses a greater and more uniform number of surface receptors per cell. The increased level is stable over many cell divisions, and over many half-lives for the metabolic degradation of the surface receptors. Selection was performed by adhesion to a surface coated with a monoclonal antibody to a surface epitope on the muscle receptor, followed by expansion of the most tightly attached population of cells. Studies of the selected cells show that the surface receptors contain all four subunits of the muscle nicotinic receptor, and the functional properties of the receptors appear normal. The metabolic stability of the surface receptors is not altered, while the amount of mRNA for the subunits is increased in the selected population of cells. These observations indicate that the more likely reason for increased expression is a transcriptional effect, and that translational or posttranslational changes are unlikely.

A sequence in the main cytoplasmic loop of the alpha subunit is required for assembly of mouse muscle nicotinic acetylcholine receptor

We have investigated the role of intracellular cytoplasmic sequences in the assembly of the mouse muscle nicotinic acetylcholine receptor (AChR) transiently expressed in COS cells. A chimeric protein in which the region from M1 to M4 of the alpha subunit was replaced by the corresponding region in the beta subunit was unable to support AChR assembly when substituted for the alpha subunit; a chimeric alpha subunit containing only the long cytoplasmic loop from the beta subunit was likewise inactive. Systematic mutation of short segments of the loop identified a sequence of 17 amino acids near the C-terminal end of the loop for which the beta sequence could not be substituted. Each of the inactive chimeric and mutated alpha subunits bound alpha-bungarotoxin when expressed alone and formed a heterodimer when expressed with the delta subunit. An alpha subunit truncated after M1 formed both an alpha delta heterodimer and an alpha delta beta heterotrimer, demonstrating that the cytoplasmic loop is dispensable for the early steps of assembly. A sequence in the long cytoplasmic loop of the alpha subunit thus appears to play a role in a late step of AChR assembly.

Clustered acetylcholine receptors have two levels of organization in Xenopus muscle cells

We studied the organization of acetylcholine receptor (AChR) clusters by shearing cultured Xenopus muscle cells with a stream of buffer, and preparing rotary replicas of the exposed cytoplasmic surface of the sarcolemma. AChR clusters contained numerous particles that protruded from the sarcolemma and formed an irregular array composed of discrete aggregates. AChR were located within these particle aggregates, as shown by comparison of the replicas to labeling by fluorescent alpha-bungarotoxin, and by immunogold cytochemistry with antibodies specific for the receptor. The aggregates were cross-linked by a dense network of 7 nm filaments that replicated with the banded pattern characteristic of actin microfilaments. The organization of receptors into the small aggregates was independent of the organization of these aggregates into clusters, as alkaline extraction removed the microfilament network and disrupted the irregular array of particle aggregates, but did not disperse individual receptors from the aggregates. We conclude that two levels of interactions organize AChR clusters in Xenopus muscle cells: short-range interactions that assemble individual AChR into small aggregates, and long-range interactions, perhaps mediated by actin microfilaments, that anchor the aggregates into larger clusters.

Interaction of the 43 kd postsynaptic protein with all subunits of the muscle nicotinic acetylcholine receptor

The 43 kd postsynaptic protein (43K) plays a key role in the aggregation of muscle nicotinic acetylcholine receptors (AChRs) in the postsynaptic membrane of the neuromuscular junction. By transiently coexpressing 43K and a single AChR subunit (alpha, beta, gamma, or delta) in the quail fibroblast cell line, QT-6, we show that 43K interacts with each subunit to form cell surface clusters in which 43K and receptor subunit are precisely colocalized. Although the level of cell surface expression of single subunits is much lower than that of fully assembled receptor, the clustering of both single subunits and fully assembled AChR occurs efficiently. In addition, 43K-induced clustering is specific for AChR subunits. From these results, we conclude that each pentameric AChR has five potential sites for interacting with 43K during cluster formation.

The N-terminal domains of acetylcholine receptor subunits contain recognition signals for the initial steps of receptor assembly

Ligand-gated ion channels are oligomeric membrane proteins in which homologous subunits specifically recognize one another and assemble around an aqueous pore. To identify domains responsible for the specificity of subunit association, we used a dominant-negative assay in which truncated subunits of the mouse muscle acetylcholine receptor (AChR) were coexpressed with the four wild-type subunits in transfected COS cells. Fragments of the alpha, delta, and gamma subunits consisting solely of the extracellular N-terminal domain blocked surface expression of the AChR and the formation of alpha delta heterodimers, an early step in the assembly pathway of the AChR. Immunoprecipitation and sucrose gradient sedimentation experiments showed that an N-terminal fragment of the alpha subunit forms a specific complex with the intact delta subunit. Thus the extracellular N-terminal domain of the alpha, delta, and gamma subunits contains the information necessary for specific subunit association.

Identification of two amino acid residues in the epsilon subunit that promote mammalian muscle acetylcholine receptor assembly in COS cells

We have used a species difference in epsilon subunits of the acetylcholine receptor (AChR) to investigate regions of the subunit protein that are important in receptor assembly. Upon transient transfection of COS cells, mouse epsilon subunit cDNA is approximately 10 times more effective than that of the rat in supporting expression of surface AChRs when the other subunits are from either mouse or rat. In cells transfected with only alpha and epsilon subunit cDNAs, the formation of an alpha epsilon heterodimer, a presumed assembly intermediate, is also less efficient with rat than with mouse epsilon subunit. By site-directed mutagenesis, we have found that these differences can be accounted for by 2 amino acid differences in the N-terminal domain at positions 106 and 115 of the rat and mouse epsilon subunits, suggesting that the region near these 2 amino acid residues is important for AChR assembly.

Properties of embryonic and adult muscle acetylcholine receptors transiently expressed in COS cells

We used transient transfection in COS cells to compare the properties of mouse muscle acetylcholine receptors (AChRs) containing alpha, beta, delta, and either gamma or epsilon subunits. gamma- and epsilon-AChRs had identical association rates for binding 125I-alpha-bungarotoxin, and identical curves for inhibition of toxin binding by d-tubocurarine, but epsilon-AChRs had a significantly longer half-time of turnover in the membrane than gamma-AChRs. A myasthenic serum specific for the embryonic form of the AChR reduced toxin binding to gamma-, but not epsilon-AChRs. The gamma-AChRs had channel characteristics of embryonic AChRs, whereas the major class of epsilon-AChR channels had the characteristics of adult AChRs. Two minor channel classes with smaller conductances were also seen with epsilon-AChR. Thus, some, but not all, of the differences between AChRs at adult endplates and those in the extrasynaptic membrane can be explained by the difference in subunit composition of gamma- and epsilon-AChRs.

Immunolocalization of hyalin in sea urchin eggs and embryos using an antihyalin-specific monoclonal antibody

Monoclonal antibodies were raised against purified cortical secretory vesicles (CVs) from the eggs of Strongylocentrotus purpuratus. One of the monoclonal antibodies (MAb 69-10, an IgA) was shown by immunofluorescence labeling of intact and detergent-lysed CVs to be directed against a CV content antigen. Immunoblot analysis of CVs revealed that MAb 69-10 bound to a major CV polypeptide with an Mr similar to that of hyalin (i.e., 300,000). MAb 69-10 was subsequently shown to bind to purified hyalin prepared from S. purpuratus and to cross react with hyalin prepared from Lytechinus pictus. Immunogold labeling on thin sections of unfertilized S. purpuratus eggs showed that hyalin was localized to the electron-lucent portion of CVs. This result is in agreement with the labeling pattern obtained by Hylander and Summers (Dev Biol 93:368-380, 1982) using polyclonal antihyalin antibodies. In fertilized eggs and later-stage embryos, hyalin was observed to be located on the external surface of the embryo. MAb 69-10 should be useful in studies of the structure of hyalin and its function in morphogenesis.

Main immunogenic region of Torpedo electroplax and human muscle acetylcholine receptor: localization and microheterogeneity revealed by the use of synthetic peptides

Most anti-nicotinic acetylcholine receptor (AChR) antibodies in myasthenia gravis are directed against an immunodominant epitope or epitopes [main immunogenic region (MIR)] on the AChR alpha-subunit. Thirty-two synthetic peptides, corresponding to the complete Torpedo alpha-subunit sequence and to a segment of human muscle alpha-subunit, were used to map the epitopes for 11 monoclonal antibodies (mAbs) directed against the Torpedo and/or the human MIR and for a panel of anti-AChR mAbs directed against epitopes on the alpha-subunit other than the MIR. A main constituent loop of the MIR was localized within residues alpha 67-76. Residues 70 and 75, which are different in the Torpedo and human alpha-subunits, seem to be crucial in determining the binding profile for several mAbs whose binding to the peptides correlated very well with their binding pattern to native Torpedo and human AChRs. This strongly supports the identification of the peptide loop alpha 67-76 as the actual location of the MIR on the intact AChR molecule. Residues 75 and 76 were necessary for binding of some mAbs and irrelevant for others, in agreement with earlier suggestions that the MIR comprises overlapping epitopes. Structural predictions for the sequence segment alpha 67-76 indicate that this segment has a relatively high segmental mobility and a very strong turning potential centered around residues 68-71. The most stable structure predicted for this segment, in both the Torpedo and human alpha-subunits, is a hairpin loop, whose apex is a type I beta-turn and whose arms are beta-strands. This loop is highly hydrophilic, and its apex is negatively charged. All these structural properties have been proposed as characteristic of antibody binding sites. We also localized the epitopes for mAbs against non-MIR regions. Among these, the epitope for a monoclonal antibody (mAb 13) that noncompetitively inhibits channel function was localized within residues alpha 331-351.

Molecular basis of the two nonequivalent ligand binding sites of the muscle nicotinic acetylcholine receptor

We have stably expressed in fibroblasts different pairs of alpha and non-alpha subunits of the mouse muscle nicotinic acetylcholine receptor (AChR). The gamma and delta, but not the beta, subunits associated efficiently with the alpha subunit, and they extensively modified its binding characteristics. The alpha gamma and alpha delta complexes formed distinctly different high affinity binding sites for the competitive antagonist d-tubocurarine that, together, completely accounted for the two nonequivalent antagonist binding sites in native AChR. The alpha delta complex and native AChR had similar affinities for the agonist carbamylcholine. In contrast, although the alpha gamma complex contains the higher affinity competitive antagonist binding site, it had an affinity for carbamylcholine that was an order of magnitude less than that of the alpha delta complex or the AChR. The comparatively low agonist affinity of the alpha gamma complex may represent an allosterically regulated binding site in the native AChR. These data support a model of two nonequivalent binding sites within the AChR and imply that the basis for this nonequivalence is the association of the alpha subunit with the gamma or delta subunit.

Purification and characterization of a cortical secretory vesicle membrane fraction

A membrane fraction has been prepared by sucrose density gradient fractionation of purified cortical secretory vesicles from the eggs of the sea urchin Strongylocentrotus purpuratus. The purified cortical vesicle membrane fraction has a phospholipid to protein ratio of 1.76 and exhibits a morphology typical of biological membranes as seen by electron microscopy. The protein composition of the purified membranes was analyzed by SDS-polyacrylamide gel electrophoresis and shown to be distinct from that of eggs, cell surface complex, cortical vesicles, fertilization product, and yolk platelets. Alkaline extraction (pH 11.0) of peripheral membrane proteins increased the phospholipid to protein ratio to 2.55 and removed several polypeptides. Immunoblot analysis of the isolated cortical vesicle membrane fraction revealed low levels of contamination with two major cortical vesicle content proteins. Fractions enriched in egg plasma membranes and yolk platelet membranes also have been isolated and compared with the cortical vesicle membranes by SDS-polyacrylamide gel electrophoresis. The protein compositions of the three membrane fractions were found to contain very little overlap, indicating that the cortical vesicle membrane preparation is relatively free of contamination from these likely noncortical vesicle sources of membrane. Both the plasma membrane and cortical vesicle membrane samples were found by immunoblotting to contain actin.

The alpha 1 and alpha 2 polypeptides of the dihydropyridine-sensitive calcium channel differ in developmental expression and tissue distribution

The dihydropyridine (DHP) binding complex isolated from skeletal muscle contains two large proteins, alpha 1 and alpha 2, and at least two smaller polypeptides. The alpha 1 subunit has a primary structure expected for ion channels and is a functional component of a DHP-sensitive, voltage-activated calcium channel. The functions of the alpha 2 protein and the smaller polypeptides are unknown. We prepared monoclonal antibodies to the alpha 1 and alpha 2 polypeptides and studied the developmental appearance and tissue distribution of these two proteins. In rat skeletal muscle, the levels of alpha 1 are quite low during the first 10 days after birth, then rise dramatically, and, by day 20, approach those found in adult muscle. In direct contrast, alpha 2 is present in substantial amounts in rat muscle at birth and increases only slightly during this same period of development. Furthermore, alpha 1 is detected only in skeletal muscle, whereas alpha 2 is present in a variety of tissues. These results provide evidence for the segregation of these two polypeptides and suggest that the function of alpha 2 is not limited to that associated with the DHP-sensitive calcium channel.

Assembly and N-glycosylation of all ACh receptor subunits are required for their efficient insertion into plasma membranes

Various combinations of synthetic acetylcholine receptor (AChR) subunit mRNAs were injected into Xenopus oocytes, and assembly of incomplete AChRs and their insertion into the plasma membrane was studied. Assembly of incomplete AChRs is not greatly affected by the absence of one or two of the other subunits. In contrast, the membrane insertion of incomplete AChRs is profoundly reduced as compared with complete AChRs. The role of N-glycosylation on the assembly of AChR subunits, and on their insertion into plasma membranes, was also studied by using the Xenopus oocyte expression system and tunicamycin. Assembly of non-N-glycosylated AChR subunits occurs in tunicamycin-treated oocytes, but these subunits remain in intracellular compartments, suggesting that N-glycosylation of AChR subunits is not a prerequisite for receptor assembly, but is required for their efficient insertion into the plasma membrane.

Calcitonin gene-related peptide regulates phosphorylation of the nicotinic acetylcholine receptor in rat myotubes

The nicotinic acetylcholine receptor (AChR) is a substrate for at least three different protein kinases, and phosphorylation of the receptor has been shown to increase its rate of desensitization. However, the first messengers that regulate AChR phosphorylation have not yet been identified. This study demonstrates that calcitonin gene-related peptide (CGRP), a neuropeptide present in the axon terminals of the neuromuscular junction, regulates phosphorylation of the AChR in primary rat myotube cultures. CGRP, in the presence of the phosphodiesterase inhibitor Ro 20-1724, increased phosphorylation of the alpha and delta subunits of the AChR. CGRP-induced phosphorylation of the AChR had the same subunit specificity and temporal sequence as previously observed using forskolin or cAMP analogs. Phosphorylation of the AChR in the presence of CGRP appears to be mediated by CGRP-stimulated increases in cAMP levels leading to activation of cAMP-dependent protein kinase. The present results, taken together with the recent demonstration that CGRP increases the rate of AChR desensitization in mouse myotubes, suggest that CGRP may play a physiological role as a regulator of AChR desensitization by modulating AChR phosphorylation at the neuromuscular junction.

Identification of a major polypeptide component of the sea urchin fertilization envelope

A monoclonal antibody (MAb No. 25-16), raised against purified cortical secretory vesicles (CVs) from the eggs of Strongylocentrotus purpuratus, has been used to identify a previously uncharacterized CV-derived polypeptide component of the sea urchin fertilization envelope (FE). MAb No. 25-16, an IgG1, bound to a group of proteins with Mr approximately 200,000 on immunoblots of CVs. This same group of proteins also was detected in fertilization product and in soft FEs prepared from early embryos, indicating that the antigen is released at fertilization by CV exocytosis and becomes incorporated into the FE. The multiple components recognized by MAb No. 25-16 apparently did not result from proteolysis during sample preparation or differential N-linked glycosylation. No simplification of the SDS-gel or immunoblot patterns was observed when samples of fertilization product or cell surface complex were prepared in the presence of a cocktail of protease inhibitors; nor was a change in mobility of any of the antigen forms detected following treatment with endoglycosidase F. Upon partial denaturation and reduction of the protein by incubation at room temperature in the presence of SDS and dithiothreitol, the antigen was shown to undergo a decrease in relative mobility on SDS-PAGE. Complete reduction and denaturation, by boiling in dithiothreitol-containing SDS sample buffer or by an on-blot reduction technique, resulted in loss of the epitope. The protein component recognized by MAb No. 25-16 underwent a striking increase in mobility on SDS-PAGE after chelation of calcium ions with EGTA. Immunogold labeling on thin sections of unfertilized eggs revealed that the antigen is located in the spiral lamellar cores of all CVs. In fertilized eggs, fixed 5 min after insemination, the antigen was detected in the FE. Based on these biochemical and immunological data, we suggest that the antigen recognized by MAb No. 25-16 is released exocytotically from the CVs into the perivitelline space at fertilization and becomes incorporated into the FE. The abundance of this antigen suggests that it may function as a structural component of the FE.

Immunochemical identification of desmin in Torpedo postsynaptic membranes and at the rat neuromuscular junction

Preparations of acetylcholine receptor-rich (AChR-rich) postsynaptic membranes from electric tissue of electric rays often contain an Mr 55,000 protein (55kD protein) that has not been previously characterized. Using a monoclonal antibody (MAb 1403) against the 55kD protein from Torpedo californica and a pan-specific, anti-intermediate filament antibody (Pruss et al., 1981; Cell 27:419-428), we show that the 55kD protein has the properties expected of Torpedo desmin. By the electron microscope immunogold method applied to perfusion-fixed electric tissue, MAb 1403 labeled only cytoplasmic filaments in the electroplax. These filaments were neither more concentrated nor arranged detectably differently in postsynaptic regions relative to nonpostsynaptic regions. The 55kD protein could also be fractionated away from isolated postsynaptic membranes by gradient centrifugation. The protein is thus a minor component of the postsynaptic membrane in situ and after isolation. On semithin cryosections of rat skeletal muscle, on the other hand, MAb 1403, which recognizes rat desmin but not rat vimentin, gave strong fluorescent labeling of the postsynaptic region, weaker labeling of the Z-line, and still weaker labeling of the cell surface immediately surrounding extra-junctional nuclei. The pattern of postsynaptic labeling suggests that desmin, presumably in the form of intermediate filaments, occurs near the AChR-rich crests of the junctional folds, but is particularly concentrated among and around the ends of the folds. Similar results were obtained with a second monoclonal antibody raised against authentic desmin. These results suggest that desmin intermediate filaments may have an important role in organization of the postsynaptic cytoplasm in rat muscle.

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.

Immunochemical detection of proteins biotinylated on nitrocellulose replicas

A sensitive method for staining proteins after transfer from polyacrylamide gels to nitrocellulose paper is described. Transferred proteins are first derivatized by reaction of the nitrocellulose replica with sulfosuccinimidobiotin and are then reacted sequentially with streptavidin, rabbit anti-streptavidin, and horseradish peroxidase-conjugated goat anti-rabbit IgG antibody. Application of the enzyme substrate, alpha-chloronaphthol, produces dark protein bands against a white background. The binding of streptavidin to the proteins is dependent on biotin derivatization and is inhibited by biotinylated bovine serum albumin or 10 nM biotin. The procedure permits detection of less than 5 ng of transferred protein in a single band and is thus 5-10 times more sensitive than horseradish peroxidase-conjugated avidin alone. For bovine serum albumin, the method is comparable in sensitivity to silver staining of protein in polyacrylamide gels.

Monoclonal antibodies to acetylcholine receptor secreted by human x human hybridomas derived from lymphocytes of a patient with myasthenia gravis

Peripheral blood lymphocytes of a patient with myasthenia gravis (MG) were fused to the non-secreting human lymphoblastoid line HuNSI to produce human x human hybridomas that secrete monoclonal antibodies (mAb) to acetylcholine receptor (AChR). Screening of hybridomas for antibody production involved an enzyme-linked immunosorbent (ELISA) assay with AChR from Torpedo californica (TAChR). 25 of 302 wells tested (8.3%) were positive for anti-AChR antibody production and have been stable in their secretion of mAb for eleven months. Nine lines have been studied in detail. All produced IgM mAb, and most had greater activity against membrane-bound TAChR, than against solubilized TAChR. For anti-AChR clones, the mAb concentration in culture supernatants ranged from 2 to 33 micrograms/ml. Saturation curves of binding to TAChR performed on 4 lines demonstrated dissociation constants (Kds) estimated to range from 0.1-1.0 nM. The patient whose lymphocytes were used in this study had a serum anti-AChR antibody concentration of 243nM against human AChR and 15nM against AChR from T. californica. The results demonstrate the feasibility of producing stable human x human hybridomas secreting mAb to the autoantigen from the peripheral blood of patients with organ-specific autoimmune diseases. The mAb produced here may prove to be useful in analyzing, and possibly treating, the autoimmune phenomena in MG.

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.

Characteristics of monoclonal antibodies to denatured Torpedo and to native calf acetylcholine receptors: species, subunit and region specificity

Seventy-five monoclonal antibodies (mAbs) to sodium dodecyl sulfate-denatured Torpedo californica (66 mAbs) and intact fetal calf (9 mAbs) acetylcholine receptor (AChR) were produced. These mAbs were characterized for subunit, region and species specificity, for Ig class and subclass, for protein A binding and for antigen-crosslinking capacity. Fourteen were identified as anti-alpha, 35 were anti-beta, 8 were anti-gamma and 15 were anti-delta. None of the 11 anti-alpha derived from denatured AChR bound to the main immunogenic region (MIR) as judged by antibody competition assays. This contrasts with previous results using mAbs against native AChr, the majority of which bind to the MIR. Thirty-eight mAbs crossreacted with some or all of the tested AChRs from fish electric organs and mammalian muscles in addition to the immunogen. Eight anti-alpha, anti-beta and 1 anti-delta mAbs showed good to excellent autoantibody activity. Analysis by sucrose gradient centrifugation of some AChR-mAb complexes revealed that some mAbs form intermolecular and others form intramolecular crosslinkings of the AChR. The described mAbs have proven valuable tools in AChR and myasthenia gravis research.