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

The postsynaptic 43K protein clusters muscle nicotinic acetylcholine receptors in Xenopus oocytes

Nicotinic acetylcholine receptors (AChRs) are localized at high concentrations in the postsynaptic membrane of the neuromuscular junction. A peripheral membrane protein of Mr 43,000 (43K protein) is closely associated with AChRs and has been proposed to anchor receptors at postsynaptic sites. We have used the Xenopus oocyte expression system to test the idea that the 43K protein clusters AChRs. Mouse muscle AChRs expressed in oocytes after injection of RNA encoding receptor subunits are uniformly distributed in the surface membrane. Coinjection of AChR RNA and RNA encoding the mouse muscle 43K protein causes AChRs to form clusters of 0.5-1.5 microns diameter. AChR clustering is not a consequence of increased receptor expression in the surface membrane or nonspecific clustering of all membrane proteins. The 43K protein is colocalized with AChRs in clusters when the two proteins are expressed together and forms clusters of similar size even in the absence of AChRs. These results provide direct evidence that the 43K protein causes clustering of AChRs and suggest that regulation of 43K protein clustering may be a key step in neuromuscular synaptogenesis.

Asymmetric distribution of dystrophin in developing and adult Torpedo marmorata electrocyte: evidence for its association with the acetylcholine receptor-rich membrane

Dystrophin has been shown to occur in Torpedo electrocyte [Chang, H. W., Bock, E. & Bonilla, E. (1989) J. Biol. Chem. 264, 20831-20834], a highly polarized syncytium that is embryologically derived from skeletal muscle and displays functionally distinct plasma membrane domains on its innervated and noninnervated faces. In the present study, we investigated the subcellular distribution of dystrophin in the adult electrocyte from Torpedo marmorata and the evolution of its distribution during embryogenesis. Immunofluorescence experiments performed on adult electrocytes with a polyclonal antibody directed against chicken dystrophin revealed that dystrophin immunoreactivity codistributed exclusively with the acetylcholine receptor along the innervated membrane. At the ultrastructural level, dystrophin immunoreactivity appears confined to the face of the subsynaptic membrane exposed to the cytoplasm. In developing electrocytes (45-mm embryo), dystrophin is already detectable at the acetylcholine receptor-rich ventral pole of the cells before the entry of the electromotor axons. Furthermore, we show that dystrophin represents a major component of purified membrane fractions rich in acetylcholine receptor. A putative role of dystrophin in the organization and stabilization of the subsynaptic membrane domain of the electrocyte is discussed.

Association of cytoskeletal proteins with newly formed acetylcholine receptor aggregates induced by embryonic brain extract

Aggregates of acetylcholine receptors (AChR) in muscle cell membranes are associated with accumulations of certain cytoskeletal and peripheral membrane proteins. We treated cultured rat myotubes briefly with embryonic brain extract (EBX) to promote AChR aggregation and determined the distribution of several of these proteins at early stages of aggregation. EBX-treated and control cultures were stained with tetramethylrhodamine-alpha-bungarotoxin to identify AChR aggregates and were then frozen and sectioned on a cryostat. These sections were stained with primary antibodies and fluoresceinated secondary antibodies to localize cytoskeletal proteins. The distributions of AChRs and cytoskeletal proteins was examined qualitatively and analyzed by a semiquantitative assay. Qualitatively, the 43K protein had a distribution that was virtually identical to that of AChR in both control and EBX-treated cultures, and it always colocalized with early AChR aggregates. The 58K protein similarly colocalized with early AChR aggregates, but it was also in aggregate-free areas of muscle membrane. The association of vinculin with the aggregates was quantitatively similar to that of the 43K and 58K proteins, but, qualitatively, its distribution did not follow that of the AChR as closely. Like the 58K protein and vinculin, alpha-actinin, filamin, and actin were concentrated in AChR aggregates and were also enriched elsewhere. However, they were less closely associated with the aggregates, both quantitatively and qualitatively. These results show that AChR aggregates induced by EBX tend to be enriched in the same cytoskeletal proteins that are present at the neuromuscular junction in vivo and at AChR clusters formed at sites of cell-substrate adhesion in vitro. Semiquantitative analysis also revealed that the fractional area of the cell surface associated with vinculin, alpha-actinin, and the 58K protein was the same in controls and EBX-treated myotubes, although the area enriched in AChR and the 43K protein increased about three-fold upon EBX treatment. These results suggest that AChR aggregates may form preferentially in membrane regions that are already enriched in these proteins.

A novel 87,000-Mr protein associated with acetylcholine receptors in Torpedo electric organ and vertebrate skeletal muscle

To identify proteins associated with nicotinic postsynaptic membranes, mAbs have been prepared to proteins extracted by alkaline pH or lithium diiodosalicylate from acetylcholine receptor-rich (AChR) membranes of Torpedo electric organ. Antibodies were obtained that recognized two novel proteins of 87,000 Mr and a 210,000:220,000 doublet as well as previously described proteins of 43,000 Mr, 58,000 (51,000 in our gel system), 270,000, and 37,000 (calelectrin). The 87-kD protein copurified with acetylcholine receptors and with 43- and 51-kD proteins during equilibrium centrifugation on continuous sucrose gradients, whereas a large fraction of the 210/220-kD protein was separated from AChRs. The 87-kD protein remained associated with receptors and 43-kD protein during velocity sedimentation through shallow sucrose gradients, a procedure that separated a significant amount of 51-kD protein from AChRs. The 87- and 270-kD proteins were cleaved by Ca++-activated proteases present in crude preparations and also in highly purified postsynaptic membranes. With the exception of anti-37-kD antibodies, some of the monoclonals raised against Torpedo proteins also recognized determinants in frozen sections of chick and/or rat skeletal muscle fibers and in permeabilized chick myotubes grown in vitro. Anti-87-kD sites were concentrated at chick and rat endplates, but the antibodies also recognized determinants present at lower site density in the extrasynaptic membrane. Anti-210:220-kD labeled chick endplates, but studies of neuron-myotube cocultures showed that this antigen was located on neurites rather than the postsynaptic membrane. As reported in other species, 43-kD determinants were restricted to chick endplates and anti-51-kD and anti-270-kD labeled extrasynaptic as well as synaptic membranes. None of the cross reacting antibodies recognized determinants on intact (unpermeabilized) myotubes, so the antigens must be located on the cytoplasmic aspect of the surface membrane. The role that each intracellular determinant plays in AChR immobilization at developing and mature endplates remains to be investigated.

Presence of a protein immunologically related to lamin B in the postsynaptic membrane of Torpedo marmorata electrocyte

The Torpedo electrocyte is a flattened syncytium derived from skeletal muscle, characterized by two functionally distinct plasma membrane domains. The electrocyte is filled up with a transversal network of intermediate filaments (IF) of desmin which contact in an end-on fashion both sides of the cell. In this work, we show that polyclonal antibodies specific for lamin B recognizes a component of the plasma membrane of Torpedo electrocyte. This protein which thus shares epitopes with lamin B has a relative molecular mass of 54 kD, an acidic IP of 5.4. It is localized exclusively on the cytoplasmic side of the innervated membrane of the electrocyte at sites of IF-membrane contacts. Since our previous work showed that the noninnervated membrane contains ankyrin (Kordeli, E., J. Cartaud, H. O. Nghiêm, L. A. Pradel, C. Dubreuil, D. Paulin, and J.-P. Changeux. 1986. J. Cell Biol. 102:748-761), the present results suggest that desmin filaments may be anchored via the 54-kD protein to the innervated membrane and via ankyrin to the noninnervated membrane. These findings would represent an extension of the model proposed by Georgatos and Blobel (Georgatos, S. D., and G. Blobel. 1987a. J. Cell Biol. 105:105-115) in which type III intermediate size filaments are vectorially inserted to plasma and nuclear membranes by ankyrin and lamin B, respectively.

Distribution of Na+ channels and ankyrin in neuromuscular junctions is complementary to that of acetylcholine receptors and the 43 kd protein

We have used immunogold electron microscopy to study the organization of the acetylcholine receptor, 43 kd protein, voltage-sensitive Na+ channel, and ankyrin in the postsynaptic membrane of the rat neuromuscular junction. The acetylcholine receptor and the 43 kd protein are concentrated at the crests of the postsynaptic folds, coextensive with the subsynaptic density. In contrast, Na+ channels and ankyrin are concentrated in the membranes of the troughs and in perijunctional membranes, both characterized by discontinuous submembrane electron-dense plaques. This configuration of interspersed postsynaptic membrane domains enriched in either Na+ channels or acetylcholine receptors may facilitate the initiation of the muscle action potential. Furthermore, the results support the involvement of ankyrin in immobilizing Na+ channels in specific membrane domains, analogous to the proposed involvement of the 43 kd protein in acetylcholine receptor immobilization.

Expression of RNA transcripts for the postsynaptic 43 kDa protein in innervated and denervated rat skeletal muscle

A cDNA clone encoding the mouse muscle postsynaptic 43 kDa protein was isolated and sequenced. The amino acid sequence of this protein, which is closely associated with nicotinic acetylcholine receptors at Torpedo electrocyte and vertebrate skeletal muscle synapses, is very similar in different species. A cysteine-rich region homologous to part of the regulatory domain of protein kinase C may be important in interactions of this protein with the lipid bilayer. RNA transcripts for the 43 kDa protein increase only 2-3 fold after denervation of rat skeletal muscle, in sharp contrast to the alpha-subunit of the muscle nicotinic receptor which increases more than 30-fold. Thus, the expression of these two proteins is regulated by different mechanisms.

Cytoskeletal architecture of neuromuscular junction: localization of vinculin

Cytoskeletons underneath the postsynaptic membrane of neuromuscular junctions were studied by using a quick-freeze deep-etch method and immunoelectron microscopy of ultrathin frozen sections. In a quick-freeze deep-etched replica of fresh, unfixed muscles, 8.9 +/- 1.5-nm particles were present on the true postsynaptic membrane surface. Underneath this receptor-rich postsynaptic membrane, networks of fine filaments were observed. These cytoskeletal networks were more clearly observed in extracted samples. In these samples, diameters of the filaments which formed networks were measured. In the platinum replica, three kinds of filament were recognized--12 nm, 9 nm, and 7 nm in diameter. The 12-nm filament seemed to correspond to the intermediate filament. The other two filaments formed meshworks between intermediate filaments and plasma membrane. In ultrathin frozen sections vinculin label was localized just beneath the plasma membrane. Thirty-six percent of the label was within 18 nm from the cytoplasmic side of the plasma membrane and 50% was within 30 nm. Taking the size of the vinculin molecule into account, it was concluded that vinculin is localized just beneath the plasma membrane and might play some role in anchoring filaments which formed meshworks underneath the plasma membrane.

Clusters of 43-kDa protein are absent from genetic variants of C2 muscle cells with reduced acetylcholine receptor expression

Genetic variants of the C2 muscle cell line were used to investigate the relation between acetylcholine receptor (AChR) clustering and clustering of the 43-kDa protein. Two variants that express severely reduced amounts of the alpha subunit of the AChR and consequently lack AChR clusters were found also to lack clusters of the 43-kDa protein. The amount of 43-kDa protein in the variants measured by immunoassay was reduced to about one-third the levels found in wild-type cells. The beta subunit of the AChR was reduced to a similar extent. Northern blot analysis showed that neither the 43-kDa protein mRNA nor the beta subunit mRNA was reduced in the variants. Taken together, these results suggest that the amounts of beta subunit and 43-kDa protein may be regulated coordinately by a post-transcriptional mechanism.

An unusual beta-spectrin associated with clustered acetylcholine receptors

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane is an early event in the formation of the neuromuscular junction. The mechanism of clustering is still unknown, but is generally believed to be mediated by the postsynaptic cytoskeleton. We have identified an unusual isoform of beta-spectrin which colocalizes with AChR in AChR clusters isolated from rat myotubes in vitro. A related antigen is present postsynaptically at the neuromuscular junction of the rat. Immunoprecipitation, peptide mapping and immunofluorescence show that the beta-spectrin in AChR clusters resembles but is distinct from the beta-spectrin of human erythrocytes. alpha-Spectrin appears to be absent from AChR clusters. Semiquantitative immunofluorescence techniques indicate that there are from two to seven beta-spectrin molecules present for every clustered AChR, the higher values being obtained from rapidly prepared clusters, the lower values from clusters that require several minutes or more for isolation. Upon incubation of isolated AChR clusters for 1 h at room temperature, beta-spectrin is slowly depleted and the AChR redistribute into microaggregates. The beta-spectrin that remains associated with the myotube membrane is concentrated at these microaggregates. beta-Spectrin is quantitatively lost from clusters upon digestion with chymotrypsin, which causes AChR to redistribute in the plane of the membrane. These results suggest that AChR in clusters is closely linked to an unusual isoform of beta-spectrin.

Myristic acid is the NH2-terminal blocking group of the 43-kDa protein of Torpedo nicotinic post-synaptic membranes

The NH2-terminal blocking group of the 43-kDa peripheral membrane protein (43-kDa protein) of Torpedo post-synaptic membranes has been identified as myristic acid. To identify that blocking group pure 43-kDa protein was digested with trypsin and the blocked tryptic peptide was isolated by reverse phase HPLC. That peptide coeluted with and had the same amino acid composition as a synthetic peptide, myristoyl-Gly-Gln-Asp-Gln-Thr-Lys, the structure of the amino terminus predicted from the protein sequence deduced from a cDNA clone. The presence of myristate was confirmed by the precise molecular mass of the peptide, 886.5266, determined by fast atom bombardment mass spectroscopy.

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.

Expression of RAPsyn (43K protein) and nicotinic acetylcholine receptor genes is not coordinately regulated in mouse muscle

RAPsyn (also known as 43K protein), a mouse muscle protein localized to the synaptic membrane, is thought to be involved in the localization of nicotinic acetylcholine receptors at the neuromuscular junction. We have characterized the transcriptional regulation of the RAPsyn gene and the synthesis of the RAPsyn protein during muscle cell differentiation. Nuclear run-on experiments and RNAase protection analyses showed that mRNA encoding RAPsyn, but not the acetylcholine receptor subunits, is present in undifferentiated muscle cells. The RAPsyn protein present in undifferentiated and differentiated muscle cells cannot be distinguished by peptide maps, turnover rates, cellular subfractionation, or ability to incorporate myristate. Whereas the amount of acetylcholine receptor subunit mRNA is increased approximately 100-fold after denervation, the amount of RAPsyn mRNA is increased just 2- to 3-fold. We conclude that the expression of RAPsyn and the acetylcholine receptor is not coordinately regulated in mouse muscle.

Asynchronous assembly of the acetylcholine receptor and of the 43-kD nu1 protein in the postsynaptic membrane of developing Torpedo marmorata electrocyte

The assembly of the nicotinic acetylcholine receptor (AchR) and the 43-kD protein (v1), the two major components of the post synaptic membrane of the electromotor synapse, was followed in Torpedo marmorata electrocyte during embryonic development by immunocytochemical methods. At the first developmental stage investigated (45-mm embryos), accumulation of AchR at the ventral pole of the newly formed electrocyte was observed within columns before innervation could be detected. No concomitant accumulation of 43-kD immunoreactivity in AchR-rich membrane domains was observed at this stage, but a transient asymmetric distribution of the extracellular protein, laminin, which paralleled that of the AchR, was noticed. At the subsequent stage studied (80-mm embryos), codistribution of the two proteins was noticed on the ventral face of the cell. Intracellular pools of AchR and 43-kD protein were followed at the EM level in 80-mm electrocytes. AchR immunoreactivity was detected within membrane compartments, which include the perinuclear cisternae of the endoplasmic reticulum and the plasma membrane. On the other hand, 43-kD immunoreactivity was not found associated with the AchR in the intracellular compartments of the cell, but codistributed with the AchR at the level of the plasma membrane. The data reported in this study suggest that AchR clustering in vivo is not initially determined by the association of the AchR with the 43-kD protein, but rather relies on AchR interaction with extracellular components, for instance from the basement membrane, laid down in the tissue before the entry of the electromotor nerve endings.

Molecular events in synaptogenesis: nerve-muscle adhesion and postsynaptic differentiation

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane of newly innervated muscle fibers is one of the earliest events in the development of the vertebrate neuromuscular junction. Here, we describe two hypotheses that can account for AChR clustering in response to innervation. The "trophic factor" hypothesis proposes that the neuron releases a soluble factor that interacts with the muscle cell in a specific manner and that this interaction results in the local accumulation of AChR. The "contact and adhesion" hypothesis proposes that the binding of the nerve to the muscle cell surface is itself sufficient to induce AChR clustering, without the participation of soluble factors. We present a model for the molecular assembly of AChR clusters based on the contact and adhesion hypothesis. The model involves the sequential assembly of three distinct membrane domains. The first domain to form serves to attach microfilaments to the cytoplasmic surface of the muscle cell membrane at sites of muscle-nerve adhesion. The second domain to form is clathrin-coated membrane; it serves as a site of insertion of additional membrane elements, including AChR. Upon insertion of AChR into the cell surface, a membrane skeleton assembles by anchoring itself to the AChR. The skeleton, composed in part of actin and spectrin, binds and immobilizes significant numbers of AChR, thereby forming the third membrane domain of the AChR cluster. We make several predictions that should distinguish this model of AChR clustering from one that invokes soluble, trophic factors.

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.

Visualization of the cytoplasmic surface of Torpedo postsynaptic membranes by freeze-etch and immunoelectron microscopy

The synapse-specific Mr 43,000 protein (43K protein) and the acetylcholine receptor were visualized by freeze-etch immunoelectron microscopy in preparations of purified Torpedo postsynaptic membranes. Vesicles were immobilized on glass and then sheared open by sonication to expose the cytoplasmic surface. Membranes were labeled with monoclonal antibodies to the 43K protein or the acetylcholine receptor. The cytoplasmic surface was devoid of filamentous structure, and the 43K protein and the cytoplasmic projection of the acetylcholine receptor were associated with prominent surface particles. Acetylcholine receptor and 43K protein, in membrane surfaces in direct contact with glass coated with polyornithine, segregated into dense particle aggregates separated by smooth membrane patches, whereas those in contact with glass coated with Alcian Blue underwent little or no detectable rearrangement. After treatment of vesicles at alkaline pH to remove the 43K protein, the cytoplasmic surfaces were still covered by a dense array of particles that were more uniform in shape and appeared slightly shorter than those seen on unextracted membranes, but similar in height to the extracellular projection. Monoclonal antibodies to the acetylcholine receptor labeled these particles, while antibodies to 43K protein did not. We conclude that the 43K protein is in direct association with the receptor and that complexes of the receptor and 43K protein can undergo surface-induced lateral redistribution. In addition, the cytoplasmic projection of the acetylcholine receptor is sufficiently large to be readily detected by freeze-etch electron microscopy and is similar in height to the extracellular projection.

A postsynaptic Mr 58,000 (58K) protein concentrated at acetylcholine receptor-rich sites in Torpedo electroplaques and skeletal muscle

In the study of proteins that may participate in the events responsible for organization of macromolecules in the postsynaptic membrane, we have used a mAb to an Mr 58,000 protein (58K protein) found in purified acetylcholine receptor (AChR)-enriched membranes from Torpedo electrocytes. Immunogold labeling with the mAb shows that the 58K protein is located on the cytoplasmic side of Torpedo postsynaptic membranes and is most concentrated near the crests of the postjunctional folds, i.e., at sites of high AChR concentration. The mAb also recognizes a skeletal muscle protein with biochemical characteristics very similar to the electrocyte 58K protein. In immunofluorescence experiments on adult mammalian skeletal muscle, the 58K protein mAb labels endplates very intensely, but staining of extrasynaptic membrane is also seen. Endplate staining is not due entirely to membrane infoldings since a similar pattern is seen in neonatal rat diaphragm in which postjunctional folds are shallow and rudimentary, and in chicken muscle, which lacks folds entirely. Furthermore, clusters of AChR that occur spontaneously on cultured Xenopus myotomal cells and mouse muscle cells of the C2 line are also stained more intensely than the surrounding membrane with the 58K mAb. Denervation of adult rat diaphragm muscle for relatively long times causes a dramatic decrease in the endplate staining intensity. Thus, the concentration of this evolutionarily conserved protein at postsynaptic sites may be regulated by innervation or by muscle activity.

300-kD subsynaptic protein copurifies with acetylcholine receptor-rich membranes and is concentrated at neuromuscular synapses

Acetylcholine receptor-rich membranes from the electric organ of Torpedo californica are enriched in the four different subunits of the acetylcholine receptor and in two peripheral membrane proteins at 43 and 300 kD. We produced monoclonal antibodies against the 300-kD protein and have used these antibodies to determine the location of the protein, both in the electric organ and in skeletal muscle. Antibodies to the 300-kD protein were characterized by Western blots, binding assays to isolated membranes, and immunofluorescence on tissue. In Torpedo electric organ, antibodies to the 300-kD protein stain only the innervated face of the electrocytes. The 300-kD protein is on the intracellular surface of the postsynaptic membrane, since antibodies to the 300-kD protein bind more efficiently to saponin-permeabilized, right side out membranes than to intact membranes. Some antibodies against the Torpedo 300-kD protein cross-react with amphibian and mammalian neuromuscular synapses, and the cross-reacting protein is also highly concentrated on the intracellular surface of the post-synaptic membrane.

The relationship of the postsynaptic 43K protein to acetylcholine receptors in receptor clusters isolated from cultured rat myotubes

We have examined the relationship of acetylcholine receptors (AChR) to the Mr 43,000 receptor-associated protein (43K) in the AChR clusters of cultured rat myotubes. Indirect immunofluorescence revealed that the 43K protein was concentrated at the AChR domains of the receptor clusters in intact rat myotubes, in myotube fragments, and in clusters that had been purified approximately 100-fold by extraction with saponin. The association of the 43K protein with clustered AChR was not affected by buffers of high or low ionic strength, by alkaline pHs up to 10, or by chymotrypsin at 10 micrograms/ml. However, the 43K protein was removed from clusters with lithium diiodosalicylate or at alkaline pH (greater than 10). Upon extraction of 43K, several changes were observed in the AChR population. Receptors redistributed in the plane of the muscle membrane in alkali-extracted samples. The number of binding sites accessible to an anti-AChR monoclonal antibody directed against cytoplasmic epitopes (88B) doubled. Receptors became more susceptible to digestion by chymotrypsin, which destroyed the binding sites for the 88B antibody only after 43K was extracted. These results suggest that in isolated AChR clusters the 43K protein covers part of the cytoplasmic domain of AChR and may contribute to the unique distribution of this membrane protein.

The Torpedo electrocyte: a model system for the study of receptor-cytoskeleton interactions

We have used the electrocyte of Torpedo electric organ as a model system for the study of AchR stabilization in the postsynaptic membrane. Attention was focused on membrane cytoskeleton interactions in particular on a peripheral protein of 43 KD that is believed to participate in AchR immobilization. Using immunocytochemical methods, we have shown that the cortical skeleton in Torpedo electrocyte displays a local differentiation proper for each specialized domain of the plasma membrane. In the postsynaptic membrane, characterized by an accumulation and a geometrical organization of the receptors in the plane of the membrane, the 43 KD protein participates in a submembraneous coating or "postsynaptic densities" that strictly codistribute with the AchR. The 43 KD protein might also account for the anchoring of intermediate-sized filaments. The organization of the postsynaptic domain appears readily different from that of the non-innervated one where the membrane folds are maintained by a cortical meshwork of cytoskeletal proteins such as ankyrin, spectrin and oligomeric actin. In conclusion, the asymmetrical organization of the cortical skeleton in the electrocyte offers a unique opportunity for the study of the specific aspects of membrane-skeleton interactions that take place in the postsynaptic domain.

Actin at receptor-rich domains of isolated acetylcholine receptor clusters

Acetylcholine receptor (AChR) clusters of cultured rat myotubes, isolated by extraction with saponin (Bloch, R. J., 1984, J. Cell Biol. 99:984-993), contain a polypeptide that co-electrophoreses with purified muscle actins. A monoclonal antibody against actin reacts in immunoblots with this polypeptide and with purified actins. In indirect immunofluorescence, the antibody stains isolated AChR clusters only at AChR domains, strips of membrane within clusters that are rich in receptor. It also stains the postsynaptic region of the neuromuscular junction of adult rat skeletal muscle. Semiquantitative immunofluorescence analyses show that labeling by antiactin of isolated analyses show that labeling by antiactin of isolated AChR clusters is specific and saturable and that it varies linearly with the amount of AChR in the cluster. Filaments of purified gizzard myosin also bind preferentially at AChR-rich regions, and this binding is inhibited by MgATP. These experiments suggest that actin is associated with AChR-rich regions of receptor clusters. Depletion of actin by extraction of isolated clusters at low ionic strength selectively releases the actin-like polypeptide from the preparation. Simultaneously, AChRs redistribute within the plane of the membrane of the isolated clusters. Similarly, brief digestion with chymotrypsin reduces immunofluorescence staining and causes AChR redistribution. Treatments that deplete AChR from clusters in intact cells also reduce immunofluorescent staining for actin in isolated muscle membrane fragments. Upon reversal of these treatments, cluster reformation occurs in regions of the membrane that also stain for actin. I conclude that actin is associated with AChR domains and that changes in this association are accompanied by changes in the organization of isolated AChR clusters.

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

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

The subsynaptic 43-kDa protein is concentrated at developing nerve-muscle synapses in vitro

A 43-kDa peripheral membrane protein is known to copurify with acetylcholine receptor (AcChoR)-rich membranes isolated from the electric organ of Torpedo californica. Immunoelectron microscopy and crosslinking studies have demonstrated that this 43-kDa protein is closely associated with the cytoplasmic domain(s) of the AcChoR and suggest that the 43-kDa protein could regulate the distribution of the AcChoR in the postsynaptic membrane. This paper demonstrates that this postsynaptic protein appears at developing neuromuscular synapses in Xenopus nerve/muscle cocultures as early as AcChoRs become clustered at synaptic sites. Moreover, this protein is concentrated at AcChoR clusters that occur on noninnervated muscle cells. The close spatial and temporal relationship of this subsynaptic protein and AcChoR clusters is consistent with a role for the 43-kDa protein in the formation and/or stabilization of AcChoR clusters.

Association of the postsynaptic 43K protein with newly formed acetylcholine receptor clusters in cultured muscle cells

The postsynaptic membrane from Torpedo electric organ contains, in addition to the acetylcholine receptor (AChR), a major peripheral membrane protein of approximately 43,000 mol wt (43K protein). Previous studies have shown that this protein is closely associated with AChR and may be involved in anchoring receptors to the postsynaptic membrane. In this study, binding sites for monoclonal antibodies (mabs) to the 43K protein have been compared to the distribution of AChR in Xenopus laevis muscle cells in culture. In double label immunofluorescence experiments, clusters of AChR that occur spontaneously on these cells were stained with anti-43K mabs. Newly formed receptor clusters induced with positive polypeptide-coated latex beads were also stained with anti-43K mabs as early as 12 h after the application of the beads. Exact correspondence in the distribution of the anti-43K protein binding sites and the AChR was found in both types of clusters. These results suggest that the 43K protein becomes associated with AChR clusters during a period of active postsynaptic membrane differentiation. Thus, this protein may participate in the clustering process.

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

Two monoclonal antibodies (mabs) previously prepared against Torpedo acetylcholine receptor are shown to recognize a synthetic nonadecapeptide corresponding to lys360-glu377 of the gamma subunit. The reaction was demonstrated by solid-phase enzyme-linked immunoabsorbent assays, by inhibition of binding of the mabs to receptor, and by immunoprecipitation of the peptide conjugated to bovine serum albumin. Immunogold electron microscopy on isolated postsynaptic membranes from Torpedo showed that both mabs bind to intracellular epitopes on the receptor. These results establish that amino acid residues 360-377 of the receptor gamma-subunit, and probably the analogous region of the delta-subunit, reside on the cytoplasmic side of the membrane. Since the primary structures of all four subunits suggest a common transmembrane arrangement, the corresponding domains of the alpha- and beta-subunits are probably also cytoplasmic.

Interaction of the 43K protein with components of Torpedo postsynaptic membranes

Interactions of the major Mr 43 000 peripheral membrane protein (43K protein) with components of Torpedo postsynaptic membranes have been examined. Treatment of membranes with copper o-phenanthroline promotes the polymerization of 43K protein to dimers and higher oligomers. These high molecular weight forms of 43K protein can be converted to monomers by reduction with dithiothreitol and do not contain any of the other major proteins found in these membranes, including the subunits of the acetylcholine receptor, as shown by immunoblotting with monoclonal antibodies. To study directly its interactions with the membrane, the 43K protein was radioiodinated and purified by immunoaffinity chromatography. Purified 43K protein binds tightly to pure liposomes of various compositions in a manner that is not inhibited by KCl concentrations up to 0.75 M. The binding can be reversed by adjusting the pH of the reaction to 11, the same treatment that removes 43K protein from postsynaptic membranes. Unlabeled 43K protein solubilized from Torpedo membranes with cholate can be reconstituted with exogenously added lipids in the absence of the receptor. The results suggest that 43K protein molecules are amphipathic and that they may interact with each other and with the lipid bilayer. These interactions cannot explain the coextensive distribution of 43K proteins with acetylcholine receptors in situ. However, they could account for the association of the 43K protein with the postsynaptic membrane and may contribute to the maintenance of the structure of the cytoplasmic specialization of which this protein is a major component.