Structural details of membrane-bound acetylcholine receptor from Tropedo marmorata

Structure and function meet at the nicotinic acetylcholine receptor-lipid interface

The nicotinic acetylcholine receptor (nAChR) is a transmembrane protein that mediates fast intercellular communication in response to the endogenous neurotransmitter acetylcholine. It is the best characterized and archetypal molecule in the superfamily of pentameric ligand-gated ion channels (pLGICs). As a typical transmembrane macromolecule, it interacts extensively with its vicinal lipid microenvironment. Experimental evidence provides a wealth of information on receptor-lipid crosstalk: the nAChR exerts influence on its immediate membrane environment and conversely, the lipid moiety modulates ligand binding, affinity state transitions and gating of ion translocation functions of the receptor protein. Recent cryogenic electron microscopy (cryo-EM) studies have unveiled the occurrence of sites for phospholipids and cholesterol on the lipid-exposed regions of neuronal and electroplax nAChRs, confirming early spectroscopic and affinity labeling studies demonstrating the close contact of lipid molecules with the receptor transmembrane segments. This new data provides structural support to the postulated "lipid sensor" ability displayed by the outer ring of M4 transmembrane domains and their modulatory role on nAChR function, as we postulated a decade ago. Borrowing from the best characterized nAChR, the electroplax (muscle-type) receptor, and exploiting new structural information on the neuronal nAChR, it is now possible to achieve an improved depiction of these sites. In combination with site-directed mutagenesis, single-channel electrophysiology, and molecular dynamics studies, the new structural information delivers a more comprehensive portrayal of these lipid-sensitive loci, providing mechanistic explanations for their ability to modulate nAChR properties and raising the possibility of targetting them in disease.

Single-Particle Reconstruction of Biological Molecules-Story in a Sample (Nobel Lecture)

Pictures tell a thousand words: The development of single-particle cryo-electron microscopy set the stage for high-resolution structure determination of biological molecules. In his Nobel lecture, J. Frank describes the ground-breaking discoveries that have enabled the development of cryo-EM. The method has taken biochemistry into a new era.

High-Affinity α-Conotoxin PnIA Analogs Designed on the Basis of the Protein Surface Topography Method

Despite some success for small molecules, elucidating structure-function relationships for biologically active peptides - the ligands for various targets in the organism - remains a great challenge and calls for the development of novel approaches. Some of us recently proposed the Protein Surface Topography (PST) approach, which benefits from a simplified representation of biomolecules' surface as projection maps, which enables the exposure of the structure-function dependencies. Here, we use PST to uncover the "activity pattern" in α-conotoxins - neuroactive peptides that effectively target nicotinic acetylcholine receptors (nAChRs). PST was applied in order to design several variants of the α-conotoxin PnIA, which were synthesized and thoroughly studied. Among the best was PnIA[R9, L10], which exhibits nanomolar affinity for the α7 nAChR, selectivity and a slow wash-out from this target. Importantly, these mutations could hardly be delineated by "standard" structure-based drug design. The proposed combination of PST with a set of experiments proved very efficient for the rational construction of new bioactive molecules.

Single-particle reconstruction of biological macromolecules in electron microscopy--30 years

This essay gives the autho's personal account on the development of concepts underlying single-particle reconstruction, a technique in electron microscopy of macromolecular assemblies with a remarkable record of achievements as of late. The ribosome proved to be an ideal testing ground for the development of specimen preparation methods, cryo-EM techniques, and algorithms, with discoveries along the way as a rich reward. Increasingly, cryo-EM and single-particle reconstruction, in combination with classification techniques, is revealing dynamic information on functional molecular machines uninhibited by molecular contacts.

Structural answers and persistent questions about how nicotinic receptors work

The electron diffraction structure of nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata and the X-ray crystallographic structure of acetylcholine binding protein (AChBP) are providing new answers to persistent questions about how nAChRs function as biophysical machines and as participants in cellular and systems physiology. New high-resolution information about nAChR structures might come from advances in crystallography and NMR, from extracellular domain nAChRs as high fidelity models, and from prokaryotic nicotinoid proteins. At the level of biophysics, structures of different nAChRs with different pharmacological profiles and kinetics will help describe how agonists and antagonists bind to orthosteric binding sites, how allosteric modulators affect function by binding outside these sites, how nAChRs control ion flow, and how large cytoplasmic domains affect function. At the level of cellular and systems physiology, structures of nAChRs will help characterize interactions with other cellular components, including lipids and trafficking and signaling proteins, and contribute to understanding the roles of nAChRs in addiction, neurodegeneration, and mental illness. Understanding nAChRs at an atomic level will be important for designing interventions for these pathologies.

The electron microscopy of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is a glycoprotein occurring in the electric tissue of the electric ray Torpedo sp. and the electric eel Electrophorus electricus in postsynaptic membranes in high densities. Since these membranes can be easily prepared they have been, since their discovery, a favourable object for electron microscopists. The receptor protein appears in negatively stained membranes as a ring with a diameter of about 75 A. With improved techniques of preparing membranes which contain the receptor molecules in two-dimensional crystalline arrays and especially with computer-aided image processing, the ring appeared as an arrangement of five maxima (representing probably the five receptor subunits) with a five-fold axis of pseudosymmetry perpendicular to the membrane plane. At present the resolution obtained is better than 20 A, enough to depict the receptor's overall shape and dimensions but not enough to resolve functional moieties, as for example the selectivity filter and the gating device of the ion channel, which is an integral part of the receptor complex. The receptor-rich membranes turned out to be models for developing and comparing image processing methods. In this article some of these methods, especially the Circular Harmonic Averaging method, are critically reviewed.

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.

Insulin receptor protein rendered visible in triton X-114 membranes

Insulin receptor protein has been visualized by electron microscopy. This was rendered possible after the molecules had been extensively purified and had been reinserted into Triton X-114 membranes in a situation similar to that found in biological membranes. Freeze fracturing then revealed the insulin receptor molecules as particles randomly distributed in the fracture plane of the artificial membranes. The sizes of the particles are in the range expected from biochemical data.

Quaternary structure of the acetylcholine receptor

The five membrane-spanning subunits of the acetylcholine receptor have been resolved in electron microscope images and are shown to lie at pentagonally symmetrical positions around the channel over a large fraction of their length. The channel consists of a wide synaptic portion and a narrow portion extending through the membrane into the interior of the cell.

Evidence that receptors mediating central synaptic potentials extend beyond the postsynaptic density

Physiological recordings and computer simulations of unitary inhibitory postsynaptic potentials in the Mauthner cell of the goldfish central nervous system have been used to estimate the expected size of the postsynaptic receptor matrix at individual junctions. Simultaneous pre- and postsynaptic recordings were used to determine the kinetic parameters of the quantal responses under normal conditions and in the presence of strychnine, a competitive antagonist of glycine, which is the putative transmitter at these synapses. Calculations indicate that if the postsynaptic density, which has a radius of 0.1 micron, were to accommodate the population of channels estimated to be opened during a quantal response, the glycine binding site density in that region would be unrealistically high. Computer simulation of the quantal responses included transmitter diffusion, transmitter-receptor interactions, and channel activation under conditions including both normal and lowered binding site densities, the latter corresponding to the experimental data obtained with strychnine. The data indicate that the synaptic receptors involved in generating unitary responses are widely distributed to include regions located outside the junctional area, which directly faces the presynaptic release sites. We further suggest that the receptor matrix is surrounded by a restricted diffusional space; this geometrical organization may underlie the finding that response rise times are relatively independent of receptor binding site densities.

Tubular crystals of acetylcholine receptor

Well-ordered tubular crystals of acetylcholine receptor were obtained from suspensions of Torpedo marmorata receptor-rich vesicles. They are composed of pairs of oppositely oriented molecules arranged on the surface lattice with the symmetry of the plane group p2 (average unit cell dimensions: a = 90 A, b = 162 A, gamma = 117 degrees). The receptor in this lattice has an asymmetric distribution of mass around its perimeter, yet a regular pentagonal shape; thus its five transmembrane subunits appear to have different lengths, but approximately equal cross sections. The tubes grow by lateral aggregation on the vesicle surface of ribbons of the paired molecules. Both ribbons and tubes were sensitive to dispersal by the disulphide reductant, dithiothreitol. This observation and other evidence suggest that the basic pairing interaction in the tubes may be that of the physiological dimer, involving contact between delta-subunits.

Functional and structural analysis of acetylcholine receptor-rich membranes after negative staining

Phosphotungstate (pH 7.4) used for negative staining of membranes from Torpedo electric tissue rich in acetylcholine receptor does not affect binding properties and cation permeability of the receptor and its ion channel. Uranyl salts, frequently used for negative staining, precipitate the receptor-rich membranes making measurements of ligand binding and ion-permeability regulation impossible. The gross ultrastructure in the two stains is not significantly different, but for future high-resolution electron microscopy aiming at visualizing structural details of functional receptor molecules it is necessary to resort to a stain preserving native and active receptor. Uranyl salts are not applicable for this purpose. The electron micrographs obtained with phosphotungstate reveal two distinct structures in the receptor-rich membrane: a closed ring ('doughnut') and an open ring ('horseshoe'), with a ratio of abundance of about 3:2.

Statistical significance of molecule projections by single particle averaging

A statistical test based on a multiple comparison of Friedman's rank sums has been developed to determine the statistical significance of molecule projections obtained by non-crystallographic averaging techniques. The test, applied to electron microscope images of the acetylcholine receptor molecule, confirms the significance and quoted resolution of averages by Zingsheim et al. (1980, 1982a, b).

Conformational properties of the neurotoxins and cytotoxins isolated from Elapid snake venoms

The review will critically assess the information available on the conformation of homologous neurotoxins and cytotoxins isolated from Elapid snakes. Particular attention will be given to the dynamics of the molecules in solution because there is the possibility that defined intramolecular rearrangements are involved at the sites of action. Such properties will be then reconciled with the known X-ray crystallographic and sequence data in order to derive likely structure-activity relationships.

Computer averaging of single molecules of alpha 2-macroglobulin and the alpha 2-macroglobulin/trypsin complex

From electron micrographs single molecules of alpha 2-macroglobulin in the "closed" form, the "open" form and as the trypsin complex have been computer averaged. The molecular images are discussed. Molecules of the electrophoretically fast migrating "F-form" have the "closed" form. In the case of the alpha 2-macroglobulin/trypsin complex the two attached trypsin molecules are located very near to each other and in the central part of the alpha 2-macroglobulin molecule.

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

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

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

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

Three-dimensional structure of proteins determined by electron microscopy

Recent developments in specimen preparation and image processing techniques have made it possible to determine the three-dimensional structure of proteins by electron microscopy. Periodic supramolecular aggregates of the protein under investigation are requiring to minimize radiation damage and to maximize the signal-to-noise ratio of structural detail. Useful information about the fine structure of the protein (e.g. binding sites for interacting molecules, antigenic determinants) can often be obtained by stoichiometric labeling of the ordered arrays with interacting molecules or antibody fragments, and computing difference maps from the reconstructions of the labeled and native structures. The use of this approach to molecular structure determination of proteins will be discussed in light of our work with bacteriophage and actin.

Rotational molecular dynamics of the membrane-bound acetylcholine receptor revealed by phosphorescence spectroscopy

The rotational mobility of the membrane-bound acetylcholine receptor from Torpedo marmorata has been studied by phosphorescence anisotropy techniques using eosin-5'-isothiocyanate and eosin-5'-iodoacetamide derivatives of the alpha-neurotoxin of Bungarus multicinctus (alpha-bungarotoxin). Normal membranes, those depleted of nonreceptor peripheral peptides by alkaline treatment, as well as membranes subjected to various chemical modifications of the thiol groups, have been characterized. Rotational correlation times (10--26 mus) compatible with the motion of individual 9-S monomeric receptor species of Mr 250 000 were observed upon reduction of the membranes with dithiothreitol or by raising the temperature to 39 degrees C in the case of alkaline-treated membranes. Membranes prepared throughout in N-ethylmaleimide, which yield upon detergent solubilization a predominant 13-S dimeric species and which are not depleted of the nonreceptor v-peptide (Mr 43 000) by alkaline treatment, are relatively more 'rigid' than normal membranes and do not show the anisotropy components characteristic of the monomeric receptor. The influence of the v-peptide on the structural stability of the receptor oligomeric forms and the thiol-dependent interconversions are thus reflected in the time-dependent spectroscopic measurements.

Molecular forms and hydrodynamic properties of acetylcholine receptor from electric tissue

We have studied purified acetylcholine receptor proteins from Electrophorus electricus and Torpedo marmorata which function in both binding and reconstitution experiments. The molecular properties of these receptor-channel complexes were analyzed under non-denaturing conditions by polyacrylamide gradient gel electrophoresis, ultracentrifugation sedimentation studies and laser light scattering. The purified receptor proteins exist in different interconvertible forms depending on both the type and concentration of detergent present, and the presence or absence of an intersubunit disulfide bridge. Receptor purified in the absence of sulfhydryl-blocking agents exists in two monomeric and two dimeric forms at very low detergent concentrations (0.01-0.05% Tween 80). At intermediate detergent concentrations (0.4% Triton X-100) one monomeric and one dimeric form are present. Only the monomeric form remains at high levels of detergent (2% Triton X-100). This form has a sedimentation coefficient of 9.29 S, as measured by ultracentrifugation using Schlieren optics. If receptor is purified in the presence of sulfhydryl-blocking agents, conversion of dimers into monomers by high concentrations of detergent does not occur. Disulfide-reducing agents convert dimers into monomers independent of whether the receptor's free sulfhydryl groups are blocked or not. These findings imply that dimer formation is primarily due to hydrophobic interactions between monomers. When these interactions are reduced by high levels of detergent the intersubunit disulfide bridge is dissociated at the expense of newly formed interasubunit ones, as long as the sulfhydryl group(s) have remained unblocked. Monomers and dimers bind alpha-cobratoxin with the same affinity and kinetics. Dimers of disulfide-linked monomers are not required for the reconstitution of a functional ion-translocation system. The presence of EDTA during the purification of receptor proteins (as recommended by many groups to inhibit proteolysis) adversely influences the activity of the receptor in channel gating. These observations are discussed in terms of the requirements for a purification procedure yielding receptor preparations unaffected by proteolysis and functioning in both binding and ion translocation. The molecular weight of the receptor monomer was determined by several independent techniques and yielded values in the range of 250-300 x 10(3). With a translational diffusion constant D20, w = 2.95 x 10(-7) cm2 s-1 and a sedimentation coefficient s20, w = 9.29 S, the frictional coefficient ratio f/f0 = 1.51 was calculated for the receptor monomer from Torpedo marmorata. This indicates a considerable asymmetry in the shape of the detergent-solubilized receptor.

Structural models of the nicotinic acetylcholine receptor and its toxin-binding sites

Models of the protein structure of agonist-, competitive antagonist-, and snake neurotoxin-binding sites were designed using the sequence of the first 54 residues of the acetylcholine receptor (AChR) alpha subunit from Torpedo californica. These models are based on the premise that the N-terminal portions of the subunits form the outermost extracellular surface of the AChR and that agonists bind to this portion. The models were developed by predicting the secondary structure of the alpha-subunit N-terminal segment from its sequence, then using these predictions to fold the segment into tertiary structures that should bind snake neurotoxins, agonists, and antagonists. Possible gating mechanisms and quaternary structures are suggested by the proposed tertiary structures of the subunits. Experiments are suggested to test aspects of the models.

Consequences of alkaline treatment for the ultrastructure of the acetylcholine-receptor-rich membranes from Torpedo marmorata electric organ

After fixation with glutaraldehyde and impregnation with tannic acid, the membrane that underlies the nerve terminals in Torpedo marmorata electroplaque presents a typical asymmetric triple-layered structure with an unusual thickness; in addition, it is coated with electron-dense material on its inner, cytoplasmic face. Filamentous structures are frequently found attached to these "subsynaptic densities." The organization of the subsynaptic membrane is partly preserved after homogenization of the electric organ and purification of acetylcholine-receptor (AchR)-rich membrane fragments. In vitro treatment at pH 11 and 4 degrees C of these AchR-rich membranes releases an extrinsic protein of 43,000 mol wt and at the same time causes the complete disappearance of the cytoplasmic condensations. Freeze-etching of native membrane fragments discloses remnants of the ribbonlike organization of the AchR rosettes. This organization disappears ater alkaline treatment and is replaced by a network which is not observed after rapid freezing and, therefore, most likely results from the lateral redistribution of the AchR rosettes during condition of slow freezing. A dispersion of the AchR rosettes in the plane of the membrane also occurs after fusion of the pH 11-treated fragments with phospholipid vesicles. These results are interpreted in terms of a structural stabilization and immobilization of the AchR by the 43,000-Mr protein binding to the inner face of the subsynaptic membrane.

Crystalline arrays of membrane-bound acetylcholine receptor

Electron micrographs of tubular structures with a crystalline arrangement of membrane-bound acetylcholine receptor oligomers have been analyzed by digital image reconstruction. The receptor molecules are oriented synaptic side out, and in projection they appear to be asymmetric and have a defined orientation. All four subunits are contained in the oligomers as demonstrated by immunoelectron microscopy; these structures therefore appear to be suitable for subunit localization in the oligomer.

Functional reassembly of membrane proteins in planar lipid bilayers

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