Functional stability of Torpedo acetylcholine receptor. Effects of protease treatment

The Cys-loop pentameric ligand-gated ion channel receptors: 50 years on

This year, 2011, the Department of Pharmacology at the University of Alberta celebrated its 50th anniversary. This timeframe covers nearly the entire history of Cys-loop pentameric ligand-gated ion channel (pLGIC) research. In this review we consider how major technological advancements affected our current understanding of pLGICs, and highlight the contributions made by members of our department. The individual at the center of our story is Susan Dunn; her passing earlier this year has robbed the Department of Pharmacology and the research community of a most insightful colleague. Her dissection of ligand interactions with the nAChR, together with their interpretation, was the hallmark of her extensive collaborations with Michael Raftery. Here, we highlight some electrophysiological studies from her laboratory over the last few years, using the technique that she introduced to the department in Edmonton, the 2-electrode voltage-clamp of Xenopus oocytes. Finally, we discuss some single-channel studies of the anionic GlyR and GABA(A)R that prefaced the introduction of this technique to her laboratory.

T Cells and Cytokines in the Pathogenesis of Acquired Myasthenia Gravis

Although the symptoms of myasthenia gravis (MG) and experimental MG (EAMG) are caused by autoantibodies, CD4(+) T cells specific for the target antigen, the nicotinic acetylcholine receptor, and the cytokines they secrete, have an important role in these diseases. CD4(+) T cells have a pathogenic role, by permitting and facilitating the synthesis of high-affinity anti-AChR antibodies. Th1 CD4(+) cells are especially important because they drive the synthesis of anti-AChR complement-fixing IgG subclasses. Binding of those antibodies to the muscle AChR at the neuromuscular junction will trigger the complement-mediated destruction of the postsynaptic membrane. Thus, IL-12, a crucial cytokine for differentiation of Th1 cells, is necessary for development of EAMG. Th2 cells secrete different cytokines, with different effects on the pathogenesis of EAMG. Among them, IL-10, which is a potent growth and differentiation factor for B cells, facilitates the development of EAMG. In contrast, IL-4 appears to be involved in the differentiation of AChR-specific regulatory CD4(+) T cells, which can prevent the development of EAMG and its progression to a self-maintaining, chronic autoimmune disease. Studies on the AChR-specific CD4(+) cells commonly present in the blood of MG patients support a crucial role of CD4(+) T cells in the development of MG. Circumstantial evidence supports a pathogenic role of IL-10 also in human MG. On the other hand, there is no direct or circumstantial evidence yet indicating a role of IL-4 in the modulatory or immunosuppressive circuits in MG.

The effect of proteolytic enzymes on the alpha9-nicotinic receptor-mediated response in isolated frog vestibular hair cells

In frog vestibular organs, efferent neurons exclusively innervate type II hair cells. Acetylcholine, the predominant efferent transmitter, acting on acetylcholine receptors of these hair cells ultimately inhibits and/or facilitates vestibular afferent firing. A coupling between alpha9-nicotinic acetylcholine receptors (alpha9nAChR) and apamin-sensitive, small-conductance, calcium-dependent potassium channels (SK) is thought to drive the inhibition by hyperpolarizing hair cells thereby decreasing their release of transmitter onto afferents. The presence of alpha9nAChR in these cells was demonstrated using pharmacological, immunocytochemical, and molecular biological techniques. However, fewer than 10% of saccular hair cells dissociated using protease VIII, protease XXIV, or papain responded to acetylcholine during perforated-patch clamp recordings. When present, these responses were invariably transient, small in amplitude, and difficult to characterize. In contrast, the majority of saccular hair cells ( approximately 90%) dissociated using trypsin consistently responded to acetylcholine with an increase in outward current and concomitant hyperpolarization. In agreement with alpha9nAChR pharmacology obtained in other hair cells, the acetylcholine response in saccular hair cells was reversibly antagonized by strychnine, curare, tetraethylammonium, and apamin. Brief perfusions with either protease or papain permanently abolished the alpha9-nicotinic response in isolated saccular hair cells. These enzymes when inactivated became completely ineffective at abolishing the alpha9-nicotinic response, suggesting an enzymatic interaction with the alpha9nAChR and/or downstream effector. The mechanism by which these enzymes render saccular hair cells unresponsive to acetylcholine remains unknown, but it most likely involves proteolysis of alpha9nAChR, SK, or both.

Absence of IFN-gamma or IL-12 has different effects on experimental myasthenia gravis in C57BL/6 mice

Immunization with acetylcholine receptor (AChR) causes experimental myasthenia gravis (EMG). Th1 cells facilitate EMG development. IFN-gamma and IL-12 induce Th1 responses: we investigated whether these cytokines are necessary for EMG development. We immunized wild-type (WT) C57BL/6 mice and IFN-gamma and IL-12 knockout mutants (IFN-gamma-/-, IL-12-/-) with Torpedo AChR (TAChR). WT and IFN-gamma-/- mice developed EMG with similar frequency, IL-12-/-mice were resistant to EMG. All strains synthesized anti-AChR Ab that were not IgM or IgE. WT mice had anti-AChR IgG1, IgG2b, and IgG2c, IFN-gamma-/- mice had significantly less IgG2c, and IL-12-/- mice less IgG2b and IgG2c. All mice had IgG bound to muscle synapses, but only WT and IFN-gamma-/- mice had complement; WT mice had both IgG2b and IgG2c, IFN-gamma-/- only IgG2b, and IL-12-/- neither IgG2b nor IgG2c. CD4+ cells from all AChR-immunized mice proliferated in response to AChR and recognized similar epitopes. After stimulation with TAChR, CD4+ cells from IFN-gamma-/- mice secreted less IL-2 and similar amounts of IL-4 and IL-10 as WT mice. CD4+ cells from IL-12-/- mice secreted less IFN-gamma, but more IL-4 and IL-10 than WT mice, suggesting that they developed a stronger Th2 response to TAChR. The EMG resistance of IL-12-/- mice is likely due to both reduction of anti-TAChR Ab that bind complement and sensitization of modulatory Th2 cells. The reduced Th1 function of IFN-gamma-/- mice does not suffice to reduce all complement-fixing IgG subclasses, perhaps because as in WT mice a protective Th2 response is missing.

A nicotinic acetylcholine receptor regulating cell adhesion and motility is expressed in human keratinocytes

Acetylcholine is synthesized and released by human epidermal keratinocytes and modulates the adhesion and motility of these cells. To understand the molecular basis of the effects of acetylcholine on keratinocytes, we investigated the presence, pharmacology, structure, and function of nicotinic acetylcholine receptors in human epidermal keratinocytes. Patch-clamp studies indicated that keratinocytes express acetylcholine receptors with ion gating and pharmacologic properties similar to those observed so far only in neurons, and containing the alpha 3 subunit. Specific binding of the receptor-specific ligand 125I-kappa-bungarotoxin revealed approximately 5500 binding sites per cell on undifferentiated keratinocytes in cell cultures and approximately 35,400 binding sites per cell on mature keratinocytes freshly isolated from human neonatal foreskins. Antibody binding and polymerase chain reaction experiments demonstrated the presence of alpha 3, beta 2, and beta 4 nicotinic receptor subunits. Binding of subunit-specific antibodies indicated that nicotinic receptors were associated with the suprabasal keratinocytes in epidermis and localized to the cell membranes of differentiated keratinocytes in cell cultures. Acetylcholine and the nicotinic agonist nicotine increased cell-substrate and cell-cell adherence of cultured keratinocytes and stimulated their lateral migration. The specific antagonists kappa-bungarotoxin and mecamylamine caused cell detachment and abolished migration. Thus, a nicotinic receptor expressed in keratinocytes may mediate acetylcholine control of keratinocyte adhesion and motility.

Evidence that the atypical 5-HT3 receptor ligand, [3H]-BRL46470, labels additional 5-HT3 binding sites compared to [3H]-granisetron

1. The radioligand binding characteristics of the 3H-derivative of the novel 5-HT3 receptor antagonist BRL46470 were investigated and directly compared to the well characterized 5-HT3 receptor radioligand [3H]-granisetron, in tissue homogenates prepared from rat cerebral cortex/hippocampus, rat ileum, NG108-15 cells, HEK-5-HT3As cells and human putamen. 2. In rat cerebral cortex/hippocampus, rat ileum, NG108-15 cell and HEK-5-HT3As cell homogenates, [3H]-BRL46470 bound with high affinity (Kd (nM): 1.57 +/- 0.18, 2.49 +/- 0.30, 1.84 +/- 0.27, 3.46 +/- 0.36, respectively; mean +/- s.e. mean, n = 3-4) to an apparently homogeneous saturable population of sites (Bmax (fmol mg-1 protein): 102 +/- 16, 44 +/- 4, 968 +/- 32 and 2055 +/- 105, respectively; mean +/- s.e. mean, n = 3-4) but failed to display specific binding in human putamen homogenates. 3. In the same homogenates of rat cerebral cortex/hippocampus, rat ileum, NG108-15 cells, HEK-5-HT3As cells and human putamen as used for the [3H]-BRL46470 studies, [3H]-granisetron also bound with high affinity (Kd (nM): 1.55 +/- 0.61, 2.31 +/- 0.44, 1.89 +/- 0.36, 2.03 +/- 0.42 and 6.46 +/- 2.58 respectively; mean +/- s.e. mean, n = 3-4) to an apparently homogeneous saturable population of sites (Bmax (fmol mg-1 protein): 39 +/- 4, 20 +/- 2, 521 +/- 47, 870 +/- 69 and 18 +/- 2, respectively; mean +/- s.e. mean, n = 3-4). 4. Competition studies with a range of structurally different 5-HT3 receptor ligands indicated that in both rat cerebral cortex/hippocampus and rat ileum homogenates, [3H]-BRL46470 binding exhibited a pharmacological profile consistent with the labelling the 5-HT3 receptor with compounds competing with Hill coefficients close to unity.5 In HEK-5-HT3As cell homogenates, [3H]-BRL46470 and [3H]-granisetron associated rapidly((3.84+/-0.4)106 M-1S-1 and (5.85+/-0.2)106 M-1S-1, respectively, mean+/-s.e.mean, n=3-4) in an apparently monophasic manner. Following the establishment of equilibrium, both [3H]-BRL46470 and [3H]-granisetron at a saturating concentration ([3H]-BRL46470 approximately 16 nM; [3H]-granisetron approximately 18 nM) and at a sub-Kd concentration (approximately 1 nm for both radioligands)dissociated biphasically in HEK-5-HT3As cell homogenates (saturating concentration; [3H]-BRL464704.05 x 10-3+/-2.53 x I0-3 s-1 and 5.83 x 10-5+0.91 x I0-5 s-1; [3H]-granisetron 3.20 x 10-3+ 1.70 x IO-3 s-1 and18.58 x 10-5 +/- 4.19 x I0-5 s-1: sub-Kd concentration; [3H]-BRL46470 2.47 x 10-3+/- 1.18 x 10-3 s-1 and 9.30x 10-5+/-2.59x 10-5 S-1; [3H]-granisetron 65.91 x 10-3+/-22.14x I0-3 s-1 and 49.96x 10-5+/-12.26x 10-5s- 1 mean+/- s.e.mean, n = 4-8) when induced by a 300 fold dilution in ice-cold Tris/Krebs.6 In conclusion, the present study provides evidence that [3H]-BRL46470 specifically labels the 5-HT3receptor in rat cerebral cortex/hippocampus, rat ileum, NG108-15 cell and HEK-5-HT3As cell homogenates, but fails to label the 5-HT3 receptor expressed in human putamen. Whilst the pharmacological profile of the site labelled by [3H]-BRL46470 is directly comparable to that labelled by [3H]-granisetron, [3H]-BRL46470 consistently labelled approximately twice the density of sites compared to [3H]-granisetron in the same tissue homogenates prepared from rat cortex/hippocampus, ratileum, NG108-15 cells and HEK-5-HT3As cells.

Nonequivalence of alpha-bungarotoxin binding sites in the native nicotinic receptor molecule

In the native, membrane-bound form of the nicotinic acetylcholine receptor (M-AcChR) the two sites for the cholinergic antagonist alpha-bungarotoxin (alpha-BGT) have different binding properties. One site has high affinity, and the M-AcChR/alpha-BGT complexes thus formed dissociate very slowly, similar to the complexes formed with detergent-solubilized AcChR (S-AcChR). The second site has much lower affinity (KD approximately 59 +/- 35 nM) and forms quickly reversible complexes. The nondenaturing detergent Triton X-100 is known to solubilize the AcChR in a form unable, upon binding of cholinergic ligands, to open the ion channel and to become desensitized. Solubilization of the AcChR in Triton X-100 affects the binding properties of this second site and converts it to a high-affinity, slowly reversible site. Prolonged incubation of M-AcChR at 4 degrees C converts the low-affinity site to a high-affinity site similar to those observed in the presence of Triton X-100. Although the two sites have similar properties when the AcChR is solubilized in Triton X-100, their nonequivalence can be demonstrated by the effect on alpha-BGT binding of concanavalin A, which strongly reduces the association rate of one site only. The Bmax of alpha-BGT to either Triton-solubilized AcChR or M-AcChR is not affected by the presence of concanavalin A. Occupancy of the high-affinity, slowly reversible site in M-AcChR inhibits the Triton X-100 induced conversion to irreversibility of the second site. At difference with alpha-BGT, the long alpha-neurotoxin from Naja naja siamensis venom (alpha-NTX) binds with high affinity and in a very slowly reversible fashion to two sites in the M-AcChR (Conti-Tronconi & Raftery, 1986). We confirm here that Triton-solubilized AcChR or M-AcChR binds in a very slowly reversible fashion the same amount of alpha-NTX.(ABSTRACT TRUNCATED AT 250 WORDS)

Two-dimensional 1H-NMR study of synthetic peptides containing the main immunogenic region of the Torpedo acetylcholine receptor

A comparative 1H-NMR spectral study of a synthetic decapeptide containing the main immunogenic region of the Torpedo acetylcholine receptor (AChR; WNPADYGGIK, representing the alpha 67-76 fragment of Torpedo AChR) with four analogous peptides (WNP3-D5YGGIK, WNPAA5YGGIK, WNPADYGGA9K, and WNPD4DYGGV9K) has been carried out in dimethyl sulfoxide. One- and two-dimensional nmr experiments [correlated spectroscopy (COSY), relayed COSY, and phase-sensitive nuclear Overhauser enhancement spectroscopy (NOESY)] were performed to obtain complete assignments of the proton resonances. The presence of strong and multiple short- and long-range NOEs, and especially a strong long-range NOE between the two Asn2-C alpha H and Gly7-C alpha H protons, argues in favor of a rigid folded structure in all five cases. Temperature dependence measurements indicate the existence of three intramolecular interactions involving the Asp3, Gly8, and Lys10 amide protons.

Identification of subunits of acetylcholine receptor that interact with a cholesterol photoaffinity probe

All four subunits of the acetylcholine receptor in membrane vesicles isolated from Torpedo californica have been labeled with [3H]cholesteryl diazoacetate. As this probe incorporates into lipid bilayers analogously to cholesterol, this result indicates that acetylcholine receptor interacts with cholesterol. This investigation also demonstrates that this probe is a useful reagent for studying the interaction of cholesterol with membrane proteins.

Acetylcholine receptor dimers are stabilized by extracellular disulfide bonding

Torpedo acetylcholine receptor (AcChR) exists predominantly as dimers, formed by two monomers held together by a disulfide bridge(s). The dimers are easily cleaved to monomers by reducing agents. 2-mercaptoethanesulfonic acid is shown to be a membrane-impermeant reducing agent which cleaves receptor dimers when it is present only on the outside of intact membrane vesicles. There is no increase in the extent of cleavage when 2-mercaptoethanesulfonic acid is also loaded inside the vesicles. Therefore the disulfide bond(s) involved in the dimerization of the Torpedo acetylcholine receptor is (are) formed by cysteine residues which are exposed on the extracellular side of the membrane.

Subcellular localization of creatine kinase in Torpedo electrocytes: association with acetylcholine receptor-rich membranes

Creatine kinase (CK, EC 2.7.3.2) has recently been identified as the intermediate isoelectric point species (pl 6.5-6.8) of the Mr 40,000-43,000 nonreceptor, peripheral v-proteins in Torpedo marmorata acetylcholine receptor-rich membranes (Barrantes, F. J., G. Mieskes, and T. Wallimann, 1983, Proc. Natl. Acad. Sci. USA, 80: 5440-5444). In the present study, this finding is substantiated at the cellular and subcellular level of the T. marmorata electric organ by immunofluorescence and by protein A-gold labeling of either ultrathin cryosections of electrocytes or purified receptor-membrane vesicles that use subunit-specific anti-chicken creatine kinase antibodies. The muscle form of the kinase, on the one hand, is present throughout the entire T. marmorata electrocyte except in the nuclei. The brain form of the kinase, on the other hand, is predominantly located on the ventral, innervated face of the electrocyte, where it is closely associated with both surfaces of the postsynaptic membrane, and secondarily in the synaptic vesicles at the presynaptic terminal. Labeling of the noninnervated dorsal membrane is observed at the invaginated sac system. In the case of purified acetylcholine receptor-rich membranes, antibodies specific for chicken B-CK label only one face of the isolated vesicles. No immunoreaction is observed with anti-chicken M-CK antibodies. A discussion follows on the possible implications of these localizations of creatine kinase in connection with the function of the acetylcholine receptor at the postsynaptic membrane, the Na/K ATPase at the dorsal electrocyte membrane, and the ATP-dependent transmitter release at the nerve ending.

Image analysis of the heavy form of the acetylcholine receptor from Torpedo marmorata

The structure of the heavy (H) form of the acetylcholine receptor, which comprises two covalently linked 250,000 Mr oligomers, has been investigated by numerical analysis of electron microscope images. Na-cholate solubilized Torpedo marmorata H-form receptor was reintegrated into artificial lipid vesicles and negatively stained with uranyl acetate prior to imaging in a conventional transmission microscope. The reconstituted preparations exhibited the standard polypeptide composition of the purified receptor (alpha 2 beta gamma delta) and the same transmembrane arrangement as in the native subsynaptic membrane. Covalent disulfide linkage between the two oligomers took place exclusively through the delta chains. In agreement with previous work (Cartaud et al., 1980) the H-form appeared as "doublets" of two coplanar 9 nm rosettes at a center-to-center distance of 9.2 +/- 1.1 nm. The relative angular orientation of the two rosettes in a doublet was examined by correlation analysis in the real space. It exhibited a marked variability, few of the doublets featuring any kind of symmetry, suggesting that the two oligomers of a doublets are connected via an extended and flexible chain or loop. The area of contact between the two rosettes of a doublet therefore does not necessarily represent a reliable clue as to the location of the delta chain within the structure. Averaged images obtained after reorientation and summation of up to 132 rosettes revealed the three major peaks and the two grooves already observed in previous studies. Two additional smaller peaks were identified. Tentative assignment of structural details to individual subunits was deduced from an examination of alpha-bungarotoxin-labeled doublets. The alpha subunits, which carry part or all of the acetylcholine binding sites, are probably located in nonadjacent positions in the vicinity of the newly found peaks. This assignment is consistent with the image analysis of receptor-toxin complexes recently reported by Zingsheim et al. (1982b).

Ligand-induced variations in the reactivity of thio groups of the alpha-subunit of the acetylcholine receptor from Torpedo californica

We have studied alkylation of the acetylcholine receptor by N-[3H]ethylmaleimide ([3H]NEM) under various conditions. The radiolabeled preparations were submitted to sodium dodecyl sulfate-polyacrylamide gel electrophoresis to separate the receptor complex into subunits, and the incorporation of 3H into each type of chain was determined. We found the following: (i) When cysteines of native receptor in intact membranes were reacted with [3H]NEM, only the beta-subunit was labeled; the extent of alkylation did not change significantly if cholinergic effectors were present during this reaction. (ii) When the disulfide bonds of the receptor were reduced with dithiothreitol (DTT), the alpha- and beta-chains were labeled with [3H]NEM. The presence of receptor agonists and competitive antagonists during alkylation significantly altered the labeling patterns. Gallamine and hexamethonium markedly enhanced, while carbamylcholine and decamethonium markedly lessened, labeling of the alpha-subunit. Choline, d-tubocurarine, and alpha-neurotoxin induced small, but significant decreases in alkylation of the alpha-subunit, while procaine had no effect. (iii) When the same ligands were present during the reduction step, subsequent labeling with [3H]NEM produced patterns similar to those described in (ii). We also investigated the effects of gallamine and hexamethonium on reduction of the disulfide bond located near the acetylcholine binding site by using the affinity alkylating reagent (bromoacetyl)choline (BAC). Gallamine (0.1 mM) was able to increase the rate of reduction of this particular disulfide bond 3-fold in comparison to the control. In these experiments, alkylation by BAC blocked 50% of the toxin binding sites. Hexamethonium (1 mM) had a similar effect.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Exposure of acetylcholine receptor to the lipid bilayer

All four subunits of the acetylcholine receptor in membrane fragments isolated from T. californica have been labeled with a photolabile hydrophobic probe, [3H]adamantanediazirine, which selectively labels regions of integral membrane proteins in contact with the hydrocarbon core of the lipid bilayer. As all of the homologous subunits are exposed to the lipid bilayer, it is probable that they each interact with the surrounding membrane in a similar fashion.

Structural homology of Torpedo californica acetylcholine receptor subunits

The nicotinic acetylcholine receptor (AChR) from the electroplax of the ray Torpedo californica is composed of five subunits present in a molar stoichiometry of alpha 2 beta gamma delta (refs 1-3) and contains both the binding site for the neurotransmitter and the cation gating unit (reviewed in refs 4-6). We have recently elucidated the complete primary structures of the alpha-, beta- and delta-subunit precursors of the T. californica AChR by cloning and sequencing cDNAs for these polypeptides. Here, we report the whole primary structure of the gamma-subunit precursor of the AChR deduced from the nucleotide sequence of the cloned cDNA. Comparison of the amino acid sequences of the four subunits reveals marked homology among them. The close resemblance among the hydrophilicity profiles and predicted secondary structures of all the subunits suggests that these polypeptides are oriented in a pseudosymmetric fashion across the membrane. Each subunit contains four putative transmembrane segments that may be involved in the ionic channel. The transmembrane topology of the subunit molecules has also been inferred.

Differentiation-dependent changes of nicotinic synapse-associated proteins

Developmentally regulated changes were followed by analyzing the protein composition in vivo of the electric organ of Torpedo marmorata. A 45 000-Mr component, most likely a form of actin, is found to decrease during synaptogenesis, whereas a 43 000-Mr component increases significantly at later embryonic stages, to become the most abundant protein of electric organ. The 43 000-Mr polypeptides are heterogeneous in their solubilization properties and isoelectric points. Translation in vitro of mRNA isolated from embryonic electric organ shows that the appearance of these proteins during development is regulated by the amount of translatable mRNA available. The close correlation between the translatable amounts of mRNA in vitro and the protein synthesis observed in vivo during synaptogenesis suggests that the functional maturation of the electric organ is linked to the appearance of 43 000-Mr polypeptides.

Binding of phencyclidine to the detergent solubilized acetylcholine receptor from Torpedo marmorata

The binding of phencyclidine to the acetylcholine receptor from Torpedo marmorata electroplaque was measured following solubilization of the receptor in sodium cholate followed by the exchange of cholate for Tween 80. In both the membrane-bound and solubilized AChR, the addition of cholinergic agonists simultaneously with the addition of PCP results in a 100 to 1000 fold increase in the PCP association rate and a 5 to 10 fold increase in the dissociation rate as compared to the unliganded AChR or AChR equilibrated with agonist prior to PCP addition. In addition, the number of binding sites and the pharmacological properties of the binding are not markedly changed in the soluble receptor. These results suggest that the acetylcholine receptor can undergo similar conformational transitions in the membrane-bound and the Tween 80 solubilized form and that phencyclidine can monitor these transitions in both cases.

Subunit structure of the acetylcholine receptor from Electrophorus electricus

The amino-terminal amino acid sequences of the four major peptides (Mr 41,000, 50,000, 55,000, and 62,000) present in purified preparations of Electrophorus electricus nicotinic acetylcholine receptor (AcChoR) have been determined for 24 cycles by automated sequence analysis procedures yielding four unique polypeptide sequences. The sequences showed a high degree of similarity, having identical residues in a number of positions ranging between 37% and 50% for specific pairs of subunits. Comparison of the sequences obtained with those of the subunits of similar molecular weight from Torpedo californica AcChoR revealed an even higher degree of homology (from 46% to 71%) for these two highly diverged species. Simultaneous sequence analysis of the amino termini present in native, purified Electrophorus AcChoR showed that these four related sequences were the only ones present and that they occur in a ratio of 2:1:1:1, with the smallest subunit ("alpha 1") being present in two copies. Genealogical analysis suggests that the subunits of both Torpedo and Electrophorus AcChoRs derive from a common ancestral gene, the divergence having occurred early in the evolution of the receptor. This shared ancestry and the very early divergence of the four subunits, as well as the highly conserved structure of the AcChoR complex along animal evolution, suggest that each of the subunits evolved to perform discrete crucial roles in the physiological function of the AcChoR.

Mammalian muscle acetylcholine receptor purification and characterization

Nicotinic acetylcholine receptor (AcChR) was purified from fetal calf muscle by an affinity chromatographic method utilizing alpha-neurotoxin from Naja naja siamensis as an immobilized ligand. Preparations of AcChR with an average specific activity of 5 nmol of alpha-toxin bound/mg of protein were obtained, i.e., 75% of the theoretical specific activity assuming identity with Torpedo AcChR. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified AcChR consistently showed the presence of five polypeptides, having apparent Mr's of 42 000, 44 000, 49 000, 55 000, and 58 000, respectively. The peptide of Mr 44K was demonstrated to be actin. The amino acid composition of fetal calf AcChR was shown to be similar to that of Torpedo AcChR. In addition, calf AcChR contained large amounts of amino sugars. The sedimentation coefficient of the purified calf AcChR was found to be 9.25 +/- 0.25, i.e., similar to the monomeric form of electric organ AcChR. Determination of the isoelectric point of alpha-bungarotoxin/calf AcChR complexes revealed the presence of two charged forms, having pI values of 5.16 +/- 0.13 and 6.05 +/- 0.18, respectively.