Assessment of the number of free cysteines and isolation and identification of cystine-containing peptides from acetylcholine receptor

Effects of P-Aminophenyl Dichloroarsine on Reduced High-affinity [3H]Nicotine Binding Sites from Chick Brain: A Covalent, Yet Reversible, Agent for Neuronal Nicotinic Receptors

Neuronal nicotinic acetylcholine receptor (nAChR) alpha-subunits contain a conserved disulphide that is essential for function. Here, we have examined the effects of sulphydryl redox reagents on [3H]nicotine binding to chick brain nAChR immunoisolated with the monoclonal antibody mAb35. The disulphide reducing agent, dithiothreitol (DTT), inhibited [3H]nicotine binding [50% inhibitory concentration (IC50)=146 microM] but this effect was reversed (93 +/- 1.5%) by subsequent reoxidation with 1 mM dithio-bis(nitrobenzoic acid) (DTNB). The trivalent arsenical, p-aminophenyl dichloroarsine (APA), which reacts with pairs of spatially close sulphydryls, was a potent inhibitor of reoxidation by DTNB (IC50=35 nM). However, application of the 'anti-arsenical', 2,3-dimercaptopropane sulphonic acid (DMPS), restored agonist binding after APA treatment (50% effective concentration=120 microM). Paradoxically, DMPS was also found to be a potent oxidizing agent of these receptors. Affinity alkylation of reduced nAChRs with bromoacetylcholine (BAC; 100 microM) irreversibly blocked nicotine binding (>90%). We propose (but have not proven) that APA interacts with the cysteines homologous to Cys192 - 193 in Torpedo AChRs, since APA pretreatment of reduced neuronal receptors protected against irreversible BAC alkylation, as shown by subsequent reversal of DMPS (2 mM; 20 min). This study illustrates the potent and reversible nature of the arsenical's covalent interaction with an isolated nAChR and suggests that modified arsenicals could be useful nAChR probes.

The role of the cystine loop in acetylcholine receptor assembly

Nicotinic acetylcholine receptors (AChRs) are composed of alpha, beta, gamma, and delta subunits, assembled into alpha2betagammadelta pentamers. A highly conserved feature of ionotropic neurotransmitter receptors, such as AChRs, is a 15-amino acid cystine "loop." We find that an intact cystine loop is necessary for complete AChR assembly. By preventing formation of the loop with 5 mM dithiothreitol, AChR subunits assemble into alphabetagamma trimers, but the subsequent steps in assembly are blocked. When alpha subunit loop cysteines are mutated to serines, assembly is blocked at the same step as with dithiothreitol. In contrast, when beta subunit loop cysteines are mutated to serines, assembly is blocked at a later step, i.e. after assembly of alphabetagammadelta tetramers and before the addition of the second alpha subunit. After formation of the cystine loop, the alpha subunit undergoes a conformational change, which buries the loop. This conformational change is concurrent with the step in assembly blocked by removal of the disulfide bond of the cystine loop, i.e. after assembly of alphabetagamma trimers and before the addition of the delta subunit. The data indicate that the alpha subunit conformational change involving the cystine loop is key to a series of folding events that allow the addition of unassembled subunits.

N-glycosylation at the conserved sites ensures the expression of properly folded functional ACh receptors

The role of the conserved carbohydrate moiety in the expression of complete acetylcholine receptor (AChR), alpha2 beta gamma delta was re-investigated by expressing additional site-directed mutant subunits, lacking an N-glycosylation site, in Xenopus oocytes. All mutant subunits were stably expressed and appeared to associate with other normal subunits; however, removal of carbohydrate on the alpha subunit inhibited the formation of 125I-alpha-bungarotoxin (alpha-BuTX) binding sites and functional ACh-gated ion channels. 125I-alpha-BuTX binding to AChRs was also significantly reduced by removal of the conserved carbohydrate on the gamma or delta subunits. Immunoprecipitation with monoclonal antibodies that recognize the two distinct alpha-BuTX sites on the AChR indicated that the mutant gamma subunit did not interfere with efficient formation of the alpha-BuTX binding site at the alpha/delta interface, but loss of the carbohydrate did interfere with formation of the alpha-BuTX binding site at the alpha/mutant gamma interface. A similar result was obtained with the mutant delta subunit. Furthermore, the mutant gamma and mutant delta subunits were not incorporated efficiently into the mature (correct tertiary conformation capable of alpha-BuTX binding) alpha beta delta or alpha beta gamma complexes, respectively. Since both mutant gamma and mutant delta subunits were capable of assembling with the alpha subunits (immature assembly), these results suggest that the formation of the two alpha-BuTX binding sites requires correct folding of the alpha gamma and alpha delta complexes, which is aided by the conserved carbohydrate on the gamma and delta subunits. Electrophysiological experiments demonstrated that functional receptors containing mutant subunits were produced, but the functional properties of the mutant receptors were differentially altered, depending on the subunit mutated. Together, our results suggest that N-glycosylation of AChR subunits ensures the correct folding of important functional domains and expression of proper functional receptors in the plasma membrane.

A conserved disulfide loop facilitates conformational maturation in the subunits of the acetylcholine receptor

To examine the structural determinants for the assembly of ligand-gated receptors, we constructed mutant alpha, beta, gamma and delta subunits of the Torpedo acetylcholine receptor (AChR), lacking one of the conserved cysteine residues which forms a 13-amino acid disulfide loop in the amino terminal domain of each subunit. Mutant subunits were co-expressed with complementary wild-type subunits in Xenopus oocytes. Using subunit-specific antisera and monoclonal antibodies that recognize the two distinct alpha-bungarotoxin (alpha-BuTX) sites on the AChR, we were able to distinguish immature subunit associations from conformationally mature AChR complexes. Removal of the disulfide loop on the alpha subunit completely destroyed the formation of the two toxin-binding sites, while removal of the structure on the beta subunit had little effect. While mutant gamma and delta subunits were capable of forming associations (immature assembly) with other subunits, the formation of alpha-BTX sites between alpha and mutant gamma or mutant delta subunits was diminished. Interestingly, assembly of alpha beta gamma subunits remained efficient in the presence of mutant delta subunits, whereas assembly of alpha beta delta subunits was inefficient in the presence of mutant gamma subunits. Thus, these results indicate that the formation of the disulfide loop facilitates the conformational maturation of alpha gamma and alpha delta complexes, which may be conditional for correct subunit coupling in assembling receptors. Furthermore, it seems likely that the correct coupling between the alpha and gamma subunits is the most important step in subunit assembly.

Processive post-translational modification. Vitamin K-dependent carboxylation of a peptide substrate

Mass spectrometry has been used to demonstrate that vitamin K-dependent carboxylation is a processive post-translational modification (i.e. multiple carboxylations occur during a single association between enzyme and substrate). Purified vitamin K-dependent carboxylase can carboxylate as many as 12 glutamate residues in FIXQ/S, a peptide substrate based on amino acids -18 to 41 of the human blood clotting enzyme factor IX. Mass spectrometry was used to determine the number of gamma-carboxyl groups added to FIXQ/S by the carboxylase during an in vitro reaction. Despite the fact that most substrate molecules in a reaction were uncarboxylated, almost all carboxylated FIXQ/S molecules were carboxylated many times. This observation can only be explained by two types of mechanisms. In a processive mechanism, multiple carboxylations could occur during a single substrate binding event. Alternatively, a distributive mechanism could result in the observed behavior if the initial carboxylation event results in a substrate that is additionally carboxylated far more efficiently than the uncarboxylated FIXQ/S. Kinetic experiments and arguments were used to show that the vitamin K-dependent carboxylase is not distributive but rather is one of the first well documented examples of an enzyme that catalyzes a processive post-translation modification.

Extracytoplasmic disposition of lysine beta 165 of acetylcholine receptor

The location, with respect to the membrane, of Lys 165 in the folded beta polypeptide of native nicotinic acetylcholine receptor has been determined by site-directed immunochemistry. Sealed, right-side-out vesicles rich in acetylcholine receptor were modified with pyridoxal phosphate and sodium [3H]-borohydride. Saponin was added to one portion of the vesicles to make them permeable to the pyridoxal phosphate and sodium borohydride; the other portion was modified in the absence of saponin. Both samples were then exhaustively succinylated and digested with trypsin and thermolysin to produce the peptide LDAKGER, which contains Lys beta 165. The digests were passed over an immunoadsorbent specific for peptides with the sequence LDAXGER, where X represents any modified or unmodified amino acid, and specifically bound peptides were eluted with 0.1 M sodium phosphate, pH 2.5. The eluates were submitted to high-pressure liquid chromatography, and two peptides, N epsilon-phospho[3H]pyridoxalLDAKGER and N epsilon-succinylLDAKGER, modified at the epsilon amino group of lysine with pyridoxal phosphate and sodium [3H]-borohydride or succinic anhydride, respectively, were identified by comparison to standards. The relative specific radioactivity of N epsilon-phospho[3H]pyridoxalLDAKGER modified in the presence or absence of saponin, respectively, was 0.9 +/- 0.4. The incorporation of phospho[3H]pyridoxyl groups into Lys alpha 380, a residue located on the cytoplasmic surface of acetylcholine receptor, was also monitored. The relative specific radioactivity of the peptide that contains the modified Lys alpha 380, N epsilon-phospho[3H]pyridoxalGVKYIAE, increased 3.6-fold when the modification was performed in the presence of saponin. This result verifies that the vesicles used in these experiments were sealed and right-side-out. Because the incorporation of [3H]pyridoxyl groups into Lys beta 165 is the same in the presence or absence of saponin, Lys beta 165 must have been located on the outside surface of the sealed, right-side-out vesicles, and therefore on the extracytoplasmic surface of native acetylcholine receptor.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

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

alpha-Bungarotoxin binding to two acetylcholine receptor alpha-peptides and their methylmercury-modified analogs: intrinsic phosphorescence and optically detected magnetic resonance studies

Phosphorescence and optically detected magnetic resonance (ODMR) have been used to characterize two synthetic peptides, alpha 181-198 and alpha 185-196, of the major binding determinant of the alpha-acetylcholine receptor (AChR) of Torpedo californica and its interaction with alpha-bungarotoxin (BgTX) using Trp as an intrinsic probe. BgTX conformational changes are suggested upon complexation with the peptides. Methylmercury-modified peptides show conformational heterogeneity which brings some of the modified Cys residues into proximity of peptide Trp(s). These modified peptides, when bound to BgTX, undergo structural changes which remove the tagged Cys from its close contact with the Trp residue(s) of the peptide.

Species- and subtype-specific recognition by antibody WF6 of a sequence segment forming an alpha-bungarotoxin binding site on the nicotinic acetylcholine receptor alpha subunit

The monoclonal antibody WF6 competes with acetylcholine and alpha-bungarotoxin (alpha-BGT) for binding to the Torpedo nicotinic acetylcholine receptor (nAChR) alpha 1 subunit. Using synthetic peptides corresponding to the complete Torpedo nAChR alpha 1 subunit, we previously mapped a continuous epitope recognized by WF6, and the prototope for alpha-BGT, to the sequence segment alpha 1(181-200). Single amino acid substitution analogs have been used as an initial approach to determine the critical amino acids for WF6 and alpha-BGT binding. In the present study, we continue our analysis of the structural features of the WF6 epitope by comparing its cross-reactivity with synthetic peptides corresponding to the alpha 1 subunits from the muscle nAChRs of different species, the rat brain alpha 2, alpha 3, alpha 4 and alpha 5 nAChR subtypes, and the chick brain alpha-BGT binding protein subunits, alpha BGTBP alpha 1 and alpha BGTBP alpha 2. Our results indicate that WF6 is able to cross-react with the muscle alpha 1 subunits of different species by virtue of conservation of several critical amino acid residues between positions 190-198 of the alpha 1 subunit. These studies further define the essential structural features of the sequence segment alpha 1(181-200) required to form the epitope for WF6.

Structural determinants within residues 180-199 of the rodent alpha 5 nicotinic acetylcholine receptor subunit involved in alpha-bungarotoxin binding

Synthetic peptides corresponding to sequence segments of the nicotinic acetylcholine receptor (nAChR) alpha subunits have been used to identify regions that contribute to formation of the binding sites for cholinergic ligands. We have previously defined alpha-bungarotoxin (alpha-BTX) binding sequences between residues 180 and 199 of a putative rat neuronal nAChR alpha subunit, designated alpha 5 [McLane, K. E., Wu, X., & Conti-Tronconi, B. M. (1990) J. Biol. Chem. 265, 9816-9824], and between residues 181 and 200 of the chick neuronal alpha 7 and alpha 8 subunits [McLane, K. E., Wu, X., Schoepfer, R., Lindstrom, J., & Conti-Tronconi, B. M. (1991) J. Biol. Chem. (in press)]. These sequences are relatively divergent compared with the Torpedo and muscle nAChR alpha 1 alpha-BTX binding sites, which indicates a serious limitation of predicting functional domains of proteins based on homology in general. Given the highly divergent nature of the alpha 5 sequence, we were interested in determining the critical amino acid residues for alpha-BTX binding. In the present study, the effects of single amino acid substitutions of Gly or Ala for each residue of the rat alpha 5(180-199) sequence were tested, using a competition assay, in which peptides compete for 125I-alpha-BTX binding with native Torpedo nAChR.(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping of the acetylcholine binding site of the nicotinic acetylcholine receptor: [3H]nicotine as an agonist photoaffinity label

The agonist [3H]nicotine was used as a photoaffinity label for the acetylcholine binding sites on the Torpedo nicotinic acetylcholine receptor (AChR). [3H]nicotine binds at equilibrium with Keq = 0.6 microM to the agonist binding sites. Irradiation with 254-nm light of AChR-rich membranes equilibrated with [3H]nicotine resulted in covalent incorporation into the alpha- and gamma-subunits, which was inhibited by agonists and competitive antagonists but not by noncompetitive antagonists. Inhibition of labeling by d-tubocurarine demonstrated that the alpha-subunit was labeled via both agonist sites but the gamma-subunit was labeled only via the site that binds d-tubocurarine with high affinity. Within the alpha-subunit, 93% of the labeling was contained within a 20-kDa Staphylococcus aureus V8 proteolytic fragment beginning at Ser-173. Sequence analysis of this peptide indicated that approximately 80% of the incorporation was into Tyr-198, approximately 13% was into Cys-192, and approximately 7% was into Tyr-190. Chymotryptic digestion of the alpha-subunit confirmed that Tyr-198 was the principal amino acid labeled by [3H]nicotine. This confirmation required a novel radio-sequencing strategy employing omicron-phthalaldehyde, since the efficiency of photolabeling was low (approximately 1.0%) and the labeled chymotryptic peptide was not isolated in sufficient quantity to be identified by mass. [3H]Nicotine, which is the first photoaffinity agonist used, labels primarily Tyr-198 in contrast to competitive antagonist affinity labels, which label primarily Tyr-190 and Cys-192/Cys-193.

Acetylcholine receptor channel gating and conductance involve extracellular disulfide bond(s)

Disulfide bonds are critical determinants of the function of the acetylcholine receptor at the vertebrate neuromuscular junction. In the present study, the role of these bonds in acetylcholine receptor channel gating and conductance was investigated at the single channel level. Disulfide bond reducing agents decreased the single channel conductance of both ligand-gated and spontaneously opening acetylcholine receptor channels, indicating that the observed decrease in conductance is not due to blockade of the channel lumen by agonist molecules. In addition, the reducing agents increased the opening frequency of both liganded and unliganded acetylcholine receptor channels. Use of inside-out patches and both membrane permeant and impermeant reducing agents demonstrated that the disulfide bonds involved are all extracellular. These findings indicate that both channel gating and conductance involve conformational changes in extracellular regions of the acetylcholine receptor.

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

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

Changes in channel properties of acetylcholine receptors during the time course of thiol chemical modifications

The changes occurring during chemical modification of thiol groups in single acetylcholine receptor (AChR) channels of BC3H-11 cells were examined by the patch-clamp technique in the "cell-attached" configuration. Treatment with either 1 mM or 5 mM dithiothreitol or with 5 mM N-ethylmaleimide (NEM) does not cause significant changes in the conductance and mean open time of the channels. However, reduction with dithiothreitol followed by alkylation with NEM produces modifications of AChRs. Under these conditions, channels activated by 2 microM acetylcholine show decreased open times (about 15-fold shorter for the most-modified AChRs) and a slight reduction in single-channel current. Both changes are dependent on the time of exposure and concentration of NEM. The rate of occurrence of openings, however, changes little during NEM treatment. When reduced and alkylated AChRs are activated by 100 microM acetylcholine, clusters of short openings separated by silent periods of about 1 s are observed. The channel-open probability, determined for openings within a cluster, is decreased by about 10-fold when compared with control receptors. The observations at high agonist concentration indicate that the modified AChR is still able to undergo desensitization.

Structural characterization by mass spectrometry of native and recombinant human relaxin

Mass spectrometry has played a key role in characterizing the primary structure of native and recombinant relaxin, a peptide hormone that induces ripening of the cervix prior to childbirth. The peptide is composed of two chains, A and B, and is formed from a single-chain prohormone, as is insulin. Aside from conserved cysteines, though, it has little sequence homology with insulin. Due to the small amounts of native peptide initially available (less than 10 pmol), traditional techniques could not provide information on the blocked A-chain sequence, on the carboxyterminal sequences, nor on other possible post-translational modifications. Mass measurements by fast atom bombardment (FAB) were made on reduced human relaxin isolated from corpora lutea. The detection limit by FAB for reduced relaxin was 500 fmol. The B-chain was four amino acids shorter than expected from comparison of the previously known cDNA sequence with homologous rat and porcine sequences. The A-chain, as predicted, was 24 amino acids in length and had a pyroglutamic acid residue on the amino-terminus. The purified samples were homogeneous with no other post-translational modifications. The recombinant relaxin molecule was also extensively characterized by mass spectrometry. In addition to the intact molecule, all tryptic peptides were characterized by FAB. A capillary high-performance liquid chromatography continuous-flow FAB system, developed for high-sensitivity peptide mapping, aided in these analyses. Finally, the three disulfide bonds were shown by tandem mass spectrometry to match those of insulin.

Functional role of the cysteine 451 thiol group in the M4 helix of the gamma subunit of Torpedo californica acetylcholine receptor

Previous chemical modification studies of the acetylcholine receptor [Yee, A.S., Corey, D.E., & McNamee, M.G. (1986) Biochemistry 25, 2110-2119] were extended by using fluorescent N-pyrenylmaleimide to alkylate purified Torpedo californica nicotinic acetylcholine receptor (AChR). Peptide sequencing of the tryptic fragments of the labeled AChR gamma subunit identified cysteines 416, 420, and 451 as the modified residues. The functional role of Cys-451 in the M4 transmembrane domain of the AChR gamma subunit was further investigated by studying the functional consequences of the site-specific mutation of this cysteine to either serine or tryptophan by using AChR mRNAs injected into Xenopus laevis oocytes. Both mutants displayed about 50% reduction in the normalized channel activity of the receptor measured as the ACh-induced conductance per femtomole of surface alpha-bungarotoxin binding sites. However, the mutations did not change other AChR functional properties such as agonist binding ability, the slow phase of desensitization, and blockade by competitive and noncompetitive antagonists. The significant reduction in AChR ion channel activity associated with the above point mutations, especially the simple change of the -SH group on Cys-451 to the -OH group, suggests that this thiol group in the M4 helix of gamma subunit may play an important role in AChR ion channel function. Previous site-directed mutations of the Cys-416 and -420 residues showed a decreased response when both of these residues were changed to phenylalanine, but not when they were changed to serine [Pradier, L., Yee, A.S., & McNamee, M.G. (1989) Biochemistry 28, 6562-6571].(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a disulfide between cysteine 214 and cysteine 277 in the beta subunit of native (Na+ + K+)ATPase

Two peptides, produced during tryptic digestion and thermolytic digestion, respectively, and containing the same intact disulfide from the beta polypeptide of (Na+ + K+)ATPase from Torpedo californica, were isolated and unambiguously identified. The disulfide is between Cysteine 214 and Cysteine 277.