The Helix‐M2 Model of the Ion Channel of the Nicotinic Acetylcholine Receptor

Hucho, Ferdinand

doi:10.1002/bbpc.198800251

Cited by 5 publications

(11 citation statements)

References 26 publications

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“…Although the authors tried to label the open channel conformation, addition of carbachol as performed in their studies presumably resulted in rapid desensitization and channel closure rather than in channel opening. Because in the closed state molecules cannot pass the channel pore, PhTX bound from the cytosolic side would not be able to compete with ethidium for binding at the high-affinity NCI site, which is known to be located in the extracellular part of the ion channel [13]. Based on our results that PhTXs and poly(methylene tetramine)s displace ethidium bound at the high-affinity NCI site in the channel's closed conformation we conclude that polyamine derivatives are orientated toward the extracellular side of the ion channel.…”

Section: Discussionmentioning

confidence: 99%

“…On the other hand, several high‐affinity binding sites for noncompetitive inhibitors (NCIs), such as triphenylmethylphosphonium (TPMP + ), have been found within the ion channel on the M2 transmembrane domain [11,12]. Luminal NCIs are assumed to enter the open channel and to bind to different rings within the selectivity filter, thereby inhibiting the ion conductance by sterically plugging the channel pore [13].…”

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confidence: 99%

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Structure–activity relationship and site of binding of polyamine derivatives at the nicotinic acetylcholine receptor

Bixel¹,

Krauß²,

Liu

et al. 2000

European Journal of Biochemistry

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Several wasp venoms contain philanthotoxins (PhTXs), natural polyamine amides, which act as noncompetitive inhibitors (NCIs) on the nicotinic acetylcholine receptor (nAChR). Effects of varying the structure of PhTXs and poly(methylene tetramine)s on the binding affinity have been investigated. Using the fluorescent NCI ethidium in a displacement assay K app values of these compounds have been determined. We found that an increase in size of the PhTX's hydrophobic head group significantly increased the binding affinity, while inserting positive charge almost completely destroyed it. Elongating the PhTX polyamine chain by introducing an additional aminomethylene group decreased the binding affinity, whereas a terminal lysine improved it. In general, poly(methylene tetramine)s showed higher binding affinities than PhTX analogues.The stoichiometry of PhTX binding was determined to be two PhTX molecules per receptor monomer. PhTXs appeared to bind to a single class of nonallosterically interacting binding sites and bound PhTX was found to be completely displaced by well-characterized luminal NCIs. To elucidate the site of PhTX binding, a photolabile, radioactive PhTX derivative was photocross-linked to the nAChR in its closed channel conformation resulting in labeling yields for the two a and the b, g and d subunits of 10.4, 11.1, 4.0 and 7.4%, respectively.Based on these findings we suggest that PhTXs and poly(methylene tetramine)s enter the receptor's ionic channel from the extracellular side. The hydrophobic head groups most likely bind to the high-affinity NCI site, while the positively charged polyamine chains presumably interact with the negatively charged selectivity filter located deep in the channel lumen.

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Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

Structure–activity relationship and site of binding of polyamine derivatives at the nicotinic acetylcholine receptor

Bixel¹,

Krauß²,

Liu

et al. 2000

European Journal of Biochemistry

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“…Upon photoactivation it reacted covalently with the Ser262 of the Torpedo receptor's 6 subunit (Giraudat et al, 1986). As we shall see in the next section, this site was also identified by means of triphenylmethyl phosphonium (TPMP'), another NCI , as part of the channel wall, which led to the helix-M2 model of the receptor's ion channel Hucho and Hilgenfeld, 1989). In addition to sites within the channel NCIs can act at the lipid-protein interface.…”

Section: The Regulatory Domain: the Allosteric Modelmentioning

confidence: 95%

“…The channel formed by the five helices M2 is funnel-shaped ( Fig. 9) (Hucho and Hilgenfeld, 1989). This can be concluded from the three available diameters at the channel entrance (2.0-2.5 nm, visible by electron microscopy; Unwin, 1993), at the reaction site of the noncompetitive channel blockers (the diameter of TPMP' as determined by X-ray crystallography is 1.15 nm; McPhail et al, 1971) and at the narrowest part, where the diameter of the largest permeating cation is 0.64 nm (Huang et a]., 1978).…”

Section: The Ion Channel: the Helix-m2 Modelmentioning

confidence: 99%

The Emerging Three‐Dimensional Structure of a Receptor

Hucho

Tsetlin

Machold

1996

European Journal of Biochemistry

203

184

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The nicotinic acetylcholine receptor is the neurotransmitter receptor with the most-characterized protein structure. The amino acid sequences of its five subunits have been elucidated by cDNA cloning and sequencing. Its shape and dimensions (approximately 12.5 nmX8 nm) were deduced from electronmicroscopy studies. Its subunits are arranged around a five-fold axis of pseudosymmetry in the order (clockwise) aI,yaLGP. Its two agonistkompetitive-antagonist-binding sites have been localized by photolabelling studies to a deep gorge between the subunits near the membrane surface. Its ion channel is formed by five membrane-spanning (M2) helices that are contributed by the five subunits. This finding has been generalized as the Helix M2 model for the superfamily of ligand-gated ion channels. The binding site for regulatory non-competitive antagonists has been localized by photolabelling and site-directed-mutagenesis studies within this ion channel.Therefore a three-dimensional image of the nicotinic acetylcholine receptor is emerging, the most prominent feature of which is an active site that combines the agonist/competitive-antagonist-binding sites, the regulatory site and the ion channel within a relatively narrow space close to and within the bilayer membrane.

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“…69,70 Luminal noncompetitive antagonists are assumed to enter the open channel and to bind deep in the ion channel close to the negatively charged selectivity filter, thereby inhibiting the ion flow. 71 Therefore, it may well be that the positively charged backbone of tetraamines is able to interact with the negatively charged amino acid residues located at the constriction of the channel and on the extracellular side of this constriction as well, provided that the distance separating the amine functions of the ligand fits the distance between the carboxylate residues of the receptor.…”

Section: -61mentioning

confidence: 99%

Polymethylene tetraamine backbone as template for the development of biologically active polyamines

Melchiorre

Antonello

Banzi

et al. 2002

Medicinal Research Reviews

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The concept that polyamines may represent a universal template in the receptor recognition process is embodied in the design of ligands for different biological targets. As a matter of fact, the insertion of different pharmacophores onto the polymethylene tetraamine backbone can tune both affinity and selectivity for any given receptor. The application of this approach provided a prospect of modifying benextramine (1). structure to achieve specific recognition of muscarinic receptors that led to the discovery of methoctramine (2). which is widely used as a pharmacological tool for muscarinic receptor characterization. In turn, appropriate structural modifications performed on the structure of methoctramine led to the discovery of new polyamines endowed with high affinity and selectivity for (a). muscarinic receptor subtypes, (b). G(i) proteins, and (c). muscle-type nicotinic receptors. Thus, polyamines tripitramine (9) and spirotramine (33), among others, were designed, which were shown to be highly selective for muscarinic M(2) and M(1) receptors, respectively. Several polyamines have been discovered, which inhibit noncompetitively a closed state of the nicotinic receptor. These ligands, such as 66, resulted in important tools for elucidating the mode and site of interaction of polyamines with the ion channel. It was discovered that reducing the flexibility of the diaminohexane spacer of methoctramine led to polyamines, such as 70, which are endowed with a biological profile significantly different from that of the prototype. Most likely, tetraamine (70) is a potent activator of G(i) proteins. Finally, the universal template approach formed the basis for modifying benextramine (1) structure to the design of ligands, which display affinity for acetylcholinesterase and muscarinic M(2) receptors. Thus, these polyamines, such as caproctamine (78), could have potential in the investigation of Alzheimer disease.

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The Helix‐M2 Model of the Ion Channel of the Nicotinic Acetylcholine Receptor

Abstract: Experimental data describing the ion channel which is part of the acetylcholine receptor are reviewed. The Helix M 2 model is discussed, which proposes that the channel wall is formed by homologous sequences of receptor subunits.

Cited by 5 publications

References 26 publications

Structure–activity relationship and site of binding of polyamine derivatives at the nicotinic acetylcholine receptor

Structure–activity relationship and site of binding of polyamine derivatives at the nicotinic acetylcholine receptor

The Emerging Three‐Dimensional Structure of a Receptor

Polymethylene tetraamine backbone as template for the development of biologically active polyamines

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