Amino acid residues forming the interface of a neuronal nicotinic acetylcholine receptor with kappa-bungarotoxin: a study using single residue substituted peptide analogs

Nicotinic acetylcholine receptors of adrenal chromaffin cells

In the adrenal medulla, acetylcholine released by the sympathetic splanchnic nerves activates neuronal-type nicotinic acetylcholine receptors (nAChRs) on the membrane of chromaffin cells which liberate catecholamines into the bloodstream in preparation for the fight and flight reactions. On adrenal chromaffin cells the main class of nAChRs is a pentameric assembly of alpha3 and beta4 subunits that forms ion channels which produce membrane depolarization by increasing Na+, K+ and Ca2+ permeability. Homomeric alpha7 nicotinic receptors are expressed in a species-dependent manner and do not contribute to catecholamine secretion. Chromaffin cell nAChRs rapidly activate and desensitize with full recovery on washout. nAChR activity is subjected to various types of dynamic regulation. It is allosterically modulated by the endogenous neuropeptide substance P that stabilizes receptors in their desensitized state, thus depressing their responsiveness. The full-length peptide CGRP acts as a negative allosteric modulator by inhibiting responses without changing desensitization, whereas its N-terminal fragments act as positive allosteric modulators to transiently enhance nAChR function. nAChR expression increases when cells are chronically exposed to either selective antagonists or agonists such as nicotine, a protocol mimicking the condition of chronic heavy smokers. In this case, large upregulation of nAChRs occurs even though most of the extra nAChRs remain inside the cells, creating a mismatch between the increase in total nAChRs and increase in functional nAChRs on the cell surface. These findings highlight the plastic properties of cholinergic neurotransmission in the adrenal medulla to provide robust mechanisms for adapting catecholamine release to acute and chronic changes in sympathetic activity.

Binding of native kappa-neurotoxins and site-directed mutants to nicotinic acetylcholine receptors

The kappa-neurotoxins are useful ligands for the pharmacological characterization of nicotinic acetylcholine receptors because they are potent antagonists at only a subgroup of these receptors containing either alpha 3- or alpha 4-subunits (IC50 < or = 100 nM). Four of these highly homologous, 66 amino acid peptides have been purified from the venom of Bungarus multicinctus (kappa-bungarotoxin (kappa-Bgt), kappa 2-Bgt, kappa 3-Bgt] and Bungarus flaviceps [kappa-Fvt)]. Two approaches were taken to examine the binding of these toxins to nicotinic receptors. First, venom-derived kappa-Fvt and kappa-Bgt were radioiodinated and the specific binding was measured of these toxins to overlapping synthetic peptides (16-20 amino acids in length) prepared based on the known sequence of the nicotinic receptor alpha 3-subunit. At least two main regions of interaction between the toxins and the receptor subunit were identified, both lying in the N-terminal region of the subunit that is exposed to the extracellular space. The second approach examined the importance of several sequence position in kappa-Bgt for binding to alpha 3-containing receptors in autonomic ganglia and alpha 1-containing muscle receptors. This was done using site-directed mutants of kappa-Bgt produced by an Escherichia coli expression system. Arg-34 and position 36 were important for binding to both receptor subtypes, while replacing Gln-26 with Trp-26 (an invariant in alpha-neurotoxins) increased affinity for the muscle receptor by 8-fold. The results confirm that kappa-neurotoxins bind potently to the alpha 3-subunit and bind with considerably reduced affinity (Kd approximately 10 microM) to muscle receptors. Site-directed mutagenesis of recombinant kappa-Bgt is thus an important approach for the study of structure-function relationships between kappa-Bgt and nicotinic receptors.

Role of hydrophobic and hydrophilic forces in peptide-protein interaction: new advances

A considerable part of important biological processes is governed by the noncovalent association of peptides and proteins. Various types of intermolecular forces may be involved in the formation of these molecular assemblies. This review gives a brief account of the physicochemical bases of interactive forces, with special emphasis on their impact on various peptide-protein interactions; summarizes the newest biochemical and biophysical methods for the study of such interactions; and discusses the role of various hydrophilic and hydrophobic forces in peptide-protein interactions in various fields of life sciences, such as immunology, enzymology, receptor binding, and toxicology.

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.

Amino acid residues within the sequence region alpha 55-74 of Torpedo nicotinic acetylcholine receptor interacting with antibodies to the main immunogenic region and with snake alpha-neurotoxins

The sequence region 55-74 of the alpha-subunit of the acetylcholine receptor (AChR) from Torpedo californica electroplax comprises the amino-terminal end of a sequence segment--residues alpha 67-76--forming the main immunogenic region (MIR), which is most frequently recognized by anti-AChR autoantibodies in myasthenia gravis. The synthetic sequence alpha 55-74 of Torpedo AChR binds alpha-bungarotoxin (alpha BTX), suggesting that amino acid residues within this sequence region may contribute to formation of an alpha BTX binding site. Using single-residue substituted synthetic analogues of the sequence alpha 55-74 of Torpedo AChR, in which each residue was sequentially substituted by either glycine or alanine, we sought identification of the amino acids involved in interaction with alpha-neurotoxins and with three different anti-MIR monoclonal antibodies (mAbs 6, 22, and 198). Substitution of Arg55, Arg57, Trp60, Arg64, Leu65, Arg66, Trp67, or Asn68 strongly inhibited alpha-toxin binding, whereas substitutions of Ile61, Val63, Pro69, Ala70, Asp71, or Tyr72 had marginal effects. Substitutions within the region alpha 68-72 significantly diminished binding of anti-MIR mAbs, although residue preferences differed among mAbs. Further, substituting Trp60 substantially reduced binding of mAb 198, and moderately affected binding of mAb 6, and substitution of Asp62 slightly but consistently affected binding of mAbs 6 and 22.