Ligand Gated Ion Channels

Ligand-gated ion channels represent a large, evolutionarily related group of intrinsic membrane proteins that form multisubunit complexes and transduce the binding of small agonists into transient openings of ion channels.

The nicotinic acetylcholine receptor is the prototypic ligand-gated cation-selective ion channel and the availability of high affinity antagonists, the alpha-neurotoxins, has greatly facilitated the biochemical and molecular characterization of this receptor / channel complex.

The snake venom alpha-neurotoxins such as alpha-bungarotoxin (BGTX) competitively block or occlude the binding of agonists to the muscle-type acetylcholine receptor. Other comptetive antagonists which target the muscle-type nicotinic receptor include the larkspur toxin, methyllycaconitine, the compound dihydro beta erythoidine (DHBE) and d-Tubocurarine. A major determinant of the antagonist binding site, which likely overlaps the agonist binding site, is thought to reside on the alpha subunit between residues 173 and 204. The evidence for this is based on the following :

• small synthetic peptides corresponding to this region bind BGTX.
• acetylcholine receptors from BGTX-resistant animals and from neuronal acetylcholine receptors which are insensitive to BGTX contain altered sequences within this region (eg. Barchan et al 1992)
• the shortest peptide retaining high binding activity for BGTX is the heptapeptide alpha-189-195.
• synthetic peptide analogue studies and site-directed mutagenesis of recombinant proteins suggest that residues Tyr-189 and Tyr-190 play an important function in BGTX binding.
• two-dimensional nuclear magnetic resonance (NMR) characterization of the stoichiometric complex formed between BGTX and a synthetic dodecapeptide (alpha185-196) corresponding to a functionally important region on the alpha-subunit from Torpedo californica electric organ tissue indicates that BGTX undergoes a conformational change upon peptide binding indicating that alpha-subunit residues 186-190 are on the extracellular surface of the receptor.

Ref: Basus, V.J., Song, G. and Hawrot, E. (1995) NMR solution structure of an alpha-bungarotoxin/nicotinic receptor peptide complex. Biochemistry 32: 12290-12298.

The neuronal-type nicotinic receptor is also a pentamer but has no gamma or delta subunits. It is comprised of only alpha and beta subunits.

BGL,October-95