Structural homology of Torpedo californica acetylcholine receptor subunits

Abstract

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.

Localisation of the main immunogenic region of the nicotinic acetylcholine receptor

“…The sequences of all 4 chains of Torpedo nAChR have been determined by the use of cloning techniques (review [7]). Based on these data, it was initially proposed that the MIR might be contained in the region of residues 161-166 of the a-chain [8,9]. Using synthetic peptides, we have presented data suggesting that this is unlikely [lo], this subsequently being confirmed by other workers [ll].…”

Antigenic sites of the nicotinic acetylcholine receptor cannot be predicted from the hydrophilicity profile

“…The peptide sequence 161-166 of the a-chain of the nAChR has been proposed as a major antigenic site [5,8,9]. We here show that a synthetic peptide consisting of residues 15 1 -169 (this sequence being identical in T. californica and T. marmorata) does not bind to antibodies to the nAChR, clearly demonstrating that this site is not an important antigenic site on the molecule.…”

“…Extreme conservation of the primary structure of the chain is found across these different species. Based on the hydrophilicity profile of the amino acid sequence of the a-chain of T. californica, it has been proposed that a region consisting of residues 161-166 might correspond to the MIR [5, 8,9]. We have synthesised a peptide containing the sequence 151-169 of the chain of Torpedo and here report that this site does not represent an important antigenic site on the nAChR.…”

Neurotoxin-Binding Site on The Acetylcholine Receptor

“…The negatively charged residues at pcsitions 175 and 200 are also present on at least two of the three other subunits constituting the6 Torpedo californica AChR (32). Thus, only the negative charge at residue 130 us unique to the a-subunit and, therefore, represents a ikely candidate for anionic subsite.…”

“…Several models of the secondary structure of the a-subunit of the AC'R have be.n proposed (11,28,(31)(32)(33)(34)(35).…”