Mapping the agonist binding site of the nicotinic acetylcholine receptor by cysteine scanning mutagenesis: antagonist footprint and secondary structure prediction.
To further define the surface of the Torpedo californica nicotinic acetylcholine receptor (nAChR) contributing to the agonist binding site structure, we used the substituted Cys accessibility method to identify novel residues and determined the "footprint" of residues protected from modification by the reversible competitive antagonist d-tubocurarine (dTC). nAChRs containing single Cys substitutions within regions of the alpha- or gamma-subunit primary structure known to contribute to the agonist binding site were expressed in Xenopus laevis oocytes. Cys substitutions in binding site segments A (alphaTyr-93 and alphaAsn-94), C (alphaTyr-198), and D (gammaGlu-57) had been shown previously to be accessible for modification. We now introduced cysteines from alphaAsp-195 to alphaIle-201 and from gammaAla-106 to gammaAsp-113 and identified positions accessible for modification in segments C (alphaAsp-195, alphaThr-196, alphaPro-197, alphaAsp-200, and alphaIle-201) and E (gammaAsn-107 and gammaLeu-109). dTC protected against alkylation in segments D (gammaGlu-57) and E (gammaLeu-109) but not in segment A (alphaTyr-93 and alphaAsn-94). In segment C, dTC protection experiments revealed a pattern in which every other residue (alpha196, alpha198, and alpha200, but not alpha197 or alpha201) was protected from alkylation. This pattern of protection provides evidence that bound dTC is near amino acids in segments C, D, and E but not in segment A, and identifies a beta-strand surface within segment C contributing to the binding site. These results are discussed in terms of a homology model, based on the molluscan acetylcholine binding protein crystal structure, of the T. californica nAChR agonist binding site.
Sullivan
Deirdre
D
Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA.
Chiara
David C
DC
Cohen
Jonathan B
JB
D019943:Amino Acid Substitution; D000818:Animals; D001667:Binding, Competitive; D003545:Cysteine / Q000235:genetics / Q000378:metabolism*; D005020:Ethyl Methanesulfonate / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology; D008698:Mesylates / Q000737:chemistry / Q000494:pharmacology; D016296:Mutagenesis; D018722:Nicotinic Agonists / Q000737:chemistry / Q000494:pharmacology*; D009865:Oocytes; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011978:Receptors, Nicotinic / Q000737:chemistry / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D014101:Torpedo; D014982:Xenopus laevis
D008698:Mesylates; D018722:Nicotinic Agonists; D011978:Receptors, Nicotinic; C093171:methanethiosulfonate ethylammonium; C109154:(2-(trimethylammonium)ethyl)methanethiosulfonate; D005020:Ethyl Methanesulfonate; D003545:Cysteine