Determinants Involved in the Affinity of α-Conotoxins GI and SI for the Muscle Subtype of Nicotinic Acetylcholine Receptors

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Abstract
Nicotinic acetylcholine receptors from muscle contain two functionally active and pharmacologically distinct acetylcholine-binding sites located at the α/γ and α/δ subunit interfaces. The α-conotoxins are competitive antagonists of nicotinic receptors and can be highly site-selective, displaying greater than 10 000-fold differences in affinities for the two acetylcholine-binding sites on a single nicotinic receptor. The higher affinity site for α-conotoxins GI, MI, and SI is the α/δ site on mouse muscle-derived BC3H-1 receptors. However, α-conotoxins GI and MI exhibit higher affinity for the other site (α/γ site) on nicotinic receptors from Torpedocalifornica electric organ. α-Conotoxin SI does not distinguish between the two acetylcholine-binding sites on Torpedo receptors. In this study, α-conotoxins [K10H]SI and [K10N]SI displayed wild-type affinity for the two acetylcholine-binding sites on BC3H-1 receptors but a 10−20-fold decrease in apparent affinity at one of the two acetylcholine-binding sites on Torpedo receptors. α-Conotoxin [P9K]SI displayed a selective and dramatic increase in the apparent affinity for the α/δ site of BC3H-1 receptors and for the α/γ site of Torpedo receptors. α-Conotoxin [R9A]GI displayed a reduction in affinity for both acetylcholine-binding sites on BC3H-1 receptors, although the extent of its selectivity for the α/δ site was retained. α-Conotoxin [R9A]GI also displayed a loss of affinity for the two acetylcholine-binding sites on Torpedo receptors, but its site-selectivity was apparently abolished. These results indicate that positions 9 and 10 in α-conotoxins GI and SI are involved in complex species- and subunit-dependent interactions with nicotinic receptors.