Properties of an alpha9 alpha10 nicotinic receptor mutated in the channel domain

PLAZAS, PV; KATZ, E; ELGOYHEN, AB

Villa Carlos Paz, Cordoba, Argentina

Congreso; XXXVII Annual Meeting of the Argentine Society for Biochemistry and Molecular Biology Research; 2001

Nicotinic acetylcholine receptors (nAChRs) form part of a gene superfamily, which includes g-aminobutiric acid type A (GABAA), serotonine type 3 and glycine receptors. The putative channel-forming MII domains of these receptors contain a highly conserved leucine residue (Leu9´) that is suggested to point into the closed channel. Cloning of the a9 and a10 subunits added two peculiar members to the family of nAChRs, since they show less than 39% of sequence identity with all known nAChRs subunits. Recombinant expression of the a9 and a10 nAChRs subunits yields functional homomeric a9 and heteromeric a9a10 receptors that are activated by ACh but not by nicotine. It is postulated that both subunits are main components of the cochlear and vestibular cholinergic receptor. The aim of this work was to study the function of this leucine ring in the a9a10 receptor. cDNAs for rat nicotinic a9 and a10 subunits where the amino acid Leu9´ was mutated to threonine were expressed in Xenopus oocytes. When compared to WT receptors, ACh-evoked currents through a9a10(Leu9´Thr) receptors exhibited low desensitization kinetics and a high apparent affinity for this agonist. Furthermore, it possessed a paradoxical pharmacological profile: nicotine, an antagonist of the WT receptor, elicited ionic currents when applied to this mutant. One plausible interpretation of these results is that the replacement of Leu9´ by threonine renders a channel, which is permeable in one of its desensitized conformations.