PT  - JOURNAL ARTICLE
AU  - Anthony G. Hope
AU  - Delia Belelli
AU  - Ian D. Mair
AU  - Jeremy J. Lambert
AU  - John A. Peters
TI  - Molecular Determinants of (+)-Tubocurarine Binding at Recombinant 5-Hydroxytryptamine<sub>3A</sub> Receptor Subunits
AID  - 10.1124/mol.55.6.1037
DP  - 1999 Jun 01
TA  - Molecular Pharmacology
PG  - 1037--1043
VI  - 55
IP  - 6
4099  - http://molpharm.aspetjournals.org/content/55/6/1037.short
4100  - http://molpharm.aspetjournals.org/content/55/6/1037.full
SO  - Mol Pharmacol1999 Jun 01; 55
AB  - The 5-hydroxytryptamine type 3 (5-HT3) receptor is a transmitter-gated ion channel mediating neuronal excitation. The receptor native to neurons, or as a homopentameric assembly of 5-HT3A receptor subunits, displays a species-dependent pharmacology exemplified by a 1800-fold difference in the potency of (+)-tubocurarine [(+)-Tc] as an antagonist of the current response mediated by mouse and human receptor orthologs. Here, we attempt to identify amino acid residues involved in binding (+)-Tc by use of chimeric and mutant 5-HT3A subunits of mouse and human expressed in Xenopus laevis oocytes. Replacement of the entire extracellular N-terminal domain of the mouse 5-HT3A(m5-HT3A) subunit by that of the human ortholog and vice versa exchanged the differential potency of (+)-Tc, demonstrating the ligand binding site to be contained wholly within this region. Mutagenesis of multiple amino acid residues within a putative binding domain that exchanged nonconserved residues between mouse and human receptors shifted the apparent affinity of (+)-Tc in a reciprocal manner. The magnitude of the shift increased with the number of residues (3, 5, or 7) exchanged, with septuple mutations of m5-HT3A and human 5-HT3A subunits producing a 161-fold decrease and 53-fold increase in the apparent affinity of (+)-Tc, respectively. The effect of point mutations was generally modest, the exception being m5-HT3A D206E, which produced a 9-fold decrease in apparent affinity. We conclude that multiple amino acids within a binding loop of human and mouse 5-HT3Asubunits influence the potency of (+)-Tc.