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Vol. 55, Issue 6, 1037-1043, June 1999
Department of Pharmacology and Neuroscience, Ninewells Hospital and
Medical School, The University of Dundee, Dundee, United Kingdom
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-HT3A
subunits influence the potency of (+)-Tc.
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