1. Neurosciences Institute, Department of Pharmacology and Neuroscience, Ninewells Hospital and Medical School, The University of Dundee, Dundee DD1 9SY, UK
2. The Institute for Genomic Research, 9712 Medical Center Drive, Rockville, Maryland 20850, USA

Correspondence to: John A. Peters¹ Email: j.a.peters@dundee.ac.uk

Abstract

5-Hydroxytryptamine type 3 (5-HT₃) receptors are cation-selective transmitter-gated ion channels of the Cys-loop superfamily^{1, 2, 3, 4, 5, 6, 7, 8, 9}. The single-channel conductance of human recombinant 5-HT₃ receptors assembled as homomers of 5-HT_3A subunits, or heteromers of 5-HT_3A and 5-HT_3B subunits, are markedly different, being 0.4 pS (refs 6, 9) and 16 pS (ref. 7), respectively. Paradoxically, the channel-lining M2 domain of the 5-HT_3A subunit would be predicted to promote cation conduction, whereas that of the 5-HT_3B subunit would not⁷. Here we describe a determinant of single-channel conductance that can explain these observations. By constructing chimaeric 5-HT_3A and 5-HT_3B subunits we identified a region (the 'HA-stretch')¹⁰ within the large cytoplasmic loop of the receptor that markedly influences channel conductance. Replacement of three arginine residues unique to the HA-stretch of the 5-HT_3A subunit by their 5-HT_3B subunit counterparts increased single-channel conductance 28-fold. Significantly, ultrastructural studies of the Torpedo nicotinic acetylcholine receptor¹¹ indicate that the key residues might frame narrow openings that contribute to the permeation pathway. Our findings solve the conundrum of the anomalously low conductance of homomeric 5-HT_3A receptors and indicate an important function for the HA-stretch in Cys-loop transmitter-gated ion channels.