Structural features of the ligand-binding domain of the serotonin 5HT(3) receptor

The nicotinic acetylcholine receptor (AChR) and the serotonin type 3 recept or (5HT(3)R) are members of the Ligand-gated ion channel gene family, Both receptors are inhibited by nanomolar concentrations of d-tubocurarine (cura re) in a competitive fashion. Chemical labeling studies on the AChR have id entified tryptophan residues on the gamma (gamma Trp-55) and delta (delta T rp-57) subunits that interact with curare, Comparison of the sequences of t hese two subunits with the 5HT(3)R shows that a tryptophan residue is found in the homologous position in the 5HT(3)R (Trp-89), suggesting that this r esidue may be involved in curare-5HT(3)R interactions. Site-directed mutage nesis at position Trp-89 markedly reduces the affinity of the 5HT(3)R for t he antagonists curare and granisetron but has little effect on the affinity for the agonist serotonin, To further examine the role of this region of t he receptor in ligand-receptor interactions, alanine-scanning mutagenesis a nalysis of the region centered on Trp-89 (Thr-85 to Trp-94) was carried out , and the ligand binding properties of the mutant receptors were determined . Within this region of the receptor, curare affinity is reduced by substit ution only at Trp-89, whereas serotonin affinity is reduced only by substit ution at Arg-91, On the other hand, granisetron affinity is reduced by subs titutions at Trp-89, Arg-91, and Tyr-93, This differential effect of substi tutions on ligand affinity suggests that different ligands may have differe nt points of interaction within the ligand-binding pocket. In addition, the every-other-residue periodicity of the effects on granisetron affinity str ongly suggests that this region of the ligand-binding site of the 5HT(3)R ( and by inference, other members of the ligand-gated ion channel family) is in a beta-strand conformation.