Identification of critical residues in loop E in the 5-HT3ASR binding site

The serotonin type 3 receptor (5-HT3R) is a member of a superfamily of ligand gated ion channels. All members of this family share a large degree of sequence homology and presumably significant structural similarity. A large number of studies have explored the structure-function relationships of members of this family, particularly the nicotinic and GABA receptors. This information can be utilized to gain additional insights into specific structural and functional features of other receptors in this family. Thirteen amino acids in the mouse 5-HT3ASR that correspond to the putative E binding loop of the nicotinic alpha7 receptor were chosen for mutagenesis. Due to the presence of a highly conserved glycine in this region, it has been suggested that this binding loop is comprised of a hairpin turn and may form a portion of the ligand-binding site in this ion channel family. Mutation of the conserved glycine (G147) to alanine eliminated binding of the 5-HT3R antagonist [3H]granisetron. Three tyrosine residues (Y140, Y142 and Y152) also significantly altered the binding of 5-HT3R ligands. Mutations in neighboring residues had little or no effect on binding of these ligands to the 5-HT3ASR. Our data supports a role for the putative E-loop region of the 5-HT3R in the binding of 5-HT, mCPBG, d-tc and lerisetron. 5-HT and mCPBG interact with Y142, d-tc with Y140 and lerisetron with both Y142 and Y152. Our data also provides support for the hypothesis that this region of the receptor is present in a loop structure.