Two Amino Acid Residues Contribute to a Cation-π Binding Interaction in the Binding Site of an Insect GABA Receptor

Cys-loop receptor binding sites characteristically possess an aromatic box, where several aromatic amino acid residues surround the bound ligand. A cation-pi interaction between one of these residues and the natural agonist is common, although the residue type and location are not conserved. Even in the closely related vertebrate GABA(A) and GABA(C) receptors, residues in distinct locations perform this role: in GABAA receptors, a Tyr residue in loop A forms a cation-pi interaction with GABA, while in GABA(C) receptors it is a loop B residue. GABA-activated Cys-loop receptors also exist in invertebrates, where they have distinct pharmacologies and are the target of a range of pesticides. Here we examine the location of GABA in an insect binding site by incorporating a series of fluorinated Phe derivatives into the receptor binding pocket using unnatural amino acid mutagenesis, and evaluating the resulting receptors when expressed in Xenopus oocytes. A homology model suggests that two aromatic residues (in loops B and C) are positioned such that they could contribute to a cation-pi interaction with the primary ammonium of GABA, and the data reveal a clear correlation between the GABA EC(50) and the cation-pi binding ability both at Phe206 (loop B) and Tyr254 (loop C), demonstrating for the first time the contribution of two aromatic residues to a cation-pi interaction in a Cys-loop receptor.