On high- and low-affinity agonist sites in GABAA receptors

GABA(A) receptors are activated via low-affinity binding sites for the agonists GABA or muscimol. Evidence has been provided that the amino acid residue alpha(1)F64 located at the beta(2)(+)/alpha(1)( -) subunit interface forms part of this binding site. In radioactive ligand binding studies the agonist [H-3] muscimol has been found to interact with the receptor via a high-affinity binding site. This site has been interpreted as a conformational variant of the low-affinity site. Alternatively, the high-affinity binding site has been located to the alpha(1)(+)/beta(2)(-) interface and the homologous residue to alpha(1)F64, beta(2)Y62 has been proposed to constitute an important part of this site. Here we investigated the effect of the point mutation alpha(1)F64L and the homologous mutation beta(2)Y62L on agonist and antagonist binding and functional properties in alpha(1)beta(2)gamma(2) GABA(A) receptors. While the mutation in the alpha(1) subunit had drastic consequences on all studied properties, including desensitization, the mutation in the beta(2) subunit had little consequence. Our observations are relevant for the relative location of high- and low-affinity agonist sites in GABA(A) receptors.