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J Biol Chem, Vol. 275, Issue 19, 14198-14204, May 12, 2000

Tyrosine 62 of the -Aminobutyric Acid Type A Receptor 2 Subunit Is an Important Determinant of High Affinity Agonist Binding^*

J. Glen Newell^§, Martin Davies, Alan N. Bateson^¶**, and Susan M. J. Dunn^¶§§

From the  Department of Pharmacology, ^¶ Division of Neuroscience, and the  Department of Psychiatry, 9-70 Medical Sciences Building, University of Alberta, Edmonton, Alberta T6G 2H7, Canada

The -aminobutyric acid type A receptor (GABA_AR) carries both high (K_D = 10-30 nM) and low (K_D = 0.1-1.0 µM) affinity binding sites for agonists. We have used site-directed mutagenesis to identify a specific residue in the rat 2 subunit that is involved in high affinity agonist binding. Tyrosine residues at positions 62 and 74 were mutated to either phenylalanine or serine and the effects on ligand binding and ion channel activation were investigated after the expression of mutant subunits with wild-type 1 and 2 subunits in tsA201 cells or in Xenopus oocytes. None of the mutations affected [³H]Ro15-4513 binding or impaired allosteric interactions between the low affinity GABA and benzodiazepine sites. Although mutations at position 74 had little effect on [³H]muscimol binding, the Y62F mutation decreased the affinity of the high affinity [³H]muscimol binding sites by ~6-fold, and the Y62S mutation led to a loss of detectable high affinity binding sites. After expression in oocytes, the EC₅₀ values for both muscimol and GABA-induced activation of Y62F and Y62S receptors were increased by 2- and 6-fold compared with the wild-type. We conclude that Tyr-62 of the  subunit is an important determinant for high affinity agonist binding to the GABA_A receptor.

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