Received for publication May 8, 2008.
Revised June 24, 2008.
Accepted for publication July 2, 2008.

Structural determinants for antagonist pharmacology that distinguishes the 1 GABA_C receptor from GABA_A receptors

Abstract

-aminobutyric acid (GABA) receptor types C (GABA_CR) and A (GABA_AR) are both GABA-gated chloride channels that are distinguished by their distinct competitive antagonist properties. The structural mechanism underlying these distinct properties is not well understood. In this study, using previously identified binding residues as a guide, we made individual or combined mutations of 9 binding residues in the 1 GABA_CR subunit to their counterparts in the 122 GABA_AR or reverse mutations in 1 or 2 subunits. The mutants were expressed in Xenopus oocytes and tested for sensitivities of GABA-induced currents to the GABA_A and GABA_C receptor antagonists. The results revealed that bicuculline insensitivity of the 1 GABA_CR was mainly determined by Y106, F138 and F240 residues. Gabazine insensitivity of the 1 GABA_CR was highly dependent on Y102, Y106, and F138. The sensitivity of the 1 GABA_CR to 3-APA and its analogue 3-APMPA mainly depended on residues Y102, V140, FYS240-242, and F138. Thus, the residues of Y102, Y106, F138, and F240 in the 1 GABA_CR are major determinants for its antagonist properties distinct from those in the GABA_AR. In addition, V140 in the GABA_CR also contributes to the 3-APA binding. In conclusion, we have identified the key structural elements underlying distinct antagonist properties for the GABA_CR. The mechanistic insights were further extended and discussed in the context of antagonists docking to the homology models of GABA_A or GABA_C receptors.

Key words: GABAA, GABAC, Func. analysis receptor/ion channel mutants, Mutagenesis/Chimeric approaches