Comparative models of GABAA receptor extracellular and transmembrane domains: important insights in pharmacology and function.

Comparative models of the extracellular and transmembrane domains of GABAA receptors in the agonist-free state were generated based on the recently published structures of the nicotinic acetylcholine receptor. The models were validated by computational methods, and their reliability was estimated by analyzing conserved and variable elements of the cys-loop receptor topology. In addition, the methodological limits in the interpretation of such anion channel receptor models are discussed. Alignment ambiguities in the helical domain were resolved for helix 3 by placing two gaps into the linker connecting helices 2 and 3. The resulting models were shown to be consistent with a wide range of pharmacological and mutagenesis data from GABAA and glycine receptors. The loose packing of the models results in a large amount of solvent-accessible space and offers a natural explanation for the rich pharmacology and the great flexibility of these receptors that are known to exist in numerous drug-induced conformational states. Putative drug binding pockets found within and between subunits are described, and amino acid residues important for the action and subtype selectivity of volatile and intravenous anesthetics, barbiturates, and furosemide are shown to be part of these pockets. The entire helical domain, however, seems to be crucial not only for binding of drugs but also for transduction of binding to gating or of allosteric modulation. These models can now be used to design new experiments for clarification of pharmacological and structural questions as well as for investigating and visualizing drug induced conformational changes.