Background:
The ρ GABAC ion channel receptors are members of the GABAergic system which also includes GABAA and GABAB receptors (1). Although the fact that ρ subunits are structurally related to GABAA subunits, it has a distinct functional and pharmacological properties (2). The research under this study contributes to understanding the unique mechanism of ρ subunits by studying site-specific mutations on the action of various GABAC partial agonists.
Methods:
1. Molecular biology; used to make DNA and RNA of wild type and mutants subunits.
2. Site-directed mutagenesis; to prepare mutant receptors.
3. Two Electrode voltage clamp electrophysiology to measure the response of receptors to partial agonists.
Results and Discussion:
The GABA analogues glycine (3) and valeric acid were found to partially activate the GABAC homopentameric ion channel receptors (Figure 1). Those partial agonists were also applied on mutated GABAC T244S receptors. Interestingly, at mutant receptors both ligands act as antagonist. The conversion of partial agonist’s activity when the residue mutated to serine indicates the important role of loop C ‘threonine 244’ residue in binding of ligand leading to the channel activation.
Threonine 244 residue is in a direct contact with ligand in the extracellular domain binding pocket. When threonine is mutated to serine, the methyl group is removed which leads to less hydrophobic interaction and more free space for ligands in the binding pocket. Ligands such as glycine and valeric acid may have more freedom in the binding pocket which may lead to lack of forming the right stabilization and then channel activation, but rather compete with GABA as antagonist.
In conclusion, structural chemistry studies of various partial agonists on GABAC wild type and some particular mutants of interest would help in exploring the role of binding residue and enhance our understanding to the particular receptors.