Comparative surface accessibility of a pore-lining threonine residue (T6′) in the glycine and GABAA receptors

Abstract

The substituted cysteine accessibility method was used to probe the surface exposure of a pore-lining threonine residue (T6′) common to both the glycine receptor (GlyR) and γ-aminobutyric acid, type A receptor (GABAAR) chloride channels. This residue lies close to the channel activation gate, the ionic selectivity filter, and the main pore blocker binding site. Despite their high amino acid sequence homologies and common role in conducting chloride ions, recent studies have suggested that the GlyRs and GABAARs have divergent open state pore structures at the 6′ position. When both the human α1T6′C homomeric GlyR and the rat α1T6′C heteromeric GABAAR were expressed in human embryonic kidney 293 cells, their 6′ residue surface accessibilities differed significantly in the closed state. However, when a soluble cysteine-modifying compound was applied in the presence of saturating agonist concentrations, both receptors were locked into the open state. This action was not induced by oxidizing agents in either receptor. These results provide evidence for a conserved pore opening mechanism in anion-selective members of the ligand-gated ion channel family. The results also indicate that the GABAAR pore structure at the 6′ level may vary between different expression systems.