The Cys-loop superfamily of ligand-gated ion channels: the impact of receptor structure on function

The Cys-loop receptors constitute an important superfamily of LGICs (ligand-gated ion channels) comprising receptors for acetylcholine, 5-HT₃ (5-hydroxytryptamine; 5-HT₃ receptors), glycine and GABA (γ-aminobutyric acid; GABA_A receptors). A vast knowledge of the structure of the Cys-loop superfamily and its impact on channel function have been accrued over the last few years, leading to exciting new proposals on how ion channels open and close in response to agonist binding. Channel opening is initiated by the extracellular association of agonists to discrete binding pockets, leading to dramatic conformational changes, culminating in the opening of a central ion pore. The importance of channel structure is exemplified in the allosteric modulation of channel function by the binding of other molecules to distinct sites on the channel, which exerts an additional level of control on their function. The subsequent conformational changes (gating) lead to channel opening and ion transport. Following channel pore opening, ion selectivity is determined by receptor structure in, and around, the ion pore. As a final level of control, cytoplasmic determinants control the magnitude (conductance) of ion flow into the cell. Thus the Cys-loop receptors are complex molecular motors, with moving parts, which can transduce extracellular signals across the plasma membrane. Once the full mechanical motions involved are understood, it may be possible to design sophisticated therapeutic agents to modulate their activity, or at least be able to throw a molecular spanner into the works!