Long-range coupling in an allosteric receptor revealed by mutant cycle analysis

The functional coupling of residues that are far apart in space is the quintessential property of allosteric proteins. For example, in Cys-loop receptors, the gating of an intrinsic ion channel is allosterically regulated by the binding of small molecule neurotransmitters 50-60 A from the channel gate. Some residues near the binding site must have as their primary function the communication of the binding event to the gating region. These gating pathway residues are essential to function, but their identification and characterization can be challenging. This work introduces a simple strategy, derived from mutant cycle analysis, for identifying gating pathway residues using macroscopic measurements alone. In the exemplar Cys-loop receptor, the nicotinic acetylcholine receptor, a well-characterized reporter mutation (betaL9'S) known to impact gating, was combined with mutations of target residues in the ligand-binding domain hypothesized or previously found to be functionally significant. A mutant cycle analysis of the macroscopic EC(50) measurements can then provide insights into the role of the target residue. This new method, elucidating long-range functional coupling in allosteric receptors, can be applied to several reporter mutations in a wide variety of receptors to identify previously characterized and novel mutations that impact the gating pathway. We support our interpretation of macroscopic data with single-channel studies. Elucidating long-range functional coupling in allosteric receptors should be broadly applicable to determining functional roles of residues in allosteric receptors.