Transmembrane allosteric energetics characterization for strong coupling between proton and potassium ion binding in the KcsA channel

The slow spontaneous inactivation of potassium channels exhibits classic signatures of transmembrane allostery. A variety of data support a model in which the loss of K+ ions from the selectivity filter is a major factor in promoting inactivation, which defeats transmission, and is allosterically coupled to protonation of key channel activation residues, more than 30 angstrom from the K+ ion binding site. We show that proton binding at the intracellular pH sensor perturbs the potassium affinity at the extracellular selectivity filter by more than three orders of magnitude for the full-length wildtype KcsA, a pH-gated bacterial channel, in membrane bilayers. Studies of F103 in the hinge of the inner helix suggest an important role for its bulky sidechain in the allosteric mechanism; we show that the energetic strength of coupling of the gates is strongly altered when this residue is mutated to alanine. These results provide quantitative site-specific measurements of allostery in a bilayer environment, and highlight the power of describing ion channel gating through the lens of allosteric coupling.