Conformational equilibrium shift underlies altered K⁺ channel gating as revealed by NMR

The potassium ion (K⁺) channel plays a fundamental role in controlling K⁺ permeation across the cell membrane and regulating cellular excitabilities. Mutations in the transmembrane pore reportedly affect the gating transitions of K⁺ channels, and are associated with the onset of neural disorders. However, due to the lack of structural and dynamic insights into the functions of K⁺ channels, the structural mechanism by which these mutations cause K⁺ channel dysfunctions remains elusive. Here, we used nuclear magnetic resonance spectroscopy to investigate the structural mechanism underlying the decreased K⁺-permeation caused by disease-related mutations, using the prokaryotic K⁺ channel KcsA. We demonstrated that the conformational equilibrium in the transmembrane region is shifted toward the non-conductive state with the closed intracellular K⁺-gate in the disease-related mutant. We also demonstrated that this equilibrium shift is attributable to the additional steric contacts in the open-conductive structure, which are evoked by the increased side-chain bulkiness of the residues lining the transmembrane helix. Our results suggest that the alteration in the conformational equilibrium of the intracellular K⁺-gate is one of the fundamental mechanisms underlying the dysfunctions of K⁺ channels caused by disease-related mutations.