What defines a gate? The enigmas of K⁺ channel structural mechanics

Potassium (K⁺) channels vary in shape and size. Commonly, they form a functional pore through the membrane as an assembly of four subunits. For the predominant family of K⁺ channels at the plant plasma membrane, each subunit comprises a polypeptide of six transmembrane helices with a canonical voltage-sensor domain fused to a pair of pore-lining helices. These constructs give rise to a wide range of behaviours that are nonetheless 'hard-wired' within the channel protein structure. With cryo-EM data now available for four arabidopsis (Arabidopsis thaliana) K⁺ channels, it nonetheless remains an enigma how the structural mechanics give rise to channels that open-or gate-for K⁺ flux across widely differing voltages and how ligands, especially K⁺ itself, regulate this flux.