Channel function reconstitution and re‐animation: a single‐channel strategy in the postcrystal age

The most essential properties of ion channels for their physiologically relevant functions are ion‐selective permeation and gating. Among the channel species, the potassium channel is primordial and the most ubiquitous in the biological world, and knowledge of this channel underlies the understanding of features of other ion channels. The strategy applied to studying channels changed dramatically after the crystal structure of the potassium channel was resolved. Given the abundant structural information available, we exploited the bacterial KcsA potassium channel as a simple model channel. In the postcrystal age, there are two effective frameworks with which to decipher the functional codes present in the channel structure, namely reconstitution and re‐animation. Complex channel proteins are decomposed into essential functional components, and well‐examined parts are rebuilt for integrating channel function in the membrane (reconstitution). Permeation and gating are dynamic operations, and one imagines the active channel by breathing life into the ‘frozen’ crystal (re‐animation). Capturing the motion of channels at the single‐molecule level is necessary to characterize the behaviour of functioning channels. Advanced techniques, including diffracted X‐ray tracking, lipid bilayer methods and high‐speed atomic force microscopy, have been used. Here, I present dynamic pictures of the KcsA potassium channel from the submolecular conformational changes to the supramolecular collective behaviour of channels in the membrane. These results form an integrated picture of the active channel and offer insights into the processes underlying the physiological function of the channel in the cell membrane.