Structural and Thermodynamic Properties of Selective Ion Binding in a K+ Channel

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Abstract
Thermodynamic measurements of ion binding to the Streptomyces lividans K+ channel were carried out using isothermal titration calorimetry, whereas atomic structures of ion-bound and ion-free conformations of the channel were characterized by x-ray crystallography. Here we use these assays to show that the ion radius dependence of selectivity stems from the channel's recognition of ion size (i.e., volume) rather than charge density. Ion size recognition is a function of the channel's ability to adopt a very specific conductive structure with larger ions (K+, Rb+, Cs+, and Ba2+) bound and not with smaller ions (Na+, Mg2+, and Ca2+). The formation of the conductive structure involves selectivity filter atoms that are in direct contact with bound ions as well as protein atoms surrounding the selectivity filter up to a distance of 15 Å from the ions. We conclude that ion selectivity in a K+ channel is a property of size-matched ion binding sites created by the protein structure. The exquisite selectivity of potassium ion (K+) channels in cellular membranes allows them to pass K+ ions while restricting the closely related sodium (Na+) ions, and thereby maintain the electrical potential across cellular membranes. In this study, we address the fundamental question: how does the K+ channel discriminate between K+ and Na+ ions? Past studies have relied on nonequilibrium measurements of ionic current flow. We measured heat exchange associated with ion binding to the channel under equilibrium conditions and determined crystal structures of ion-bound and ion-free forms of the channel. By studying a series of alkali metal and alkaline earth cations, we documented the effect of varying systematically the ionic charge and radius, and we discovered that the K+ channel recognizes an ion's size rather than its electric field strength. By analyzing the structures, we show that the channel's ability to recognize an ion's size is a function of protein atoms that are both near to and far away from the ion binding sites. This study opens a new window into ion selectivity in channels and also contributes to our expanding knowledge of the emerging role of long-range interactions in ligand recognition.