Structural Basis for Ion Conductance and Selectivity in Sodium and Calcium Channels

Abstract

Voltage‐gated sodium and calcium channels generate rapid electrical signals and local calcium transients that initiate key physiological events in many cell types. High selectivity for sodium and calcium is essential to this signaling process. The high‐resolution structure of a bacterial ancestor of voltage‐gated sodium and calcium channels (NavAb) at 2.7 Å resolution reveals the structural basis for ion conductance and selectivity. The NavAb selectivity filter is short, ~4.6 Å wide, and water‐filled, with four glutamate side‐chains that form a high field‐strength site surrounding the entry to the ion‐conduction pathway. NavAb is highly selective for sodium over calcium, with P(Na):P(Ca)=30. Molecular dynamics studies show that sodium is conducted as a hydrated cation and interacts sequentially with three sites in the ion selectivity filter during conduction. Addition of two more negative charges on the extracellular side of the ion selectivity filter increases P(Ca):P(Na)=400, a 12,000‐fold change. Addition of a single negative charge on the extracellular side of the high field‐strength site accounts for a 1000‐fold increase in calcium selectivity by itself. Structural analysis reveals bound calcium ions in the extracellular vestibule and at three sites in the ion selectivity filter. These results define the mechanism of ion conduction and selectivity for calcium through interactions of the hydrated cation at 3 sequential ion coordination sites within the pore. Binding sites for calcium and the pore‐blocking ions cadmium and manganese are illuminated at atomic resolution by x‐ray crystallography. Overall, these results reveal the molecular basis for electrical signaling and give a high‐resolution view of the ion conductance and selectivity of sodium and calcium channels.