Voltage-gated calcium (Ca_v) channels are transmembrane proteins that allow the conduction of calcium ions into cells in response to changes in cell membrane voltage. Ca_v channels can select Ca²⁺ over Na⁺ at a ratio of 1000:1, even though the extracellular concentration of Ca²⁺ is 70-fold that of Na⁺. This high Ca²⁺ selectivity is believed to be associated with the presence of four residues bearing acidic side chains (the “EEEE” locus in L-type Ca_v channels) within the selectivity filter. Many efforts have been devoted to understanding where and how Ca²⁺ ions bind with the selectivity filter of Ca_v channels. For instance, early site-directed mutagenesis analyses predicted that the EEEE locus forms a single high-affinity site that can bind with multiple Ca²⁺ ions. However, the molecular detail of such binding is ambiguous.

The cover image for the February 7 issue of Biophysical Journal is an artistic rendering of the Ca_v1.1 channel (shown in surface representation), an L-type Ca_v channel prevalently expressed in skeletal muscle, embedded in a cell membrane (gray van der Waals spheres). Ca_v1.1 is essential for the excitation-contraction coupling of skeletal muscles. In this image, repeats Ⅰ–III of the Ca_v1.1 channel are displayed in different colors (blue, tan, and yellow), and repeat IV is hidden. In the lower right corner is a locally enlarged view of the selectivity filter, showing how Ca²⁺ ions bind to the EEEE locus, and an adjacent residue, Asp614 (shown in licorice representation). Our extensive molecular dynamics simulations based on the recently available cryo-EM structure of Ca_v1.1 have identified the existence of two Ca²⁺-binding sites in the selectivity filter and unveiled their molecular environment. This image was created by using VMD (visual molecular dynamics) software and post-processed by using Photoshop.

Our simulation results reveal on the molecular level how the two Ca²⁺ ions are coordinated by the EEEE locus via the spontaneous rearrangement of the side chains of some key residues. The findings are crucial for uncovering the mechanism underlying the rapid, highly selective transport of Ca²⁺ ions through Ca_v channels.

For more information about the work of our group, please visit https://ins.nuaa.edu.cn.

—Junliang Zhu, Hu Qiu, and Wanlin Guo