Probing ion binding in the selectivity filter of the Ca1.1 channel with molecular dynamics.

Ca1.1 is the voltage-gated calcium channel essential for the contraction of skeletal muscles upon membrane potential changes. Structural determination of the Ca1.1 channel opens the avenue toward understanding of the structure-function relationship of voltage-gated calcium channels. Here, we show that there exist two Ca-binding sites, termed S1 and S2, within the selectivity filter of Ca1.1 through extensive molecular dynamics simulations on various initial ion arrangement configurations. The formation of both binding sites is associated with the four Glu residues (Glu292/614/1014/1323) that constitute the so-called EEEE locus. At the S1 site near the extracellular side, the Ca ion is coordinated with the negatively charged carboxylic groups of these Glu residues and of the Asp615 residue either in a direct way or via an intermediate water molecule. At the S2 site, Ca binding shows two distinct states: an upper state involving two out of the four Glu residues in the EEEE locus and a lower state involving only one Glu residue. In addition, there exist two recruitment sites for Ca above the entrance of the filter. These findings promote the understanding of mechanism for ion permeation and selectivity in calcium channels.