Identification of a Determinant of High Affinity Calcium Binding in the Selectivity Filter of a Mammalian Calcium Channel

Voltage-gated calcium channels (CaVs) provide the primary source of calcium influx in excitable cells and couple electrical signals to chemical signaling cascades. Due to CaV size and the difficulty of expressing CaVs at quantities sufficient for high-resolution determination, detailed structural information is limited to isolated cytoplasmic domains. However, CaVs are homologous to voltage-gated sodium channels (NaVs) and NaV structure can serve as a template for CaV structure. We determined the structure of the closed conformation of NaVAe1p, a pore only bacterial NaV derived from NaVAe1, an Alkalilimnicola ehrlichei bacterial NaV. This structure reveals the site of a putative calcium ion at the extracellular mouth of the selectivity filter liganded by four serines. At the equivalent site in mammalian calcium channel selectivity filters, there is a conserved aspartate in one of the calcium channel domains. Our functional studies show that this aspartate is a previously unknown determinant of CaV high affinity calcium binding in the mammalian calcium channel CaV1.2. These findings show the extent of similarities between bacterial sodium channels and eukaryotic voltage gated channels and shed new light on the selectivity filter in mammalian calcium channels.