The Cation Selectivity Filter of the Bacterial Sodium Channel, NaChBac

The Bacillus halodurans voltage-gated sodium-selective channel (NaChBac) (Ren, D., B. Navarro, H. Xu, L. Yue, Q. Shi, and D.E. Clapham. 2001b. Science. 294:2372–2375), is an ideal candidate for high resolution structural studies because it can be expressed in mammalian cells and its functional properties studied in detail. It has the added advantage of being a single six transmembrane (6TM) orthologue of a single repeat of mammalian voltage-gated Ca²⁺ (Ca_V) and Na⁺ (Na_V) channels. Here we report that six amino acids in the pore domain (LESWAS) participate in the selectivity filter. Replacing the amino acid residues adjacent to glutamatic acid (E) by a negatively charged aspartate (D; LEDWAS) converted the Na⁺-selective NaChBac to a Ca²⁺- and Na⁺-permeant channel. When additional aspartates were incorporated (LDDWAD), the mutant channel resulted in a highly expressing voltage-gated Ca²⁺-selective conductance.