Ca 2+ efflux facilitated by co-transport of inorganic phosphate anion in the H + /Ca 2+ antiporter YfkE.

Ca 2+ is an important signaling messenger. In microorganisms, fungi, and plants, H + /Ca 2+ antiporters (CAX) are known to play key roles in the homeostasis of intracellular Ca 2+ by catalyzing its efflux across the cell membrane. Here, we reveal that the bacterial CAX homolog YfkE transports Ca 2+ in two distinct modes: a low-flux H + /Ca 2+ exchange mode and a high-flux mode in which Ca 2+ and phosphate ions are co-transported (1:1) in exchange for H + . Coupling with phosphate greatly accelerates the Ca 2+ efflux activity of YfkE. Our studies reveal that Ca 2+ and phosphate bind to adjacent sites in a central translocation pathway and lead to mechanistic insights that explain how this CAX alters its conserved alpha-repeat motifs to adopt phosphate as a specific "transport chaperon" for Ca 2+ translocation. This finding uncovers a co-transport mechanism within the CAX family that indicates this class of proteins contributes to the cellular homeostasis of both Ca 2+ and phosphate.