YfkE, a protein from Bacillus subtilis, exhibits homology to the Ca2+:Cation Antiporter (CaCA) Family. In a fluorescence-based assay of everted membrane vesicles prepared from Na+(Ca2+)/H+ antiporter-defective mutant Escherichia coli KNabc, YfkE exhibited robust Ca2+/H+ antiport activity, with a K (m) for Ca2+ estimated at 12.5 mu M at pH 8.5 and 113 mu M at pH 7.5. Neither Na+ nor K+ served as a substrate. Mg2+ also did not serve as a substrate, but inhibited the Ca2+/H+ antiporter activity. The Ca2+ transport capability of YfkE was also observed directly by transport assays in everted membrane vesicles using radiolabeled Ca-45(2+). Transcriptional analysis from the putative yfkED operon using beta-garactosidase activity as a reporter revealed that both of the yfkE and yfkD genes are regulated by forespore-specific sigma factor, SigG, and the general stress response regulator, SigB. These results suggest that YfkE may be needed for Ca2+ signaling in the sporulation or germination process in B. subtilis. ChaA is proposed as the designation for YfkE of B. subtilis.