Halotolerant cyanobacterium Aphanothece halophytica contains an Na+/H+ antiporter, homologous to eukaryotic ones, with novel ion specificity affected by C-terminal tail

Recently, a cyanobacterium Synechocystis sp. PCC 6803 has been shown to contain an Na+/H+ antiporter gene homologous to plants (SOS1 and AtNHX1 from Arabidopsis) and mammalians (NHEs from human) but not to Escherichia coli (nhaA and nhaB). Here, we examined whether a halotolerant cyanobacterium Aphanothece halophytica has homologous genes. It turned out that A. halophytica contains an Na+/H+ antiporter homologous to plants, mammalians, and some bacteria (nhaP from Pseudomonas and synnhaP from Synechocystis) but with novel ion specificity. Its gene product, ApNhaP (Na+/H+ antiporter from Aphanothece halophytica), exhibited the Na+/H+ antiporter activity over a wide pH range between 5 and 9 and complemented the Na+-sensitive phenotype of the antiporter-deficient E. coli mutant. The ApNhaP had virtually no activity for the Li+/H+ antiporter but showed high Ca2+/H+ antiporter activity at alkaline pH. The ApNhaP complemented the Ca2+-sensitive phenotype of the E. coli mutant but not the Li+-sensitive phenotype. The replacement of a long C-terminal tail of ApNhaP with that of Synechocystis altered the ion specificity of the antiporter. These results suggest that the ion specificity of an Na+/H+ antiporter is partly determined by the structural properties of the C-terminal tail, which was well exemplified in the case of A. halophytica.