Isolation and characterization of Na+/H+ antiporter gene from a halotolerant cyanobacterium Aphanothece halophytica

Abstract

A putative Na+/H+ antiporter gene whose deduced amino acid sequence was highly homologous to the NhaP antiporter from Pseudomonas aeruginosa and SOS1 antiporter from Arabidopsis was isolated from a halotolerant cyanobacterium Aphanothece halophytica. The Aphanothece NhaP antiporter (ApNhaP) was expressed in E. coli mutant cells which were deficient in Na+/H+ antiporters. It was found that the ApNhaP complemented the NaCl-sensitive phenotype of the E. coli mutant. Membrane vesicles prepared from the E. coli mutant transformed with the ApNhaP exhibited the Na+/H+ antiporter activitiy, but did not show the Li+/H+ antiporter activity. The Na+/H+ antiporter activity was very high between pH 5 and 9. A topological model suggests that Asp139 in ApNhaP is conserved in NhaP, SOS1 and AtNHX1, and play an important role for the exchange activity. Thus, it appeared that the Na+/H+ antiporter from Aphanothece had unique properties among the eucaryotic Na+/H+ antiporters.