A novel Na+(K+)/H+ antiporter plays an important role in the growth of Acetobacter tropicalis SKU1100 at high temperatures via regulation of cation and pH homeostasis

A gene encoding a putative Na+/H+ antiporter was previously proposed to be involved in the thermotolerance mechanism of Acetobacter tropicalis SKU 1100. The results of this study show that disruption of this antiporter gene impaired growth at high temperatures with an external pH > 6.5. The growth impairment at high temperatures was much more severe in the absence of Na+ (with only the presence of K+); under these conditions, cells failed to grow even at 30° C and neutral to alkaline pH values, suggesting that this protein is also important for K+ tolerance. Functional analysis with inside-out membrane vesicles from wild type and mutant strains indicated that the antiporter, At-NhaK2 operates as an alkali cation/proton antiporter for ions such as Na+, K+, Li+, and Rb+ at acidic to neutral pH values (6.5-7.5). The membrane vesicles were also shown to contain a distinct pH-dependent Na+(specific)/H+ antiporter(s) that might function at alkaline pH values. In addition, phylogenetic analysis showed that At-NhaK2 is a novel type of Na+/H+ antiporter belonging to a phylogenetically distinct new clade. These data demonstrate that At-NhaK2 functions as a Na+(K+)/H+ antiporter and is essential for K+ and pH homeostasis during the growth of A. tropicalis SKU1100, especially at higher temperatures.