Na+-stimulated ATPase of alkaliphilic halotolerant cyanobacterium Aphanothece halophytica translocates Na+ into proteoliposomes via Na+ uniport mechanism

<p>Abstract</p> <p>Background</p> <p>When cells are exposed to high salinity conditions, they develop a mechanism to extrude excess Na⁺from cells to maintain the cytoplasmic Na⁺concentration. Until now, the ATPase involved in Na⁺transport in cyanobacteria has not been characterized. Here, the characterization of ATPase and its role in Na⁺transport of alkaliphilic halotolerant <it>Aphanothece halophytica </it>were investigated to understand the survival mechanism of <it>A. halophytica </it>under high salinity conditions.</p> <p>Results</p> <p>The purified enzyme catalyzed the hydrolysis of ATP in the presence of Na⁺but not K⁺, Li⁺and Ca²⁺. The apparent K_{<it>m </it>}values for Na⁺and ATP were 2.0 and 1.2 mM, respectively. The enzyme is likely the F₁F₀-ATPase based on the usual subunit pattern and the protection against <it>N,N'</it>-dicyclohexylcarbodiimide inhibition of ATPase activity by Na⁺in a pH-dependent manner. Proteoliposomes reconstituted with the purified enzyme could take up Na⁺upon the addition of ATP. The apparent K_{<it>m </it>}values for this uptake were 3.3 and 0.5 mM for Na⁺and ATP, respectively. The mechanism of Na⁺transport mediated by Na⁺-stimulated ATPase in <it>A. halophytica </it>was revealed. Using acridine orange as a probe, alkalization of the lumen of proteoliposomes reconstituted with Na⁺-stimulated ATPase was observed upon the addition of ATP with Na⁺but not with K⁺, Li⁺and Ca²⁺. The Na⁺- and ATP-dependent alkalization of the proteoliposome lumen was stimulated by carbonyl cyanide <it>m </it>- chlorophenylhydrazone (CCCP) but was inhibited by a permeant anion nitrate. The proteoliposomes showed both ATPase activity and ATP-dependent Na⁺uptake activity. The uptake of Na⁺was enhanced by CCCP and nitrate. On the other hand, both CCCP and nitrate were shown to dissipate the preformed electric potential generated by Na⁺-stimulated ATPase of the proteoliposomes.</p> <p>Conclusion</p> <p>The data demonstrate that Na⁺-stimulated ATPase from <it>A. halophytica</it>, a likely member of F-type ATPase, functions as an electrogenic Na⁺pump which transports only Na⁺upon hydrolysis of ATP. A secondary event, Na⁺- and ATP-dependent H⁺efflux from proteoliposomes, is driven by the electric potential generated by Na⁺-stimulated ATPase.</p

Halotolerant Cyanobacterium Aphanothece halophytica Contains an Na+-dependent F1F0-ATP Synthase with a Potential Role in Salt-stress Tolerance*

Aphanothece halophytica is a halotolerant alkaliphilic cyanobacterium that can grow in media of up to 3.0 m NaCl and pH 11. Here, we show that in addition to a typical H+-ATP synthase, Aphanothece halophytica contains a putative F1F0-type Na+-ATP synthase (ApNa+-ATPase) operon (ApNa+-atp). The operon consists of nine genes organized in the order of putative subunits β, ϵ, I, hypothetical protein, a, c, b, α, and γ. Homologous operons could also be found in some cyanobacteria such as Synechococcus sp. PCC 7002 and Acaryochloris marina MBIC11017. The ApNa+-atp operon was isolated from the A. halophytica genome and transferred into an Escherichia coli mutant DK8 (Δatp) deficient in ATP synthase. The inverted membrane vesicles of E. coli DK8 expressing ApNa+-ATPase exhibited Na+-dependent ATP hydrolysis activity, which was inhibited by monensin and tributyltin chloride, but not by the protonophore, carbonyl cyanide m-chlorophenyl hydrazone (CCCP). The Na+ ion protected the inhibition of ApNa+-ATPase by N,N′-dicyclohexylcarbodiimide. The ATP synthesis activity was also observed using the Na+-loaded inverted membrane vesicles. Expression of the ApNa+-atp operon in the heterologous cyanobacterium Synechococcus sp. PCC 7942 showed its localization in the cytoplasmic membrane fractions and increased tolerance to salt stress. These results indicate that A. halophytica has additional Na+-dependent F1F0-ATPase in the cytoplasmic membrane playing a potential role in salt-stress tolerance