Poster: Salinity

Abs # 151: Functional characterization of NhaP type Na⁺ /H⁺ antiporter

Presenter:	Takabe, Teruhiro , takabe@ccmfs.meijo-u.ac.jp
Authors	Takabe, Teruhiro  (A)   Waditee, Rungaroon  (A)   Hibino, Takashi  (A)   Nakamura, Tatsunosuke  (B)   Incharoensakdi, Aran  (C)   Takano, Jun  (D)
Affiliations:	(A): Meijo University (B): Niigata University of Pharmacy and Applied Life Sciences (C): Chulalongkorn University (D): Shimadzu Company

Recently, cyanobacteria have been shown to contain Na⁺ /H⁺ antiporters homologous to some bacteria (NhaP from Pseudomonas), plants (SOS1 and AtNHX1) and mammalians (NHEs), but not to E. coli (NhaA and NhaB). We isolated a homologous gene from a halotolerant cyanobacterium Aphanothece halophytica. Its gene product, ApNhaP, exhibited the Na⁺ /H⁺ antiporter activity over a wide range of pH between 5 and 9 and complemented the Na⁺-sensitive phenotype of the antiporter deficient E. coli mutant. In contrast to SynNhaP from Synechocystis PCC 6803, ApNhaP had virtually no activity of 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 SynNhaP altered the ion specificity of antiporter. These results suggest that the ion specificity of NhaP antiporter is partly determined by the structural properties of the C-terminal tail. Performances on the salt tolerance by a fresh water cyanobacterium Synechococcus PCC 7942 transformed with the genes involved in the synthesis of ApNhaP, betaine, catalase, and chaperone were examined. Compared with the expression of betaine, catalase, and chaperone, the expression of ApNhaP drastically improved the salt tolerance of freshwater cyanobacterium. The cells expressing ApNhaP could grow in BG 11 medium containing 0.5 M NaCl as well as in sea water. These results demonstrate a key role of sodium ion exclusion by ApNhaP for the salt tolerance of photosynhetic organisms.