Identification of Essential Amino Acid Residues of the NorM Na super(+)/Multidrug Antiporter in Vibrio parahaemolyticus

• Published in: Journal of bacteriology 2005-03, Vol.187 (5), p.1552-1558
• Main Authors: Otsuka, Masato, Yasuda, Makoto, Morita, Yuji, Otsuka, Chie, Tsuchiya, Tomofusa, Omote, Hiroshi, Moriyama, Yoshinori
• Format: Article
• Language: English
• Subjects: Vibrio parahaemolyticus
• ISSN: 0021-9193

NorM is a member of the multidrug and toxic compound extrusion (MATE) family and functions as a Na super(+)/multidrug antiporter in Vibrio parahaemolyticus, although the underlying mechanism of the Na super(+)/multidrug antiport is unknown. Acidic amino acid residues Asp32, Glu251, and Asp367 in the transmembrane region of NorM are conserved in one of the clusters of the MATE family. In this study, we investigated the role(s) of acidic amino acid residues Asp32, Glu251, and Asp367 in the transmembrane region of NorM by site-directed mutagenesis. Wild- type NorM and mutant proteins with amino acid replacements D32E (D32 to E), D32N, D32K, E251D, E251Q, D367A, D367E, D367N, and D367K were expressed and localized in the inner membrane of Escherichia coli KAM32 cells, while the mutant proteins with D32A, E251A, and E251K were not. Compared to cells with wild-type NorM, cells with the mutant NorM protein exhibited reduced resistance to kanamycin, norfloxacin, and ethidium bromide, but the NorM D367E mutant was more resistant to ethidium bromide. The NorM mutant D32E, D32N, D32K, D367A, and D367K cells lost the ability to extrude ethidium ions, which was Na super(+) dependent, and the ability to move Na super(+), which was evoked by ethidium bromide. Both E251D and D367N mutants decreased Na super(+)-dependent extrusion of ethidium ions, but ethidium bromide-evoked movement of Na super(+) was retained. In contrast, D367E caused increased transport of ethidium ions and Na super(+). These results suggest that Asp32, Glu251, and Asp367 are involved in the Na super(+)- dependent drug transport process.