Abstract
Na+/H+ exchange is essential for survival of all organisms, having a role in the regulation of the intracellular Na+ concentration, pH and cell volume. Furthermore, Na+/H+ exchangers were shown to be involved in the virulence of the bacterium Yersinia pestis, indicating they might be potential targets for novel antibiotic treatments. The model system for Na+/H+ exchangers is the NhaA transporter from Escherichia coli, EcNhaA. Therefore, the general transport mechanism of NhaA exchangers is currently well characterized. However, much less is known about NhaB exchangers, with only a limited number of studies available. The pathogen Klebsiella pneumoniae, which is a major source of nosocomial infection, possesses three electrogenic Na+/H+ exchangers, KpNhaA1, KpNhaA2 and KpNhaB, none of which have been previously investigated. Our aim in this study was to functionally characterize KpNhaB using solid supported membrane-based electrophysiology as the main investigation technique, and thus provide the first electrophysiological investigation of an NhaB Na+/H+ exchanger. We found that NhaB can be described by the same competition-based mechanism that was shown to be valid for electrogenic NhaA and NapA, and for electroneutral NhaP Na+/H+ exchangers. For comparison we also characterized the activity of KpNhaA1 and KpNhaA2 and found that the three exchangers have complementary activity profiles, which is likely a survival advantage for K. pneumoniae when faced with environments of different salinity and pH. This underlines their importance as potential antibiotic drug targets.
Highlights
Na+/H+ exchangers are ubiquitous in nature as they ensure that organisms are capable of regulating their intracellular Na+ concentration, pH and volume [1]
The monomeric forms of the proteins migrated on the gel to masses lower than their calculated molecular weights (KpNhaB– 57 kDa; KpNhaA1–43 kDa; KpNhaA2–41 kDa), which is typical for membrane proteins [35]
E. coli strains deficient in EcNhaA and EcNhaB such as the EP432 strain [43] or the KNabc strain that we employed in this work [44] do not survive under conditions of high Na+ or Li+ concentration
Summary
Na+/H+ exchangers are ubiquitous in nature as they ensure that organisms are capable of regulating their intracellular Na+ concentration, pH and volume [1]. The best studied Na+/H+ exchangers belong to the cation proton antiporter (CPA) superfamily, which includes both electrogenic and electroneutral members [2]. To this date, the crystal structures of four CPA Na+/H+ exchangers have been solved, beginning with the prototype of the family, NhaA from E. coli (EcNhaA) [3] and continuing in recent years with the structures of NapA from T. Competition-based transport mechanism in NhaB from K. pneumoniae collection and analysis, decision to publish, or preparation of the manuscript
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