Critical involvement of the E373-D434 region in the acid sensitivity of a NhaB-type Na(+)/H(+) antiporter from Vibrio alginolyticus

Na+-NQR Confers Aminoglycoside Resistance via the Regulation of l-Alanine Metabolism

The Na⁺-NQR complex functions as a unique redox-driven sodium pump, generating membrane potential directly. However, whether it mediates generation of membrane potential indirectly is unknown. The present study shows that the Na⁺-NQR complex impacts membrane potential through other antiporter families Atp and Mnh. It proceeds by ATP and then cAMP/CRP regulon, which inhibits l-alanine catabolism and promotes l-alanine anabolism. When the Na⁺-NQR complex is reduced as in antibiotic-resistant bacteria, l-alanine is depressed, which is related to the antibiotic resistance phenotypes. However, exogenous l-alanine reverts the phenotype and promotes antibiotic-mediated killing. These findings suggest a novel mechanism by which the Na⁺-NQR system regulates antibiotic resistance via l-alanine metabolism in a cAMP/CRP complex-dependent manner.

Sodium-translocating NADH:quinone oxidoreductase (Na⁺-NQR) functions as a unique redox-driven sodium pump, generating membrane potential, which is related to aminoglycoside antibiotic resistance. However, whether it modulates other metabolisms to confer antibiotic resistance is unknown. The present study showed that loss of nqrA or nqrF led to differential metabolomes with elevated resistance to aminoglycoside antibiotics. Decreased alanine, aspartate, and glutamate metabolism and depressed abundance of alanine were characterized as the most impacted pathway and crucial biomarker, respectively. Further data showed that higher viability was detected in ΔnqrA and ΔnqrF mutant strains than their parent strain ATCC 33787 in the presence of gentamicin but recovered by exogenous l-alanine. It proceeds by the following events. The loss of nqrA or nqrF led to the decrease of membrane potential, ATPase activity, and then ATP and cyclic AMP (cAMP), which reduced the cAMP/CRP (cAMP receptor protein) complex. The reduced cAMP/CRP complex promoted l-alanine catabolism and inhibited l-alanine anabolism, causing reduced levels of alanine. Reduced alanine affected the expression of antiporter families Atp and Mnh genes. Our results suggest a novel mechanism by which the Na⁺-NQR system regulates antibiotic resistance via l-alanine metabolism in a cAMP/CRP complex-dependent manner.

Mutation of two key aspartate residues alters stoichiometry of the NhaB Na+/H+ exchanger from Klebsiella pneumoniae

Bacterial NhaB Na⁺/H⁺ exchangers belonging to the Ion Transporter superfamily are poorly characterized in contrast to Na⁺/H⁺ exchangers of the Cation Proton Antiporter superfamily which have NhaA from Escherichia coli as a prominent member. For a more detailed understanding of the intricacies of the exchanger’s transport mechanism, mutational studies are essential. Therefore, we mutated two protonatable residues present in the putative transmembrane region of NhaB from Klebsiella pneumoniae (KpNhaB), which could serve as substrate binding sites, Asp146 and Asp404, to either glutamate or alanine and analyzed transport function and stability of the mutants using electrophysiological and fluorimetric techniques. While mutation of either Asp residue to Glu only had slight to moderate effects on the transport activity of the exchanger, the mutations D404A and D146A, in particular, had more profound effects on the transport function. Furthermore, a double mutant, D146A/D404A, exhibited a remarkable behavior at alkaline pH, where recorded electrical currents changed polarity, showing steady-state transport with a stoichiometry of H⁺:Na⁺ < 1, as opposed to the H⁺:Na⁺ > 1 stoichiometry of the WT. Thus, we showed that Asp146 and Asp404 are part of the substrate binding site(s) of KpNhaB and engineered a Na⁺/H⁺ exchanger with a variable stoichiometry.

Competition is the basis of the transport mechanism of the NhaB Na+/H+ exchanger from Klebsiella pneumoniae

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.

Diversities and similarities in pH dependency among bacterial NhaB-like Na+/H+ antiporters

NhaB-like antiporters were the second described class of Na(+)/H(+) antiporters, identified in bacteria more than 20 years ago. While nhaB-like gene sequences have been found in a number of bacterial genomes, only a few of the NhaB-like antiporters have been functionally characterized to date. Although earlier studies have identified a few pH-sensitive and -insensitive NhaB-like antiporters, the mechanisms that determine their pH responses still remain elusive. In this study, we sought to investigate the diversities and similarities among bacterial NhaB-like antiporters, with particular emphasis on their pH responsiveness. Our phylogenetic analysis of NhaB-like antiporters, combined with pH profile analyses of activities for representative members of several phylogenetic groups, demonstrated that NhaB-like antiporters could be classified into three distinct types according to the degree of their pH dependencies. Interestingly, pH-insensitive NhaB-like antiporters were only found in a limited proportion of enterobacterial species, which constitute a subcluster that appears to have diverged relatively recently among enterobacterial NhaB-like antiporters. Furthermore, kinetic property analyses of NhaB-like antiporters at different pH values revealed that the degree of pH sensitivity of antiport activities was strongly correlated with the magnitude of pH-dependent change in apparent Km values, suggesting that the dramatic pH sensitivities observed for several NhaB-like antiporters might be mainly due to the significant increases of apparent Km at lower pH. These results strongly suggested the possibility that the loss of pH sensitivity of NhaB-like antiporters had occurred relatively recently, probably via accumulation of the mutations that impair pH-dependent change of Km in the course of molecular evolution.