Prokaryotic Solute/Sodium Symporters: Versatile Functions and Mechanisms of a Transporter Family.

The solute/sodium symporter family (SSS family; TC 2.A.21; SLC5) consists of integral membrane proteins that use an existing sodium gradient to drive the uphill transport of various solutes, such as sugars, amino acids, vitamins, or ions across the membrane. This large family has representatives in all three kingdoms of life. The human sodium/iodide symporter (NIS) and the sodium/glucose transporter (SGLT1) are involved in diseases such as iodide transport defect or glucose-galactose malabsorption. Moreover, the bacterial sodium/proline symporter PutP and the sodium/sialic acid symporter SiaT play important roles in bacteria-host interactions. This review focuses on the physiological significance and structural and functional features of prokaryotic members of the SSS family. Special emphasis will be given to the roles and properties of proteins containing an SSS family domain fused to domains typically found in bacterial sensor kinases.

Core Transmembrane Domain 6 Plays a Pivotal Role in the Transport Cycle of the Sodium/Proline Symporter PutP.

Crystal structures of transporters with a LeuT-type structural fold assign core transmembrane domain 6 (TM6') a central role in substrate binding and translocation. Here, the function of TM6' in the sodium/proline symporter PutP, a member of the solute/sodium symporter family, was investigated. A complete scan of TM6' identified eight amino acids as particularly important for PutP function. Of these residues, Tyr-248, His-253, and Arg-257 impact sodium binding, whereas Arg-257 and Ala-260 may participate in interactions leading to closure of the inner gate. Furthermore, the previous suggestion of an involvement of Trp-244, Tyr-248, and Pro-252 in proline binding is further supported. In addition, substitution of Gly-245, Gly-247, and Gly-250 affects the amount of PutP in the membrane. A Cys accessibility analysis suggests an involvement of the inner half of TM6' in the formation of a hydrophilic pathway that is open to the inside in the absence of ligands and closed in the presence of sodium and proline. In conclusion, the results demonstrate that TM6' plays a central role in substrate binding and release on the inner side of the membrane also in PutP and extend the knowledge on functionally relevant amino acids in transporters with a LeuT-type structural fold.

Glu-311 in External Loop 4 of the Sodium/Proline Transporter PutP Is Crucial for External Gate Closure.

The available structural information on LeuT and structurally related transporters suggests that external loop 4 (eL4) and the outer end of transmembrane domain (TM) 10' participate in the reversible occlusion of the outer pathway to the solute binding sites. Here, the functional significance of eL4 and the outer region of TM10' are explored using the sodium/proline symporter PutP as a model. Glu-311 at the tip of eL4, and various amino acids around the outer end of TM10' are identified as particularly crucial for function. Substitutions at these sites inhibit the transport cycle, and affect in part ligand binding. In addition, changes at selected sites induce a global structural alteration in the direction of an outward-open conformation. It is suggested that interactions between the tip of eL4 and the peptide backbone at the end of TM10' participate in coordinating conformational alterations underlying the alternating access mechanism of transport. Together with the structural information on LeuT-like transporters, the results further specify the idea that common design and functional principles are maintained across different transport families.

Identification of a second substrate-binding site in solute-sodium symporters.

The structure of the sodium/galactose transporter (vSGLT), a solute-sodium symporter (SSS) from Vibrio parahaemolyticus, shares a common structural fold with LeuT of the neurotransmitter-sodium symporter family. Structural alignments between LeuT and vSGLT reveal that the crystallographically identified galactose-binding site in vSGLT is located in a more extracellular location relative to the central substrate-binding site (S1) in LeuT. Our computational analyses suggest the existence of an additional galactose-binding site in vSGLT that aligns to the S1 site of LeuT. Radiolabeled galactose saturation binding experiments indicate that, like LeuT, vSGLT can simultaneously bind two substrate molecules under equilibrium conditions. Mutating key residues in the individual substrate-binding sites reduced the molar substrate-to-protein binding stoichiometry to ~1. In addition, the related and more experimentally tractable SSS member PutP (the Na(+)/proline transporter) also exhibits a binding stoichiometry of 2. Targeting residues in the proposed sites with mutations results in the reduction of the binding stoichiometry and is accompanied by severely impaired translocation of proline. Our data suggest that substrate transport by SSS members requires both substrate-binding sites, thereby implying that SSSs and neurotransmitter-sodium symporters share common mechanistic elements in substrate transport.

Initial steps of photosystem II de novo assembly and preloading with manganese take place in biogenesis centers in Synechocystis.

In the cyanobacterium Synechocystis sp PCC 6803, early steps in thylakoid membrane (TM) biogenesis are considered to take place in specialized membrane fractions resembling an interface between the plasma membrane (PM) and TM. This region (the PratA-defined membrane) is defined by the presence of the photosystem II (PSII) assembly factor PratA (for processing-associated TPR protein) and the precursor of the D1 protein (pD1). Here, we show that PratA is a Mn(2+) binding protein that contains a high affinity Mn(2+) binding site (K(d) = 73 µM) and that PratA is required for efficient delivery of Mn(2+) to PSII in vivo, as Mn(2+) transport is retarded in pratA(-). Furthermore, ultrastructural analyses of pratA(-) depict changes in membrane organization in comparison to the wild type, especially a semicircle-shaped structure, which appears to connect PM and TM, is lacking in pratA(-). Immunogold labeling located PratA and pD1 to these distinct regions at the cell periphery. Thus, PratA is necessary for efficient delivery of Mn(2+) to PSII, leading to Mn(2+) preloading of PSII in the periplasm. We propose an extended model for the spatial organization of Mn(2+) transport to PSII, which is suggested to take place concomitantly with early steps of PSII assembly in biogenesis centers at the cell periphery.

The ABC transporter family efflux pump PvdRT-OpmQ of Pseudomonas putida KT2440: purification and initial characterization.

Tripartite efflux systems of the ABC-type family transport a variety of substrates and contribute to the antimicrobial resistance of Gram-negative bacteria. PvdRT-OpmQ, a member of this family, is thought to be involved in the secretion of the newly synthesized and recycled siderophore pyoverdine in Pseudomonas species. Here, we purified and characterized the inner membrane component PvdT and the periplasmic adapter protein PvdR of the plant growth-promoting soil bacterium Pseudomonas putida KT2440. We show that PvdT possesses an ATPase activity that is stimulated by the addition of PvdR. In addition, we provide the first biochemical evidence for direct interactions between pyoverdine and PvdRT.

The RND efflux system ParXY affects siderophore secretion in Pseudomonas putida KT2440.

IMPORTANCE: Gram-negative bacteria from the Pseudomonas group are survivors in various environmental niches. For example, the bacteria secrete siderophores to capture ferric ions under deficiency conditions. Tripartite efflux systems are involved in the secretion of siderophores, which are also important for antibiotic resistance. For one of these efflux systems, the resistance-nodulation-cell division transporter ParXY from the model organism Pseudomonas putida KT2440, we show that it influences the secretion of the siderophore pyoverdine in addition to its already known involvement in antibiotic resistance. Phenotypically, its role in pyoverdine secretion is only apparent when other pyoverdine secretion systems are inactive. The results confirm that the different tripartite efflux systems have overlapping substrate specificities and can at least partially functionally substitute for each other, especially in important physiological activities such as supplying the cell with iron ions. This fact must be taken into account when developing specific inhibitors for tripartite efflux systems.

Contribution of Uncharacterized Target Genes of MxtR/ErdR to Carbon Source Utilization by Pseudomonas putida KT2440.

MxtR/ErdR is a two-component system that has been previously described as a regulator of the utilization of acetate in Vibrio cholerae and in some Pseudomonas species. Regulation is achieved by controlling the expression of the acs gene (acetyl-coenzyme A [CoA] synthetase). However, the physiological significance of other identified target genes is not fully understood. Here, we investigated the role of pp_0154 (scpC) and pp_0354/pp_0353 in the soil bacterium Pseudomonas putida KT2440. To this end, the genes were individually deleted and complemented in trans. Then, the growth of the resulting strains on different carbon sources was analyzed. To obtain information on protein function, a bioinformatic analysis was performed, and ScpC was purified and characterized in vitro. Our results indicated that scpC is important for P. putida KT2440 to cope with high concentrations of acetate. The encoded enzyme catalyzes the transfer of coenzyme A between acetate and succinate. On the contrary, pp_0353 and pp_0354 proved to be unimportant for the growth of the strain on acetate under our conditions. Extending the phenotypic analysis to other carbon sources led to the discovery that mxtR, erdR, and pp_0353 are important for the utilization of pyruvate as a carbon source. Taken together, the findings of this study expand the knowledge about the role of the MxtR/ErdR two-component system in carbon source utilization and about the specific functions of its target genes. IMPORTANCE MxtR/ErdR and homologous two-component systems play important roles in the regulatory networks that control cell metabolism and influence bacterial-host interactions. Using the MxtR/ErdR two-component system of the plant growth-promoting soil bacterium Pseudomonas putida KT2440 as a model, this work elucidates the function of previously uncharacterized target genes of MxtR/ErdR and extends the knowledge of the physiological significance of the two-component system. Our results suggest that the target gene scpC encodes an acetate:succinate CoA transferase that is involved in the detoxification of acetate when it is present in large amounts. Furthermore, it is shown that MxtR/ErdR controls the metabolism of not only acetate but also pyruvate. This control involves the target gene pp_0353 (putative exonuclease). These findings may facilitate the optimization of P. putida KT2440 as a chassis for biotechnological applications and may contribute to a better understanding of the regulatory network of pathogens like Pseudomonas aeruginosa.

Involvement of the MxtR/ErdR (CrbS/CrbR) Two-Component System in Acetate Metabolism in Pseudomonas putida KT2440.

MxtR/ErdR (also called CrbS/CrbR) is a two-component system previously identified as important for the utilization of acetate in Vibrio cholerae and some Pseudomonas species. In addition, evidence has been found in Pseudomonas aeruginosa for a role in regulating the synthesis and expression, respectively, of virulence factors such as siderophores and RND transporters. In this context, we investigated the physiological role of the MxtR/ErdR system in the soil bacterium Pseudomonas putida KT2440. To that end, mxtR and erdR were individually deleted and the ability of the resulting mutants to metabolize different carbon sources was analyzed in comparison to wild type. We also assessed the impact of the deletions on siderophore production, expression of mexEF-oprN (RND transporter), and the biocontrol properties of the strain. Furthermore, the MxtR/ErdR-dependent expression of putative target genes and binding of ErdR to respective promoter regions were analyzed. Our results indicated that the MxtR/ErdR system is active and essential for acetate utilization in P. putida KT2440. Expression of scpC, pp_0354, and acsA-I was stimulated by acetate, while direct interactions of ErdR with the promoter regions of the genes scpC, pp_0354, and actP-I were demonstrated by an electromobility shift assay. Finally, our results suggested that MxtR/ErdR is neither involved in regulating siderophore production nor the expression of mexEF-oprN in P. putida KT2440 under the conditions tested.

HutT functions as the major L-histidine transporter in Pseudomonas putida KT2440.

Histidine is an important carbon and nitrogen source of γ-proteobacteria and can affect bacteria-host interactions. The mechanisms of histidine uptake are only partly understood. Here, we analyze functional properties of the putative histidine transporter HutT of the soil bacterium Pseudomonas putida. The hutT gene is part of the histidine utilization operon, and the gene product belongs to the amino acid-polyamine-organocation (APC) family of secondary transporters. Deletion of hutT severely impairs growth of P. putida on histidine, suggesting that the encoded transporter is the major histidine uptake system of P. putida. Transport experiments with cells and purified and reconstituted protein indicate that HutT functions as a high-affinity histidine : proton symporter with high specificity for the amino acid. Substitution analyses identified amino acids crucial for HutT function.

Resistance to Bipyridyls Mediated by the TtgABC Efflux System in Pseudomonas putida KT2440.

Resistance-nodulation-division (RND) transporters are involved in antibiotic resistance and have a broad substrate specificity. However, the physiological significance of these efflux pumps is not fully understood. Here, we have investigated the role of the RND system TtgABC in resistance to metal ion chelators in the soil bacterium Pseudomonas putida KT2440. We observed that the combined action of an RND inhibitor and the chelator 2,2'-bipyridyl inhibited bacterial growth. In addition, the deletion of ttgB made the strain susceptible to 2,2'-bipyridyl and natural bipyridyl derivatives such as caerulomycin A, indicating that TtgABC is required for detoxification of compounds of the bipyridyl family. Searching for the basis of growth inhibition by bipyridyls, we found reduced adenosine triphosphate (ATP) levels in the ttgB mutant compared to the wild type. Furthermore, the expression of genes related to iron acquisition and the synthesis of the siderophore pyoverdine were reduced in the mutant compared to the wild type. Investigating the possibility that 2,2'-bipyridyl in the ttgB mutant mediates iron accumulation in cells (which would cause the upregulation of genes involved in oxidative stress via the Fenton reaction), we measured the expression of genes coding for proteins involved in intracellular iron storage and the response to oxidative stress. However, none of the genes was significantly upregulated. In a further search for a possible link between 2,2'-bipyridyl and the observed phenotypes, we considered the possibility that the ion chelator limits the intracellular availability of metabolically important metal ions. In this context, we found that the addition of copper restores the growth of the ttgB mutant and the production of pyoverdine, suggesting a relationship between copper availability and iron acquisition. Taken together, the results suggest that detoxification of metal chelating compounds of the bipyridyl family produced by other bacteria or higher ordered organisms is one of the native functions of the RND efflux pump TtgABC. Without the efflux pump, these compounds may interfere with cell ion homeostasis with adverse effects on cell metabolism, including siderophore production. Finally, our results suggest that TtgABC is involved in resistance to bile salts and deoxycholate.

Transport and kinase activities of CbrA of Pseudomonas putida KT2440.

The CbrA/CbrB system is a two-component signal transduction system known to participate in the regulation of the cellular carbon/nitrogen balance and to play a central role in carbon catabolite repression in Pseudomonas species. CbrA is composed of a domain with similarity to proteins of the solute/sodium symporter family (SLC5) and domains typically found in bacterial sensor kinases. Here, the functional properties of the sensor kinase CbrA and its domains are analyzed at the molecular level using the system of the soil bacterium P. putida KT2440 as a model. It is demonstrated that CbrA can bind and transport L-histidine. Transport is specific for L-histidine and probably driven by an electrochemical proton gradient. The kinase domain is not required for L-histidine uptake by the SLC5 domain of CbrA, and has no significant impact on transport kinetics. Furthermore, it is shown that the histidine kinase can autophosphorylate and transfer the phosphoryl group to the response regulator CbrB. The SLC5 domain is not essential for these activities but appears to modulate the autokinase activity. A phosphatase activity of CbrA is not detected. None of the activities is significantly affected by L-histidine. The results demonstrate that CbrA functions as a L-histidine transporter and sensor kinase.

Involvement of MexS and MexEF-OprN in Resistance to Toxic Ion Chelators in Pseudomonas putida KT2440.

Bacteria must be able to cope with harsh environments to survive. In Gram-negative bacteria like Pseudomonas species, resistance-nodulation-division (RND) transporters contribute to this task by pumping toxic compounds out of cells. Previously, we found that the RND system TtgABC of Pseudomonas putida KT2440 confers resistance to toxic metal chelators of the bipyridyl group. Here, we report that the incubation of a ttgB mutant in medium containing 2,2'-bipyridyl generated revertant strains able to grow in the presence of this compound. This trait was related to alterations in the pp_2827 locus (homolog of mexS in Pseudomonas aeruginosa). The deletion and complementation of pp_2827 confirmed the importance of the locus for the revertant phenotype. Furthermore, alteration in the pp_2827 locus stimulated expression of the mexEF-oprN operon encoding an RND efflux pump. Deletion and complementation of mexF confirmed that the latter system can compensate the growth defect of the ttgB mutant in the presence of 2,2'-bipyridyl. To our knowledge, this is the first report on a role of pp_2827 (mexS) in the regulation of mexEF-oprN in P. putida KT2440. The results expand the information about the significance of MexEF-OprN in the stress response of P. putida KT2440 and the mechanisms for coping with bipyridyl toxicity.

Backbone structure of transmembrane domain IX of the Na+/proline transporter PutP of Escherichia coli.

The backbone structure is determined by site-directed spin labeling, double electron electron resonance measurements of distances, and modeling in terms of a helix-loop-helix construct for a transmembrane domain that is supposed to line the translocation pathway in the 54.3 kDa Na(+)/proline symporter PutP of Escherichia coli. The conformational distribution of the spin labels is accounted for by a rotamer library. An ensemble of backbone models with a root mean-square deviation of less than 2 A is obtained. These models exhibit a pronounced kink near residue T341, which is involved in substrate binding. The kink may be associated with a hinge that allows the protein to open and close an inwardly oriented cavity.

PvdRT-OpmQ and MdtABC-OpmB efflux systems are involved in pyoverdine secretion in Pseudomonas putida KT2440.

Fluorescent pseudomonads produce and secrete a siderophore termed pyoverdine to capture iron when it becomes scarce. The molecular basis of pyoverdine secretion is only partially understood. Here, we investigate the role of the putative PvdRT-OpmQ and MdtABC-OpmB efflux systems in pyoverdine secretion in the soil bacterium Pseudomonas putida KT2440. Expression from the respective promoters is stimulated by iron limitation albeit to varying degrees. Deletion of pvdRT-opmQ leads to reduced amounts of pyoverdine in the medium and decreased growth under iron limitation. Deletion of mdtABC-opmB does not affect growth. However, when both systems are deleted, strong effects on growth and pyoverdine secretion (yellow colony phenotype, less pyoverdine in medium, more pyoverdine in the periplasm) are observed. Overexpression of pvdRT-opmQ causes the opposite effect. These results provide first evidence for an involvement of the multidrug efflux system MdtABC-OpmB in pyoverdine secretion. In addition, the PvdRT-OpmQ system was shown to contribute to pyoverdine secretion in P. putida KT2440, extending previous investigations on its role in Pseudomonas species. Since the double deletion mutant still secrets pyoverdine, at least one additional efflux system participates in the transport of the siderophore. Furthermore, our results suggest a contribution of both efflux systems to ampicillin resistance.

Comparison of the functional properties of trimeric and monomeric CaiT of Escherichia coli.

Secondary transporters exist as monomers, dimers or higher state oligomers. The significance of the oligomeric state is only partially understood. Here, the significance of the trimeric state of the L-carnitine/γ-butyrobetaine antiporter CaiT of Escherichia coli was investigated. Amino acids important for trimer stability were identified and experimentally verified. Among others, CaiT-D288A and -D288R proved to be mostly monomeric in detergent solution and after reconstitution into proteoliposomes, as shown by blue native gel electrophoresis, gel filtration, and determination of intermolecular distances. CaiT-D288A was fully functional with kinetic parameters similar to the trimeric wild-type. Significant differences in amount and stability in the cell membrane between monomeric and trimeric CaiT were not observed. Contrary to trimeric CaiT, addition of substrate had no or only a minor effect on the tryptophan fluorescence of monomeric CaiT. The results suggest that physical contacts between protomers are important for the substrate-induced changes in protein fluorescence and the underlying conformational alterations.

Interactions mediated by a public good transiently increase cooperativity in growing Pseudomonas putida metapopulations.

Bacterial communities have rich social lives. A well-established interaction involves the exchange of a public good in Pseudomonas populations, where the iron-scavenging compound pyoverdine, synthesized by some cells, is shared with the rest. Pyoverdine thus mediates interactions between producers and non-producers and can constitute a public good. This interaction is often used to test game theoretical predictions on the "social dilemma" of producers. Such an approach, however, underestimates the impact of specific properties of the public good, for example consequences of its accumulation in the environment. Here, we experimentally quantify costs and benefits of pyoverdine production in a specific environment, and build a model of population dynamics that explicitly accounts for the changing significance of accumulating pyoverdine as chemical mediator of social interactions. The model predicts that, in an ensemble of growing populations (metapopulation) with different initial producer fractions (and consequently pyoverdine contents), the global producer fraction initially increases. Because the benefit of pyoverdine declines at saturating concentrations, the increase need only be transient. Confirmed by experiments on metapopulations, our results show how a changing benefit of a public good can shape social interactions in a bacterial population.

Non-Selective Evolution of Growing Populations.

Non-selective effects, like genetic drift, are an important factor in modern conceptions of evolution, and have been extensively studied for constant population sizes (Kimura, 1955; Otto and Whitlock, 1997). Here, we consider non-selective evolution in the case of growing populations that are of small size and have varying trait compositions (e.g. after a population bottleneck). We find that, in these conditions, populations never fixate to a trait, but tend to a random limit composition, and that the distribution of compositions "freezes" to a steady state. This final state is crucially influenced by the initial conditions. We obtain these findings from a combined theoretical and experimental approach, using multiple mixed subpopulations of two Pseudomonas putida strains in non-selective growth conditions (Matthijs et al, 2009) as model system. The experimental results for the population dynamics match the theoretical predictions based on the Pólya urn model (Eggenberger and Pólya, 1923) for all analyzed parameter regimes. In summary, we show that exponential growth stops genetic drift. This result contrasts with previous theoretical analyses of non-selective evolution (e.g. genetic drift), which investigated how traits spread and eventually take over populations (fixate) (Kimura, 1955; Otto and Whitlock, 1997). Moreover, our work highlights how deeply growth influences non-selective evolution, and how it plays a key role in maintaining genetic variability. Consequently, it is of particular importance in life-cycles models (Melbinger et al, 2010; Cremer et al, 2011; Cremer et al, 2012) of periodically shrinking and expanding populations.

The sodium/substrate symporter family: structural and functional features.

Members of the sodium/substrate symporter family (SSSF, TC 2.A.21) catalyze the uptake of a wide variety of solutes including sugars, proline, pantothenate, and iodide into cells of pro- and eukaryotic origin. Extensive analyses of the topology of different SSSF proteins suggest an arrangement of 13 transmembrane domains as a common topological motif. Regions involved in sodium and/or substrate binding were identified. Furthermore, protein chemical and spectroscopic studies reveal ligand-induced structural alterations which are consistent with close interactions between the sites of sodium and substrate binding, thereby supporting an ordered binding mechanism for transport.