The Escherichia coli AmtB protein as a model system for understanding ammonium transport by Amt and Rh proteins

Biological ammonium transporters from the Amt/Mep/Rh superfamily: mechanism, energetics, and technical limitations

The exchange of ammonium across cellular membranes is a fundamental process in all domains of life and is facilitated by the ubiquitous Amt/Mep/Rh transporter superfamily. Remarkably, despite a high structural conservation in all domains of life, these proteins have gained various biological functions during evolution. It is tempting to hypothesise that the physiological functions gained by these proteins may be explained at least in part by differences in the energetics of their translocation mechanisms. Therefore, in this review, we will explore our current knowledge of energetics of the Amt/Mep/Rh family, discuss variations in observations between different organisms, and highlight some technical drawbacks which have hampered effects at mechanistic characterisation. Through the review, we aim to provide a comprehensive overview of current understanding of the mechanism of transport of this unique and extraordinary Amt/Mep/Rh superfamily of ammonium transporters.

Evolutionary aspects of gastrointestinal tract microbiome-host interaction underlying gastrointestinal barrier integrity

In the host sustenance and homeostasis, the microbiome is a key component in the functional system. Throughout ontogenetic development, microbiome including that of the gastrointestinal tract (GIT) is the vital factor that ensures not only host functioning, but also its interaction with environment. To uncover the mechanisms underlying GIT microbiome showing a decisive influence on host organism, a systematic approach is needed, because diverse microorganisms are predominantly localized in different parts of the GIT. Recently, a new interdisciplinary direction of science, nanobioinformatics that has been extensively developed considers gene networks as the major object of study representing a coordinated group of genes that functionally account for formation and phenotypic disclosure of various host traits. Here, an important place should be provided to the genetically determined level of the gastrointestinal tract microbiome, its interaction at the level of the host food systems. There have been increasing evidence indicating that the microbiome is directly involved in the pathogenesis of host diseases showing a multi-layered interaction with host metabolic and immune systems. At the same time, the microbial community is unevenly distributed throughout the gastrointestinal tract, and its different portions are variously active while interacting with the host immune system. The architecture of interaction between the microbiome and host cells is extremely complex, and the interaction of individual cells, at the same time, varies greatly. Bacteria colonizing the crypts of the small intestine regulate enterocyte proliferation by affecting DNA replication and gene expression, while bacteria at the tip of the intestinal villi mediate gene expression responsible for metabolism and immune response. Enterocytes and Paneth cells, in turn, regulate the vital activity of the community of microorganisms through the production of polysaccharides (carbohydrates) and antibacterial factors on their surface. Thus, the integrity of the gastrointestinal barrier (GIB) is maintained, which protects the body from infections and inflammation, while violation of its integrity leads to a number of diseases. It has been shown that depending on the dominance of certain types of bacteria the microbiome can maintain or disrupt GIB integrity. The structural and functional GIB integrity is important for body homeostasis. To date, at least 50 proteins have been characterized as being involved in the structural and functional integrability of tight junctions between gastrointestinal tract epithelial cells. The current review comprehensively discusses such issues and presents original research carried out at various facilities of translational biomedicine.

Nutrient Limitation Causes Differential Expression of Transport- and Metabolism Genes in the Compartmentalized Anammox Bacterium Kuenenia stuttgartiensis

Anaerobic ammonium-oxidizing (anammox) bacteria, members of the "Candidatus Brocadiaceae" family, play an important role in the nitrogen cycle and are estimated to be responsible for about half of the oceanic nitrogen loss to the atmosphere. Anammox bacteria combine ammonium with nitrite and produce dinitrogen gas via the intermediates nitric oxide and hydrazine (anammox reaction) while nitrate is formed as a by-product. These reactions take place in a specialized, membrane-enclosed compartment called the anammoxosome. Therefore, the substrates ammonium, nitrite and product nitrate have to cross the outer-, cytoplasmic-, and anammoxosome membranes to enter or exit the anammoxosome. The genomes of all anammox species harbor multiple copies of ammonium-, nitrite-, and nitrate transporter genes. Here we investigated how the distinct genes for ammonium-, nitrite-, and nitrate- transport were expressed during substrate limitation in membrane bioreactors. Transcriptome analysis of Kuenenia stuttgartiensis planktonic cells showed that four of the seven ammonium transporter homologs and two of the nine nitrite transporter homologs were significantly upregulated during ammonium-limited growth, while another ammonium transporter- and four nitrite transporter homologs were upregulated in nitrite limited growth conditions. The two nitrate transporters were expressed to similar levels in both conditions. In addition, genes encoding enzymes involved in the anammox reaction were differentially expressed, with those using nitrite as a substrate being upregulated under nitrite limited growth and those using ammonium as a substrate being upregulated during ammonium limitation. Taken together, these results give a first insight in the potential role of the multiple nutrient transporters in regulating transport of substrates and products in and out of the compartmentalized anammox cell.

Identification and functional analysis of glutamine transporter in Streptococcus mutans

Background

Streptococcus mutans, a biofilm-forming bacterium, possesses several transporters that function as import/export molecules. Among them, the PII protein family is composed of members that regulate glutamine synthesis in bacterial species.

Objective

In this study, we characterized the function of the glutamine transporter in S. mutans MT8148.

Methods

The SMU.732 gene, corresponding to glnP in S. mutans, is homologous to the glutamine transporter gene in Bacillus subtilis. We constructed a glnP-inactivated mutant strain (GEMR) and a complement strain (comp-GEMR) and evaluated their biological functions.

Results

Growth of GEMR was similar in the presence and absence of glutamine, whereas the growth rates of MT8148 and comp-GEMR were significantly lower in the presence of glutamine as compared to its absence. Furthermore, biofilms formed by MT8148 and comp-GEMR were significantly thicker than that formed by GEMR, while the GEMR strain showed a significantly lower survival rate in an acidic environment than the other strains. Addition of n-phenyl-2-naphthylamine, used to label of the membrane, led to increased fluorescence intensity of MT8148 and GEMR, albeit that was significantly lower in the latter.

Conclusions

These results suggest that glnP is associated with glutamine transport in S. mutans, especially the import of glutamine involved in biofilm formation.

The lipid environment determines the activity of the Escherichia coli ammonium transporter AmtB

The movement of ammonium across biologic membranes is a fundamental process in all living organisms and is mediated by the ubiquitous ammonium transporter/methylammonium permease/rhesus protein (Amt/Mep/Rh) family of transporters. Recent structural analysis and coupled mass spectrometry studies have shown that the Escherichia coli ammonium transporter AmtB specifically binds 1-palmitoyl-2-oleoyl phosphatidylglycerol (POPG). Upon POPG binding, several residues of AmtB undergo a small conformational change, which stabilizes the protein against unfolding. However, no studies have so far been conducted, to our knowledge, to explore whether POPG binding to AmtB has functional consequences. Here, we used an in vitro experimental assay with purified components, together with molecular dynamics simulations, to characterize the relation between POPG binding and AmtB activity. We show that the AmtB activity is electrogenic. Our results indicate that the activity, at the molecular level, of Amt in archaebacteria and eubacteria may differ. We also show that POPG is an important cofactor for AmtB activity and that, in the absence of POPG, AmtB cannot complete the full translocation cycle. Furthermore, our simulations reveal previously undiscovered POPG binding sites on the intracellular side of the lipid bilayer between the AmtB subunits. Possible molecular mechanisms explaining the functional role of POPG are discussed.-Mirandela, G. D., Tamburrino, G., Hoskisson, P. A., Zachariae, U., Javelle, A. The lipid environment determines the activity of the Escherichia coli ammonium transporter AmtB.

Merging In-Solution X-ray and Neutron Scattering Data Allows Fine Structural Analysis of Membrane-Protein Detergent Complexes

In-solution small-angle X-ray and neutron scattering (SAXS/SANS) have become popular methods to characterize the structure of membrane proteins, solubilized by either detergents or nanodiscs. SANS studies of protein-detergent complexes usually require deuterium-labeled proteins or detergents, which in turn often lead to problems in their expression or purification. Here, we report an approach whose novelty is the combined analysis of SAXS and SANS data from an unlabeled membrane protein complex in solution in two complementary ways. First, an explicit atomic analysis, including both protein and detergent molecules, using the program WAXSiS, which has been adapted to predict SANS data. Second, the use of MONSA which allows one to discriminate between detergent head- and tail-groups in an ab initio approach. Our approach is readily applicable to any detergent-solubilized protein and provides more detailed structural information on protein-detergent complexes from unlabeled samples than SAXS or SANS alone.

Molecular characterization of two Rhesus glycoproteins from the euryhaline freshwater white-rimmed stingray, Himantura signifer, and changes in their transcript levels and protein abundance in the gills, kidney, and liver during brackish water acclimation

Himantura signifer is a freshwater stingray which inhabits rivers in Southeast Asia. It is ammonotelic in fresh water, but retains the capacities of urea synthesis and ureosmotic osmoregulation to survive in brackish water. This study aimed to elucidate the roles of Rhesus glycoproteins (Rhgp), which are known to transport ammonia, in conserving nitrogen (N) in H. signifer during brackish water acclimation when N became limited resulting from increased hepatic urea synthesis. The complete coding sequence of rhbg from H. signifer consisted of 1383 bp, encoding 460 amino acids with an estimated molecular mass of 50.5 kDa, while that of rhcg comprised 1395 bp, encoding for 464 amino acids with an estimated molecular mass of 50.8 kDa. The deduced amino sequences of Rhbg and Rhcg contained ammonia binding sites, which could recruit NH₄⁺ to be deprotonated, and a hydrophobic pore with two histidine residues, which could mediate the transport of NH₃. Our results indicated for the first time that brackish water acclimation resulted in significant decreases in the expression levels of rhbg/Rhbg and rhcg/Rhcg in the gills of H. signifer, which offered a mechanistic explanation of brackish water-related decreased ammonia excretion reported elsewhere. Furthermore, rhbg/Rhbg expression levels increased significantly in the liver of H. signifer during brackish water acclimation, indicating that the ammonia produced by extra-hepatic tissues and released into the blood could be channeled into the liver for increased urea synthesis. Overall, these results lend support to the proposition that H. signifer becomes N-limited upon utilizing urea as an osmolyte in brackish water.

Comparative genomics unravels metabolic differences at the species and/or strain level and extremely acidic environmental adaptation of ten bacteria belonging to the genus Acidithiobacillus

Members of the Acidithiobacillus genus are widely found in extreme environments characterized by low pH and high concentrations of toxic substances, thus it is necessary to identify the cellular mechanisms needed to cope with these harsh conditions. Pan-genome analysis of ten bacteria belonging to the genus Acidithiobacillus suggested the existence of core genome, most of which were assigned to the metabolism-associated genes. Additionally, the unique genes of Acidithiobacillus ferrooxidans were much less than those of other species. A large proportion of Acidithiobacillus ferrivorans-specific genes were mapped especially to metabolism-related genes, indicating that diverse metabolic pathways might confer an advantage for adaptation to local environmental conditions. Analyses of functional metabolisms revealed the differences of carbon metabolism, nitrogen metabolism, and sulfur metabolism at the species and/or strain level. The findings also showed that Acidithiobacillus spp. harbored specific adaptive mechanisms for thriving under extreme environments. The genus Acidithiobacillus had the genetic potential to resist and metabolize toxic substances such as heavy metals and organic solvents. Comparison across species and/or strains of Acidithiobacillus populations provided a deeper appreciation of metabolic differences and environmental adaptation, as well as highlighting the importance of cellular mechanisms that maintain the basal physiological functions under complex acidic environmental conditions.

Antennal-expressed ammonium transporters in the malaria vector mosquito Anopheles gambiae

The principal Afrotropical malaria vector mosquito, Anopheles gambiae remains a significant threat to human health. In this anthropophagic species, females detect and respond to a range of human-derived volatile kairomones such as ammonia, lactic acid, and other carboxylic acids in their quest for blood meals. While the molecular underpinnings of mosquito olfaction and host seeking are becoming better understood, many questions remain unanswered. In this study, we have identified and characterized two candidate ammonium transporter genes, AgAmt and AgRh50 that are expressed in the mosquito antenna and may contribute to physiological and behavioral responses to ammonia, which is an important host kairomone for vector mosquitoes. AgAmt transcripts are highly enhanced in female antennae while a splice variant of AgRh50 appears to be antennal-specific. Functional expression of AgAmt in Xenopus laevis oocytes facilitates inward currents in response to both ammonium and methylammonium, while AgRh50 is able to partially complement a yeast ammonium transporter mutant strain, validating their conserved roles as ammonium transporters. We present evidence to suggest that both AgAmt and AgRh50 are in vivo ammonium transporters that are important for ammonia sensitivity in An. gambiae antennae, either by clearing ammonia from the sensillar lymph or by facilitating sensory neuron responses to environmental exposure. Accordingly, AgAmt and AgRh50 represent new and potentially important targets for the development of novel vector control strategies.

Identification and functional analysis of an ammonium transporter in Streptococcus mutans

Streptococcus mutans, a Gram-positive bacterium, is considered to be a major etiologic agent of human dental caries and reported to form biofilms known as dental plaque on tooth surfaces. This organism is also known to possess a large number of transport proteins in the cell membrane for export and import of molecules. Nitrogen is an essential nutrient for Gram-positive bacteria, though alternative sources such as ammonium can also be utilized. In order to obtain nitrogen for macromolecular synthesis, nitrogen-containing compounds must be transported into the cell. However, the ammonium transporter in S. mutans remains to be characterized. The present study focused on characterizing the ammonium transporter gene of S. mutans and its operon, while related regulatory genes were also analyzed. The SMU.1658 gene corresponding to nrgA in S. mutans is homologous to the ammonium transporter gene in Bacillus subtilis and SMU.1657, located upstream of the nrgA gene and predicted to be glnB, is a member of the PII protein family. Using a nrgA-deficient mutant strain (NRGD), we examined bacterial growth in the presence of ammonium, calcium chloride, and manganese sulfate. Fluorescent efflux assays were also performed to reveal export molecules associated with the ammonium transporter. The growth rate of NRGD was lower, while its fluorescent intensity was much higher as compared to the parental strain. In addition, confocal laser scanning microscopy revealed that the structure of biofilms formed by NRGD was drastically different than that of the parental strain. Furthermore, transcriptional analysis showed that the nrgA gene was co-transcribed with the glnB gene. These results suggest that the nrgA gene in S. mutans is essential for export of molecules and biofilm formation.

The last generation of bacterial growth in limiting nutrient

Background

Bacterial growth as a function of nutrients has been studied for decades, but is still not fully understood. In particular, the growth laws under dynamically changing environments have been difficult to explore, because of the rapidly changing conditions. Here, we address this challenge by means of a robotic assay and measure bacterial growth rate, promoter activity and substrate level at high temporal resolution across the entire growth curve in batch culture. As a model system, we study E. coli growing under nitrogen or carbon limitation, and explore the dynamics in the last generation of growth where nutrient levels can drop rapidly.

Results

We find that growth stops abruptly under limiting nitrogen or carbon, but slows gradually when nutrients are not limiting. By measuring growth rate at a 3 min time resolution, and inferring the instantaneous substrate level, s, we find that the reduction in growth rate μ under nutrient limitation follows Monod’s law, μ=μ0sks+s. By following promoter activity of different genes we found that the abrupt stop of growth under nitrogen or carbon limitation is accompanied by a pulse-like up-regulation of the expression of genes in the relevant nutrient assimilation pathways. We further find that sharp stop of growth is conditional on the presence of regulatory proteins in the assimilation pathway.

Conclusions

The observed sharp stop of growth accompanied by a pulsed expression of assimilation genes allows bacteria to compensate for the drop in nutrients, suggesting a strategy used by the cells to prolong exponential growth under limiting substrate.

Flux balance analysis of ammonia assimilation network in E. coli predicts preferred regulation point

Nitrogen assimilation is a critical biological process for the synthesis of biomolecules in Escherichia coli. The central ammonium assimilation network in E. coli converts carbon skeleton α-ketoglutarate and ammonium into glutamate and glutamine, which further serve as nitrogen donors for nitrogen metabolism in the cell. This reaction network involves three enzymes: glutamate dehydrogenase (GDH), glutamine synthetase (GS) and glutamate synthase (GOGAT). In minimal media, E. coli tries to maintain an optimal growth rate by regulating the activity of the enzymes to match the availability of the external ammonia. The molecular mechanism and the strategy of the regulation in this network have been the research topics for many investigators. In this paper, we develop a flux balance model for the nitrogen metabolism, taking into account of the cellular composition and biosynthetic requirements for nitrogen. The model agrees well with known experimental results. Specifically, it reproduces all the (15)N isotope labeling experiments in the wild type and the two mutant (ΔGDH and ΔGOGAT) strains of E. coli. Furthermore, the predicted catalytic activities of GDH, GS and GOGAT in different ammonium concentrations and growth rates for the wild type, ΔGDH and ΔGOGAT strains agree well with the enzyme concentrations obtained from western blots. Based on this flux balance model, we show that GS is the preferred regulation point among the three enzymes in the nitrogen assimilation network. Our analysis reveals the pattern of regulation in this central and highly regulated network, thus providing insights into the regulation strategy adopted by the bacteria. Our model and methods may also be useful in future investigations in this and other networks.

Control of AmtB-GlnK complex formation by intracellular levels of ATP, ADP, and 2-oxoglutarate

P(II) proteins are one of the most widespread families of signal transduction proteins in nature, being ubiquitous throughout bacteria, archaea, and plants. They play a major role in coordinating nitrogen metabolism by interacting with, and regulating the activities of, a variety of enzymes, transcription factors, and membrane transport proteins. The regulatory properties of P(II) proteins derive from their ability to bind three effectors: ATP, ADP, and 2-oxoglutarate. However, a clear model to integrate physiological changes with the consequential structural changes that mediate P(II) interaction with a target protein has so far not been developed. In this study, we analyzed the fluctuations in intracellular effector pools in Escherichia coli during association and dissociation of the P(II) protein GlnK with the ammonia channel AmtB. We determined that key features promoting AmtB-GlnK complex formation are the rapid drop in the 2-oxoglutarate pool upon ammonium influx and a simultaneous, but transient, change in the ATP/ADP ratio. We were also able to replicate AmtB-GlnK interactions in vitro using the same effector combinations that we observed in vivo. This comprehensive data set allows us to propose a model that explains the way in which interactions between GlnK and its effectors influence the conformation of GlnK and thereby regulate its interaction with AmtB.

The Rh protein family: gene evolution, membrane biology, and disease association

The Rh (Rhesus) genes encode a family of conserved proteins that share a structural fold of 12 transmembrane helices with members of the major facilitator superfamily. Interest in this family has arisen from the discovery of Rh factor's involvement in hemolytic disease in the fetus and newborn, and of its homologs widely expressed in epithelial tissues. The Rh factor and Rh-associated glycoprotein (RhAG), with epithelial cousins RhBG and RhCG, form four subgroups conferring upon vertebrates a genealogical commonality. The past decade has heralded significant advances in understanding the phylogenetics, allelic diversity, crystal structure, and biological function of Rh proteins. This review describes recent progress on this family and the molecular insights gleaned from its gene evolution, membrane biology, and disease association. The focus is on its long evolutionary history and surprising structural conservation from prokaryotes to humans, pointing to the importance of its functional role, related to but distinct from ammonium transport proteins.

Generation and characterisation of Rhd and Rhag null mice

Mouse Rhd* and Rhag* genes were targeted using insertional vectors; the resulting knockout mice, and double-knockout descendants, were analysed. Rhag glycoprotein deficiency entailed defective assembly of the erythroid Rh complex with complete loss of Rh and intercellular adhesion molecule 4 (ICAM-4), but not CD47, expression. Absence of the Rh protein induced a loss of ICAM-4, and only a moderate reduction of Rhag expression. Double knockout phenotype was similar to that of Rhag targeted mice. Rhd and Rhag deficient mice exhibited neither the equivalent of human Rh(null) haemolytic anaemia nor any clinical or cellular abnormalities. Rhd-/- and Rhag-/- erythrocytes showed decreased basal adhesion to an endothelial cell line resulting from defective ICAM-4 membrane expression. There was no difference in recovery from phenylhydrazine-induced haematopoietic stress for double knockout mice as compared to controls, suggesting that ICAM-4 might be dispensable during stress erythropoiesis. Ammonia and methylammonia transport in erythrocytes was severely impaired in Rhag-/- but only slightly in Rhd-/- animals that significantly expressed Rhag, supporting the view that RhAG and Rhag, but not Rh, may act as ammonium transporters in human and mouse erythrocytes. These knockout mice should prove useful for further dissecting the physiological roles of Rh and Rhag proteins in the red cell membrane.

Comparative genomic analysis of carbon and nitrogen assimilation mechanisms in three indigenous bioleaching bacteria: predictions and validations

BACKGROUND

Carbon and nitrogen fixation are essential pathways for autotrophic bacteria living in extreme environments. These bacteria can use carbon dioxide directly from the air as their sole carbon source and can use different sources of nitrogen such as ammonia, nitrate, nitrite, or even nitrogen from the air. To have a better understanding of how these processes occur and to determine how we can make them more efficient, a comparative genomic analysis of three bioleaching bacteria isolated from mine sites in Chile was performed. This study demonstrated that there are important differences in the carbon dioxide and nitrogen fixation mechanisms among bioleaching bacteria that coexist in mining environments.

RESULTS

In this study, we probed that both Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans incorporate CO2 via the Calvin-Benson-Bassham cycle; however, the former bacterium has two copies of the Rubisco type I gene whereas the latter has only one copy. In contrast, we demonstrated that Leptospirillum ferriphilum utilizes the reductive tricarboxylic acid cycle for carbon fixation. Although all the species analyzed in our study can incorporate ammonia by an ammonia transporter, we demonstrated that Acidithiobacillus thiooxidans could also assimilate nitrate and nitrite but only Acidithiobacillus ferrooxidans could fix nitrogen directly from the air.

CONCLUSION

The current study utilized genomic and molecular evidence to verify carbon and nitrogen fixation mechanisms for three bioleaching bacteria and provided an analysis of the potential regulatory pathways and functional networks that control carbon and nitrogen fixation in these microorganisms.

The 1.3-A resolution structure of Nitrosomonas europaea Rh50 and mechanistic implications for NH3 transport by Rhesus family proteins

The Rhesus (Rh) proteins are a family of integral membrane proteins found throughout the animal kingdom that also occur in a number of lower eukaryotes. The significance of Rh proteins derives from their presence in the human red blood cell membrane, where they constitute the second most important group of antigens used in transfusion medicine after the ABO group. Rh proteins are related to the ammonium transport (Amt) protein family and there is considerable evidence that, like Amt proteins, they function as ammonia channels. We have now solved the structure of a rare bacterial homologue (from Nitrosomonas europaea) of human Rh50 proteins at a resolution of 1.3 A. The protein is a trimer, and analysis of its subunit interface strongly argues that all Rh proteins are likely to be homotrimers and that the human erythrocyte proteins RhAG and RhCE/D are unlikely to form heterooligomers as previously proposed. When compared with structures of bacterial Amt proteins, NeRh50 shows several distinctive features of the substrate conduction pathway that support the concept that Rh proteins have much lower ammonium affinities than Amt proteins and might potentially function bidirectionally.

Structure of the Nitrosomonas europaea Rh protein

Amt/MEP/Rh proteins are a family of integral membrane proteins implicated in the transport of NH3, CH(2)NH2, and CO2. Whereas Amt/MEP proteins are agreed to transport ammonia (NH3/NH4+), the primary substrate for Rh proteins has been controversial. Initial studies suggested that Rh proteins also transport ammonia, but more recent evidence suggests that they transport CO2. Here we report the first structure of an Rh family member, the Rh protein from the chemolithoautotrophic ammonia-oxidizing bacterium Nitrosomonas europaea. This Rh protein exhibits a number of similarities to its Amt cousins, including a trimeric oligomeric state, a central pore with an unusual twin-His site in the middle, and a Phe residue that blocks the channel for small-molecule transport. However, there are some significant differences, the most notable being the presence of an additional cytoplasmic C-terminal alpha-helix, an increased number of internal proline residues along the transmembrane helices, and a specific set of residues that appear to link the C-terminal helix to Phe blockage. This latter linkage suggests a mechanism in which binding of a partner protein to the C terminus could regulate channel opening. Another difference is the absence of the extracellular pi-cation binding site conserved in Amt/Mep structures. Instead, CO2 pressurization experiments identify a CO2 binding site near the intracellular exit of the channel whose residues are highly conserved in all Rh proteins, except those belonging to the Rh30 subfamily. The implications of these findings on the functional role of the human Rh antigens are discussed.

Ternary complex formation between AmtB, GlnZ and the nitrogenase regulatory enzyme DraG reveals a novel facet of nitrogen regulation in bacteria

Ammonium movement across biological membranes is facilitated by a class of ubiquitous channel proteins from the Amt/Rh family. Amt proteins have also been implicated in cellular responses to ammonium availability in many organisms. Ammonium sensing by Amt in bacteria is mediated by complex formation with cytosolic proteins of the P(II) family. In this study we have characterized in vitro complex formation between the AmtB and P(II) proteins (GlnB and GlnZ) from the diazotrophic plant-associative bacterium Azospirillum brasilense. AmtB-P(II) complex formation only occurred in the presence of adenine nucleotides and was sensitive to 2-oxoglutarate when Mg(2+) and ATP were present, but not when ATP was substituted by ADP. We have also shown in vitro complex formation between GlnZ and the nitrogenase regulatory enzyme DraG, which was stimulated by ADP. The stoichiometry of this complex was 1:1 (DraG monomer : GlnZ trimer). We have previously reported that in vivo high levels of extracellular ammonium cause DraG to be sequestered to the cell membrane in an AmtB and GlnZ-dependent manner. We now report the reconstitution of a ternary complex involving AmtB, GlnZ and DraG in vitro. Sequestration of a regulatory protein by the membrane-bound AmtB-P(II) complex defines a new regulatory role for Amt proteins in Prokaryotes.

Inhibitory complex of the transmembrane ammonia channel, AmtB, and the cytosolic regulatory protein, GlnK, at 1.96 A

Ammonia conductance is highly regulated. A P(II) signal transduction protein, GlnK, is the final regulator of transmembrane ammonia conductance by the ammonia channel AmtB in Escherichia coli. The complex formed between AmtB and inhibitory GlnK at 1.96-A resolution shows that the trimeric channel is blocked directly by GlnK and how, in response to intracellular nitrogen status, the ability of GlnK to block the channel is regulated by uridylylation/deuridylylation at Y51. ATP and Mg(2+) augment the interaction of GlnK. The hydrolyzed product, adenosine 5'-diphosphate orients the surface of GlnK for AmtB blockade. 2-Oxoglutarate diminishes AmtB/GlnK association, and sites for 2-oxoglutarate are evaluated.

The Amt/MEP/Rh Family: Structure of AmtB and the Mechanism of Ammonia Gas Conduction

The atomic structures of the first members of the Amt/MEP/Rh family show that they are 11-crossing membrane proteins that form trimers in the membrane. Each monomer supports a hydrophobic channel that conducts NH3but not any water or ions. The reprotonation of NH3on the receiving side raises the pH on that side in the absence of metabolism of NH3, and there is no transfer of protons through the protein.