The Rh (rhesus) blood group polypeptides are related to NH4+ transporters

Periplasmic vestibule plays an important role for solute recruitment, selectivity, and gating in the Rh/Amt/MEP superfamily

AmtB, a member of the Rh/Amt/MEP superfamily, is responsible for ammonia transport in Escherichia coli. The ammonia pathway in AmtB consists of a narrow hydrophobic lumen in between hydrophilic periplasmic and cytoplasmic vestibules. A series of molecular dynamics simulations (greater than 0.4 μs in total) were performed to determine the mechanism of solute recruitments and selectivity by the periplasmic vestibule. The results show that the periplasmic vestibule plays a crucial role in solute selectivity, and its solute preferences follow the order of NH4(+) > NH3 > CO2. Based on our results, NH4(+) recruitment is initiated by its interaction with either E70 or E225, highly conserved residues located at the entrance of the vestibule. Subsequently, the backbone carbonyl groups at the periplasmic vestibule direct NH4(+) to the conserved aromatic cage at the bottom of the vestibule (known as the Am1 site). The umbrella sampling simulations suggest that the conserved residue D160 is not directly involved in the ammonia conduction; rather its main function is to keep the structure of periplasmic vestibule intact. The MD simulations also revealed that two partially stacked phenyl rings of F107 and F215, separating the periplasmic vestibule from the hydrophobic lumen, flip open and closed simultaneously with a frequency of approximately 10(8) flipping events per second. These results show how the periplasmic vestibule selectively recruits NH4(+) to the Am1 site, and also that the synchronized flipping of two phenyl rings potentially facilitates the solute transition from the periplasmic vestibule to the hydrophobic lumen in the Rh/Amt/MEP superfamily.

Molecular genetics and clinical applications for RH

Rhesus is the clinically most important protein-based blood group system. It represents the largest number of antigens and the most complex genetics of the 30 known blood group systems. The RHD and RHCE genes are strongly homologous. Some genetic complexity is explained by their close chromosomal proximity and unusual orientation, with their tail ends facing each other. The antigens are expressed by the RhD and the RhCE proteins. Rhesus exemplifies the correlation of genotype and phenotype, facilitating the understanding of general genetic mechanisms. For clinical purposes, genetic diagnostics of Rhesus antigens will improve the cost-effective development of transfusion medicine.

Potentials of mean force and permeabilities for carbon dioxide, ammonia, and water flux across a Rhesus protein channel and lipid membranes

As a member of the ubiquitous ammonium transporter/methylamine permease/Rhesus (Amt/MEP/Rh) family of membrane protein channels, the 50 kDa Rhesus channel (Rh50) has been implicated in ammonia (NH(3)) and, more recently, also in carbon dioxide (CO(2)) transport. Here we present molecular dynamics simulations of spontaneous full permeation events of ammonia and carbon dioxide across Rh50 from Nitrosomonas europaea. The simulations show that Rh50 is functional in its crystallographic conformation, without the requirement for a major conformational change or the action of a protein partner. To assess the physiological relevance of NH(3) and CO(2) permeation across Rh50, we have computed potentials of mean force (PMFs) and permeabilities for NH(3) and CO(2) flux across Rh50 and compare them to permeation through a wide range of lipid membranes, either composed of pure lipids or composed of lipids plus an increasing cholesterol content. According to the PMFs, Rh50 is expected to enhance NH(3) flux across dense membranes, such as membranes with a substantial cholesterol content. Although cholesterol reduces the intrinsic CO(2) permeability of lipid membranes, the CO(2) permeabilities of all membranes studied here are too high to allow significant Rh50-mediated CO(2) flux. The increased barrier in the PMF for water permeation across Rh50 shows that Rh50 discriminates 40-fold between water and NH(3). Thus, Rh50 channels complement aquaporins, allowing the cell to regulate water and NH(3) flux independently. The PMFs for methylamine and NH(3) are virtually identical, suggesting that methylamine provides an excellent model for NH(3) in functional experiments.

Ammonia and urea excretion in the Pacific hagfish Eptatretus stoutii: Evidence for the involvement of Rh and UT proteins

The nature of ammonia and urea excretion was examined in the Pacific hagfish (Eptatretus stoutii), which, under resting conditions, excreted similar quantities of nitrogen as either ammonia or urea. In the presence of high external ammonia (HEA) concentrations, ammonia was taken up at high rates and then excreted at similarly high rates upon return to normal water. However, although elevated by HEA, plasma ammonia levels were maintained at approximately 1-4 μmolNg⁻¹, reflecting time-dependent decreases in the rates of ammonia uptake, the possible conversion of ammonia to urea, and the potential active excretion of ammonia against a gradient. Internal injections of NH₄Cl caused marked increases in the rate of ammonia excretion and a delayed increase in urea excretion that may have resulted from increasing urea levels in the plasma. Conversely, when the rate of urea excretion was reduced in the presence of 0.1 mM phloretin, ammonia excretion was significantly elevated. Rates of urea excretion were initially increased by approximately 1000-fold following internal urea injections while the presence of high external urea levels (5-100 mM final concentration) resulted in associated linear increases in plasma urea levels. Using hagfish skin mounted in Ussing chambers, the rate of diffusion of ammonia across the skin exceeded that of urea by approximately four times when equivalent gradients were imposed. Based on western blotting and immunocytochemistry, hagfish gill appears to possess Rh proteins (Rhag, Rhbg and Rhcg1) and urea transporter proteins. Despite the tolerance of hagfish to high levels of ammonia and urea, it is suggested that the presence of ammonia and urea transporter proteins may be required during the period of time hagfish spend in burrows or while feeding, when conditions of high ammonia and/or urea might be encountered.

Cloning and functional expression of Rh50-like glycoprotein, a putative ammonia channel, in Aedes albopictus mosquitoes

Evidence has shown that female mosquitoes can deaminate more than 80% of the ingested bloodmeal protein amino acids, and thus lead to a massive amount of ammonia production. Ammonia transport is a critical step for detoxifying ammonia in organisms. Here we characterized a putative ammonia channel gene, Rhesus (Rh) 50 glycoprotein, from Aedes albopictus (AalRh50) and determined the difference of its expression profile in different tissues at both message and protein levels as well as its response to a blood meal. We showed that AalRh50 shares a low identity with E. coli ammonia transporter (EcoAmtB), but higher identities with human RhBG and Drosophila Rh50 genes. The analysis of ammonia-conductance sites indicates that AalRh50 has residue substitutions of S237L (equivalent to S219 in AmtB) in the external vestibule, F127I (equivalent to F107 in AmtB) in the pore entrance, and S281N (equivalent to S263 in AmtB) in the internal vestibule, which could alter or reduce ammonia-conductance activity. The results from quantitative real-time-PCR and immunohistochemistry revealed that AalRh50 is expressed at significantly higher levels in the head, Malpighian tubules, and thorax of the non-blood-fed females, suggesting that AalRh50 might play roles in maintaining normal neurotransmitter metabolism, acid-base balance, and flight energy production in different tissues of mosquitoes at the non-blood-fed condition. A blood meal significantly increases AalRh50 expression in midgut, fat body, and Malpighian tubules from 3 or 6 to 24h post feeding, indicating that AalRh50 plays an important role in detoxification of excess systemic ammonia of female adults during the gonotrophic cycle.

Sharpey-Schafer lecture: gas channels

The traditional dogma has been that all gases diffuse through all membranes simply by dissolving in the lipid phase of the membrane. Although this mechanism may explain how most gases move through most membranes, it is now clear that some membranes have no demonstrable gas permeability, and that at least two families of membrane proteins, the aquaporins (AQPs) and the Rhesus (Rh) proteins, can each serve as pathways for the diffusion of both CO₂ and NH₃. The knockout of RhCG in the renal collecting duct leads to the predicted consequences in acid–base physiology, providing a clear-cut role for at least one gas channel in the normal physiology of mammals. In our laboratory, we have found that surface-pH (pH_S) transients provide a sensitive approach for detecting CO₂ and NH₃ movement across the cell membranes of Xenopus oocytes. Using this approach, we have found that each tested AQP and Rh protein has its own characteristic CO₂/NH₃ permeability ratio, which provides the first demonstration of gas selectivity by a channel. Our preliminary AQP1 data suggest that all the NH₃ and less than half of the CO₂ move along with H₂O through the four monomeric aquapores. The majority of CO₂ takes an alternative route through AQP1, possibly the central pore at the four-fold axis of symmetry. Preliminary data with two Rh proteins, bacterial AmtB and human erythroid RhAG, suggest a similar story, with all the NH₃ moving through the three monomeric NH₃ pores and the CO₂ taking a separate route, perhaps the central pore at the three-fold axis of symmetry. The movement of different gases via different pathways is likely to underlie the gas selectivity that these channels exhibit.

Rh glycoprotein expression is modulated in pufferfish (Takifugu rubripes) during high environmental ammonia exposure

Rhesus (Rh) protein involvement in ammonia transport processes in freshwater fish has received considerable attention; however, parallel investigations in seawater species are scant. We exposed pufferfish to high environmental ammonia (HEA; 1 and 5 mmol l–1 NH4HCO3) and evaluated the patterns of ammonia excretion and gill Rh mRNA and protein expression. Gill H+-ATPase, NHE1, NHE2, NHE3, Na+/K+-ATPase (NKA), Na+/K+/2Cl– co-transporter (NKCC1) mRNA, H+-ATPase activity, NKA protein and activity, were also quantified. Activation of NKA by NH4+ was demonstrated in vitro. The downregulation of Rhbg mRNA and simultaneous upregulations of Rhcg1, H+-ATPase, NHE3, NKA, NKCC1 mRNA, H+-ATPase activity, and NKA protein and activity levels suggested that during HEA, ammonia excretion was mediated mainly by mitochondria-rich cells (MRCs) driven by NKA with basolateral NH4+ entry via NKA and/or NKCC1, and apical NH3 extrusion via Rhcg1. Reprotonation of NH3 by NHE3 and/or H+-ATPase would minimise back flux through the Rh channels. Downregulated Rhbg and Rhag mRNA observed in the gill during HEA suggests a coordinated protective response to minimise the influx of external ammonia via the pavement cells and pillar cells, respectively, while routing ammonia excretion through the MRCs. Exposure to hypercapnia (1% CO2 in air) resulted in downregulated gill and erythrocyte Rhag mRNA. Surprisingly, Rhag, Rhbg, Rhcg1 and Rhcg2 proteins responded to both hypercapnia and HEA with changes in their apparent molecular masses. A dual NH3/CO2 transport function of the pufferfish Rh proteins is therefore suggested. The results support and extend an earlier proposed model of pufferfish gill ammonia excretion that was based on immunolocalisation of the Rh proteins. Passive processes and/or Rhbg and Rhcg2 in the pavement cells may maintain basal levels of plasma ammonia but elevated levels may require active excretion via NKA and Rhcg1 in the MRCs.

Function of human Rh based on structure of RhCG at 2.1 A

In humans, NH(3) transport across cell membranes is facilitated by the Rh (rhesus) family of proteins. Human Rh C glycoprotein (RhCG) forms a trimeric complex that plays an essential role in ammonia excretion and renal pH regulation. The X-ray crystallographic structure of human RhCG, determined at 2.1 A resolution, reveals the mechanism of ammonia transport. Each monomer contains 12 transmembrane helices, one more than in the bacterial homologs. Reconstituted into proteoliposomes, RhCG conducts NH(3) to raise internal pH. Models of the erythrocyte Rh complex based on our RhCG structure suggest that the erythrocytic Rh complex is composed of stochastically assembled heterotrimers of RhAG, RhD, and RhCE.

Functional characterization of Rhesus glycoproteins from an ammoniotelic teleost, the rainbow trout, using oocyte expression and SIET analysis

Recent experimental evidence from rainbow trout suggests that gill ammonia transport may be mediated in part via Rhesus (Rh) glycoproteins. In this study we analyzed the transport properties of trout Rh proteins (Rhag, Rhbg1, Rhbg2, Rhcg1, Rhcg2, Rh30-like) expressed in Xenopus oocytes, using the radiolabeled ammonia analogue [14C]methylamine, and the scanning ion electrode technique (SIET). All of the trout Rh proteins, except Rh30-like, facilitated methylamine uptake. Uptake was saturable, with Km values ranging from 4.6 to 8.9 mmol l−1. Raising external pH from 7.5 to 8.5 resulted in 3- to 4-fold elevations in Jmax values for methylamine; Km values were unchanged when expressed as total or protonated methylamine. Efflux of methylamine was also facilitated in Rh-expressing oocytes. Efflux and influx rates were stimulated by a pH gradient, with higher rates observed with steeper H+ gradients. NH4Cl inhibited methylamine uptake in oocytes expressing Rhbg1 or Rhcg2. When external pH was elevated from 7.5 to 8.5, the Ki for ammonia against methylamine transport was 35–40% lower when expressed as total ammonia or NH4+, but 5- to 6-fold higher when expressed as NH3. With SIET we confirmed that ammonia uptake was facilitated by Rhag and Rhcg2, but not Rh30-like proteins. Ammonia uptake was saturable, with a comparable Jmax but lower Km value than for total or protonated methylamine. At low substrate concentrations, the ammonia uptake rate was greater than that of methylamine. The Km for total ammonia (560 μmol l−1) lies within the physiological range for trout. The results are consistent with a model whereby NH4+ initially binds, but NH3 passes through the Rh channels. We propose that Rh glycoproteins in the trout gill are low affinity, high capacity ammonia transporters that exploit the favorable pH gradient formed by the acidified gill boundary layer in order to facilitate rapid ammonia efflux when plasma ammonia concentrations are elevated.

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.

Functional reconstitution into liposomes of purified human RhCG ammonia channel

BACKGROUND

Rh glycoproteins (RhAG, RhBG, RhCG) are members of the Amt/Mep/Rh family which facilitate movement of ammonium across plasma membranes. Changes in ammonium transport activity following expression of Rh glycoproteins have been described in different heterologous systems such as yeasts, oocytes and eukaryotic cell lines. However, in these complex systems, a potential contribution of endogenous proteins to this function cannot be excluded. To demonstrate that Rh glycoproteins by themselves transport NH(3), human RhCG was purified to homogeneity and reconstituted into liposomes, giving new insights into its channel functional properties.

METHODOLOGY/PRINCIPAL FINDINGS

An HA-tag introduced in the second extracellular loop of RhCG was used to purify to homogeneity the HA-tagged RhCG glycoprotein from detergent-solubilized recombinant HEK293E cells. Electron microscopy analysis of negatively stained purified RhCG-HA revealed, after image processing, homogeneous particles of 9 nm diameter with a trimeric protein structure. Reconstitution was performed with sphingomyelin, phosphatidylcholine and phosphatidic acid lipids in the presence of the C(12)E(8) detergent which was subsequently removed by Biobeads. Control of protein incorporation was carried out by freeze-fracture electron microscopy. Particle density in liposomes was a function of the Lipid/Protein ratio. When compared to empty liposomes, ammonium permeability was increased two and three fold in RhCG-proteoliposomes, depending on the Lipid/Protein ratio (1/300 and 1/150, respectively). This strong NH(3) transport was reversibly inhibited by mercuric and copper salts and exhibited a low Arrhenius activation energy.

CONCLUSIONS/SIGNIFICANCE

This study allowed the determination of ammonia permeability per RhCG monomer, showing that the apparent Punit(NH3) (around 1x10(-3) microm(3)xs(-1)) is close to the permeability measured in HEK293E cells expressing a recombinant human RhCG (1.60x10(-3) microm(3)xs(-1)), and in human red blood cells endogenously expressing RhAG (2.18x10(-3) microm(3)xs(-1)). The major finding of this study is that RhCG protein is active as an NH(3) channel and that this function does not require any protein partner.

Branchial ammonia excretion in the Asian weatherloach Misgurnus anguillicaudatus

The weatherloach, Misgurnus anguillicaudatus, is a freshwater, facultative air-breathing fish that lives in streams and rice paddy fields, where it may experience drought and/or high environmental ammonia (HEA) conditions. The aim of this study was to determine what roles branchial Na(+)/K(+)-ATPase, H(+)-ATPase, and Rhcg have in ammonia tolerance and how the weatherloach copes with ammonia loading conditions. The loach's high ammonia tolerance was confirmed as was evident from its high 96 h LC(50) value and high tissue tolerance to ammonia. The weatherloach does not appear to make use of Na(+)/NH(4)(+)-ATPase facilitated transport to excrete ammonia when exposed to HEA or to high environmental pH since no changes in activity were observed. Using immunofluorescence microscopy, distinct populations of vacuolar (V)-type H(+)-ATPase and Na(+)/K(+)-ATPase immunoreactive cells were identified in branchial epithelia, with apical and basolateral staining patterns, respectively. Rhesus C glycoprotein (Rhcg1), an ammonia transport protein, immunoreactivity was also found in a similar pattern as H(+)-ATPase. Rhcg1 (Slc42a3) mRNA expression also increased significantly during aerial exposure, although not significantly under ammonia loading conditions. The colocalization of H(+)-ATPase and Rhcg1 to the similar non-Na(+)/K(+)-ATPase immunoreactive cell type would support a role for H(+)-ATPase in ammonia excretion via Rhcg by NH(4)(+) trapping. The importance of gill boundary layer acidification in net ammonia excretion was confirmed in this fish; however, it was not associated with an increase in H(+)-ATPase expression, since tissue activity and protein levels did not increase with high environmental pH and/or HEA. However the V-ATPase inhibitor, bafilomycin, did decrease net ammonia flux whereas other ion transport inhibitors (amiloride, SITS) had no effect. H(+)-ATPase inhibition also resulted in a consequent elevation in plasma ammonia levels and a decrease in the net acid flux. In gill, aerial exposure was also associated with a significant increase in membrane fluidity (or increase in permeability) which would presumably enhance NH(3) permeation through the plasma membrane. Taken together, these results indicate the gill of the weatherloach is responsive to aerial conditions that would aid ammonia excretion.

The responses of zebrafish (Danio rerio) to high external ammonia and urea transporter inhibition: nitrogen excretion and expression of rhesus glycoproteins and urea transporter proteins

While adult zebrafish, Danio rerio, possess ammonia and urea transporters (Rh and UT proteins, respectively) in a number of tissues, they are most heavily concentrated within the gills. UT has a diffuse expression pattern within Na+-K+-ATPase (NKA)-type mitochondrion-rich cells and Rh proteins form a network similar to the arrangement seen in pufferfish gills (Nakada et al., 2007b). Rhag expression appeared to be limited to the pillar cells lining the blood spaces of the lamellae while Rhbg was localized to the outer layer of both the lamellae and the filament, upon the pavement cells. Exposure to high external ammonia (HEA) or phloretin increased tissue levels of ammonia and urea, respectively, in adult and juvenile zebrafish; however, the responses to these stressors were age dependent. HEA increased mRNA levels for a number of Rh proteins in embryos and larvae but did not elicit similar effects in adult gills, which appear to compensate for the unfavourable ammonia excretory gradient by increasing expression of V-type H+-ATPase. Phloretin exposure increased UT mRNA levels in embryos and larvae but was without effect in adult gill tissue. Surprisingly, in both adults and juveniles, HEA increased the mRNA expression of UT and phloretin increased the mRNA expression of Rh proteins. These results imply that, in zebrafish, there may be a tighter link between ammonia and urea excretion than is thought to occur in most teleosts.

Sensing, physiological effects and molecular response to elevated CO2 levels in eukaryotes

Carbon dioxide (CO₂) is an important gaseous molecule that maintains biosphere homeostasis and is an important cellular signalling molecule in all organisms. The transport of CO₂ through membranes has fundamental roles in most basic aspects of life in both plants and animals. There is a growing interest in understanding how CO₂ is transported into cells, how it is sensed by neurons and other cell types and in understanding the physiological and molecular consequences of elevated CO₂ levels (hypercapnia) at the cell and organism levels. Human pulmonary diseases and model organisms such as fungi, C. elegans, Drosophila and mice have been proven to be important in understanding of the mechanisms of CO₂ sensing and response.

Functional analysis of human RhCG: comparison with E. coli ammonium transporter reveals similarities in the pore and differences in the vestibule

Rh glycoproteins are members of the ammonium transporter (Amt)/methylamine permease (Mep)/Rh family facilitating movement of NH3 across plasma membranes. Homology models constructed on the basis of the experimental structures of Escherichia coli AmtB and Nitrosomonas europaea Rh50 indicated a channel structure for human RhA (RhAG), RhB (RhBG), and RhC (RhCG) glycoproteins in which external and internal vestibules are linked by a pore containing two strictly conserved histidines. The pore entry is constricted by two highly conserved phenylalanines, “twin-Phe.” In this study, RhCG function was investigated by stopped-flow spectrofluorometry measuring kinetic pH variations in HEK293E cells in the presence of an ammonium gradient. The apparent unitary NH3 permeability of RhCG was determined and was found to be close to that of AmtB. With a site-directed mutagenesis approach, critical residues involved in Rh NH3 channel activity were highlighted. In the external vestibule, the importance of both the charge and the conformation of the conserved aspartic acid was shown. In contrast to AmtB, individual mutations of each phenylalanine of the twin-Phe impaired the function while the removal of both resulted in recovery of the transport activity. The impact of the mutations suggests that, although having a common function and a similar channel structure, bacterial AmtB and human Rh vary in several aspects of the NH3 transport mechanisms.

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus(Rh) glycoproteins

Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells,whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H+ excretion and Na+uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na+ uptake and acid excretion. In this model, Rhag facilitates NH3 flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical “Na+/NH +4 exchange complex” consisting of several membrane transporters (Rhcg, V-type H+-ATPase, Na+/H+ exchanger NHE-2 and/or NHE-3, Na+ channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase(CA-2) and basolateral Na+/HCO –3cotransporter (NBC-1) and Na+/K+-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH3 by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H+ ions created by the catalysed or non-catalysed hydration of expired metabolic CO2. Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO2 transporters,their responses to feeding, and their roles in early life stages prior to the full development of gills.

Expression of rh glycoproteins in the Mammalian kidney

Ammonia metabolism is a fundamental process in the maintenance of life in all living organisms. Recent studies have identified ammonia transporter family proteins in yeast (Mep), plants (Amt), and mammals (Rh glycoproteins). In mammalian kidneys, where ammonia metabolism and transport are critically important for the regulation of systemic acid-base homeostasis, basolateral Rh B glycoprotein and apical/basolateral Rh C glycoprotein are expressed along the distal nephron segments. Data from experimental animal models and knockout mice suggest that the Rh glycoproteins appear to mediate important roles in urinary ammonia excretion.

RhCG is the major putative ammonia transporter expressed in the human kidney, and RhBG is not expressed at detectable levels

Rhesus glycoprotein homologs RhAG, RhBG, and RhCG comprise a recently identified branch of the Mep/Amt ammonia transporter family. Animal studies have shown that RhBG and RhCG are present in the kidney distal tubules. Studies in mouse and rat tissue suggest a basolateral localization for RhBG in cells of the distal tubules including the alpha-intercalated cells (alpha-IC), but no localization of RhBG has been reported in human tissue. To date RhCG localization has been described as exclusively apical plasma membrane in mouse and rat kidney, or apical and basolateral in humans, and some mouse and rat tissue studies. We raised novel antibodies to RhBG and RhCG to examine their localization in the human kidney. Madin-Darby canine kidney (MDCKI) cell lines stably expressing human green fluorescent protein-tagged RhBG or RhCG and human tissue lysates were used to demonstrate the specificity of these antibodies for detecting RhBG and RhCG. Using immunoperoxidase staining and antigen liberation techniques, both apical and basolateral RhCG localization was observed in the majority of the cells of the distal convoluted tubule and IC of the connecting tubule and collecting duct. Confocal microscopic imaging of normal human kidney cryosections showed that RhCG staining was predominantly localized to the apical membrane in these cells with some basolateral and intracellular staining evident. A proportion of RhCG staining labeled kAE1-positive cells, confirming that RhCG is localized to the alpha-IC cells. Surprisingly, no RhBG protein was detectable in the human kidney by Western blot analysis of tissue lysates, or by immunohistochemistry or confocal microscopy of tissue sections. The same antibodies, however, could detect RhBG in rat tissue. We conclude that under normal conditions, RhCG is the major putative ammonia transporter expressed in the human kidney and RhBG is not expressed at detectable levels.

An Rh1-GFP fusion protein is in the cytoplasmic membrane of a white mutant strain of Chlamydomonas reinhardtii

The major Rhesus (Rh) protein of the green alga Chlamydomonas reinhardtii, Rh1, is homologous to Rh proteins of humans. It is an integral membrane protein involved in transport of carbon dioxide. To localize a fusion of intact Rh1 to the green fluorescent protein (GFP), we used as host a white (lts1) mutant strain of C. reinhardtii, which is blocked at the first step of carotenoid biosynthesis. The lts1 mutant strain accumulated normal amounts of Rh1 heterotrophically in the dark and Rh1-GFP was at the periphery of the cell co-localized with the cytoplasmic membrane dye FM4-64. Although Rh1 carries a potential chloroplast targeting sequence at its N-terminus, Rh1-GFP was clearly not associated with the chloroplast envelope membrane. Moreover, the N-terminal half of the protein was not imported into chloroplasts in vitro and N-terminal regions of Rh1 did not direct import of the small subunit of ribulose bisphosphate carboxylase (SSU). Despite caveats to this interpretation, which we discuss, current evidence indicates that Rh1 is a cytoplasmic membrane protein and that Rh1-GFP is among the first cytoplasmic membrane protein fusions to be obtained in C. reinhardtii. Although lts1 (white) mutant strains cannot be used to localize proteins within sub-compartments of the chloroplast because they lack thylakoid membranes, they should nonetheless be valuable for localizing many GFP fusions in Chlamydomonas.

Phosphorylation and ankyrin-G binding of the C-terminal domain regulate targeting and function of the ammonium transporter RhBG

RhBG, a human member of the Amt/Mep/Rh/superfamily of ammonium transporters, has been shown to facilitate NH(3) transport and to be anchored to the basolateral plasma membrane of kidney epithelial cells, via ankyrin-G. We showed here that triple alanine substitution of the (419)FLD(421) sequence, which links the cytoplasmic C-terminal domain of RhBG to ankyrin-G, not only disrupted the interaction of RhBG with the spectrin-based skeleton but also delayed its cell surface expression, decreased its plasma membrane stability, and abolished its NH(3) transport function in epithelial cell lines. Similarly, we demonstrated that both anchoring to the membrane skeleton and ammonium transport activity are regulated by the phosphorylation status of the C-terminal tail of RhBG. Tyrosine 429, which belongs to the previously reported YED basolateral targeting signal of RhBG, was demonstrated to be phosphorylated in vitro using purified Src and Syk kinases and ex vivo by analyzing the effect of pervanadate treatment on wild-type RhBG or Y429A mutants. Then, we showed that Y429D and Y429E mutations, mimicking constitutive phosphorylation, abolished NH(3) transport and enhanced Triton X-100 solubilization of RhBG from the cell membrane. In contrast, the nonphosphorylated/nonphosphorylatable Y429A and Y429F mutants behaved the same as wild-type RhBG. Conversely, Y/A or Y/F but not Y/E or Y/D mutations of residue 429 abolished the exclusive basolateral localization of RhBG in polarized epithelial cells. All these results led to a model in which targeting and ammonium transport function of RhBG are regulated by both phosphorylation and membrane skeleton binding of the C-terminal cytoplasmic domain.

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.

Amt2 permease is required to induce ammonium-responsive invasive growth and mating in Cryptococcus neoformans

The conserved AmtB/Mep/Rh family of proteins mediate the transport of ammonium across cellular membranes in a wide range of organisms. Certain fungal members of this group are required to initiate filamentous growth. We have investigated the functions of two members of the AmtB/Mep/Rh family from the pathogenic basidiomycete Cryptococcus neoformans. Amt1 and Amt2 are low- and high-affinity ammonium permeases, respectively, and a mutant lacking both permeases is unable to grow under ammonium-limiting conditions. AMT2 is transcriptionally induced in response to nitrogen limitation, whereas AMT1 is constitutively expressed. Single and double amt mutants exhibit wild-type virulence in two models of cryptococcosis. Consistent with this, the formation of two C. neoformans virulence factors, cell wall melanin and the extracellular polysaccharide capsule, is not impaired in cells lacking either or both of the Amt1 and Amt2 permeases. Amt2 is, however, required for the initiation of invasive growth of haploid cells under low-nitrogen conditions and for the mating of wild-type cells under the same conditions. We propose that Amt2 may be a new fungal ammonium sensor and an element of the signaling cascades that govern the mating of C. neoformans in response to environmental nutritional cues.

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.

The Amt/Mep/Rh family of ammonium transport proteins

The Amt/Mep/Rh family of integral membrane proteins comprises ammonium transporters of bacteria, archaea and eukarya, as well as the Rhesus proteins found in animals. They play a central role in the uptake of reduced nitrogen for biosynthetic purposes, in energy metabolism, or in renal excretion. Recent structural information on two prokaryotic Amt proteins has significantly contributed to our understanding of this class, but basic questions concerning the transport mechanism and the nature of the transported substrate, NH3 or [NH4(+)], remain to be answered. Here we review functional and structural studies on Amt proteins and discuss the bioenergetic issues raised by the various mechanistic proposals present in the literature.

Evolution and functional characterization of the RH50 gene from the ammonia-oxidizing bacterium Nitrosomonas europaea

The family of ammonia and ammonium channel proteins comprises the Amt proteins, which are present in all three domains of life with the notable exception of vertebrates, and the homologous Rh proteins (Rh50 and Rh30) that have been described thus far only in eukaryotes. The existence of an RH50 gene in bacteria was first revealed by the genome sequencing of the ammonia-oxidizing bacterium Nitrosomonas europaea. Here we have used a phylogenetic approach to study the evolution of the N. europaea RH50 gene, and we show that this gene, probably as a component of an integron cassette, has been transferred to the N. europaea genome by horizontal gene transfer. In addition, by functionally characterizing the Rh50(Ne) protein and the corresponding knockout mutant, we determined that NeRh50 can mediate ammonium uptake. The RH50(Ne) gene may thus have replaced functionally the AMT gene, which is missing in the genome of N. europaea and may be regarded as a case of nonorthologous gene displacement.

Correlation between serology and genetics of weak D types in Denmark

BACKGROUND

To date more than 100 variant D types have been reported and the frequencies vary among populations. Blood donor typing should reveal all donors expressing D antigens, while patient typing should prevent the development of anti-D in patients with a D- or variant D blood type. Serotyping is the standard method to assign transfusion strategies, whereas molecular classification offers a more specific grouping of weak and partial D.

STUDY DESIGN AND METHODS

Blood donor and patient samples with discrepant results of D phenotyping were collected to investigate the frequency of weak D subtypes in Denmark and to evaluate currently used serologic methods.

RESULTS

Nine different weak D types were identified among the 101 samples. Weak D Types 1, 2, and 3 constituted 80 percent of the analyzed samples and 10 percent of the samples identified as weak D from serology were actually partial D.

CONCLUSION

The distribution of weak D types in Denmark was found to resemble the distribution in Northern Germany in respect to the three most prevalent weak D types. Correctly defining all samples that show weak reactions in D typing as weak D or partial D is not possible with serotyping alone; genotyping offers the only exact categorization. Serology is superior for routine blood typing, however.

Ammonia excretion in rainbow trout (Oncorhynchus mykiss): evidence for Rh glycoprotein and H+-ATPase involvement

Branchial ammonia transport in freshwater teleosts is not well understood. Most studies conclude that NH(3) diffuses out of the gill and becomes protonated to NH(4)(+) in an acidified gill boundary layer. Rhesus (Rh) proteins are new members of the ammonia transporter superfamily and rainbow trout possess genes encoding for Rh30-like1 and Rhcg2. We identified seven additional full-length trout Rh cDNA sequences: one Rhag and two each of Rhbg, Rhcg1, and Rh30-like. The mRNA expression of Rhbg, Rhcg1, and Rhcg2 was examined in trout tissues (blood, brain, eye, gill, heart, intestine, kidney, liver, muscle, skin, spleen) exposed to high external ammonia (HEA; 1.5 mmol/l NH(4)HCO(3), pH 7.95, 15 degrees C). Rhbg was expressed in all tissues, Rhcg1 was expressed in brain, gill, liver, and skin, and Rhcg2 was expressed in gill and skin. Brain Rhbg and Rhcg1 were downregulated, blood Rh30-like and Rhag were downregulated, and skin Rhbg and Rhcg2 were upregulated with HEA. After an initial uptake of ammonia into the fish during HEA, excretion was reestablished, coinciding with upregulations of gill Rh mRNA in the pavement cell fraction: Rhcg2 at 12 and 48 h, and Rhbg at 48 h. NHE2 expression remained unchanged, but upregulated H(+)-ATPase (V-type, B-subunit) and downregulated carbonic anhydrase (CA2) expression and activity were noted in the gill and again expression changes occurred in pavement cells, and not in mitochondria-rich cells. Together, these results indicate Rh glycoprotein involvement in ammonia transport and excretion in the rainbow trout while underscoring the significance of gill boundary layer acidification by H(+)-ATPase.

Structural and mechanistic aspects of Amt/Rh proteins

Amt/Rh proteins, which mediate movement of ammonium across cell membranes, are spread throughout the three kingdoms of life. Most functional studies on various members of the family have been performed using cellular assays in heterologous expression systems, which are, however, not very well suited for detailed mechanistic studies. Although now generally considered to be ammonia conducting channels, based on a number of experimental studies and structural insights, the possibility remains that some plant Amts facilitate net ammonium ion transport. The Escherichia coli channel AmtB has become the model system of choice for analysis of the mechanism of ammonia conductance, increasingly also through molecular dynamics simulations. Further progress in a more detailed mechanistic understanding of these proteins requires a reliable in vitro assay using purified protein, allowing quantitative kinetic measurements under a variety of experimental conditions for different Amt/Rh proteins, including mutants. Here, we critically review the existing functional data in the context of the most interesting and unresolved mechanistic questions and we present our results, obtained using an in vitro assay set up with the purified E. coli channel AmtB.

The yeast ammonium transport protein Mep2 and its positive regulator, the Npr1 kinase, play an important role in normal and pseudohyphal growth on various nitrogen media through retrieval of excreted ammonium

Three ammonium transport systems of the Mep/Amt/Rh superfamily contribute to ammonium uptake for use as a nitrogen source in Saccharomyces cerevisiae. A specific sensor role has further been proposed for Mep2 in the stimulation of pseudohyphal development during ammonium limitation. Optimal ammonium transport by the Mep proteins requires the Npr1 kinase, a potential target of the target-of-rapamycin signalling pathway. We show here that the growth impairment of cells lacking Npr1 on many nitrogen sources is shared by cells deprived of the three Mep proteins and is a consequence of deficient ammonium retrieval. Expression of a newly isolated Npr1-independent and hyperactive Mep2 in cells lacking Npr1 and/or the Mep proteins restores growth on low ammonium but also on other nitrogen sources. This hyperactive Mep2 variant efficiently counteracts ammonium excretion. Hence, ammonium uptake activity plays an important role in compensating for leakage of catabolic ammonium. Our data also reveal that the requirement of Npr1 for ammonium-induced pseudohyphal growth is an indirect consequence of its necessity for Mep2-mediated ammonium transport. Finally, we show that Mep2 participates, through ammonium leakage compensation, in pseudohyphal growth induced by amino acid starvation. This argues further in favour of tight coupling of Mep2 transport and sensor functions.

Structure of GlnK1 with bound effectors indicates regulatory mechanism for ammonia uptake

A binary complex of the ammonia channel Amt1 from Methanococcus jannaschii and its cognate P(II) signalling protein GlnK1 has been produced and characterized. Complex formation is prevented specifically by the effector molecules Mg-ATP and 2-ketoglutarate. Single-particle electron microscopy of the complex shows that GlnK1 binds on the cytoplasmic side of Amt1. Three high-resolution X-ray structures of GlnK1 indicate that the functionally important T-loop has an extended, flexible conformation in the absence of Mg-ATP, but assumes a compact, tightly folded conformation upon Mg-ATP binding, which in turn creates a 2-ketoglutarate-binding site. We propose a regulatory mechanism by which nitrogen uptake is controlled by the binding of both effector molecules to GlnK1. At normal effector levels, a 2-ketoglutarate molecule binding at the apex of the compact T-loop would prevent complex formation, ensuring uninhibited ammonia uptake. At low levels of Mg-ATP, the extended loops would seal the ammonia channels in the complex. Binding of both effector molecules to P(II) signalling proteins may thus represent an effective feedback mechanism for regulating ammonium uptake through the membrane.

The structure and function of the Rh antigen complex

The Rh system is one of the most important and complex blood group systems because of the large number of antigens and the serious complications for the fetus of a woman sensitized by transfusion or pregnancy. Major advances in our understanding of the Rh system have occurred with the cloning of the genes and with functional evidence that the Rh blood group proteins belong to an ancient family of membrane proteins involved in ammonia transport. The arrangement and configuration of the genes at the RH locus promotes genetic exchange, generating new antigens. Importantly, RH genetic testing can now be applied to clinical transfusion medicine and prenatal practice. This includes testing for RHD zygosity, confirmation or resolution of D antigen status, and detection of altered RHD and RHCE genes in individuals at risk for producing antibodies to high-incidence Rh antigens, particularly sickle cell disease (SCD) patients. The Rh proteins form a core complex that is critical to the structure of the erythrocyte membrane, and they may play a physiologic role in the sequestration of blood ammonia. The Rh family of proteins now includes non-erythroid homologs present in many other tissues, and comparative genomics reveal Rh homologs in all domains of life.

Ammonium channel expression is essential for brain development and function in the larva ofCiona intestinalis

Ammonium uptake into the cell is known to be mediated by ammonium transport (Amt) proteins, which are present in all domains of life. The physiological role of Amt proteins remains elusive; indeed, loss-of-function experiments suggested that Amt proteins do not play an essential role in bacteria, yeast, and plants. Here we show that the reverse holds true in the tunicate Ciona intestinalis. The genome of C. intestinalis contains two AMT genes, Ci-AMT1a and Ci-AMT1b, which we show derive from an ascidian-specific gene duplication. We analyzed Ci-AMT expression during embryo development. Notably, Ci-AMT1a is expressed in the larval brain in a small number of cells defining a previously unseen V-shaped territory; these cells connect the brain cavity to the external environment. We show that the knockdown of Ci-AMT1a impairs the formation of the brain cavity and consequently the function of the otolith, the gravity-sensing organ contained in it. We speculate that the normal mechanical functioning (flotation and free movement) of the otolith may require a close regulation of ammonium salt(s) concentration in the brain cavity, because ammonium is known to affect both fluid density and viscosity; the cells forming the V territory may act as a conduit in achieving such a regulation.

Molecular mechanisms of renal ammonia transport

Acid-base homeostasis to a great extent relies on renal ammonia metabolism. In the past several years, seminal studies have generated important new insights into the mechanisms of renal ammonia transport. In particular, the theory that ammonia transport occurs almost exclusively through nonionic NH(3) diffusion and NH(4)(+) trapping has given way to a model postulating that a variety of proteins specifically transport NH(3) and NH(4)(+) and that this transport is critical for normal ammonia metabolism. Many of these proteins transport primarily H(+) or K(+) but also transport NH(4)(+). Nonerythroid Rh glycoproteins transport ammonia and may represent critical facilitators of ammonia transport in the kidney. This review discusses the underlying aspects of renal ammonia transport as well as specific proteins with important roles in renal ammonia transport.

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.

Ammonium Homeostasis and Human Rhesus Glycoproteins

The brain ammonium production is detoxified by astrocytes, the gut ammonium production is detoxified by hepatic cells, and the renal ammonium production plays a major role in renal acid excretion. As a result of ammonium handling in these organs, the ammonium and pH values are strictly regulated in plasma. Up until recently, it was accepted that mammalian cell transmembrane ammonium transport was due to NH(4)(+) transport by non-specific transporting systems, and to non-ionic NH(3) diffusion, whereas lower organisms (such as bacteria, yeasts and plants) were endowed with specific ammonium transporters (Amts). Sequence homologies between Amts and human Rhesus (Rh) glycoproteins (RhAG, from erythroid cells, and RhBG and RhCG from epithelial cells) raised the hypothesis that Rh glycoproteins act as specific ammonium transporters, further sustained by the polarized distribution of RhBG and RhCG in gut, kidney and liver. Results from functional studies agree that Rh glycoproteins are the first ammonium transporters reported in mammals. However, the nature of the transported specie(s) is much debated: in particular, it is proposed that Rh glycoproteins mediate a direct NH(3) transport, or that they mediate an indirect NH(3) transport (resulting from NH(4)(+) for H(+) exchange). Direct NH(3) transport (associated or not with NH(4)(+) transport) raises the exciting hypothesis that Rh glycoproteins may also transport other gases than NH(3) (namely, CO(2)).

Effect of AmtB homologues on the post-translational regulation of nitrogenase activity in response to ammonium and energy signals in Rhodospirillum rubrum

The AmtB protein transports uncharged NH(3) into the cell, but it also interacts with the nitrogen regulatory protein P(II), which in turn regulates a variety of proteins involved in nitrogen fixation and utilization. Three P(II) homologues, GlnB, GlnK and GlnJ, have been identified in the photosynthetic bacterium Rhodospirillum rubrum, and they have roles in at least four overlapping and distinct functions, one of which is the post-translational regulation of nitrogenase activity. In R. rubrum, nitrogenase activity is tightly regulated in response to addition or energy depletion (shift to darkness), and this regulation is catalysed by the post-translational regulatory system encoded by draTG. Two amtB homologues, amtB(1) and amtB(2), have been identified in R. rubrum, and they are linked with glnJ and glnK, respectively. Mutants lacking AmtB(1) are defective in their response to both addition and darkness, while mutants lacking AmtB(2) show little effect on the regulation of nitrogenase activity. These responses to darkness and appear to involve different signal transduction pathways, and the poor response to darkness does not seem to be an indirect result of perturbation of internal pools of nitrogen. It is also shown that AmtB(1) is necessary to sequester detectable amounts GlnJ to the cell membrane. These results suggest that some element of the AmtB(1)-P(II) regulatory system senses energy deprivation and a consistent model for the integration of nitrogen, carbon and energy signals by P(II) is proposed. Other results demonstrate a degree of specificity in interaction of AmtB(1) with the different P(II) homologues in R. rubrum. Such interaction specificity might be important in explaining the way in which P(II) proteins regulate processes involved in nitrogen acquisition and utilization.