Molecular and functional studies of the gamma subunit of the sodium pump

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Knockdown of the Sodium/Potassium ATPase Subunit Beta 2 Reduces Egg Production in the Dengue Vector, Aedes aegypti

The Na+/K+ ATPase (NKA) is present in the cellular membrane of most eukaryotic cells. It utilizes energy released by ATP hydrolysis to pump sodium ions out of the cell and potassium ions into the cell, which establishes and controls ion gradients. Functional NKA pumps consist of three subunits, alpha, beta, and FXYD. The alpha subunit serves as the catalytic subunit while the beta and FXYD subunits regulate the proper folding and localization, and ion affinity of the alpha subunit, respectively. Here we demonstrate that knockdown of NKA beta subunit 2 mRNA (nkaβ2) reduces fecundity in female Ae. aegypti. We determined the expression pattern of nkaβ2 in several adult mosquito organs using qRT-PCR. We performed RNAi-mediated knockdown of nkaβ2 and assayed for lethality, and effects on female fecundity. Tissue expression levels of nkaβ2 mRNA were highest in the ovaries with the fat body, midgut and thorax having similar expression levels, while Malpighian tubules had significantly lower expression. Survival curves recorded post dsRNA injection showed a non-significant decrease in survival of nkaβ2 dsRNA-injected mosquitoes compared to GFP dsRNA-injected mosquitoes. We observed a significant reduction in the number of eggs laid by nkaβ2 dsRNA-injected mosquitoes compared to control mosquitoes. These results, coupled with the tissue expression profile of nkaβ2, indicate that this subunit plays a role in normal female Ae. aegypti fecundity. Additional research needs to be conducted to determine the exact role played by NKAβ2 in mosquito post-blood meal nutrient sensing, transport, yolk precursor protein (YPP) synthesis and yolk deposition.

Na+/K+-ATPase as a Target of Cardiac Glycosides for the Treatment of SARS-CoV-2 Infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), identified for the first time in Wuhan, China, causes coronavirus disease 2019 (COVID-19), which moved from epidemic status to becoming a pandemic. Since its discovery in December 2019, there have been countless cases of mortality and morbidity due to this virus. Several compounds such as chloroquine, hydroxychloroquine, lopinavir-ritonavir, and remdesivir have been tested as potential therapies; however, no effective treatment is currently recommended by regulatory agencies. Some studies on respiratory non-enveloped viruses such as adenoviruses and rhinovirus and some respiratory enveloped viruses including human respiratory syncytial viruses, influenza A, parainfluenza, SARS-CoV, and SARS-CoV-2 have shown the antiviral activity of cardiac glycosides, correlating their effect with Na⁺/K⁺-ATPase (NKA) modulation. Cardiac glycosides are secondary metabolites used to treat patients with cardiac insufficiency because they are the most potent inotropic agents. The effects of cardiac glycosides on NKA are dependent on cell type, exposure time, and drug concentration. They may also cause blockage of Na⁺ and K⁺ ionic transport or trigger signaling pathways. The antiviral activity of cardiac glycosides is related to cell signaling activation through NKA inhibition. Nuclear factor kappa B (NFκB) seems to be an essential transcription factor for SARS-CoV-2 infection. NFκB inhibition by cardiac glycosides interferes directly with SARS-CoV-2 yield and inflammatory cytokine production. Interestingly, the antiviral effect of cardiac glycosides is associated with tyrosine kinase (Src) activation, and NFκB appears to be regulated by Src. Src is one of the main signaling targets of the NKA α-subunit, modulating other signaling factors that may also impair viral infection. These data suggest that Src-NFκB signaling modulated by NKA plays a crucial role in the inhibition of SARS-CoV-2 infection. Herein, we discuss the antiviral effects of cardiac glycosides on different respiratory viruses, SARS-CoV-2 pathology, cell signaling pathways, and NKA as a possible molecular target for the treatment of COVID-19.

Cardiac Glycosides in Human Physiology and Disease: Update for Entomologists

Cardiac glycosides, cardenolides and bufadienolides, are elaborated by several plant or animal species to prevent grazing or predation. Entomologists have characterized several insect species that have evolved the ability to sequester these glycosides in their tissues to reduce their palatability and, thus, reduce predation. Cardiac glycosides are known to interact with the sodium- and potassium-activated adenosine triphosphatase, or sodium pump, through a specific receptor-binding site. Over the last couple of decades, and since entomologic studies, it has become clear that mammals synthesize endogenous cardenolides that closely resemble or are identical to compounds of plant origin and those sequestered by insects. The most important of these are ouabain-like compounds. These compounds are essential for the regulation of normal ionic physiology in mammals. Importantly, at physiologic picomolar or nanomolar concentrations, endogenous ouabain, a cardenolide, stimulates the sodium pump, activates second messengers, and may even function as a growth factor. This is in contrast to the pharmacologic or toxic micromolar or milimolar concentrations achieved after consumption of exogenous cardenolides (by consuming medications, plants, or insects), which inhibit the pump and result in either a desired medical outcome, or the toxic consequence of sodium pump inhibition.

RH421 binds into the ATP-binding site on the Na+/K+-ATPase

The Na⁺/K⁺-ATPase plays a key role in ion transport across the plasma membrane of all animal cells. The voltage-sensitive styrylpyrimidium dye RH421 has been used in several laboratories for monitoring of Na⁺/K⁺-ATPase kinetics. It is known, that RH421 can interact with the enzyme and it can influence its activity at micromolar concentrations, but structural details of this interaction are only poorly understood. Experiments with isolated large cytoplasmic loop (C45) of Na⁺/K⁺-ATPase revealed that RH421 can interact with this part of the protein with dissociation constant 1μM. The Trp-to-RH421 FRET performed on six single-tryptophan mutants revealed that RH421 binds directly into the ATP-binding site. This conclusion was further supported by results from molecular docking, site-directed mutagenesis and by competitive experiments using ATP. Experiments with C45/DPPC mixture revealed that RH421 can bind to both C45 and lipids, but only the former interaction was influenced by the presence of ATP.

Transcriptome analysis reveals potential mechanisms underlying differential heart development in fast- and slow-growing broilers under heat stress

BACKGROUND

Modern fast-growing broilers are susceptible to heart failure under heat stress because their relatively small hearts cannot meet increased need of blood pumping. To improve the cardiac tolerance to heat stress in modern broilers through breeding, we need to find the important genes and pathways that contribute to imbalanced cardiac development and frequent occurrence of heat-related heart dysfunction. Two broiler lines - Ross 708 and Illinois - were included in this study as a fast-growing model and a slow-growing model respectively. Each broiler line was separated to two groups at 21 days posthatch. One group was subjected to heat stress treatment in the range of 35-37 °C for 8 h per day, and the other was kept in thermoneutral condition. Body and heart weights were measured at 42 days posthatch, and gene expression in left ventricles were compared between treatments and broiler lines through RNA-seq analysis.

RESULTS

Body weight and normalized heart weight were significantly reduced by heat stress only in Ross broilers. RNA-seq results of 44 genes were validated using Biomark assay. A total of 325 differentially expressed (DE) genes were detected between heat stress and thermoneutral in Ross 708 birds, but only 3 in Illinois broilers. Ingenuity pathway analysis (IPA) predicted dramatic changes in multiple cellular activities especially downregulation of cell cycle. Comparison between two lines showed that cell cycle activity is higher in Ross than Illinois in thermoneutral condition but is decreased under heat stress. Among the significant pathways (P < 0.01) listed for different comparisons, "Mitotic Roles of Polo-like Kinases" is always ranked first.

CONCLUSIONS

The increased susceptibility of modern broilers to cardiac dysfunction under heat stress compared to slow-growing broilers could be due to diminished heart capacity related to reduction in relative heart size. The smaller relative heart size in Ross heat stress group than in Ross thermoneutral group is suggested by the transcriptome analysis to be caused by decreased cell cycle activity and increased apoptosis. The DE genes in RNA-seq analysis and significant pathways in IPA provides potential targets for breeding of heat-tolerant broilers with optimized heart function.

Protein phosphatase 2A interacts with the Na,K-ATPase and modulates its trafficking by inhibition of its association with arrestin

BACKGROUND

The P-type ATPase family constitutes a collection of ion pumps that form phosphorylated intermediates during ion transport. One of the best known members of this family is the Na⁺,K⁺-ATPase. The catalytic subunit of the Na⁺,K⁺-ATPase includes several functional domains that determine its enzymatic and trafficking properties.

METHODOLOGY/PRINCIPAL FINDINGS

Using the yeast two-hybrid system we found that protein phosphatase 2A (PP2A) catalytic C-subunit is a specific Na⁺,K⁺-ATPase interacting protein. PP-2A C-subunit interacted with the Na⁺,K⁺-ATPase, but not with the homologous sequences of the H⁺,K⁺-ATPase. We confirmed that the Na⁺,K⁺-ATPase interacts with a complex of A- and C-subunits in native rat kidney. Arrestins and G-protein coupled receptor kinases (GRKs) are important regulators of G-protein coupled receptor (GPCR) signaling, and they also regulate Na⁺,K⁺-ATPase trafficking through direct association. PP2A inhibits association between the Na⁺,K⁺-ATPase and arrestin, and diminishes the effect of arrestin on Na⁺,K⁺-ATPase trafficking. GRK phosphorylates the Na⁺,K⁺-ATPase and PP2A can at least partially reverse this phosphorylation.

CONCLUSIONS/SIGNIFICANCE

Taken together, these data demonstrate that the sodium pump belongs to a growing list of ion transport proteins that are regulated through direct interactions with the catalytic subunit of a protein phosphatase.

Protective effect of 2,2'-dithienyl diselenide on kainic acid-induced neurotoxicity in rat hippocampus

In this study, we investigated the effects of 2,2'-dithienyl diselenide (DTDS), an organoselenium compound, against seizures induced by kainic acid (KA) in rats. Rats were pretreated with DTDS (50 or 100 mg/kg) by oral route 1 h before KA injection (10 mg/kg, intraperitoneal). Our results showed that DTDS (100 mg/kg) was effective in increasing latency for the onset of the first clonic seizure episode induced by KA, as well as in decreasing the appearance of seizures and the Racine's score. DTDS also caused a decrease in the excitatory electroencephalographic (EEG) changes, resulting from KA exposure in hippocampus and cerebral cortex of rats. Besides, elevated reactive species (RS) and carbonyl protein levels and Na(+), K(+)-ATPase activity in hippocampus of rats treated with KA were ameliorated by DTDS (50 and 100 mg/kg). Lastly, as evidenced by Cresyl-Violet stain, DTDS (100 mg/kg) elicited a protective effect against KA-induced neurodegeneration in rat hippocampus 7 days after KA injection. In conclusion, the present study showed that DTDS attenuated KA-induced status epilepticus in rats and the subsequent hippocampal damage.

The beta1 subunit of the Na,K-ATPase pump interacts with megalencephalic leucoencephalopathy with subcortical cysts protein 1 (MLC1) in brain astrocytes: new insights into MLC pathogenesis

Megalencephalic leucoencephalopathy with subcortical cysts (MLC) is a rare congenital leucodystrophy caused by mutations in MLC1, a membrane protein of unknown function. MLC1 expression in astrocyte end-feet contacting blood vessels and meninges, along with brain swelling, fluid cysts and myelin vacuolation observed in MLC patients, suggests a possible role for MLC1 in the regulation of fluid and ion homeostasis and cellular volume changes. To identify MLC1 direct interactors and dissect the molecular pathways in which MLC1 is involved, we used NH2-MLC1 domain as a bait to screen a human brain library in a yeast two-hybrid assay. We identified the β1 subunit of the Na,K-ATPase pump as one of the interacting clones and confirmed it by pull-downs, co-fractionation assays and immunofluorescence stainings in human and rat astrocytes in vitro and in brain tissue. By performing ouabain-affinity chromatography on astrocyte and brain extracts, we isolated MLC1 and the whole Na,K-ATPase enzyme in a multiprotein complex that included Kir4.1, syntrophin and dystrobrevin. Because Na,K-ATPase is involved in intracellular osmotic control and volume regulation, we investigated the effect of hypo-osmotic stress on MLC1/Na,K-ATPase relationship in astrocytes. We found that hypo-osmotic conditions increased MLC1 membrane expression and favoured MLC1/Na,K-ATPase-β1 association. Moreover, hypo-osmosis induced astrocyte swelling and the reversible formation of endosome-derived vacuoles, where the two proteins co-localized. These data suggest that through its interaction with Na,K-ATPase, MLC1 is involved in the control of intracellular osmotic conditions and volume regulation in astrocytes, opening new perspectives for understanding the pathological mechanisms of MLC disease.

Differential cellular expression of FXYD1 (phospholemman) and FXYD2 (gamma subunit of Na, K-ATPase) in normal human tissues: a study using high density human tissue microarrays

FXYD proteins have been proposed to function as regulators of Na, K-ATPase function by lowering affinities of the system for potassium and sodium. However, their distribution in normal human tissues has not been studied. We have therefore used immunohistochemistry and semi-quantitative histomorphometric analysis to determine the relative expression at the protein level and distribution of FXYD1 (phospholemman) and FXYD2 (gamma subunit of Na, K-ATPase) in human Tissue MicroArrays (TMAs). Expression of FXYD1 was abundant in heart, kidney, placenta, skeletal muscle, gastric and anal mucosa, small intestine and colon. Lower FXYD1 expression was detected in uterine, intestinal and bladder smooth muscle, choroid plexus, liver, gallbladder, spleen, breast, prostate and epididymis. The tissue distribution of FXYD2 was less extensive compared to that of FXYD1. There was an abundant expression in kidney and choroid plexus and moderate expression in placenta, amniotic membranes, breast epithelium, salivary glands, pancreas and uterine endometrium. Weaker FXYD2 expression was detected in the adrenal medulla, liver, gallbladder, bladder and pancreas. The common denominator in the distribution of FXYD1 and FXYD2 was expression in highly active transport epithelia of the kidney, choroid plexus, placenta and salivary glands. This study reveals, in human tissues, the specific expression of FXYD proteins, which may associate with Na, K-ATPase in selected cell types and modulate its catalytic properties.

Branchial FXYD protein expression in response to salinity change and its interaction with Na+/K+-ATPase of the euryhaline teleost Tetraodon nigroviridis

Na+/K+-ATPase (NKA) is a ubiquitous membrane-bound protein crucial for teleost osmoregulation. The enzyme is composed of two essential subunits, a catalytic alpha subunit and a glycosylated beta subunit which is responsible for membrane targeting of the enzyme. In mammals, seven FXYD members have been found. FXYD proteins have been identified as the regulatory protein of NKA in mammals and elasmobranchs, it is thus interesting to examine the expression and functions of FXYD protein in the euryhaline teleosts with salinity-dependent changes of gill NKA activity. The present study investigated the expression and distribution of the FXYD protein in gills of seawater (SW)- or freshwater (FW)-acclimated euryhaline pufferfish (Tetraodon nigroviridis). The full-length pufferfish FXYD gene (pFXYD) was confirmed by RT-PCR. pFXYD was found to be expressed in many organs including gills of both SW and FW pufferfish. pFXYD mRNA abundance in gills, determined by real-time PCR, was significantly higher in FW fish than in SW fish. An antiserum raised against a partial amino acid sequence of pFXYD was used for the immunoblots of gill homogenates and a major band at 13 kDa was detected. The relative amounts of pFXYD protein and mRNA in gills of SW and FW pufferfish were identical, but opposite to the expression levels of NKA. Immunofluorescent staining of frozen sections demonstrated that pFXYD was colocalized to NKA-immunoreactive cells in the gill filaments. In addition, interaction between pFXYD and NKA was demonstrated by co-immunoprecipitation. Taken together, salinity-dependent expression of pFXYD protein and NKA, as well as the evidence for their colocalization and interaction in pufferfish gills suggested that pFXYD regulates NKA activity in gills of euryhaline teleosts upon salinity challenge.

Thyroid hormone rapidly stimulates alveolar Na,K-ATPase by activation of phosphatidylinositol 3-kinase

PURPOSE OF REVIEW

Nongenomic actions of 3,3',5-triiodo-L-thyronine (T3) occur quite rapidly usually via activation of signaling cascades. In this review, we focus on recent advances made in the understanding of activation of the phosphatidylinositol 3-kinase pathway by T3 in alveolar epithelial cells, resulting in upregulation of Na,K-ATPase hydrolytic activity and potential physiological significance of this finding.

RECENT FINDINGS

T3 stimulates the Src family of kinases. Activation of Src-kinase and phosphatidylinositol 3-kinase/protein kinase B is required for the T3-induced stimulation of alveolar epithelial Na,K-ATPase activity in rat alveolar epithelial cells. The stimulation does not require transcription. This T3-sensitive Na,K-ATPase stimulation in rat alveolar epithelial cells is switched on late in gestation. In skin fibroblasts phosphatidylinositol 3-kinase is also involved in the nongenomic T3 stimulation of ZAK1-4alpha protein expression, an endogenous calcineurin inhibitor.

SUMMARY

T3 plays an important role in cell survival and differentiation. Nongenomic regulation of phosphatidylinositol 3-kinase and downstream molecules by T3 is being recognized in different tissues. Upregulation of alveolar Na,K-ATPase is one such molecule, which plays an important role in removal of edema fluid from the alveolar space. These effects are rapid and do not require direct nuclear gene transcription.

Association of renal Na,K-ATPase alpha-subunit with the beta- and gamma-subunits based on cryoelectron microscopy

Na,K-ATPase transports Na(+) and K(+) across cell membranes and consists of alpha- and beta-subunits. Na,K-ATPase also associates with small FXYD proteins that regulate the activity of the pump. We have used cryoelectron microscopy of two-dimensional crystals including data to 8 A resolution to determine the three-dimensional (3-D) structure of renal Na,K-ATPase containing FXYD2, the gamma-subunit. A homology model for the alpha-subunit was calculated from a Ca(2+)-ATPase structure and used to locate the additional beta- and gamma-subunits present in the 3-D map of Na,K-ATPase. Based on the 3-D map, the beta-subunit is located close to transmembrane helices M8 and M10 and the gamma-subunit is adjacent to helices M2 and M9 of the alpha-subunit.

Fluorescence spectroscopic studies of pressure effects on Na+,K(+)-ATPase reconstituted into phospholipid bilayers and model raft mixtures

To contribute to the understanding of membrane protein function upon application of pressure as relevant for understanding, for example, the physiology of deep sea organisms or for baroenzymological biotechnical processes, we investigated the influence of hydrostatic pressure on the activity of Na+,K+-ATPase enriched in the plasma membrane from rabbit kidney outer medulla using a kinetic assay that couples ATP hydrolysis to NADH oxidation. The data show that the activity of Na+,K+-ATPase is reversibly inhibited by pressures below 2 kbar. At higher pressures, the enzyme is irreversibly inactivated. To be able to explore the effect of the lipid matrix on enzyme activity, the enzyme was also reconstituted into various lipid bilayer systems of different chain length, conformation, phase state, and heterogeneity including model raft mixtures. To yield additional information on the conformation and phase state of the lipid bilayer systems, generalized polarization values by the Laurdan fluorescence technique were determined as well. Incorporation of the enzyme leads to a significant increase of the lipid chain order. Generally, similar to the enzyme activity in the natural plasma membrane, high hydrostatic pressures lead to a decline of the activity of the enzyme reconstituted into the various lipid bilayer systems, and in most cases, a multi-phasic behavior is observed. Interestingly, in the low-pressure region, around 100 bar, a significant increase of activity is observed for the enzyme reconstituted into DMPC and DOPC bilayers. Above 100-200 bar, this activity enhancement is followed by a steep decrease of activity up to about 800 bar, where a more or less broad plateau value is reached. The enzyme activity decreases to zero around 2 kbar for all reconstituted systems measured. A different scenario is observed for the effect of pressure on the enzyme activity in the model raft mixture. The coexistence of liquid-ordered and liquid-disordered domains with the possibility of lipid sorting in this lipid mixture leads to a reduced pressure sensitivity in the medium-pressure range. The decrease of ATPase activity may be induced by an increasing hydrophobic mismatch, leading to a decrease of the conformational dynamics of the protein and eventually subunit rearrangement. High pressures, above about 2.2 kbar, irreversibly change protein conformation, probably because of the dissociation and partial unfolding of the subunits.

[FXYD proteins: novel regulators of Na,K-ATPase]

Members of the FXYD protein family are small membrane proteins which are characterized by an FXYD motif, two conserved glycines and a serine residue. FXYD proteins show a tissue-specific distribution. Recent evidence suggests that 6 out of 7 FXYD proteins, FXYD1 (phospholemman), FXYD2 (gamma subunit of Na,K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), FXYD5 (Ric) and FXYD7 associate with Na,K-ATPase and modulate its transport properties e.g. its Na+ and/or its K+ affinity in a distinct way. These results highlight the complex regulation of Na+ and K+ transport which is necessary to ensure proper tissue functions such as renal Na+-reabsorption, muscle contractility and neuronal excitability. Moreover, mutation of a conserved glycine residue into an arginine residue in FXYD2 has been linked to cases of human hypomagnesemia indicating that dysregulation of Na,K-ATPase by FXYD proteins may be implicated in pathophysiological states. A better characterization of this novel regulatory mechanism of Na,K-ATPase may help to better understand its role in physiological and pathophysiological conditions.

Fluorometric measurements of intermolecular distances between the alpha- and beta-subunits of the Na+/K+-ATPase

The Na+/K+-ATPase maintains the physiological Na+ and K+ gradients across the plasma membrane in most animal cells. The functional unit of the ion pump is comprised of two mandatory subunits including the alpha-subunit, which mediates ATP hydrolysis and ion translocation, as well as the beta-subunit, which acts as a chaperone to promote proper membrane insertion and trafficking in the plasma membrane. To examine the conformational dynamics between the alpha- and beta-subunits of the Na+/K+-ATPase during ion transport, we have used fluorescence resonance energy transfer, under voltage clamp conditions on Xenopus laevis oocytes, to differentiate between two models that have been proposed for the relative orientation of the alpha- and beta-subunits. These experiments were performed by measuring the time constant of irreversible donor fluorophore destruction with fluorescein-5-maleimide as the donor fluorophore and in the presence or absence of tetramethylrhodamine-6-maleimide as the acceptor fluorophore following labeling on the M3-M4 or M5-M6 loop of the alpha-subunit and the beta-subunit. We have also used fluorescence resonance energy transfer to investigate the relative movement between the two subunits as the ion pump shuttles between the two main conformational states (E1 and E2) as described by the Albers-Post scheme. The results from this study have identified a model for the orientation of the beta-subunit in relation to the alpha-subunit and suggest that the alpha- and beta-subunits move toward each other during the E2 to E1 conformational transition.

Role of FXYD proteins in ion transport

The FXYD proteins are a family of seven homologous single transmembrane segment proteins (FXYD1-7), expressed in a tissue-specific fashion. The FXYD proteins modulate the function of Na,K-ATPase, thus adapting kinetic properties of active Na+ and K+ transport to the specific needs of different cells. Six FXYD proteins are known to interact with Na,K-ATPase and affect its kinetic properties in specific ways. Although effects of FXYD proteins on parameters such as K(1/2)Na+, K(1/2)K+, K(m)ATP, and V(max) are modest, usually twofold, these effects may have important long-term consequences for homeostasis of cation balance. In this review we summarize basic features of FXYD proteins and present recent evidence for functional effects, structure-function relations and structural interactions with Na,K-ATPase. We then discuss possible physiological roles, based on in vitro observations and newly available knockout mice models. Finally, we also consider evidence that FXYD proteins affect functioning of other ion transport systems.

FXYD proteins: new regulators of Na-K-ATPase

FXYD proteins belong to a family of small-membrane proteins. Recent experimental evidence suggests that at least five of the seven members of this family, FXYD1 (phospholemman), FXYD2 (gamma-subunit of Na-K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), and FXYD7, are auxiliary subunits of Na-K-ATPase and regulate Na-K-ATPase activity in a tissue- and isoform-specific way. These results highlight the complexity of the regulation of Na+ and K+ handling by Na-K-ATPase, which is necessary to ensure appropriate tissue functions such as renal Na+ reabsorption, muscle contractility, and neuronal excitability. Moreover, a mutation in FXYD2 has been linked to cases of human hypomagnesemia, indicating that perturbations in the regulation of Na-K-ATPase by FXYD proteins may be critically involved in pathophysiological states. A better understanding of this novel regulatory mechanism of Na-K-ATPase should help in learning more about its role in pathophysiological states. This review summarizes the present knowledge of the role of FXYD proteins in the modulation of Na-K-ATPase as well as of other proteins, their regulation, and their structure-function relationship.

Structure-function relationships in the NA+,K+-pump

The Na,K-pump was discovered about 50 years ago. Since then there has been a methodic investigation of its structure and functional characteristics. The development of the Albers-Post model for the transport cycle was a milestone that provided the framework for detailed understanding of the transport process. The pump is composed of 2 subunits that exist in the membrane as an alphabeta heterodimer. All known enzymatic functions of the pump occur through the alpha subunit. Although necessary for activity, the complete role of the beta subunit is not understood fully. Numerous studies have established that the alphabeta protomer is the minimal functional unit needed to perform the Albers-Post reaction cycle. However, higher orders of aggregation [(alphabeta)n] are commonly detected. There is little evidence that oligomerization has functional consequence for ion transport. The Na+,K+-adenosine triphosphatase (ATPase) is a member of the P-type ATPase family of transporters. Proteins within this family have common amino acid sequence motifs that share functional characteristics and structure. Low-resolution 3-dimensional reconstruction of 2-dimensional crystal diffractions provide evidence for the similarity in tertiary structure of the alpha subunit and the Ca2+ATPase (a closely related P-type ATPase). The spatial location of the beta subunit also is obvious in these reconstructions. Recent high-resolution reconstructions from 3-dimensional crystals of the Ca2+ATPase provide structural details at the atomic level. It now is possible to interpret structurally some of the key steps in the Albers-Post reaction. Some of these high-resolution interpretations are translatable to the Na+,K+-ATPase, but a high-resolution structure of the Na,K-pump is needed for the necessary details of those aspects that are unique to this transporter.

Structural interactions between FXYD proteins and Na+,K+-ATPase: alpha/beta/FXYD subunit stoichiometry and cross-linking

Interactions of rat FXYD4 (corticosteroid hormone-induced factor (CHIF)), FXYD2 (gamma), or FXYD1 (phospholemman (PLM)) proteins with rat alpha1 subunits of Na(+),K(+)-ATPase have been analyzed by co-immunoprecipitation and covalent cross-linking. In detergent-solubilized membranes from HeLa cells expressing both gamma and CHIF or CHIF and hemagglutinin A-tagged CHIF, mixed complexes of CHIF and gamma or CHIF and hemagglutinin A-tagged CHIF with alpha/beta subunits are undetectable. This implies that the alpha/beta/FXYD protomer is the major species in detergent solution. A lipid-soluble cysteine-cysteine bifunctional reagent, dibromobimane, cross-links CHIF to alpha in colonic membranes but not gamma or PLM to alpha in kidney or heart membranes, respectively. Sequence comparisons of the FXYD proteins suggested that Cys-49 in the trans-membrane segment of CHIF could be involved. In detergent-solubilized HeLa cell membranes, dibromobimane cross-links wild-type CHIF to alpha but not the C49F mutant, and also the corresponding F36C mutant but not wild-type gammab, and F48C but not wild-type PLM. C140S, C338A, C804A, and C966S mutants of the alpha subunit have been expressed. Only the C140S mutant prevents cross-linking with CHIF. The data demonstrated the proximity of trans-membrane segments of CHIF, gamma, and PLM to M2 of alpha. Molecular modeling is consistent with location of the trans-membrane segment of all FXYD proteins between M2, M6, and M9 and the proximity of Cys-49 of CHIF or Phe-36 of gamma with Cys-140 of M2. Cross-linking also demonstrated CHIF-alpha and CHIF-beta proximities in extra-membrane regions, similar to the evidence for gamma-alpha and gamma-beta cross-links.

Stabilization of trypsin by association to plasma membranes: Implications for tryptic cleavage of membrane-bound Na,K-ATPase

Tryptic cleavage has been a potential method for studying the structure and mechanism of many membrane transport proteins. Here, we report tight association of trypsin to pig kidney plasma membranes enriched in Na,K-ATPase. Trypsin also associated with protein-free vesicles prepared from plasma membrane lipids. Membrane-associated trypsin was found to be highly resistant to autolysis and insensitive to inhibition by PMSF. Na,K-ATPase substrate ions differentially influenced the level of trypsin membrane association. Thus, NaCl significantly increased trypsin membrane association compared to KCl. The ions seem to exert direct effects on the membrane independent of their effects on protein conformation. Bicarbonate anions, which detach peripheral membrane proteins, efficiently released trypsin from the membrane. Trypsin membrane association was found to enhance the cleavage of the Na,K-ATPase gamma-subunit. Comparison between membranes from shark rectal gland and pig kidney showed that trypsin association was significantly higher in the former. This was found to be partly due to the presence of higher cholesterol levels in the membrane. In conclusion, the differential membrane association of trypsin may affect the outcome of proteolytic cleavage of membrane-bound proteins.

Na,K-ATPase Subunit Heterogeneity as a Mechanism for Tissue-Specific Ion Regulation

The Na,K-ATPase comprises a family of isozymes that catalyze the active transport of cytoplasmic Na+ for extracellular K+ at the plasma membrane of cells. Isozyme diversity for the Na,K-ATPase results from the association of different molecular forms of the alpha (alpha1, alpha2, alpha3, and alpha4) and beta (beta1, beta2, and beta3) subunits that constitute the enzyme. The various isozymes are characterized by unique enzymatic properties and a highly regulated pattern of expression that depends on cell type, developmental stage, and hormonal stimulation. The molecular complexity of the Na,K-ATPase goes beyond its alpha and beta isoforms and, in certain tissues, other accessory proteins associate with the enzyme. These small membrane-bound polypeptides, known as the FXYD proteins, modulate the kinetic characteristics of the Na,K-ATPase. The experimental evidence available suggests that the molecular and functional heterogeneity of the Na,K-ATPase is a physiologically relevant event that serves the specialized functions of cells. This article focuses on the functional properties, regulation, and the biological relevance of the Na,K-ATPase isozymes as a mechanism for the tissue-specific control of Na+ and K+ homeostasis.

Influence of nitric oxide donors on the intrinsic fluorescence of Na+,K+-ATPase and effects on the membrane lipids

Effects of the nitric oxide donors S-nitroso-glutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) on Na+,K+-ATPase-rich membrane fragments purified from pig kidney outer medulla were studied using intrinsic fluorescence and ESR of spin-labeled membranes. These S-nitrosothiols differently affected the intrinsic fluorescence of Na+,K+-ATPase: GSNO induced a partial quenching, whereas SNAP produced no alteration. Quenching can be due to a direct modification of exposed tryptophan residues or to an indirect effect caused by reactions of nitrogen oxide reactive species with other residues or even with the membrane lipids. Pre-incubation of Na+,K+-ATPase with 0.4mM GSNO resulted in a modest inhibition of ATPase activity (about 24%) measured under optimal conditions. Stearic acid spin-labeled at the 14th carbon atom (14-SASL) was used to investigate membrane fluidity and the protein-lipid interface. SNAP slightly increased the mobility of bulk lipids from Na+,K+-ATPase-rich membranes, but did not change the fraction of bulk to protein-interacting lipids. Conversely, treatment with GSNO extinguished the ESR signals from 14-SASL, indicating generation of free radicals with high affinity for the lipid moiety. Our results demonstrated that membranes influence bioavailability of reactive nitrogen species and bias the activity of different S-nitrosothiols.

Interaction of FXYD10 (PLMS) with Na,K-ATPase from shark rectal glands. Close proximity of Cys74 of FXYD10 to Cys254 in the a domain of the alpha-subunit revealed by intermolecular thiol cross-linking

FXYD domain-containing proteins are tissue-specific regulators of the Na,K-ATPase that have been shown to have significant physiological implications. Information about the sites of interaction between some FXYD proteins and subunits of the Na,K-ATPase is beginning to emerge. We previously identified an FXYD protein in plasma membranes from shark rectal gland cells and demonstrated that this protein (FXYD10) modulates shark Na,K-ATPase activity. The present study was undertaken to identify the location of the C-terminal domain of FXYD10 on the alpha-subunit of Na,K-ATPase, using covalent cross-linking combined with proteolytic cleavage. Treatment of Na,K-ATPase-enriched membranes with the homobifunctional thiol cross-linker 1,4-bismaleimidyl-2,3-dihydroxybutane resulted in cross-linking of FXYD10 to the alpha-subunit. Cross-linking was not affected by preincubation with sodium or potassium but was significantly reduced after pre-incubation with the non-hydrolyzable ATP analog beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP). A peptic assay was developed, in which pepsin treatment of Na,K-ATPase at low pH resulted in extensive cleavage of the alpha-subunit while FXYD10 was left intact. Proteolytic fragments of control and cross-linked preparations were isolated by immunoprecipitation and analyzed by gel electrophoresis. A proteolytic fragment containing FXYD10 cross-linked to a fragment from the alpha-subunit could be localized on SDS gels. Sequencing of this fragment showed the presence of FXYD10 as well as a fragment within the A domain of the alpha-subunit comprising 33 amino acids, including a single Cys residue, Cys254. Thus, regulation of Na,K-ATPase by FXYD10 occurs in part via cytoplasmic interaction of FXYD10 with the A domain of the shark alpha-subunit.

Na,K-ATPase from mice lacking the gamma subunit (FXYD2) exhibits altered Na+ affinity and decreased thermal stability

The gamma subunit of the Na,K-ATPase, a 7-kDa single-span membrane protein, is a member of the FXYD gene family. Several FXYD proteins have been shown to bind to Na,K-ATPase and modulate its properties, and each FXYD protein appears to alter enzyme kinetics differently. Different results have sometimes been obtained with different experimental systems, however. To test for effects of gamma in a native tissue environment, mice lacking a functional gamma subunit gene (Fxyd2) were generated. These mice were viable and without observable pathology. Prior work in the mouse embryo showed that gamma is expressed at the blastocyst stage. However, there was no delay in blastocele formation, and the expected Mendelian ratios of offspring were obtained even with Fxyd2-/- dams. In adult Fxyd2-/- mouse kidney, splice variants of gamma that have different nephron segment-specific expression patterns were absent. Purified gamma-deficient renal Na,K-ATPase displayed higher apparent affinity for Na+ without significant change in apparent affinity for K+. Affinity for ATP, which was expected to be decreased, was instead slightly increased. The results suggest that regulation of Na+ sensitivity is a major functional role for this protein, whereas regulation of ATP affinity may be context-specific. Most importantly, this implies that gamma and other FXYD proteins have their effects by local and not global conformation change. Na,K-ATPase lacking the gamma subunit had increased thermal lability. Combined with other evidence that gamma participates in an early step of thermal denaturation, this indicates that FXYD proteins may play an important structural role in the enzyme complex.

The Effect of the Gamma Modulator on Na/K Pump Activity of Intact Mammalian Cells

This study concerns the modulatory effects of the gamma modulator of the Na/K pump, in particular whether the effects seen in previous experiments with isolated membranes are relevant to Na/K pump behavior in intact mammalian cells. For this purpose, HeLa cells previously transfected with the rat Na/K catalytic subunit were used. The results show that both variants of the regulator, gammaa and gammab, decrease the apparent affinity of the pump for Na(+) and cause a modest increase in apparent ATP affinity as seen in measurements of ouabain-sensitive (86)Rb(K(+)) influx into cells in which ATP was varied using antimycin A and glucose. Equivalent results had been obtained previously in our analyses of Na,K-ATPase activity of membrane fragments, i.e., an increase in K(0.5(Na)) at high K(+) concentration and a decrease in K'(ATP). Comparison of clones of gamma-transfected and mock-transfected cells (with similar V(max) values) indicated that gamma causes a modest approximately 30% increase in the steady-state concentration of intracellular Na(+). Furthermore, for both gammaa and gammab, values of intracellular Na(+) were similar to those predicted from the kinetic constants, K(0.5(Na)) and V(max). Finally, there was a gamma-mediated increase in apparent affinity for extracellular K(+), which had not been detected in assays of permeabilized membranes.

Effects of warm acclimation on Na+,K+-ATPase α-subunit expression in chloride cells of Antarctic fish

The teleosts Trematomus bernacchii thrive in southern oceanic waters with temperatures below 0 degrees C. These fish have serum osmolalities almost double those found in fish of temperate waters, thereby lowering their serum's freezing point and the energy needed for ionic homeostasis. Upon warm acclimation to 4 degrees C, T. bernacchii decrease their serum osmolality and increase the Na+,K+-ATPase activity in their gills. Na+,K+-ATPase alpha1-, alpha2-, and alpha3-subunit isoforms are expressed in the gills of T. bernacchii and it is thought that Na+,K+-ATPase subunit composition in chloride cells changes with warm acclimation. Using immunohistochemistry, we compared the number of chloride cells expressing various alpha-isoforms of the Na+,K+-ATPase in the gills of cold- and warm-acclimated T. bernacchii. We found no change in the number of alpha2- or alpha3-immunopositive cells in warm-acclimated fish gills or in the number of cells immunopositive for the Na+,K+,2Cl- cotransporter. However, the number of pan-alpha-immunopositive (recognizing all three alpha-isoforms) and alpha1-immunopositive cells both increased in warm-acclimated fish. This suggests that changes in the number of alpha1-isoform-expressing chloride cells could contribute to the increased Na+,K+-ATPase activity that occurs with warm-acclimation.

Hypertrophy, increased ejection fraction, and reduced Na-K-ATPase activity in phospholemman-deficient mice

Phospholemman (FXYD1), a 72-amino acid transmembrane protein abundantly expressed in the heart and skeletal muscle, is a major substrate for phosphorylation in the cardiomyocyte sarcolemma. Biochemical, cellular, and electrophysiological studies have suggested a number of possible roles for this protein, including ion channel modulator, taurine-release channel, Na(+)/Ca(2+) exchanger modulator, and Na-K-ATPase-associated subunit. We have generated a phospholemman-deficient mouse. The adult null mice exhibited increased cardiac mass, larger cardiomyocytes, and ejection fractions that were 9% higher by magnetic resonance imaging compared with wild-type animals. Notably, this occurred in the absence of hypertension. Total Na-K-ATPase activity was 50% lower in the phospholemman-deficient hearts. Expression (per unit of membrane protein) of total Na-K-ATPase was only slightly diminished, but expression of the minor alpha(2)-isoform, which has been specifically implicated in the control of contractility, was reduced by 60%. The absence of phospholemman thus results in a complex response, including a surprisingly large reduction in intrinsic Na-K-ATPase activity, changes in Na-K-ATPase isoform expression, increase in ejection fraction, and increase in cardiac mass. We hypothesize that a primary effect of phospholemman is to modulate the Na-K-ATPase and that its reduced activity initiates compensatory responses.

Identification of potential regulatory sites of the Na+,K+-ATPase by kinetic analysis

Kinetic models are presented that allow the Na(+),K(+)-ATPase steady-state turnover number to be estimated at given intra- and extracellular concentrations of Na(+), K(+), and ATP. Based on experimental transient kinetic data, the models utilize either three or four steps of the Albers-Post scheme, that is, E(2) --> E(1), E(1) --> E(2)P (or E(1) --> E(1)P and E(1)P --> E(2)P), and E(2)P --> E(2), which are the major rate-determining steps of the enzyme cycle. On the time scale of these reactions, the faster binding steps of Na(+), K(+), and ATP to the enzyme are considered to be in equilibrium. Each model was tested by comparing calculations of the steady-state turnover from rate constants and equilibrium constants for the individual partial reactions with published experimental data of the steady-state activity at varying Na(+) and K(+) concentrations. To provide reasonable agreement between the calculations and the experimental data, it was found that Na(+)/K(+) competition for cytoplasmic binding sites was an essential feature required in the model. The activity was also very dependent on the degree of K(+)-induced stimulation of the reverse reaction E(1) --> E(2). Taking into account the physiological substrate concentrations, the models allow the most likely potential sites of short-term Na(+),K(+)-ATPase regulation to be identified. These were found to be (a) the cytoplasmic Na(+) and K(+) binding sites, via changes in Na(+) or K(+) concentration or their dissociation constants, (b) ATP phosphorylation (as a substrate), via a change in its rate constant, and (c) the position of the E(2)<==>E(1) equilibrium.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

Modulation of Na,K-ATPase by the gamma subunit: studies with transfected cells and transmembrane mimetic peptides

The enzymatic activity of the Na,K-ATPase, or sodium pump, is modulated by members of the so-called FXYD family of transmembrane proteins. The best characterized member, FXYD2, also referred to as the gamma subunit, has been shown to decrease the apparent Na+ affinity and increase the apparent ATP affinity of the pump. The effect on ATP affinity had been ascribed to the cytoplasmic C-terminal end of the protein, whereas recent observations suggest that the transmembrane (TM) segment of gamma mediates the Na+ affinity effect. Here we use a novel approach involving synthetic transmembrane mimetic peptides to demonstrate unequivocally that the TM domain of gamma effects the shift in apparent Na+ affinity. Specifically, we show that incubation of these peptides with membranes containing alphabeta pumps modulates Na+ affinity in a manner similar to transfected full-length gamma subunit. Using mutated gamma peptides and transfected proteins, we also show that a specific glycine residue, Gly-41, which is associated with a form of familial renal hypomagnesemia when mutated to Arg, is important for this kinetic effect, whereas Gly-35, located on an alternate face of the transmembrane helix, is not. The peptide approach allows for the analysis of mutants that fail to be expressed in a transfected system.

Structure and mechanism of Na,K-ATPase: functional sites and their interactions

The cell membrane Na,K-ATPase is a member of the P-type family of active cation transport proteins. Recently the molecular structure of the related sarcoplasmic reticulum Ca-ATPase in an E1 conformation has been determined at 2.6 A resolution. Furthermore, theoretical models of the Ca-ATPase in E2 conformations are available. As a result of these developments, these structural data have allowed construction of homology models that address the central questions of mechanism of active cation transport by all P-type cation pumps. This review relates recent evidence on functional sites of Na,K-ATPase for the substrate (ATP), the essential cofactor (Mg(2+) ions), and the transported cations (Na(+) and K(+)) to the molecular structure. The essential elements of the Ca-ATPase structure, including 10 transmembrane helices and well-defined N, P, and A cytoplasmic domains, are common to all PII-type pumps such as Na,K-ATPase and H,K-ATPases. However, for Na,K-ATPase and H,K-ATPase, which consist of both alpha- and beta-subunits, there may be some detailed differences in regions of subunit interactions. Mutagenesis, proteolytic cleavage, and transition metal-catalyzed oxidative cleavages are providing much evidence about residues involved in binding of Na(+), K(+), ATP, and Mg(2+) ions and changes accompanying E1-E2 or E1-P-E2-P conformational transitions. We discuss this evidence in relation to N, P, and A cytoplasmic domain interactions, and long-range interactions between the active site and the Na(+) and K(+) sites in the transmembrane segments, for the different steps of the catalytic cycle.

Structure/function studies of the gamma subunit of the Na,K-ATPase

The Na,K-ATPase gamma subunit is present primarily in kidney as two splice variants, gammaa and gammab, which differ only at their extracellular N-termini. Two distinct effects of gamma are seen in biochemical Na,K-ATPase assays of mammalian (HeLa) cells transfected with gammaa or gammab, namely, (i) a decrease in K'(ATP) probably secondary to a shift in steady-state E(1) <--> E(2) poise in favor of E(1) and (ii) an increase in cytoplasmic K(+)/Na(+) antagonism seen as an increase in K'(Na) at high K(+) concentration. Mutagenesis experiments involving alterations in extramembranous regions of gamma indicate that different regions mediate the aforementioned distinct effects and that the effects appear to be long range. Studies of ouabain-sensitive fluxes with intact cells confirm the gamma effects seen with membranes and also suggest an additional effect (increase) in apparent affinity for extracellular K(+). Alteration in gamma function was also evidenced in the behavior of a G41 -->R mutation within the transmembrane domain of gamma. G41R is associated with autosomal dominant renal magnesium wasting. Our studies show that this mutation in the gammab variant retards trafficking of gamma, but not alphabeta pumps, to the cell surface and abolishes functional effects of gamma, consistent with the conclusion that the Mg(2+) transport defect is secondary to loss of gamma modulation of Na,K-ATPase function.

FXYD proteins as regulators of the Na,K-ATPase in the kidney

The FXYD gene family has seven members in mammals and others in fish. Five of these (FXYD1, FXYD2, FXYD4, FXYD7, and PLMS from shark) have been shown to alter the activity of the Na,K-ATPase, as described by other papers in this volume. The gene structure of FXYD family members suggests assembly from protein domain modules and gene duplication. The gamma subunit is unique in the family for having alternative splice variants in the coding region and can be posttranslationally modified with different final consequences for enzyme properties. The nonoverlapping distribution of gamma and CHIF (FXYD4) in kidney helps to explain physiological differences in Na(+) affinity among nephron segments. We also detected phospholemman (FXYD1) in kidney. By immunofluorescence, it was found in extraglomerular mesangial cells (EM cells) of the juxtaglomerular apparatus and in the afferent arteriole. Contrary to many reports that only alpha1 and beta1 are expressed in the kidney, we found that alpha2 and beta2 are present, although not in any nephron segment. Both were detected in arterioles, and beta2 was found in the EM cells. In contrast, alpha1, beta1, and gamma were found in adjacent macula densa. Phospholemman, alpha2, and beta2 are proposed to have distinct roles in regulating the sodium pump in structures involved in tubuloglomerular feedback.

Biochemistry of Na,K-ATPase

The Na,K-ATPase or sodium pump carries out the coupled extrusion and uptake of Na and K ions across the plasma membranes of cells of most higher eukaryotes. It is a member of the P-type ATPase superfamily. This heterodimeric integral membrane protein is composed of a 100-kDa alpha-subunit with ten transmembrane segments and a heavily glycosylated beta subunit of about 55 kDa, which is a type II membrane protein. Current ideas on how the protein achieves active transport are based on a fusion of results of transport physiology, protein chemistry, and heterologous expression of mutant proteins. Recently acquired high resolution structural information provides an important new avenue for a more complete understanding of this protein. In this review, the current status of knowledge of Na,K-ATPase is discussed, and areas where there is still considerable uncertainty are highlighted.

The ATP-Mg2+ binding site and cytoplasmic domain interactions of Na+,K+-ATPase investigated with Fe2+-catalyzed oxidative cleavage and molecular modeling

This work utilizes Fe(2+)-catalyzed cleavages and molecular modeling to obtain insight into conformations of cytoplasmic domains and ATP-Mg(2+) binding sites of Na(+),K(+)-ATPase. In E(1) conformations the ATP-Fe(2+) complex mediates specific cleavages at 712VNDS (P domain) and near 440VAGDA (N domain). In E(2)(K), ATP-Fe(2+) mediates cleavages near 212TGES (A domain), near 440VAGDA, and between residues 460-490 (N domain). Cleavages at high ATP-Fe(2+) concentrations do not support suggestions for two ATP sites. A new reagent, fluorescein-DTPA, has been synthesized. The fluorescein-DTPA-Fe(2+) complex mediates cleavages similar to those mediated by ATP-Fe(2+). The data suggest the existence of N to P domain interactions in E(1)Na, with bound ATP-Fe(2+) or fluorescein-DPTA-Fe(2+), A-N, and A-P interactions in E(2)(K), and provide testable constraints for model building. Molecular models based on the Ca(2+)-ATPase structure are consistent with the predictions. Specifically, high-affinity ATP-Mg(2+) binding in E(1) is explained with the N domain tilted ca. 80 degrees toward the P domain, by comparison with well-separated N and P domains in the Ca-ATPase crystal structure. With ATP-Mg(2+) docked, bound Mg(2+) is close to both D710 (in 710DGVNDS) and D443 (in 440VAGDASE). D710 is known to be crucial for Mg(2+) binding. The cleavage and modeling data imply that D443 could also be a candidate for Mg(2+) binding. Comparison of E(1).ATP,Mg(2+) and E(2) models suggests an explanation of the high or low ATP affinities, respectively. We propose a scheme of ATP-Mg(2+) and Mg(2+) binding and N, P, and A domain interactions in the different conformations of the catalytic cycle.

Distinct regulatory effects of the Na,K-ATPase gamma subunit

The two variants of the gamma subunit of the rat renal sodium pump, gamma(a) and gamma(b), have similar effects on the Na,K-ATPase. Both increase the affinity for ATP due to a shift in the enzyme's E(1) <--> E(2) conformational equilibrium toward E(1). In addition, both increase K(+) antagonism of cytoplasmic Na(+) activation. To gain insight into the structural basis for these distinct effects, extramembranous N-terminal and C-terminal mutants of gamma were expressed in rat alpha1-transfected HeLa cells. At the N terminus, the variant-distinct region was deleted (gammaNDelta7) or replaced by alanine residues (gammaN7A). At the C terminus, four (gamma(a)CDelta4) or ten (gamma(a)CDelta10) residues were deleted. None of these mutations abrogates the K(+)/Na(+) antagonism as evidenced in a similar increase in K'(Na) seen at high (100 mm) K(+) concentration. In contrast, the C-terminal as well as N-terminal deletions (gammaNDelta7, gamma(a)CDelta4, and gamma(a)CDelta10) abolished the decrease in K'(ATP) seen with wild-type gamma(a) or gamma(b). It is concluded that different regions of the gamma chain mediate the distinct functional effects of gamma, and the effects can be long-range. In the transmembrane region, the impact of G41R replacement was analyzed since this mutation is associated with autosomal dominant renal Mg(2+)-wasting in man (Meij, I. C., Koenderink, J. B., van Bokhoven, H., Assink, K. F. H., Groenestege, W. T., de Pont, J. J. H. H. M., Bindels, R. J. M., Monnens, L. A. H., Van den Heuvel, L. P. W. J., and Knoers, N. V. A. M. (2000) Nat. Genet. 26, 265-266). The results show that Gly-41 --> Arg prevents trafficking of gamma but not alphabeta pumps to the cell surface and abrogates functional effects of gamma on alphabeta pumps. These findings underscore a potentially important role of gamma in affecting solute transport, in this instance Mg(2+) reabsorption, consequent to its primary effect on the sodium pump.

Differential regulation of renal Na,K-ATPase by splice variants of the gamma subunit

Sodium and potassium-exchanging adenosine triphosphatase (Na,K-ATPase) in the kidney is associated with the gamma subunit (gamma, FXYD2), a single-span membrane protein that modulates ATPase properties. Rat and human gamma occur in two splice variants, gamma(a) and gamma(b), with different N termini. Here we investigated their structural heterogeneity and functional effects on Na,K-ATPase properties. Both forms were post-translationally modified during in vitro translation with microsomes, indicating that there are four possible forms of gamma. Site-directed mutagenesis revealed Thr(2) and Ser(5) as potential sites for post-translational modification. Similar modification can occur in cells, with consequences for Na,K-ATPase properties. We showed previously that stable transfection of gamma(a) into NRK-52E cells resulted in reduction of apparent affinities for Na(+) and K(+). Individual clones differed in gamma post-translational modification, however, and the effect on Na(+) affinity was absent in clones with full modification. Here, transfection of gamma(b) also resulted in clones with or without post-translational modification. Both groups showed a reduction in Na(+) affinity, but modification was required for the effect on K(+) affinity. There were minor increases in ATP affinity. The physiological importance of the reduction in Na(+) affinity was shown by the slower growth of gamma(a), gamma(b), and gamma(b') transfectants in culture. The differential influence of the four structural variants of gamma on affinities of the Na,K-ATPase for Na(+) and K(+), together with our previous finding of different distributions of gamma(a) and gamma(b) along the rat nephron, suggests a highly specific mode of regulation of sodium pump properties in kidney.