Primary structure of the alpha-subunit of Torpedo californica (Na+ + K+)ATPase deduced from cDNA sequence

Effects of seawater acclimation on two Na+/K+-ATPase α-subunit isoforms in the gills of the marble goby, Oxyeleotris marmorata

The marble goby, Oxyeleotris marmorata, is a freshwater teleost, but can acclimate progressively to survive in seawater (salinity 30). As an obligatory air-breather, it can also survive long periods of emersion. Two isoforms of Na⁺/K⁺-ATPase (nka) α-subunit, nkaα1 and nkaα3, but not nkaα2, had been cloned from the gills of O. marmorata. The cDNA sequence of nkaα1 consisted of 3069 nucleotides, coding for 1023 amino acids (112.5 kDa), whereas nkaα3 consisted of 2976 nucleotides, coding for 992 amino acids (109.5 kDa). As only one form of branchial Nkaα1 was identified using molecular cloning in this study, O. marmorata lacks specific freshwater- and seawater-type Nkaα isoforms as demonstrated by some other euryhaline fish species. The nkaα1 transcript level was about 2.5-fold higher than that of nkaα3 in the gills of freshwater O. marmorata. During exposure to seawater, the branchial transcript level of nkaα1 increased significantly on day 1 (~3.3-fold) and day 6 (~2.6-fold). By contrast, the branchial transcript level of nkaα3 increased significantly on day 1 (~2.6-fold), but not on day 6, of seawater exposure. Six days of exposure to seawater also led to significant increases in protein abundances of Nkaα1 (~6.9-fold) and Nkaα3 (~2.8-fold) in the gills of O. marmorata. Hence, the mRNA and protein expressions of both nkaα1/Nkaα1 and nkaα3/Nkaα3 were up-regulated in O. marmorata during seawater acclimation. This could explain why V_max increases but K_m for Na⁺ and K⁺ remain unchanged in Nka extracted from the gills of O. marmorata acclimated to seawater as reported previously.

Computer modelling reveals new conformers of the ATP binding loop of Na+/K+-ATPase involved in the transphosphorylation process of the sodium pump

Hydrolysis of ATP by Na⁺/K⁺-ATPase, a P-Type ATPase, catalyzing active Na⁺ and K⁺ transport through cellular membranes leads transiently to a phosphorylation of its catalytical α-subunit. Surprisingly, three-dimensional molecular structure analysis of P-type ATPases reveals that binding of ATP to the N-domain connected by a hinge to the P-domain is much too far away from the Asp³⁶⁹ to allow the transfer of ATP’s terminal phosphate to its aspartyl-phosphorylation site. In order to get information for how the transfer of the γ-phosphate group of ATP to the Asp³⁶⁹ is achieved, analogous molecular modeling of the M₄–M₅ loop of ATPase was performed using the crystal data of Na⁺/K⁺-ATPase of different species. Analogous molecular modeling of the cytoplasmic loop between Thr³³⁸ and Ile⁷⁶⁰ of the α₂-subunit of Na⁺/K⁺-ATPase and the analysis of distances between the ATP binding site and phosphorylation site revealed the existence of two ATP binding sites in the open conformation; the first one close to Phe⁴⁷⁵ in the N-domain, the other one close to Asp³⁶⁹ in the P-domain. However, binding of Mg²⁺•ATP to any of these sites in the “open conformation” may not lead to phosphorylation of Asp³⁶⁹. Additional conformations of the cytoplasmic loop were found wobbling between “open conformation” <==> “semi-open conformation <==> “closed conformation” in the absence of 2Mg²⁺•ATP. The cytoplasmic loop’s conformational change to the “semi-open conformation”—characterized by a hydrogen bond between Arg⁵⁴³ and Asp⁶¹¹—triggers by binding of 2Mg²⁺•ATP to a single ATP site and conversion to the “closed conformation” the phosphorylation of Asp³⁶⁹ in the P-domain, and hence the start of Na⁺/K⁺-activated ATP hydrolysis.

Nonhuman genetics. Genomic basis for the convergent evolution of electric organs

Little is known about the genetic basis of convergent traits that originate repeatedly over broad taxonomic scales. The myogenic electric organ has evolved six times in fishes to produce electric fields used in communication, navigation, predation, or defense. We have examined the genomic basis of the convergent anatomical and physiological origins of these organs by assembling the genome of the electric eel (Electrophorus electricus) and sequencing electric organ and skeletal muscle transcriptomes from three lineages that have independently evolved electric organs. Our results indicate that, despite millions of years of evolution and large differences in the morphology of electric organ cells, independent lineages have leveraged similar transcription factors and developmental and cellular pathways in the evolution of electric organs.

Branchial Na(+):K(+):2Cl(-) cotransporter 1 and Na(+)/K(+)-ATPase α-subunit in a brackish water-type ionocyte of the euryhaline freshwater white-rimmed stingray, Himantura signifer

Himantura signifer is a freshwater stingray which inhabits rivers in Southeast Asia. It can survive in brackish water but not seawater. In brackish water, it becomes partially ureosmotic, but how it maintains its plasma hypoionic to the external medium is enigmatic because of the lack of a rectal gland. Here, we report for the first time the expression of Na(+):K(+):2Cl(-) cotransporter 1 (nkcc1) in the gills of freshwaterH. signifer, and its moderate up-regulation (~2-fold) in response to brackish water (salinity 20) acclimation. The absence of the Ste20-related proline-alanine-rich kinase and oxidation stress response kinase 1 interaction site from the N-terminus of H. signifer Nkcc1 suggested that it might not be effectively activated by stress kinases in response to salinity changes as in more euryhaline teleosts. The increased activity of Nkcc1 during salt excretion in brackish water would lead to an influx of Na(+) into ionocytes, and the maintenance of intracellular Na(+) homeostasis would need the cooperation of Na(+)/K(+)-ATPase (Nka). We demonstrated for the first time the expression of nkaα1, nkaα2 and nkaα3 in the gills of H. signifer, and the up-regulation of the mRNA expression of nkaα3 and the overall protein abundance of Nkaα in response to acclimation to brackish water. Immunofluorescence microscopy revealed the presence of a sub-type of ionocyte, co-expressing Nkcc1 and Nkaα, near the base of the secondary lamellae in the gills of H. signifer acclimated to brackish water, but this type of ionocyte was absent from the gills of fish kept in fresh water. Hence, there could be a change in the function of the gills of H. signifer from salt absorption to salt excretion during brackish water acclimation in the absence of a functioning rectal gland.

Roles of three branchial Na(+)-K(+)-ATPase α-subunit isoforms in freshwater adaptation, seawater acclimation, and active ammonia excretion in Anabas testudineus

Three Na(+)-K(+)-ATPase (nka) α-subunit isoforms, nka α1a, nka α1b, and nka α1c, were identified from gills of the freshwater climbing perch Anabas testudineus. The cDNA sequences of nka α1a and nka α1b consisted of 3,069 bp, coding for 1,023 amino acids, whereas nka α1c was shorter by 22 nucleotides at the 5' end. In freshwater, the quantity of nka α1c mRNA transcripts present in the gills was the highest followed by nka α1a and nka α1b that was almost undetectable. The mRNA expression of nka α1a was downregulated in the gills of fish acclimated to seawater, indicating that it could be involved in branchial Na(+) absorption in a hypoosmotic environment. By contrast, seawater acclimation led to an upregulation of the mRNA expression of nka α1b and to a lesser extent nka α1c, indicating that they could be essential for ion secretion in a hyperosmotic environment. More importantly, ammonia exposure led to a significant upregulation of the mRNA expression of nka α1c, which might be involved in active ammonia excretion. Both seawater acclimation and ammonia exposure led to significant increases in the protein abundance and changes in the kinetic properties of branchial Na(+)-K(+)-ATPase (Nka), but they involved two different types of Nka-immunoreactive cells. Since there was a decrease in the effectiveness of NH(4)(+) to substitute for K(+) to activate branchial Nka from fish exposed to ammonia, Nka probably functioned to remove excess Na(+) and to transport K(+) instead of NH(4)(+) into the cell to maintain intracellular Na(+) and K(+) homeostasis during active ammonia excretion.

Neutral endopeptidase neprilysin is copurified with Na,K-ATPase from rabbit outer medulla and hydrolyzes its α-subunit

Preparations of Na,K-ATPase from outer medulla of rabbit kidney purified in accordance with the method of P. L. Jorgensen were shown to contain as admixture a protease that moves with α-subunit (~100 kDa) as a single protein band during one-dimensional SDS-PAGE. The electro-elution of proteins of this band from polyacrylamide gel results in the appearance of two protein fragments (~67 and 55 kDa) that are stained with polyclonal antibodies against Na,K-ATPase α-subunit. Liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis showed that the neutral membrane-bound endopeptidase neprilysin is located in one protein band together with the Na,K-ATPase α-subunit. Addition of thiorphan, a specific inhibitor of neutral endopeptidase, eliminates proteolysis of the α-subunit. The data demonstrate that Na,K-ATPase α-subunit may be a natural target for neprilysin.

The nicotinic acetylcholine receptor and the Na,K-ATPase alpha2 isoform interact to regulate membrane electrogenesis in skeletal muscle

The nicotinic acetylcholine receptor (nAChR) and the Na,K-ATPase functionally interact in skeletal muscle (Krivoi, I. I., Drabkina, T. M., Kravtsova, V. V., Vasiliev, A. N., Eaton, M. J., Skatchkov, S. N., and Mandel, F. (2006) Pflugers Arch. 452, 756-765; Krivoi, I., Vasiliev, A., Kravtsova, V., Dobretsov, M., and Mandel, F. (2003) Ann. N.Y. Acad. Sci. 986, 639-641). In this interaction, the specific binding of nanomolar concentrations of nicotinic agonists to the nAChR stimulates electrogenic transport by the Na,K-ATPase alpha2 isozyme, causing membrane hyperpolarization. This study examines the molecular nature and membrane localization of this interaction. Stimulation of Na,K-ATPase activity by the nAChR does not require ion flow through open nAChRs. It can be induced by nAChR desensitization alone, in the absence of nicotinic agonist, and saturates when the nAChR is fully desensitized. It is enhanced by noncompetitive blockers of the nAChR (proadifen, QX-222), which promote non-conducting or desensitized states; and retarded by tetracaine, which stabilizes the resting nAChR conformation. The interaction operates at the neuromuscular junction as well as on extrajunctional sarcolemma. The Na,K-ATPase alpha2 isozyme is enriched at the postsynaptic neuromuscular junction and co-localizes with nAChRs. The nAChR and Na,K-ATPase alpha subunits specifically coimmunoprecipitate with each other, phospholemman, and caveolin-3. In a purified membrane preparation from Torpedo californica enriched in nAChRs and the Na,K-ATPase, a ouabain-induced conformational change of the Na,K-ATPase enhances a conformational transition of the nAChR to a desensitized state. These results suggest a mechanism by which the nAChR in a desensitized state with high apparent affinity for agonist interacts with the Na,K-ATPase to stimulate active transport. The interaction utilizes a membrane-delimited complex involving protein-protein interactions, either directly or through additional protein partners. This interaction is expected to enhance neuromuscular transmission and muscle excitation.

Long-term terbutaline exposure stimulates alpha1-Na+-K+-ATPase expression at posttranscriptional level in rat fetal distal lung epithelial cells

Transepithelial Na(+) transport through epithelial Na(+) channels (ENaC) on the apical membrane and Na(+)-K(+)-ATPase activity on the basolateral membrane of distal lung epithelial cells are critical for alveolar fluid clearance. Acute exposure to beta-adrenergic agonists stimulates lung fluid clearance by increasing Na(+) transport. We investigated the effects of chronic exposure to the beta(2)-adrenergic agonist terbutaline on the transepithelial Na(+) transport in rat fetal distal lung epithelia (FDLE). FDLE monolayers exposed to 10(-4) M terbutaline for 48 h had significantly increased propanolol-blockable transepithelial total and amiloride-sensitive short-circuit current (I(sc)); however, when these chronically exposed monolayers were acutely exposed to additional beta-agonists and intracellular cAMP upregulators, there was no further increase in I(sc). Monolayers exposed to terbutaline for >48 h had I(sc) similar to control cells. Ouabain-sensitive Na(+)-K(+)-ATPase activity was increased in 48-h terbutaline-exposed FDLE whose apical membranes were permeabilized with nystatin. In contrast, terbutaline did not increase amiloride-sensitive apical membrane I(sc) in FDLE whose basolateral membranes were permeabilized with nystatin. Terbutaline treatment did not affect alpha-, beta-, or gamma-ENaC mRNA or alpha-ENaC protein steady-state levels, but increased total cellular levels and rate of synthesis of alpha(1)-Na(+)-K(+)-ATPase protein in FDLE in the absence of any change in alpha(1)-Na(+)-K(+)-ATPase mRNA. Total cellular beta(1)-Na(+)-K(+)-ATPase mRNA and protein levels were not affected by terbutaline. These data suggest that FDLE have different responses from adult type II epithelial cells when chronically exposed to terbutaline, and their increased transepithelial Na(+) transport occurs via a posttranscriptional increase in alpha(1)-Na(+)-K(+)-ATPase expression.

Thick ascending limb: the Na(+):K (+):2Cl (-) co-transporter, NKCC2, and the calcium-sensing receptor, CaSR

The thick ascending limb of Henle's loop is a nephron segment that is vital to the formation of dilute and concentrated urine. This ability is accomplished by a consortium of functionally coupled proteins consisting of the apical Na(+):K(+):2Cl(-) co-transporter, the K(+) channel, and basolateral Cl(-) channel that mediate electroneutral salt absorption. In thick ascending limbs, salt absorption is importantly regulated by the calcium-sensing receptor. Genetic or pharmacological disruption impairing the function of any of these proteins results in Bartter syndrome. The thick ascending limb is also an important site of Ca(2+) and Mg(2+) absorption. Calcium-sensing receptor activation inhibits cellular Ca(2+) absorption induced by parathyroid hormone, as well as passive paracellular Ca(2+) transport. The present review discusses these functions and their genetic and molecular regulation.

Structural and mechanistic implications of the amino acid sequence of calcium-transporting ATPases

Work is reviewed in which the amino acid sequences of two Ca2+-transporting ATPases of sarcoplasmic reticulum (SR) from slow (or cardiac) and fast skeletal muscle were determined from the nucleotide sequences of cloned cDNAs. Analysis of hydrophobicity and secondary structure, combined with the known shape derived from electron micrographs, leads to a model of five domains with functional implications. The major globular part of the molecule is in the cytoplasm and consists of one antiparallel and two parallel beta-sheet domains. One of the latter binds ATP, which, in the presence of Ca2+, phosphorylates an aspartic acid on the other domain. It is proposed that subsequent kinase-like movements are transmitted to the SR membrane via a penta-helical, calcium-binding stalk. The Ca2+ is first trapped and then translocated via the ten helices which constitute the transmembrane (channel) region. The difference in requirements for counter ions between the Ca2+- and Na+/K+-ATPases can be explained in terms of differing charge distributions in this channel.

Comparative effects of aluminum and ouabain on synaptosomal choline uptake, acetylcholine release and (Na+/K+)ATPase

Closing the gap between adverse health effects of aluminum and its mechanisms of action still represents a huge challenge. Cholinergic dysfunction has been implicated in neuronal injury induced by aluminum. Previously reported data also indicate that in vivo and in vitro exposure to aluminum inhibits the mammalian (Na(+)/K(+))ATPase, an ubiquitous plasma membrane pump. This study was undertaken with the specific aim of determining whether in vitro exposure to AlCl(3) and ouabain, the foremost utilized selective inhibitor of (Na(+)/K(+))ATPase, induce similar functional modifications of cholinergic presynaptic nerve terminals, by comparing their effects on choline uptake, acetylcholine release and (Na(+)/K(+))ATPase activity, on subcellular fractions enriched in synaptic nerve endings isolated from rat brain, cuttlefish optic lobe and torpedo electric organ. Results obtained show that choline uptake by rat synaptosomes was inhibited by submillimolar AlCl(3), whereas the amount of choline taken up by synaptosomes isolated from cuttlefish and torpedo remained unchanged. Conversely, choline uptake was reduced by ouabain to a large extent in all synaptosomal preparations analyzed. In contrast to ouabain, which modified the K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions, AlCl(3) induced reduction of stimulated acetylcholine release was only observed when rat synaptosomes were challenged. Finally, it was observed that the aluminum effect on cuttlefish and torpedo synaptosomal (Na(+)/K(+))ATPase activity was slight when compared to its inhibitory action on mammalian (Na(+)/K(+))ATPase. In conclusion, inhibition of (Na(+)/K(+))ATPase by AlCl(3) and ouabain jeopardized the high-affinity (Na(+)-dependent, hemicholinium-3 sensitive) uptake of choline and the Ca(2+)-dependent, K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions. The effects of submillimolar AlCl(3) on choline uptake and acetylcholine release only resembled those of ouabain when rat synaptosomes were assayed. Therefore, important differences were found between the species regarding the cholinotoxic action of aluminum. The variability of (Na(+)/K(+))ATPase sensitivity to aluminum of cholinergic neurons might contribute to their differential susceptibility to this neurotoxic agent.

Expression of Na(+)/K(+)-ATPase alpha-subunit mRNA during embryonic development of the crayfish Astacus leptodactylus

Astacus leptodactylus is a decapod crustacean fully adapted to freshwater where it spends its entire life cycle after hatching under huge osmoconcentration differences between the hemolymph and surrounding freshwater. We investigated the expression of mRNA encoding one ion transport-related protein, Na(+)/K(+)-ATPase alpha-subunit, and one putative housekeeping gene, beta-actin, during crayfish ontogenesis using quantitative real-time PCR. A 216-amino acid part of the open reading frame region of the cDNA coding for the Na(+)/K(+)-ATPase alpha-subunit was sequenced from total embryo, juvenile and adult gill tissues. The predicted amino acid sequence showed a high percentage similarity to those of other invertebrates (up to 95%) and vertebrates (up to 69%). beta-actin expression exhibited modest changes through embryonic development and early post-embryonic stage. The Na(+)/K(+)-ATPase alpha-subunit gene was expressed in all studied stages from metanauplius to juvenile. Two peaks of expression were observed: one in young embryos at 25% of embryonic development (EI=100 mum), and one in embryos just before hatching (at EI=420 mum), continuing in the freshly hatched juveniles. The Na(+)/K(+)-ATPase expression profile during embryonic development is time-correlated with the occurrence of other features, including ontogenesis of excretory antennal glands and differentiation of gill ionocytes linked to hyperosmoregulation processes and therefore involved in freshwater adaptation.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Immunolocalization of Na+,K+-ATPase in the branchial cavity during the early development of the crayfish Astacus leptodactylus (Crustacea, Decapoda)

The ontogeny of osmoregulation was examined in the branchial cavity of embryonic and early post-embryonic stages of the crayfish Astacus leptodactylus maintained in freshwater, at the sub-cellular level through the detection of the sodium-potassium adenosine triphosphatase (Na(+),K(+)-ATPase). The embryonic rate of development was calculated according to the eye index (EI) which was 430-450 microm at hatching. The distribution of the enzyme was identified by immunofluorescence microscopy using a monoclonal antibody IgGalpha5 raised against the avian alpha-subunit of the Na(+),K(+)-ATPase. Immunoreactivity staining, indicating the presence of Na(+), K(+)-ATPase appeared in the gills of late embryos (EI>/=400 microm), i.e. a few days before hatching time, and steadily increased throughout the late embryonic and early post-embryonic development. The appearance of the enzyme correlates with the ability to osmoregulate which also occurs late in the embryonic development at EI 410-420 microm and with tissue differentiation within the gill filaments. These observations indicate that the physiological shift from osmoconforming embryos to hyper-regulating late embryos and post-hatching stages in freshwater must originate partly from the differentiation in the gill epithelia of ionocytes which are the site of ion pumping, as suggested by the location of Na(+),K(+)-ATPase. Only the gills were immunostained and a lack of specific staining was noted in the lamina and the branchiostegites. Therefore, osmoregulation through Na(+)active uptake is likely achieved in embryos at the gill level; all the newly formed gills in embryos function in ion regulation; other parts of the branchial chamber such as the branchiostegites and lamina do not appear to be involved in osmoregulation.

Molecular biology of major components of chloride cells

Current understanding of chloride cells (CCs) is briefly reviewed with emphasis on molecular aspects of their channels, transporters and regulators. Seawater-type and freshwater-type CCs have been identified based on their shape, location and response to different ionic conditions. Among the freshwater-type CCs, subpopulations are emerging that are implicated in the uptake of Na(+), Cl(-) and Ca(2+), respectively, and can be distinguished by their shape of apical crypt and affinity for lectins. The major function of the seawater CC is transcellular secretion of Cl(-), which is accomplished by four major channels and transporters: (1). CFTR Cl(-) channel, (2). Na(+),K(+)-ATPase, (3). Na(+)/K(+)/2Cl(-) cotransporter and (4). a K(+) channel. The first three components have been cloned and characterized, but concerning the K(+) channel that is essential for the continued generation of the driving force by Na(+),K(+)-ATPase, only one candidate is identified. Although controversial, freshwater CCs seem to perform the uptake of Na(+), Cl(-) and Ca(2+) in a manner analogous to but slightly different from that seen in the absorptive epithelia of mammalian kidney and intestine since freshwater CCs face larger concentration gradients than ordinary epithelial cells. The components involved in these processes are beginning to be cloned, but their CC localization remains to be established definitively. The most important yet controversial issue is the mechanism of Na(+) uptake. Two models have been postulated: (i). the original one involves amiloride-sensitive electroneutral Na(+)/H(+) exchanger (NHE) with the driving force generated by Na(+),K(+)-ATPase and carbonic anhydrase (CA) and (ii). the current model suggests that Na(+) uptake occurs through an amiloride-sensitive epithelial sodium channel (ENaC) electrogenically coupled to H(+)-ATPase. While fish ENaC remains to be identified by molecular cloning and database mining, fish NHE has been cloned and shown to be highly expressed on the apical membrane of CCs, reviving the original model. The CC is also involved in acid-base regulation. Analysis using Osorezan dace (Tribolodon hakonensis) living in a pH 3.5 lake demonstrated marked inductions of Na(+),K(+)-ATPase, CA-II, NHE3, Na(+)/HCO(3)(-) cotransporter-1 and aquaporin-3 in the CCs on acidification, leading to a working hypothesis for the mechanism of Na(+) retention and acid-base regulation.

Ion transport proteins and aquaporin water channels in the kidney of amphibians from different habitats

Amphibians are known to spend part of their life on land and return to water to reproduce. However, some urodeles spend their entire life in water, while others succeed in completely avoiding water even during reproduction. Osmoregulatory mechanisms must therefore be different in the diverse environmental conditions of their respective life histories. The architecture of the kidney is similar in all amphibians; as a consequence the ion-water equilibrium must be regulated in the different environmental conditions. We investigated the immunolocalisation of Na(+)/K(+)/Cl(-) cotransport proteins, sodium pump and water-channel proteins (aquaporins) in aquatic Amphiuma means means, Rana dalmatina, a species that returns to water to reproduce, and Speleomantes genei, a completely terrestrial species. The investigation was carried out with immunohistochemical methods using antibodies to Na(+)/K(+)/Cl(-) cotransport protein NKCC1 T4, Na(+)/K(+)ATPase alpha-subunit, water-channel aquaporin 3 and the inner mitochondrial membrane (AMA). Cotransport proteins and sodium pump, involved in ion reabsorption, are widely distributed in A. means and R. dalmatina and confined to the distal segment in S. genei; conversely water channels, involved in water reabsorption, are limited to the collecting duct in A. means and R. dalmatina and distributed in the proximal and collecting ducts in S. genei.

Invited review: Clearance of lung liquid during the perinatal period

At birth, the distal lung epithelium undergoes a profound phenotypic switch from secretion to absorption in the course of adaptation to air breathing. In this review, we describe the developmental regulation of key membrane transport proteins and the way in which epinephrine, oxygen, glucocorticoids, and thyroid hormones interact to bring about this crucial change in function. Evidence from molecular, transgenic, cell culture, and whole lung studies is presented, and the clinical consequences of the failure of the physiological mechanisms that underlie perinatal lung liquid absorption are discussed.

Na(+), Cl(-), Ca(2+) and Zn(2+) transport by fish gills: retrospective review and prospective synthesis

The secondary active Cl(-) secretion in seawater (SW) teleost fish gills and elasmobranch rectal gland involves basolateral Na(+),K(+)-ATPase and NKCC, apical membrane CFTR anion channels, and a paracellular Na(+)-selective conductance. In freshwater (FW) teleost gill, the mechanism of NaCl uptake is more controversial and involves apical V-type H(+)-ATPase linked to an apical Na(+) channel, apical Cl(-)-HCO-3 exchange and basolateral Na(+),K(+)-ATPase. Ca(2+) uptake (in FW and SW) is via Ca(2+) channels in the apical membrane and Ca(2+)-ATPase in the basolateral membrane. Mainly this transport occurs in mitochondria rich (MR) chloride cells, but there is a role for the pavement cells also. Future research will likely expand in two major directions, molded by methodology: first in physiological genomics of all the transporters, including their expression, trafficking, operation, and regulation at the molecular level, and second in biotelemetry to examine multivariable components in behavioral physiological ecology, thus widening the integration of physiology from the molecular to the environmental levels while deepening understanding at all levels.

Protein kinase C phosphorylation of purified Na,K-ATPase: C-terminal phosphorylation sites at the alpha- and gamma-subunits close to the inner face of the plasma membrane

The alpha-subunit of the Na,K-ATPase is phosphorylated at specific sites by protein kinases A and C. Phosphorylation by protein kinase C (PKC) is restricted to the N terminus and takes place to a low stoichiometry, except in rat. Here we show that the alpha-subunit of shark Na,K-ATPase can be phosphorylated by PKC at C-terminal sites to stoichiometric levels in the presence of detergents. Two novel phosphorylation sites are possible candidates for this PKC phosphorylation: Thr-938 in the M8/M9 loop located very close to the PKA site, and Ser-774, in the proximal part of the M5/M6 hairpin. Both sites are highly conserved in all known alpha-subunits, indicating a physiological role. A similar pattern of detergent-mediated phosphorylation by PKC was found in pig kidney Na,K-ATPase alpha-subunit. Interestingly, the kidney-specific gamma-subunit was phosphorylated by PKC in the presence of detergent. The close proximity of the novel PKC sites to the membrane suggests that targeting proteins to tether PKC into the membrane phase is important in controlling the in vivo phosphorylation of this novel class of membrane-adjacent PKC sites. It is suggested that in purified preparations where functional targeting may be impaired detergents are needed to expose the sites.

Na++K+-ATPase in gills of the blue crabCallinectes sapidus: cDNA sequencing and salinity-related expression of α-subunit mRNA and protein

Many studies have shown that hyperosmoregulation in euryhaline crabs is accompanied by enhanced Na++K+-ATPase activity in the posterior gills, but it remains unclear whether the response is due to regulation of pre-existing enzyme or to increased gene transcription and mRNA translation. To address this question, the complete open reading frame and 3′ and 5′ untranslated regions of the mRNA coding for the α-subunit of Na++K+-ATPase from the blue crab Callinectes sapidus were amplified by reverse transcriptase/polymerase chain reaction (RT-PCR) and sequenced. The resulting 3828-nucleotide cDNA encodes a putative 1039-amino-acid protein with a predicted molecular mass of 115.6 kDa. Hydrophobicity analysis of the amino acid sequence indicated eight membrane-spanning regions, in agreement with previously suggested topologies. The α-subunit amino acid sequence is highly conserved among species, with the blue crab sequence showing 81–83 % identity to those of other arthropods and 74–77 % identity to those of vertebrate species. Quantitative RT-PCR analysis showed high levels of α-subunit mRNA in posterior gills 6–8 compared with anterior gills 3–5. Western blots of gill plasma membranes revealed a single Na++K+-ATPase α-subunit protein band of the expected size. The posterior gills contained a much higher level of α-subunit protein than the anterior gills, in agreement with previous measurements of enzyme activity. Immunocytochemical analysis showed that the Na++K+-ATPase α-subunit protein detected by α5 antibody is localized to the basolateral membrane region of gill epithelial cells. Transfer of blue crabs from 35 to 5 ‰ salinity was not accompanied by notable differences in the relative proportions of α-subunit mRNA and protein in the posterior gills, suggesting that the enhanced Na++K+-ATPase enzyme activity that accompanies the hyperosmoregulatory response may result from post-translational regulatory processes.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

Mutations of either or both Cys876 and Cys888 residues of sarcoplasmic reticulum Ca2+-ATPase result in a complete loss of Ca2+ transport activity without a loss of Ca2+-dependent ATPase activity. Role of the CYS876-CYS888 disulfide bond

Disulfide-containing peptides in pepsin digest of sarcoplasmic reticulum vesicles were identified by using a fluorogenic thiol-specific reagent 4-fluoro-7-sulfamoylbenzofurazan and a reductant tributylphosphine. Sequencing of the purified peptides revealed the presence of a Cys(876)-Cys(888) disulfide bond on the luminal loop connecting the 7th and 8th transmembrane helices (loop 7-8) of the Ca(2+)-ATPase (SERCA1a). We substituted either or both of these cysteine residues with alanine and made three mutants (C876A, C888A, C876A/C888A), in which the disulfide bond is disrupted. The mutants and the wild type were expressed in COS-1 cells, and functional analysis was performed with the microsomes isolated from the cells. Electrophoresis performed under reducing and non-reducing conditions confirmed the presence of Cys(876)-Cys(888) disulfide bond in the expressed wild type. All the three mutants possessed high Ca(2+)-ATPase activity. In contrast, no Ca(2+) transport activity was detected with these mutants. These mutants formed almost the same amount of phosphoenzyme intermediate as the wild type from ATP and from P(i). Detailed kinetic analysis showed that the three mutants hydrolyze ATP in the mechanism well accepted for the Ca(2+)-ATPase; activation of the catalytic site upon high affinity Ca(2+) binding, formation of ADP-sensitive phosphoenzyme, subsequent rate-limiting transition to ADP-insensitive phosphoenzyme, and hydrolysis of the latter phosphoenzyme. It is likely that the pathway for delivery of Ca(2+) from the binding sites into the lumen of vesicles is disrupted by disruption of the Cys(876)-Cys(888) disulfide bond, and therefore that the loop 7-8 having the disulfide bond is important for formation of the proper structure of the Ca(2+) pathway.

Identification and characterization of membrane-associated polypeptides in Torpedo nicotinic acetylcholine receptor-rich membranes by hydrophobic photolabeling

To identify membrane-associated polypeptides present in Torpedo nicotinic acetylcholine receptor (AChR)-rich membranes, we used hydrophobic photolabeling with [(3)H]diazofluorene ([(3)H]DAF) and 1-azidopyrene (1-AP) to tag the membrane proteins which were then identified by amino-terminal sequence analysis of labeled fragments isolated from proteolytic digests by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by reverse-phase high-performance liquid chromatography. In addition to AChR subunits, identified polypeptides include the 95 kDa alpha-subunit of the (Na(+)+K(+))-ATPase, the 89 kDa voltage-gated chloride channel (CLC-0), the 105 kDa SITS-binding protein, and 32 and 34 kDa polypeptides identified as Torpedo homologues of the mitochondrial membrane ATP/ADP carrier protein and the voltage-dependent anion channel (VDAC), respectively. Further, individual amino acids that reacted with [(3)H]DAF and therefore likely to be in contact with lipid were identified in the transmembrane segment M3 of the alpha-subunit of the (Na(+)+K(+))-ATPase and in a putative transmembrane beta-strand in VDAC. Collectively these results demonstrate that [(3)H]DAF/1-AP photolabeling provides an effective method for tagging the membrane-associated segments of polypeptides in a way that makes it easy to isolate the labeled polypeptide or polypeptide fragments by fluorescence and then to identify amino acids at the lipid-protein interface by (3)H release.

The functional unit of Na,K-ATPase is a monomeric alphabeta protomer

The ouabain-resistant and ouabain-sensitive alpha-subunit cRNAs in various molar ratios were injected into Xenopus oocytes together with cRNA for the beta-subunit. The ouabain-resistant ATPase activity, as well as ouabain-resistant Rb+ uptake, of the injected oocytes increased linearly with increasing the amount of cRNA for the ouabain-resistant alpha-subunit. When a functionless mutant was used instead of the ouabain-sensitive alpha-subunit, similar results were obtained in ATPase activity and Rb+ uptake. These results indicate that a monomeric alphabeta protomer is a functional unit of membrane-bound Na,K-ATPase, even if the enzyme exists structurally as a diprotomer or higher oligomers in membranes.

Deletions or specific substitutions of a few residues in the NH(2)-terminal region (Ala(3) to Thr(9)) of sarcoplasmic reticulum Ca(2+)-ATPase cause inactivation and rapid degradation of the enzyme expressed in COS-1 cells

Amino acid residues in the NH(2)-terminal region (Glu(2) - Ala(14)) of adult fast twitch skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) were deleted or substituted, and the mutants were expressed in COS-1 cells. Deletion of any single residue in the Ala(3)-Ser(6) region or deletion of two or more consecutive residues in the Ala(3)-Thr(9) region caused strongly reduced expression. Substitution mutants A4K, A4D, and H5K also showed very low expression levels. Deletion of any single residue in the Ala(3)-Ser(6) region caused only a small decrease in the specific Ca(2+) transport rate/mg of SERCA1a protein. In contrast, other mutants showing low expression levels had greatly reduced specific Ca(2+) transport rates. In vitro expression experiments indicated that translation, transcription, and integration into the microsomal membranes were not impaired in the mutants that showed very low expression levels in COS-1 cells. Pulse-chase experiments using [(35)S]methionine/cysteine labeling of transfected COS-1 cells demonstrated that degradation of the mutants showing low expression levels was substantially faster than that of the wild type. Lactacystin, a specific inhibitor of proteasome, inhibited the degradation accelerated by single-residue deletion of Ala(3). These results suggest that the NH(2)-terminal region (Ala(3) -Thr(9)) of SERCA1a is sensitive to the endoplasmic reticulum-mediated quality control and is thus critical for either correct folding of the SERCA1a protein or stabilization of the correctly folded SERCA1a protein or both.

Hormonal modulation of branchial Na+-K+-ATPase subunit mRNA in a marine teleost Sparus sarba

The effect of hormone treatment on the abundance of Na+-K+-ATPase alpha- and beta-subunit mRNA in Sparus sarba branchial tissue was investigated. Groups of seawater (33/1000) and hypo-osmotic (6/1000) acclimated fish were injected daily, with either saline, cortisol, recombinant bream growth hormone (rbGH) or ovine prolactin (oPRL). Total RNA from branchial tissue was analyzed by Northern blotting using PCR amplified Na+-K+-ATPase alpha- and beta-subunit cDNA clones. Na+-K+-ATPase alpha- and beta- subunit transcripts of 3.3kb and 2.4kb respectively, were detected and their abundance, after hormone treatment was assessed using RNA dot blots. The abundance of subunit mRNAs increased 1.4-1.9 fold, relative to controls, after cortisol treatment. The alpha:beta mRNA ratio also increased in cortisol treated seawater acclimated fish. Growth hormone treatment did not cause any significant changes in Na+-K+-ATPase subunit mRNA, whereas prolactin significantly reduced alpha-subunit mRNA levels by approximately 0.5 fold in both seawater and hypo-osmotic conditions. The data from this study add further support to the generally accepted roles that cortisol and prolactin have in the modulation of Na+-K+-ATPase activity. It can be concluded from this study that S. sarba branchial Na+-K+-ATPase subunit expression is multihormonally regulated.

Mutations of Arg198 in sarcoplasmic reticulum Ca2+-ATPase cause inhibition of hydrolysis of the phosphoenzyme intermediate formed from inorganic phosphate

Arg198 of sarcoplasmic reticulum Ca2+-ATPase was substituted with lysine, glutamine, glutamic acid, alanine, and isoleucine by site-directed mutagenesis. Kinetic analysis was performed with microsomal membranes isolated from COS-1 cells which were transfected with the mutated cDNAs. The rate of dephosphorylation of the ADP-insensitive phosphoenzyme was determined by first phosphorylating the Ca2+-ATPase with 32Pi and then diluting the sample with non-radioactive Pi. This rate was reduced substantially in the mutant R198Q, more strongly in the mutants R198A and R1981, and most strongly in the mutant R198E, but to a much lesser extent in R198K. The reduction in the rate of dephosphorylation was consistent with the observed decrease in the turnover rate of the Ca2+-ATPase accompanied by the steady-state accumulation of the ADP-insensitive phosphoenzyme formed from ATP. These results indicate that the positive charge and high hydrophilicity of Arg198 are critical for rapid hydrolysis of the ADP-insensitive phosphoenzyme.

Affinity labeling of two nucleotide sites on Na,K-ATPase using 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-[alpha-32P]diphosphate (TNP-8N3-[alpha-32P]ADP) as a photoactivatable probe. Label incorporation before and after blocking the high affinity ATP site with fluorescein isothiocyanate

ATP and its analogues act on the minimal functional unit of Na, K-ATPase, the alpha beta protomer, with high and low affinity effects. Fluorescein isothiocyanate (FITC) irreversibly blocks the high affinity, or catalytic, ATP site, and yet the surviving K+-phosphatase activity of soluble FITC-modified alphabeta protomers can be photoinactivated by 2'(3')-O-trinitrophenyl (TNP)-8N3-ADP (Ward, D. G., and Cavieres, J. D. (1998) J. Biol. Chem. 273, 14277-14284). We have now used TNP-8N3-[alpha-32P]ADP as a photoaffinity label for Na,K-ATPase. The native enzyme can be photolabeled at 5 microM TNP-8N3-[alpha-32P]ADP, and ATP or FITC treatment prevents labeling of the alpha chain. At 25 microM, however, TNP-8N3-[alpha-32P]ADP can be incorporated in the FITC-modified alpha chain, concurrently with the inactivation of the K+-phosphatase activity, to an extrapolated level of 0.5-1.2 mol of 32P-probe per mol of alpha chain. Photoinactivation and labeling are prevented by TNP-ADP, vanadate, or strophanthidin and are promoted by Na+ or Mg2+, but not K+. The cation effects suggest that the fluorescein-modified enzyme incorporates the TNP-8N3-[alpha-32P]ADP. Mg complex preferentially, and the free probe when in the E1 enzyme form and after occupation of a low-affinity Na+ site. Partial trypsinolysis reveals that the point of TNP-8N3-[alpha-32P]ADP attachment is on the C-terminal 58-kDa fragment of the FITC-modified alpha chain. The affinity labeling of the fluorescein enzyme by TNP-8N3-[alpha-32P]ADP endorses the view that two nucleotide sites can be occupied simultaneously in each alpha subunit of Na,K-ATPase.

Photoinactivation of fluorescein isothiocyanate-modified Na,K-ATPase by 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-diphosphate. Abolition of E1 and E2 partial reactions by sequential block of high and low affinity nucleotide sites

The Na,K-ATPase activity of the sodium pump exhibits apparent multisite kinetics toward ATP, a feature that is inherent to the minimal enzyme unit, the alpha beta protomer. We have argued that this should arise from separate catalytic and noncatalytic sites on the alpha beta protomer as fluorescein isothiocyanate (FITC) blocks a high affinity ATP site on all alpha subunits and yet the modified Na, K-ATPase retains a low affinity response to nucleotides (Ward, D. G., and Cavieres, J. D. (1996) J. Biol. Chem. 271, 12317-12321). We now find that 2'(3')-O-(2,4,6-trinitrophenyl)8-azido-adenosine 5'-diphosphate (TNP-8N3-ADP), a high affinity photoactivatable analogue of ATP, can inhibit the K+-phosphatase activity of the FITC-modified enzyme during assays in dimmed light. The inhibition occurs with a Ki of 140 microM at 20 mM K+; it requires the adenine ring as 2'(3')-O-(2,4 6-trinitrophenyl) (TNP)-UDP or TNP-uridine are less potent and 2,4,6-trinitrobenzene-sulfonate is ineffective. Under irradiation with UV light, TNP-8N3-ADP inactivates the K+-phosphatase activity of the fluorescein-enzyme and also its phosphorylation by [32P]Pi. The photoinactivation process is stimulated by Na+ or Mg2+, and is inhibited by K+ or excess TNP-ADP. In the presence of 50 mM Na+ and 1 mM Mg2+, TNP-8N3-ADP photoinactivates with a K0.5 of 15 microM. Furthermore, TNP-8N3-ADP photoinactivates the FITC-modified, solubilized alpha beta protomers, even more effectively than the membrane-bound fluorescein-enzyme. These results strongly suggest that catalytic and allosteric ATP sites coexist on the alpha beta protomer of Na,K-ATPase.

A P-type ATPase from the aquatic fungus Blastocladiella emersonii similar to animal Na,K-ATPases

We have cloned a P-type ATPase gene from the aquatic fungus Blastocladiella emersonii (BePAT1) using a probe obtained with degenerate oligonucleotides, corresponding to two amino acid sequences highly conserved among all P-type ATPase isoforms, and the polymerase chain reaction technique. Nucleotide sequence analysis revealed a 3.4 kb open reading frame encoding a putative peptide of 1080 amino acid residues with a calculated molecular mass of 119 kDa, which presents all diagnostic features of P-type transporting ATPases. Comparison to other members of the family and phylogenetic analyses have shown that the BePAT1 protein belongs to the subfamily of Na,K- and H,K-ATPases, indicating that the divergence between the alpha-subunit of the Na,K-ATPase and other members of the P-type ATPase family has occurred before the divergence between the animal and fungal lineages in evolution.

Insect Na(+)/K(+)-ATPase

Na(+)/K(+)-ATPase (sodium/potassium pump) is a P-type ion-motive ATPase found in the plasma membranes of animal cels. In vertebrates, the functions of this enzyme in nerves, heart and kidney are well characterized and characteristics a defined by different isoforms. In contrast, despite different tissue distributions, insects possess a single isoform of the alpha-subunit. A comparison of insect and vertebrate Na(+)/K(+)-ATPases reveals that although the mode of action and structure are very highly conserved, the specific roles of the enzyme in most tissues varies. However, the enzyme is essential for the function of nerve cells, and in this respect Na(+)/K(+)-ATPase appears to be fundamental in metazoan evolution.

Probing of the membrane topology of sarcoplasmic reticulum Ca2+-ATPase with sequence-specific antibodies. Evidence for plasticity of the c-terminal domain

The topology of Ca2+-ATPase in sarcoplasmic reticulum (SR) vesicles was investigated with the aid of sequence-specific antibodies, produced against oligopeptides corresponding to sequences close to the membranous portions of the protein. The antisera in competitive enzyme-linked immunosorbent assays only reacted with intact SR vesicles to a limited extent, but most epitopic regions were exposed by low concentrations of nondenaturing detergent, octaethylene glycol dodecyl ether (C12E8) or after removal of cytosolic regions by proteinase K. In particular, these treatments exposed the loop regions in the C-terminal domain, including L7-8, the loop region located between transmembrane segments M7 and M8, with a putative intravesicular position, which had immunochemical properties very similar to those of the C terminus with a documented cytosolic exposure. In contrast to this, the reactivity of the N-terminal intravesicular loop regions L1-2 and L3-4 was only increased by C12E8 treatment but not by proteinase K proteolysis. Complexation of Ca2+-ATPase with beta,gamma-CrATP stabilized the C-terminal domain of Ca2+-ATPase against proteinase K proteolysis and reaction with most of the antisera, but immunoreactivity was maintained by the L6-7 and L7-8 loops. Immunoelectron microscopic analyses of vesicles following negative staining, thin sectioning, and the SDS-digested freeze-fracture labeling method suggested that the L7-8 epitope, in contrast to L6-7 and the C terminus, can be exposed on either the intravesicular or cytosolic side of the membrane. A preponderant intravesicular location of L7-8 in intact vesicles is suggested by the susceptibility of this region to proteolytic cleavage after disruption of the vesicular barrier with C12E8 and in symmetrically reconstituted Ca2+-ATPase proteoliposomes. In conclusion, our data suggest an adaptable membrane insertion of the C-terminal Ca2+-ATPase domain, which under some conditions permits sliding of M8 through the membrane with cytosolic exposure of L7-8, of possible functional significance in connection with Ca2+ translocation. On the technical side, our data emphasize that extreme caution is needed when using nondenaturing detergents or other treatments like EGTA at alkaline pH to open up vesicles for probing of intravesicular location with antibodies.

Characterization of the secondary structure and assembly of the transmembrane domains of trypsinized Na,K-ATPase by Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy has been used to compare native Na,K-ATPase-containing membranes with those trypsinized in the presence of either Rb+ or Na+ ions to remove the extramembranous parts of the protein. The protein secondary structure content deduced from the amide I band is approximately 30-35% alpha-helix, 37-40% beta-structure, and 13-15% random coil for native membranes from shark rectal gland and from pig kidney, in both the Na- and K-forms. Trypsinization in either Rb+ (a K+ congener) or Na+ removes approximately 35% of the amide I band intensity of native membranes from shark rectal gland. The protein secondary structural content of the trypsinized membranes lies in the range of approximately 23-32% alpha-helix, 37-46% beta-structure, and 12-18% random coil for the shark and kidney enzymes. The distribution of intensity between the bands corresponding to protonated and deuterium-exchanged alpha-helices, and between the component bands attributed to beta-structure, changes considerably on trypsinization, in the direction of a greater proportion of protonated alpha-helix and a broader range of frequencies for beta-structure. The kinetics of deuteration of the slowly exchanging population of protein amide groups is also changed on trypsinization. The mean rate constant for deuteration of trypsinized membranes is approximately half that for native membranes, whereas the proportion of amides contributing to this population increases on trypsinization. The temperature dependence of the amide I band in the Fourier transform infrared spectra indicates that the onset of thermal denaturation occurs at 58 degrees C for native membranes (in either Na+ or K+) and for membranes trypsinized in Rb+, but the major denaturation event for membranes trypsinized in Na+ occurs at approximately 84 degrees C. These results correlate with the functional properties of the intramembranous section of the enzyme.

Modification of arginine-198 in sarcoplasmic reticulum Ca2+-ATPase by 1,2-cyclohexanedione causes inhibition of formation of the phosphoenzyme intermediate from inorganic phosphate

Sarcoplasmic reticulum vesicles were modified with 1,2-cyclohexanedione (CHD), a specific arginine-modifying reagent, in sodium borate (pH 8.0 or 8.8). Phosphoenzyme formation from Pi in the Ca2+-ATPase (reversal of hydrolysis of the phosphoenzyme intermediate) was almost completely inhibited by the modification with CHD. Tight binding of F- and Mg2+ and high affinity binding of vanadate in the presence of Mg2+, either of which produces a transition state analog for phosphoenzyme formation from the magnesium-enzyme-phosphate complex, were also markedly inhibited. In contrast, phosphoenzyme formation from acetyl phosphate in the forward reaction was unaffected. The enzyme was appreciably protected by tight binding of F- and Mg2+ or by high affinity binding of vanadate in the presence of Mg2+, but not by the presence of 20 mM MgCl2 alone or 150 mM Pi alone, against the CHD-induced inhibition of phosphoenzyme formation from Pi. Peptide mapping of the tryptic digests, detection of peptides containing CHD-modified arginyl residues with Girard's reagent T, sequencing, and mass spectrometry showed that Arg-198 was a single major residue protected by tight binding of F- and Mg2+ against the modification with CHD. These results indicate that modification of Arg-198 with CHD is responsible for at least a part (the portion reduced by the transition state analogs) of the CHD-induced inhibition of phosphoenzyme formation from Pi and suggest that Arg-198 is located in or close to the catalytic site in the transition state for phosphoenzyme formation from the magnesium-enzyme-phosphate complex.

cDNA cloning of Na+, K(+)-ATPase alpha-subunit from embryos of the sea urchin, Hemicentrotus pulcherrimus

Na+, K(+)-ATPase alpha-subunit cDNA of the sea urchin, Hemicentrotus pulcherrimus, was obtained by twice screening prism and gastrula lambda gt10 cDNA libraries using an oligonucleotide probe derived from a mostly conserved region, FSBA (5'-p-(fluorosulfonyl)-benzoyladenosine) binding site of cation transport ATPases. The 5'-end of the non-coding region was determined by primer extension and the region was amplified by 5'-RACE method. The sea urchin alpha-subunit cDNA consists of 4401 nucleotides and encodes 1038 amino acid residues (MW, 114 kDa). The predicted primary structure, except N-terminal region, has similar degree of high homology to various metazoan Na+, K(+)-ATPase alpha-subunits. Alignment of amino acid sequence and a hydropathy profile also predicts eight putative transmembrane segments at least. The phylogenetic tree suspected from alignment of amino acid sequences of 21 species suggests that sea urchin and vertebrate Na+, K(+)-ATPase alpha-subunits seem to have evolved from a common origin, before vertebrate alpha-subunit divided into three isoforms.

Identification of arginyl residues located at the ATP binding site of sarcoplasmic reticulum Ca2+-ATPase. Modification with 1,2-cyclohexanedione

Sarcoplasmic reticulum vesicles were treated with 1, 2-cyclohexanedione (CHD) in sodium borate (pH 8.0). The Ca2+-ATPase activity was completely inhibited. Inhibition of Mg.ATP and Mg.ADP binding to the high affinity ATP binding site as well as inhibition of phosphorylation with ATP occurred simultaneously with the inhibition of the Ca2+-ATPase activity. Phosphorylation with acetyl phosphate was not inhibited. The Ca2+-ATPase was strongly protected by Mg.ATP, Mg.ADP, and Mg.AMP against this inhibition. Binding of acetyl phosphate or Pi to the enzyme gave no protection. Phosphorylation with acetyl phosphate also had no protective effect. Peptide mapping of the tryptic digests, detection of peptides containing CHD-modified arginyl residues with Girard's reagent T, and sequencing revealed that Arg-489, Arg-505, and Arg-678 were modified with CHD. Arg-489 and Arg-678 were almost completely protected by Mg.ATP against this modification, but partially protected by prelabeling with fluorescein 5-isothiocyanate, which occupies the adenosine binding region in the ATP binding site. In contrast, Arg-505 was slightly protected by Mg.ATP and almost completely protected by prelabeling with fluorescein 5-isothiocyanate. Taken together, these findings suggest that Arg-489 and Arg-678 are located in or near the region occupied by the triphosphate moiety of ATP, either or both of these residues being in or close to the region occupied by the alpha-phosphoryl group in the high affinity ATP binding site and involved in the CHD-induced inhibition of this enzyme and that Arg-505 is very close to (but slightly out of) the adenosine binding region in the ATP binding site. The acetyl phosphatase activity and phosphorylation with Pi were also inhibited by the CHD treatment, but the inhibitions were considerably slower than those described above. This suggests that the arginyl residues involved in these inhibitions are distinct from that involved in the inhibition of the Ca2+-ATPase activity.

Amino-terminal processing of the catalytic subunit from Na(+)-K(+)-ATPase

The first five amino acids of the catalytic alpha 1-subunit predicted from its cDNA are not found in purified mammalian Na(+)-K(+)-ATPase, suggesting co- or posttranslational cleavage. To facilitate evaluation of amino-terminal structure and the cleavage process, we developed a site-directed antibody (anti-VGR) specific for the first nine residues of nascent alpha 1 from rat. In immunoblots of polypeptides generated by in vitro translation, anti-VGR detected a prominent band with a mobility appropriate for the alpha 1-subunit (100 kDa). Immunoblots of total protein from various rat organs, however, revealed no significant binding, implying that virtually all the alpha 1-subunit expressed in vivo was modified. We also assessed amino-terminal structure in various heterologous expression systems. Binding of anti-VGR was observed in Escherichia coli transformed with a vector containing an alpha 1/troponin fusion protein and in insect cells infected with baculovirus containing full-length alpha 1 or alpha 1T. This suggests that modification of the introduced alpha 1 in these expression systems was absent or different from that in mammals. In contrast, green monkey kidney cells (COS-1) transfected with alpha 1 did not reveal significant binding of the antibody, indicating that the introduced isoform was processed appropriately. These results demonstrate that the structure of the alpha 1-subunit's amino terminus differs among various expression systems. The results further imply that efficient co- or posttranslational processing of nascent alpha 1 is conserved among various organs within the rat, yet the required modification enzymes are not present in distant phyla.

Photolabile amiloride derivatives as cation site probes of the Na,K-ATPase

Treatment of purified canine renal Na,K-ATPase with a range of photoactivatable amiloride derivatives results in inhibition of ATPase activity prior to illumination. Inhibition by amiloride derivatives substituted on a guanidium N could not be prevented by the presence of either K or Na; however, these cations could protect the enzyme against inhibition by derivatives substituted on the 5-position of the pyrazine ring. In the case of 5-(N-ethyl-[2'-methoxy-4'-nitrobenzyl])amiloride (NENMBA), the presence of monovalent cations (Na, K, and Rb) protected the enzyme effectively against inhibition, with concentrations in the millimolar range. ATP did not prevent inhibition; furthermore, native and NENMBA-treated enzyme exhibited normal levels of high affinity [3H]ADP (and hence ATP) binding. The rate of inhibition increased with increasing concentrations of NENMBA. Extensive washing of NENMBA-inhibited enzyme did not restore ATPase activity, showing that NENMBA has an extremely slow off-rate for dissociation from its inhibitory site. Partially inhibited enzyme could be rapidly pelleted and resuspended in NENMBA-free buffer and inhibition was observed to continue, albeit at a somewhat diminished rate, suggesting that NENMBA gains access to its inhibitory site after partitioning into the lipid phase rather than directly from the aqueous solution. Photolysis of NENMBA-inhibited enzyme resulted in covalent incorporation of the reagent into the alpha-subunit of the Na,K-ATPase, as observed by separation of labeled protein on a Laemmli gel and Western analysis using a polyclonal amiloride antibody. Almost all of the covalent labeling could be prevented by the presence of Rb in the incubation and labeling medium. These results suggest that NENMBA inhibits the Na, K-ATPase by disruption of the cation transport domain rather than the catalytic domain of the enzyme and that it promises to be a useful tool for cation site localization.

Localization of Na+, K(+)-ATPase in tissues of rabbit and teleosts using an antiserum directed against a partial sequence of the alpha-subunit

A specific polyclonal antibody against Na+, K(+)-ATPase alpha-subunit was developed using a synthetic oligopeptide as antigen. By Western blot analysis under non-reducing conditions, this antibody recognized a protein band of approximately 150 kDa corresponding to the intact form (alpha beta-complex) of Na+, K(+)-ATPase in rabbit kidney. Furthermore, this antibody recognized a 150 kDa protein band corresponding to the intact form of Na+, K(+)-ATPase and some bands of about 60-65 kDa corresponding to fragments of the alpha-subunit in gill and kidney of masu salmon. This antibody did not recognize the alpha-subunit under reducing conditions. By immunohistochemical analysis, cells immunoreactive with this antibody were observed in renal tubular epithelial cells in kidney sections of rabbit, masu salmon, eel and rockfish. In addition, large spherical eosinophilic cells in gills of masu salmon, eel and rockfish were immunoreactive with the antibody. It is likely that these immunoreactive cells correspond to gill chloride cells. These data indicate that this antibody is a useful tool for studying changes in and the function of Na+, K(+)-ATPase during osmoregulatory adaptation in a variety of fish species.

Functional expression of gastric H+,K(+)-ATPase and site-directed mutagenesis of the putative cation binding site and catalytic center

Gastric H+,K(+)-ATPase was functionally expressed in the human kidney HEK293 cell line. The expressed enzyme catalyzed ouabain-resistant K(+)-dependent ATP hydrolysis. The K(+)-ATPase activity was inhibited by SCH 28090, a specific inhibitor of gastric proton pump, in a dose-dependent manner. By using this functional expression system in combination with site-directed mutagenesis, we investigated effects of mutations in the putative cation binding site and the catalytic center of the gastric H+,K(+)-ATPase. In Na+,K(+)-ATPase, the glutamic acid residue in the 4th transmembrane segment is regarded as one of the residues responsible for the K(+)-induced conformational change (Kuntzweiler, T. A., Wallick, E. T., Johnson, C. L., and Lingrel, J. B. (1995) J. Biol. Chem. 270, 2993-3000). When the corresponding glutamic acid (Glu-345) of H+,K(+)-ATPase was mutated to aspartic acid, lysine, or valine, the SCH 28080-sensitive K(+)-ATPase activity was abolished. However, when this residue was replaced by glutamine, about 50% of the activity was retained. This mutant showed a 10-fold lower affinity for K+ (Km = 2.6 mM) compared with the wild-type enzyme (Km = 0.24 mm). Thus, Glu-345 is important in determining the K+ affinity of H+,K(+)-ATPase. When the aspartic acid residue in the phosphorylation site was mutated to glutamic acid, this mutant showed no SCH 28080-sensitive K(+)-ATPase activity. Thus, amino acid replacement of the phosphorylation site is not tolerated and a stringent structure appears to be required for enzyme activity. When the lysine residue in the fluorescein isothiocyanate binding site (part of ATP binding site) was mutated to arginine, asparagine, or glutamic acid, the SCH 28080-sensitive K(+)-ATPase activity was eliminated. However, the mutant in which this residue was changed to glutamine had about 30% of the activity, suggesting that amino acid replacement of this site is tolerated to a certain extent.

4-hydroxynonenal inhibits Na(+)-K(+)-ATPase

4-Hydroxynonenal binds rapidly to Na(+)-K(+)-ATPase, and this was accompanied by a decrease in measurable sulfhydryl groups and a loss of enzyme activity. The I50 value for Na(+)-K(+)-ATPase inhibition by 4-hydroxynonenal was found to be 120 microM. Although the sulfhydryl groups could be completely restored with beta-mercaptoethanol during the reaction of the Na(+)-K(+)-ATPase-HNE-adduct, the Na(+)-K(+)-ATPase activity was only partially restored by this reducing agent. A combination of hydroxylamine and beta-mercaptoethanol yielded the greatest recovery of enzyme activity, 85% of original. Thus, 4-hydroxynonenal binding to Na(+)-K(+)-ATPase led to an irreversible decrease of enzyme activity under the conditions employed. It is hypothesized that 4-hydroxynonenal reacts with sulfhydryls at sites on the enzyme that are inaccessible by beta-mercaptoethanol. Furthermore, evidence was obtained that 4-hydroxynonenal reacts with other amino acids such as lysine to form adducts that also interfere with protein function.