News Physiol Sci 18: 119-124, 2003;
doi:10.1152/nips.01434.2003
1548-9213/03 $5.00
News in Physiological Sciences, Vol. 18, No. 3, 119-124,
June 2003
© 2003 Int. Union Physiol. Sci./Am. Physiol. Soc.
Functional Modulation of the Sodium Pump: The Regulatory Proteins "Fixit"
Flemming Cornelius and
Yasser A. Mahmmoud
Department of Biophysics, University of Aarhus, DK-8000 Aarhus, Denmark
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Abstract
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Proteins of the FXYD family act as tissue-specific regulators of the Na-K-ATPase. They are small hydrophobic type I proteins with a single-transmembrane span containing an extracellular invariant FXYD sequence. FXYD proteins are not an integral part of the Na-K-ATPase but function to modulate its catalytic properties by molecular interactions with specific Na-K-ATPase domains.
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Introduction
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The Na-K-ATPase is the integral membrane enzyme responsible for the active coupled transport of Na+ and K+ across the plasma membranes of animal cells. The enzyme is activated by the simultaneous presence of Na+ and K+ and is specifically inhibited by the plant glycoside ouabain. The generation and maintenance of the electrochemical gradients for Na+ and K+ requires energy that is derived from the hydrolysis of ATP, which in humans can amount to 25% of the basal metabolic rate. These interconnected functions are the basis for its equivalent designation as the Na/K pump or Na-K-ATPase.
The Na-K-ATPase is a heterodimer composed of a catalytic 
-subunit, which undergoes conformational changes that couple ATP hydrolysis to ion transport, and the ß-subunit, which is responsible for maturation, assembly, and membrane targeting of the enzyme. The Na-K-ATPase is a member of P-type ATPases in which a transfer of the 
-phosphate from ATP to an active site in the enzyme is taking place. Other members of this family include the sarco(endo)plasmic reticulum Ca-ATPase (SERCA) and the gastric H-K-ATPase.
The ion gradients maintained by the Na-K-ATPase, together with the different permeability of the cell membrane to Na+ and K+, form the basis for the resting membrane potential, counteract the Donnan effect (which would otherwise lead to cell lysis) and provide the energy for many co- and countertransport systems. In excitable tissue of nerves and muscles, it reestablishes the ion gradients that are dissipated after depolarization. In transport epithelia of kidney, intestine, and secretory glands, its polarized localization to basolateral membrane compartments provides the driving force for water and salt transport. Thus the Na-K-ATPase is of paramount importance for cell homeostasis and is as such under strict hormonal control, as described in the excellent review by Therien and Blostein (15), where references up to the year 2000 can be found.
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Acute regulation of Na-K-ATPase
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Two levels of hormonal actions have been defined, namely rapid (short-term) and sustained (long-term) regulation. Hormones that exert rapid regulation respond to, e.g., variable salt intake or heavy exercise. Those hormones affect the activity of Na-K-ATPase already located in the plasma membrane or regulate the location of Na-K-ATPase by membrane trafficking. On the other hand, hormones that exert sustained effects are those controlling gene expression levels.
The regulation of Na-K-ATPase is a very complex process taking place at many different levels, and the sequence of events occurring from hormone binding at the extracellular side of the cell to the direct effects on Na-K-ATPase activity is largely unknown. Many signaling pathways initiated by the action of such hormones terminate by targeting protein kinases (serine/threonine kinases like PKA, PKC, and PKG as well as tyrosine kinases) or phosphatases to the enzyme controlling its state of phosphorylation.
Recently, considerable interest has been directed at elucidating the role of protein-protein interactions in regulation of Na-K-ATPase activity. Indeed, certain proteins apart from protein kinases and protein phosphatases bind to the Na-K-ATPase, including structural proteins like ankyrin, actin, and adducin. Besides, a small hydrophobic protein called the -subunit was originally shown to associate with the kidney Na-K-ATPase and to modulate its activity. The -subunit is a member of a family collectively known as the FXYD (pronounced "fixit") protein family (14) because of the presence of conserved amino acids in its signature motif (see Fig. 1
). The family includes phospholemman (PLM or FXYD1), the -subunit (FXYD2), mammary tumor protein (MAT-8 or FXYD3), channel-inducing factor (CHIF or FXYD4), related ion channel protein (RIC or FYXD5), as well as FXYD6 and FXYD7, which have not yet been characterized at the protein level. Recently, a PLM-like protein from shark (PLMS) was shown to associate with the Na-K-ATPase (9). This has now led to the suggestion that members of this protein family are tissue-specific regulators of the Na-K-ATPase.
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FIGURE 1. Alignment of sequences of the currently known 7 mammalian FXYD proteins. Conserved residues are in red. All except FXYD2 and FXYD7 contain a candidate cleavable NH2-terminal signal (in blue). The PKA/PKC multisite phosphorylation domain of phospholemman (PLM) is framed. All sequences are from human.
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The structure of FXYD proteins
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The FXYD family consists of small hydrophobic proteins, except for FXYD5, which contains an NH2-terminal extension. In Fig. 1
, the sequences of the seven known mammalian FXYD proteins are aligned. They are all type I membrane proteins with a single transmembrane domain, an extracellular NH2 terminus containing the family signature FXYD, and a cytoplasmic COOH terminus. Except for FXYD2 and FXYD7, all are predicted to contain a cleavable NH2-terminal signal peptide.
PLM and PLMS are the only members known to contain a COOH-terminal domain that is phosphorylated by PKA and PKC. However, as seen from Fig. 1, all known mammalian FXYDs contain close to the membrane face a cytoplasmic serine residue, which lies within a conventional PKC phosphorylation motif. In -subunit from pig kidney, this site can be phosphorylated by PKC in vitro (9), but the physiological significance is unknown. FXYD7 contains three putative PKC sites at the cytoplasmic domain, but it is unknown whether they are physiological substrates for phosphorylation.
Several of the FXYD proteins (FXYD1, 2, 4, and 7) are resolved on SDS-PAGE as multiple or broad diffuse bands, and co- or posttranslational processing seems to be a common feature. The genomic organization of the large FXYD1 and FXYD2 genes demonstrate the existence of multiple exons, and to date two splice variants of -subunit are known in humans, a and b, with small different NH2 termini (1, 8). Both splice variants can be further modified by posttranslational processing (1).
PLMS and -subunit have both been shown to form oligomers in native membranes (9), which may be stabilized by transmembrane leucine-isoleucine zippers (lz). Indeed, this tendency for formation of oligomeric structures may explain why several FXYD proteins (FXYD1, 2, 3, and 4 but not 7) induce an increase in ion conductance after incorporation or expression in cell membranes, but it is still controversial whether this tendency for self-association has any physiological significance.
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The functional interaction of FXYD proteins with Na-K-ATPase
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The main function of the Na-K-ATPase is to ensure, within some dynamic limits, a low intracellular Na+ concentration. One way to ensure this is by substrate regulation of the enzyme activity by cytoplasmic Na+ along a sigmoid activation curve with half-maximal activation in the range of the normal intracellular Na+ concentration of 2–10 mM. Another way to adapt to special tissue requirements is by specific expression of various isoforms of the Na-K-ATPase (of which 4 are currently known) with altered properties. However, direct tissue-specific modulation of the enzyme may be needed in some cases in which dynamic changes are encountered. One way to affect enzyme activity might be to achieve regulatory control of one or more rate-determining steps of the reaction mechanism. For Na-K-ATPase, the main rate-limiting step is the deocclusion of K+ to the cytoplasmic face induced by low-affinity ATP and Na+ binding (step 1 in Fig. 2). However, the subsequent phosphorylation reactions are also considered partially rate determining.
The FXYD proteins are not an integral part of the Na-K-ATPase ß-complex, which alone exhibits both catalytic activity and ion-transport capacity. Rather, FXYD modulation of enzyme activity is achieved by intermolecular interactions, which must be envisaged to change some molecular rearrangements within the ß-complex accompanying the enzyme reaction. Results are only just beginning to arrive to show how the interaction between FXYD and the Na-K-ATPase actually takes place at the molecular level. Since the first molecular model from X-ray crystallography of the P-type SERCA arrived, it has become clear that the catalytic -subunit is segregated into three major cytoplasmic domains: a nucleotide-binding domain (N domain), a phosphorylation domain (P domain), and a domain that coordinates the catalytic reaction with ion transport (A domain). Furthermore, ion binding to a transmembrane domain (TM domain) is taking place. An essential feature of the model is that rather large domain movements take place during enzyme turnover (Fig. 2). By analogy to the recent model for SERCA by Toyoshima et al. (16), the binding of Na+ to the enzyme in the E1 conformation with bound ATP (E1ATPNa3) is relayed by TM4 and TM5 to a P-domain rotation, which activates the aspartate residue in the active site in the P domain. The flexible N domain, which is connected to the P domain via a hinge region, donates the -phosphate of ATP to the activated aspartate residue, which is phosphorylated, forming E1~PNa3. Following this the A domain, which is connected to transmembrane helices TM1, 2, and 3, rotates ~90° to allow the conserved TGES sequence to approach the aspartyl phosphate. The rotated A domain causes a rearrangement of TM1–3 and rotates the P domain via the connected TM4 and TM5. This changes the enzyme conformation to E2-P and exposes the cation sites to the extracellular side, where the three Na+ are exchanged for two K+. The E2-P bond then hydrolyzes, and the two K+ become occluded in the E2 form of the enzyme. Low-affinity binding of MgATP again changes the enzyme conformation, releases the two K+ to the cytoplasmic side, and rotates the A and P domains. The enzyme is now ready to receive another three Na+.
Table 1 summarizes some functional effects of FXYD regulation of Na-K-ATPase activity. As seen, effects on the maximum catalytic activity, on apparent ion affinities, and on apparent ATP affinity are encountered. The main effects of -subunit, CHIF, and PLM on Na-K-ATPase deduced from coexpression experiments are on the apparent cytoplasmic Na+ affinity and with variable intensity on the extracellular K+ affinity, whereas FXYD7 only affects the extracellular K+ affinity. The ATP affinity is increased by -subunit, resulting from stabilization of the E1 conformation, whereas CHIF does not affect ATP affinity. It might seem natural for a transport protein primarily responsible for regulating intracellular Na+ to be modulated by controlling its Na+-binding affinity. However, a change in apparent affinity of a substrate measured at steady state does not necessarily indicate that the intrinsic binding affinity for that substrate has changed. Indeed, any change in the E1/E2 conformational equilibrium is likely to change the apparent affinities for cations and ATP, as well as maximum turnover (Vmax). Thus, regarding -subunit, the decreased Na+ affinity has been explained by an increased competition between Na+ and K+ at the cytoplasmic side (15). However, other experiments seem to show kinetic effects more complex than simple Na+-K+ competition (1). A persisting difficulty encountered in coexpression experiments is that changes in Vmax are not easily detected due to variable specific activity in different clones, and a kinetic interpretation of a change in apparent substrate affinity can depend on whether or not a change in Vmax has occurred.
It is still controversial which steps in the reaction mechanism are affected and how this is achieved at the molecular level by interaction with the FXYD proteins. In vitro experiments using kidney membrane preparations treated with anti--subunit antiserum to perturb interaction between -subunit and Na-K-ATPase resulted in inhibition and increased ATP affinity (15); however, lower antibody concentrations reversed these effects (9). This could indicate successive neutralization of several interaction domains between -subunit and the Na-K-ATPase. Indeed, mutational studies have shown that different regions of the -subunit molecule mediate distinct functional effects on Na-K-ATPase activity (13). Thus deletion of a small portion of the COOH terminus or of the splice variant-specific NH2 terminus both eliminated the effects of -subunit on apparent ATP affinity but not the K+ effect on the apparent cytoplasmic Na+ affinity. Interestingly, a point mutation in a highly conserved transmembrane glycine residue of the -subunit gene is associated with renal Mg2+ deficiency (10). This mutation was shown to cause a defect in the -subunit trafficking (10, 13), abrogating its effect on the Na-K-ATPase. Regarding CHIF, the presence of the FXYD motif appeared to be important for long-term, stable association with the Na-K-ATPase. The positively charged amino acids in the cytoplasmic COOH terminus of CHIF were found necessary for functional effects on Na-K-ATPase (3).
Considering the hydrophobic nature of the FXYD proteins, it is conceivable that additional molecular interactions with the -subunit must exist along the TM domains in the membrane phase. Such hydrophobic interactions in the TM domain are probably stabilized by lz motifs present in the FXYD proteins and similar conserved lz motifs found in Na-K-ATPase, most evident in TM1 and TM2 (5). Support for this comes from experiments in which the interactions between the FXYD proteins and the Na-K-ATPase are disrupted or destabilized by treatment with low concentrations of nonionic detergents. Thus it is conceivable that association between the FXYD proteins and the Na-K-ATPase is anchored via hydrophobic interaction in the membrane phase and that modulation of activity is by specific interactions at the cytoplasmic region of the enzyme (Fig. 2), a hypothesis compatible with the very variable length and nonhomologous sequences of the COOH-terminal portions of the FXYD proteins (cf. Fig. 1).
Although the regulatory functions of -subunit, CHIF, and FXYD7 might be controlled by the level of expression, PLM and PLMS, on the other hand, are known to be substrates for PKA and PKC, and their effects on the Na-K-ATPase may be regulated by phosphorylation. The mechanism of interaction of PLMS and Na-K-ATPase has been studied in a nonmammalian transport tissue, the salt gland of sharks (9). Thus PKC phosphorylation of PLMS was demonstrated to significantly increase the maximum Na-K-ATPase activity without affecting the apparent affinities for Na+ or K+ at the conditions used. This was probably achieved as a result of a release of the interaction between the COOH-terminal end of PLMS with the Na-K-ATPase, because selective truncation of the COOH-terminal domain of PLMS had similar effects on the Na-K-ATPase (5). The increase in Na-K-ATPase activity following PLMS truncation was accompanied by an increase in the rate of phosphorylation, suggesting that association of the COOH terminus of PLMS with the -subunit restricted the freely movable N and/or A domains, thus leading to inhibition of the enzyme (cf. Fig. 2).
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Tissue-specific regulation by FXYD proteins
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FXYD proteins are abundant in tissues involved in active ion transport. Below follows a description of the main functional regulations exerted by the known FXYD proteins. Table 1 summarizes the expression pattern of FXYD protein in different tissues, the Na-K-ATPase isoforms they associate with, and their main functional effects.
-Subunit (FXYD2) and CHIF (FXYD4) are nephron segment-specific regulators in kidney.
The kidney is a spectacular example of how structure and function interact to solve very complex tasks. The nephron carries out variable functions along its length, regulating water and salt balance, and is subject to complex hormonal regulation. A protein of major importance in the functions of the nephron is the Na-K-ATPase. Its function is intimately connected with its location exclusively to basolateral membrane compartments. The distribution of Na-K-ATPase along the nephron as well as its activity vary, and intrinsic properties like Na+ affinity also seem to differ in different segments. The basis for this difference is apparently not a specific localization of isoforms along the nephron, because 1/ß1 seems to be the only isoform complex present in kidney tubule. However, recently two FXYD proteins that are kidney-specific regulators of Na-K-ATPase, namely FXYD2 (-subunit) and FXYD4 (CHIF), show a segment-specific localization that can account for this functional differentiation (cf. Fig. 3).
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FIGURE 3. Sketch to show the differential expression of Na-K-ATPase 1- and -subunits and channel-inducing factor (CHIF) along the kidney nephron. The Na-K-ATPase is expressed together with a in low levels in the proximal convoluted tubule (PCT), proximal straight tubule (PST), and cortical collecting ducts (CCD). In the descending (DTL) or ascending (ATL) thin limb of Henle, as well as in medullary collecting ducts (MCD), very low levels of are found. The highest expression levels of Na-K-ATPase are in the medullary and cortical thick ascending limbs (MTAL and CTAL, respectively) and distal convoluted and connecting tubules (DCT and CNT, respectively). Here it is coexpressed with a + b in the inner stripe of MTAL, which shifts towards b further on and in CTAL. In DCT and CNT, b predominates. CHIF, on the other hand, is located specifically to collecting ducts.
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The -subunit was the first FXYD protein demonstrated to be specifically associated with Na-K-ATPase, and it was later shown by coexpression studies to modulate the Na-K-ATPase, increasing the apparent affinity for extracellular K+ in the presence of Na+ in a voltage-dependent way. The first indication that CHIF interacted with Na-K-ATPase came from studies demonstrating coimmunoprecipitation of CHIF and Na-K-ATPase -subunit in colon membranes. Further studies showed that CHIF increases the apparent affinity of Na-K-ATPase for cytoplasmic Na+ and affected the affinity for extracellular K+, depending on Na+ and membrane potential (3, 7).
Immunofluorescence studies have demonstrated colocalization of both -subunit and CHIF with Na-K-ATPase in kidney, whereas the distribution of -subunit along the nephron seems to be complementary to that of CHIF (see Fig. 3), which is exclusively expressed in collecting ducts (2, 7, 12). In line with this, coimmunoprecipitation has demonstrated the presence of /ß/-subunit and /ß/CHIF in kidney membranes but no mixed complexes containing a combination of both FXYD proteins (7). The situation is even more complex, because the two splice variants a and b with small differences in the NH2-terminal sequence also show a differential segment-specific localization, although with some overlap, as well as small differences in their functional effects on the Na-K-ATPase -subunit (2). The latter may depend on posttranslational modifications, the nature of which are not yet resolved.
Functionally, the -subunit and CHIF also seem to modulate the Na-K-ATPase in a complementary way (Table 1). The main functional property modulated by both FXYD proteins seems to be the apparent cytoplasmic Na+ affinity of the Na-K-ATPase, but whereas -subunit decreases the affinity for cytoplasmic Na+, CHIF increases the Na+ affinity. In addition, -subunit increased the apparent ATP affinity due to a shift in the E1/E2 equilibrium toward E1. These functional effects of -subunit and CHIF probably reflect adaptations in different segments of the nephron to changing conditions. In segments like the medullar thick ascending limb of Henle’s loop where Na+ reabsorption is intense, an increased affinity for ATP induced by -subunit could help compensate for fluctuations in the ATP concentration, e.g., accompanying anoxic events. Likewise, the decreased Na+ affinity would increase the regulatory power of the Na-K-ATPase but still allow it to work around K0.5 at increasing intracellular Na+. On the other hand, a CHIF-induced increased Na+ affinity in cortical and medullary collecting ducts would increase Na+ pumping efficiency at low intracellular Na+. Also, CHIF has been implicated in K+ homeostasis of the collecting ducts and its regulation by mineral corticoids, since CHIF mRNA levels increase significantly in response to a low-K+ diet. These effects are consistent with recent studies using CHIF knockout mice showing an increased urine volume under Na+ deprivation and K+ loading.
PLM (FXYD1) is a tissue-specific regulator of Na-K-ATPase in cardiac and skeletal muscle.
PLM was first cloned in cardiac tissue as a PKA and PKC substrate and suggested to play an important role in heart physiology (11). However, until recently it was not known whether or not PLM was associated with another protein. PLM was shown to induce a Cl- conductance when expressed in Xenopus oocytes, suggesting that it may function as a channel or channel regulator.
A possible relationship between PLM and the Na-K-ATPase was first realized when a PLMS from Squalus acanthias was shown to specifically associate and modulate Na-K-ATPase functions (9). The rectal gland of the shark is the transport epithelium responsible for NaCl clearance and contains high levels of Na-K-ATPase located to the basolateral membranes, providing the driving force for the likewise basolateral Na-K-2Cl cotransporter. A basolateral K+ channel recycles K+, whereas CFTR moves Cl- across the apical membrane. In vitro phosphorylation of shark Na-K-ATPase membranes by PKA or PKC revealed a phosphoprotein band with an electrophoretic mobility of 15 kDa in SDS gels, and the NH2-terminal sequence showed a close homology to PLM and to the -subunit of Na-K-ATPase sharing the FXYD family motif. PLMS coimmunoprecipitated and coexpressed with the 3-like subunit of shark Na-K-ATPase, whereas phosphorylation of PLMS by protein kinase weakened its association with the Na-K-ATPase, relieving the inhibition of hydrolytic activity (cf. Fig. 2). Coexpression experiments subsequently demonstrated the specific interaction between mammalian PLM and the -subunit of Na-K-ATPase (6). The main functional effect of PLM seems to be a lowering by a factor of two of the intracellular Na+ affinity, whereas the Vmax was unchanged as deduced from voltage-clamp experiments on the coexpressed system. The effects, if any, of protein kinase phosphorylation of the mammalian PLM on Na-K-ATPase activity has not yet been investigated.
FXYD7 is a brain-specific Na-K-ATPase regulator.
FXYD6 and FXYD7 were both identified from expressed sequence tag analysis and have not yet been characterized at the protein level (14). Recently, FXYD7 was demonstrated to be expressed exclusively in the brain (4). Like the other FXYD proteins, it is a type I protein, but a signal peptide is apparently not present. Coexpression experiments of Na-K-ATPase and FXYD7 in Xenopus oocytes and isoform-specific immunoprecipitation experiments with brain microsomes suggest that FXYD7 associates specifically with 1/ß complexes, although 1 and 2 are both present in the brain. In patch-clamp experiments using coexpressed FXYD7 and Na-K-ATPase, FXYD7 decreases the apparent affinity for extracellular K+ of the 1/ß1 complex by a factor of two, and this effect was ascribed to an effect on the intrinsic K+ binding affinity (4). Moreover, FXYD7 decreased the enzyme turnover at increasing negative membrane potentials, indicating additional effects on the Na+ translocation process. The FXYD7-induced decrease in K+ affinity could be important to adapt the Na-K-ATPase to increased extracellular K+ concentration during neuronal activity.
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Acknowledgments
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We thank Kathleen J. Sweadner, Laboratory of Membrane Biology, Massachusetts General Hospital, for very helpful comments on the manuscript.
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References
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- Arystarkhova E, Donnet C, Asinovski NK, and Sweadner KJ. Differential regulation of renal Na,K-ATPase by splice variants of the -subunit. J Biol Chem 277: 10162–10172, 2002.[Abstract/Free Full Text]
- Arystarkhova H, Wetzel RK, and Sweadner KJ. Distribution and oligomeric association of splice forms of Na+-K+-ATPase regulatory -subunit in rat kidney. Am J Physiol Renal Physiol 282: F393–F407, 2002.[Abstract/Free Full Text]
- Béguin P, Crambert G, Guennoun S, Garty H, Horisberger JD, and Gerring K. CHIF, a member of the FXYD protein family, is a regulator of Na,K-ATPase distinct from the -subunit. EMBO J 20: 3993–4002, 2001.[ISI][Medline]
- Béguin P, Crambert G, Monnet-Tschudi F, Uldry M, Horisberger JD, Garty H, and Geering K. FXYD7 is a brain-specific regulator of Na,K-ATPase 1-ß isozymes. EMBO J 21: 3264–3273, 2002.[ISI][Medline]
- Cornelius F and Mahmmoud YA. Themes in ion pump regulation. Ann NY Acad Sci USA. In press.
- Crambert G, Füzesi M, Garty H, Karlish S, and Geering K. Phospholemman (FXYD1) associates with Na,K-ATPase and regulates its transport properties. Proc Natl Acad Sci USA 99: 11476–11481, 2002.[Abstract/Free Full Text]
- Garty H, Lindzen M, Scanzano R, Aizman R, Fuzesi M, Goldshleger R, Farman N, Blostein R, and Karlish SJD. A functional interaction between CHIF and Na-K-ATPase: implication for regulation by FXYD proteins. Am J Physiol Renal Physiol 283: F607–F615, 2002.[Abstract/Free Full Text]
- Küster B, Shainskaya A, Pu HX, Goldshleger R, Blostein R, Mann M, and Karlish SJD. A new variant of the subunit of renal Na,K-ATPase. Identification by mass spectroscopy, antibody binding, and expression in cultured cells. J Biol Chem 275: 18441–18446, 2000.[Abstract/Free Full Text]
- Mahmmoud YA, Vorum H, and Cornelius F. Identification of a phospholemman-like protein from shark rectal glands. Evidence for indirect regulation of Na,K-ATPase by protein kinase C via a novel member of the FXYDY family. J Biol Chem 275: 35969–35977, 2000.[Abstract/Free Full Text]
- Meij IC, Koenderink JB, van Bokhoven H, Assink KFH, Groenestege WT, de Pont JJ, Bindels RJ, Monnens LA, van den Heuvel LP, and Knoers NV. Dominant isolated renal magnesium loss is caused by misrouting of the Na+,K+-ATPase -subunit. Nat Genet 26: 265–266, 2000.[ISI][Medline]
- Palmer CJ, Scott BT, and Jones LR. Purification and complete sequence determination of the major plasma membrane substrate for cAMP-dependent protein kinase and protein kinase C in myocardium. J Biol Chem 266: 11126–11130, 1991.[Abstract/Free Full Text]
- Pu HX, Cluzeaud F, Goldshleger R, Karlish SJD, Farman N, and Blostein R. Functional role and immunocytochemical localization of the a and b forms of the Na,K-ATPase subunit. J Biol Chem 276: 20370–20378, 2001.[Abstract/Free Full Text]
- Pu XH, Scanzano R, and Blostein R. Distinct regulatory effects of the Na,K-ATPase subunit. J Biol Chem 277: 20270–20276, 2002.[Abstract/Free Full Text]
- Sweadner KJ and Rael E. The FXYD gene family of small ion transport regulators or channels: cDNA sequence, protein signature sequence, and expression. Genomics 68: 41–56, 2000.[ISI][Medline]
- Therien AG and Blostein R. Mechanisms of sodium pump regulation. Am J Physiol Cell Physiol 279: C541–C566, 2000.[Abstract/Free Full Text]
- Toyoshima C and Nomura H. Structural changes in the calcium pump accompanying the dissociation of calcium. Nature 418: 605–611, 2002.[Medline]
Copyright © 2003 by the Int. Union Physiol. Sci./Am. Physiol. Soc.
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Introduction
Acute regulation of Na-K-ATPase
