Tissue-specific distribution and modulatory role of the gamma subunit of the Na,K-ATPase

The FXYD gene family of small ion transport regulators or channels: cDNA sequence, protein signature sequence, and expression

A gene family of small membrane proteins, represented by phospholemman and the gamma subunit of Na,K-ATPase, was defined and characterized by the analysis of more than 1000 related ESTs (expressed sequence tags). In addition to new and more complete cDNA sequence for known family members (including MAT-8, CHIF, and RIC), the findings included two new family members and new splicing variants. A large number of EST replicates made it possible to derive curated DNA sequence with higher confidence and accuracy than from the sequencing of individual clones. The family has a core motif of 35 invariant and conserved amino acids centered on a single transmembrane span. Features of each predicted protein product were compared, and tissue distributions were determined. The gene family was named FXYD (pronounced fix-id) in recognition of invariant amino acids in its signature motif. The abundant proteins are involved in the control of ion transport.

Phospholemman is a substrate for myotonic dystrophy protein kinase

The genetic abnormality in myotonic muscular dystrophy, multiple CTG repeats lie upstream of a gene that encodes a novel protein kinase, myotonic dystrophy protein kinase (DMPK). Phospholemman (PLM), a major membrane substrate for phosphorylation by protein kinases A and C, induces Cl currents (I(Cl(PLM))) when expressed in Xenopus oocytes. To test the idea that PLM is a substrate for DMPK, we measured in vitro phosphorylation of purified PLM by DMPK. To assess the functional effects of PLM phosphorylation we compared I(Cl(PLM)) in Xenopus oocytes expressing PLM alone to currents in oocytes co-expressing DMPK, and examined the effect of DMPK on oocyte membrane PLM expression. We found that PLM is indeed a good substrate for DMPK in vitro. Co-expression of DMPK with PLM in oocytes resulted in a reduction in I(Cl(PLM)). This was most likely a specific effect of phosphorylation of PLM by DMPK, as the effect was not present in oocytes expressing a phos(-) PLM mutant in which all potential phosphorylation had been disabled by Ser --> Ala substitution. The biophysical characteristics of I(Cl(PLM)) were not changed by DMPK or by the phos(-) mutation. Co-expression of DMPK reduced the expression of PLM in oocyte membranes, suggesting a possible mechanism for the observed reduction in I(Cl(PLM)) amplitude. These data show that PLM is a substrate for phosphorylation by DMPK and provide functional evidence for modulation of PLM function by phosphorylation.

A new variant of the gamma subunit of renal Na,K-ATPase. Identification by mass spectrometry, antibody binding, and expression in cultured cells

The gamma subunit is a specific regulator of Na,K-ATPase expressed mainly in kidney. On SDS-polyacryylamide gel electrophoresis, gamma runs as a doublet, but the origin and significance of the doublet is obscure. Mass spectrometry of the gamma chains of rat kidney Na, K-ATPase shows that gamma(a) (upper) has a mass of 7184.0 +/- 1 Da (carbamidomethyl cysteine), corresponding closely to that for the published sequence without the initiator methionine, while gamma(b) (lower) has a mass of 7337.9 +/- 1Da. Tryptic peptide mapping and sequencing by mass spectrometry reveals that the seven N-terminal residues of gamma(a), TELSANH, are replaced by Ac-MDRWYL in gamma(b), but otherwise the chains are identical. Antibodies raised against peptides TELSANHC and MDRWYLC recognize either gamma(a) or gamma(b) of the Na,K-ATPase, respectively. gamma(a) or gamma(b) cDNAs have been expressed in human embryonic kidney and HeLa cells. The major bands expressed correspond to gamma(a) or gamma(b) of renal Na, K-ATPase. Additional minor bands seen after transfection, namely gamma(a)' in human embryonic kidney and gamma(b)' in HeLa, are presumably cell-specific modifications. The present work clarifies earlier uncertainty regarding doublets seen in kidney and in transfected cells. In particular, the results show that renal Na, K-ATPase contains two variants of the gamma subunit with different sequences but otherwise are unmodified. We discuss the possible functional significance of the two variants.

Renal proximal tubule sodium transport and genetic mechanisms of essential hypertension

Renal sodium re-absorption is a closely regulated process serving to maintain both extracellular fluid volume and arterial blood pressure. Proteins participating in sodium re-absorption and its regulation are therefore important candidate proteins whose genes may contain sequence variation contributing to the inherited tendency for increased arterial blood pressure (essential hypertension). Important insight has come from rare forms of single-gene hypertension in human subjects and from polygenic animal models of genetic hypertension. Both indicate the primacy of altered renal function in the genesis of hypertension, and suggest that genes contributing to the disease are members of the subset of genes expressed in the kidney. This review examines evidence for abnormalities in renal sodium re-absorption in hypertension and focuses on the proximal tubule as a site of relevant dysfunction. Identification of the proteins participating in renal sodium re-absorption and its regulation, particularly those involved in the renal pressure-natriuresis mechanism, will allow gene cloning and sequencing which in turn may lead to the identification of novel gene sequence variation participating in hypertension.

Cyclophilin B expression in renal proximal tubules of hypertensive rats

Rat cyclophilin-like protein (Cy-LP) is a candidate hypertension gene initially identified by differential hybridization and implicated in renal mechanisms of salt retention and high blood pressure. We report the molecular characterization of rat cyclophilin B (CypB) and demonstrate, through sequence analysis and an allele-specific polymerase chain reaction primer assay, that CypB but not Cy-LP is expressed in rat kidney. CypB is an endoplasmic reticulum-localized prolyl-isomerase that interacts with elongation initiation factor 2-beta, an important regulator of protein translation and a central component of the endoplasmic reticulum stress response to hypoxia or ATP depletion. Active renal transport of sodium is increased in the spontaneously hypertensive rat (SHR), and there is evidence that this coincides with hypoxia and ATP depletion in the renal cortex. In the present studies we have examined expression of CypB in rat proximal tubules, which contributes to the increased renal sodium reabsorption in this model of hypertension. We report that CypB transcript abundance is significantly elevated in proximal convoluted tubules from SHR compared with the control Wistar-Kyoto strain. This upregulation occurs in weanling animals and precedes the development of hypertension, indicating that it is not a simple response to hypertension in SHR. Further, CypB expression is also higher in a proximal tubule cell line derived from SHR compared with a similar line derived from Wistar-Kyoto rats, indicating that this difference is genetically determined. No sequence differences were observed in the CypB cDNA from these 2 strains. These observations suggest that a genetically determined alteration in proximal tubules from SHR occurs that leads to increased expression of CypB. In view of evidence linking CypB to the regulation of elongation initiation factor-2, the upregulation of CypB may result from metabolic stress.

Cellular and subcellular specification of Na,K-ATPase alpha and beta isoforms in the postnatal development of mouse retina

The Na,K-ATPase is a dominant factor in retinal energy metabolism, and unique combinations of isoforms of its alpha and beta subunits are expressed in different cell types and determine its functional properties. We used isoform-specific antibodies and fluorescence confocal microscopy to determine the expression of Na,K-ATPase alpha and beta subunits in the mouse and rat retina. In the adult retina, alpha1 was found in Müller and horizontal cells, alpha2 in some Müller glia, and alpha3 in photoreceptors and all retinal neurons. beta1 was largely restricted to horizontal, amacrine, and ganglion cells; beta2 was largely restricted to photoreceptors, bipolar cells, and Müller glia; and beta3 was largely restricted to photoreceptors. Photoreceptor inner segments have the highest concentration of Na,K-ATPase in adult retinas. Isoform distribution exhibited marked changes during postnatal development. alpha3 and beta2 were in undifferentiated photoreceptor somas at birth but only later were targeted to inner segments and synaptic terminals. beta3, in contrast, was expressed late in photoreceptor differentiation and was immediately targeted to inner segments. A high level of beta1 expression in horizontal cells preceded migration, whereas increases in beta2 expression in bipolar cells occurred very late, coinciding with synaptogenesis in the inner plexiform layer. Most of the spatial specification of Na,K-ATPase isoform expression was completed before eye opening and the onset of electroretinographic responses on postnatal day 13 (P13), but quantitative increase continued until P22 in parallel with synaptogenesis.

The gamma subunit modulates Na(+) and K(+) affinity of the renal Na,K-ATPase

The Na(+),K(+)-ATPase catalyzes the active transport of ions. It has two necessary subunits, alpha and beta, but in kidney it is also associated with a 7.4-kDa protein, the gamma subunit. Stable transfection was used to determine the effect of gamma on Na, K-ATPase properties. When isolated from either kidney or transfected cells, alphabetagamma had lower affinities for both Na(+) and K(+) than alphabeta. A post-translational modification of gamma selectively eliminated the effect on Na(+) affinity, suggesting three configurations (alphabeta, alphabetagamma, and alphabetagamma*) conferring different stable properties to Na, K-ATPase. In the nephron, segment-specific differences in Na(+) affinity have been reported that cannot be explained by the known alpha and beta subunit isoforms of Na,K-ATPase. Immunofluorescence was used to detect gamma in rat renal cortex. Cortical ascending limb and some cortical collecting tubules lacked gamma, correlating with higher Na(+) affinities in those segments reported in the literature. Selective expression in different segments of the nephron is consistent with a modulatory role for the gamma subunit in renal physiology.

K(+)/Na(+) antagonism at cytoplasmic sites of Na(+)-K(+)-ATPase: a tissue-specific mechanism of sodium pump regulation

Tissue-distinct interactions of the Na(+)-K(+)-ATPase with Na(+) and K(+), independent of isoform-specific properties, were reported previously (A. G. Therien, N. B. Nestor, W. J. Ball, and R. Blostein. J. Biol. Chem. 271: 7104-7112, 1996). In this paper, we describe a detailed analysis of tissue-specific kinetics particularly relevant to regulation of pump activity by intracellular K(+), namely K(+) inhibition at cytoplasmic Na(+) sites. Our results show that the order of susceptibilities of alpha(1) pumps of various rat tissues to K(+)/Na(+) antagonism, represented by the ratio of the apparent affinity for Na(+) binding at cytoplasmic activation sites in the absence of K(+) to the affinity constant for K(+) as a competitive inhibitor of Na(+) binding at cytoplasmic sites, is red blood cell < axolemma approximately rat alpha(1)-transfected HeLa cells < small intestine < kidney < heart. In addition, we have carried out an extensive analysis of the kinetics of K(+) binding and occlusion to the cytoplasmic cation binding site and find that, for most tissues, there is a relationship between the rate of K(+) binding/occlusion and the apparent affinity for K(+) as a competitive inhibitor of Na(+) activation, the order for both parameters being heart >/= kidney > small intestine approximately rat alpha(1)-transfected HeLa cells. The notion that modulations in cytoplasmic K(+)/Na(+) antagonism are a potential mode of pump regulation is underscored by evidence of its reversibility. Thus the relatively high K(+)/Na(+) antagonism characteristic of kidney pumps was reduced when rat kidney microsomal membranes were fused into the dog red blood cell.

Altered sodium pump alpha and gamma subunit gene expression in nephron segments from hypertensive rats

OBJECTIVE

To determine the qualitative and quantitative expression of alpha and gamma sodium pump subunits in whole kidney and nephron segment RNA from Sprague Dawley rats, spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats.

DESIGN

A novel reverse transcription polymerase chain reaction technique was devised which provides accurate and precise measurement of the number of molecules of specific transcript abundance, a measurement of gene expression. This allows the quantitative comparison of multiple samples across multiple subjects and, since the estimates are accurate rather than relative, can also be used to make quantitative comparisons across expressed genes, such as isoforms and subunits of the heterotrimeric renal sodium pump.

METHODS

We examined which catalytic isoforms were expressed and then quantified transcript abundance in whole kidney and convoluted and straight segments of the proximal tubule.

RESULTS

Alpha 1 and gamma transcripts, but not alpha 2, alpha 3 or alpha 4 isoforms, were consistently observed in nephron segments. Levels of alpha 1 were lower in kidney RNA from 15-16-week-old SHR than in WKY rats of the same age (P = 0.001), but were not different between SHR and WKY in 4-5-week-old animals. No significant difference was observed in gamma subunit abundance in kidney RNA from 4-5-week-old animals; however, at 15-16 weeks, the expression in SHR was one-third that in WKY rats (P = 0.003). In proximal convoluted tubules from 4-5-week-old animals, the level of alpha 1 RNA expression was lower (P = 0.03) in SHR than in WKY rats. In addition, levels of alpha 1 in proximal straight tubule from the 4-5-week-old SHR were also lower than in WKY rats (P = 0.02). This difference was even greater in 15-16-week-old animals: in SHR, alpha 1 expression was less than 20% of the level of expression in WKY rats (P = 0.0003). Expression of the gamma subunit exhibited a similar pattern of downregulation in SHR. In RNA from proximal convoluted tubules and proximal straight tubules from both 4-5- and 15-16-week-old animals, expression of the gamma subunit was demonstrated to be significantly lower in SHR than in WKY rats.

CONCLUSION

The results indicate a coordinate reduction in the abundance of sodium pump alpha and gamma subunits in the proximal tubules of SHR, which occurs early during the development of hypertension.

Jeanne Mannery Fisher Memorial Lecture 1998. Structure-function studies of the sodium pump

The Na+, K+-ATPase is an ubiquitous plasma membrane protein complex that belongs to the P-type family of ion motive ATPases. Under normal conditions, it couples the hydrolysis of one molecule of ATP to the exchange of three Na+ for two K+ ions, thus maintaining the normal gradient of these cations in animal cells. Despite decades of investigation of its structure and function, the structural basis for its cation specificity and for conformational coupling of the scalar energy of ATP hydrolysis to the vectorial movement of Na+ and K+ have remained a major unresolved issue. This paper summarizes our recent studies concerned with these issues. The findings indicate that regions(s) of the amino terminus and first cytoplasmic (M2/M3) loop act synergistically to affect the steady-state conformational equilibrium of the enzyme. Although carboxyl- or hydroxyl-bearing amino acids comprise the cation-binding and occlusion sites, our experiments also suggest that these interactions may be modulated by juxtapositioned cytoplasmic regions.

Stimulation of ouabain binding to Na,K-ATPase in 40% dimethyl sulfoxide by a factor from Na,K-ATPase preparations

In 40% dimethyl sulfoxide (Me2SO) high-affinity ouabain (O) binding to Na,K-ATPase (E) is promoted by Mg2+ in the absence of inorganic phosphate (Pi) (Fontes et al., Biochim. Biophys. Acta 1104, 215-225, 1995). Furthermore, in Me2SO the EO complex reacts very slowly with Pi and this ouabain binding can therefore be measured by the degree of inhibition of rapid phosphoenzyme formation. Here we found that, unexpectedly, the ouabain binding decreased with the enzyme concentration in the Me2SO assay medium. We extracted the enzyme preparation with Me2SO or chloroform/methanol and demonstrated that the extracted (depleted) enzyme bound ouabain poorly. Addition of such extracts to assays with low enzyme concentration or depleted enzyme fully restored the high-affinity ouabain binding. Dialysis experiments indicated that the active principle had a molecular mass between 3.5 and 12 kDa. It was highly resistant to proteolysis. It was suggested that the active principle could either be a low-molecular-weight, proteolysis-resistant-peptide (e.g., a proteolipid) or a factor with a nonproteinaceous nature. A polyclonal antibody raised against the C-terminal 10 amino acids of the rat kidney gamma-subunit was able to recognize this low-molecular-weight peptide present in the extracts. The previously depleted enzyme displayed lower amounts of the gamma-proteolipid in comparison to the native untreated enzyme, as demonstrated by immunoreaction with the antibody.

Preparation of Na+,K+-ATPase with near maximal specific activity and phosphorylation capacity: evidence that the reaction mechanism involves all of the sites

The phosphorylation capacity of Na+,K+-ATPase preparations in common use is much less than expected on the basis of the molecular weight of the enzyme deduced from cDNA sequences. This has led to the popularity of half-of-the-sites or flip-flop models for the enzyme reaction mechanism. We have prepared Na+,K+-ATPase from nasal salt glands of salt-adapted ducks which has a phosphorylation capacity and specific activity near the theoretical maxima. Preparations with specific activities of >60 micromol (mg of protein)-1 min-1 at 37 degrees C had phosphorylation capacities of >60 nmol/mg of protein, and the rate of turnover of the enzyme was 9690 min-1, within the range reported for the enzyme from other sources. The fraction of the maximal specific activity of the enzyme compared well with the fraction of the protein on SDS-PAGE which was alpha and beta chains, especially at the highest specific activity which indicates that all of the alphabeta protomers are active. The gels of the most reactive preparations contained only alpha and beta chains, but less active preparations contained a number of extraneous proteins. The major contaminant was actin. The preparation did not contain any protein which migrated in the molecular weight range of the gamma subunit. The subunit composition of the enzyme was alpha1 and beta1 only. This is the first report of a pure, homogeneous, fully active preparation of the protein. Reaction models which incorporate a half-of-the-sites or flip-flop mechanism do not apply to this enzyme.

Expression and functional role of the gamma subunit of the Na, K-ATPase in mammalian cells

The functional role of the gamma subunit of the Na,K-ATPase was studied using rat gamma cDNA-transfected HEK-293 cells and an antiserum (gammaC33) specific for gamma. Although the sequence for gamma was verified and shown to be larger (7237 Da) than first reported, it still comprises a single initiator methionine despite the expression of a gammaC33-reactive doublet on immunoblots. Kinetic analysis of the enzyme of transfected compared with control cells and of gammaC33-treated kidney pumps shows that gamma regulates the apparent affinity for ATP. Thus, gamma-transfected cells have a decreased K'ATP as shown in measurements of (i) K'ATP of Na,K-ATPase activity and (ii) K+ inhibition of Na-ATPase at 1 microM ATP. Consistent with the behavior of gamma-transfected cells, gammaC33 pretreatment increases K'ATP of the kidney enzyme and K+ inhibition (1 microM ATP) of both kidney and gamma-transfected cells. These results are consistent with previous findings that an antiserum raised against the pig gamma subunit stabilizes the E2(K) form of the enzyme (Therien, A. G., Goldshleger, R., Karlish, S. J., and Blostein, R. (1997) J. Biol. Chem. 272, 32628-32634). Overall, our data demonstrate that gamma is a tissue (kidney)-specific regulator of the Na,K-ATPase that can increase the apparent affinity of the enzyme for ATP in a manner that is reversible by anti-gamma antiserum.

A novel small protein associated with a conjugated trienoic chromophore from membranes of scallop adductor muscle: phosphorylation by protein kinase A

Membranes enriched in sarcolemma from the cross-striated adductor muscle of the deep sea scallop have been found to contain a previously undescribed small protein of 6-8 kDa that can be released by treatment with organic solvent mixtures. This proteolipid co-purified with a non-amino acid chromophore containing a conjugated trienoic moiety. Although common in plants and algae, such a stable conjugated trienoic group is unusual for an animal cell. The N-terminal amino acid sequence of the protein was XEFQHGLFGXF/ADNIGLQ, which most strongly resembles sequences in the triacyl glycerol lipase precursor and the product of the human breast cancer susceptibility gene BRCA 1, but does not show similarity to previously described proteolipids. The protein was found to be one of the major substrates in its parent membrane for the catalytic subunit of protein kinase A, which may imply a regulatory function for this molecule.

Characterization of disulfide cross-links between fragments of proteolyzed Na,K-ATPase. Implications for spatial organization of trans-membrane helices

This study characterizes disulfide cross-links between fragments of a well defined tryptic preparation of Na,K-ATPase, 19-kDa membranes solubilized with C12E10 in conditions preserving an intact complex of fragments and Rb occlusion (Or, E., Goldshleger, R., Tal, D. M., and Karlish, S. J. D. (1996) Biochemistry 35, 6853-6864). Upon solubilization, cross-links form spontaneously between the beta subunit, 19- and 11.7-kDa fragments of the alpha subunit, containing trans-membrane segments M7-M10 and M1/M2, respectively. Treatment with Cu2+-phenanthroline (CuP) improves efficiency of cross-linking. Sequencing and immunoblot analysis have shown that the cross-linked products consist of a mixture of beta-19 kDa dimers ( approximately 65%) and beta-19 kDa-11.7 kDa trimers ( approximately 35%). The alpha-beta cross-link has been located within the 19-kDa fragment to a 6.5-kDa chymotryptic fragment containing M8, indicating that betaCys44 is cross-linked to either Cys911 or Cys930. In addition, an internal cross-link between M9 and M10, Cys964-Cys983, has been found by sequencing tryptic fragments of the cross-linked product. The M1/M2-M7/M10 cross-link has not been identified directly. However, we propose that Cys983 in M10 is cross-linked either to Cys104 in M1 or internally to Cys964 in M9. Based on this study, cross-linking induced by o-phthalaldehyde (Or, E., Goldshleger, R., and Karlish, S. J. D. (1998) Biochemistry 37, 8197-8207), and information from the literature, we propose an approximate spatial organization of trans-membrane segments of the alpha and beta subunits.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

Residue leu973 of the rat alpha1 subunit of the Na,K-ATPase is located on the cytoplasmic side of the plasma membrane

In a previous study (Yoon, K. L., and Guidotti, G., 1994 J. Biol. Chem. 269, 28249-28258), we indicated that the alpha subunit of the Na,K-ATPase has 4 transmembrane segments in the COOH terminal domain between residues Lys769 and Val939, and that both the NH2-terminus and the COOH-terminus are in the cytosol. However, there was insufficient information to determine whether there are more transmembrane segments between residues Val939 and the COOH-terminal Tyr1018. To investigate this question, we inserted the influenza virus hemagglutinin (HA)-epitope between Leu973 and Arg974 of the alpha1 chain, expressed the construct in COS-7 and HeLa cells and determined the membrane arrangement by indirect immunofluorescence. The results indicate that Leu973 is not on the extracellular surface of the plasma membrane. Thus, the alpha1 subunit is likely to possess only four complete transmembrane segments in the COOH terminal domain between residues Lys769 and Tyr1018.

Specific cross-links between fragments of proteolyzed Na,K-ATPase induced by o-phthalaldehyde

We have used o-phthalaldehyde (OPA) to cross-link adjacent fragments of "19 kDa membranes", a tryptic preparation of Na,K-ATPase lacking the ATP site but retaining cation occlusion sites. Treatment with OPA of "19 kDa membranes" or detergent-solubilized membranes containing occluded Rb ions [Or, E., Goldshleger, R., Tal, D. M., and Karlish, S. J. D. (1996) Biochemistry 35, 6853-6864] yielded cross-linked products of 25 and 31 kDa. Both species contained the 19 kDa fragment of the alpha subunit (transmembrane segments M7-M10). In addition, the 25 kDa product contained the fragment including M5-M6, while the 31 kDa product contained a 16 kDa fragment of the beta subunit. Cross-linking was unaffected by the absence or presence of ligands (Na, Rb, or Mg and ouabain). Cross-linking was largely abolished in thermally inactivated "19 kDa membranes". When proteolytic digestion of the 25 and 31 kDa products was combined with antibody binding, PKA-dependent phosphorylation, and sequencing of fragments, approximate positions of the cross-links were established. In the 25 kDa product, the cross-link was located within the short cytoplasmic segment Asn831-Arg841 of the 19 kDa fragment preceding M7 and within Ala749-Ala770 preceding M5. Thus, M7 and M5 are likely to be in close proximity. In the 31 kDa product, the cross-link was located in the extracellular loop of the alpha subunit between M7 and M8, close to residues which are known to interact with the beta subunit. Functional implications of the interactions between the fragments of the alpha (M5-M6 and M7-M10) and beta subunits are discussed.

The gamma subunit of the Na,K-ATPase induces cation channel activity

The gamma subunit of the Na,K-ATPase is a hydrophobic protein of approximately 10 kDa. The gamma subunit was expressed in Sf-9 insect cells and Xenopus oocytes to ascertain its role in Na,K-ATPase function. Immunoblotting has shown that the gamma subunit is expressed in Sf-9 cells infected with recombinant baculovirus containing the cDNA for the human gamma subunit. Confocal microscopy demonstrates that the gamma subunit can be delivered to the plasma membrane of Sf-9 cells independently of the other Na,K-ATPase subunits and that gamma colocalizes with alpha1 when these proteins are coexpressed. When Sf-9 cells were coinfected with alpha1 and gamma, antibodies to the gamma subunit were able to coimmunoprecipitate the alpha1 subunit, suggesting that gamma is able to associate with alpha1. The gamma subunit is a member of a family of single-pass transmembrane proteins that induces ion fluxes in Xenopus oocytes. Evidence that the gamma subunit is a functional component was supported by experiments showing gamma-induced cation channel activity when expressed in oocytes and increases in Na+ and K+ uptake when expressed in Sf-9 cells.