Amino-acid sequence of the beta-subunit of the (Na+ + K+)ATPase deduced from a cDNA

Functional activity of oligosaccharide-deficient (Na,K)ATPase expressed in Xenopus oocytes

(Na,K)ATPase from Torpedo californica was expressed in Xenopus laevis oocytes in the presence of tunicamycin by injecting mRNAs for the alpha- and beta-subunits derived from the cloned cDNAs into the oocytes. The oligosaccharide-deficient ATPase thus synthesized was transported to the oocyte plasma membrane, where it exhibited virtually the same ATPase activity, ouabain-binding capacity and 86Rb+ transport activity as the fully glycosylated enzyme. We conclude that the oligosaccharide chains on the beta-subunit has no effect on the catalytic activities of (Na,K)ATPase.

TRK1 encodes a plasma membrane protein required for high-affinity potassium transport in Saccharomyces cerevisiae

We identified a 180-kilodalton plasma membrane protein in Saccharomyces cerevisiae required for high-affinity transport (uptake) of potassium. The gene that encodes this putative potassium transporter (TRK1) was cloned by its ability to relieve the potassium transport defect in trk1 cells. TRK1 encodes a protein 1,235 amino acids long that contains 12 potential membrane-spanning domains. Our results demonstrate the physical and functional independence of the yeast potassium and proton transport systems. TRK1 is nonessential in S. cerevisiae and maps to a locus unlinked to PMA1, the gene that encodes the plasma membrane ATPase. Haploid cells that contain a null allele of TRK1 (trk1 delta) rely on a low-affinity transporter for potassium uptake and, under certain conditions, exhibit energy-dependent loss of potassium, directly exposing the activity of a transporter responsible for the efflux of this ion.

Heterologous expression of excitability proteins: route to more specific drugs?

Many clinically important drugs act on the intrinsic membrane proteins (ion channels, receptors, and ion pumps) that control cell excitability. A major goal of pharmacology has been to develop drugs that are more specific for a particular subtype of excitability molecule. DNA cloning has revealed that many excitability proteins are encoded by multigene families and that the diversity of previously recognized pharmacological subtypes is matched, and probably surpassed, by the diversity of messenger RNAs that encode excitability molecules. In general, the diverse subtypes retain their properties when the excitability proteins are expressed in foreign cells such as oocytes and mammalian cell lines. Such heterologous expression may therefore become a tool for testing drugs against specific subtypes. In a systematic research program to exploit this possibility, major considerations include alternative processing of messenger RNA for excitability proteins, coupling to second-messenger systems, and expression of enough protein to provide material for structural studies.

Role of the Na,K-ATPase beta-subunit in the cellular accumulation and maturation of the enzyme as assessed by glycosylation inhibitors

No functional role could yet be established for the glycosylated beta-subunit of the Na,K-ATPase. In this study, we describe the intracellular processing of the beta-subunit as a glycoprotein in toad bladder cells and the consequences of its structural perturbation with glycosylation inhibitors on the cellular expression of the alpha- and beta-subunits and on the structural and functional maturation of the enzyme. Controlled trypsinolysis of homogenates from pulse-labeled cells reveals that the beta-subunit is subjected to glycosylation-dependent structural rearrangements during its intracellular routing. Inhibition of correct terminal glycosylation of the beta-subunit with deoxynojirimycin or swainsonine has no effect on the trypsin sensitivity of the alpha-subunit, its ability to perform cation-dependent conformation changes or the cellular Na,K-ATPase activity. Acquisition of core-sugars is sufficient for the enzyme to assume its catalytic functions. On the other hand, complete inhibition of glycosylation with tunicamycin leads to a destabilization of both the beta- and the alpha-subunits as judged by their higher trypsin sensitivity. In addition, tunicamycin treatment results in a decrease of the amount of newly synthesized beta- and alpha-subunit indicating that a glycoprotein, possibly the beta-subunit itself, plays a role in the efficient accumulation of the alpha-subunit in the endoplasmic reticulum.

Increased abundance of Na+-K+-ATPase mRNAs in response to low external K+

Exposure of ARL 15 cells, an established line from adult rat liver, to external K+ concentrations less than 1 mM for 24 h increases Na+-K+ pump abundance (Na+-K+-ATPase) (J. Gen. Physiol. 87:591-606, 1986). We found that treatment of confluent monolayers of ARL 15 cells with low-K+ medium (0.65 mM) caused a 100% increase in total RNA content per plate after 24 h, as well as a 25% increase in DNA and protein content per plate. Concomitant with this growth effect, low-K+ exposure for 6 h elicited 60% increases in mRNA alpha and mRNA beta, the mRNAs that encode the constituent subunits of the Na+-K+-ATPase, in a polyadenylated RNA fraction. At 24 h, however, the abundance of mRNA alpha increased by 290%, whereas mRNA beta increased by only 70%. Moreover, in both control and low-K+-treated cells, mRNA alpha was 30-fold or more greater in abundance than mRNA beta. This discrepancy in abundance was also present in rat liver, but not in cultured MDCK cells. The differences in abundance of mRNA alpha and mRNA beta suggest that the liver may have an unusual subunit composition or biosynthetic mechanism. Nevertheless, the increases in the abundance of mRNA alpha and mRNA beta are sufficient to account for the observed 70-100% increase in Na+-K+-ATPase activity in response to low external K+.

Origin of the gamma polypeptide of the Na+/K+-ATPase

The Na+/K+-ATPase purified from lamb kidney contains a gamma polypeptide fraction which is a collection of fragments derived from the alpha and beta polypeptides of the enzyme. This fraction has the solubility characteristics of a proteolipid and was isolated either by high performance liquid chromatography (size exclusion chromatography) in 1% sodium dodecyl sulfate or by sequential organic extraction of purified lamb kidney Na+/K+-ATPase. Formation of gamma polypeptide(s) from detergent solubilized holoenzyme was accelerated by sulfhydryl containing reagents and was unaffected by addition of inhibitors of proteolytic enzymes. Treatment of the holoenzyme with the photoaffinity reagent N-(2-nitro-4-azidophenyl)[3H]ouabain ([3H]NAP-ouabain) labeled the alpha polypeptide and the gamma polypeptide fraction but not the beta polypeptide. Amino acid sequence analysis of one gamma polypeptide preparation revealed homology of one component of this fraction with the N-terminus of the beta subunit of the Na+/K+-ATPase. Amino acid analysis of two preparations of proteolipid showed similar amino acid compositions with a peptide derived from the alpha subunit. The insolubility and complexity of the gamma polypeptide(s)/proteolipid fraction appears to preclude a conclusive sequence analysis of all components of this fraction.

TRK1 encodes a plasma membrane protein required for high-affinity potassium transport in Saccharomyces cerevisiae

We identified a 180-kilodalton plasma membrane protein in Saccharomyces cerevisiae required for high-affinity transport (uptake) of potassium. The gene that encodes this putative potassium transporter (TRK1) was cloned by its ability to relieve the potassium transport defect in trk1 cells. TRK1 encodes a protein 1,235 amino acids long that contains 12 potential membrane-spanning domains. Our results demonstrate the physical and functional independence of the yeast potassium and proton transport systems. TRK1 is nonessential in S. cerevisiae and maps to a locus unlinked to PMA1, the gene that encodes the plasma membrane ATPase. Haploid cells that contain a null allele of TRK1 (trk1 delta) rely on a low-affinity transporter for potassium uptake and, under certain conditions, exhibit energy-dependent loss of potassium, directly exposing the activity of a transporter responsible for the efflux of this ion.

Precipitation of solubilized Na+/K+-ATPase by divalent cations

A method for preparation of membranous fragments of pure and highly active shark rectal gland Na+/K+-ATPase by Mn2+ precipitation of C12E8-solubilized enzyme is described. The method is rapid and inexpensive, and yields enzyme with a specific Na+/K+-ATPase activity of up to 1800 mumol/mg per h at 37 degrees C. The influence of the detergent/protein and lipid/protein ratios on the yield of membrane bound enzyme is described.

Isolation and sequence of a cDNA clone encoding the 31-kDa subunit of bovine kidney vacuolar H+-ATPase

We have isolated a cDNA encoding the 31-kDa subunit of the bovine kidney vacuolar H+-ATPase. The composite sequence contains 1219 base pairs, which includes the entire 678-base-pair coding region. A lysine-rich sequence previously found in the Na+, K+-ATPase alpha subunit and the H+,K+-ATPase was identified in the 31-kDa subunit. An RNA blot and an immunoblot demonstrated variable 31-kDa subunit expression and immunoreactivity in different tissues; the highest levels were observed in kidney medulla and brain with both types of analysis. The isolation of a cDNA for the 31-kDa subunit is an important step in understanding this subunit's role in the function and regulation of the vacuolar H+-ATPase.

All three potential N-glycosylation sites of the dog kidney (Na+ + K+)-ATPase beta-subunit contain oligosaccharide

The beta-subunit of dog kidney (Na+ + K+)-ATPase is a sialoglycoprotein and contains three potential N-glycosylation sites. In this study, the oligosaccharide chains of purified dog kidney beta-subunit were labeled with tritium by oxidation with sodium periodate or galactose oxidase followed by NaB3H4 reduction. The beta-subunit was extensively digested by trypsin and the radioactive peptides were purified by HPLC. The enzyme, glycopeptidase A, which catalyzes the removal of N-linked oligosaccharide chains and the conversion of the glycosylated Asn residue to Asp, was used to demonstrate that a number of purified beta-subunit tryptic peptides were glycosylated. Amino-acid analysis of these beta-subunit peptides following glycopeptidase-A treatment revealed the expected Asn to Asp conversion for Asn-157, Asn-192 and Asn-264, demonstrating that all three potential N-glycosylation sites of the dog kidney beta-subunit are glycosylated. In addition, amino-acid sequence data suggest that a disulfide bond exists between Cys-158 and Cys-174.

Chromosomal localization of human Na+, K+-ATPase alpha- and beta-subunit genes

Na+, K+-ATPase is a heterodimeric enzyme responsible for the active maintenance of sodium and potassium gradients across the plasma membrane. Recently, cDNAs for several tissue-specific isoforms of the larger catalytic alpha-subunit and the smaller beta-subunit have been cloned. We have hybridized rat brain and human kidney cDNA probes, as well as human genomic isoform-specific DNA fragments, to Southern filters containing panels of rodent X human somatic cell hybrid lines. The results obtained have allowed us to assign the loci for the ubiquitously expressed alpha-chain (ATP1A1) to human chromosome 1, region 1p21----cen, and for the alpha 2 isoform that predominates in neural and muscle tissues (ATP1A2) to chromosome 1, region cen----q32. A common PstI RFLP was detected with the ATP1A2 probe. The alpha 3 gene, which is expressed primarily in neural tissues (ATP1A3), was assigned to human chromosome 19. A fourth alpha gene of unknown function (alpha D) that was isolated by molecular cloning (ATP1AL1) was mapped to chromosome 13. Although evidence to date had suggested a single gene for the beta-subunit, we found hybridizing restriction fragments derived from two different human chromosomes. On the basis of knowledge of conserved linkage groups on human and murine chromosomes, we propose that the coding gene ATP 1B is located on the long arm of human chromosome 1 and that the sequence on human chromosome 4 (ATP 1BL1) is either a related gene or a pseudogene.

Topology of Na+,K+-ATPase. Identification of the extra- and intracellular hydrophilic loops of the catalytic subunit by specific antibodies

To study the topology of Na+,K+-ATPase monoclonal antibodies (MAbs) specific for membrane-bound enzyme were produced. Using immunofluorescence staining of viable cells or smears of a pig kidney embryonic (PKE) cell line, two groups of MAbs were selected, namely those binding to extra- or intracellular portions of the alpha-subunit. The extracellular location of peptide loop 804-841 linking the Vth and VIth intramembrane hydrophobic segments was proved using MAb VG2. Another MAb, IIC9, interacting with PKE cells only after membrane perforation (4% formaldehyde and 0.1% Tween-20), was shown to bind to the hydrophilic loop 868-945. The antigenic determinants recognized by MAb IIC9 and VG2 are located in peptides 887-904 and 810-825, respectively. The C-terminus of the alpha-subunit molecule was positioned on the outer side of the cytoplasmic membrane utilizing affinity-purified antibodies to the synthetic peptide corresponding to fragment 999-1008.

Differentiated analysis of the secondary structure of hydrophilic and hydrophobic regions in alpha- and beta-subunits of Na+,K+-ATPase by Raman spectroscopy

Raman spectra of active Na+,K+-ATPase from pig kidney and membrane-bound products of its two-stage trypsinolysis, including alpha-subunit hydrophobic regions as well as the intact beta-subunit and hydrophobic regions of alpha- and beta-subunits, were measured to calculate the secondary structure of hydrophilic and hydrophobic regions of the enzyme. Consequent comparison demonstrated unambiguously that (i) membrane-bound hydrophobic parts of polypeptide chains of Na+,K+-ATPase subunits are in the alpha-helical conformation; (ii) essential contents of the alpha-helix as well as beta-sheet are estimated to form the hydrophilic (mainly cytoplasmic) domain of the Na+,K+-ATPase alpha-subunit; (iii) the exoplasmic hydrophilic domain of the beta-subunit is shown to include several antiparallel beta-pleated sheets and a small amount of the alpha-helix and unordered conformations. The model of the secondary structure organization of hydrophilic domains as well as 8 hydrophobic transmembrane segments of the enzyme molecule was proposed on the basis of experimental results and predictional calculations.

Expression of functional (Na+ + K+)-ATPase from cloned cDNAs

Functional (Na+ + K+)-ATPase is formed in Xenopus oocytes injected with alpha- and beta-subunit-specific mRNAs derived from cloned Torpedo californica cDNAs. Both the mRNAs are required for the expression of functional (Na+ + K+)-ATPase.

Cloning, sequence analysis, and expression of the large subunit of the human lymphocyte activation antigen 4F2

Among the earliest expressed antigens on the surface of activated human lymphocytes is the surface antigen 4F2. We have used DNA-mediated gene transfer and fluorescence-activated cell sorting to obtain cell lines that contain the gene encoding the large subunit of the human 4F2 antigen in a mouse L-cell background. Human DNAs cloned from these cell lines were subsequently used as hybridization probes to isolate a full-length cDNA clone expressing 4F2. Sequence analysis of the coding region has revealed an amino acid sequence of 529 residues. Hydrophobicity plotting has predicted a probable structure for the protein that includes an external carboxyl terminus, an internal leader sequence, a single hydrophobic transmembrane domain, and two possible membrane-associated domains. The 4F2 cDNA detects a single 1.8-kilobase mRNA in T-cell and B-cell lines. RNA gel blot analysis of RNA derived from quiescent and serum-stimulated Swiss 3T3 fibroblasts reveals a cell-cycle modulation of 4F2 gene expression: the mRNA is present in quiescent fibroblasts but increases 8-fold 24-36 hr after stimulation, at the time of maximal DNA synthesis.

Immunochemical studies of (Na+ + K+)-ATPase using site-specific, synthetic peptide directed antibodies

Rabbit antisera were raised against a series of synthetic peptides corresponding to regions of the alpha subunit of lamb kidney (Na+ + K+)-ATPase which chemical labeling studies and hydropathy plots of the amino-acid sequence suggest are exposed, accessible regions of the enzyme and may comprise the cation selectivity region, the ATP and cardiac glycoside binding sites, and the phosphorylation site. Five of six peptides tested (11-15 residues in length) were immunogenic and the antisera to four peptides recognized the intact, electroblotted (Western blot analysis) alpha subunit. Immunization with peptides conjugated to keyhole limpet haemocyanin (KLH) produced antipeptide antibodies for seven of nine conjugates. Antisera to four peptide conjugates recognized the native enzyme, confirming predictions that these sequence regions are exposed regions of the holoenzyme. In addition, a collection of four polyclonal antisera and five monoclonal antibodies raised to native holoenzyme were tested for their ability to bind to the peptide conjugates. In this way, two NH2-terminal sequence regions (1-12 and 16-30) and the putative ATP-binding site region (496-506) were identified as epitopes of the native enzyme. These results confirm some aspects of the transmembrane folding models proposed by Shull et al. and Kawakami et al. for the membrane-bound (Na+ + K+)-ATPase.

An evaluation of the molecular clock hypothesis using mammalian DNA sequences

A statistical analysis of extensive DNA sequence data from primates, rodents, and artiodactyls clearly indicates that no global molecular clock exists in mammals. Rates of nucleotide substitution in rodents are estimated to be four to eight times higher than those in higher primates and two to four times higher than those in artiodactyls. There is strong evidence for lower substitution rates in apes and humans than in monkeys, supporting the hominoid slowdown hypothesis. There is also evidence for lower rates in humans than in apes, suggesting a further rate slowdown in the human lineage after the separation of humans from apes. By contrast, substitution rates are nearly equal in mouse and rat. These results suggest that differences in generation time or, more precisely, in the number of germline DNA replications per year are the primary cause of rate differences in mammals. Further, these differences are more in line with the neutral mutation hypothesis than if the rates are the same for short- and long-living mammals.

The three-dimensional structure of the Na,K-ATPase from electron microscopy

The structure of Na,K-ATPase has been studied by electron microscopy and image reconstruction. A three-dimensional structure of this enzyme has been obtained to an overall resolution of 2.5 nm using data from specimens of negatively stained dimer sheets tilted through a range of angles +/- 60 degrees. The reconstruction shows a complex mass distribution consisting of ribbons of paired molecules extending approximately 6.0 nm from the cytoplasmic side of the membrane. The molecular envelope consists of a massive "body" with "lobe" and "arm" structures projecting from it. The body has a columnar shape and is tilted with respect to the plane of the membrane. The region of interaction responsible for dimer formation is located between two bodies and is clearly visible in the reconstruction. It has been identified as a segment in the amino-terminal portion of the alpha subunit. The arms that interconnect the ribbons are located close to the membrane and are most probably formed by the beta subunits.

Detailed structural analysis of exposed domains of membrane-bound Na+,K+-ATPase. A model of transmembrane arrangement

Exposed regions of the alpha- and beta-subunits of membrane-bound Na+,K+-ATPase were in turn hydrolyzed with trypsin. Resistance of the beta-subunit to proteolysis was shown to be due mainly to the presence of disulfide bridge(s) in the molecule. A model for the spatial organisation of the enzyme in the membrane was proposed on the basis of detailed structural analysis of extramembrane regions of both subunits.

Regulation by aldosterone of Na+,K+-ATPase mRNAs, protein synthesis, and sodium transport in cultured kidney cells

Transepithelial Na+ reabsorption across tight epithelia is regulated by aldosterone. Mineralocorticoids modulate the expression of a number of proteins. Na+,K+-ATPase has been identified as an aldosterone-induced protein (Geering, K., M. Girardet, C. Bron, J. P. Kraehenbuhl, and B. C. Rossier, 1982, J. Biol. Chem., 257:10338-10343). Using A6 cells (kidney of Xenopus laevis) grown on filters we demonstrated by Northern blot analysis that the induction of Na+,K+-ATPase was mainly mediated by a two- to fourfold accumulation of both alpha- and beta-subunit mRNAs. The specific competitor spironolactone decreased basal Na+ transport, Na+,K+-ATPase mRNA, and the relative rate of protein biosynthesis, and it blocked the response to aldosterone. Cycloheximide inhibited the aldosterone-dependent sodium transport but did not significantly affect the cytoplasmic accumulation of Na+,K+-ATPase mRNA induced by aldosterone.

Molecular cloning and sequence analysis of the (Na+ + K+)-ATPase beta subunit from dog kidney

cDNA complementary to mRNA coding for the beta subunit of dog renal (Na+ + K+)-ATPase has been cloned into lambda gt11 and the nucleotide sequence of the DNA has been determined. The amino acid sequence of the beta subunit polypeptide has also been deduced from the DNA. The mature form of the dog kidney beta subunit contains 302 amino acids with three potential asparagine-linked attachment sites for carbohydrate. The initiation methionine is removed during processing of the polypeptide to its mature form. Although the beta subunit is an integral membrane protein there is no signal sequence for the polypeptide, and hydropathy analysis predicts that the beta subunit polypeptide spans the cell membrane only once. Secondary structure predictions and a model for the structure of the beta subunit are proposed. DNA sequencing of the 5' non-coding region of the mRNA revealed a 200 bp inverted repeat from the coding region. Blot hybridization of a fragment of the beta subunit cDNA identified a single mRNA species of 2.7 kb in dog kidney and several rat tissues. RNA from rat liver was deficient in mRNA that hybridized to the dog kidney beta subunit cDNA, although mRNA that hybridized to an alpha subunit cDNA was detected. RNA from a human hepatoma cell line, HepG2, however, contained comparable levels of mRNA for both the alpha and the beta subunits.

Pretranslational regulation of Na-K-ATPase in cultured canine kidney cells by low K+

Long-term upregulation of the sodium pump [Na-K-adenosine triphosphatase (Na-K-ATPase)] entails an increase in the number of enzyme molecules. We incubated Madin-Darby canine kidney (MDCK) cells in low K+ medium and studied the time course and magnitude of change in the relative abundance of the two Na-K-ATPase subunits (alpha and beta), in the synthesis rate of the subunits, and in the relative abundance of alpha- and beta-mRNA. When cells were incubated in medium containing 0.25 mM K+, intracellular Na+ increased from 25.2 +/- 0.9 (SE) mmol/l cell H2O to 69.8 +/- 9.6 at 4 h and 132 +/- 6 at 16 h. Cell K+ fell from 146 +/- 4 mmol/l cell H2O to 105 +/- 9 at 4 h and 42.3 +/- 4.7 at 16 h. The relative abundance of Na-K-ATPase subunits, measured with immunoblots of cell homogenates, increased such that after 24 h alpha was 1.71 +/- 0.33 and beta was 1.67 +/- 0.22 times control. After 8 h of K+ depletion, alpha-synthesis rate, measured by immunoprecipitation of pulse-labeled cells, increased to 2.30 +/- 0.50 and beta increased to 2.07 +/- 0.42 times control. The alpha- and beta-subunit mRNA abundance, measured by hybridizing alpha- and beta-cDNA probes to total RNA, increased within 30 min to 1.93 +/- 0.24 and 2.29 +/- 0.64 times control, respectively. We conclude that regulatory adjustments of Na-K-ATPase abundance involve an increase in translation after a rapid and coordinate increase in the concentrations of alpha- and beta-subunit mRNA.

Chick embryonic erythrocyte antigen--tissues sharing expression

There is an antigenic glycoprotein (Mr 48 kD) present on the surfaces of erythrocytes of embryonic and young chickens that cannot be detected on the circulating erythrocytes in adult birds. This antigen, generally defined by this differential expression, has been thought to be associated with the maturation of hematopoietic tissues. We now present evidence, based on the use of a monoclonal antibody, maEE1, and the characteristic pattern of this glycoprotein on two-dimensional (2D) gels, that this antigen, which we have named chickEE, is expressed in a number of other embryonic and adult tissues. Immunofluorescent labeling of cryosections and flow-cytometric analysis of cells labeled with maEE1 have revealed the presence of chickEE in the retina (present in all layers), in muscle tissues (present in the endomysium and within the vascular endothelium), in the liver (especially evident on the lateral surface of hepatocytes and within the sinusoids), on epithelia such as the gut and kidney tubule epithelium and within lymphoid organs (present on bursacytes, splenocytes, thymocytes and peripheral leukocytes, and again within the endothelium) of young and adult animals. The 2D gel patterns of chickEE derived from embryonic tissues (retina, hind limb, thymus and bursa) and the adult tissues (retina and spleen) are very similar to that of the embryonic erythrocyte. Thus, the extended reactivity of the monoclonal antibody to chickEE, maEE1, with additional tissues is, in at least the tissues examined, based on the presence of the chickEE glycoprotein and not on incidental cross-reactivity. The evidence presented in this paper for the widely-shared expression of chickEE antigen makes it necessary to reconsider the function of this component of the cell surface.

Tryptic and chymotryptic cleavage sites in sequence of alpha-subunit of (Na+ + K+)-ATPase from outer medulla of mammalian kidney

Localization of selective proteolytic splits in alpha-subunit of (Na+ + K+)-ATPase is important for understanding the mechanism of active Na+,K+-transport. Proteolytic fragments of alpha-subunit from pig kidney were purified by chromatography in NaDodSO4 on TSK 3000 SW columns. NH2-terminal amino acid sequences of fragments as determined in a gas phase sequenator were unambiguously located within the total sequence of alpha-subunit from sheep kidney (Shull, C.E., et al. (1985) Nature 316, 691-695) and pig kidney (Ovchinnikov, Y.A., et al. (1985) Proc. Acad. Sci. USSR 285, 1490-1495). The primary chymotryptic split in the E1-form is located between Leu-266 and Ala-267 while the tryptic cleavage site appears to be between Arg-262 and Ile-263 (Bond 3). Tryptic cleavage in the initial fast phase of inactivation of the E1-form is located between Lys-30 and Glu-31 (Bond 2). In the E2-form, primary tryptic cleavage is between Arg-438 and Ala-439 (Bond 1). Chymotryptic cleavage between Leu-266 and Ala-267 stabilizes the E1-form of the protein without affecting the sites for binding of cations or nucleotides. Titration of fluorescence responses demonstrates the importance of the NH2-terminal for E1-E2 transition. Protonation of His-13 facilitates transition from E1- to E2-forms of the protein. Removal of His-13 after cleavage of bond 2 can explain the increase in apparent affinity of the cleaved enzyme for Na+ and the shift in poise of E1-E2 equilibrium in direction of E1-forms. The NH2-terminal sequence in renal alpha-subunit is not conserved in alpha + from rat neurolemma or in alpha-subunit from Torpedo or brine shrimp. A regulatory function of the NH2-terminal part of the alpha-subunit may thus be a unique feature of the alpha-subunit in (Na+ + K+)-ATPase from mammalian kidney.