Solubilization of Na+,K+-ATPase

Overview on solubilization and lipid reconstitution of Na,K-ATPase: enzyme kinetic and biophysical characterization

Na,K-ATPase is a membrane protein which plays a vital role. It pumps Na⁺ and K⁺ ions across the cellular membranes using energy from ATP hydrolysis, and is responsible for maintaining the osmotic equilibrium and generating the membrane potential. Moreover, Na,K-ATPase has also been involved in cell signaling, interacting with partner proteins. Cardiotonic steroids bind specifically to Na,K-ATPase triggering a number of signaling pathways. Because of its importance, many efforts have been employed to study the structure and function of this protein. Difficulties associated with its removal from natural membranes and the concomitant search for appropriate replacement conditions to keep the protein in solution have presented a challenge that had to be overcome prior to carrying out biophysical and biochemical studies in vitro. In this review, we summarized all of the methods and techniques applied by our group in order to obtain information about Na,K-ATPase in respect to solubilization, reconstitution into mimetic system, influence of lipid composition, stability, oligomerization, and aggregation.

Correlation of structural and functional thermal stability of the integral membrane protein Na,K-ATPase

The membrane-bound cation-transporting P-type Na,K-ATPase isolated from pig kidney membranes is much more resistant towards thermal inactivation than the almost identical membrane-bound Na,K-ATPase isolated from shark rectal gland membranes. The loss of enzymatic activity is correlated well with changes in protein structure as determined using synchrotron radiation circular dichroism (SRCD) spectroscopy. The enzymatic activity is lost at a 12°C higher temperature for pig enzyme than for shark enzyme, and the major changes in protein secondary structure also occur at T(m)'s that are ~10-15°C higher for the pig than for the shark enzyme. The temperature optimum for the rate of hydrolysis of ATP is about 42°C for shark and about 57°C for pig, both of which are close to the temperatures for onset of thermal unfolding. These results suggest that the active site region may be amongst the earliest parts of the structure to unfold. Detergent-solubilized Na,K-ATPases from the two sources show the similar differences in thermal stability as the membrane-bound species, but inactivation occurs at a lower temperature for both, and may reflect the stabilizing effect of a bilayer versus a micellar environment.

Bulk properties of the lipid bilayer are not essential for the thermal stability of Na,K-ATPase from shark rectal gland or pig kidney

The thermal stability of Na,K-ATPase from pig kidney is markedly greater than that of Na,K-ATPase from shark salt glands. The role of the lipid bilayer is studied by solubilisation of the membrane-bound enzyme in the nonionic detergent octaethyleneglycoldodecylmonoether (C(12)E(8)), addition of excess dioleylphosphatidylcholine (DOPC) or palmitoyloleylphosphatidylcholine (POPC) and reconstitution of membranes by removal of detergent. At 54°C the reconstituted enzymatically active pig enzyme retains a high thermal stability, and reconstituted shark enzyme retains a low thermal stability, even with a 9-fold excess of DOPC. This result suggests that the origin of the difference in thermal stability is not related to bulk lipid properties of the native membranes.

Fermentable dietary fiber increases GLP-1 secretion and improves glucose homeostasis despite increased intestinal glucose transport capacity in healthy dogs

Ileal proglucagon gene expression and postprandial plasma concentrations of proglucagon-derived peptides are reported to change with the type and quantity of dietary fiber ingested by rats. Within the intestine, proglucagon encodes several proglucagon-derived peptides known to modulate intestinal absorption capacity and pancreatic insulin secretion. To determine whether the chronic ingestion of fermentable dietary fiber regulates the expression and synthesis of proglucagon-derived peptides in the distal intestine to modulate glucose homeostasis, the following study was conducted: 16 adult dogs (23 +/- 2 kg) were fed isoenergetic, isonitrogenous diets containing a mixture of high fermentable dietary fibers (HFF) or low fermentable (LFF) wood cellulose for 14 d in a randomized cross-over design. Food was withheld for 16 h before an oral glucose tolerance test was conducted supplying 2 g of glucose/kg body wt, and peripheral blood was collected via a hind-leg catheter at 0, 15, 30, 45, 60, 90 and 120 min for plasma glucose, insulin and glucagon-like peptide-1(7-36)NH2 (GLP-1) analyses. Intestinal samples were collected after the second dietary treatment. Ileal proglucagon mRNA, intestinal (GLP-1) concentrations and the integrated area under the curves (AUC) for plasma GLP-1 and insulin were greater and plasma glucose AUC was reduced when dogs were fed the HFF diet compared to the LFF diet (P < 0.05). Intestinal villi heights, brush border and basolateral glucose transporter protein abundance and jejunal transport capacities were significantly greater when dogs were fed the HFF diet than when fed the LFF diet. In conclusion, improvements in glucose homeostasis are observed in healthy dogs when they ingest fermentable fibers.

Functional regulation of reconstituted Na,K-ATPase by protein kinase A phosphorylation

Reconstituted Na+,K+-ATPase from either pig kidney or shark rectal glands was phosphorylated by cAMP dependent protein kinase, PKA. The stoichiometry was approximately 0.9 mol P(i)/mol alpha-subunit in the pig kidney enzyme and approximately 0.2 mol P(i)/mol alpha-subunit in the shark enzyme. In shark, Na+,K+-ATPase PKA phosphorylation increased the maximum hydrolytic activity for cytoplasmic Na+ activation and extracellular K+ activation without affecting the apparent K(m) values. In contrast, no significant functional effect after PKA phosphorylation was observed in pig kidney Na+,K+-ATPase.

Quantification of the Na+/K(+)-pump in solubilized tissue by the ouabain binding method coupled with high-performance gel chromatography

Membrane-bound Na+/K(+)-ATPase purified from dog kidney outer medulla was solubilized with octaethylene glycol n-dodecyl ether (C12E8) and incubated with [3H]ouabain in the presence of NaCl. ATP and MgCl2 for 10 min at 0 degrees C. The resulting enzyme was separated, by high-performance gel chromatography executed at 0.2 degrees C. Mainly into its (alpha beta)2-diprotomer and alpha beta-protomer, which both bound stoichiometrically to [3H]ouabain. The amounts of ouabain that bound to the tissue itself and its microsomes could be estimated in the same way, as [3H]ouabain was found to bind only to the diprotomer and protomer they possessed. The amounts of ouabain that bound to them in the solubilized state were at least 5-times higher than those that did so when they were non-solubilized, suggesting that the surfactant rendered the enzyme accessible to ouabain. When the solubilized tissue (138 mg ml-1 wet tissue) was reacted with ouabain in the presence of 0.1 M NaCl and 4.8 mM MgCl2 for 10 min at 0 degrees C, maximal ouabain binding was attained in the presence of 18.3 microM [3H]ouabain, 1.2 mM ATP and 3 to 5 mg ml-1 C12E8, which was common to the outer medulla and human colon cancer cells. The present method enabled the pump number in protein and tissue samples in the range 7.2 x 10(-9) (purified pump) to 1.5 x 10(-12) (cancer tissue) mol/mg protein to be estimated within 2 h.

Functional coupling of phosphorylation and nucleotide binding sites in the proteolytic fragments of Na+/K(+)-ATPase

Cleavage of the alpha-subunit of Na+/K(+)-ATPase by trypsin at Arg438-Ala439 causes enzyme inhibition which has been suggested to be due to altered alignment of phosphorylation site on the 48-kDa N-terminal fragment with nucleotide binding site on the 64-kDa C-terminal fragment. Our aims were to test this hypothesis and to assess the effect of the cleavage on the enzyme's two ATP sites. Na(+)-dependent phosphorylation of the partially cleaved enzyme by ATP showed that K0.5 values of ATP for phosphorylations of intact alpha and 48-kDa peptide were the same (0.4 microM). Unchanged interactions among the residues across the cleavage site were also indicated by data showing that reaction of fluorescein isothiocyanate with the 64-kDa peptide blocked phosphorylation of the 48-kDa peptide by ATP. ATP is known to block the reaction of fluorescein isothiocyanate with the enzyme. Experiments on the partially cleaved enzyme showed that K0.5 of ATP for protection of alpha was 30-60 microM, and the value for the protection of interacting 48-kDa and 64-kDa peptides was 1-3 mM. Evidently, while the cleavage does not affect the high affinity catalytic site, it disrupts the allosteric low affinity ATP site. Experiments on reconstituted preparations showed that the cleavage abolished ATP-dependent Na+/K+ exchange, Pi+ATP-dependent Rb+/Rb+ exchange, ATP-dependent Na+/Na+ exchange, and ADP+ATP-dependent Na+/Na+ exchange activities. Selective disruption of the low affinity ATP site accounts for the inhibitions of all functions involving K+(Rb+), based on the established role of this site in the control of K+ access channels. Cleavage-induced inhibitions of other activities, however, suggest additional roles of the low affinity ATP site in the reaction cycle.

Purification, reconstitution, and subunit composition of a voltage-gated chloride channel from Torpedo electroplax

The voltage-gated Cl- channel from Torpedo electroplax was purified in functional form by an immunoaffinity procedure. Channel activity was assayed by 36Cl- uptake into reconstituted liposomes and by direct recording after insertion into planar lipid bilayers. The purified channel displays the same "double-barreled" gating kinetics observed with native membranes, as well as the correct single-channel permeation characteristics. Preparations of active channels consist of a 90-kDa polypeptide, as expected from the known cDNA sequence. No associated subunits are present in the purified material. Direct protein sequencing confirms the absence of a cleavable signal sequence and demonstrates an N-terminus at Ser-2 of the cDNA-derived sequence. This "ClC-0" protein is lightly glycosylated, losing only approximately 2 kDa of sugar upon treatment with endoglycosidase H or N-glycanase. Most if not all of this glycosylation is found on Asn-365. This result necessitates revision of current transmembrane topology proposals, which have placed this residue on the cytoplasmic side of the membrane. Sedimentation in sucrose density gradients under activity-preserving conditions suggests the ClC-0 channel is slightly larger than the Na/K-ATPase alpha/beta-protomer (approximately equal to 150 kDa) and substantially smaller than the reduced form of the nicotinic acetylcholine receptor (approximately equal to 300 kDa). The detergent-solubilized ClC-0 channel, which invariably displays two Cl- diffusion pores in the active complex, is therefore built most likely as a homodimer of the 90-kDa protein purified here.

Rapid purification of the gastric H+/K(+)-ATPase complex by tomato-lectin affinity chromatography

We have previously shown that tomato lectin binds specifically to the 60-90 kDa membrane glycoprotein of parietal cell tubulovesicles, the beta-subunit of the gastric H+/K(+)-ATPase (proton pump) [Callaghan, Toh, Pettitt, Humphris & Gleeson (1990) J. Cell Sci. 95, 563-576; Toh, Gleeson, Simpson, Mortiz, Callaghan, Goldkorn, Jones, Martinelli, Mu, Humphris, Pettitt, Mori, Masuda, Sobieszczuk, Weinstock, Mantamadiotis & Baldwin (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6418-6422]. Here we have exploited this interaction for the development of a rapid single-step chromatography procedure for the purification of an active pig gastric proton pump complex. Initially, H+/K(+)-ATPase-enriched membranes, prepared from pig gastric microsomes by density-gradient centrifugation, were extracted in 1% Triton X-100 and passed through a 1 ml tomato lectin-Sepharose 4B column. The bound material, eluted with 20 mM-chitotriose, showed a major band with an apparent molecular mass of 95 kDa, and a faint broad band of 60-90 kDa, by SDS/PAGE. N-Glycanase treatment of the bound material resulted in the appearance of a 35 kDa band, the size of the protein core of the 60-90 kDa glycoprotein beta-subunit. The two components were identified as the 95 kDa alpha-subunit and the 60-90 kDa beta-subunit of the gastric H+/K(+)-ATPase, by immunoreactivity with monospecific antibodies, and by tryptic peptide sequences of the tomato-lectin-bound material. The beta-subunit was present in approximately equimolar amounts to the catalytic alpha-subunit. Whereas the gastric H+/K(+)-ATPase was not active after solubilization in 1% Triton X-100, solubilization of density-gradient-purified membranes in the non-ionic detergent, C12E8, followed by chromatography of the extract on tomato lectin-Sepharose 4B, resulted in the purification of the gastric H+/K(+)-ATPase complex which exhibited K(+)-dependent phosphatase activity. This is the first report of a rapid purification of a partially active solubilized gastric H+/K(+)-ATPase complex.

Ankyrin links fodrin to the alpha subunit of Na,K-ATPase in Madin-Darby canine kidney cells and in intact renal tubule cells

In nonerythroid cells the distribution of the cortical membrane skeleton composed of fodrin (spectrin), actin, and other proteins varies both temporally with cell development and spatially within the cell and on the membrane. In monolayers of Madin-Darby canine kidney (MDCK) cells, it has previously been shown that fodrin and Na,K-ATPase are codistributed asymmetrically at the basolateral margins of the cell, and that the distribution of fodrin appears to be regulated posttranslationally when confluence is achieved (Nelson, W. J., and P. I. Veshnock. 1987. J. Cell Biol. 104:1527-1537). The molecular mechanisms underlying these changes are poorly understood. We find that (a) in confluent MDCK cells and intact kidney proximal tubule cells, Na,K-ATPase, fodrin, and analogues of human erythrocyte ankyrin are precisely colocalized in the basolateral domain at the ultrastructural level. (b) This colocalization is only achieved in MDCK cells after confluence is attained. (c) Erythrocyte ankyrin binds saturably to Na,K-ATPase in a molar ratio of approximately 1 ankyrin to 4 Na,K-ATPase's, with a kD of 2.6 microM. (d) The binding of ankyrin to Na,K-ATPase is inhibited by the 43-kD cytoplasmic domain of erythrocyte band 3. (e) 125I-labeled ankyrin binds to the alpha subunit of Na,K-ATPase in vitro. There also appears to be a second minor membrane protein of approximately 240 kD that is associated with both erythrocyte and kidney membranes that binds 125I-labeled ankyrin avidly. The precise identity of this component is unknown. These results identify a molecular mechanism in the renal epithelial cell that may account for the polarized distribution of the fodrin-based cortical cytoskeleton.