Residues of the fourth transmembrane segments of the Na,K-ATPase and the gastric H,K-ATPase contribute to cation selectivity

Interplay of potassium channel, gastric parietal cell and proton pump in gastrointestinal physiology, pathology and pharmacology

Gastric acid secretion plays a pivotal role in the physiology of gastrointestinal tract. The functioning of the system encompasses a P2 ATPase pump (which shuttles electroneutral function at low pH) along with different voltage sensitive/neutral ion channels, cytosolic proteins, acid sensor receptors as well hormonal regulators. The increased acid secretion is a pathological marker of several diseases like peptic ulcer, gastroesophageal reflux disease (GERD), chronic gastritis, and the bug Helicobacter pylori (H. pylori) has also a critical role, which altogether affects the patient's quality of life. This review comprehensively describes about the nature of potassium ion channel and its mediators, the different clinical strategy to control acid rebound, and some basic experimental observations performed to study the interplay of ion channels, pumps, as well as mediators during acid secretion. Different aspects of regulation of gastric acid secretion have been focused either in terms of physiology of secretion or molecular interactions. The importance of H pylori infection and its treatment have also been discussed. Furthermore, the relevance of calcium signaling during acid secretion has been reviewed. The entire theme will make anyone to understand in details about the gastric secretion machinery in general.

Preactivation of AMPK by metformin may ameliorate the epithelial cell damage caused by renal ischemia

Alterations in epithelial cell polarity and in the subcellular distributions of epithelial ion transport proteins are key molecular consequences of acute kidney injury and intracellular energy depletion. AMP-activated protein kinase (AMPK), a cellular energy sensor, is rapidly activated in response to renal ischemia, and we demonstrate that its activity is upregulated by energy depletion in Madin-Darby canine kidney (MDCK) cells. We hypothesized that AMPK activity may influence the maintenance or recovery of epithelial cell organization in mammalian renal epithelial cells subjected to energy depletion. MDCK cells were ATP depleted through a 1-h incubation with antimycin A and 2-deoxyglucose. Immunofluoresence localization demonstrated that this regimen induces mislocalization of the Na-K-ATPase from its normal residence at the basolateral plasma membrane to intracellular vesicular compartments. When cells were pretreated with the AMPK activator metformin before energy depletion, basolateral localization of Na-K-ATPase was preserved. In MDCK cells in which AMPK expression was stably knocked down with short hairpin RNA, preactivation of AMPK with metformin did not prevent Na-K-ATPase redistribution in response to energy depletion. In vivo studies demonstrate that metformin activated renal AMPK and that treatment with metformin before renal ischemia preserved cellular integrity, preserved Na-K-ATPase localization, and led to reduced levels of neutrophil gelatinase-associated lipocalin, a biomarker of tubular injury. Thus AMPK may play a role in preserving the functional integrity of epithelial plasma membrane domains in the face of energy depletion. Furthermore, pretreatment with an AMPK activator before ischemia may attenuate the severity of renal tubular injury in the context of acute kidney injury.

H,K-ATPase protein localization and Kir4.1 function reveal concordance of three axes during early determination of left-right asymmetry

Consistent laterality is a fascinating problem, and study of the Xenopus embryo has led to molecular characterization of extremely early steps in left-right patterning: bioelectrical signals produced by ion pumps functioning upstream of asymmetric gene expression. Here, we reveal a number of novel aspects of the H+/K+-ATPase module in chick and frog embryos. Maternal H+/K+-ATPase subunits are asymmetrically localized along the left-right, dorso-ventral, and animal-vegetal axes during the first cleavage stages, in a process dependent on cytoskeletal organization. Using a reporter domain fused to molecular motors, we show that the cytoskeleton of the early frog embryo can provide asymmetric, directional information for subcellular transport along all three axes. Moreover, we show that the Kir4.1 potassium channel, while symmetrically expressed in a dynamic fashion during early cleavages, is required for normal LR asymmetry of frog embryos. Thus, Kir4.1 is an ideal candidate for the K+ ion exit path needed to allow the electroneutral H+/K+-ATPase to generate voltage gradients. In the chick embryo, we show that H+/K+-ATPase and Kir4.1 are expressed in the primitive streak, and that the known requirement for H+/K+-ATPase function in chick asymmetry does not function through effects on the circumferential expression pattern of Connexin43. These data provide details crucial for the mechanistic modeling of the physiological events linking subcellular processes to large-scale patterning and suggest a model where the early cytoskeleton sets up asymmetric ion flux along the left-right axis as a system of planar polarity functioning orthogonal to the apical-basal polarity of the early blastomeres.

The renal H+, K+-ATPases as therapeutic targets

The kidney is an important regulatory organ responsible for maintaining constant blood volume and composition despite wide variations in the intake of food and water. Throughout the nephron, the functional unit of the kidney, there is a wide variety of proteins that function to add additional waste products and to recover needed materials from the lumen filtrate. The collecting duct of the nephron is the primary renal location for the H+, K+-ATPases, a group of ion pumps that function in both acid/base balance and potassium homeostasis. This review summarizes the present understanding of the structure and functions for the different subtypes of the H+, K+-ATPases under specific physiologic conditions. The obstacles in determining the pharmacologic properties of the different subtypes are considered and future directions for the inhibition and/or stimulation of the H+, K+-ATPases are evaluated.

The future of the pump

Since it was discovered 3 decades ago the H,K-ATPase has come to be recognized as the key both to the generation and pharmacologic suppression of gastric acid secretion. Although 30 years of concerted research has answered many questions, it is perhaps not surprising that these efforts have raised many new and crucial issues that await elucidation. These can be divided into 5 broad categories that relate to structure, mechanism, regulation, trafficking, and macromolecular interactions. It is probably safe to predict that the growing sophistication of x-ray crystallographic techniques will yield a picture of the pump's molecular structure in the near future. These insights will, in turn, illuminate the details of the process through which enzymatic hydrolysis is coupled to ion translocation with unprecedented clarity. The gastric parietal cell employs an extremely complicated system of receptors, kinases, and second messengers to maintain tight control over pump function. Upon activation, this cell also performs a massive and elegant membrane trafficking transformation that plays a critical role in the regulatory process. Finally, it is becoming clear that every ion transport protein is a component in a large macromolecular complex whose constituents help to determine all of the transport system's fundamental physiologic properties. These are the major topics that will drive H,K pump research in the future, and it is likely that their resolution will create the foundations for the next generation of therapies aimed at controlling gastric acid secretion and its clinical consequences.

SGK1 activates Na+-K+-ATPase in amphibian renal epithelial cells

Serum- and glucocorticoid-induced kinase 1 (SGK1) is thought to be an important regulator of Na+reabsorption in the kidney. It has been proposed that SGK1 mediates the effects of aldosterone on transepithelial Na+transport. Previous studies have shown that SGK1 increases Na+transport and epithelial Na+channel (ENaC) activity in the apical membrane of renal epithelial cells. SGK1 has also been implicated in the modulation of Na+-K+-ATPase activity, the transporter responsible for basolateral Na+efflux, although this observation has not been confirmed in renal epithelial cells. We examined Na+-K+-ATPase function in an A6 renal epithelial cell line that expresses SGK1 under the control of a tetracycline-inducible promoter. The results showed that expression of a constitutively active mutant of SGK1 (SGK1TS425D) increased the transport activity of Na+-K+-ATPase 2.5-fold. The increase in activity was a direct consequence of activation of the pump itself. The onset of Na+-K+-ATPase activation was observed between 6 and 24 h after induction of SGK1 expression, a delay that is significantly longer than that required for activation of ENaC in the same cell line (1 h). SGK1 and aldosterone stimulated the Na+pump synergistically, indicating that the pathways mediated by these molecules operate independently. This observation was confirmed by demonstrating that aldosterone, but not SGK1TS425D, induced an ∼2.5-fold increase in total protein and plasma membrane Na+-K+-ATPase α1-subunit abundance. We conclude that aldosterone increases the abundance of Na+-K+-ATPase, whereas SGK1 may activate existing pumps in the membrane in response to chronic or slowly acting stimuli.

Proton binding sites and conformational analysis of H+K(+)-ATPase

It is proposed that the hydronium ion, H3O+, binds to the E1 conformation of the alpha-subunit of gastric proton pump. The H3O+ binding cavities are characterized parametrically based on valence, sequence, geometry, and size considerations from comparative modeling. The cavities have scope for accommodating monovalent cations of different ionic radii. The H3O+ transport is proposed to be aided by arenes which are arranged regularly along the pump starting from N-domain through the transmembrane region. Step-by-step structural changes accompanying H3O+ occlusion are studied in detail. The observations corroborate well with earlier experimental studies.

Reconstruction of the Complete Ouabain-binding Pocket of Na,K-ATPase in Gastric H,K-ATPase by Substitution of Only Seven Amino Acids

Although cardiac glycosides have been used as drugs for more than 2 centuries and their primary target, the sodium pump (Na,K-ATPase), has already been known for 4 decades, their exact binding site is still elusive. In our efforts to define the molecular basis of digitalis glycosides binding we started from the fact that a closely related enzyme, the gastric H,K-ATPase, does not bind glycosides like ouabain. Previously, we showed that a chimera of these two enzymes, in which only the M3-M4 and M5-M6 hairpins were of Na,K-ATPase, bound ouabain with high affinity (Koenderink, J. B., Hermsen, H. P. H., Swarts, H. G. P., Willems, P. H. G. M., and De Pont, J. J. H. H. M. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 11209-11214). We also demonstrated that only three amino acids (Phe(783), Thr(797), and Asp(804)) present in the M5-M6 hairpin of Na,K-ATPase were sufficient to confer high affinity ouabain binding to a chimera which contained in addition the M3-M4 hairpin of Na,K-ATPase (Qiu, L. Y., Koenderink, J. B., Swarts, H. G., Willems, P. H., and De Pont, J. J. H. H. M. (2003) J. Biol. Chem. 278, 47240-47244). To further pinpoint the ouabain-binding site here we used a chimera-based loss-of-function strategy and identified four amino acids (Glu(312), Val(314), Ile(315), Gly(319)), all present in M4, as being important for ouabain binding. In a final gain-of-function study we showed that a gastric H,K-ATPase that contained Glu(312), Val(314), Ile(315), Gly(319), Phe(783), Thr(797), and Asp(804) of Na,K-ATPase bound ouabain with the same affinity as the native enzyme. Based on the E(2)P crystal structure of Ca(2+)-ATPase we constructed a homology model for the ouabain-binding site of Na,K-ATPase involving all seven amino acids as well as several earlier postulated amino acids.

The C-terminal tail of the polycystin-1 protein interacts with the Na,K-ATPase alpha-subunit

Polycystin-1 (PC-1) is the product of the PKD1 gene, which is mutated in autosomal dominant polycystic kidney disease. We show that the Na,K-ATPase alpha-subunit interacts in vitro and in vivo with the final 200 amino acids of the polycystin-1 protein, which constitute its cytoplasmic C-terminal tail. Functional studies suggest that this association may play a role in the regulation of the Na,K-ATPase activity. Chinese hamster ovary cells stably expressing the entire PC-1 protein exhibit a dramatic increase in Na,K-ATPase activity, although the kinetic properties of the enzyme remain unchanged. These data indicate that polycystin-1 may contribute to the regulation of Na,K-ATPase activity in kidneys in situ, thus modulating renal tubular fluid and electrolyte transport.

Involvement in K+ access of Leu318 at the extracellular domain flanking M3 and M4 of the Na+,K+-ATPase α-subunit

The effect of point mutation in the sequence 316TWLE319, which occurs in the extracellular loop flanking the third (M3) and the fourth (M4) transmembrane segment (L3/4) of the Na+,K+-ATPase alpha-subunit, was examined. Mutation of Glu319 to Asp yielded an enzyme with full activity, whereas substituting Glu319 to Ala resulted in a severe loss of activity. A negative charge was introduced along the sequence, one residue at a time, from Thr316 to Leu318 (by E-scanning) in the mutant construct with Glu319 already mutated to Gln. The activity that had been reduced to 60% by the mutation of Glu319 to Gln was restored upon the introduction of a negative charge by E-scanning. When Leu318 was replaced by Glu in a series of scanning experiments, the K+ sensitivity of the ATPase activity was lowered. The lowering of K+ sensitivity was further demonstrated when a mutation of Leu318 to Glu was introduced into the wild-type enzyme. Furthermore, mutants with Leu318 to Gln, Arg, and Phe displayed lower K+ sensitivity similar to that of Leu318 to Glu mutant. Leu318 may be in access path for K+, and any substitution at this position may interfere with access of K+ from outside the cell.

Thr-774 (transmembrane segment M5), Val-920 (M8), and Glu-954 (M9) are involved in Na+ transport, and Gln-923 (M8) is essential for Na,K-ATPase activity

The highly conserved amino acids of rat Na,K-ATPase, Thr-774 in the transmembrane helices M5, Val-920 and Gln-923 in M8, and Glu-953 and Glu-954 in M9, the side chains of which appear to be in close proximity, were mutated, and the resulting proteins, T774A, E953A/K, and E954A/K, V920E and Q923N/E/D/L, were expressed in HeLa cells. Ouabain-resistant cell lines were obtained from T774A, V920E, E953A, and E954A, whereas Q923N/E/D/L, E953K, and E954K could only be transiently expressed as fusion proteins with an enhanced green fluorescent protein. The apparent K0.5 values for Na+, as estimated by the Na+-dependent phosphoenzyme formation (K0.5(Na,EP)) or Na,K-ATPase activity (K(0.5)(Na,ATPase)), were increased by around 2 approximately 8-fold in the case of T774A, V920E, and E954A. The apparent K0.5 values for K+, as estimated by the Na,K-ATPase (K0.5(K,ATPase)) or p-nitrophenylphosphatase activity (K0.5(K,pNPPase)), were affected only slightly by the 3 mutations, except that V920E showed a 1.7-fold increase in the K0.5(K,ATPase). The apparent K0.5 values for ATP (K0.5(EP)), as estimated by phosphorylation (a high affinity ATP effect), were increased by 1.6 approximately 2.6-fold in the case of T774A, V920E, and E954A. Those estimated by Na,K-ATPase activity (K0.5(ATPase)) and ATP-induced inhibition (K(i,0.5)(pNPPase)) of K-pNPPase activity (low affinity ATP effects) were, respectively, increased by 1.8-fold and unchanged in the case of T774A but decreased by 2- and 4.8-fold in the case of V920E and were slightly changed and increased by 1.7-fold in the case of E954A. The E953A showed little significant change in the apparent affinities. These results suggest that Gln-923 in M8 is crucial for the active transport of Na+ and/or K+ across membranes and that the side chain oxygen atom of Thr-774 in M5, the methyl group(s) of Val-920 in M8, and the carboxyl oxygen(s) of Glu-954 in M9 mainly play some role in the transport of Na+ and also in the high and low affinity ATP effects rather than the transport of K+.

Residues within transmembrane domains 4 and 6 of the Na,K-ATPase alpha subunit are important for Na+ selectivity

The Na,K- and H,K-ATPases are plasma membrane enzymes responsible for the active exchange of extracellular K(+) for cytoplasmic Na(+) or H(+), respectively. At present, the structural determinants for the specific function of these ATPases remain poorly understood. To investigate the cation selectivity of these ATPases, we constructed a series of Na,K-ATPase mutants in which residues in the membrane spanning segments of the alpha subunit were changed to the corresponding residues common to gastric H,K-ATPases. Thus, mutants were created with substitutions in transmembrane domains TM1, TM4, TM5, TM6, TM7, and TM8 independently or together (designated TMAll). The function of each mutant was assessed after coexpression with the beta subunit in Sf-9 cells using baculoviruses. The enzymatic properties of TM1, TM7, and TM8 mutants were similar to the wild-type Na,K-ATPase, and while TM5 showed modest changes in apparent affinity for Na(+), TM4, TM6, and TMAll displayed an abnormal activity. This resulted in a Na(+)-independent hydrolysis of ATP, a 2-fold higher K(0.5) for Na(+) activation, and the ability to function at low pH. These results suggest a loss of discrimination for Na(+) over H(+) for the enzymes. In addition, TM4, TM6, and TMAll mutants exhibited a 1.5-fold lower affinity for K(+) and a 4-5-fold decreased sensitivity to vanadate. Altogether, these results provide evidence that residues in transmembrane domains 4 and 6 of the alpha subunit of the Na,K-ATPase play an important role in determining the specific cation selectivity of the enzyme and also its E1/E2 conformational equilibrium.

Structural and conformational changes concomitant with the E1–E2 transition in H+K+-ATPase: a comparative protein modeling study

Comparative modeling studies on conserved regions of the gastric H(+)K(+)-ATPase reveal that the E1-E2 conformational transition induces significant tertiary structural changes while conserving the secondary structure. The residues 516-530 of the cytoplasmic domain and TM10 within the transmembrane (TM) regions undergo maximum tertiary structural changes. The luminal regions exhibit comparatively lesser tertiary structural deviations. Residues 249-304 show maximum secondary structural deviation in the conformational transition. The Cys-815 and Cys-323 residues involved in inhibitor binding are found to have smaller buried side chain areas in the E1 conformation compared to E2. Retention of activity correlates well with the buried side chain area when selected amino acid residues in TM6 are mutated using modeling techniques with bulkier amino acid residues. Conformational specificity for ion binding is corroborated with the fraction of side chains exposed to polar atoms of the residues E345, D826, V340, A341, V343, and E822.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

The fourth transmembrane segment of the Na,K-ATPase alpha subunit: a systematic mutagenesis study

The Na,K-ATPase is a major ion-motive ATPase of the P-type family responsible for many aspects of cellular homeostasis. To determine the structure of the pathway for cations across the transmembrane portion of the Na,K-ATPase, we mutated 24 residues of the fourth transmembrane segment into cysteine and studied their function and accessibility by exposure to the sulfhydryl reagent 2-aminoethyl-methanethiosulfonate. Accessibility was also examined after treatment with palytoxin, which transforms the Na,K-pump into a cation channel. Of the 24 tested cysteine mutants, seven had no or a much reduced transport function. In particular cysteine mutants of the highly conserved "PEG" motif had a strongly reduced activity. However, most of the non-functional mutants could still be transformed by palytoxin as well as all of the functional mutants. Accessibility, determined as a 2-aminoethyl-methanethiosulfonate-induced reduction of the transport activity or as inhibition of the membrane conductance after palytoxin treatment, was observed for the following positions: Phe(323), Ile(322), Gly(326), Ala(330), Pro(333), Glu(334), and Gly(335). In accordance with a structural model of the Na,K-ATPase obtained by homology modeling with the two published structures of sarcoplasmic and endoplasmic reticulum calcium ATPase (Protein Data Bank codes 1EUL and 1IWO), the results suggest the presence of a cation pathway along the side of the fourth transmembrane segment that faces the space between transmembrane segments 5 and 6. The phenylalanine residue in position 323 has a critical position at the outer mouth of the cation pathway. The residues thought to form the cation binding site II ((333)PEGL) are also part of the accessible wall of the cation pathway opened by palytoxin through the Na,K-pump.

Do H+ ions obscure electrogenic Na+ and K+ binding in the E1 state of the Na,K-ATPase?

In contrast to other P-type ATPases, the Na,K-ATPase binding and release of ions on the cytoplasmic side, to the state called E1, is not electrogenic with the exception of the third Na+. Since the high-resolution structure of the closely related SR Ca-ATPase in state E1 revealed the ion-binding sites deep inside the transmembrane part of the protein, the missing electrogenicity in state E1 can be explained by an obscuring counter-movement of H+ ions. Evidence for such a mechanism is presented by analysis of pH effects on Na+ and K+ binding and by electrogenic H+ movements in the E1 conformation of the Na,K-ATPase.

Structural and functional insights of Wilson disease copper-transporting ATPase

Wilson disease is an autosomal recessive disorder of copper metabolism. The gene for this disorder has been cloned and identified to encode a copper-transporting ATPase (ATP7B), a member of a large family of cation transporters, the P-type ATPases. In addition to the core elements common to all P-type ATPases, the Wilson copper-transporting ATPase has a large cytoplasmic N-terminus comprised six heavy metal associated (HMA) domains, each of which contains the copper-binding sequence motif GMT/HCXXC. Extensive studies addressing the functional, regulatory, and structural aspects of heavy metal transport by heavy metal transporters in general, have offered great insights into copper transport by Wilson copper-transporting ATPase. The findings from these studies have been used together with homology modeling of the Wilson disease copper-transporting ATPases based on the X-ray structure of the sarcoplasmic reticulum (SR) calcium-ATPase, to present a hypothetical model of the mechanism of copper transport by copper-transporting ATPases.

Extracellular domains, transmembrane segments, and intracellular domains interact to determine the cation selectivity of Na,K- and gastric H,K-ATPase

We have previously reported that three residues of the fourth transmembrane segment (TM4) of the Na,K- and gastric H,K-ATPase alpha-subunits appear to play a major role in the distinct cation selectivities of these pumps [Mense, M., et al. (2000) J. Biol. Chem. 275, 1749-1756]. Substituting these three residues in the Na,K-ATPase sequence with their H,K-ATPase counterparts (L319F, N326Y, T340S) and replacing the TM3-TM4 ectodomain sequence with that of the H,K-ATPase alpha-subunit result in a pump that exhibits 50% of its maximal ATPase activity in the absence of Na(+) when the assay is performed at pH 6.0. This effect is not seen when the ectodomain alone is replaced. To gain more insight into the contributions of the three residues to establishing the selectivity of these pumps for Na(+) ions versus protons, we generated Na,K-ATPase constructs in which these residues are replaced by their H,K-ATPase counterparts either singly or in combinations. Surprisingly, none of the point mutants nor even the triple mutant was able to hydrolyze ATP at pH 6.0 at a rate greater than 20% of their respective V(max)s. For the point mutants L319F and N326Y, protons seem to competitively inhibit ATP hydrolysis at pH 6.0, based on the low apparent affinity for Na(+) ions at pH 6.0 compared to pH 7.5. It would appear, therefore, that the cation selectivity of Na,K- and H,K-ATPase is generated through a cooperative effort between residues of transmembrane segments and the flanking loops that connect these transmembrane domains. This view is further supported by homology modeling of the Na,K-ATPase based on the crystal structure of the SERCA pump.

Alanine-scanning mutagenesis of the sixth transmembrane segment of gastric H+,K+-ATPase alpha-subunit

The sixth transmembrane (M6) segment of the catalytic subunit plays an important role in the ion recognition and transport in the type II P-type ATPase families. In this study, we singly mutated all amino acid residues in the M6 segment of gastric H(+),K(+)-ATPase alpha-subunit with alanine, expressed the mutants in HEK-293 cells, and studied the effects of the mutation on the functions of H(+),K(+)-ATPase; overall K(+)-stimulated ATPase, phosphorylation, and dephosphorylation. Four mutants, L819A, D826A, I827A, and L833A, completely lost the K(+)-ATPase activity. Mutant L819A was phosphorylated but hardly dephosphorylated in the presence of K(+), whereas mutants D826A, I827A, and L833A were not phosphorylated from ATP. We found that almost all of these amino acid residues, which are important for the function, are located on the same side of the alpha-helix of the M6 segment. In addition, we found that amino acids involved in the phosphorylation are located exclusively in the cytoplasmic half of the M6 segment and those involved in the K(+)-dependent dephosphorylation are in the luminal half. Several mutants such as I821A, L823A, T825A, and P829A partly retained the K(+)-ATPase activity accompanying the decrease in the rate of phosphorylation.

Ion pumps in polarized cells: sorting and regulation of the Na+, K+- and H+, K+-ATPases

The physiologic function of an ion transport protein is determined, in part, by its subcellular localization and by the cellular mechanisms that modulate its activity. The Na(+),K(+)-ATPase and the H(+),K(+)-ATPases are closely related members of the P-type family of ion transporting ATPases. Despite their homology, these pumps are sorted to different domains in polarized epithelial cells, and their enzymatic activities are subject to distinct regulatory pathways. The molecular signals responsible for these properties have begun to be elucidated. It appears that a complex array of inter- and intramolecular interactions govern trafficking, distribution, and catalytic capacities of these proteins.

Chimeras of X+, K+-ATPases. The M1-M6 region of Na+, K+-ATPase is required for Na+-activated ATPase activity, whereas the M7-M10 region of H+, K+-ATPase is involved in K+ de-occlusion

In this study we reveal regions of Na(+),K(+)-ATPase and H(+),K(+)-ATPase that are involved in cation selectivity. A chimeric enzyme in which transmembrane hairpin M5-M6 of H(+),K(+)-ATPase was replaced by that of Na(+),K(+)-ATPase was phosphorylated in the absence of Na(+) and showed no K(+)-dependent reactions. Next, the part originating from Na(+),K(+)-ATPase was gradually increased in the N-terminal direction. We demonstrate that chimera HN16, containing the transmembrane segments one to six and intermediate loops of Na(+),K(+)-ATPase, harbors the amino acids responsible for Na(+) specificity. Compared with Na(+),K(+)-ATPase, this chimera displayed a similar apparent Na(+) affinity, a lower apparent K(+) affinity, a higher apparent ATP affinity, and a lower apparent vanadate affinity in the ATPase reaction. This indicates that the E(2)K form of this chimera is less stable than that of Na(+),K(+)-ATPase, suggesting that it, like H(+),K(+)-ATPase, de-occludes K(+) ions very rapidly. Comparison of the structures of these chimeras with those of the parent enzymes suggests that the C-terminal 187 amino acids and the beta-subunit are involved in K(+) occlusion. Accordingly, chimera HN16 is not only a chimeric enzyme in structure, but also in function. On one hand it possesses the Na(+)-stimulated ATPase reaction of Na(+),K(+)-ATPase, while on the other hand it has the K(+) occlusion properties of H(+),K(+)-ATPase.

Amino acids involved in sodium interaction of murine type II Na(+)-P(i) cotransporters expressed in Xenopus oocytes

Type IIa and IIb Na+-Pi cotransporters are highly conserved proteins expressed in brush border membranes of proximal tubules and small intestine, respectively. The kinetics of IIa and IIb differ significantly: type IIb is saturated at lower concentrations of Na+ and Pi. To define the domain responsible for the difference in Na+ affinity we constructed several mouse IIa-IIb chimeras as well as site-directed mutagenized cotransporters. Pi uptake activity was determined after injection of cRNAs into Xenopus laevis oocytes. From the chimera experiments we concluded that the domain containing part of the second intracellular loop, the fifth transmembrane domain (TD) and part of the third extracellular loop determines the specific Na+ activation properties for both types of cotransporter. Within this domain only a few residues located in the fifth TD are not conserved between type IIa and IIb. Site-directed mutagenesis on non-conserved residues was performed. Substitution of F402 of IIa by the corresponding L418 from IIb yielded a cotransporter that behaved like the IIb. On the other hand, substitution of the specific L418 of IIb by the corresponding F402 of IIa produced a cotransporter with a Na+ activation similar to IIa. (Single letter amino acid nomenclature is used throughout the paper.) These data suggest that the specific Na+ activation properties exhibited by type IIa and type IIb Na+-Pi cotransporters are at least in part due to the presence of a specific amino acid (F402 in IIa, and L418 in IIb) within the fifth TD of the protein.

The cell biology of ion pumps: sorting and regulation

The physiologic function of an ion pump is determined, in part, by its subcellular localization and by the cellular mechanisms that modulate its activity. The Na,K-ATPase and the gastric H,K-ATPase are two closely related members of the P-type family of ion transporting ATPases. Despite their homology, these pumps are sorted to different domains in polarized epithelial cells and their enzymatic activities are subject to distinct regulatory pathways. The molecular signals responsible for these properties have begun to be elucidated. It appears that a complex array of inter- and intra-molecular interactions govern these proteins' trafficking, distribution and catalytic capacity.

A transmembrane segment determines the steady-state localization of an ion-transporting adenosine triphosphatase

The H,K-adenosine triphosphatase (ATPase) of gastric parietal cells is targeted to a regulated membrane compartment that fuses with the apical plasma membrane in response to secretagogue stimulation. Previous work has demonstrated that the alpha subunit of the H, K-ATPase encodes localization information responsible for this pump's apical distribution, whereas the beta subunit carries the signal responsible for the cessation of acid secretion through the retrieval of the pump from the surface to the regulated intracellular compartment. By analyzing the sorting behaviors of a number of chimeric pumps composed of complementary portions of the H, K-ATPase alpha subunit and the highly homologous Na,K-ATPase alpha subunit, we have identified a portion of the gastric H,K-ATPase, which is sufficient to redirect the normally basolateral Na,K-ATPase to the apical surface in transfected epithelial cells. This motif resides within the fourth of the H,K-ATPase alpha subunit's ten predicted transmembrane domains. Although interactions with glycosphingolipid-rich membrane domains have been proposed to play an important role in the targeting of several apical membrane proteins, the apically located chimeras are not found in detergent-insoluble complexes, which are typically enriched in glycosphingolipids. Furthermore, a chimera incorporating the Na, K-ATPase alpha subunit fourth transmembrane domain is apically targeted when both of its flanking sequences derive from H,K-ATPase sequence. These results provide the identification of a defined apical localization signal in a polytopic membrane transport protein, and suggest that this signal functions through conformational interactions between the fourth transmembrane spanning segment and its surrounding sequence domains.