Mutation of Gly-94 in transmembrane segment M1 of Na+,K+-ATPase interferes with Na+ and K+ binding in E2P conformation

Cryo-electron microscopy of Na+ ,K+ -ATPase reveals how the extracellular gate locks in the E2·2K+ state

Na⁺ ,K⁺ -ATPase (NKA) is one of the most important members of the P-type ion-translocating ATPases and plays a pivotal role in establishing electrochemical gradients for Na⁺ and K⁺ across the cell membrane. Presented here is a 3.3 Å resolution structure of NKA in the E2·2K⁺ state solved by cryo-electron microscopy. It is a stable state with two occluded K⁺ after transferring three Na⁺ into the extracellular medium and releasing inorganic phosphate bound to the cytoplasmic P domain. We describe how the extracellular ion pathway wide open in the E2P state becomes closed and locked in E2·2K⁺ , linked to events at the phosphorylation site more than 50 Å away. We also show, though at low resolution, how ATP binding to NKA in E2·2K⁺ relaxes the gating machinery and thereby accelerates the transition into the next step, that is, the release of K⁺ into the cytoplasm, more than 100 times.

Understanding the impacts of missense mutations on structures and functions of human cancer-related genes: A preliminary computational analysis of the COSMIC Cancer Gene Census

Genomics and genome screening are proving central to the study of cancer. However, a good appreciation of the protein structures coded by cancer genes is also invaluable, especially for the understanding of functions, for assessing ligandability of potential targets, and for designing new drugs. To complement the wealth of information on the genetics of cancer in COSMIC, the most comprehensive database for cancer somatic mutations available, structural information obtained experimentally has been brought together recently in COSMIC-3D. Even where structural information is available for a gene in the Cancer Gene Census, a list of genes in COSMIC with substantial evidence supporting their impacts in cancer, this information is quite often for a single domain in a larger protein or for a single protomer in a multiprotein assembly. Here, we show that over 60% of the genes included in the Cancer Gene Census are predicted to possess multiple domains. Many are also multicomponent and membrane-associated molecular assemblies, with mutations recorded in COSMIC affecting such assemblies. However, only 469 of the gene products have a structure represented in the PDB, and of these only 87 structures have 90-100% coverage over the sequence and 69 have less than 10% coverage. As a first step to bridging gaps in our knowledge in the many cases where individual protein structures and domains are lacking, we discuss our attempts of protein structure modelling using our pipeline and investigating the effects of mutations using two of our in-house methods (SDM2 and mCSM) and identifying potential driver mutations. This allows us to begin to understand the effects of mutations not only on protein stability but also on protein-protein, protein-ligand and protein-nucleic acid interactions. In addition, we consider ways to combine the structural information with the wealth of mutation data available in COSMIC. We discuss the impacts of COSMIC missense mutations on protein structure in order to identify and assess the molecular consequences of cancer-driving mutations.

Ion Pathways in the Na+/K+-ATPase

Na⁺/K⁺-ATPase (NKA) is an essential cation pump protein responsible for the maintenance of the sodium and potassium gradients across the plasma membrane. Recently published high-resolution structures revealed amino acids forming the cation binding sites (CBS) in the transmembrane domain and variable position of the domains in the cytoplasmic headpiece. Here we report molecular dynamic simulations of the human NKA α1β1 isoform embedded into DOPC bilayer. We have analyzed the NKA conformational changes in the presence of Na⁺- or K⁺-cations in the CBS, for various combinations of the cytoplasmic ligands, and the two major enzyme conformations in the 100 ns runs (more than 2.5 μs of simulations in total). We identified two novel cytoplasmic pathways along the pairs of transmembrane helices TM3/TM7 or TM6/TM9 that allow hydration of the CBS or transport of cations from/to the bulk. These findings can provide a structural explanation for previous mutagenesis studies, where mutation of residues that are distal from the CBS resulted in the alteration of the enzyme affinity to the transported cations or change in the enzyme activity.

Neurological disease mutations of α3 Na+,K+-ATPase: Structural and functional perspectives and rescue of compromised function

Na⁺,K⁺-ATPase creates transmembrane ion gradients crucial to the function of the central nervous system. The α-subunit of Na⁺,K⁺-ATPase exists as four isoforms (α1-α4). Several neurological phenotypes derive from α3 mutations. The effects of some of these mutations on Na⁺,K⁺-ATPase function have been studied in vitro. Here we discuss the α3 disease mutations as well as information derived from studies of corresponding mutations of α1 in the light of the high-resolution crystal structures of the Na⁺,K⁺-ATPase. A high proportion of the α3 disease mutations occur in the transmembrane sector and nearby regions essential to Na⁺ and K⁺ binding. In several cases the compromised function can be traced to disturbance of the Na⁺ specific binding site III. Recently, a secondary mutation was found to rescue the defective Na⁺ binding caused by a disease mutation. A perspective is that it may be possible to develop an efficient pharmaceutical mimicking the rescuing effect.

HMA6 and HMA8 are two chloroplast Cu+-ATPases with different enzymatic properties

Copper (Cu) plays a key role in the photosynthetic process as cofactor of the plastocyanin (PC), an essential component of the chloroplast photosynthetic electron transfer chain. Encoded by the nuclear genome, PC is translocated in its apo-form into the chloroplast and the lumen of thylakoids where it is processed to its mature form and acquires Cu. In Arabidopsis, Cu delivery into the thylakoids involves two transporters of the PIB-1 ATPases family, heavy metal associated protein 6 (HMA6) located at the chloroplast envelope and HMA8 at the thylakoid membrane. To gain further insight into the way Cu is delivered to PC, we analysed the enzymatic properties of HMA8 and compared them with HMA6 ones using in vitro phosphorylation assays and phenotypic tests in yeast. These experiments reveal that HMA6 and HMA8 display different enzymatic properties: HMA8 has a higher apparent affinity for Cu(+) but a slower dephosphorylation kinetics than HMA6. Modelling experiments suggest that these differences could be explained by the electrostatic properties of the Cu(+) releasing cavities of the two transporters and/or by the different nature of their cognate Cu(+) acceptors (metallochaperone/PC).

A novel cholinergic action of alcohol and the development of tolerance to that effect in Caenorhabditis elegans

Understanding the genes and mechanisms involved in acute alcohol responses has the potential to allow us to predict an individual's predisposition to developing an alcohol use disorder. To better understand the molecular pathways involved in the activating effects of alcohol and the acute functional tolerance that can develop to such effects, we characterized a novel ethanol-induced hypercontraction response displayed by Caenorhabditis elegans. We compared body size of animals prior to and during ethanol treatment and showed that acute exposure to ethanol produced a concentration-dependent decrease in size followed by recovery to their untreated size by 40 min despite continuous treatment. An increase in cholinergic signaling, leading to muscle hypercontraction, is implicated in this effect because pretreatment with mecamylamine, a nicotinic acetylcholine receptor (nAChR) antagonist, blocked ethanol-induced hypercontraction, as did mutations causing defects in cholinergic signaling (cha-1 and unc-17). Analysis of mutations affecting specific subunits of nAChRs excluded a role for the ACR-2R, the ACR-16R, and the levamisole-sensitive AChR and indicated that this excitation effect is dependent on an uncharacterized nAChR that contains the UNC-63 α-subunit. We performed a forward genetic screen and identified eg200, a mutation that affects a conserved glycine in EAT-6, the α-subunit of the Na(+)/K(+) ATPase. The eat-6(eg200) mutant fails to develop tolerance to ethanol-induced hypercontraction and remains contracted for at least 3 hr of continuous ethanol exposure. These data suggest that cholinergic signaling through a specific α-subunit-containing nAChR is involved in ethanol-induced excitation and that tolerance to this ethanol effect is modulated by Na(+)/K(+) ATPase function.

Somatic ATP1A1 , ATP2B3 , and KCNJ5 Mutations in Aldosterone-Producing Adenomas

Aldosterone-producing adenomas (APAs) cause a sporadic form of primary aldosteronism and somatic mutations in the KCNJ5 gene, which encodes the G-protein–activated inward rectifier K + channel 4, GIRK4, account for ≈40% of APAs. Additional somatic APA mutations were identified recently in 2 other genes, ATP1A1 and ATP2B3 , encoding Na + /K + -ATPase 1 and Ca 2+ -ATPase 3, respectively, at a combined prevalence of 6.8%. We have screened 112 APAs for mutations in known hotspots for genetic alterations associated with primary aldosteronism. Somatic mutations in ATP1A1 , ATP2B3 , and KCNJ5 were present in 6.3%, 0.9%, and 39.3% of APAs, respectively, and included 2 novel mutations (Na + /K + -ATPase p.Gly99Arg and GIRK4 p.Trp126Arg). CYP11B2 gene expression was higher in APAs harboring ATP1A1 and ATP2B3 mutations compared with those without these or KCNJ5 mutations. Overexpression of Na + /K + -ATPase p.Gly99Arg and GIRK4 p.Trp126Arg in HAC15 adrenal cells resulted in upregulation of CYP11B2 gene expression and its transcriptional regulator NR4A2. Structural modeling of the Na + /K + -ATPase showed that the Gly99Arg substitution most likely interferes with the gateway to the ion binding pocket. In vitro functional assays demonstrated that Gly99Arg displays severely impaired ATPase activity, a reduced apparent affinity for Na + activation of phosphorylation and K + inhibition of phosphorylation that indicate decreased Na + and K + binding, respectively. Moreover, whole cell patch-clamp studies established that overexpression of Na + /K + -ATPase Gly99Arg causes membrane voltage depolarization. In conclusion, somatic mutations are common in APAs that result in an increase in CYP11B2 gene expression and may account for the dysregulated aldosterone production in a subset of patients with sporadic primary aldosteronism.

Expression of mutant α1 Na/K-ATPase defective in conformational transition attenuates Src-mediated signal transduction

The α1 Na/K-ATPase possesses both pumping and signaling functions. Using purified enzyme we found that the α1 Na/K-ATPase might interact with and regulate Src activity in a conformation-dependent manner. Here we further explored the importance of the conformational transition capability of α1 Na/K-ATPase in regulation of Src-related signal transduction in cell culture. We first rescued the α1-knockdown cells by wild-type rat α1 or α1 mutants (I279A and F286A) that are known to be defective in conformational transition. Stable cell lines with comparable expression of wild type α1, I279A, and F286A were characterized. As expected, the defects in conformation transition resulted in comparable degree of inhibition of pumping activity in the mutant-rescued cell lines. However, I279A was more effective in inhibiting basal Src activity than either the wild-type or the F286A. Although much higher ouabain concentration was required to stimulate Src in I279A-rescued cells, extracellular K(+) was comparably effective in regulating Src in both control and I279A cells. In contrast, ouabain and extracellular K(+) failed to produce detectable changes in Src activity in F286A-rescued cells. Furthermore, expression of either mutant inhibited integrin-induced activation of Src/FAK pathways and slowed cell spreading processes. Finally, the expression of these mutants inhibited cell growth, with I279A being more potent than that of F286A. Taken together, the new findings suggest that the α1 Na/K-ATPase may be a key player in dynamic regulation of cellular Src activity and that the capability of normal conformation transition is essential for both pumping and signaling functions of α1 Na/K-ATPase.

Inhibition of phosphorylation of na+,k+-ATPase by mutations causing familial hemiplegic migraine

The neurological disorder familial hemiplegic migraine type II (FHM2) is caused by mutations in the α2-isoform of the Na(+),K(+)-ATPase. We have studied the partial reaction steps of the Na(+),K(+)-pump cycle in nine FHM2 mutants retaining overall activity at a level still compatible with cell growth. Although it is believed that the pathophysiology of FHM2 results from reduced extracellular K(+) clearance and/or changes in Na(+) gradient-dependent transport processes in neuroglia, a reduced affinity for K(+) or Na(+) is not a general finding with the FHM2 mutants. Six of the FHM2 mutations markedly affect the maximal rate of phosphorylation from ATP leading to inhibition by intracellular K(+), thereby likely compromising pump function under physiological conditions. In mutants R593W, V628M, and M731T, the defective phosphorylation is caused by local perturbations within the Rossmann fold, possibly interfering with the bending of the P-domain during phosphoryl transfer. In mutants V138A, T345A, and R834Q, long range effects reaching from as far away as the M2 transmembrane helix perturb the function of the catalytic site. Mutant E700K exhibits a reduced rate of E(2)P dephosphorylation without effect on phosphorylation from ATP. An extremely reduced vanadate affinity of this mutant indicates that the slow dephosphorylation reflects a destabilization of the phosphoryl transition state. This seems to be caused by insertion of the lysine between two other positively charged residues of the Rossmann fold. In mutants R202Q and T263M, effects on the A-domain structure are responsible for a reduced rate of the E(1)P to E(2)P transition.

A structural overview of the plasma membrane Na+,K+-ATPase and H+-ATPase ion pumps

Plasma membrane ATPases are primary active transporters of cations that maintain steep concentration gradients. The ion gradients and membrane potentials derived from them form the basis for a range of essential cellular processes, in particular Na(+)-dependent and proton-dependent secondary transport systems that are responsible for uptake and extrusion of metabolites and other ions. The ion gradients are also both directly and indirectly used to control pH homeostasis and to regulate cell volume. The plasma membrane H(+)-ATPase maintains a proton gradient in plants and fungi and the Na(+),K(+)-ATPase maintains a Na(+) and K(+) gradient in animal cells. Structural information provides insight into the function of these two distinct but related P-type pumps.

Cyclopiazonic acid is complexed to a divalent metal ion when bound to the sarcoplasmic reticulum Ca2+-ATPase

We have determined the structure of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) in an E2.P(i)-like form stabilized as a complex with MgF(4)(2-), an ATP analog, adenosine 5'-(beta,gamma-methylene)triphosphate (AMPPCP), and cyclopiazonic acid (CPA). The structure determined at 2.5A resolution leads to a significantly revised model of CPA binding when compared with earlier reports. It shows that a divalent metal ion is required for CPA binding through coordination of the tetramic acid moiety at a characteristic kink of the M1 helix found in all P-type ATPase structures, which is expected to be part of the cytoplasmic cation access pathway. Our model is consistent with the biochemical data on CPA function and provides new measures in structure-based drug design targeting Ca(2+)-ATPases, e.g. from pathogens. We also present an extended structural basis of ATP modulation pinpointing key residues at or near the ATP binding site. A structural comparison to the Na(+),K(+)-ATPase reveals that the Phe(93) side chain occupies the equivalent binding pocket of the CPA site in SERCA, suggesting an important role of this residue in stabilization of the potassium-occluded E2 state of Na(+),K(+)-ATPase.

Red blood cell Na pump: Insights from species differences

The red blood cell membrane is specialized to exchange chloride and bicarbonate; usually the pH gradient, the chloride ratio, and the membrane potential are tightly coupled. We review the evidence that led to the ability to separately vary inside and outside pH in red cells. The effect of pH on Na pump activity and on the selectivity of the inside and the outside transport sites is reviewed. In red blood cells, at high pH, the outside site is not selective. An increase in protons leads to an increase in K(+) affinity, thus making the site more selective. The pK for this site is different in rats and humans; because of the high conservation of residues in these two species, there are only a few possible residues that can account for this difference. On the inside, work from unsided preparations suggests that, at high pH, the transport site is highly selective for Na(+). Once again, an increase in protons leads to an increase in K(+) affinity, but now the result is a less selective site. During their maturation, reticulocytes lose many membrane proteins. The type and fractional loss is species dependent. For example, most reticulocytes lose most of their Na pumps, retaining about 100 pumps per cell, but animals from the order Carnivora lose all their pumps. We review some of the evidence that PKC phosphorylation of N-terminus serines is responsible for endocytosis in other cell types and species variation in this region.

The structure of the Na+,K+-ATPase and mapping of isoform differences and disease-related mutations

The Na+,K+-ATPase transforms the energy of ATP to the maintenance of steep electrochemical gradients for sodium and potassium across the plasma membrane. This activity is tissue specific, in particular due to variations in the expressions of the alpha subunit isoforms one through four. Several mutations in alpha2 and 3 have been identified that link the specific function of the Na+,K+-ATPase to the pathophysiology of neurological diseases such as rapid-onset dystonia parkinsonism and familial hemiplegic migraine type 2. We show a mapping of the isoform differences and the disease-related mutations on the recently determined crystal structure of the pig renal Na+,K+-ATPase and a structural comparison to Ca2+-ATPase. Furthermore, we present new experimental data that address the role of a stretch of three conserved arginines near the C-terminus of the alpha subunit (Arg1003-Arg1005).

Review. Peering into an ATPase ion pump with single-channel recordings

In principle, an ion channel needs no more than a single gate, but a pump requires at least two gates that open and close alternately to allow ion access from only one side of the membrane at a time. In the Na+,K+-ATPase pump, this alternating gating effects outward transport of three Na+ ions and inward transport of two K+ ions, for each ATP hydrolysed, up to a hundred times per second, generating a measurable current if assayed in millions of pumps. Under these assay conditions, voltage jumps elicit brief charge movements, consistent with displacement of ions along the ion pathway while one gate is open but the other closed. Binding of the marine toxin, palytoxin, to the Na+,K+-ATPase uncouples the two gates, so that although each gate still responds to its physiological ligand they are no longer constrained to open and close alternately, and the Na+,K+-ATPase is transformed into a gated cation channel. Millions of Na+ or K+ ions per second flow through such an open pump-channel, permitting assay of single molecules and allowing unprecedented access to the ion transport pathway through the Na+,K+-ATPase. Use of variously charged small hydrophilic thiol-specific reagents to probe cysteine targets introduced throughout the pump's transmembrane segments allows mapping and characterization of the route traversed by transported ions.

The ion pathway through the opened Na(+),K(+)-ATPase pump

P-type ATPases pump ions across membranes, generating steep electrochemical gradients that are essential for the function of all cells. Access to the ion-binding sites within the pumps alternates between the two sides of the membrane to avoid the dissipation of the gradients that would occur during simultaneous access. In Na(+),K(+)-ATPase pumps treated with the marine agent palytoxin, this strict alternation is disrupted and binding sites are sometimes simultaneously accessible from both sides of the membrane, transforming the pumps into ion channels (see, for example, refs 2, 3). Current recordings in these channels can monitor accessibility of introduced cysteine residues to water-soluble sulphydryl-specific reagents. We found previously that Na(+),K(+) pump-channels open to the extracellular surface through a deep and wide vestibule that emanates from a narrower pathway between transmembrane helices 4 and 6 (TM4 and TM6). Here we report that cysteine scans from TM1 to TM6 reveal a single unbroken cation pathway that traverses palytoxin-bound Na(+),K(+) pump-channels from one side of the membrane to the other. This pathway comprises residues from TM1, TM2, TM4 and TM6, passes through ion-binding site II, and is probably conserved in structurally and evolutionarily related P-type pumps, such as sarcoplasmic- and endoplasmic-reticulum Ca(2+)-ATPases and H(+),K(+)-ATPases.

Identification and function of a cytoplasmic K+ site of the Na+, K+ -ATPase

A cytoplasmic nontransport K(+)/Rb(+) site in the P-domain of the Na(+), K(+)-ATPase has been identified by anomalous difference Fourier map analysis of crystals of the [Rb(2)].E(2).MgF(4)(2-) form of the enzyme. The functional roles of this third K(+)/Rb(+) binding site were studied by site-directed mutagenesis, replacing the side chain of Asp(742) donating oxygen ligand(s) to the site with alanine, glutamate, and lysine. Unlike the wild-type Na(+), K(+)-ATPase, the mutants display a biphasic K(+) concentration dependence of E(2)P dephosphorylation, indicating that the cytoplasmic K(+) site is involved in activation of dephosphorylation. The affinity of the site is lowered significantly (30-200-fold) by the mutations, the lysine mutation being most disruptive. Moreover, the mutations accelerate the E(2) to E(1) conformational transition, again with the lysine substitution resulting in the largest effect. Hence, occupation of the cytoplasmic K(+)/Rb(+) site not only enhances E(2)P dephosphorylation but also stabilizes the E(2) dephosphoenzyme. These characteristics of the previously unrecognized nontransport site make it possible to account for the hitherto poorly understood trans-effects of cytoplasmic K(+) by the consecutive transport model, without implicating a simultaneous exposure of the transport sites toward the cytoplasmic and extracellular sides of the membrane. The cytoplasmic K(+)/Rb(+) site appears to be conserved among Na(+), K(+)-ATPases and P-type ATPases in general, and its mode of operation may be associated with stabilizing the loop structure at the C-terminal end of the P6 helix of the P-domain, thereby affecting the function of highly conserved catalytic residues and promoting helix-helix interactions between the P- and A-domains in the E(2) state.

Roles of transmembrane segment M1 of Na+,K+-ATPase and Ca2-ATPase, the gatekeeper and the pivot

In this review we summarize mutagenesis work on the structure-function relationship of transmembrane segment M1 in the Na+,K+-ATPase and the sarco(endo)plasmic reticulum Ca2+-ATPase. The original hypothesis that charged residues in the N-terminal part of M1 interact with the transported cations can be rejected. On the other hand hydrophobic residues in the middle part of M1 turned out to play crucial roles in Ca2+ interaction/occlusion in Ca2+-ATPase and K+ interaction/occlusion in Na+,K+-ATPase. Leu65 of the Ca2+-ATPase and Leu99 of the Na+,K+-ATPase, located at homologous positions in M1, function as gate-locking residues that restrict the mobility of the side chain of the cation binding/gating residue of transmembrane segment M4, Glu309/Glu329. A pivot formed between a pair of a glycine and a bulky residue in M1 and M3 seems critical to the opening of the extracytoplasmic gate in both the Ca2+-ATPase and the Na+,K+-ATPase.

FXYD5 modulates Na+ absorption and is increased in cystic fibrosis airway epithelia

FXYD5, also known as dysadherin, belongs to a family of tissue-specific regulators of the Na(+)-K(+)-ATPase. We determined the kinetic effects of FXYD5 on Na(+)-K(+)-ATPase pump activity in stably transfected Madin-Darby canine kidney cells. FXYD5 significantly increased the apparent affinity for Na(+) twofold and decreased the apparent affinity for K(+) by 60% with a twofold increase in V(max) of K(+), a pattern that would increase activity and Na(+) removal from the cell. To test the effect of increased Na(+) uptake on FXYD5 expression, we analyzed Madin-Darby canine kidney cells stably transfected with an inducible vector expressing all three subunits of the epithelial Na(+) channel (ENaC). Na(+)-K(+)-ATPase activity increased sixfold after 48-h ENaC induction, but FXYD5 expression decreased 75%. FXYD5 expression was also decreased in lung epithelia from mice that overexpress ENaC, suggesting that chronic Na(+) absorption by itself downregulates epithelial FXYD5 expression. Patients with cystic fibrosis (CF) display ENaC-mediated hyperabsorption of Na(+) in the airways, accompanied by increased Na(+)-K(+)-ATPase activity. However, FXYD5 was significantly increased in the lungs and nasal epithelium of CF mice as assessed by RT-PCR, immunohistochemistry, and immunoblot analysis (P < 0.001). FXYD5 was also upregulated in nasal scrapings from human CF patients compared with controls (P < 0.02). Treatment of human tracheal epithelial cells with a CFTR inhibitor (I-172) confirmed that loss of CFTR function correlated with increased FXYD5 expression (P < 0.001), which was abrogated by an inhibitor of NF-kappaB. Thus FXYD5 is upregulated in CF epithelia, and this change may exacerbate the Na(+) hyperabsorption and surface liquid dehydration observed in CF airway epithelia.

Crystal structure of the sodium-potassium pump

The Na+,K+-ATPase generates electrochemical gradients for sodium and potassium that are vital to animal cells, exchanging three sodium ions for two potassium ions across the plasma membrane during each cycle of ATP hydrolysis. Here we present the X-ray crystal structure at 3.5 A resolution of the pig renal Na+,K+-ATPase with two rubidium ions bound (as potassium congeners) in an occluded state in the transmembrane part of the alpha-subunit. Several of the residues forming the cavity for rubidium/potassium occlusion in the Na+,K+-ATPase are homologous to those binding calcium in the Ca2+-ATPase of sarco(endo)plasmic reticulum. The beta- and gamma-subunits specific to the Na+,K+-ATPase are associated with transmembrane helices alphaM7/alphaM10 and alphaM9, respectively. The gamma-subunit corresponds to a fragment of the V-type ATPase c subunit. The carboxy terminus of the alpha-subunit is contained within a pocket between transmembrane helices and seems to be a novel regulatory element controlling sodium affinity, possibly influenced by the membrane potential.

Importance of Leu99 in Transmembrane Segment M1 of the Na+,K+-ATPase in the Binding and Occlusion of K+

Twenty-six point mutations were introduced into the N-terminal and middle parts of transmembrane segment M1 of the Na+, K+ -ATPase and its cytosolic extension. None of the alterations to charged and polar residues in the N-terminal part of M1 and its cytosolic extension had any major effect on the cation binding properties, thus rejecting the hypothesis that these residues are involved in cation selectivity. By contrast, specific residues in the middle part of M1, particularly Leu(99), were found critical to K+ interaction of the enzyme. Hence, mutation L99A reduced the affinity for K+ activation of E2P dephosphorylation 17-fold, and L99F reduced the equilibrium level of the K+-occluded intermediate [K2]E2 and increased the rate of K+ deocclusion 39-fold, i.e. more than seen for mutation E329Q of the cation-binding glutamate in M4. L99Q affected K+ interaction in yet another way, the equilibrium level of [K2]E2 being slightly increased despite an increased rate of K+ deocclusion, suggesting that the K+ ions leave and enter the occlusion pocket more frequently than in the wild type. L99Q furthermore affected the ability to discriminate between Na+ and K+ on the extracellular side. Our findings can be explained by a structural model in which Leu(99) and Glu(329) interact and cooperate in K+ binding and gating of the K+ sites. The disturbance of K+ interaction in mutants with alteration to Leu(91), Phe(95), Ser(96), or Leu(98) could be a consequence of the roles of these residues in positioning the M1 helix optimally for the interaction between Leu(99) and Glu(329). Phe(95) may serve to stabilize the pivot for movement of M1 through interaction with Ile(287) in M3.

Mutations Phe785Leu and Thr618Met in Na+,K+-ATPase, associated with familial rapid-onset dystonia parkinsonism, interfere with Na+ interaction by distinct mechanisms

The Na(+),K(+)-ATPase plays key roles in brain function. Recently, missense mutations in the Na(+),K(+)-ATPase were found associated with familial rapid-onset dystonia parkinsonism (FRDP). Here, we have characterized the functional consequences of FRDP mutations Phe785Leu and Thr618Met. Both mutations lead to functionally altered, but active, Na(+),K(+)-pumps, that display reduced apparent affinity for cytoplasmic Na(+), but the underlying mechanism differs between the mutants. In Phe785Leu, the interaction of the E(1) form with Na(+) is defective, and the E(1)-E(2) equilibrium is not displaced. In Thr618Met, the Na(+) affinity is reduced because of displacement of the conformational equilibrium in favor of the K(+)-occluded E(2)(K(2)) form. In both mutants, K(+) interaction at the external activating sites of the E(2)P phosphoenzyme is normal. The change of cellular Na(+) homeostasis is likely a major factor contributing to the development of FRDP in patients carrying the Phe785Leu or Thr618Met mutation. Phe785Leu moreover interferes with Na(+) interaction on the extracellular side and reduces the affinity for ouabain significantly. Analysis of two additional Phe(785) mutants, Phe785Leu/Leu786Phe and Phe785Tyr, demonstrated that the aromatic function of the side chain, as well as its exact position, is critical for Na(+) and ouabain binding. The effects of substituting Phe(785) could be explained by structural modeling, demonstrating that Phe(785) participates in a hydrophobic network between three transmembrane segments. Thr(618) is located in the cytoplasmic part of the molecule near the catalytic site, and the structural modeling indicates that the Thr618Met mutation interferes with the bonding pattern in the catalytic site in the E(1) form, thereby destabilizing E(1) relative to E(2)(K(2)).