Na,K-ATPase with Disrupted Na Binding Sites I and III Binds Na with Increased Affinity at Site II and Undergoes Na-Activated Phosphorylation with ATP.

Na,K-ATPase actively extrudes three cytoplasmic Na ions in exchange for two extracellular K ions for each ATP hydrolyzed. The atomic structure with bound Na identifies three Na sites, named I, II, and III. It has been proposed that site III is the first to be occupied and site II last, when Na binds from the cytoplasmic side.

On the track of the lipid transport pathway of the phospholipid flippase ATP8A2 - Mutation analysis of residues of the transmembrane segments M1, M2, M3 and M4.

P4-ATPases, also known as flippases, translocate specific lipids from the exoplasmic leaflet to the cytoplasmic leaflet of biological membranes, thereby generating an asymmetric lipid distribution essential for numerous cellular functions. A debated issue is which pathway within the protein the lipid substrate follows during the translocation. Here we present a comprehensive mutational screening of all amino acid residues in the transmembrane segments M1, M2, M3, and M4 of the flippase ATP8A2, thus allowing the functionally important residues in these transmembrane segments to be highlighted on a background of less important residues.

Crystal structures of Na ,K -ATPase reveal the mechanism that converts the K -bound form to Na -bound form and opens and closes the cytoplasmic gate.

Na ,K -ATPase (NKA) plays a pivotal role in establishing electrochemical gradients for Na and K across the cell membrane by alternating between the E1 (showing high affinity for Na and low affinity for K ) and E2 (low affinity to Na and high affinity to K ) forms. Presented here are two crystal structures of NKA in E1·Mg and E1·3Na states at 2.9 and 2.

Temperature instability of a mutation at a multidomain junction in Na,K-ATPase isoform ATP1A3 (p.Arg756His) produces a fever-induced neurological syndrome.

ATP1A3 encodes the α3 isoform of Na,K-ATPase. In the brain, it is expressed only in neurons. Human ATP1A3 mutations produce a wide spectrum of phenotypes, but particular syndromes are associated with unique substitutions.

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.

Role of a conserved ion-binding site tyrosine in ion selectivity of the Na+/K+ pump.

The essential transmembrane Na+ and K+ gradients in animal cells are established by the Na+/K+ pump, a P-type ATPase that exports three Na+ and imports two K+ per ATP hydrolyzed. The mechanism by which the Na+/K+ pump distinguishes between Na+ and K+ at the two membrane sides is poorly understood. Crystal structures identify two sites (sites I and II) that bind Na+ or K+ and a third (site III) specific for Na+.

Cryoelectron microscopy of Na,K-ATPase in the two E2P states with and without cardiotonic steroids.

Cryoelectron microscopy (cryo-EM) was applied to Na+,K+-ATPase (NKA) to determine the structures of two E2P states, one (E2PATP) formed by ATP and Mg2+ in the forward reaction, and the other (E2PPi) formed by inorganic phosphate (Pi) and Mg2+ in the backward reaction, with and without ouabain or istaroxime, representatives of classical and new-generation cardiotonic steroids (CTSs). These two E2P states exhibit different biochemical properties. In particular, K+-sensitive acceleration of the dephosphorylation reaction is not observed with E2PPi, attributed to the presence of a Mg2+ ion in the transmembrane cation binding sites.

ATP1A2- and ATP1A3-associated early profound epileptic encephalopathy and polymicrogyria.

Constitutional heterozygous mutations of ATP1A2 and ATP1A3, encoding for two distinct isoforms of the Na+/K+-ATPase (NKA) alpha-subunit, have been associated with familial hemiplegic migraine (ATP1A2), alternating hemiplegia of childhood (ATP1A2/A3), rapid-onset dystonia-parkinsonism, cerebellar ataxia-areflexia-progressive optic atrophy, and relapsing encephalopathy with cerebellar ataxia (all ATP1A3). A few reports have described single individuals with heterozygous mutations of ATP1A2/A3 associated with severe childhood epilepsies. Early lethal hydrops fetalis, arthrogryposis, microcephaly, and polymicrogyria have been associated with homozygous truncating mutations in ATP1A2.

Binding of cardiotonic steroids to Na,K-ATPase in the E2P state.

The sodium pump (Na, K-ATPase, NKA) is vital for animal cells, as it actively maintains Na and K electrochemical gradients across the cell membrane. It is a target of cardiotonic steroids (CTSs) such as ouabain and digoxin. As CTSs are almost unique strong inhibitors specific to NKA, a wide range of derivatives has been developed for potential therapeutic use.

Distinct effects of Q925 mutation on intracellular and extracellular Na and K binding to the Na, K-ATPase.

Three Na sites are defined in the Na-bound crystal structure of Na, K-ATPase. Sites I and II overlap with two K sites in the K-bound structure, whereas site III is unique and Na specific. A glutamine in transmembrane helix M8 (Q925) appears from the crystal structures to coordinate Na at site III, but does not contribute to K coordination at sites I and II.

Asparagine 905 of the mammalian phospholipid flippase ATP8A2 is essential for lipid substrate-induced activation of ATP8A2 dephosphorylation.

The P-type ATPase protein family includes, in addition to ion pumps such as Ca-ATPase and Na,K-ATPase, also phospholipid flippases that transfer phospholipids between membrane leaflets. P-type ATPase ion pumps translocate their substrates occluded between helices in the center of the transmembrane part of the protein. The large size of the lipid substrate has stimulated speculation that flippases use a different transport mechanism.

Functional consequences of the CAPOS mutation E818K of Na,K-ATPase.

The cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) syndrome is caused by the single mutation E818K of the α3-isoform of Na,K-ATPase. Here, using biochemical and electrophysiological approaches, we examined the functional characteristics of E818K, as well as of E818Q and E818A mutants. We found that these amino acid substitutions reduce the apparent Na affinity at the cytoplasmic-facing sites of the pump protein and that this effect is more pronounced for the lysine and glutamine substitutions (3-4-fold) than for the alanine substitution.

Germline De Novo Mutations in ATP1A1 Cause Renal Hypomagnesemia, Refractory Seizures, and Intellectual Disability.

Over the last decades, a growing spectrum of monogenic disorders of human magnesium homeostasis has been clinically characterized, and genetic studies in affected individuals have identified important molecular components of cellular and epithelial magnesium transport. Here, we describe three infants who are from non-consanguineous families and who presented with a disease phenotype consisting of generalized seizures in infancy, severe hypomagnesemia, and renal magnesium wasting. Seizures persisted despite magnesium supplementation and were associated with significant intellectual disability.

Disease mutations reveal residues critical to the interaction of P4-ATPases with lipid substrates.

Phospholipid flippases (P-ATPases) translocate specific phospholipids from the exoplasmic to the cytoplasmic leaflet of membranes. While there is good evidence that the overall molecular structure of flippases is similar to that of P-type ATPase ion-pumps, the transport pathway for the "giant" lipid substrate has not been determined. ATP8A2 is a flippase with selectivity toward phosphatidylserine (PS), possessing a net negatively charged head group, whereas ATP8B1 exhibits selectivity toward the electrically neutral phosphatidylcholine (PC).

The α2β2 isoform combination dominates the astrocytic Na /K -ATPase activity and is rendered nonfunctional by the α2.G301R familial hemiplegic migraine type 2-associated mutation.

Synaptic activity results in transient elevations in extracellular K , clearance of which is critical for sustained function of the nervous system. The K clearance is, in part, accomplished by the neighboring astrocytes by mechanisms involving the Na /K -ATPase. The Na /K -ATPase consists of an α and a β subunit, each with several isoforms present in the central nervous system, of which the α2β2 and α2β1 isoform combinations are kinetically geared for astrocytic K clearance.

Arginine substitution of a cysteine in transmembrane helix M8 converts Na+,K+-ATPase to an electroneutral pump similar to H+,K+-ATPase.

Na,K-ATPase and H,K-ATPase are electrogenic and nonelectrogenic ion pumps, respectively. The underlying structural basis for this difference has not been established, and it has not been revealed how the H,K-ATPase avoids binding of Na at the site corresponding to the Na-specific site of the Na,K-ATPase (site III). In this study, we addressed these questions by using site-directed mutagenesis in combination with enzymatic, transport, and electrophysiological functional measurements.

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.

Glutamate transporter activity promotes enhanced Na /K -ATPase-mediated extracellular K management during neuronal activity.

Key Points: Management of glutamate and K in brain extracellular space is of critical importance to neuronal function. The astrocytic α2β2 Na /K -ATPase isoform combination is activated by the K transients occurring during neuronal activity. In the present study, we report that glutamate transporter-mediated astrocytic Na transients stimulate the Na /K -ATPase and thus the clearance of extracellular K .

Importance of a Potential Protein Kinase A Phosphorylation Site of Na+,K+-ATPase and Its Interaction Network for Na+ Binding.

The molecular mechanism underlying PKA-mediated regulation of Na(+),K(+)-ATPase was explored in mutagenesis studies of the potential PKA site at Ser-938 and surrounding charged residues. The phosphomimetic mutations S938D/E interfered with Na(+) binding from the intracellular side of the membrane, whereas Na(+) binding from the extracellular side was unaffected. The reduction of Na(+) affinity is within the range expected for physiological regulation of the intracellular Na(+) concentration, thus supporting the hypothesis that PKA-mediated phosphorylation of Ser-938 regulates Na(+),K(+)-ATPase activity in vivo Ser-938 is located in the intracellular loop between transmembrane segments M8 and M9.

Specific mutations in mammalian P4-ATPase ATP8A2 catalytic subunit entail differential glycosylation of the accessory CDC50A subunit.

P4-ATPases, or flippases, translocate phospholipids between the two leaflets of eukaryotic biological membranes. They are essential to the physiologically crucial phospholipid asymmetry and involved in severe diseases, but their molecular structure and mechanism are still unresolved. Here, we show that in an extensive mutational alanine screening of the mammalian flippase ATP8A2 catalytic subunit, five mutations stand out by leading to reduced glycosylation of the accessory subunit CDC50A.

Rescue of Na+ affinity in aspartate 928 mutants of Na+,K+-ATPase by secondary mutation of glutamate 314.

The Na(+),K(+)-ATPase binds Na(+) at three transport sites denoted I, II, and III, of which site III is Na(+)-specific and suggested to be the first occupied in the cooperative binding process activating phosphorylation from ATP. Here we demonstrate that the asparagine substitution of the aspartate associated with site III found in patients with rapid-onset dystonia parkinsonism or alternating hemiplegia of childhood causes a dramatic reduction of Na(+) affinity in the α1-, α2-, and α3-isoforms of Na(+),K(+)-ATPase, whereas other substitutions of this aspartate are much less disruptive. This is likely due to interference by the amide function of the asparagine side chain with Na(+)-coordinating residues in site III.

Distinct neurological disorders with ATP1A3 mutations.

Genetic research has shown that mutations that modify the protein-coding sequence of ATP1A3, the gene encoding the α3 subunit of Na(+)/K(+)-ATPase, cause both rapid-onset dystonia parkinsonism and alternating hemiplegia of childhood. These discoveries link two clinically distinct neurological diseases to the same gene, however, ATP1A3 mutations are, with one exception, disease-specific. Although the exact mechanism of how these mutations lead to disease is still unknown, much knowledge has been gained about functional consequences of ATP1A3 mutations using a range of in-vitro and animal model systems, and the role of Na(+)/K(+)-ATPases in the brain.

Critical roles of isoleucine-364 and adjacent residues in a hydrophobic gate control of phospholipid transport by the mammalian P4-ATPase ATP8A2.

P4-ATPases (flippases) translocate specific phospholipids such as phosphatidylserine from the exoplasmic leaflet of the cell membrane to the cytosolic leaflet, upholding an essential membrane asymmetry. The mechanism of flipping this giant substrate has remained an enigma. We have investigated the importance of amino acid residues in transmembrane segment M4 of mammalian P4-ATPase ATP8A2 by mutagenesis.

Relationship between intracellular Na+ concentration and reduced Na+ affinity in Na+,K+-ATPase mutants causing neurological disease.

The neurological disorders familial hemiplegic migraine type 2 (FHM2), alternating hemiplegia of childhood (AHC), and rapid-onset dystonia parkinsonism (RDP) are caused by mutations of Na(+),K(+)-ATPase α2 and α3 isoforms, expressed in glial and neuronal cells, respectively. Although these disorders are distinct, they overlap in phenotypical presentation. Two Na(+),K(+)-ATPase mutations, extending the C terminus by either 28 residues ("+28" mutation) or an extra tyrosine ("+Y"), are associated with FHM2 and RDP, respectively.

Molecular cloning and characterization of porcine Na⁺/K⁺-ATPase isoforms α1, α2, α3 and the ATP1A3 promoter.

Na⁺/K⁺-ATPase maintains electrochemical gradients of Na⁺ and K⁺ essential for a variety of cellular functions including neuronal activity. The α-subunit of the Na⁺/K⁺-ATPase exists in four different isoforms (α1-α4) encoded by different genes. With a view to future use of pig as an animal model in studies of human diseases caused by Na⁺/K⁺-ATPase mutations, we have determined the porcine coding sequences of the α1-α3 genes, ATP1A1, ATP1A2, and ATP1A3, their chromosomal localization, and expression patterns.