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

Hypoxia-induced oxidative stress and transcriptome changes in the mud crab (Scylla paramamosain)

Mud crab (Scylla paramamosain) is an economically important cultured species in China. Hypoxia is a major environmental stressor during mud crab culture. In the present study, we investigated the oxidative stress and transcriptome changes in the gills of mud crab after intermediate hypoxia stress with dissolved oxygen (DO) 3.0 ± 0.2 mg/L (named as "DO3") and acute hypoxia stress with DO 1.0 ± 0.2 mg/L (named as "DO1") for 0, 3, 6, 12 and 24 h. The superoxide dismutase (SOD) activity of DO1 increased significantly at 3, 6 and 24 h after hypoxia stress, while SOD activity of DO3 increased significantly at 6 and 24 h. The total antioxidant capacity (T-AOC) increased significantly at 6, 12 and 24 h after hypoxia stress. The malondialdehyde (MDA) concentration of DO1 increased significantly at 6, 12 and 24 h after hypoxia stress, while MDA concentration of DO3 only increased significantly at 6 h. The lactate dehydrogenase (LDH) activity of DO1 increased significantly at 3, 6, 12 and 24 h after hypoxia stress, while LDH activity of DO3 increased significantly at 12 and 24 h. Transcriptomic analysis was conducted at 24 h of gill tissues after hypoxia stress. A total of 1052 differentially expressed genes (DEGs) were obtained, including 394 DEGs between DO1 and DO3, 481 DEGs between DO1 and control group, 177 DEGs between DO3 and control group. DEGs were enriched in the pathways related to metabolism, immune functions, ion transport, and signal transduction. Transcriptional analysis showed that glycolysis and tricarboxylic acid cycle genes were the key factors in regulating the adaptation of mud crab to hypoxia stress.

An Intracellular Pathway Controlled by the N-terminus of the Pump Subunit Inhibits the Bacterial KdpFABC Ion Pump in High K+ Conditions

The heterotetrameric bacterial KdpFABC transmembrane protein complex is an ion channel-pump hybrid that consumes ATP to import K⁺ against its transmembrane chemical potential gradient in low external K⁺ environments. The KdpB ion-pump subunit of KdpFABC is a P-type ATPase, and catalyses ATP hydrolysis. Under high external K⁺ conditions, K⁺ can diffuse into the cells through passive ion channels. KdpFABC must therefore be inhibited in high K⁺ conditions to conserve cellular ATP. Inhibition is thought to occur via unusual phosphorylation of residue Ser162 of the TGES motif of the cytoplasmic A domain. It is proposed that phosphorylation most likely traps KdpB in an inactive E1-P like conformation, but the molecular mechanism of phosphorylation-mediated inhibition remains unknown. Here, we employ molecular dynamics (MD) simulations of the dephosphorylated and phosphorylated versions of KdpFABC to demonstrate that phosphorylated KdpB is trapped in a conformation where the ion-binding site is hydrated by an intracellular pathway between transmembrane helices M1 and M2 which opens in response to the rearrangement of cytoplasmic domains resulting from phosphorylation. Cytoplasmic access of water to the ion-binding site is accompanied by a remarkable loss of secondary structure of the KdpB N-terminus and disruption of a key salt bridge between Glu87 in the A domain and Arg212 in the P domain. Our results provide the molecular basis of a unique mechanism of regulation amongst P-type ATPases, and suggest that the N-terminus has a significant role to play in the conformational cycle and regulation of KdpFABC.

Structure of the human plasma membrane Ca2+-ATPase 1 in complex with its obligatory subunit neuroplastin

Plasma membrane Ca²⁺-ATPases (PMCAs) are key regulators of global Ca²⁺ homeostasis and local intracellular Ca²⁺ dynamics. Recently, Neuroplastin (NPTN) and basigin were identified as previously unrecognized obligatory subunits of PMCAs that dramatically increase the efficiency of PMCA-mediated Ca²⁺ clearance. Here, we report the cryo-EM structure of human PMCA1 (hPMCA1) in complex with NPTN at a resolution of 4.1 Å for the overall structure and 3.9 Å for the transmembrane domain. The single transmembrane helix of NPTN interacts with the TM_8-9-linker and TM10 of hPMCA1. The subunits are required for the hPMCA1 functional activity. The NPTN-bound hPMCA1 closely resembles the E1-Mg²⁺ structure of endo(sarco)plasmic reticulum Ca²⁺ ATPase and the Ca²⁺ site is exposed through a large open cytoplasmic pathway. This structure provides insight into how the subunits bind to the PMCAs and serves as an important basis for understanding the functional mechanisms of this essential calcium pump family.

The plasma membrane Ca²⁺ ATPase (PMCA) is essential for maintaining Ca²⁺ homeostasis in eukaryotic cells, and neuroplastin (NPTN) was recently identified as an obligatory subunit of PMCA. Here the authors present the cryo-EM structure of NPTN bound to human PMCA1, which reveals that the NPTN transmembrane (TM) helix interacts with TM10 and the TM8-9-linker of PMCA1.

Improved Model of Proton Pump Crystal Structure Obtained by Interactive Molecular Dynamics Flexible Fitting Expands the Mechanistic Model for Proton Translocation in P-Type ATPases

The plasma membrane H⁺-ATPase is a proton pump of the P-type ATPase family and essential in plants and fungi. It extrudes protons to regulate pH and maintains a strong proton-motive force that energizes e.g., secondary uptake of nutrients. The only crystal structure of a H⁺-ATPase (AHA2 from Arabidopsis thaliana) was reported in 2007. Here, we present an improved atomic model of AHA2, obtained by a combination of model rebuilding through interactive molecular dynamics flexible fitting (iMDFF) and structural refinement based on the original data, but using up-to-date refinement methods. More detailed map features prompted local corrections of the transmembrane domain, in particular rearrangement of transmembrane helices 7 and 8, and the cytoplasmic N- and P-domains, and the new model shows improved overall quality and reliability scores. The AHA2 structure shows similarity to the Ca²⁺-ATPase E1 state, and provides a valuable starting point model for structural and functional analysis of proton transport mechanism of P-type H⁺-ATPases. Specifically, Asp684 protonation associated with phosphorylation and occlusion of the E1P state may result from hydrogen bond interaction with Asn106. A subsequent deprotonation associated with extracellular release in the E2P state may result from an internal salt bridge formation to an Arg655 residue, which in the present E1 state is stabilized in a solvated pocket. A release mechanism based on an in-built counter-cation was also later proposed for Zn²⁺-ATPase, for which structures have been determined in Zn²⁺ released E2P-like states with the salt bridge interaction formed.

Sequential substitution of K(+) bound to Na(+),K(+)-ATPase visualized by X-ray crystallography

Na⁺,K⁺-ATPase transfers three Na⁺ from the cytoplasm into the extracellular medium and two K⁺ in the opposite direction per ATP hydrolysed. The binding and release of Na⁺ and K⁺ are all thought to occur sequentially. Here we demonstrate by X-ray crystallography of the ATPase in E2·MgF₄²⁻·2K⁺, a state analogous to E2·Pi·2K⁺, combined with isotopic measurements, that the substitution of the two K⁺ with congeners in the extracellular medium indeed occurs at different rates, substantially faster at site II. An analysis of thermal movements of protein atoms in the crystal shows that the M3–M4E helix pair opens and closes the ion pathway leading to the extracellular medium, allowing K⁺ at site II to be substituted first. Taken together, these results indicate that site I K⁺ is the first cation to bind to the empty cation-binding sites after releasing three Na⁺.

The Na⁺,K⁺-ATPase moves three Na⁺ ions out of the cell and transfers two K⁺ ions in the opposite direction. Here the authors use X-ray crystallography to look at the substitution of two bound K⁺ with those in the medium and show that it occurs sequentially through a narrow gate.

Somatic mutations of the ATP1A1 gene and aldosterone-producing adenomas

Primary aldosteronism is the most common form of secondary hypertension. It affects approximately 10% of patients with hypertension and causes greater cardiovascular morbidity and mortality compared to essential hypertension of similar severity and duration. The cause of primary aldosteronism in about half of these patients is an aldosterone-producing adenoma; over half of these adenomas have mutations in one of several ion channels and pumps, including the potassium channel KCNJ5, calcium channel Cav1.3, α1 subunit of the sodium potassium ATPase, and membrane calcium ATPase 3. This review concentrates on the molecular and physiological mechanisms by which mutations of the ATP1A1 gene increase aldosterone production.

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. In the related ion pumps Na(+),K(+)-ATPase and Ca(2+)-ATPase, M4 moves during the enzyme cycle, carrying along the ion bound to a glutamate. In ATP8A2, the corresponding residue is an isoleucine, which recently was found mutated in patients with cerebellar ataxia, mental retardation, and dysequilibrium syndrome. Our analyses of the lipid substrate concentration dependence of the overall and partial reactions of the enzyme cycle in mutants indicate that, during the transport across the membrane, the phosphatidylserine head group passes near isoleucine-364 (I364) and that I364 is critical to the release of the transported lipid into the cytosolic leaflet. Another M4 residue, N359, is involved in recognition of the lipid substrate on the exoplasmic side. Our functional studies are supported by structural homology modeling and molecular dynamics simulations, suggesting that I364 and adjacent hydrophobic residues function as a hydrophobic gate that separates the entry and exit sites of the lipid and directs sequential formation and annihilation of water-filled cavities, thereby enabling transport of the hydrophilic phospholipid head group in a groove outlined by the transmembrane segments M1, M2, M4, and M6, with the hydrocarbon chains following passively, still in the membrane lipid phase.

Hyperactivation of the human plasma membrane Ca2+ pump PMCA h4xb by mutation of Glu99 to Lys

The transport of calcium to the extracellular space carried out by plasma membrane Ca(2+) pumps (PMCAs) is essential for maintaining low Ca(2+) concentrations in the cytosol of eukaryotic cells. The activity of PMCAs is controlled by autoinhibition. Autoinhibition is relieved by the binding of Ca(2+)-calmodulin to the calmodulin-binding autoinhibitory sequence, which in the human PMCA is located in the C-terminal segment and results in a PMCA of high maximal velocity of transport and high affinity for Ca(2+). Autoinhibition involves the intramolecular interaction between the autoinhibitory domain and a not well defined region of the molecule near the catalytic site. Here we show that the fusion of GFP to the C terminus of the h4xb PMCA causes partial loss of autoinhibition by specifically increasing the Vmax. Mutation of residue Glu(99) to Lys in the cytosolic portion of the M1 transmembrane helix at the other end of the molecule brought the Vmax of the h4xb PMCA to near that of the calmodulin-activated enzyme without increasing the apparent affinity for Ca(2+). Altogether, the results suggest that the autoinhibitory interaction of the extreme C-terminal segment of the h4 PMCA is disturbed by changes of negatively charged residues of the N-terminal region. This would be consistent with a recently proposed model of an autoinhibited form of the plant ACA8 pump, although some differences are noted.

A conserved asparagine in a P-type proton pump is required for efficient gating of protons

The minimal proton pumping machinery of the Arabidopsis thaliana P-type plasma membrane H(+)-ATPase isoform 2 (AHA2) consists of an aspartate residue serving as key proton donor/acceptor (Asp-684) and an arginine residue controlling the pKa of the aspartate. However, other important aspects of the proton transport mechanism such as gating, and the ability to occlude protons, are still unclear. An asparagine residue (Asn-106) in transmembrane segment 2 of AHA2 is conserved in all P-type plasma membrane H(+)-ATPases. In the crystal structure of the plant plasma membrane H(+)-ATPase, this residue is located in the putative ligand entrance pathway, in close proximity to the central proton donor/acceptor Asp-684. Substitution of Asn-106 resulted in mutant enzymes with significantly reduced ability to transport protons against a membrane potential. Sensitivity toward orthovanadate was increased when Asn-106 was substituted with an aspartate residue, but decreased in mutants with alanine, lysine, glutamine, or threonine replacement of Asn-106. The apparent proton affinity was decreased for all mutants, most likely due to a perturbation of the local environment of Asp-684. Altogether, our results demonstrate that Asn-106 is important for closure of the proton entrance pathway prior to proton translocation across the membrane.

Ion pathways in the sarcoplasmic reticulum Ca2+-ATPase

The sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) is a transmembrane ion transporter belonging to the P(II)-type ATPase family. It performs the vital task of re-sequestering cytoplasmic Ca(2+) to the sarco/endoplasmic reticulum store, thereby also terminating Ca(2+)-induced signaling such as in muscle contraction. This minireview focuses on the transport pathways of Ca(2+) and H(+) ions across the lipid bilayer through SERCA. The ion-binding sites of SERCA are accessible from either the cytoplasm or the sarco/endoplasmic reticulum lumen, and the Ca(2+) entry and exit channels are both formed mainly by rearrangements of four N-terminal transmembrane α-helices. Recent improvements in the resolution of the crystal structures of rabbit SERCA1a have revealed a hydrated pathway in the C-terminal transmembrane region leading from the ion-binding sites to the cytosol. A comparison of different SERCA conformations reveals that this C-terminal pathway is exclusive to Ca(2+)-free E2 states, suggesting that it may play a functional role in proton release from the ion-binding sites. This is in agreement with molecular dynamics simulations and mutational studies and is in striking analogy to a similar pathway recently described for the related sodium pump. We therefore suggest a model for the ion exchange mechanism in P(II)-ATPases including not one, but two cytoplasmic pathways working in concert.

Na(+),K (+)-ATPase as a docking station: protein-protein complexes of the Na(+),K (+)-ATPase

The Na(+),K(+)-ATPase, or sodium pump, is well known for its role in ion transport across the plasma membrane of animal cells. It carries out the transport of Na(+) ions out of the cell and of K(+) ions into the cell and thus maintains electrolyte and fluid balance. In addition to the fundamental ion-pumping function of the Na(+),K(+)-ATPase, recent work has suggested additional roles for Na(+),K(+)-ATPase in signal transduction and biomembrane structure. Several signaling pathways have been found to involve Na(+),K(+)-ATPase, which serves as a docking station for a fast-growing number of protein interaction partners. In this review, we focus on Na(+),K(+)-ATPase as a signal transducer, but also briefly discuss other Na(+),K(+)-ATPase protein-protein interactions, providing a comprehensive overview of the diverse signaling functions ascribed to this well-known enzyme.

The sarcoplasmic Ca2+-ATPase: design of a perfect chemi-osmotic pump

The sarcoplasmic (SERCA 1a) Ca2+-ATPase is a membrane protein abundantly present in skeletal muscles where it functions as an indispensable component of the excitation-contraction coupling, being at the expense of ATP hydrolysis involved in Ca2+/H+ exchange with a high thermodynamic efficiency across the sarcoplasmic reticulum membrane. The transporter serves as a prototype of a whole family of cation transporters, the P-type ATPases, which in addition to Ca2+ transporting proteins count Na+, K+-ATPase and H+, K+-, proton- and heavy metal transporting ATPases as prominent members. The ability in recent years to produce and analyze at atomic (2·3-3 Å) resolution 3D-crystals of Ca2+-transport intermediates of SERCA 1a has meant a breakthrough in our understanding of the structural aspects of the transport mechanism. We describe here the detailed construction of the ATPase in terms of one membraneous and three cytosolic domains held together by a central core that mediates coupling between Ca2+-transport and ATP hydrolysis. During turnover, the pump is present in two different conformational states, E1 and E2, with a preference for the binding of Ca2+ and H+, respectively. We discuss how phosphorylated and non-phosphorylated forms of these conformational states with cytosolic, occluded or luminally exposed cation-binding sites are able to convert the chemical energy derived from ATP hydrolysis into an electrochemical gradient of Ca2+ across the sarcoplasmic reticulum membrane. In conjunction with these basic reactions which serve as a structural framework for the transport function of other P-type ATPases as well, we also review the role of the lipid phase and the regulatory and thermodynamic aspects of the transport mechanism.

A new Italian FHM2 family: clinical aspects and functional analysis of the disease-associated mutation

OBJECTIVE

To describe a new FHM kindred, and to analyse the functional consequences of the disease-associated ATP1A2 p.G301R mutation in human cellular models grown at 37°C.

PATIENTS AND METHODS

Seven patients were clinically evaluated and gave informed consent for molecular analysis. Extra-pyramidal rigidity of the limbs was present in four subjects and in three of them tongue apraxia was also observed. ATP1A2 and CACNA1A were analysed by direct sequencing. Functional consequences of the mutation were investigated by cell viability assays, Western blots, and immunocytochemistry. Three-dimensional models of the human Na(+)/K(+)-ATPase α2 subunit were generated by homology modelling using SWISS-MODEL.

FINDINGS

Analysis of ATP1A2 showed a heterozygous mutation, c.901G>A predicting the replacement of arginine for glycine at residue 301 (p.G301R). Functional analysis suggested that the mutation completely abolished Na(+)/K(+)-ATPase function.

CONCLUSIONS

The phenotypic spectrum of our FHM2 family includes some peculiar features. Functional data confirm that Na(+)/K(+)-ATPase haploinsufficiency caused by the ATP1A2 p.G301R mutation is responsible for FHM in the described family.

In and out of the cation pumps: P-type ATPase structure revisited

Active transport across membranes is a crucial requirement for life. P-type ATPases build up electrochemical gradients at the expense of ATP by forming and splitting a covalent phosphoenzyme intermediate, coupled to conformational changes in the transmembrane section where the ions are translocated. The marked increment during the last three years in the number of crystal structures of P-type ATPases has greatly improved our understanding of the similarities and differences of pumps with different ion specificities, since the structures of the Ca2+-ATPase, the Na+,K+-ATPase and the H+-ATPase can now be compared directly. Mechanisms for ion gating, charge neutralization and backflow prevention are starting to emerge from comparative structural analysis; and in combination with functional studies of mutated pumps this provides a framework for speculating on how the ions are bound and released as well as on how specificity is achieved.