Structure and function of the P-type ATPases

Cloning and Characterization of Two Putative P-Type ATPases from the Marine Microalga Dunaliella maritima Similar to Plant H+-ATPases and Their Gene Expression Analysis under Conditions of Hyperosmotic Salt Shock

The green microalga genus Dunaliella is mostly comprised of species that exhibit a wide range of salinity tolerance, including inhabitants of hyperhaline reservoirs. Na+ content in Dunaliella cells inhabiting saline environments is maintained at a fairly low level, comparable to that in the cells of freshwater organisms. However, despite a long history of studying the physiological and molecular mechanisms that ensure the ability of halotolerant Dunaliella species to survive at high concentrations of NaCl, the question of how Dunaliella cells remove excess Na+ ions entering from the environment is still debatable. For thermodynamic reasons it should be a primary active mechanism; for example, via a Na+-transporting ATPase, but the molecular identification of Na+-transporting mechanism in Dunaliella has not yet been carried out. Formerly, in the euryhaline alga D. maritima, we functionally identified Na+-transporting P-type ATPase in experiments with plasma membrane (PM) vesicles which were isolated from this alga. Here we describe the cloning of two putative P-type ATPases from D. maritima, DmHA1 and DmHA2. Phylogenetic analysis showed that both ATPases belong to the clade of proton P-type ATPases, but the similarity between DmHA1 and DmHA2 is not high. The expression of DmHA1 and DmHA2 in D. maritima cells under hyperosmotic salt shock was studied by qRT-PCR. Expression of DmHA1 gene decreases and remains at a relatively low level during the response of D. maritima cells to hyperosmotic salt shock. In contrast, expression of DmHA2 increases under hyperosmotic salt shock. This indicates that DmHA2 is important for overcoming hyperosmotic salt stress by the algal cells and as an ATPase it is likely directly involved in transport of Na+ ions. We assume that it is the DmHA2 ATPase that represents the Na+-transporting ATPase.

Cu transporter protein CrpF protects against Cu-induced toxicity in Fusarium oxysporum

Cu is an essential trace element for cell growth and proliferation. However, excess of Cu accumulation leads to cellular toxicity. Thus, precise and tight regulation of Cu homeostasis processes, including transport, delivery, storage, detoxification, and efflux machineries, is required. Moreover, the maintenance of Cu homeostasis is critical for the survival and virulence of fungal pathogens. Cu homeostasis has been extensively studied in mammals, bacteria, and yeast, but it has not yet been well documented in filamentous fungi. In the present work, we investigated Cu tolerance in the filamentous fungus Fusarium oxysporum by analysing the Cu transporter coding gene crpF, previously studied in Aspergillus fumigatus. The expression studies demonstrated that crpF is upregulated in the presence of Cu and its deletion leads to severe sensitivity to low levels of CuSO₄ in F. oxysporum. Targeted deletion of crpF did not significantly alter the resistance of the fungus to macrophage killing, nor its pathogenic behaviour on the tomato plants. However, the targeted deletion mutant ΔcrpF showed increased virulence in a murine model of systemic infection compared to wild-type strain (wt).

Multifunction of the ER P-Type Calcium Pump Spf1 During Hyphal Development in Candida albicans

Candida albicans is one of the most important fungal pathogens. Hyphal development is required for the virulence of this pathogen. Our previous study has revealed that Spf1, an ER P-type calcium pump, plays an important role in hyphal development. However, the detailed mechanisms by which this protein functions in this process remain to be investigated. In this study, we found that loss of Spf1 led to decreased growth biomass under the hypha-inducing condition, suggesting a role of this protein in maintaining hyphal growth rate. Actin staining further revealed that the spf1Δ/Δ mutant showed attenuated tip-localization of actin patches and the defect in transport of both the chitin synthase Chs3 and the hypha-related factor Hwp1, implying that Spf1 functions in polarized growth of the hyphae by regulating actin organization and consequent polarized transport of morphogenesis-associated factors. Moreover, deletion of SPF1 led to abnormal vacuolar morphology under the hypha-inducing condition, which may also contribute to the defect of hyphal development in the spf1Δ/Δ mutant. This study revealed the pleiotropic role of Spf1-regulated calcium homeostasis in controlling hyphal development in C. albicans.

The P-type ATPase Spf1 is required for endoplasmic reticulum functions and cell wall integrity in Candida albicans

Endoplasmic reticulum (ER) is crucial for protein folding, glycosylation and secretion in eukaryotic organisms. These important functions are supported by high levels of Ca(2+) in the ER. We have recently identified a putative ER Ca(2+) pump in Candida albicans, called Spf1, which plays key roles in maintenance of cellular Ca(2+) homeostasis, morphogenesis and virulence. In this study, we purified Spf1 and confirmed that it is a P-type ATPase, suggesting its role in maintaining high levels of ER Ca(2+). Disruption of SPF1 caused severe defects in glycosylation of the ER-localized protein Cdc101 and secretory acid phosphatase, and a decrease in expression of SEC61 which encodes an important ER protein. Moreover, the spf1Δ/Δ mutant showed increased sensitivity to cell wall stresses, abnormal cell wall composition, delayed cell wall reconstruction and decreased flocculation and adherence, indicating its defect in cell wall integrity (CWI). We also revealed that disruption of SPF1 has an impact on gene expression related to CWI and morphogenesis. This study provides evidence that Spf1, as a P-type ATPase, is essential for ER functions and consequent CWI, implicating a role of ER Ca(2+) homeostasis in C. albicans physiology.

Characterization of cellular protective effects of ATP13A2/PARK9 expression and alterations resulting from pathogenic mutants

Mutations in ATP13A2, which encodes a lysosomal P-type ATPase of unknown function, cause an autosomal recessive parkinsonian syndrome. With mammalian cells, we show that ATP13A2 expression protects against manganese and nickel toxicity, in addition to proteasomal, mitochondrial, and oxidative stress. Consistent with a recessive mode of inheritance of gene defects, disease-causing mutations F182L and G504R are prone to misfolding and do not protect against manganese and nickel toxicity because they are unstable as a result of degradation via the endoplasmic reticulum-associated degradation (ERAD)-proteasome system. The protective effects of ATP13A2 expression are not due to inhibition of apoptotic pathways or a reduction in typical stress pathways, insofar as these pathways are still activated in challenged ATP13A2-expressing cells; however, these cells display a dramatic reduction in the accumulation of oxidized and damaged proteins. These data indicate that, contrary to a previous suggestion, ATP13A2 is unlikely to convey cellular resilience simply by acting as a lysosomal manganese transporter. Consistent with the recent identification of an ATP13A2 recessive mutation in Tibetan terriers that develop neurodegeneration with neuronal ceroid lipofucinoses, our data suggest that ATP13A2 may function to import a cofactor required for the function of a lysosome enzyme(s).

Apical targeting and Golgi retention signals reside within a 9-amino acid sequence in the copper-ATPase, ATP7B

ATP7B is a copper-transporting P-type ATPase present predominantly in liver. In basal copper, hepatic ATP7B is in a post-trans-Golgi network (TGN) compartment where it loads cytoplasmic Cu(I) onto newly synthesized ceruloplasmin. When copper levels rise, the protein redistributes via unique vesicles to the apical periphery where it exports intracellular Cu(I) into bile. We want to understand the mechanisms regulating the copper-sensitive trafficking of ATP7B. Earlier, our laboratory reported the presence of apical targeting/TGN retention information within residues 1-63 of human ATP7B; deletion of these residues resulted in a mutant protein that was not efficiently retained in the post-TGN in low copper and constitutively trafficked to the basolateral membrane of polarized, hepatic WIF-B cells with and without copper (13). In this study, we used mutagenesis and adenovirus infection of WIF-B cells followed by confocal immunofluorescence microscopy analysis to identify the precise retention/targeting sequences in the context of full-length ATP7B. We also analyzed the expression of selected mutants in livers of copper-deficient and -loaded mice. Our combined results clearly demonstrate that nine amino acids, F(37)AFDNVGYE(45), comprise an essential apical targeting determinant for ATP7B in elevated copper and participate in the TGN retention of the protein under low-copper conditions. The signal is novel, does not require phosphorylation, and is highly conserved in approximately 24 species of ATP7B. Furthermore, N41S, which is part of the signal we identified, is the first and only Wilson disease-causing missense mutation in residues 1-63 of ATP7B. Expression of N41S-ATP7B in WIF-B cells severely disabled the targeting and retention of the protein. We present a working model of how this physiologically relevant signal might work.

Niemann-Pick C1 protein transports copper to the secretory compartment from late endosomes where ATP7B resides

Wilson disease is a genetic disorder characterized by the accumulation of copper in the body by defective biliary copper excretion. Wilson disease gene product (ATP7B) functions in copper incorporation to ceruloplasmin (Cp) and biliary copper excretion. However, copper metabolism in hepatocytes has been still unclear. Niemann-Pick disease type C (NPC) is a lipid storage disorder and the most commonly mutated gene is NPC1 and its gene product NPC1 is a late endosome protein and regulates intracellular vesicle traffic. In the present study, we induced NPC phenotype and examined the localization of ATP7B and secretion of holo-Cp, a copper-binding mature form of Cp. The vesicle traffic was modulated using U18666A, which induces NPC phenotype, and knock down of NPC1 by RNA interference. ATP7B colocalized with the late endosome markers, but not with the trans-Golgi network markers. U18666A and NPC1 knock down decreased holo-Cp secretion to culture medium, but did not affect the secretion of other secretory proteins. Copper accumulated in the cells after the treatment with U18666A. These findings suggest that ATP7B localizes in the late endosomes and that copper in the late endosomes is transported to the secretory compartment via NPC1-dependent pathway and incorporated into apo-Cp to form holo-Cp.

Nonlinear relaxation dynamics in elastic networks and design principles of molecular machines

Analyzing nonlinear conformational relaxation dynamics in elastic networks corresponding to two classical motor proteins, we find that they respond by well defined internal mechanical motions to various initial deformations and that these motions are robust against external perturbations. We show that this behavior is not characteristic for random elastic networks. However, special network architectures with such properties can be designed by evolutionary optimization methods. Using them, an example of an artificial elastic network, operating as a cyclic machine powered by ligand binding, is constructed.

Copper binding to the N-terminal metal-binding sites or the CPC motif is not essential for copper-induced trafficking of the human Wilson protein (ATP7B)

The Wilson protein (ATP7B) is a copper-translocating P-type ATPase that mediates the excretion of excess copper from hepatocytes into bile. Excess copper causes the protein to traffic from the TGN (trans-Golgi network) to subapical vesicles. Using site-directed mutagenesis, mutations known or predicted to abrogate catalytic activity (copper translocation) were introduced into ATP7B and the effect of these mutations on the intracellular trafficking of the protein was investigated. Mutation of the critical aspartic acid residue in the phosphorylation domain (DKTGTIT) blocked copper-induced redistribution of ATP7B from the TGN, whereas mutation of the phosphatase domain [TGE (Thr-Gly-Glu)] trapped ATP7B at cytosolic vesicular compartments. Our findings demonstrate that ATP7B trafficking is regulated with its copper-translocation cycle, with cytosolic vesicular localization associated with the acyl-phosphate intermediate. In addition, mutation of the six N-terminal metal-binding sites and/or the trans-membrane CPC (Cys-Pro-Cys) motif did not suppress the constitutive vesicular localization of the ATP7B phosphatase domain mutant. These results suggested that copper co-ordination by these sites is not essential for trafficking. Importantly, copper-chelation studies with these mutants clearly demonstrated a requirement for copper in ATP7B trafficking, suggesting the presence of an additional copper-binding site(s) within the protein. The results presented in this report significantly advance our understanding of the regulatory mechanism that links copper-translocation activity with copper-induced intracellular trafficking of ATP7B, which is central to hepatic and hence systemic copper homoeostasis.

NH2-terminal signals in ATP7B Cu-ATPase mediate its Cu-dependent anterograde traffic in polarized hepatic cells

Cu is an essential cofactor of cellular proteins but is toxic in its free state. The hepatic Cu-ATPase ATP7B has two functions in Cu homeostasis: it loads Cu+ onto newly synthesized apoceruloplasmin in the secretory pathway, thereby activating the plasma protein; and it participates in the excretion of excess Cu+ into the bile. To carry out these two functions, the membrane protein responds to changes in intracellular Cu levels by cycling between the Golgi and apical region. We used polarized hepatic WIF-B cells and high-resolution confocal microscopy to map the itinerary of endogenous and exogenous ATP7B under different Cu conditions. In Cu-depleted cells, ATP7B resided in a post-trans-Golgi network compartment that also contained syntaxin 6, whereas in Cu-loaded cells, the protein relocated to unique vesicles very near to the apical plasma membrane as well as the membrane itself. To determine the role of ATP7B's cytoplasmic NH2 terminus in regulating its intracellular movements, we generated seven mutations/deletions in this large [approximately 650 amino acid (AA)] domain and analyzed the Cu-dependent behavior of the mutant ATP7B proteins in WIF-B cells. Truncation of the ATP7B NH2 terminus up to the fifth copper-binding domain (CBD5) yielded an active ATPase that was insensitive to cellular Cu levels and constitutively trafficked to the opposite (basolateral) plasma membrane domain. Fusion of the NH2-terminal 63 AA of ATP7B to the truncated protein restored both its Cu responsiveness and correct intracellular targeting. These results indicate that important targeting information is contained in this relatively short sequence, which is absent from the related CuATPase, ATP7A.

Elucidation of the Na+, K+-ATPase digitalis binding site

Despite controversy over their use and the potential for toxic side effects, cardiac glycosides have remained an important clinical component for the treatment for congestive heart failure (CHF) and supraventricular arrhythmias since the effects of Digitalis purpurea were first described in 1785. While there is a wealth of information available with regard to the effects of these drugs on their pharmacological receptor, the Na(+), K(+)-ATPase, the exact molecular mechanism of digitalis binding and inhibition of the enzyme has remained elusive. In particular, the absence of structural knowledge about Na(+), K(+)-ATPase has thwarted the development of improved therapeutic agents with larger therapeutic indices via rational drug design approaches. Here, we propose a binding mode for digoxin and several analogues to the Na(+), K(+)-ATPase. A 3D-structural model of the extracellular loop regions of the catalytic alpha1-subunit of the digitalis-sensitive sheep Na(+), K(+)-ATPase was constructed from the crystal structure of an E(1)Ca(2+) conformation of the SERCA1a and a consensus orientation for digitalis binding was inferred from the in silico docking of a series of steroid-based cardiotonic compounds. Analyses of species-specific enzyme affinities for ouabain were also used to validate the model and, for the first time, propose a detailed model of the digitalis binding site.

How do P-Type ATPases transport ions?

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 of one or two ion species across the respective membrane. The pump function of the investigated pumps is described by a so-called Post-Albers cycle. Main features of the pumping process are (1) a Ping-Pong mechanism, i.e. both transported ion species are transferred successively and in opposite direction across the membrane, (2) the transport process for each ion species consists of a sequence of reaction steps, which are ion binding, ion occlusion, conformational transition of the protein, successive deocclusion of the ions and release to the other side of the membrane. (3) Recent experimental evidence shows that the ion-binding sites are placed in the transmembrane section of the proteins and that ion movements occur preferentially during the ion binding and release processes. The main features of the mechanism include narrow access channels from both sides, one gate per access channel, and an ion-binding moiety that is adapted specifically to the ions that are transported, and differently in both principal conformations.

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 ATPases: a new family for a family-based drug design approach

The rapid discovery of new drugs is greatly facilitated when a family of related proteins is targeted with a similar approach in chemistry. Few protein families have so far been investigated using this kind of 'family-based' approach. Therefore, to increase the size of our Pharmacopeia and to cure human diseases more efficiently, new druggable protein families must be identified. It is shown in this review that ATPases are very good candidates for a family-based approach. The human proteome contains many ATPases, which are involved in several diseases. All the ATPases contain a nucleotide-binding site, and it is therefore possible to target all of them with a single strategy in chemistry; the design of competitive ATP inhibitors. Moreover, because a similar approach has been conducted with the protein kinases, the compound libraries and the knowledge developed in the kinase field can be directly applied to the ATPases.

Ralstonia metallidurans, a bacterium specifically adapted to toxic metals: towards a catalogue of metal-responsive genes

Ralstonia metallidurans, formerly known as Alcaligenes eutrophus and thereafter as Ralstonia eutropha, is a beta-Proteobacterium colonizing industrial sediments, soils or wastes with a high content of heavy metals. The type strain CH34 carries two large plasmids (pMOL28 and pMOL30) bearing a variety of genes for metal resistance. A chronological overview describes the progress made in the knowledge of the plasmid-borne metal resistance mechanisms, the genetics of R. metallidurans CH34 and its taxonomy, and the applications of this strain in the fields of environmental remediation and microbial ecology. Recently, the sequence draft of the genome of R. metallidurans has become available. This allowed a comparison of these preliminary data with the published genome data of the plant pathogen Ralstonia solanacearum, which harbors a megaplasmid (of 2.1 Mb) carrying some metal resistance genes that are similar to those found in R. metallidurans CH34. In addition, a first inventory of metal resistance genes and operons across these two organisms could be made. This inventory, which partly relied on the use of proteomic approaches, revealed the presence of numerous loci not only on the large plasmids pMOL28 and pMOL30 but also on the chromosome. It suggests that metal-resistant Ralstonia, through evolution, are particularly well adapted to the harsh environments typically created by extreme anthropogenic situations or biotopes.

Chloride ATPase pumps in nature: do they exist?

Five widely documented mechanisms for chloride transport across biological membranes are known: anion-coupled antiport, Na+ and H(+)-coupled symport, Cl- channels and an electrochemical coupling process. These transport processes for chloride are either secondarily active or are driven by the electrochemical gradient for chloride. Until recently, the evidence in favour of a primary active transport mechanism for chloride has been inconclusive despite numerous reports of cellular Cl(-)-stimulated ATPases coexisting, in the same tissue, with uphill ATP-dependent chloride transport. Cl(-)-stimulated ATPase activity is a ubiquitous property of practically all cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase pump activity. Recent studies of Cl(-) -stimulated ATPase activity and ATP-dependent chloride transport in the same plasma membrane system, including liposomes, strongly suggest a mediation by the ATPase in the net movement of chloride up its electrochemical gradient across the plasma membrane structure. Contemporary evidence points to the existence of Cl(-)-ATPase pumps; however, these primary active transporters exist as either P-, F- or V-type ATPase pumps depending upon the tissue under study.

Molecular architecture of the phosphorylation region of the yeast plasma membrane H+-ATPase

The molecular architecture of the yeast plasma membrane H(+)-ATPase phosphorylation region was explored by Fe(2+)-catalyzed cleavage. An ATP-Mg(2+).Fe(2+) complex was found to act as an affinity cleavage reagent in the presence of dithiothreitol/H(2)O(2). Selective enzyme cleavage required bound adenine nucleotide, either ATP or ADP, in the presence of Mg(2+). The fragment profile included a predominant N-terminal 61-kDa fragment, a minor 37-kDa fragment, and three prominent C-terminal fragments of 39, 36, and 30 kDa. The 61-kDa N-terminal and 39-kDa C-terminal fragments were predicted to originate from cleavage within the conserved MLT(558)GDAVG sequence. The 37-kDa fragment was consistent with cleavage within the S4/M4 sequence PVGLPA(340)V, while the 30-kDa and 36-kDa C-terminal fragments appeared to originate from cleavage in or around sequences D(646)TGIAVE and DMPGS(595)ELADF, respectively. The latter are spatially close to the highly conserved motif GD(634)GVND(638)APSL and conserved residues Thr(558) and Lys(615), which have been implicated in coordinating Mg(2+) and ATP. Overall, these results demonstrate that Fe(2+) associated with ATP and Mg(2+) acts as an affinity cleavage agent of the H(+)-ATPase with backbone cleavage occurring in conserved regions known to coordinate metal-nucleotide complexes. This study provides support for a three-dimensional organization of the phosphorylation region of the yeast plasma membrane H(+)-ATPase that is consistent with, but not identical to, typical P-type enzymes.

Modeling the three-dimensional structure of H+-ATPase of Neurospora crassa

Homology modeling in combination with transmembrane topology predictions are used to build the atomic model of Neurospora crassa plasma membrane H+-ATPase, using as template the 2.6 A crystal structure of rabbit sarcoplasmic reticulum Ca2+-ATPase [Toyoshima, C., Nakasako, M., Nomura, H. & Ogawa, H. (2000) Nature 405, 647-655]. Comparison of the two calcium-binding sites in the crystal structure of Ca2+-ATPase with the equivalent region in the H+-ATPase model shows that the latter is devoid of most of the negatively charged groups required to bind the cations, suggesting a different role for this region. Using the built model, a pathway for proton transport is then proposed from computed locations of internal polar cavities, large enough to contain at least one water molecule. As a control, the same approach is applied to the high-resolution crystal structure of halorhodopsin and the proton pump bacteriorhodopsin. This revealed a striking correspondence between the positions of internal polar cavities, those of crystallographic water molecules and, in the case of bacteriorhodopsin, the residues mediating proton translocation. In our H+-ATPase model, most of these cavities are in contact with residues previously shown to affect coupling of proton translocation to ATP hydrolysis. A string of six polar cavities identified in the cytoplasmic domain, the most accurate part of the model, suggests a proton entry path starting close to the phosphorylation site. Strikingly, members of the haloacid dehalogenase superfamily, which are close structural homologs of this domain but do not share the same function, display only one polar cavity in the vicinity of the conserved catalytic Asp residue.

Cod1p/Spf1p is a P-type ATPase involved in ER function and Ca2+ homeostasis

The internal environment of the ER is regulated to accommodate essential cellular processes, yet our understanding of this regulation remains incomplete. Cod1p/Spf1p belongs to the widely conserved, uncharacterized type V branch of P-type ATPases, a large family of ion pumps. Our previous work suggested Cod1p may function in the ER. Consistent with this hypothesis, we localized Cod1p to the ER membrane. The cod1Delta mutant disrupted cellular calcium homeostasis, causing increased transcription of calcium-regulated genes and a synergistic increase in cellular calcium when paired with disruption of the Golgi apparatus-localized Ca2+ pump Pmr1p. Deletion of COD1 also impaired ER function, causing constitutive activation of the unfolded protein response, hypersensitivity to the glycosylation inhibitor tunicamycin, and synthetic lethality with deletion of the unfolded protein response regulator HAC1. Expression of the Drosophila melanogaster homologue of Cod1p complemented the cod1Delta mutant. Finally, we demonstrated the ATPase activity of the purified protein. This study provides the first biochemical characterization of a type V P-type ATPase, implicates Cod1p in ER function and ion homeostasis, and indicates that these functions are conserved among Cod1p's metazoan homologues.

Functional properties of the copper-transporting ATPase ATP7B (the Wilson's disease protein) expressed in insect cells

Copper-transporting ATPase ATP7B is essential for normal distribution of copper in human cells. Mutations in the ATP7B gene lead to copper accumulation in a number of tissues and to a severe multisystem disorder, known as Wilson's disease. Primary sequence analysis suggests that the copper-transporting ATPase ATP7B or the Wilson's disease protein (WNDP) belongs to the large family of cation-transporting P-type ATPases, however, the detailed characterization of its enzymatic properties has been lacking. Here, we developed a baculovirus-mediated expression system for WNDP, which permits direct and quantitative analysis of catalytic properties of this protein. Using this system, we provide experimental evidence that WNDP has functional properties characteristic of a P-type ATPase. It forms a phosphorylated intermediate, which is sensitive to hydroxylamine, basic pH, and treatments with ATP or ADP. ATP stimulates phosphorylation with an apparent K(m) of 0.95 +/- 0.25 microm; ADP promotes dephosphorylation with an apparent K(m) of 3.2 +/- 0.7 microm. Replacement of Asp(1027) with Ala in a conserved sequence motif DKTG abolishes phosphorylation in agreement with the proposed role of this residue as an acceptor of phosphate during the catalytic cycle. Catalytic phosphorylation of WNDP is inhibited by the copper chelator bathocuproine; copper reactivates the bathocuproine-treated WNDP in a specific and cooperative fashion confirming that copper is required for formation of the acylphosphate intermediate. These studies establish the key catalytic properties of the ATP7B copper-transporting ATPase and provide a foundation for quantitative analysis of its function in normal and diseased cells.

The reproductive importance of P-type ATPases

P-type ATPases are integral membrane proteins that use the free energy of ATP hydrolysis to generate transmembrane electrochemical ion gradients to support a variety of cellular processes. They have eight signature motifs, eight or ten transmembrane domains, highly conserved phosphorylation and ATP-binding sites, and similar hydropathic profiles. This review summarizes recent insights in the relationship of P-type ATPases to successful reproduction, and the hormone dependence of some family members. Because protein topology is central to understanding the pump action of this family of enzymes, this review also describes the dramatic change in the primary structure of one family member that may mediate transcription in the uterus.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

Cloning and characterization of an atypical Type IV P-type ATPase that binds to the RING motif of RUSH transcription factors

RUSH proteins are SWI/SNF-related transcription factors with RING finger signatures near their COOH termini. Long suspected of mediating protein-protein interactions, the RING motif was used to clone a binding partner. The RING finger binding protein (RFBP) is a Type IV P-type ATPase, a putative phospholipid pump, with conserved sequences for two loop segments, an ATP-binding site, a phosphorylation domain, and transmembrane passes potentially involved in substrate binding and translocation. However, RFBP differs from all other Type IV P-type ATPases in three ways. It has only three of four highly conserved NH(2)-terminal transmembrane passes, it is located in the inner nuclear membrane, and it binds the RING domain. Topographically the orientation of the adjacent hydrophilic domains and the determinants of transport specificity are altered. As a result, the small, hydrophilic loop extends into the perinuclear space that is contiguous with the lumen of the endoplasmic reticulum. The large, conformationally flexible loop extends into the nucleoplasm to contact euchromatin. Competitive reverse transcriptase-polymerase chain reaction and high performance liquid chromatography analysis revealed that endometrial RFBP mRNA expression is hormonally regulated. The physical association of a hormone-dependent RING finger-binding protein with transcriptionally active chromatin supports the speculation that RFBP plays a role in the subnuclear trafficking of transcription factors with RING motifs.

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

Autoinhibition of a calmodulin-dependent calcium pump involves a structure in the stalk that connects the transmembrane domain to the ATPase catalytic domain

The regulation of Ca(2+)-pumps is important for controlling [Ca(2+)] in the cytosol and organelles of all eukaryotes. Here, we report a genetic strategy to identify residues that function in autoinhibition of a novel calmodulin-activated Ca(2+)-pump with an N-terminal regulatory domain (isoform ACA2 from Arabidopsis). Mutant pumps with constitutive activity were identified by complementation of a yeast (K616) deficient in two Ca(2+)-pumps. Fifteen mutations were found that disrupted a segment of the N-terminal autoinhibitor located between Lys(23) and Arg(54). Three mutations (E167K, D219N, and E341K) were found associated with the stalk that connects the ATPase catalytic domain (head) and with the transmembrane domain. Enzyme assays indicated that the stalk mutations resulted in calmodulin-independent activity, with V(max), K(mATP), and K(mCa(2+)) similar to that of a pump in which the N-terminal autoinhibitor had been deleted. A highly conservative substitution at Asp(219) (D219E) still produced a deregulated pump, indicating that the autoinhibitory structure in the stalk is highly sensitive to perturbation. In plasma membrane H(+)-ATPases from yeast and plants, similarly positioned mutations resulted in hyperactive pumps. Together, these results suggest that a structural feature of the stalk is of general importance in regulating diverse P-type ATPases.

Gene knockout studies of Ca2+-transporting ATPases

The biochemical functions of intracellular and plasma membrane Ca2+-transporting ATPases in the control of cytosolic and organellar Ca2+ levels are well established, but the physiological roles of specific isoforms are less well understood. There appear to be three different types of Ca2+ pumps in mammalian tissues: the sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs), which sequester Ca2+ within the endoplasmic or sarcoplasmic reticulum, the plasma membrane Ca2+-ATPases (PMCAs), which extrude Ca2+ from the cell, and the putative secretory pathway Ca2+-ATPase (SPCA), the function of which is poorly understood. This review describes the results of recent analyses of mouse models with null mutations in the genes encoding SERCA and PMCA isoforms and genetic studies of SERCA and SPCA dysfunction in both humans and model organisms. These studies are yielding important insights regarding the physiological functions of individual Ca2+-transporting ATPases in vivo.

The mechanochemistry of V-ATPase proton pumps

The vacuolar H(+)-ATPases (V-ATPases) are a universal class of proton pumps that are structurally similar to the F-ATPases. Both protein families are characterized by a membrane-bound segment (V(o), F(o)) responsible for the translocation of protons, and a soluble portion, (V(1), F(1)), which supplies the energy for translocation by hydrolyzing ATP. Here we present a mechanochemical model for the functioning of the V(o) ion pump that is consistent with the known structural features and biochemistry. The model reproduces a variety of experimental measurements of performance and provides a unified view of the many mechanisms of intracellular pH regulation.