Expression in high yield of pig alpha 1 beta 1 Na,K-ATPase and inactive mutants D369N and D807N in Saccharomyces cerevisiae

Evolution of the sodium pump

Eukaryotic plasma membranes (PMs) are energized by electrogenic P-type ATPases that generate either Na+ or H+ motive forces to drive Na+ and H+ dependent transport processes, respectively. For this purpose, animal rely on Na+/K+-ATPases whereas fungi and plants employ PM H+-ATPases. Prokaryotes, on the other hand, depend on H+ or Na+-motive electron transport complexes to energize their cell membranes. This raises the question as to why and when electrogenic Na+ and H+ pumps evolved? Here it is shown that prokaryotic Na+/K+-ATPases have near perfect conservation of binding sites involved in coordination of three Na+ and two K+ ions. Such pumps are rare in Eubacteria but are common in methanogenic Archaea where they often are found together with P-type putative PM H+-ATPases. With some exceptions, Na+/K+-ATPases and PM H+-ATPases are found everywhere in the eukaryotic tree of life, but never together in animals, fungi and land plants. It is hypothesized that Na+/K+-ATPases and PM H+-ATPases evolved in methanogenic Archaea to support the bioenergetics of these ancestral organisms, which can utilize both H+ and Na+ as energy currencies. Both pumps must have been simultaneously present in the first eukaryotic cell, but during diversification of the major eukaryotic kingdoms, and at the time animals diverged from fungi, animals kept Na+/K+-ATPases but lost PM H+-ATPases. At the same evolutionary branch point, fungi did loose Na+/K+-ATPases, and their role was taken over by PM H+-ATPases. An independent but similar scenery emerged during terrestrialization of plants: they lost Na+/K+-ATPases but kept PM H+-ATPases.

Principles to recover copper-conducting CTR proteins for the purpose of structural and functional studies

Transition metals such as copper and zinc are essential elements required for the survival of most organisms, from bacteria to humans. Yet, elevated levels of these elements are highly toxic. The Copper TRansporter protein family (CTRs) represents the only identified copper uptake proteins in eukaryotes and hence serves as key components for the maintenance of appropriate levels of the metal. Moreover, CTRs have been proposed to serve as an entry point into cells of certain cancer drugs and to constitute attractive drug-targets for novel antifungals. Nevertheless, the structure, function, and regulation of the CTRs remain elusive, limiting valuable information also for applied sciences. To this end, here we report procedures to isolate a range of CTR members using Saccharomyces cerevisiae as a production host, focusing on three homologs, human CTR1, human CTR2, and Candida albicans CTR. Using forms C-terminally-linked to a protease cleavage sequence, Green Fluorescent Protein (GFP), and a His-tag, assessment of the localization, quantification and purification was facilitated. Cellular accumulation of the proteins was investigated via live-cell imaging. Detergents compatible with acceptable solubilization yields were identified and fluorescence-detection size-exclusion-chromatography (F-SEC) revealed preferred membrane extraction conditions for the targets. For purification purposes, the solubilized CTR members were subjected to affinity chromatography and SEC, reaching near homogeneity. The quality and quantity of the CTRs studied will permit downstream efforts to uncover imperative biophysical aspects of these proteins, paving the way for subsequent drug-discovery studies.

pXOOY: A dual-function vector for expression of membrane proteins in Saccharomyces cerevisiae and Xenopus laevis oocytes

On the quest for solving structures of membrane proteins by X-ray crystallography or cryo-EM, large quantities of ultra-pure protein are a paramount prerequisite. Obtaining enough protein of such high standard is not a trivial task, especially for difficult-to-express membrane proteins. Producing membrane protein for structural studies is often performed in Escherichia coli or Saccharomyces cerevisiae and is frequently complemented with functional studies. Ion channels and electrogenic receptors are traditionally studied in terms of their electrophysiological behavior, which cannot be performed in neither E. coli nor yeast. Therefore, they are frequently characterized in mammalian cells or in Xenopus laevis oocytes. To avoid generating two different plasmids, we here describe the construction of a dual-function plasmid, pXOOY, for membrane protein production in yeast and for electrophysiology in oocytes. pXOOY was constructed such that all elements required for oocyte expression were copied from the dual Xenopus-mammalian vector pXOOM and meticulously introduced into the high-yield yeast expression vector pEMBLyex4. pXOOY is thus designed to preserve the high yield of protein from pEMBLyex4 while simultaneously permitting in vitro transcription for expression in oocytes. We evaluated the performance of pXOOY by comparing expression of two yeast codon optimized human potassium channels, ohERG and ohSlick (Slo2.1) from pXOOY to expression of these channels obtained from the reference vectors pEMBLyex4 and pXOOM. Our proof-of-concept study indicates that accumulation in PAP1500 yeast cells was higher when the channels were expressed from pXOOY, which was verified both qualitatively and quantitatively. Two-electrode voltage clamp measurements in oocytes showed that the pXOOY constructs encoding ohERG and ohSlick gave currents with full preservation of electrophysiological characteristics. Our results show that it is possible to design a dual-function Xenopus-yeast vector without compromising expression in yeast and simultaneously maintaining channel activity in oocytes.

Heterologous (Over) Expression of Human SoLute Carrier (SLC) in Yeast: A Well-Recognized Tool for Human Transporter Function/Structure Studies

For more than 20 years, yeast has been a widely used system for the expression of human membrane transporters. Among them, more than 400 are members of the largest transporter family, the SLC superfamily. SLCs play critical roles in maintaining cellular homeostasis by transporting nutrients, ions, and waste products. Based on their involvement in drug absorption and in several human diseases, they are considered emerging therapeutic targets. Despite their critical role in human health, a large part of SLCs' is 'orphans' for substrate specificity or function. Moreover, very few data are available concerning their 3D structure. On the basis of the human health benefits of filling these knowledge gaps, an understanding of protein expression in systems that allow functional production of these proteins is essential. Among the 500 known yeast species, S. cerevisiae and P. pastoris represent those most employed for this purpose. This review aims to provide a comprehensive state-of-the-art on the attempts of human SLC expression performed by exploiting yeast. The collected data will hopefully be useful for guiding new attempts in SLCs expression with the aim to reveal new fundamental data that could lead to potential effects on human health.

Phospholipids alter activity and stability of mitochondrial membrane-bound ubiquitin ligase MARCH5

This study shows that lipids can act as regulators for the ubiquitination process and can control the stability and activity of a membrane-embedded E3 ubiquitin ligase.

Mitochondrial homeostasis is tightly controlled by ubiquitination. The mitochondrial integral membrane ubiquitin ligase MARCH5 is a crucial regulator of mitochondrial membrane fission, fusion, and disposal through mitophagy. In addition, the lipid composition of mitochondrial membranes can determine mitochondrial dynamics and organelle turnover. However, how lipids influence the ubiquitination processes that control mitochondrial homeostasis remains unknown. Here, we show that lipids common to the mitochondrial membranes interact with MARCH5 and affect its activity and stability depending on the lipid composition in vitro. As the only one of the tested lipids, cardiolipin binding to purified MARCH5 induces a significant decrease in thermal stability, whereas stabilisation increases the strongest in the presence of phosphatidic acid. Furthermore, we observe that the addition of lipids to purified MARCH5 alters the ubiquitination pattern. Specifically, cardiolipin enhances auto-ubiquitination of MARCH5. Our work shows that lipids can directly affect the activity of ubiquitin ligases and suggests that the lipid composition in mitochondrial membranes could control ubiquitination-dependent mechanisms that regulate the dynamics and turnover of mitochondria.

Production of an Active, Human Membrane Protein in Saccharomyces cerevisiae: Full-Length FICD

The human Fic domain-containing protein (FICD) is a type II endoplasmic reticulum (ER) membrane protein that is important for the maintenance of ER proteostasis. Structural and in vitro biochemical characterisation of FICD AMPylase and deAMPylase activity have been restricted to the soluble ER-luminal domain produced in Escherichia coli. Information about potentially important features, such as structural motifs, modulator binding sites or other regulatory elements, is therefore missing for the approximately 100 N-terminal residues including the transmembrane region of FICD. Expressing and purifying the required quantity and quality of membrane proteins is demanding because of the low yields and poor stability often observed. Here, we produce full-length FICD by combining a Saccharomyces cerevisiae-based platform with green fluorescent protein (GFP) tagging to optimise the conditions for expression, solubilisation and purification. We subsequently employ these conditions to purify milligram quantities of His-tagged FICD per litre of culture, and show that the purified, detergent-solubilised membrane protein is an active deAMPylating enzyme. Our work provides a straightforward methodology for producing not only full-length FICD, but also other membrane proteins in S. cerevisiae for structural and biochemical characterisation.

Recombinant production of membrane proteins in yeast

Biochemical pathways are compartmentalized in living cells. This permits each cell to maintain chemical compositions that differ between the cytosol, intracellular organelles and the external environment. Achieving this requires each compartment to be very selective in what is allowed to enter and leave. Nature has solved this by surrounding each cell and each organelle with a virtually solute impermeable lipid membrane, embedded with integral membrane proteins that mediate strictly controlled trans-membrane movement of matter and information. Access to pure and active integral membrane proteins is therefore required to comprehend membrane biology, ultimately through high-resolution structures of the membrane proteome and, therefore, also for our understanding of physiology. Unfortunately, apart from a few exceptions, membrane proteins cannot be purified from native tissue but need to be produced recombinantly, which is eminently challenging. This chapter shows how we have engineered yeast to provide high levels of prime quality membrane proteins of prokaryotic, archaeal or eukaryotic origin for structural biology.

Cyclohexyl-α maltoside as a highly efficient tool for membrane protein studies

Membrane proteins (MPs) constitute a large fraction of the proteome, but exhibit physicochemical characteristics that impose challenges for successful sample production crucial for subsequent biophysical studies. In particular, MPs have to be extracted from the membranes in a stable form. Reconstitution into detergent micelles represents the most common procedure in recovering MPs for subsequent analysis. n-dodecyl-β-D-maltoside (DDM) remains one of the most popular conventional detergents used in production of MPs. Here we characterize the novel DDM analogue 4-trans-(4-trans-propylcyclohexyl)-cyclohexyl α-maltoside (t-PCCαM), possessing a substantially lower critical micelle concentration (CMC) than the parental compound that represents an attractive feature when handling MPs. Using three different types of MPs of human and prokaryotic origin, i.e., a channel, a primary and a secondary active transporter, expressed in yeast and bacterial host systems, respectively, we investigate the performance of t-PCCαM in solubilization and affinity purification together with its capacity to preserve native fold and activity. Strikingly, t-PCCαM displays favorable behavior in extracting and stabilizing the three selected targets. Importantly, t-PCCαM promoted extraction of properly folded protein, enhanced thermostability and provided negatively-stained electron microscopy samples of promising quality. All-in-all, t-PCCαM emerges as competitive surfactant applicable to a broad portfolio of challenging MPs for downstream structure-function analysis.

Saccharomyces cerevisiae as a superior host for overproduction of prokaryotic integral membrane proteins

Integral membrane proteins (IMPs) constitute ~30% of all proteins encoded by the genome of any organism and Escherichia coli remains the first-choice host for recombinant production of prokaryotic IMPs. However, the expression levels of prokaryotic IMPs delivered by this bacterium are often low and overproduced targets often accumulate in inclusion bodies. The targets are therefore often discarded to avoid an additional and inconvenient refolding step in the purification protocol. Here we compared expression of five prokaryotic (bacterial and archaeal) IMP families in E. coli and Saccharomyces cerevisiae. We demonstrate that our S. cerevisiae-based production platform is superior in expression of four investigated IMPs, overall being able to deliver high quantities of active target proteins. Surprisingly, in case of the family of zinc transporters (Zrt/Irt-like proteins, ZIPs), S. cerevisiae rescued protein expression that was undetectable in E. coli. We also demonstrate the effect of localization of the fusion tag on expression yield and sample quality in detergent micelles. Lastly, we present a road map to achieve the most efficient expression of prokaryotic IMPs in our yeast platform. Our findings demonstrate the great potential of S. cerevisiae as host for high-throughput recombinant overproduction of bacterial and archaeal IMPs for downstream biophysical characterization.

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S. cerevisiae is superior to E. coli in expressing correctly folded and active IMPs.

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S. cerevisiae completely rescues the expression of the family of zinc transporters.

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Localization of the fusion tag affects expression yields and protein quality.

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We provide a roadmap to efficient expression of prokaryotic IMPs in S. cerevisiae.

Overproduction of Human Zip (SLC39) Zinc Transporters in Saccharomyces cerevisiae for Biophysical Characterization

Zinc constitutes the second most abundant transition metal in the human body, and it is implicated in numerous cellular processes, including cell division, DNA and protein synthesis as well as for the catalytic activity of many enzymes. Two major membrane protein families facilitate zinc homeostasis in the animal kingdom, i.e., Zrt/Irt-like proteins (ZIPs aka solute carrier 39, SLC39, family) and Zn transporters (ZnTs), essentially conducting zinc flux in the opposite directions. Human ZIPs (hZIPs) regulate import of extracellular zinc to the cytosol, being critical in preventing overaccumulation of this potentially toxic metal, and crucial for diverse physiological and pathological processes, including development of neurodegenerative disorders and several cancers. To date, our understanding of structure-function relationships governing hZIP-mediated zinc transport mechanism is scarce, mainly due to the notorious difficulty in overproduction of these proteins for biophysical characterization. Here we describe employment of a Saccharomyces cerevisiae-based platform for heterologous expression of hZIPs. We demonstrate that yeast is able to produce four full-length hZIP members belonging to three different subfamilies. One target (hZIP1) is purified in the high quantity and homogeneity required for the downstream biochemical analysis. Our work demonstrates the potential of the described production system for future structural and functional studies of hZIP transporters.

Purification and initial characterization of Plasmodium falciparum K+ channels, PfKch1 and PfKch2 produced in Saccharomyces cerevisiae

Resistance towards known antimalarial drugs poses a significant problem, urging for novel drugs that target vital proteins in the malaria parasite Plasmodium falciparum. However, recombinant production of malaria proteins is notoriously difficult. To address this, we have investigated two putative K⁺ channels, PfKch1 and PfKch2, identified in the P. falciparum genome. We show that PfKch1 and PfKch2 and a C-terminally truncated version of PfKch1 (PfKch1¹⁻¹⁰⁹⁴) could indeed be functionally expressed in vivo, since a K⁺-uptake deficient Saccharomyces cerevisiae strain was complemented by the P. falciparum cDNAs. PfKch1¹⁻¹⁰⁹⁴-GFP and GFP-PfKch2 fusion proteins were overexpressed in yeast, purified and reconstituted in lipid bilayers to determine their electrophysiological activity. Single channel conductance amounted to 16 ± 1 pS for PfKch1¹⁻¹⁰⁹⁴-GFP and 28 ± 2 pS for GFP-PfKch2. We predicted regulator of K⁺-conductance (RCK) domains in the C-terminals of both channels, and we accordingly measured channel activity in the presence of Ca^2+.

Yeast recombinant production of intact human membrane proteins with long intrinsically disordered intracellular regions for structural studies

Membrane proteins exist in lipid bilayers and mediate solute transport, signal transduction, cell-cell communication and energy conversion. Their activities are fundamental for life, which make them prominent subjects of study, but access to only a limited number of high-resolution structures complicates their mechanistic understanding. The absence of such structures relates mainly to difficulties in expressing and purifying high quality membrane protein samples in large quantities. An additional layer of complexity stems from the presence of intra- and/or extra-cellular domains constituted by unstructured intrinsically disordered regions (IDR), which can be hundreds of residues long. Although IDRs form key interaction hubs that facilitate biological processes, these are regularly removed to enable structural studies. To advance mechanistic insight into intact intrinsically disordered membrane proteins, we have developed a protocol for their purification. Using engineered yeast cells for optimized expression and purification, we have purified to homogeneity two very different human membrane proteins each with >300 residues long IDRs; the sodium proton exchanger 1 and the growth hormone receptor. Subsequent to their purification we have further explored their incorporation into membrane scaffolding protein nanodiscs, which will enable future structural studies.

Characterization of Hailey-Hailey Disease-mutants in presence and absence of wild type SPCA1 using Saccharomyces cerevisiae as model organism

Hailey-Hailey disease is an autosomal genetic disease caused by mutations in one of the two ATP2C1 alleles encoding the secretory pathway Ca²⁺/Mn²⁺-ATPase, hSPCA1. The disease almost exclusively affects epidermis, where it mainly results in acantholysis of the suprabasal layers. The etiology of the disease is complex and not well understood. We applied a yeast based complementation system to characterize fourteen disease-causing ATP2C1 missense mutations in presence or absence of wild type ATP2C1 or ATP2A2, encoding SERCA2. In our yeast model system, mutations in ATP2C1 affected Mn²⁺ transport more than Ca²⁺ transport as twelve out of fourteen mutations were unable to complement Mn²⁺ sensitivity while thirteen out of fourteen to some extent complemented the high Ca²⁺requirement. Nine out of fourteen mutations conferred a cold sensitive complementation capacity. In absence of a wild type ATP2C1 allele, twelve out of fourteen mutations induced an unfolded protein response indicating that in vivo folding of hSPCA1 is sensitive to disease causing amino acid substitutions and four of the fourteen mutations caused the hSPCA1 protein to accumulate in the vacuolar membrane. Co-expression of either wild type ATP2C1 or ATP2A2 prevented induction of the unfolded protein response and hSPCA1 mis-localization.

Structure of the human ClC-1 chloride channel

ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue ("fast gate") known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-β-synthase (CBS) domains and the intracellular vestibule ("slow gating"). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1-related diseases.

Purification of Functional Human TRP Channels Recombinantly Produced in Yeast

(1) Background: Human transient receptor potential (TRP) channels constitute a large family of ion-conducting membrane proteins that allow the sensation of environmental cues. As the dysfunction of TRP channels contributes to the pathogenesis of many widespread diseases, including cardiac disorders, these proteins also represent important pharmacological targets. TRP channels are typically produced using expensive and laborious mammalian or insect cell-based systems. (2) Methods: We demonstrate an alternative platform exploiting the yeast Saccharomyces cerevisiae capable of delivering high yields of functional human TRP channels. We produce 11 full-length human TRP members originating from four different subfamilies, purify a selected subset of these to a high homogeneity and confirm retained functionality using TRPM8 as a model target. (3) Results: Our findings demonstrate the potential of the described production system for future functional, structural and pharmacological studies of human TRP channels.

Human adipose glycerol flux is regulated by a pH gate in AQP10

Obesity is a major threat to global health and metabolically associated with glycerol homeostasis. Here we demonstrate that in human adipocytes, the decreased pH observed during lipolysis (fat burning) correlates with increased glycerol release and stimulation of aquaglyceroporin AQP10. The crystal structure of human AQP10 determined at 2.3 Å resolution unveils the molecular basis for pH modulation-an exceptionally wide selectivity (ar/R) filter and a unique cytoplasmic gate. Structural and functional (in vitro and in vivo) analyses disclose a glycerol-specific pH-dependence and pinpoint pore-lining His80 as the pH-sensor. Molecular dynamics simulations indicate how gate opening is achieved. These findings unravel a unique type of aquaporin regulation important for controlling body fat mass. Thus, targeting the cytoplasmic gate to induce constitutive glycerol secretion may offer an attractive option for treating obesity and related complications.

Purification and functional comparison of nine human Aquaporins produced in Saccharomyces cerevisiae for the purpose of biophysical characterization

The sparse number of high-resolution human membrane protein structures severely restricts our comprehension of molecular physiology and ability to exploit rational drug design. In the search for a standardized, cheap and easily handled human membrane protein production platform, we thoroughly investigated the capacity of S. cerevisiae to deliver high yields of prime quality human AQPs, focusing on poorly characterized members including some previously shown to be difficult to isolate. Exploiting GFP labeled forms we comprehensively optimized production and purification procedures resulting in satisfactory yields of all nine AQP targets. We applied the obtained knowledge to successfully upscale purification of histidine tagged human AQP10 produced in large bioreactors. Glycosylation analysis revealed that AQP7 and 12 were O-glycosylated, AQP10 was N-glycosylated while the other AQPs were not glycosylated. We furthermore performed functional characterization and found that AQP 2, 6 and 8 allowed flux of water whereas AQP3, 7, 9, 10, 11 and 12 also facilitated a glycerol flux. In conclusion, our S. cerevisiae platform emerges as a powerful tool for isolation of functional, difficult-to-express human membrane proteins suitable for biophysical characterization.

Characterization of ATP7A missense mutants suggests a correlation between intracellular trafficking and severity of Menkes disease

Menkes disease (MD) is caused by mutations in ATP7A, encoding a copper-transporting P-type ATPase which exhibits copper-dependent trafficking. ATP7A is found in the Trans-Golgi Network (TGN) at low copper concentrations, and in the post-Golgi compartments and the plasma membrane at higher concentrations. Here we have analyzed the effect of 36 ATP7A missense mutations identified in phenotypically different MD patients. Nine mutations identified in patients with severe MD, virtually eliminated ATP7A synthesis, in most cases due to aberrant RNA splicing. A group of 21 predominantly severe mutations led to trapping of the protein in TGN and displayed essentially no activity in a yeast-based functional assay. These were predicted to inhibit the catalytic phosphorylation of the protein. Four mutants showed diffuse post-TGN localization, while two displayed copper dependent trafficking. These six variants were identified in patients with mild MD and typically displayed activity in the yeast assay. The four post-TGN located mutants were presumably affected in the catalytic dephosphorylation of the protein. Together these results indicate that the severity of MD correlate with cellular localization of ATP7A and support previous studies indicating that phosphorylation is crucial for the exit of ATP7A from TGN, while dephosphorylation is crucial for recycling back to TGN.

Scaffolded Antigens in Yeast Cell Particle Vaccines Provide Protection against Systemic Polyoma Virus Infection

Background. U65, a self-aggregating peptide scaffold, traps fused protein antigens in yeast cells. Conversion to Yeast Cell Particle (YCP) vaccines by partial removal of surface mannoproteins exposes β-glucan, mediating efficient uptake by antigen-presenting cells (APCs). YCP vaccines are inexpensive, capable of rapid large-scale production and have potential for both parenteral and oral use. Results. YCP processing by alkaline hydrolysis exposes up to 20% of the glucan but converts scaffolded antigen and internal yeast proteins into a common aggregate, preventing selective yeast protein removal. For U65-green fluorescent protein (GFP) or U65-Apolipoprotein A1 (ApoA1) subcutaneous vaccines, maximal IgG responses in mice required 10% glucan exposure. IgG responses to yeast proteins were 5-fold lower. Proteolytic mannoprotein removal produced YCPs with only 6% glucan exposure, insufficiently porous for selective removal of even native yeast proteins. Vaccine efficacy was reduced 10-fold. Current YCP formulations, therefore, are not suitable for human use but have considerable potential for use in feed animal vaccines. Significantly, a YCP vaccine expressing a GFP fusion to VP1, the murine polyoma virus major capsid protein, after either oral or subcutaneous administration, protected mice against an intraperitoneal polyoma virus challenge, reducing viral DNA levels in spleen and liver by >98%.

Coordinated Overexpression in Yeast of a P4-ATPase and Its Associated Cdc50 Subunit: The Case of the Drs2p/Cdc50p Lipid Flippase Complex

Structural and functional characterization of integral membrane proteins requires milligram amounts of purified sample. Unless the protein you are studying is abundant in native membranes, it will be critical to overexpress the protein of interest in a homologous or heterologous way, and in sufficient quantities for further purification. The situation may become even more complicated if you chose to investigate the structure and function of a complex of two or more membrane proteins. Here, we describe the overexpression of a yeast lipid flippase complex, namely the P4-ATPase Drs2p and its associated subunit Cdc50p, in a coordinated manner. Moreover, we can take advantage of the fact that P4-ATPases, like most other P-type ATPases, form an acid-stable phosphorylated intermediate, to verify that the expressed complex is functional.

High yield purification of full-length functional hERG K+ channels produced in Saccharomyces cerevisiae

The hERG potassium channel is essential for repolarization of the cardiac action potential. Due to this vital function, absence of unintended and potentially life-threatening interactions with hERG is required for approval of new drugs. The structure of hERG is therefore one of the most sought-after. To provide purified hERG for structural studies and new hERG biomimetic platforms for detection of undesirable interactions, we have developed a hERG expression platform generating unprecedented amounts of purified and functional hERG channels. Full-length hERG, with or without a C-terminally fused green fluorescent protein (GFP) His ₈-tag was produced from a codon-optimized hERG cDNA in Saccharomyces cerevisiae. Both constructs complemented the high potassium requirement of a knock-out Saccharomyces cerevisiae strain, indicating correct tetramer assembly in vivo. Functionality was further demonstrated by Astemizole binding to membrane embedded hERG-GFP-His ₈ with a stoichiometry corresponding to tetramer assembly. The 156 kDa hERG-GFP protein accumulated to a membrane density of 1.6%. Fluorescence size exclusion chromatography of hERG-GFP-His ₈ solubilized in Fos-Choline-12 supplemented with cholesteryl-hemisuccinate and Astemizole resulted in a monodisperse elution profile demonstrating a high quality of the hERG channels. hERG-GFP-His ₈ purified by Ni-affinity chromatography maintained the ability to bind Astemizole with the correct stoichiometry indicating that the native, tetrameric structure was preserved. To our knowledge this is the first reported high-yield production and purification of full length, tetrameric and functional hERG. This significant breakthrough will be paramount in obtaining hERG crystal structures, and in establishment of new high-throughput hERG drug safety screening assays.

Overexpression of membrane proteins from higher eukaryotes in yeasts

Heterologous expression and characterisation of the membrane proteins of higher eukaryotes is of paramount interest in fundamental and applied research. Due to the rather simple and well-established methods for their genetic modification and cultivation, yeast cells are attractive host systems for recombinant protein production. This review provides an overview on the remarkable progress, and discusses pitfalls, in applying various yeast host strains for high-level expression of eukaryotic membrane proteins. In contrast to the cell lines of higher eukaryotes, yeasts permit efficient library screening methods. Modified yeasts are used as high-throughput screening tools for heterologous membrane protein functions or as benchmark for analysing drug-target relationships, e.g., by using yeasts as sensors. Furthermore, yeasts are powerful hosts for revealing interactions stabilising and/or activating membrane proteins. We also discuss the stress responses of yeasts upon heterologous expression of membrane proteins. Through co-expression of chaperones and/or optimising yeast cultivation and expression strategies, yield-optimised hosts have been created for membrane protein crystallography or efficient whole-cell production of fine chemicals.

Role of vesicular nucleotide transporter VNUT (SLC17A9) in release of ATP from AR42J cells and mouse pancreatic acinar cells

ATP is released from cells in response to various stimuli. Our previous studies on pancreas indicated that pancreatic acini could be major stores of secreted ATP. In the present study, our aim was to establish the role of the vesicular nucleotide transporter (VNUT), SLC17A9, in storage and release of ATP. Freshly prepared acini from mice and AR42J rat acinar cells were used in this study. We illustrate that in AR42J cells, quinacrine (an ATP store marker) and Bodipy ATP (a fluorescent ATP analog) co-localized with VNUT-mCherry to vesicles/granules. Furthermore, in acini and AR42J cells, a marker of the zymogen granule membranes, Rab3D, and VNUT co-localized. Dexamethasone treatment of AR42J cells promoted formation of acinar structures, paralleled by increased amylase and VNUT expression, and increased ATP release in response to cholinergic stimulation. Mechanical stimulus (pressure) and cell swelling also induced ATP release, but this was not influenced by dexamethasone, most likely indicating different non-zymogen-related release mechanism. In conclusion, we propose that VNUT-dependent ATP release pathway is associated with agonist-induced secretion process and downstream purinergic signalling in pancreatic ducts.

Saccharomyces cerevisiae-based platform for rapid production and evaluation of eukaryotic nutrient transporters and transceptors for biochemical studies and crystallography

To produce large quantities of high quality eukaryotic membrane proteins in Saccharomyces cerevisiae, we modified a high-copy vector to express membrane proteins C-terminally-fused to a Tobacco Etch Virus (TEV) protease detachable Green Fluorescent Protein (GFP)-8His tag, which facilitates localization, quantification, quality control, and purification. Using this expression system we examined the production of a human glucose transceptor and 11 nutrient transporters and transceptors from S. cerevisiae that have not previously been overexpressed in S. cerevisiae and purified. Whole-cell GFP-fluorescence showed that induction of GFP-fusion synthesis from a galactose-inducible promoter at 15°C resulted in stable accumulation of the fusions in the plasma membrane and in intracellular membranes. Expression levels of the 12 fusions estimated by GFP-fluorescence were in the range of 0.4 mg to 1.7 mg transporter pr. liter cell culture. A detergent screen showed that n-dodecyl-ß-D-maltopyranoside (DDM) is acceptable for solubilization of the membrane-integrated fusions. Extracts of solubilized membranes were prepared with this detergent and used for purifications by Ni-NTA affinity chromatography, which yielded partially purified full-length fusions. Most of the fusions were readily cleaved at a TEV protease site between the membrane protein and the GFP-8His tag. Using the yeast oligopeptide transporter Ptr2 as an example, we further demonstrate that almost pure transporters, free of the GFP-8His tag, can be achieved by TEV protease cleavage followed by reverse immobilized metal-affinity chromatography. The quality of the GFP-fusions was analysed by fluorescence size-exclusion chromatography. Membranes solubilized in DDM resulted in preparations containing aggregated fusions. However, 9 of the fusions solubilized in DDM in presence of cholesteryl hemisuccinate and specific substrates, yielded monodisperse preparations with only minor amounts of aggregated membrane proteins. In conclusion, we developed a new effective S. cerevisiae expression system that may be used for production of high-quality eukaryotic membrane proteins for functional and structural analysis.

Effects of obesity and estradiol on Na+/K+-ATPase and their relevance to cardiovascular diseases

Obesity is associated with aberrant sodium/potassium-ATPase (Na(+)/K(+)-ATPase) activity, apparently linked to hyperglycemic hyperinsulinemia, which may repress or inactivate the enzyme. The reduction of Na(+)/K(+)-ATPase activity in cardiac tissue induces myocyte death and cardiac dysfunction, leading to the development of myocardial dilation in animal models; this has also been documented in patients with heart failure (HF). During several pathological situations (cardiac insufficiency and HF) and in experimental models (obesity), the heart becomes more sensitive to the effect of cardiac glycosides, due to a decrease in Na(+)/K(+)-ATPase levels. The primary female sex steroid estradiol has long been recognized to be important in a wide variety of physiological processes. Numerous studies, including ours, have shown that estradiol is one of the major factors controlling the activity and expression of Na(+)/K(+)-ATPase in the cardiovascular (CV) system. However, the effects of estradiol on Na(+)/K(+)-ATPase in both normal and pathological conditions, such as obesity, remain unclear. Increasing our understanding of the molecular mechanisms by which estradiol mediates its effects on Na(+)/K(+)-ATPase function may help to develop new strategies for the treatment of CV diseases. Herein, we discuss the latest data from animal and clinical studies that have examined how pathophysiological conditions such as obesity and the action of estradiol regulate Na(+)/K(+)-ATPase activity.

Recombinant production of human Aquaporin-1 to an exceptional high membrane density in Saccharomyces cerevisiae

In the present paper we explored the capacity of yeast Saccharomyces cerevisiae as host for heterologous expression of human Aquaporin-1. Aquaporin-1 cDNA was expressed from a galactose inducible promoter situated on a plasmid with an adjustable copy number. Human Aquaporin-1 was C-terminally tagged with yeast enhanced GFP for quantification of functional expression, determination of sub-cellular localization, estimation of in vivo folding efficiency and establishment of a purification protocol. Aquaporin-1 was found to constitute 8.5 percent of total membrane protein content after expression at 15°C in a yeast host over-producing the Gal4p transcriptional activator and growth in amino acid supplemented minimal medium. In-gel fluorescence combined with western blotting showed that low accumulation of correctly folded recombinant Aquaporin-1 at 30°C was due to in vivo mal-folding. Reduction of the expression temperature to 15°C almost completely prevented Aquaporin-1 mal-folding. Bioimaging of live yeast cells revealed that recombinant Aquaporin-1 accumulated in the yeast plasma membrane. A detergent screen for solubilization revealed that CYMAL-5 was superior in solubilizing recombinant Aquaporin-1 and generated a monodisperse protein preparation. A single Ni-affinity chromatography step was used to obtain almost pure Aquaporin-1. Recombinant Aquaporin-1 produced in S. cerevisiae was not N-glycosylated in contrast to the protein found in human erythrocytes.

Covalent binding of cisplatin impairs the function of Na(+)/K(+)-ATPase by binding to its cytoplasmic part

This study was aimed at verifying the hypothesis that acute kidney failure accompanying cisplatin administration in the cancer therapy could be due to cisplatin interaction with the cytoplasmic part of Na(+)/K(+)-ATPase. Our results demonstrated that cisplatin-binding caused inhibition of Na(+)/K(+)-ATPase, in contrast to other platinated chemotherapeutics such as carboplatin and oxaliplatin, which are known to be much less nephrotoxic. To acquire more detailed structural information, we performed a series of experiments with the isolated large cytoplasmic segment connecting transmembrane helices 4 and 5 (C45 loop) of Na(+)/K(+)-ATPase. Electrochemistry showed that cisplatin is bound to the cysteine residues of the C45 loop, mass spectrometry revealed a modification of the C45 peptide fragment GSHMASLEAVETLGSTSTICSDK, which contains the conserved phosphorylated residue Asp369. Hence, we hypothesize that binding of cisplatin to Cys367 can cause sterical obstruction during the phosphorylation or dephosphorylation step of the Na(+)/K(+)-ATPase catalytic cycle.

Saccharomyces cerivisiae as a model system for kidney disease: what can yeast tell us about renal function?

Ion channels, solute transporters, aquaporins, and factors required for signal transduction are vital for kidney function. Because mutations in these proteins or in associated regulatory factors can lead to disease, an investigation into their biogenesis, activities, and interplay with other proteins is essential. To this end, the yeast, Saccharomyces cerevisiae, represents a powerful experimental system. Proteins expressed in yeast include the following: 1) ion channels, including the epithelial sodium channel, members of the inward rectifying potassium channel family, and cystic fibrosis transmembrane conductance regulator; 2) plasma membrane transporters, such as the Na(+)-K(+)-ATPase, the Na(+)-phosphate cotransporter, and the Na(+)-H(+) ATPase; 3) aquaporins 1-4; and 4) proteins such as serum/glucocorticoid-induced kinase 1, phosphoinositide-dependent kinase 1, Rh glycoprotein kidney, and trehalase. The variety of proteins expressed and studied emphasizes the versatility of yeast, and, because of the many available tools in this organism, results can be obtained rapidly and economically. In most cases, data gathered using yeast have been substantiated in higher cell types. These attributes validate yeast as a model system to explore renal physiology and suggest that research initiated using this system may lead to novel therapeutics.

High sensibility to reactivation by acidic lipids of the recombinant human plasma membrane Ca2+-ATPase isoform 4xb purified from Saccharomyces cerevisiae

The human plasma membrane Ca2+ pump (isoform 4xb) was expressed in Saccharomyces cerevisiae and purified by calmodulin-affinity chromatography. Under optimal conditions the recombinant enzyme (yPMCA) hydrolyzed ATP in a Ca2+ dependent manner at a rate of 15 micromol/mg/min. The properties of yPMCA were compared to those of the PMCA purified from human red cells (ePMCA). The mobility of yPMCA in SDS-PAGE was the expected for the hPMCA4xb protein but slightly lower than that of ePMCA. Both enzymes achieved maximal activity when supplemented with acidic phospholipids. However, while ePMCA in mixed micelles of phosphatidylcholine-detergent had 30% of its maximal activity, the yPMCA enzyme was nearly inactive. Increasing the phosphatidylcholine content of the micelles did not increase the activity of yPMCA but the activity in the presence of phosphatidylcholine improved by partially removing the detergent. The reactivation of the detergent solubilized yPMCA required specifically acidic lipids and, as judged by the increase in the level of phosphoenzyme, it involved the increase in the amount of active enzyme. These results indicate that the function of yPMCA is highly sensitive to delipidation and the restitution of acidic lipids is needed for a functional enzyme.

D443 of the N domain of Na+,K+-ATPase interacts with the ATP-Mg2+ complex, possibly via a second Mg2+ ion

This paper provides evidence for an interaction of D443 in the N domain of Na(+),K(+)-ATPase with a Mg(2+) ion. Wild-type, D443N/A/C and S445A mutants of porcine Na(+),K(+)-ATPase (alpha1beta1) have been expressed in Pichia pastoris. By comparison with wild-type, D443N reduces the turn-over rate by about 40%. Binding affinity of ATP, measured directly, was not affected by D443N, D443A, or D443C mutations. AMP-PNP-Fe(2+)-catalyzed oxidative cleavage of Na(+),K(+)-ATPase produces two characteristic fragments, at (708)VNDS (P domain) and near (440)VAGDA (N domain), respectively. In the D443N and D443A mutants, both cleavages are suppressed, indicating an interaction between the residues with AMP-PNP-Fe(2+) bound. Previous work suggested that with ATP-Fe(2+) bound the N and P domains come into proximity, both D710 and D443 making contact with a single Fe(2+) (or Mg(2+)) ion. However, the crystal structure of Ca(2+)-ATPase with bound AMP-PCP and Mg(2+) confirm the involvement of D703 (D710) but show that E439 (D443) is too far to make contact with the Mg(2+). By contrast, in the crystal structure with bound ADP, AlF(4), and Mg(2+), representing the E(1)-P conformation, two Mg(2+) ions were observed. Significantly, ADP-Fe(2+)-mediated oxidative cleavage of renal Na,K-ATPase produces the fragment near (440)VAGDA (N domain), while the cleavage at (708)VNDS (P domain) is almost completely absent. The results are explained economically by the hypothesis that ATP is bound with two Mg(2+) (Fe(2+)) ions, a "catalytic" Mg(2+) interacting with D710 via the gamma phosphate and a "structural" Mg(2+) interacting with D443 via the alpha and beta phosphates and a water molecule, respectively.

Heterologous expression of membrane and soluble proteins derepresses GCN4 mRNA translation in the yeast Saccharomyces cerevisiae

This paper describes the first physiological response at the translational level towards heterologous protein production in Saccharomyces cerevisiae. In yeast, the phosphorylation of eukaryotic initiation factor 2alpha (eIF-2alpha) by Gcn2p protein kinase mediates derepression of GCN4 mRNA translation. Gcn4p is a transcription factor initially found to be required for transcriptional induction of genes responsible for amino acid or purine biosynthesis. Using various GCN4-lacZ fusions, knockout yeast strains, and anti-eIF-2alpha-P/anti-eIF-2alpha antibodies, we observed that heterologous expression of the membrane-bound alpha1beta1 Na,K-ATPase from pig kidney, the rat pituitary adenylate cyclase seven-transmembrane-domain receptor, or a 401-residue soluble part of the Na,K-ATPase alpha1 subunit derepressed GCN4 mRNA translation up to 70-fold. GCN4 translation was very sensitive to the presence of heterologous protein, as a density of 1 per thousand of heterologous membrane protein derepressed translation maximally. Translational derepression of GCN4 was not triggered by misfolding in the endoplasmic reticulum, as expression of the wild type or temperature-sensitive folding mutants of the Na,K-ATPase increased GCN4 translation to the same extent. In situ activity of the heterologously expressed protein was not required for derepression of GCN4 mRNA translation, as illustrated by the expression of an enzymatically inactive Na,K-ATPase. Two- to threefold overexpression of the highly abundant and plasma membrane-located endogenous H-ATPase also induced GCN4 translation. Derepression of GCN4 translation required phosphorylation of eIF-2alpha, the tRNA binding domain of Gcn2p, and the ribosome-associated proteins Gcn1p and Gcn20p. The increase in Gcn4p density in response to heterologous expression did not induce transcription from the HIS4 promoter, a traditional Gcn4p target.

Purification of Na+,K+-ATPase Expressed in Pichia pastoris Reveals an Essential Role of Phospholipid-Protein Interactions

Na+,K+-ATPase (porcine alpha/his10-beta) has been expressed in Pichia Pastoris, solubilized in n-dodecyl-beta-maltoside and purified to 70-80% purity by nickel-nitrilotriacetic acid chromatography combined with size exclusion chromatography. The recombinant protein is inactive if the purification is done without added phospholipids. The neutral phospholipid, dioleoylphosphatidylcholine, preserves Na+,K+-ATPase activity of protein prepared in a Na+-containing medium, but activity is lost in a K+-containing medium. By contrast, the acid phospholipid, dioleoylphosphatidylserine, preserves activity in either Na+- or K+-containing media. In optimal conditions activity is preserved for about 2 weeks at 0 degrees C. Both recombinant Na+,K+-ATPase and native pig kidney Na+,K+-ATPase, dissolved in n-dodecyl-beta-maltoside, appear to be mainly stable monomers (alpha/beta) as judged by size exclusion chromatography and sedimentation velocity. Na+,K+-ATPase activities at 37 degrees C of the size exclusion chromatography-purified recombinant and renal Na+,K+-ATPase are comparable but are lower than that of membrane-bound renal Na+,K+-ATPase. The beta subunit is expressed in Pichia Pastoris as two lightly glycosylated polypeptides and is quantitatively deglycosylated by endoglycosidase-H at 0 degrees C, to a single polypeptide. Deglycosylation inactivates Na+,K+-ATPase prepared with dioleoylphosphatidylcholine, whereas dioleoylphosphatidylserine protects after deglycosylation, and Na+,K+-ATPase activity is preserved. This work demonstrates an essential role of phospholipid interactions with Na+,K+-ATPase, including a direct interaction of dioleoylphosphatidylserine, and possibly another interaction of either the neutral or acid phospholipid. Additional lipid effects are likely. A role for the beta subunit in stabilizing conformations of Na+,K+-ATPase (or H+,K+-ATPase) with occluded K+ ions can also be inferred. Purified recombinant Na+,K+-ATPase could become an important experimental tool for various purposes, including, hopefully, structural work.

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.

Expression of Na+,K+-ATPase in Pichia pastoris: analysis of wild type and D369N mutant proteins by Fe2+-catalyzed oxidative cleavage and molecular modeling

Na+,K+-ATPase (pig alpha1,beta1) has been expressed in the methylotrophic yeast Pichia pastoris. A protease-deficient strain was used, recombinant clones were screened for multicopy genomic integrants, and protein expression, and time and temperature of methanol induction were optimized. A 3-liter culture provides 300-500 mg of membrane protein with ouabain binding capacity of 30-50 pmol mg-1. Turnover numbers of recombinant and renal Na+,K+-ATPase are similar, as are specific chymotryptic cleavages. Wild type (WT) and a D369N mutant have been analyzed by Fe2+- and ATP-Fe2+-catalyzed oxidative cleavage, described for renal Na+,K+-ATPase. Cleavage of the D369N mutant provides strong evidence for two Fe2+ sites: site 1 composed of residues in P and A cytoplasmic domains, and site 2 near trans-membrane segments M3/M1. The D369N mutation suppresses cleavages at site 1, which appears to be a normal Mg2+ site in E2 conformations. The results suggest a possible role of the charge of Asp369 on the E1 <--> E2 conformational equilibrium. 5'-Adenylyl-beta,gamma-imidodi-phosphate(AMP-PNP)-Fe2+-catalyzed cleavage of the D369N mutant produces fragments in P (712VNDS) and N (near 440VAGDA) domains, described for WT, but only at high AMP-PNP-Fe2+ concentrations, and a new fragment in the P domain (near 367CSDKTGT) resulting from cleavage. Thus, the mutation distorts the active site. A molecular dynamic simulation of ATP-Mg2+ binding to WT and D351N structures of Ca2+-ATPase (analogous to Asp369 of Na+,K+-ATPase) supplies possible explanations for the new cleavage and for a high ATP affinity, which was observed previously for the mutant. The Asn351 structure with bound ATP-Mg2+ may resemble the transition state of the WT poised for phosphorylation.

Transmission of E1-E2 structural changes in response to Na+ or K+ binding in Na,K-ATPase

The extensive E(1)-E(2) conformational changes in response to Na(+) or K(+) binding in the absence of other ligands must be driven by motion of the side chains contributing to cation coordination, but the differences in structure of Na(+) and K(+) sites have not been resolved. The recent high resolution structure model of the E(2) conformation of Ca-ATPase offers the first opportunity to examine and model the changes accompanying the adjustment of the cation sites from an E(1) form with specificity for Na(+) to an E(2) form with specificity for K(+). The model of the E(2) form provides a remarkable fit to the data of direct Tl(+) or K(+) binding after site-directed mutagenesis of residues Asp804 and Asp808 in M6, Glu 779, Gln776, and Ser775 in M5, and Glu327 in M4. Cytoplasmic domain movements during E(1) <--> E(2) conformational transition can be monitored by proteolytic cleavage. Protection of the chymotrypsin-sensitive bond at Leu266 in L2/3 and rotation of the A domain is more complete in the E(2)Mg-vanadate-ouabain complex than in the E(2)[2K] form.

Specific lipid requirements of membrane proteins--a putative bottleneck in heterologous expression

Membrane proteins are mostly protein-lipid complexes. For more than 30 examples of membrane proteins from prokaryotes, yeast, plant and mammals, the importance of phospholipids and sterols for optimal activity is documented. All crystallized membrane protein complexes show defined lipid-protein contacts. In addition, lipid requirements may also be transitory and necessary only for correct folding and intercellular transport. With respect to specific lipid requirements of membrane proteins, the phospholipid and glycolipid as well as the sterol content of the host cell chosen for heterologous expression should be carefully considered. The lipid composition of bacteria, archaea, yeasts, insects,Xenopus oocytes, and typical plant and mammalian cells are given in this review. A few examples of heterologous expression of membrane proteins, where problems of specific lipid requirements have been noticed or should be thought of, have been chosen.

Overproduction in yeast and rapid and efficient purification of the rabbit SERCA1a Ca(2+)-ATPase

Large amounts of heterologous C-terminally his-tagged SERCA1a Ca(2+)-ATPase were expressed in yeast using a galactose-regulated promoter and purified by Ni(2+) affinity chromatography followed by Reactive red chromatography. Optimizing the number of galactose inductions and increasing the amount of Gal4p transcription factor improved expression. Lowering the temperature from 28 degrees C to 18 degrees C during expression enhanced the recovery of solubilized and active Ca(2+)-ATPase. In these conditions, a 4 l yeast culture produced 100 mg of Ca(2+)-ATPase, 60 and 22 mg being pelleted with the heavy and light membrane fractions respectively, representing 7 and 1.7% of total proteins. The Ca(2+)-ATPase expressed in light membranes was 100% solubilized with L-alpha-lysophosphatidylcholine (LPC), 50% with n-dodecyl beta-D-maltoside (DM) and 25% with octaethylene glycol mono-n-dodecyl ether (C(12)E(8)). Compared to LPC, DM preserved specific activity of the solubilized Ca(2+)-ATPase during the chromatographic steps. Starting from 1/6 (3.8 mg) of the total amount of Ca(2+)-ATPase expressed in light membranes, 800 microg could be routinely purified to 50% purity by metal affinity chromatography and then 200 microg to 70% with Reactive red chromatography. The purified Ca(2+)-ATPase displayed the same K(m) for calcium and ATP as the native enzyme but a reduced specific activity ranging from 4.5 to 7.3 micromol ATP hydrolyzed/min/mg Ca(2+)-ATPase. It was stable and active for several days at 4 degrees C or after removal of DM with Bio-beads and storage at -80 degrees C.

Importance of Na,K-ATPase residue alpha 1-Arg544 in the segment Arg544-Asp567 for high-affinity binding of ATP, ADP, or MgATP

To identify residues involved in ATP binding in the N-domain of the alpha1-subunit of Na,K-ATPase, mutations were directed to the segment Arg(544)-Asp(567), a beta-strand-loop-helix structure with Arg(544) positioned at the mouth of the ATP-binding pocket near the interface to the P-domain. Substitution of Arg(544) with Gln abolished high-affinity binding of free ATP, while substitution with lysine reduced ADP affinity with minor effects on ATP binding. The contribution of Arg(544) to the change in free energy of ATP binding was estimated to 6.9 kJ/mol (DeltaDeltaG(b)) from double mutations with Asp(369) and to 7.8 kJ/mol from the MgATP dependence of phosphorylation. The phosphorylation data show that binding of Mg(2+) may increase the apparent affinity of wild-type enzyme for ATP [K(1/2)(ATP) 12 nM]. Moderately reduced affinities for ATP were seen after mutations of Asp(555), Glu(556), Asp(565), or Asp(567) with DeltaDeltaG(b) approximately equals 0.5-3 kJ/mol. Mutations of Cys(549) did not affect ATP binding. In conclusion, Arg(544) is important for binding of ATP or ADP, probably by stabilizing the beta- or gamma-phosphate moieties and aligning the gamma-phosphate for interaction with the carboxylate group of Asp(369).

Nucleotide Binding to Na,K-ATPase: The Role of Electrostatic Interactions

The contribution of electrostatic forces to the interaction of Na,K-ATPase with adenine nucleotides was investigated by studying the effect of ionic strength on nucleotide binding. At pH 7.0 and 20 degrees C, there was a qualitative correlation between the equilibrium dissociation constant (K(d)) values for ATP, ADP, and MgADP and their total charges. All K(d) values increased with increasing ionic strength. According to the Debye-Hückel theory, this suggests that the nucleotide binding site and its ligands have "effective" charges of opposite signs. However, quantitative analysis of the dependence on ionic strength shows that the product of the effective electrostatic charges on the ligand and the binding site is the same for all nucleotides, and is therefore independent of the total charge of the nucleotide. The data suggest that association of nucleotides with Na,K-ATPase is governed by a partial charge rather than the total charge of the nucleotide. This charge, interacting with positive charges on the protein, is probably the one corresponding to the alpha-phosphate of the nucleotide. Dissociation rate constants measured in complementary transient kinetic experiments were 13 s(-1) for ATP and 27 s(-1) for ADP, independent of the ionic strength in the range 0.1-0.5 M. This implies similar association rate constants for the two nucleotides (about 40 x 10(6) M(-1) s(-1) at I = 0.1 M). The results suggest that long-range Coulombic forces, affecting association rates, are not the main contributors to the observed differences in affinities, and that local interactions, affecting dissociation rates, may play an even greater role.

Role of phylogenetically conserved amino acids in folding of Na,K-ATPase

This paper focuses on the amino acid sequence 708-TGDGVNDSPALKK in pig kidney Na,K-ATPase as one of the best conserved among P-type ATPases. In Ca-ATPase this sequence forms a strand-loop-helix structure as part of a Rossman fold next to the phosphorylation site. Substitution of polar residues in the investigated sequence interfered with high-level accumulation of mutant protein. Mutant alpha1-subunit protein only accumulated in membranes from yeast cells grown at 15 degrees C whereas wild-type protein accumulated at both 15 and 35 degrees C. A systematic screen for the molecular mechanism behind lack of accumulation of mutant protein at 35 degrees C showed that transcription and translation were unaffected by the mutations. To demonstrate in vivo protein folding problems, an unfolded protein response reporter system was constructed in yeast. In this strain, only expression of mutant Na,K-ATPase alpha1-subunit caused induction of the unfolded protein response at 35 degrees C, indicating folding problems in the ER. Lowering the expression temperature to 15 degrees C prevented induction of the unfolded protein response after mutant protein expression, indicating correct folding at this temperature. At the permissive temperature mutant proteins were able to escape the endoplasmic reticulum quality control, reach the plasma membrane, and bind ouabain with high affinity. Since mutants in the 708-TGDGVNDSPALKK segment had a thermo inactivation profile identical to that of wild type, they were classified as temperature-sensitive synthesis mutants. The results indicate that this segment contributes side chains of importance for overall folding and maturation of Na,K-ATPase and all other P-type ATPases.

Structure-function relationships of Na(+), K(+), ATP, or Mg(2+) binding and energy transduction in Na,K-ATPase

The focus of this article is on progress in establishing structure-function relationships through site-directed mutagenesis and direct binding assay of Tl(+), Rb(+), K(+), Na(+), Mg(2+) or free ATP at equilibrium in Na,K-ATPase. Direct binding may identify residues coordinating cations in the E(2)[2K] or E(1)P[3Na] forms of the ping-pong reaction sequence and allow estimates of their contributions to the change of Gibbs free energy of binding. This is required to understand the molecular basis for the pronounced Na/K selectivity at the cytoplasmic and extracellular surfaces. Intramembrane Glu(327) in transmembrane segment M4, Glu(779) in M5, Asp(804) and Asp(808) in M6 are essential for tight binding of K(+) and Na(+). Asn(324) and Glu(327) in M4, Thr(774), Asn(776), and Glu(779) in 771-YTLTSNIPEITP of M5 contribute to Na(+)/K(+) selectivity. Free ATP binding identifies Arg(544) as essential for high affinity binding of ATP or ADP. In the 708-TGDGVND segment, mutations of Asp(710) or Asn(713) do not interfere with free ATP binding. Asp(710) is essential and Asn(713) is important for coordination of Mg(2+) in the E(1)P[3Na] complex, but they do not contribute to Mg(2+) binding in the E(2)P-ouabain complex. Transition to the E(2)P form involves a shift of Mg(2+) coordination away from Asp(710) and Asn(713) and the two residues become more important for hydrolysis of the acyl phosphate bond at Asp(369).

Importance of conserved alpha -subunit segment 709GDGVND for Mg2+ binding, phosphorylation, and energy transduction in Na,K-ATPase

The segment (708)TGDGVNDSPALKK(720) in the alpha-subunit P domain of Na,K-ATPase is highly conserved among cation pumps, but little is known about its role in binding of Mg(2+) or ATP and energy transduction. Here, 11 mutations of polar residues are expressed at reduced temperature in yeast with preserved capacities for high affinity binding of ouabain and ATP, whereas the Thr(708) --> Ser mutation and alterations of Asp(714) abolish all catalytic reactions. In mutations of Asp(710) and Asn(713), ATP affinity is preserved or increased, whereas Na,K-ATPase activity is severely reduced. Assay of phosphorylation from ATP in the presence of oligomycin shows that Asp(710) contributes to coordination of Mg(2+) during transfer of gamma-phosphate to Asp(369) in the high energy Mg.E(1)P[3Na] intermediate and that Asn(713) is involved in these processes. In contrast, Asp(710) and Asp(713) do not contribute to Mg(2+) binding in the E(2)P.ouabain complex. Transition to E(2)P thus involves a shift of Mg(2+) coordination away from Asp(710) and Asn(713), and the two residues become more important for hydrolysis of the acyl phosphate bond at Asp(369). The Asp(710) --> Ala mutation blocks interaction with vanadate, whereas Asn(713) --> Ala interferes with phosphorylation from P(i) of the E(2).ouabain complex, showing that the GDGVND segment is required for stabilization of the transition state and for the phosphorylation reaction. The Asp(710) --> Ala mutation also interferes with transmission of structural changes to the ouabain site and reduces the affinity for binding of Tl(+) 2- to 3-fold, suggesting a role in transmission of K(+) stimulation of phospho-enzyme hydrolysis from transmembrane segment 5 to the P domain.

Site-directed chemical labeling of extracellular loops in a membrane protein. The topology of the Na,K-ATPase alpha-subunit

We have mapped the membrane topology of the renal Na,K-ATPase alpha-subunit by using a combination of introduced cysteine mutants and surface labeling with a membrane impermeable Cys-directed reagent, N-biotinylaminoethyl methanethiosulfonate. To begin our investigation, two cysteine residues (Cys(911) and Cys(964)) in the wild-type alpha-subunit were substituted to create a background mutant devoid of exposed cysteines (Lutsenko, S., Daoud, S., and Kaplan, J. H. (1997) J. Biol. Chem. 272, 5249-5255). Into this background construct were then introduced single cysteines in each of the five putative extracellular loops (P118C, T309C, L793C, L876C, and M973C) and the resulting alpha-subunit mutants were co-expressed with the beta-subunit in baculovirus-infected insect cells. All of our expressed Na,K-ATPase mutants were functionally active. Their ATPase, phosphorylation, and ouabain binding activities were measured, and the turnover of the phosphoenzyme intermediate was close to the wild-type enzyme, suggesting that they are folded properly in the infected cells. Incubation of the insect cells with the cysteine-selective reagent revealed essentially no labeling of the alpha-subunit of the background construct and labeling of all five mutants with single cysteine residues in putative extracellular loops. Two additional mutants, V969C and L976C, were created to further define the M9M10 loop. The lack of labeling for these two mutants showed that although Met(973) is apparently exposed, Val(969) and Leu(976) are not, demonstrating that this method may also be utilized to define membrane aqueous boundaries of membrane proteins. Our labeling studies are consistent with a specific 10-transmembrane segment model of the Na,K-ATPase alpha-subunit. This strategy utilized only functional Na,K-ATPase mutants to establish the membrane topology of the entire alpha-subunit, in contrast to most previously applied methods.

Glutamic acid 472 and lysine 480 of the sodium pump alpha 1 subunit are essential for activity. Their conservation in pyrophosphatases suggests their involvement in recognition of ATP phosphates

P-type ATPases such as the Na+,K+-ATPase (sodium pump) hydrolyze ATP to pump ions through biological membranes against their electrochemical gradients. The mechanisms that couple ATP hydrolysis to the vectorial ion transport are not yet understood, but unveiling structures that participate in ATP binding and in the formation of the ionophore might help to gain insight into this process. Looking at the alpha- and beta-phosphates of ATP as a pyrophosphate molecule, we found that peptides highly conserved among all soluble inorganic pyrophosphatases are also present in ion-transporting ATPases. Included therein are Glu48 and Lys56 of the Saccharomyces cerevisiae pyrophosphatase (SCE1-PPase) that are essential for the activity of this enzyme and have been shown in crystallographic analysis to interact with phosphate molecules. To test the hypothesis that equivalent amino acids are also essential for the activity of ion-transporting ATPases, Glu472 and Lys480 of the sodium pump alpha 1 subunit corresponding to Glu48 and Lys56 of SCE1-PPase were mutated to various amino acids. Mutants of the sodium pump alpha1 subunit were expressed in yeast and analyzed for their ATPase activity and their ability to bind ouabain in the presence of either ATP, Mg2+, and Na+ or phosphate and Mg2+. All four mutants investigated, Glu472Ala, Glu472Asp, Lys480Ala, and Lys480Arg, display only a fraction of the ATPase activity obtained with the wild-type enzyme. The same applies with respect to their ability to bind ouabain, where maximum ouabain binding to the mutants accounts for only about 10% of the binding obtained with the wild-type enzyme. On the basis of our results, we conclude that Glu472 and Lys480 are essential for the activity of the sodium pump. Their function is probably to arrest the alpha- and beta-phosphate groups of ATP in a proper position prior to hydrolysis of the gamma-phosphate group. The identification of these amino acids as essential components of the ATP-recognizing mechanism of the pump has resulted in a testable hypothesis for the initial interactions of the sodium pump, and possibly of other P-type ATPases, with ATP.

Alanine scanning mutagenesis of oxygen-containing amino acids in the transmembrane region of the Na,K-ATPase

Oxygen-containing amino acids in the transmembrane region of the Na, K-ATPase alpha subunit were studied to identify residues involved in Na+ and/or K+ coordination by the enzyme. Conserved residues located in the polar face of transmembrane helices were selected using helical wheel and topological models of the enzyme. Alanine substitution of these residues were introduced into an ouabain-resistant sheep alpha1 isoform and expressed in HeLa cells. The capacity to generate essential Na+ and K+ gradients and thus support cell growth was used as an initial indication of the functionality of heterologous enzymes. Enzymes carrying alanine substitution of Ser94, Thr136, Ser140, Gln143, Glu144, Glu282, Thr334, Thr338, Thr340, Ser814, Tyr817, Glu818, Glu821, Ser822, Gln854, and Tyr994 supported cell growth, while those carrying substitutions Gln923Ala, Thr955Ala, and Asp995Ala did not. To study the effects of these latter replacements on cation binding, they were introduced into the wild-type alpha1 sheep isoform and expressed in mouse NIH3T3 cells where [3H]ouabain binding was utilized to probe the heterologous proteins. These substitutions did not affect ouabain, K+, or Na+ binding. Expression levels of these enzymes were similar to that of control. However, the level of Gln923Ala-, Thr955Ala-, or Asp995Ala-substituted enzyme at the plasma membrane was significantly lower than that of the wild-type isoform. Thus, these substitutions appear to impair the maturation process or targeting of the enzyme to the plasma membrane, but not cation-enzyme interactions. These results complete previous studies which have identified Ser755, Asp804, and Asp808 as absolutely essential for Na+ and K+ transport by the enzyme. Thus, it is significant that most transmembrane conserved-oxygen-containing residues in the Na,K-ATPase can be replaced without substantially affecting cation-enzyme interactions to the extent of preventing enzyme function. Consequently, other chemical groups, aromatic rings or backbone carbonyls, should be considered in models of cation-binding sites.

Contribution to Tl+, K+, and Na+ binding of Asn776, Ser775, Thr774, Thr772, and Tyr771 in cytoplasmic part of fifth transmembrane segment in alpha-subunit of renal Na,K-ATPase

The sequence Y771TLTSNIPEIT781P in the fifth transmembrane segment of the alpha-subunit of Na,K-ATPase is unique among cation pump proteins. Here, in search of the molecular basis for Na,K specificity, alanine and conservative substitutions were directed to six oxygen-carrying residues in this segment. The contribution of the residues to cation binding was estimated from direct binding of Tl+ [Nielsen, et al. (1998) Biochemistry 37, 1961-1968], K+ displacement of ATP binding at equilibrium, and Na+-dependent phosphorylation from ATP in the presence of oligomycin. As an intrinsic control, substitution of Thr781 had no effect on Tl+(K+) or Na+ binding. There are several novel observations from this work. First, the carboxamide group of Asn776 is equally important for binding Tl+(K+) or Na+, whereas a shift of the position of the carboxamide of Asn776 (Asn776Gln) causes a large depression of Na+ binding without affecting the binding of Tl+(K+). Second, Thr774 is important for Na+ selectivity because removal of the hydroxyl group reduces the binding of Na+ with no effect on binding of Tl+(K+). Removal of the methyl groups of Thr774 or Thr772 reduces binding of both Tl+(K+) and Na+, whereas the hydroxyl group of Thr772 does not contribute to cation binding. Furthermore, the hydroxyl groups of Ser775 and Tyr771 are important for binding both Tl+(K+) and Na+. The data suggest that rotating or tilting of the cytoplasmic part of the fifth transmembrane segment may adapt distances between coordinating groups and contribute to the distinctive Na+/K+ selectivity of the pump.