Distinctive features of catalytic and transport mechanisms in mammalian sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) and Cu+ (ATP7A/B) ATPases

Metal-Coordinated Polydopamine Structures for Tumor Imaging and Therapy

Meticulously engineered nanomaterials achieve significant advances in the diagnosis and therapy of solid tumors by improving tumor delivery efficiency; and thereby, enhancing imaging and therapeutic efficacy. Currently, polydopamine (PDA) attracts widespread attention because of its biocompatibility, simplicity of preparation, abundant surface groups, and high photothermal conversion efficiency, which can be applied in drug delivery, photothermal therapy, theranostics, and other nanomedicine fields. Inspired by PDA structures that are rich in catechol and amino functional groups that can coordinate with various metal ions, which have charming qualities and characteristics, metal-coordinated PDA structures are exploited for tumor theranostics, but are not thoroughly summarized. Herein, this review summarizes the recent progress in the fabrication of metal-coordinated PDA structures and their availabilities in tumor imaging and therapy, with further in-depth discussion of the challenges and future perspectives of metal-coordinated PDA structures, with the aim that this systematic review can promote interdisciplinary intersections and provide inspiration for the further growth and clinical translation of PDA materials.

Copper Deposition in Polydopamine Nanostructure to Promote Cuproptosis by Catalytically Inhibiting Copper Exporters of Tumor Cells for Cancer Immunotherapy

Cuproptosis is an emerging programmed cell death, displaying great potential in cancer treatment. However, intracellular copper content to induce cuproptosis is unmet, which mainly ascribes to the intracellular pumping out equilibrium mechanism by copper exporter ATP7A and ATP7B. Therefore, it is necessary to break such export balance mechanisms for desired cuproptosis. Mediated by diethyldithiocarbamate (DTC) coordination, herein a strategy to efficiently assemble copper ions into polydopamine nanostructure (PDA-DTC/Cu) for reprogramming copper metabolism of tumor is developed. The deposited Cu2+ can effectively trigger the aggregation of lipoylated proteins to induce cuproptosis of tumor cells. Beyond elevating intracellular copper accumulation, PDA-DTC/Cu enables to break the balance of copper metabolism by disrupting mitochondrial function and restricting the adenosine triphosphate (ATP) energy supply, thus catalytically inhibiting the expressions of ATP7A and ATP7B of tumor cells to enhance cuproptosis. Meanwhile, the killed tumor cells can induce immunogenic cell death (ICD) to stimulate the immune response. Besides, PDA-DTC/Cu NPs can promote the repolarization of tumor-associated macrophages (TAMs ) to relieve the tumor immunosuppressive microenvironment (TIME). Collectively, PDA-DTC/Cu presented a promising "one stone two birds" strategy to realize copper accumulation and inhibit copper export simultaneously to enhance cuproptosis for 4T1 murine breast cancer immunotherapy.

Numerical model for electrogenic transport by the ATP-dependent potassium pump KdpFABC

In vitro assays of ion transport are an essential tool for understanding molecular mechanisms associated with ATP-dependent pumps. Because ion transport is generally electrogenic, principles of electrophysiology are applicable, but conventional tools like patch-clamp are ineffective due to relatively low turnover rates of the pumps. Instead, assays have been developed to measure either voltage or current generated by transport activity of a population of molecules either in cell-derived membrane fragments or after reconstituting purified protein into proteoliposomes. In order to understand the nuances of these assays and to characterize effects of various operational parameters, we have developed a numerical model to simulate data produced by two relevant assays: fluorescence from voltage-sensitive dyes and current recorded by capacitive coupling on solid supported membranes. Parameters of the model, which has been implemented in Python, are described along with underlying principles of the computational algorithm. Experimental data from KdpFABC, a K+ pump associated with P-type ATPases, are presented, and model parameters have been adjusted to mimic these data. In addition, effects of key parameters such as nonselective leak conductance and turnover rate are demonstrated. Finally, simulated data are used to illustrate the effects of capacitive coupling on measured current and to compare alternative methods for quantification of raw data.

Tracking mitochondrial Cu(I) fluctuations through a ratiometric fluorescent probe in AD model cells: Towards understanding how AβOs induce mitochondrial Cu(I) dyshomeostasis

Mitochondrial copper signaling pathway plays a role in Alzheimer's disease (AD), especially in relevant Amyloid-β oligomers (AβOs) neurotoxicity and mitochondrial dysfunction. Clarifying the relationship between mitochondrial copper homeostasis and both of mitochondrial dysfunction and AβOs neurotoxicity is important for understanding AD pathogenesis. Herein, we designed and synthesized a ratiometric fluorescent probe CHC-NS4 for Cu(I). CHC-NS4 possesses excellent ratiometric response, high selectivity to Cu(I) and specific ability to target mitochondria. Under mitochondrial dysfunction induced by oligomycin, mitochondrial Cu(I) levels gradually increased, which may be related to inhibition of ATP7A-mediated Cu(I) exportation and/or high expression of COX. On this basis, CHC-NS4 was further utilized to visualize the fluctuations of mitochondrial Cu(I) levels during progression of AD model cells induced by AβOs. It was found that mitochondrial Cu(I) levels were gradually elevated during the AD progression, which depended on not only AβOs concentration but also incubation time. Moreover, endocytosis maybe served as a prime pathway mode for mitochondrial Cu(I) dyshomeostasis induced by AβOs during AD progression. These results have provided a novel inspiration into mitochondrial copper biology in AD pathogenesis.

Structure and mechanism of the human copper transporting ATPases: Fitting the pieces into a moving puzzle

Human copper transporters ATP7B and ATP7A deliver copper to biosynthetic pathways and maintain copper homeostasis in the cell. These enzymes combine several challenges for structural biology because they are large low abundance membrane proteins with many highly mobile domains and long disordered loops. No method has yet succeeded in solving the structure of the complete fully functional protein. Still, X-ray crystallography, Cryo-EM and NMR helped to piece together a structure based model of the enzyme activity and regulation by copper. We review the structures of ATP7B and ATP7A with an emphasis on the mechanistic insights into the unique aspects of the transport function and regulation of the human copper ATPases that have emerged from more than twenty years of research.

Functional characterization of Legionella pneumophila Cu+ transport ATPase. The activation by Cu+ and ATP

Cu⁺-ATPases are integral membrane proteins belonging to the IB subfamily of the P-type ATPases that couple Cu⁺ transport to the hydrolysis of ATP. As some structural and functional particularities arise for Cu⁺-ATPases, several authors suggest that some of the reaction steps of the Albers-Post model postulated for other P-ATPases may be different. In this work we describe a functional characterization of Legionella pneumophila Cu⁺-ATPase (LpCopA), the first P_IB-ATPase whose structure was determined by X-ray crystallography. Cu⁺-ATPase activity of the enzyme presents a maximum at ∼37 °C and pH 6.6-6.8. Phospholipids enhance LpCopA Cu⁺-ATPase activity in a non-essential mode where optimal activity is achieved at an asolectin molar fraction of 0.15 and an amphiphile-protein ratio of ~30,000. As described for other P-ATPases, Mg²⁺ acts as an essential activator. Furthermore, Cu⁺-ATPase activity dependence on [Cu⁺] and [ATP] can both be described by a sum of two hyperbolic functions. Based on that, and the [Cu⁺] and [ATP] dependencies of the best fitting parameters of the hyperbolae pointed above, we propose a minimal reaction scheme for the catalytic mechanism that shares the basic reaction steps of the Albers-Post model for P-type ATPases. The reaction scheme postulated contemplates two different binding affinities for a single ATP (apparent affinities of 0.66 and 550 μM at [Cu⁺] → ∞) and binding of at least 2 Cu⁺ with different affinities as well (apparent affinities of 1.4 and 102.5 μM at [ATP] → ∞).

Transmembrane Cu(I) P-type ATPase pumps are electrogenic uniporters

Cu(i) P-type ATPases are transmembrane primary active ion pumps that catalyze the extrusion of copper ions across cellular membranes. Their activity is critical in controlling copper levels in all kingdoms of life. Biochemical and structural characterization established the structural framework by which Cu-pumps perform their function. However, the details of the overall mechanism of transport (uniporter vs. cotransporter) and electrogenicity still remain elusive. In this work, we developed a platform to reconstitute the model Cu(i)-pump from E. coli (EcCopA) in artificial lipid bilayer small unilamellar vesicles (SUVs) to quantitatively characterize the metal substrate, putative counter-ions and charge translocation. By encapsulating in the liposome lumen fluorescence detector probes (CTAP-3, pyranine and oxonol VI) responsive to diverse stimuli (Cu(i), pH and membrane potential), we correlated substrate, secondary-ion translocation and charge movement events in EcCopA proteoliposomes. This platform centered on multiple fluorescence reporters allowed study of the mechanism and translocation kinetic parameters in real-time for wild-type EcCopA and inactive mutants. The maximal initial Cu(i) transport rate of 165 nmol Cu(i) mg-1 min-1 and KM, Cu(I) = 0.15 ± 0.07 μM was determined with this analysis. We reveal that Cu(i) pumps are primary-active uniporters and electrogenic. The Cu(i) translocation cycle does not require proton counter-transport resulting in electrogenic generation of transmembrane potential upon translocation of one Cu(i) per ATP hydrolysis cycle. Thus, mechanistic differences between Cu(i) pumps and other better characterized P-type ATPases are discussed. The platform opens the venue to study translocation events and mechanisms of transport in other transition metal P-type ATPase pumps.

The transferred translocases: An old wine in a new bottle

The role of translocases was underappreciated and was not included as a separate class in the enzyme commission until August 2018. The recent research interests in proteomics of orphan enzymes, ionomics, and metallomics along with high-throughput sequencing technologies generated overwhelming data and revamped this enzyme into a separate class. This offers a great opportunity to understand the role of new or orphan enzymes in general and specifically translocases. The enzymes belonging to translocases regulate/permeate the transfer of ions or molecules across the membranes. These enzyme entries were previously associated with other enzyme classes, which are now transferred to a new enzyme class 7 (EC 7). The entries that are reclassified are important to extend the enzyme list, and it is the need of the hour. Accordingly, there is an upgradation of entries of this class of enzymes in several databases. This review is a concise compilation of translocases with reference to the number of entries currently available in the databases. This review also focuses on function as well as dysfunction of translocases during normal and disordered states, respectively.

Protein Adsorption on Solid Supported Membranes: Monitoring the Transport Activity of P-Type ATPases

P-type ATPases are a large family of membrane transporters that are found in all forms of life. These enzymes couple ATP hydrolysis to the transport of various ions or phospholipids across cellular membranes, thereby generating and maintaining crucial electrochemical potential gradients. P-type ATPases have been studied by a variety of methods that have provided a wealth of information about the structure, function, and regulation of this class of enzymes. Among the many techniques used to investigate P-type ATPases, the electrical method based on solid supported membranes (SSM) was employed to investigate the transport mechanism of various ion pumps. In particular, the SSM method allows the direct measurement of charge movements generated by the ATPase following adsorption of the membrane-bound enzyme on the SSM surface and chemical activation by a substrate concentration jump. This kind of measurement was useful to identify electrogenic partial reactions and localize ion translocation in the reaction cycle of the membrane transporter. In the present review, we discuss how the SSM method has contributed to investigate some key features of the transport mechanism of P-type ATPases, with a special focus on sarcoplasmic reticulum Ca²⁺-ATPase, mammalian Cu⁺-ATPases (ATP7A and ATP7B), and phospholipid flippase ATP8A2.

Linking Biochemical and Structural States of SERCA: Achievements, Challenges, and New Opportunities

Sarcoendoplasmic reticulum calcium ATPase (SERCA), a member of the P-type ATPase family of ion and lipid pumps, is responsible for the active transport of Ca²⁺ from the cytoplasm into the sarcoplasmic reticulum lumen of muscle cells, into the endoplasmic reticulum (ER) of non-muscle cells. X-ray crystallography has proven to be an invaluable tool in understanding the structural changes of SERCA, and more than 70 SERCA crystal structures representing major biochemical states (defined by bound ligand) have been deposited in the Protein Data Bank. Consequently, SERCA is one of the best characterized components of the calcium transport machinery in the cell. Emerging approaches in the field, including spectroscopy and molecular simulation, now help integrate and interpret this rich structural information to understand the conformational transitions of SERCA that occur during activation, inhibition, and regulation. In this review, we provide an overview of the crystal structures of SERCA, focusing on identifying metrics that facilitate structure-based categorization of major steps along the catalytic cycle. We examine the integration of crystallographic data with different biophysical approaches and computational methods to link biochemical and structural states of SERCA that are populated in the cell. Finally, we discuss the challenges and new opportunities in the field, including structural elucidation of functionally important and novel regulatory complexes of SERCA, understanding the structural basis of functional divergence among homologous SERCA regulators, and bridging the gap between basic and translational research directed toward therapeutic modulation of SERCA.

A case of a mild Wolfram Syndrome with concomitant ATP7B mutation

Background

Wolfram Syndrome 1 (WS1) has been characterized on the basis of mutation in the WFS1 gene encoding a calcium storage wolframin endoplasmatic reticulum transmembrane glycoprotein.

Patients and Methods

We observed a WS 10-years old female subject, with Type 1 diabetes-mellitus (DM), that had compound heterozygous WSF1 mutations but without other symptoms generally observed in WS subjects, such as optic atrophy or neurodegeneration.

Results

Decreased copper, ceruloplasmin, and transferrin levels, pointing to a copper deficiency, were associated with a new c.18703A>G mutation in the ATP7B gene, while lower calcium levels were associated with WSF1 mutations. An omega-3 fatty acids therapy was administrated to the subject in the attempt to ameliorate diabetes symptoms, restored copper deficiency, and normal calcium levels.

Conclusions

This specific case report provides new insights into the potential interplay of ATP7B mutation in shaping a milder WS clinical picture.

Enhanced cellulase production by decreasing intercellular pH through H+-ATPase gene deletion in Trichoderma reesei RUT-C30

BACKGROUND

Cellulolytic enzymes produced by Trichoderma reesei are widely used for the industrial production of biofuels and chemicals from lignocellulose. We speculated that intracellular pH during the fermentation process can affect cellulase induction.

RESULTS

In this study, two H⁺-ATPase genes, tre76238 and tre78757, were first identified in T. reesei. Deletion of tre76238 and tre78757 in T. reesei RUT-C30 confirmed that tre76238 has a major function in maintaining intracellular pH, whereas tre78757 has a minor function. The tre76238 deletion strain Δ76238 displayed a high level of cellulase production using cellulase-repressive glucose as a sole carbon source, along with intracellular acid accumulation and growth retardation. Our results indicated that intracellular acid accumulation in Δ76238 stimulated a significant increase in the cytosolic Ca²⁺ levels. Ca²⁺ channels were shown to be necessary for cellulase production using glucose as the carbon source in Δ76238. Delayed Δ76238 growth could be reversed by optimizing the medium's nitrogen sources to produce ammonia for intracellular acid neutralization in the early phase. This may be useful for scale-up of cellulase production using glucose as the carbon source.

CONCLUSIONS

This study provides a new perspective for significant alterations in the cellulase expression pattern of T. reesei Δ76238, indicating a new mechanism for cellulase regulation under conditions of low intracellular pH.

Modulating Chemosensitivity of Tumors to Platinum-Based Antitumor Drugs by Transcriptional Regulation of Copper Homeostasis

Platinum (Pt)-based antitumor agents have been effective in treating many human malignancies. Drug importing, intracellular shuffling, and exporting—carried out by the high-affinity copper (Cu) transporter (hCtr1), Cu chaperone (Ato x1), and Cu exporters (ATP7A and ATP7B), respectively—cumulatively contribute to the chemosensitivity of Pt drugs including cisplatin and carboplatin, but not oxaliplatin. This entire system can also handle Pt drugs via interactions between Pt and the thiol-containing amino acid residues in these proteins; the interactions are strongly influenced by cellular redox regulators such as glutathione. hCtr1 expression is induced by acute Cu deprivation, and the induction is regulated by the transcription factor specific protein 1 (Sp1) which by itself is also regulated by Cu concentration variations. Copper displaces zinc (Zn) coordination at the zinc finger (ZF) domains of Sp1 and inactivates its DNA binding, whereas Cu deprivation enhances Sp1-DNA interactions and increases Sp1 expression, which in turn upregulates hCtr1. Because of the shared transport system, chemosensitivity of Pt drugs can be modulated by targeting Cu transporters. A Cu-lowering agent (trientine) in combination with a Pt drug (carboplatin) has been used in clinical studies for overcoming Pt-resistance. Future research should aim at further developing effective Pt drug retention strategies for improving the treatment efficacy.

Drug Interactions With the Ca2+-ATPase From Sarco(Endo)Plasmic Reticulum (SERCA)

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) is an intracellular membrane transporter that utilizes the free energy provided by ATP hydrolysis for active transport of Ca²⁺ ions from the cytoplasm to the lumen of sarco(endo)plasmic reticulum. SERCA plays a fundamental role for cell calcium homeostasis and signaling in muscle cells and also in cells of other tissues. Because of its prominent role in many physiological processes, SERCA dysfunction is associated to diseases displaying various degrees of severity. SERCA transport activity can be inhibited by a variety of compounds with different chemical structures. Specific SERCA inhibitors were identified which have been instrumental in studies of the SERCA catalytic and transport mechanism. It has been proposed that SERCA inhibition may represent a novel therapeutic strategy to cure certain diseases by targeting SERCA activity in pathogens, parasites and cancer cells. Recently, novel small molecules have been developed that are able to stimulate SERCA activity. Such SERCA activators may also offer an innovative and promising therapeutic approach to treat diseases, such as heart failure, diabetes and metabolic disorders. In the present review the effects of pharmacologically relevant compounds on SERCA transport activity are presented. In particular, we will discuss the interaction of SERCA with specific inhibitors and activators that are potential therapeutic agents for different diseases.

Preexisting domain motions underlie protonation-dependent structural transitions of the P-type Ca2+-ATPase

We have performed microsecond molecular dynamics (MD) simulations to determine the mechanism for protonation-dependent structural transitions of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), one of the most prominent members of the large P-type ATPase superfamily that transports ions across biological membranes. The release of two H+ from the transport sites activates SERCA by inducing a structural transition between low (E2) and high (E1) Ca2+-affinity states (E2-to-E1 transition), but the structural mechanism by which transport site deprotonation facilitates this transition is unknown. We performed microsecond all-atom MD simulations to determine the effects of transport site protonation on the structural dynamics of the E2 state in solution. We found that the protonated E2 state has structural characteristics that are similar to those observed in crystal structures of E2. Upon deprotonation, a single Na+ ion rapidly (<10 ns) binds to the transmembrane transport sites and induces a kink in M5, disrupts the M3-M5 interface, and increases the mobility of the M3/A-M3 linker. Principal component analysis showed that counter-rotation of the cytosolic N-A domains about the membrane normal axis, which is the primary motion driving the E2-to-E1 transition, is present in both protonated and deprotonated E2 states; however, protonation-dependent structural changes in the transmembrane domain control the hierarchical organization and amplitude of this motion. We propose that preexisting rigid-body domain motions underlie structural transitions of SERCA, where the functionally important directionality is preserved while transport site protonation controls the dominance and amplitude of motion to shift the equilibrium between the E1 and E2 states. We conclude that ligand-induced modulation of preexisting domain motions is likely a common theme in structural transitions of the P-type ATPase superfamily.

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.

Mechanisms of charge transfer in human copper ATPases ATP7A and ATP7B

ATP7A and ATP7B are Cu⁺ -transporting ATPases of subclass IB and play a fundamental role in intracellular copper homeostasis. ATP7A/B transfer Cu⁺ ions across the membrane from delivery to acceptor proteins without establishing a free Cu⁺ gradient. Transfer of copper across the membrane is coupled to ATP hydrolysis. Current measurements on solid supported membranes (SSM) were performed to investigate the mechanism of copper-related charge transfer across ATP7A and ATP7B. SSM measurements demonstrated that electrogenic copper displacement occurs within ATP7A/B following addition of ATP and formation of the phosphorylated intermediate. Comparison of the time constants for cation displacement in ATP7A/B and sarcoplasmic reticulum Ca²⁺ -ATPase is consistent with the slower phosphoenzyme formation in copper ATPases. Moreover, ATP-dependent copper transfer in ATP7A/B is not affected by varying the pH, suggesting that net proton counter-transport may not occur in copper ATPases. Platinum anticancer drugs activate ATP7A/B and are subjected to ATP-dependent vectorial displacement with a mechanism analogous to that of copper. © 2016 IUBMB Life, 69(4):218-225, 2017.

Molecular features of copper binding proteins involved in copper homeostasis

Copper has a wide and important role in biological systems, determining conformation and activity of many metalloproteins and enzymes, such as cytochrome oxidase and superoxide dismutase . Furthermore, due to its possible reactivity with nonspecific proteins and toxic effects, elaborate systems of absorption, concentration buffering, delivery to specific protein sites and elimination, require a complex system including small carriers, chaperones and active transporters. The P-type copper ATPases ATP7A and ATP7B provide an important system for acquisition, active transport, distribution and elimination of copper. Relevance of copper metabolism to human diseases and therapy is already known. It is quite certain that further studies will reveal detailed and useful information on biochemical mechanisms and relevance to diseases. © 2016 IUBMB Life, 69(4):211-217, 2017.

The promiscuous phosphomonoestearase activity of Archaeoglobus fulgidus CopA, a thermophilic Cu+ transport ATPase

Membrane transport P-type ATPases display two characteristic enzymatic activities: a principal ATPase activity provides the driving force for ion transport across biological membranes, whereas a promiscuous secondary activity catalyzes the hydrolysis of phosphate monoesters. This last activity is usually denoted as the phosphatase activity of P-ATPases. In the present study, we characterize the phosphatase activity of the Cu(+)-transport ATPase from Archaeglobus fulgidus (Af-CopA) and compare it with the principal ATPase activity. Our results show that the phosphatase turnover number was 20 times higher than that corresponding to the ATPase activity, but it is compensated by a high value of Km, producing a less efficient catalysis for pNPP. This secondary activity is enhanced by Mg(2+) (essential activator) and phospholipids (non-essential activator), and inhibited by salts and Cu(+). Transition state analysis of the catalyzed and noncatalyzed hydrolysis of pNPP indicates that Af-CopA enhances the reaction rates by a factor of 10(5) (ΔΔG(‡)=38 kJ/mol) mainly by reducing the enthalpy of activation (ΔΔH(‡)=30 kJ/mol), whereas the entropy of activation is less negative on the enzyme than in solution. For the ATPase activity, the decrease in the enthalpic component of the barrier is higher (ΔΔH(‡)=39 kJ/mol) and the entropic component is small on both the enzyme and in solution. These results suggest that different mechanisms are involved in the transference of the phosphoryl group of p-nitrophenyl phosphate and ATP.

Atomic-level mechanisms for phospholamban regulation of the calcium pump

We performed protein pKa calculations and molecular dynamics (MD) simulations of the calcium pump (sarcoplasmic reticulum Ca(2+)-ATPase (SERCA)) in complex with phospholamban (PLB). X-ray crystallography studies have suggested that PLB locks SERCA in a low-Ca(2+)-affinity E2 state that is incompatible with metal-ion binding, thereby blocking the conversion toward a high-Ca(2+)-affinity E1 state. Estimation of pKa values of the acidic residues in the transport sites indicates that at normal intracellular pH (7.1-7.2), PLB-bound SERCA populates an E1 state that is deprotonated at residues E309 and D800 yet protonated at residue E771. We performed three independent microsecond-long MD simulations to evaluate the structural dynamics of SERCA-PLB in a solution containing 100 mM K(+) and 3 mM Mg(2+). Principal component analysis showed that PLB-bound SERCA lies exclusively along the structural ensemble of the E1 state. We found that the transport sites of PLB-bound SERCA are completely exposed to the cytosol and that K(+) ions bind transiently (≤5 ns) and nonspecifically (nine different positions) to the two transport sites, with a total occupancy time of K(+) in the transport sites of 80%. We propose that PLB binding to SERCA populates a novel (to our knowledge) E1 intermediate, E1⋅H(+)771. This intermediate serves as a kinetic trap that controls headpiece dynamics and depresses the structural transitions necessary for Ca(2+)-dependent activation of SERCA. We conclude that PLB-mediated regulation of SERCA activity in the heart results from biochemical and structural transitions that occur primarily in the E1 state of the pump.

Hofmeister effect of anions on calcium translocation by sarcoplasmic reticulum Ca(2+)-ATPase

The occurrence of Hofmeister (specific ion) effects in various membrane-related physiological processes is well documented. For example the effect of anions on the transport activity of the ion pump Na⁺, K⁺-ATPase has been investigated. Here we report on specific anion effects on the ATP-dependent Ca²⁺ translocation by the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA). Current measurements following ATP concentration jumps on SERCA-containing vesicles adsorbed on solid supported membranes were carried out in the presence of different potassium salts. We found that monovalent anions strongly interfere with ATP-induced Ca²⁺ translocation by SERCA, according to their increasing chaotropicity in the Hofmeister series. On the contrary, a significant increase in Ca²⁺ translocation was observed in the presence of sulphate. We suggest that the anions can affect the conformational transition between the phosphorylated intermediates E₁P and E₂P of the SERCA cycle. In particular, the stabilization of the E₁P conformation by chaotropic anions seems to be related to their adsorption at the enzyme/water and/or at the membrane/water interface, while the more kosmotropic species affect SERCA conformation and functionality by modifying the hydration layers of the enzyme.

A sulfur-based transport pathway in Cu+-ATPases

Cells regulate copper levels tightly to balance the biogenesis and integrity of copper centers in vital enzymes against toxic levels of copper. PIB -type Cu(+)-ATPases play a central role in copper homeostasis by catalyzing the selective translocation of Cu(+) across cellular membranes. Crystal structures of a copper-free Cu(+)-ATPase are available, but the mechanism of Cu(+) recognition, binding, and translocation remains elusive. Through X-ray absorption spectroscopy, ATPase activity assays, and charge transfer measurements on solid-supported membranes using wild-type and mutant forms of the Legionella pneumophila Cu(+)-ATPase (LpCopA), we identify a sulfur-lined metal transport pathway. Structural analysis indicates that Cu(+) is bound at a high-affinity transmembrane-binding site in a trigonal-planar coordination with the Cys residues of the conserved CPC motif of transmembrane segment 4 (C382 and C384) and the conserved Met residue of transmembrane segment 6 (M717 of the MXXXS motif). These residues are also essential for transport. Additionally, the studies indicate essential roles of other conserved intramembranous polar residues in facilitating copper binding to the high-affinity site and subsequent release through the exit pathway.

Chelerythrine inhibits the sarco/endoplasmic reticulum Ca(2+)-ATPase and results in cell Ca(2+) imbalance

The isoquinoline alkaloid chelerythrine is described as an inhibitor of SERCA. The ATPase inhibition presented two non-competitive components, Ki1=1, 2 μM and Ki2=26 μM. Conversely, chelerythrine presented a dual effect on the p-nitrophenylphosphatase (pNPPase) of SERCA. Ca(2+)-dependent pNPPase was activated up to ∼5 μM chelerythrine with inhibition thereafter. Ca(2+)-independent pNPPase was solely inhibited. The phosphorylation of SERCA with ATP reached half-inhibition with 10 μM chelerythrine and did not parallel the decrease of ATPase activity. In contrast, chelerythrine up to 50 μM increased the phosphorylation by Pi. Cross-linking of SERCA with glutaraldehyde was counteracted by high concentrations of chelerythrine. The controlled tryptic digestion of SERCA shows that the low-affinity binding of chelerythrine evoked an E2-like pattern. Our data indicate a non-competitive inhibition of ATP hydrolysis that favors buildup of the E2-conformers of the enzyme. Chelerythrine as low as 0.5-1.5 μM resulted in an increase of intracellular Ca(2+) on cultured PBMC cells. The inhibition of SERCA and the loss of cell Ca(2+) homeostasis could in part be responsible for some described cytotoxic effects of the alkaloid. Thus, the choice of chelerythrine as a PKC-inhibitor should consider its potential cytotoxicity due to the alkaloid's effects on SERCA.

Biochemical characterization of P-type copper ATPases

Copper ATPases, in analogy with other members of the P-ATPase superfamily, contain a catalytic headpiece including an aspartate residue reacting with ATP to form a phosphoenzyme intermediate, and transmembrane helices containing cation-binding sites [TMBS (transmembrane metal-binding sites)] for catalytic activation and cation translocation. Following phosphoenzyme formation by utilization of ATP, bound copper undergoes displacement from the TMBS to the lumenal membrane surface, with no H⁺ exchange. Although PII-type ATPases sustain active transport of alkali/alkali-earth ions (i.e. Na⁺, Ca²⁺) against electrochemical gradients across defined membranes, PIB-type ATPases transfer transition metal ions (i.e. Cu⁺) from delivery to acceptor proteins and, prominently in mammalian cells, undergo trafficking from/to various membrane compartments. A specific component of copper ATPases is the NMBD (N-terminal metal-binding domain), containing up to six copper-binding sites in mammalian (ATP7A and ATP7B) enzymes. Copper occupancy of NMBD sites and interaction with the ATPase headpiece are required for catalytic activation. Furthermore, in the presence of copper, the NMBD allows interaction with protein kinase D, yielding phosphorylation of serine residues, ATP7B trafficking and protection from proteasome degradation. A specific feature of ATP7A is glycosylation and stabilization on plasma membranes. Cisplatin, a platinum-containing anti-cancer drug, binds to copper sites of ATP7A and ATP7B, and undergoes vectorial displacement in analogy with copper.

Translocation of platinum anticancer drugs by human copper ATPases ATP7A and ATP7B

Cisplatin, carboplatin, and oxaliplatin are widely used anticancer drugs. Their efficacy is strongly reduced by development of cell resistance. Down-regulation of CTR1 and up-regulation of the Cu-ATPases, ATP7A and ATP7B, have been associated to augmented drug resistance. To gain information on translocation of Pt drugs by human Cu-ATPases, we performed electrical measurements on the COS-1 cell microsomal fraction, enriched with recombinant ATP7A, ATP7B, and selected mutants, and adsorbed on a solid supported membrane. The experimental results indicate that Pt drugs activate Cu-ATPases and undergo ATP-dependent translocation in a fashion similar to that of Cu. We then used NMR spectroscopy and ESI-MS to determine the binding mode of these drugs to the first N-terminal metal-binding domain of ATP7A (Mnk1).

Istaroxime stimulates SERCA2a and accelerates calcium cycling in heart failure by relieving phospholamban inhibition

BACKGROUND AND PURPOSE

Calcium handling is known to be deranged in heart failure. Interventions aimed at improving cell Ca(2) (+) cycling may represent a promising approach to heart failure therapy. Istaroxime is a new luso-inotropic compound that stimulates cardiac contractility and relaxation in healthy and failing animal models and in patients with acute heart failure (AHF) syndrome. Istaroxime is a Na-K ATPase inhibitor with the unique property of increasing sarcoplasmic reticulum (SR) SERCA2a activity as shown in heart microsomes from humans and guinea pigs. The present study addressed the molecular mechanism by which istaroxime increases SERCA2a activity.

EXPERIMENTAL APPROACH

To study the effect of istaroxime on SERCA2a-phospholamban (PLB) complex, we applied different methodologies in native dog healthy and failing heart preparations and heterologous canine SERCA2a/PLB co-expressed in Spodoptera frugiperda (Sf21) insect cells.

KEY RESULTS

We showed that istaroxime enhances SERCA2a activity, Ca(2) (+) uptake and the Ca(2) (+) -dependent charge movements into dog healthy and failing cardiac SR vesicles. Although not directly demonstrated, the most probable explanation of these activities is the displacement of PLB from SERCA2a.E2 conformation, independently from cAMP/PKA. We propose that this displacement may favour the SERCA2a conformational transition from E2 to E1, thus resulting in the acceleration of Ca(2) (+) cycling.

CONCLUSIONS AND IMPLICATIONS

Istaroxime represents the first example of a small molecule that exerts a luso-inotropic effect in the failing human heart through the stimulation of SERCA2a ATPase activity and the enhancement of Ca(2) (+) uptake into the SR by relieving the PLB inhibitory effect on SERCA2a in a cAMP/PKA independent way.

Copper-transporting P-type ATPases use a unique ion-release pathway

Heavy metals in cells are typically regulated by PIB-type ATPases. The first structure of the class, a Cu(+)-ATPase from Legionella pneumophila (LpCopA), outlined a copper transport pathway across the membrane, which was inferred to be occluded. Here we show by molecular dynamics simulations that extracellular water solvated the transmembrane (TM) domain, results indicative of a Cu(+)-release pathway. Furthermore, a new LpCopA crystal structure determined at 2.8-Å resolution, trapped in the preceding E2P state, delineated the same passage, and site-directed-mutagenesis activity assays support a functional role for the conduit. The structural similarities between the TM domains of the two conformations suggest that Cu(+)-ATPases couple dephosphorylation and ion extrusion differently than do the well-characterized PII-type ATPases. The ion pathway explains why certain Menkes' and Wilson's disease mutations impair protein function and points to a site for inhibitors targeting pathogens.

Ca(2+)/H (+) exchange, lumenal Ca(2+) release and Ca (2+)/ATP coupling ratios in the sarcoplasmic reticulum ATPase

The Ca²⁺ transport ATPase (SERCA) of sarcoplasmic reticulum (SR) plays an important role in muscle cytosolic signaling, as it stores Ca²⁺ in intracellular membrane bound compartments, thereby lowering cytosolic Ca²⁺ to induce relaxation. The stored Ca²⁺ is in turn released upon membrane excitation to trigger muscle contraction. SERCA is activated by high affinity binding of cytosolic Ca²⁺, whereupon ATP is utilized by formation of a phosphoenzyme intermediate, which undergoes protein conformational transitions yielding reduced affinity and vectorial translocation of bound Ca²⁺. We review here biochemical and biophysical evidence demonstrating that release of bound Ca²⁺ into the lumen of SR requires Ca²⁺/H⁺ exchange at the low affinity Ca²⁺ sites. Rise of lumenal Ca²⁺ above its dissociation constant from low affinity sites, or reduction of the H⁺ concentration by high pH, prevent Ca²⁺/H⁺ exchange. Under these conditions Ca²⁺ release into the lumen of SR is bypassed, and hydrolytic cleavage of phosphoenzyme may yield uncoupled ATPase cycles. We clarify how such Ca²⁺pump slippage does not occur within the time length of muscle twitches, but under special conditions and in special cells may contribute to thermogenesis.

Enhanced adsorption of Ca-ATPase containing vesicles on a negatively charged solid-supported-membrane for the investigation of membrane transporters

A convenient model system for a biological membrane is a solid-supported membrane (SSM), which consists of a gold-supported alkanethiol|phospholipid bilayer. In combination with a concentration jump method, SSMs have been used for the investigation of several membrane transporters. Vesicles incorporating sarcoplasmic reticulum Ca-ATPase (SERCA) were adsorbed on a negatively charged SSM (octadecanethiol|phosphatidylserine bilayer). The current signal generated by the adsorbed vesicles following an ATP concentration jump was compared to that produced by SERCA-containing vesicles adsorbed on a conventional SSM (octadecanethiol|phosphatidylcholine bilayer). A significantly higher current amplitude was recorded on the serine-based SSM. The adsorption of SERCA-incorporating vesicles on the SSM was then characterized by surface plasmon resonance (SPR). The SPR measurements clearly indicate that in the presence of Ca(2+) and Mg(2+), the amount of adsorbed vesicles on the serine-based SSM is about twice that obtained using the conventional SSM, thereby demonstrating that the higher current amplitude recorded on the negatively charged SSM is correlated with a greater quantity of adsorbed vesicles. The enhanced adsorption of membrane vesicles on the PS-based SSM may be useful to study membrane preparations with a low concentration of transport protein generating small current signals, as in the case of various recombinantly expressed proteins.

Golgi in copper homeostasis: a view from the membrane trafficking field

Copper is essential for a variety of important biological processes as a cofactor and regulator of many enzymes. Incorporation of copper into the secreted and plasma membrane-targeted cuproenzymes takes place in Golgi, a compartment central for normal copper homeostasis. The Golgi complex harbors copper-transporting ATPases, ATP7A and ATP7B that transfer copper from the cytosol into Golgi lumen for incorporation into copper-dependent enzymes. The Golgi complex also sends these ATPases to appropriate post-Golgi destinations to ensure correct Cu fluxes in the body and to avoid potentially toxic copper accumulation. Mutations in ATP7A or ATP7B or in the proteins that regulate their trafficking affect their exit from Golgi or subsequent retrieval to this organelle. This, in turn, disrupts the homeostatic Cu balance, resulting in copper deficiency (Menkes disease) or copper overload (Wilson disease). Research over the last decade has yielded significant insights into the enzymatic properties and cell biology of the copper ATPases. However, the mechanisms through which the Golgi regulates trafficking of ATP7A/7B and, therefore, maintains Cu homeostasis remain unclear. This review summarizes current data on the role of the Golgi in Cu metabolism and outlines questions and challenges that should be addressed to understand ATP7A and ATP7B trafficking mechanisms in health and disease.

On allosteric modulation of P-type Cu(+)-ATPases

P-type ATPases perform active transport of various compounds across biological membranes and are crucial for ion homeostasis and the asymmetric composition of lipid bilayers. Although their functional cycle share principles of phosphoenzyme intermediates, P-type ATPases also show subclass-specific sequence motifs and structural elements that are linked to transport specificity and mechanistic modulation. Here we provide an overview of the Cu(+)-transporting ATPases (of subclass PIB) and compare them to the well-studied sarco(endo)plasmic reticulum Ca(2+)-ATPase (of subclass PIIA). Cu(+) ions in the cell are delivered by soluble chaperones to Cu(+)-ATPases, which expose a putative "docking platform" at the intracellular interface. Cu(+)-ATPases also contain heavy-metal binding domains providing a basis for allosteric control of pump activity. Database analysis of Cu(+) ligating residues questions a two-site model of intramembranous Cu(+) binding, and we suggest an alternative role for the proposed second site in copper translocation and proton exchange. The class-specific features demonstrate that topological diversity in P-type ATPases may tune a general energy coupling scheme to the translocation of compounds with remarkably different properties.

Ion pathways in the sarcoplasmic reticulum Ca2+-ATPase

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