Structure of the Ca2+ pump of sarcoplasmic reticulum: a view along the lipid bilayer at 9-A resolution

Role of Whole Plant Extract of Nelumbo nucifera Gaertn in the Treatment of Thrombolysis

AIM

This study aims to find out the components responsible for the antithrombotic activity of Nelumbo nucifera.

MATERIAL AND METHODS

Petroleum ether, chloroform and hydroalcoholic extracts of whole plant of Nelumbo nucifera (Lotus) were prepared and assessed for its thrombolytic, anti-platelet aggregation activity and bleeding time. The extracts were further analyzed through HPTLC and GC-MS. Statistical analysis was conducted through ANOVA trailed by Tukey's multiple comparison test test.

RESULTS

Hydroalcoholic extract showed the highest activity at the concentration of 400µg/ml in thrombolytic assay (42.03 ± 5.76), anti-platelet aggregation assay (57.93 ± 1.68) and bleeding time (70.17 ± 2.16) in comparison to clopodigrel (33.76 ± 3.43), aspirin (66.55 ± 1.86) and aspirin (93.85 ± 2.75) at the concentration of 100 µg/ml respectively. 25 peaks were identified through GC-MS, out of which, ferulic acid (14.2µ/g) and quercetin (5.4 µ/g) are active chemical compounds. HPTLC showed different chromatograms in hydroalcoholic extracts like (1) chlorogenic, (2) quercetin, (3) benzoic acid, (4) caffeic acid, (5) ferulic acid, (6) kaempferol, and (7) gallic acid.

CONCLUSION

Based on these findings, flavonoids present in hydroalcoholic extract may be developed into a drug for clinical application for the treatment of thrombosis in patients.

Intrinsically disordered HAX-1 regulates Ca2+ cycling by interacting with lipid membranes and the phospholamban cytoplasmic region

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) is a 279 amino acid protein expressed ubiquitously. In cardiac muscle, HAX-1 was found to modulate the sarcoendoplasmic reticulum calcium ATPase (SERCA) by shifting its apparent Ca2+ affinity (pCa). It has been hypothesized that HAX-1 binds phospholamban (PLN), enhancing its inhibitory function on SERCA. HAX-1 effects are reversed by cAMP-dependent protein kinase A that phosphorylates PLN at Ser16. To date, the molecular mechanisms for HAX-1 regulation of the SERCA/PLN complex are still unknown. Using enzymatic, in cell assays, circular dichroism, and NMR spectroscopy, we found that in the absence of a binding partner HAX-1 is essentially disordered and adopts a partial secondary structure upon interaction with lipid membranes. Also, HAX-1 interacts with the cytoplasmic region of monomeric and pentameric PLN as detected by NMR and in cell FRET assays, respectively. We propose that the regulation of the SERCA/PLN complex by HAX-1 is mediated by its interactions with lipid membranes, adding another layer of control in Ca2+ homeostatic balance in the heart muscle.

Essay on Biomembrane Structure

Of all the macromolecular assemblies of life, the least understood is the biomembrane. This is especially true in regard to its atomic structure. Ideas on biomembranes, developed in the last 200 years, culminated in the fluid mosaic model of the membrane. In this essay, I provide a historical outline of how we arrived at our current understanding of biomembranes and the models we use to describe them. A selection of direct experimental findings on the nano-scale structure of biomembranes is taken up to discuss their physical nature, and special emphasis is put on the surprising insights that arise from atomic scale descriptions.

Mechanism of artemisinin resistance for malaria PfATP6 L263 mutations and discovering potential antimalarials: An integrated computational approach

Artemisinin resistance in Plasmodium falciparum threatens global efforts in the elimination or eradication of malaria. Several studies have associated mutations in the PfATP6 gene in conjunction with artemisinin resistance, but the underlying molecular mechanism of the resistance remains unexplored. Associated mutations act as a biomarker to measure the artemisinin efficacy. In the proposed work, we have analyzed the binding affinity and efficacy between PfATP6 and artemisinin in the presence of L263D, L263E and L263K mutations. Furthermore, we performed virtual screening to identify potential compounds to inhibit the PfATP6 mutant proteins. In this study, we observed that artemisinin binding affinity with PfATP6 gets affected by L263D, L263E and L263K mutations. This in silico elucidation of artemisinin resistance enhanced the identification of novel compounds (CID: 10595058 and 10625452) which showed good binding affinity and efficacy with L263D, L263E and L263K mutant proteins in molecular docking and molecular dynamics simulations studies. Owing to the high propensity of the parasite to drug resistance the need for new antimalarial drugs will persist until the malarial parasites are eventually eradicated. The two compounds identified in this study can be tested in in vitro and in vivo experiments as possible candidates for the designing of new potential antimalarial drugs.

A plausible mechanism for the antimalarial activity of artemisinin: A computational approach

Artemisinin constitutes the frontline treatment to aid rapid clearance of parasitaemia and quick resolution of malarial symptoms. However, the widespread promiscuity about its mechanism of action is baffling. There is no consensus about the biochemical target of artemisinin but recent studies implicate haem and PfATP6 (a calcium pump). We investigated the role of iron and artemisinin on PfATP6, in search of a plausible mechanism of action, via density functional theory calculations, docking and molecular dynamics simulations. Results suggest that artemisinin gets activated by iron which in turn inhibits PfATP6 by closing the phosphorylation, nucleotide binding and actuator domains leading to loss of function of PfATP6 of the parasite and its death. The mechanism elucidated here should help in the design of novel antimalarials.

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

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

Phosphorylation and mutation of phospholamban alter physical interactions with the sarcoplasmic reticulum calcium pump

Phospholamban physically interacts with the sarcoplasmic reticulum calcium pump (SERCA) and regulates contractility of the heart in response to adrenergic stimuli. We studied this interaction using electron microscopy of 2D crystals of SERCA in complex with phospholamban. In earlier studies, phospholamban oligomers were found interspersed between SERCA dimer ribbons and a 3D model was constructed to show interactions with SERCA. In this study, we examined the oligomeric state of phospholamban and the effects of phosphorylation and mutation of phospholamban on the interaction with SERCA in the 2D crystals. On the basis of projection maps from negatively stained and frozen-hydrated crystals, phosphorylation of Ser16 selectively disordered the cytoplasmic domain of wild type phospholamban. This was not the case for a pentameric gain-of-function mutant (Lys27Ala), which retained inhibitory activity and remained ordered in the phosphorylated state. A partial loss-of-function mutation that altered the charge state of phospholamban (Arg14Ala) retained an ordered state, while a complete loss-of-function mutation (Asn34Ala) was also disordered. The functional state of phospholamban was correlated with an order-to-disorder transition of the phospholamban cytoplasmic domain in the 2D co-crystals. Furthermore, co-crystals of the gain-of-function mutant (Lys27Ala) facilitated data collection from frozen-hydrated crystals. An improved projection map was calculated to a resolution of 8 Å, which supports the pentamer as the oligomeric state of phospholamban in the crystals. The 2D co-crystals with SERCA require a functional pentameric form of phospholamban, which physically interacts with SERCA at an accessory site distinct from that used by the phospholamban monomer for the inhibitory association.

Structural aspects of ion pumping by Ca2+-ATPase of sarcoplasmic reticulum

Ca2+-ATPase of muscle sarcoplasmic reticulum is an ATP-powered Ca2+-pump that establishes a >10,000-fold concentration gradient across the membrane. Its crystal structures have been determined for nine different states that cover nearly the entire reaction cycle. Presented here is a brief structural account of the ion pumping process, which is achieved by a series of very large domain rearrangements.

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

In this review we summarize mutagenesis work on the structure-function relationship of transmembrane segment M1 in the Na+,K+-ATPase and the sarco(endo)plasmic reticulum Ca2+-ATPase. The original hypothesis that charged residues in the N-terminal part of M1 interact with the transported cations can be rejected. On the other hand hydrophobic residues in the middle part of M1 turned out to play crucial roles in Ca2+ interaction/occlusion in Ca2+-ATPase and K+ interaction/occlusion in Na+,K+-ATPase. Leu65 of the Ca2+-ATPase and Leu99 of the Na+,K+-ATPase, located at homologous positions in M1, function as gate-locking residues that restrict the mobility of the side chain of the cation binding/gating residue of transmembrane segment M4, Glu309/Glu329. A pivot formed between a pair of a glycine and a bulky residue in M1 and M3 seems critical to the opening of the extracytoplasmic gate in both the Ca2+-ATPase and the Na+,K+-ATPase.

Interdomain fluorescence resonance energy transfer in SERCA probed by cyan-fluorescent protein fused to the actuator domain

We have used a biosynthetically incorporated fluorescent probe to monitor domain movements involved in ion transport by the sarcoendoplasmic reticulum Ca-ATPase (SERCA) from rabbit fast-twitch skeletal muscle. X-ray crystal structures suggest that the nucleotide-binding (N) and actuator (A) domains of SERCA move apart by several nanometers upon Ca binding. To test this hypothesis, cDNA constructs were created to fuse cyan-fluorescent protein (CFP) to the N terminus of SERCA (A domain). This CFP-SERCA fluorescent fusion protein retained activity when expressed in Sf21 insect cells using the baculovirus system. Fluorescence resonance energy transfer (FRET) was used to monitor the A-N interdomain distance for CFP-SERCA selectively labeled with fluorescein isothiocyanate (FITC) at Lys 515 in the N domain. At low [Ca (2+)] (E2 biochemical state), the measured FRET efficiency between CFP (donor in A domain) and FITC (acceptor in N domain) was 0.34 +/- 0.03, indicating a mean distance of 61.6 +/- 2.0 A between probes on the two domains. An increase of [Ca (2+)] to 0.1 mM (E1-Ca biochemical state) decreased the FRET efficiency by 0.06 +/- 0.03, indicating an increase in the mean distance by 3.0 +/- 1.2 A. Quantitative molecular modeling of dual-labeled SERCA, including an accurate calculation of the orientation factor, shows that the FRET data observed in the absence of Ca is consistent with the E2 crystal structure, but the increase in distance (decrease in FRET) induced by Ca is much less than predicted by the E1 crystal structure. We conclude that the E1 crystal structure does not reflect the predominant structure of SERCA under physiological conditions in a functional membrane bilayer.

Structural analysis of 2D crystals of gastric H+,K+-ATPase in different states of the transport cycle

The H+,K+-ATPase uses ATP to pump protons across the gastric membrane. We used electron crystallography and limited trypsin proteolysis to study conformational changes in the H+,K+-ATPase. Well-ordered 2D crystals were obtained with detergent-solubilized H+,K+-ATPase at low pH in the absence of nucleotides, E1 state, and in the presence of fluoroaluminate and ADP, mimicking the E1PADP state. Projection maps obtained with frozen-hydrated two-dimensional crystals of the H+,K+-ATPase in these two states looked very similar, suggesting only small conformational changes during the transition from the E1 to the E1P x ADP state. This result differs from the X-ray crystal structures of the related ATPase SERCA, which revealed substantially different conformations in the E1 and E1P x ADP states. To further characterize the conformational changes in the H+,K+-ATPase during its transport cycle, we performed limited proteolysis with trypsin. All examined states of the H+,K+-ATPase, including the E1 and E1P x ADP states present in the 2D crystals,showed characteristic differences in the digestion patterns. While the results from the limited proteolysis experiments thus show that the H+,K+-ATPase adopts distinct conformations during different stages of the transport cycle, the projection maps indicate that the structural rearrangements in the H+,K+-ATPase are much smaller than those observed in the related SERCA ATPase.

ATP-binding to P-type ATPases as revealed by biochemical, spectroscopic, and crystallographic experiments

P-type ATPases form a large family of cation translocating ATPases. Recent progress in crystallography yielded several high-resolution structures of Ca(2+)-ATPase from sarco(endo)plasmic reticulum (SERCA) in various conformations. They could elucidate the conformational changes of the enzyme, which are necessary for the translocation of cations, or the mechanism that explains how the nucleotide binding is coupled to the cation transport. However, crystals of proteins are usually obtained only under conditions that significantly differ from the physiological ones and with ligands that are incompatible with the enzyme function, and both of these factors can inevitably influence the enzyme structure. Biochemical (such as mutagenesis, cleavage, and labeling) or spectroscopic experiments can yield only limited structural information, but this information could be considered relevant, because measurement can be performed under physiological conditions and with true ligands. However, interpretation of some biochemical or spectroscopic data could be difficult without precise knowledge of the structure. Thus, only a combination of both these approaches can extract the relevant information and identify artifacts. Briefly, there is good agreement between crystallographic and other experimental data concerning the overall shape of the molecule and the movement of cytoplasmic domains. On the contrary, the E1-AMPPCP crystallographic structure is, in details, in severe conflict with numerous spectroscopic experiments and probably does not represent the physiological state. Notably, the E1-ADP-AlF(4) structure is almost identical to the E1-AMPPCP, again suggesting that the structure is primarily determined by the crystal-growth conditions. The physiological relevance of the E2 and E2-P structures is also questionable, because the crystals were prepared in the presence of thapsigargin, which is known to be a very efficient inhibitor of SERCA. Thus, probably only crystals of E1-2Ca conformation could reflect some physiological state. Combination of biochemical, spectroscopic, and crystallographic data revealed amino acids that are responsible for the interaction with the nucleotide. High sequence homology of the P-type ATPases in the cytoplasmic domains enables prediction of the ATP-interacting amino acids also for other P-type ATPases.

Efficient determination of low-frequency normal modes of large protein structures by cluster-NMA

The structure-function relationship is critical to understanding the biologically relevant functions of protein structures. Various experimental techniques and numerical modeling methods, normal mode analysis (NMA) in particular, have been employed to gain insight into this relationship. Experimental methods are often unable to provide all the desired information and comprehensive modeling techniques are often too computationally expensive. The authors build upon and optimize their cluster normal mode analysis (cNMA) tool, which uses embedded rigid-bodies and harmonic potentials to capture the biologically significant, low-frequency, oscillations of protein structures. cNMA represents atomic details with a scalable number of degrees-of-freedom, which can be chosen independent of structure size. This representation overcomes the otherwise quadratic order memory requirements and cubic order computational complexity associated with traditional all-atom NMA. cNMA is two orders of magnitude faster than traditional all-atom NMA when clustering by residue (very high resolution) and in the more traditional application using a fixed number of clusters, cNMA computationally scales as O(n), which is two orders of complexity faster than all-atom NMA. cNMA is presented and very large example structures with up to 10(6) atoms are analyzed on a notebook PC in the time scale of minutes/hours. The resulting mode shapes help identify biologically significant, conformational pathways.

The average conformation at micromolar [Ca2+] of Ca2+-atpase with bound nucleotide differs from that adopted with the transition state analog ADP.AlFx or with AMPPCP under crystallization conditions at millimolar [Ca2+]

Crystalline forms of detergent-solubilized sarcoplasmic reticulum Ca2+-ATPase, obtained in the presence of either a substrate analog, AMPPCP, or a transition state complex, ADP.fluoroaluminate, were recently described to share the same general architecture despite the fact that, when studied in a test tube, these forms show different functional properties. Here, we show that the differences in the properties of the E1.AMPPCP and the E1.ADP.AlFx membraneous (or solubilized) forms are much less pronounced when these properties are examined in the presence of 10 mM Ca2+ (the concentration prevailing in the crystallization media) than when they are examined in the presence of the few micromolar of Ca2+ known to be sufficient to saturate the transport sites. This concerns various properties, including ATPase susceptibility to proteolytic cleavage by proteinase K, ATPase reactivity toward SH-directed Ellman's reagent, ATPase intrinsic fluorescence properties (here described for the E1.ADP.AlFx complex for the first time), and also the rates of 45Ca2+-40Ca2+ exchange at site "II." These results solve the above paradox at least partially and suggest that the presence of a previously unrecognized Ca2+ ion in the E1.AMPPCP crystals should be re-investigated. A contrario, they emphasize the fact that the average conformation of the E1.AMPPCP complex under usual conditions in the test tube differs from that found in the crystalline form. The extended conformation of nucleotide revealed by the E1.AMPPCP crystalline form might be only indicative of the requirements for further processing of the complex, toward the transition state leading to phosphorylation and Ca2+ occlusion.

The mechanics of calcium transport

With the recent atomic models for the sarcoplasmic reticulum Ca(2+)-ATPase in the Ca(2+)-bound state, the Ca(2+)-free, thapsigargin-inhibited state, and the Ca(2+)-free, vanadate-inhibited state, we are that much closer to understanding and animating the Ca(2+)-transport cycle. These "snapshots" of the Ca(2+)-transport cycle reveal an impressive breadth and complexity of conformational change. The cytoplasmic domains undergo rigid-body movements that couple the energy of ATP to the transport of Ca2+ across the membrane. Large-scale rearrangements in the transmembrane domain suggest that the Ca(2+)-binding sites may alternately cease to exist and reform during the transport cycle. Of the three cytoplasmic domains, the actuator (A) domain undergoes the largest movement, namely a 110 degrees rotation normal to the membrane. This domain is linked to transmembrane segments M1-M3, which undergo large rearrangements in the membrane domain. Together, these movements are a main event in Ca2+ transport, yet their significance is poorly understood. Nonetheless, inhibition or modulation of Ca(2+)-ATPase activity appears to target these conformational changes. Thapsigargin is a high-affinity inhibitor that binds to the M3 helix near Phe256, and phospholamban is a modulator of Ca(2+)-ATPase activity that has been cross-linked to M2 and M4. The purpose of this review is to postulate roles for the A domain and M1-M3 in Ca2+ transport and inhibition.

Structural basis of ion pumping by Ca2+-ATPase of the sarcoplasmic reticulum

The structures of the Ca2+-ATPase (SERCA1a) have been determined for five different states by X-ray crystallography. Detailed comparison of the structures in the Ca2+ bound form and unbound (but thapsigargin bound) form reveals that very large rearrangements of the transmembrane helices take place accompanying Ca2+ dissociation and binding and that they are mechanically linked with equally large movements of the cytoplasmic domains. The meanings of the rearrangements of the transmembrane helices and those of the cytoplasmic domains as well as the mechanistic roles of phosphorylation are now becoming clear. Furthermore, the roles of critical amino acid residues identified by extensive mutagenesis studies are becoming evident in terms of atomic structure.

The time-dependent distribution of phosphorylated intermediates in native sarcoplasmic reticulum Ca2+-ATPase from skeletal muscle is not compatible with a linear kinetic model

Quenched-flow mixing was used to characterize the kinetic behavior of the intermediate reactions of the skeletal muscle sarcoplasmic reticulum (SR) Ca-ATPase (SERCA1) at 2 and 21 degrees C. At 2 degrees C, phosphorylation of SR Ca-ATPase with 100 microM ATP labeled one-half of the catalytic sites with a biphasic time dependence [Mahaney, J. E., Froehlich, J. P., and Thomas, D. D. (1995) Biochemistry 34, 4864-4879]. Chasing the phosphoenzyme (EP) with 1.66 mM ADP 10 ms after the start of phosphorylation revealed mostly ADP-insensitive E2P (95% of EP(total)), consistent with its rapid formation from ADP-sensitive E1P. The consecutive relationship of the phosphorylated intermediates predicts a decrease in the proportion of E1P ([E1P]/[EP(total)]) with increasing phosphorylation time. Instead, after 10 ms the proportion of E1P increased and that of E2P decreased until they reached a constant 1:1 stoichiometry ([E1P]:[E2P] approximately 1). At 21 degrees C, phosphorylation displayed a transient overshoot associated with an inorganic phosphate (P(i)) burst, reflecting increased turnover of E2P at the higher temperature. The P(i) burst exceeded the decay of the EP overshoot, suggesting that rephosphorylation of the enzyme occurs before the recycling step (E2 --> E1). This behavior and the reversed order of accumulation of phosphorylated intermediates at 2 degrees C are not compatible with the conventional linear consecutive reaction mechanism: E1 + ATP --> E1.ATP --> E1P + ADP --> E2P --> E2.P(i) --> E1 + P(i). Solubilization of the Ca-ATPase into monomers using the nonionic detergent C(12)E(8) gave a pattern of phosphorylation in which E1P and E2P behave like consecutive intermediates. Kinetic modeling of the C(12)E(8)-solubilized SR Ca-ATPase showed that it behaves according to the conventional Ca-ATPase reaction mechanism, consistent with monomeric catalytic function. We conclude that the nonconforming features of native SERCA1 arise from oligomeric protein conformational interactions that constrain the subunits to a staggered or out-of-phase mode of operation.

Calcium activation of the Ca-ATPase enhances conformational heterogeneity between nucleotide binding and phosphorylation domains

High-resolution crystal structures obtained in two conformations of the Ca-ATPase suggest that a large-scale rigid-body domain reorientation of approximately 50 degrees involving the nucleotide-binding (N) domain is required to permit the transfer of the gamma-phosphoryl group of ATP to Asp(351) in the phosphorylation (P) domain during coupled calcium transport. However, variability observed in the orientations of the N domain relative to the P domain in the different crystal structures of the Ca-ATPase following calcium activation and the structures of other P-type ATPases suggests the presence of conformational heterogeneity in solution, which may be modulated by contact interactions within the crystal. Therefore, to address the extent of conformational heterogeneity between these domains in solution, we have used fluorescence resonance energy transfer to measure the spatial separation and conformational heterogeneity between donor (i.e., 5-[[2-[(iodoacetyl)amino]ethyl]amino]naphthalene-1-sulfonic acid) and acceptor (i.e., fluorescein 5-isothiocyanate) chromophores covalently bound to the P and N domains, respectively, within the Ca-ATPase stabilized in different enzymatic states associated with the transport cycle. In comparison to the unliganded enzyme, the spatial separation and conformational heterogeneity between these domains are unaffected by enzyme phosphorylation. However, calcium activation results in a 3.4 A increase in the average spatial separation, from 29.4 to 32.8 A, in good agreement with the 4.3 A increase in the distance estimated from high-resolution structures where these sites are respectively separated by 31.6 A (1IWO.pdb) and 35.9 A (1EUL.pdb). Thus, the crystal structures accurately reflect the average solution structures of the Ca-ATPase. These results suggest that the approximation of cytoplasmic domains accompanying calcium transport, as observed from crystal structures, occurs in solution within the context of large amplitude domain motions important for catalysis. We suggest that these domain motions enhance the rates of substrate (ATP) access and product (ADP) egress and the probability of a productive juxtaposition of the gamma-phosphoryl moiety of ATP with Asp(351) on the phosphorylation domain to facilitate enzyme phosphorylation and calcium transport.

Structure and function of the calcium pump

Active transport of cations is achieved by a large family of ATP-dependent ion pumps, known as P-type ATPases. Various members of this family have been targets of structural and functional investigations for over four decades. Recently, atomic structures have been determined for Ca2+-ATPase by X-ray crystallography, which not only reveal the architecture of these molecules but also offer the opportunity to understand the structural mechanisms by which the energy of ATP is coupled to calcium transport across the membrane. This energy coupling is accomplished by large-scale conformational changes. The transmembrane domain undergoes plastic deformations under the influence of calcium binding at the transport site. Cytoplasmic domains undergo dramatic rigid-body movements that deliver substrates to the catalytic site and that establish new domain interfaces. By comparing various structures and correlating functional data, we can now begin to associate the chemical changes constituting the reaction cycle with structural changes in these domains.

Chloride ATPase pumps in nature: do they exist?

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

Calcium binding to the transmembrane domain of the sarcoplasmic reticulum Ca2+-ATPase: insights from molecular modeling

Sarcoplasmic reticulum Ca(2+)- ATPase pumps Ca(2+) ions from muscle cells to the sarcoplasmic reticulum. Here we use molecular dynamics and electrostatic modeling to investigate structural and dynamical features of key intermediates in the Ca(2+) binding process of the protein. Structural models of the protein (containing either two, one, or no calcium ions in the transmembrane domain) are constructed based on the X-ray structure by Toyoshima et al. (Nature 2000;405:647-655). The protein is embedded in a water/octane bilayer, which mimics the water/membrane environment. Our calculations provide information on the hydration of the two Ca(2+) ions, not emerging from the X-ray structure. Furthermore, they indicate that uptake of the metal ions causes large structural rearrangements of the metal binding sites. In addition, they suggest that the two ions reach their binding sites via two specific pathways. Finally, they allow identification of residues in the outer mouth of the protein that might interact with the Ca(2+) ions during the binding process.

The ATP-Mg2+ binding site and cytoplasmic domain interactions of Na+,K+-ATPase investigated with Fe2+-catalyzed oxidative cleavage and molecular modeling

This work utilizes Fe(2+)-catalyzed cleavages and molecular modeling to obtain insight into conformations of cytoplasmic domains and ATP-Mg(2+) binding sites of Na(+),K(+)-ATPase. In E(1) conformations the ATP-Fe(2+) complex mediates specific cleavages at 712VNDS (P domain) and near 440VAGDA (N domain). In E(2)(K), ATP-Fe(2+) mediates cleavages near 212TGES (A domain), near 440VAGDA, and between residues 460-490 (N domain). Cleavages at high ATP-Fe(2+) concentrations do not support suggestions for two ATP sites. A new reagent, fluorescein-DTPA, has been synthesized. The fluorescein-DTPA-Fe(2+) complex mediates cleavages similar to those mediated by ATP-Fe(2+). The data suggest the existence of N to P domain interactions in E(1)Na, with bound ATP-Fe(2+) or fluorescein-DPTA-Fe(2+), A-N, and A-P interactions in E(2)(K), and provide testable constraints for model building. Molecular models based on the Ca(2+)-ATPase structure are consistent with the predictions. Specifically, high-affinity ATP-Mg(2+) binding in E(1) is explained with the N domain tilted ca. 80 degrees toward the P domain, by comparison with well-separated N and P domains in the Ca-ATPase crystal structure. With ATP-Mg(2+) docked, bound Mg(2+) is close to both D710 (in 710DGVNDS) and D443 (in 440VAGDASE). D710 is known to be crucial for Mg(2+) binding. The cleavage and modeling data imply that D443 could also be a candidate for Mg(2+) binding. Comparison of E(1).ATP,Mg(2+) and E(2) models suggests an explanation of the high or low ATP affinities, respectively. We propose a scheme of ATP-Mg(2+) and Mg(2+) binding and N, P, and A domain interactions in the different conformations of the catalytic cycle.

A structural model for the catalytic cycle of Ca(2+)-ATPase

Ca(2+)-ATPase is responsible for active transport of calcium ions across the sarcoplasmic reticulum membrane. This coupling involves an ordered sequence of reversible reactions occurring alternately at the ATP site within the cytoplasmic domains, or at the calcium transport sites within the transmembrane domain. These two sites are separated by a large distance and conformational changes have long been postulated to play an important role in their coordination. To characterize the nature of these conformational changes, we have built atomic models for two reaction intermediates and postulated the mechanisms governing the large structural changes. One model is based on fitting the X-ray crystallographic structure of Ca(2+)-ATPase in the E1 state to a new 6 A structure by cryoelectron microscopy in the E2 state. This fit indicates that calcium binding induces enormous movements of all three cytoplasmic domains as well as significant changes in several transmembrane helices. We found that fluorescein isothiocyanate displaced a decavanadate molecule normally located at the intersection of the three cytoplasmic domains, but did not affect their juxtaposition; this result indicates that our model likely reflects a native E2 conformation and not an artifact of decavanadate binding. To explain the dramatic structural effect of calcium binding, we propose that M4 and M5 transmembrane helices are responsive to calcium binding and directly induce rotation of the phosphorylation domain. Furthermore, we hypothesize that both the nucleotide-binding and beta-sheet domains are highly mobile and driven by Brownian motion to elicit phosphoenzyme formation and calcium transport, respectively. If so, the reaction cycle of Ca(2+)-ATPase would have elements of a Brownian ratchet, where the chemical reactions of ATP hydrolysis are used to direct the random thermal oscillations of an innately flexible molecule.

Selective Fe2+-catalyzed oxidative cleavage of gastric H+,K+-ATPase: implications for the energy transduction mechanism of P-type cation pumps

In the presence of ascorbate/H(2)O(2), Fe(2+) ions or the ATP-Fe(2+) complex catalyze selective cleavage of the alpha subunit of gastric H(+),K(+)-ATPase. The electrophoretic mobilities of the fragments and dependence of the cleavage patterns on E(1) and E(2) conformational states are essentially identical to those described previously for renal Na(+),K(+)-ATPase. The cleavage pattern of H(+),K(+)-ATPase by Fe(2+) ions is consistent with the existence of two Fe(2+) sites: site 1 within highly conserved sequences in the P and A domains, and site 2 at the cytoplasmic entrance to trans-membrane segments M3 and M1. The change in the pattern of cleavage catalyzed by Fe(2+) or the ATP-Fe(2+) complex induced by different ligands provides evidence for large conformational movements of the N, P, and A cytoplasmic domains of the enzyme. The results are consistent with the Ca(2+)-ATPase crystal structure (Protein Data Bank identification code; Toyoshima, C., Nakasako, M., Nomura, H., and Ogawa, H. (2000) Nature 405, 647-655), an E(1)Ca(2+) conformation, and a theoretical model of Ca(2+)-ATPase in an E(2) conformation (Protein Data Bank identification code ). Thus, it can be presumed that the movements of N, P, and A cytoplasmic domains, associated with the E(1) <--> E(2) transitions, are similar in all P-type ATPases. Fe(2+)-catalyzed cleavage patterns also reveal sequences involved in phosphate, Mg(2+), and ATP binding, which have not yet been shown in crystal structures, as well as changes which occur in E(1) <--> E(2) transitions, and subconformations induced by H(+),K(+)-ATPase-specific ligands such as SCH28080.

The thermal unfolding and domain structure of Na+/K+-exchanging ATPase. A scanning calorimetry study

The thermal unfolding and domain structure of Na+/K+-ATPase from pig kidney were studied by high-sensitivity differential scanning calorimetry (HS-DSC). The excess heat capacity function of Na+/K+-ATPase displays the unfolding of three cooperative domains with midpoint transition temperatures (Td) of 320.6, 327.5, 331.5 K, respectively. The domain with Td = 327.5 K was identified as corresponding to the beta subunit, while two other domains belong to the alpha subunit. The thermal unfolding of the low-temperature domain leads to large changes in the amplitude of the short-circuit current, but has no effect on the ATP hydrolysing activity. Furthermore, dithiothreitol or 2-mercaptoethanol treatment causes destruction of this domain, accompanied by significant disruption of the ion transporting function and a 25% loss of ATPase activity. The observed total unfolding enthalpy of the protein is rather low (approximately 12 J.g-1), suggesting that thermal denaturation of Na+/K+-ATPase does not lead to complete unfolding of the entire molecule. Presumably, transmembrane segments retain most of their secondary structure upon thermal denaturation. The binding of physiological ligands results in a pronounced increase in the conformational stability of both enzyme subunits.

Structural organization and energy transduction mechanism of Na+,K+-ATPase studied with transition metal-catalyzed oxidative cleavage

This chapter describes contributions of transition metal-catalyzed oxidative cleavage of Na+,K+-ATPase to our understanding of structure-function relations. In the presence of ascorbate/H2O2, specific cleavages are catalyzed by the bound metal and because more than one peptide bond close to the metal can be cleaved, this technique reveals proximity of the different cleavage positions within the native structure. Specific cleavages are catalyzed by Fe2+ bound at the cytoplasmic surface or by complexes of ATP-Fe2+, which directs the Fe2+ to the normal ATP-Mg2+ site. Fe2+- and ATP-Fe2+-catalyzed cleavages reveal large conformation-dependent changes in interactions between cytoplasmic domains, involving conserved cytoplasmic sequences, and a change of ligation of Mg2+ ions between E1P and E2P, which may be crucial in facilitating hydrolysis of E2P. The pattern of domain interactions in E1 and E2 conformations, and role of Mg2+ ions, may be common to all P-type pumps. Specific cleavages can also be catalyzed by Cu2+ ions, bound at the extracellular surfaces, or a hydrophobic Cu2+-diphenyl phenanthroline (DPP) complex, which directs the Cu2+ to the membrane-water interface. Cu2+ or Cu2+-DPP-catalyzed cleavages are providing information on alpha/beta subunit interactions and spatial organization of transmembrane segments. Transition metal-catalyzed cleavage could be widely used to investigate other P-type pumps and membrane proteins and, especially, ATP binding proteins.

Phospholamban domain IB forms an interaction site with the loop between transmembrane helices M6 and M7 of sarco(endo)plasmic reticulum Ca2+ ATPases

Transmembrane helix M6 of the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) has been shown to form a site of interaction with phospholamban (PLN). Site-directed mutagenesis was carried out in the cytoplasmic loop (L67) between M6 and M7 in SERCA1a to detect other SERCA-PLN binding sites. Mutants N810A, D813A, and R822A had diminished ability to interact functionally with PLN, but only D813A and R822A had reduced physical interaction with PLN. PLN mutants R25A, Q26A, N27A, L28A, Q29A, and N30A had enhanced physical interaction with wild-type (wt) SERCA1a, but physical interaction of these PLN mutants with SERCA1a mutants D813A and R822A was reduced about 2.5 fold (range 1.44-2.82). Exceptions were the interactions of PLN N27A and N30A with SERCA1a D813A, which were reduced by 7.3- and 5.8-fold, respectively. A superinhibitory PLN deletion mutant, PLNDelta21-29, had strong physical interactions with SERCA1a and with SERCA1a mutant D813A. Physical interactions with SERCA1a and mutant D813A were sharply diminished, however, for the PLN deletion mutant, PLNDelta21-30, lacking PLN N30. Physical interactions between SERCA1a and a PLN-cytochrome b(5) chimera containing PLN residues 1-29 were much stronger than those between a PLN-cytochrome b(5) chimera containing PLN residues 1-21 and lacking N27. These results suggest that a SERCA1-PLN interaction site occurs between L67 of SERCA1a and domain IB of PLN, which involves SERCA1a D813 and PLN N27 and N30.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

What the structure of a calcium pump tells us about its mechanism

The report of the crystal structure of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum in its Ca(2+)-bound form [Toyoshima, Nakasako and Ogawa (2000) Nature (London) 405, 647-655] provides an opportunity to interpret much kinetic and mutagenic data on the ATPase in structural terms. There are no large channels leading from the cytoplasmic surface to the pair of high-affinity Ca(2+) binding sites within the transmembrane region. One possible access pathway involves the charged residues in transmembrane alpha-helix M1, with a Ca(2+) ion passing through the first site to reach the second site. The Ca(2+)-ATPase also contains a pair of binding sites for Ca(2+) that are exposed to the lumen. In the four-site model for transport, phosphorylation of the ATPase leads to transfer of the two bound Ca(2+) ions from the cytoplasmic to the lumenal pair of sites. In the alternating four-site model for transport, phosphorylation leads to release of the bound Ca(2+) ions directly from the cytoplasmic pair of sites, linked to closure of the pair of lumenal binding sites. The lumenal pair of sites could involve a cluster of conserved acidic residues in the loop between M1 and M2. Since there is no obvious pathway from the high-affinity sites to the lumenal surface of the membrane, transport of Ca(2+) ions must involve a significant change in the packing of the transmembrane alpha-helices. The link between the phosphorylation domain and the pair of high-affinity Ca(2+) binding sites is probably provided by two small helices, P1 and P2, in the phosphorylation domain, which contact the loop between transmembrane alpha-helices M6 and M7.

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

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

Cytoplasmic interactions between phospholamban residues 1-20 and the calcium-activated ATPase of the sarcoplasmic reticulum

Phospholamban regulates the activity of the calcium-activated ATPase (CaATPase) of cardiac sarcoplasmic reticulum. Equilibrium fluorescence studies have shown that the N-terminal cytoplasmic region of phospholamban (residues 1-20, domain 1) causes a decrease in the intrinsic tryptophan fluorescence of the CaATPase. The interaction of phospholamban residues 1-20 with the CaATPase also results in spectral changes for the extrinsic chromophore FITC covalently attached to the cytoplasmic region of the calcium pump. The fluorescence changes for both reporter groups correlate with a dissociation constant of approximately 40 microM for the complex between phospholamban residues 1-20 and the CaATPase. Complex formation is notably weaker when phospholamban 1-20 is titrated into the CaATPase in the presence of calcium, with altered conformational effects resulting from binding. The interaction of domain 1 of phospholamban with the CaATPase is also reduced upon phosphorylation of phospholamban 1-20 at Ser-16. This region of phospholamban 1-20 is shown by isotope-edited NMR study to be involved in interaction with the CaATPase. Binding of the phosphorylated peptide is not abolished, however, indicating that phospholamban 1-20 remains associated with the CaATPase even after phosphorylation. The data provide direct evidence for the interaction between the cytoplasmic regions of phospholamban and the pump, and are discussed in the context of the mechanism for inhibition of cardiac CaATPase activity by phospholamban.

Predicted location and limited accessibility of protein kinase A phosphorylation site on Na-K-ATPase

Regulation of Na-K-ATPase by cAMP-dependent protein kinase occurs in a variety of tissues. Phosphorylation of the enzyme's catalytic subunit at a classical phosphorylation consensus motif has been observed with purified enzyme. Demonstration of phosphorylation at the same site in normal living cells or tissues has been more difficult, however, making it uncertain that the Na-K-ATPase is a direct physiological substrate of the kinase. Recently, the structure of the homologous sarco(endo)plasmic reticulum Ca-ATPase (SERCA1a) has been determined at 2.6 A resolution (Toyoshima C, Nakasako M, Nomura H, and Ogawa H. Nature 405: 647-655, 2000.), and the Na-K- ATPase should have the same fold. Here, the Na-K-ATPase sequence has been aligned with the Ca-ATPase structure to examine the predicted disposition of the phosphorylation site. The location is close to the membrane and partially buried by adjacent loops, and the site is unlikely to be accessible to the kinase in this conformation. Conditions that may expose the site or further bury it are discussed to highlight the issues facing future research on regulation of Na-K-ATPase by cAMP-dependent pathways.

The complex ATP-Fe(2+) serves as a specific affinity cleavage reagent in ATP-Mg(2+) sites of Na,K-ATPase: altered ligation of Fe(2+) (Mg(2+)) ions accompanies the E(1)-->E(2) conformational change

In the presence of ascorbate/H(2)O(2), ATP-Fe(2+) or AMP-PNP-Fe(2+) complexes act as affinity cleavage reagents, mediating selective cleavage of the alpha subunit of Na,K-ATPase at high affinity ATP-Mg(2+) sites. The cleavages reveal contact points of Fe(2+) or Mg(2+) ions. In E(1) and E(1)Na conformations, two major cleavages are detected within the conserved (708)TGDGVNDSPALKK sequence (at V712 and nearby), and one (E(1)Na) or two (E(1)) minor cleavages near V440. In media containing sodium and ATP, Fe(2+) substitutes for Mg(2+) in activating phosphorylation and ATP hydrolysis. In the E(1)P conformation, cleavages are the same as in E(1). Fe(2+) is not bound tightly. By contrast, in the E(2)P conformation, the pattern is different. A major cleavage occurs near the conserved sequence (212)TGES, whereas those in TGDGVNDSPALKK are less prominent. Fe(2+) is bound very tightly. On E(2)P hydrolysis, the Fe(2+) dissociates. The results are consistent with E(1)<-->E(2) conformation-dependent movements of cytoplasmic domains and sites for P(i) and Mg(2+) ions, inferred from previous Fe-cleavage experiments. Furthermore, these concepts fit well with the crystal structure of Ca-ATPase [Toyoshima, C., Nakasako, M., Nomura, H. & Ogawa, H. (2000) Nature (London) 405, 647-655]. Altered ligation of Mg(2+) ions in E(2)P may be crucial in facilitating nucleophilic attack of water on the OP bond. Mg(2+) ions may play a similar role in all P-type pumps. As affinity cleavage reagents, ATP-Fe(2+) or other nucleotide-Fe(2+) complexes could be widely used to investigate nucleotide binding proteins.

Phospholamban remains associated with the Ca2+- and Mg2+-dependent ATPase following phosphorylation by cAMP-dependent protein kinase

We have used fluorescence and spin-label EPR spectroscopy to investigate how the phosphorylation of phospholamban (PLB) by cAMP-dependent protein kinase (PKA) modifies structural interactions between PLB and the Ca(2+)- and Mg(2+)-dependent ATPase (Ca-ATPase) that result in enzyme activation. Following covalent modification of N-terminal residues of PLB with dansyl chloride or the spin label 4-isothiocyanato-2,2,6,6-tetramethylpiperidine-N-oxyl ('ITC-TEMPO'), we have co-reconstituted PLB with affinity-purified Ca-ATPase isolated from skeletal sarcoplasmic reticulum (SR) with full retention of catalytic function. The Ca(2+)-dependence of the ATPase activity of this reconstituted preparation is virtually identical with that observed using native cardiac SR before and after PLB phosphorylation, indicating that co-reconstituted sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase 1 (SERCA1) and PLB provide an equivalent experimental model for SERCA2a-PLB interactions. Phosphorylation of PLB in the absence of the Ca-ATPase results in a greater amplitude of rotational mobility, suggesting that the structural linkage between the transmembrane region and the N-terminus is destabilized. However, whereas co-reconstitution with the Ca-ATPase restricts the amplitude of rotational motion of PLB, subsequent phosphorylation of PLB does not significantly alter its rotational dynamics. Thus structural interactions between PLB and the Ca-ATPase that restrict the rotational mobility of the N-terminus of PLB are retained following the phosphorylation of PLB by PKA. On the other hand, the fluorescence intensity decay of bound dansyl is sensitive to the phosphorylation state of PLB, indicating that there are changes in the tertiary structure of PLB coincident with enzyme activation. These results suggest that PLB phosphorylation alters its structural interactions with the Ca-ATPase by inducing structural rearrangements between PLB and the Ca-ATPase within a defined complex that modulates Ca(2+)-transport function.

Sarco(endo)plasmic reticulum calcium pumps: recent advances in our understanding of structure/function and biology (review)

This review examines the structure and function of the sarco(endo)plasmic reticulum calcium pump (SERCA1a) in the light of the recent publication of the 2.6 A resolution structure of this protein, and looks at the increasing awareness of the key role played by SERCAs in calcium signalling. The roles played by the calcium pump isoforms, SERCA1a/b, SERCA2a/b and SERCA3a/b/c in cellular function are discussed, and the modulation of SERCA activity by phospholamban, sarcolipin and other modulatory influences is examined. The recent discoveries of human SERCA mutations leading to disease states is reviewed, and the insights into SERCA function using transgenic approaches are outlined.

Calcium transport across the sarcoplasmic reticulum: structure and function of Ca2+-ATPase and the ryanodine receptor

Contraction of striated muscle results from a rise in cytoplasmic calcium concentration in a process termed excitation/contraction coupling. Most of this calcium moves back and forth across the sarcoplasmic-reticulum membrane in cycles of contraction and relaxation. The channel responsible for release from the sarcoplasmic reticulum is the ryanodine receptor, whereas Ca2+-ATPase effects reuptake in an ATP-dependent manner. The structures of these two molecules have been studied by cryoelectron microscopy, with helical crystals in the case of Ca2+-ATPase and as isolated tetramers in the case of ryanodine receptor. Structures of Ca2+-ATPase at 8-A resolution reveal the packing of transmembrane helices and have allowed fitting of a putative ATP-binding domain among the cytoplasmic densities. Comparison of ATPases in different conformations gives hints about the conformational changes that accompany the reaction cycle. Structures of ryanodine receptor at 30-A resolution reveal a multitude of isolated domains in the cytoplasmic portion, as well as a distinct transmembrane assembly. Binding sites for various protein ligands have been determined and conformational changes induced by ATP, calcium and ryanodine have been characterized. Both molecules appear to use large conformational changes to couple interactions in their cytoplasmic domains with calcium transport through their membrane domains, and future studies at higher resolution will focus on the mechanisms for this coupling.

Detailed characterization of the cooperative mechanism of Ca(2+) binding and catalytic activation in the Ca(2+) transport (SERCA) ATPase

Expression of heterologous SERCA1a ATPase in Cos-1 cells was optimized to yield levels that account for 10-15% of the microsomal protein, as revealed by protein staining on electrophoretic gels. This high level of expression significantly improved our characterization of mutants, including direct measurements of Ca(2+) binding by the ATPase in the absence of ATP, and measurements of various enzyme functions in the presence of ATP or P(i). Mutational analysis distinguished two groups of amino acids within the transmembrane domain: The first group includes Glu771 (M5), Thr799 (M6), Asp800 (M6), and Glu908 (M8), whose individual mutations totally inhibit binding of the two Ca(2+) required for activation of one ATPase molecule. The second group includes Glu309 (M4) and Asn796 (M6), whose individual or combined mutations inhibit binding of only one and the same Ca(2+). The effects of mutations of these amino acids were interpreted in the light of recent information on the ATPase high-resolution structure, explaining the mechanism of Ca(2+) binding and catalytic activation in terms of two cooperative sites. The Glu771, Thr799, and Asp800 side chains contribute prominently to site 1, together with less prominent contributions by Asn768 and Glu908. The Glu309, Asn796, and Asp800 side chains, as well as the Ala305 (and possibly Val304 and Ile307) carbonyl oxygen, contribute to site 2. Sequential binding begins with Ca(2+) occupancy of site 1, followed by transition to a conformation (E') sensitive to Ca(2+) inhibition of enzyme phosphorylation by P(i), but still unable to utilize ATP. The E' conformation accepts the second Ca(2+) on site 2, producing then a conformation (E' ') which is able to utilize ATP. Mutations of residues (Asp813 and Asp818) in the M6/M7 loop reduce Ca(2+) affinity and catalytic turnover, suggesting a strong influence of this loop on the correct positioning of the M6 helix. Mutation of Asp351 (at the catalytic site within the cytosolic domain) produces total inhibition of ATP utilization and enzyme phosphorylation by P(i), without a significant effect on Ca(2+) binding.

Green fluorescent protein-tagged sarco(endo)plasmic reticulum Ca2+-ATPase overexpression in Paramecium cells: isoforms, subcellular localization, biogenesis of cortical calcium stores and functional aspects

We have followed the time-dependent transfection of Paramecium cells with a vector containing the gene of green fluorescent protein (GFP) attached to the C-terminus of the PtSERCA1 gene. The outlines of alveolar sacs (ASs) are labelled, as is the endoplasmic reticulum (ER) throughout the cell. When GFP fluorescence is compared with previous anti-PtSERCA1 antibody labelling, the much wider distribution of GFP (ER+ASs) indicates that only a small amount of SERCA molecules is normally retained in the ER. A second isoform, PtSERCA2, also occurs and its C-terminal GFP-tagging results in the same distribution pattern. However, when GFP is inserted in the major cytoplasmic loop, PtSERCA1 and two fusion proteins are mostly retained in the ER, probably because of the presence of the overt C-terminal KKXX ER-retention signal and/or masking of a signal for transfer into ASs. On the overall cell surface, new SERCA molecules seem to be permanently delivered from the ER to ASs by vesicle transport, whereas in the fission zone of dividing cells ASs may form anew. In cells overexpressing PtSERCA1 (with C-terminal GFP) in ASs, [Ca2+]i regulation during exocytosis is not significantly different from controls, probably because their Ca2+ pump has to mediate only slow reuptake.

Crystal structure of the calcium pump of sarcoplasmic reticulum at 2.6 A resolution

Calcium ATPase is a member of the P-type ATPases that transport ions across the membrane against a concentration gradient. Here we have solved the crystal structure of the calcium ATPase of skeletal muscle sarcoplasmic reticulum (SERCA1a) at 2.6 A resolution with two calcium ions bound in the transmembrane domain, which comprises ten alpha-helices. The two calcium ions are located side by side and are surrounded by four transmembrane helices, two of which are unwound for efficient coordination geometry. The cytoplasmic region consists of three well separated domains, with the phosphorylation site in the central catalytic domain and the adenosine-binding site on another domain. The phosphorylation domain has the same fold as haloacid dehalogenase. Comparison with a low-resolution electron density map of the enzyme in the absence of calcium and with biochemical data suggests that large domain movements take place during active transport.

Modeling a dehalogenase fold into the 8-A density map for Ca(2+)-ATPase defines a new domain structure

Members of the large family of P-type pumps use active transport to maintain gradients of a wide variety of cations across cellular membranes. Recent structures of two P-type pumps at 8-A resolution have revealed the arrangement of transmembrane helices but were insufficient to reveal the architecture of the cytoplasmic domains. However, recent proposals of a structural homology with a superfamily of hydrolases offer a new basis for modeling these domains. In the current work, we have extended the sequence comparison for the superfamily and delineated domains in the 8-A density map of Ca(2+)-ATPase. The homology suggests a new domain structure for Ca(2+)-ATPase and, specifically, that the phosphorylation domain adopts a Rossman fold. Accordingly, the atomic structure of L-2 haloacid dehalogenase has been fitted into the relevant domain of Ca(2+)-ATPase. The resulting model suggests the existence of two ATP sites at the interface between two domains. Based on this new model, we are able to reconcile numerous results of mutagenesis and chemical cross-linking within the catalytic domains. Furthermore, we have used the model to predict the configuration of Mg.ATP at its binding site. Based on this prediction, we propose a mechanism, involving a change in Mg(2+) liganding, for initiating the domain movements that couple sites of ion transport to ATP hydrolysis.

Thermal destabilization of transmembrane proteins by local anaesthetics

Local anaesthetics, in addition to anaesthesia, induce the synthesis of heat shock proteins (HSPs), sensitize cells to hyperthermia, and increase the aggregation of nuclear proteins during heat shock. Anaesthetics are membrane active agents, and anaesthesia appears to be due to altered ion channel activity; however, the direct effect of heat shock is protein denaturation. These observations suggest that local anaesthetics may sensitize cells to hyperthermia by interacting with and destabilizing membrane proteins such that protein denaturation is increased. It is shown, using differential scanning calorimetry (DSC), that the local anaesthetics procaine, lidocaine, tetracaine and dibucaine destabilize the transmembrane domains of the Ca2+ -ATPase of sarcoplasmic reticulum and the band III anion transporter of red blood cells. The transmembrane domain of the Ca2+ -ATPase has a transition temperature (Tm) of denaturation of 61 degrees C which is decreased, for example, to 53 degrees C by 15 mM lidocaine. The degree of destabilization (deltaTm) by each anaesthetic is proportional to the lipid to water partition coefficient, and the increased sensitization by anaesthetics with larger partition coefficients and at higher pH suggests that the uncharged forms of the anaesthetics are responsible for destabilization. A Hill analysis of deltaTm for the Ca2+ -ATPase as a function of the concentration of anaesthetic in water gives dissociation constants (Kd) on the order of 10(-4) M, if binding occurs directly from the aqueous phase. This demonstrates moderate affinity binding. However, dissociation constants of 1-3 M are obtained, if binding occurs through the lipid phase, which demonstrates low affinity binding. Thus, the interaction of local anaesthetics with the Ca2+ -ATPase may be moderately specific or non-specific depending on the mechanism of interaction. The observation that local anaesthetics also destabilize the transmembrane domain of the band III protein of erythrocytes suggests that destabilization of transmembrane proteins is a general property of anaesthetics, which is at least in part a mechanism of sensitization to hyperthermia.

Characterization of calcium, nucleotide, phosphate, and vanadate bound states by derivatization of sarcoplasmic reticulum ATPase with ThioGlo1

Sarcoplasmic reticulum vesicles were incubated with the maleimide-directed probe ThioGlo1, resulting in ATPase inactivation. Reacted ThioGlo1, revealed by its enhanced fluorescence, was found to be associated with the cytosolic but not with the membrane-bound region of the ATPase. The dependence of inactivation on ThioGlo1 concentration suggests derivatization of approximately four residues per ATPase, of which Cys(364), Cys(498), and Cys(636) were identified in prominently fluorescent peptide fragments. These cysteines reside within the phosphorylation and nucleotide-binding region of the ATPase. Accordingly, protection is observed in the presence of ATP, 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate (TNP-AMP), or an fluoroisothiocyanate label of Lys(515). Furthermore, protection is observed in the presence of vanadate (or decavanadate), but not in the presence of phosphate. Labeling occurs equally well in the presence or in the absence of Ca(2+) and thapsigargin, excluding a role of the E1-to-E2 transition in the protective effect of vanadate. It is concluded that protection by vanadate is due to formation of a pentacoordinated orthovanadate complex at the phosphorylation site, corresponding to a stable transition state analog of the phosphorylation reaction, with intermediate characteristics of the EP1 and EP2 states. The lack of protection by phosphate is attributed to instability of its complex with the enzyme (EP2). These findings are discussed with respect to different structural images obtained from diffraction studies of ATPase in the presence or in the absence of Ca(2+) and/or decavanadate (Ogawa et al., 1998, Biophys. J. 75:41-52).

Comparison of H+-ATPase and Ca2+-ATPase suggests that a large conformational change initiates P-type ion pump reaction cycles

BACKGROUND

Structures have recently been solved at 8 A resolution for both Ca2+-ATPase from rabbit sarcoplasmic reticulum and H+-ATPase from Neurospora crassa. These cation pumps are two distantly related members of the family of P-type ATPases, which are thought to use similar mechanisms to generate ATP-dependent ion gradients across a variety of cellular membranes. We have undertaken a detailed comparison of the two structures in order to describe their similarities and differences as they bear on their mechanism of active transport.

RESULTS

Our first important finding was that the arrangement of 10 transmembrane helices was remarkably similar in the two molecules. This structural homology strongly supports the notion that these pumps use the same basic mechanism to transport their respective ions. Despite this similarity in the membrane-spanning region, the cytoplasmic regions of the two molecules were very different, both in their disposition relative to the membrane and in the juxtaposition of their various subdomains.

CONCLUSIONS

On the basis of the crystallization conditions, we propose that these two crystal structures represent different intermediates in the transport cycle, distinguished by whether cations are bound to their transport sites. Furthermore, we propose that the corresponding conformational change (E2 to E1 ) has two components: the first is an inclination of the main cytoplasmic mass by 20 degrees relative to the membrane-spanning domain; the second is a rearrangement of the domains comprising the cytoplasmic part of the molecules. Accordingly, we present a rough model for this important conformational change, which relays the effects of cation binding within the membrane-spanning domain to the nucleotide-binding site, thus initiating the transport cycle.

Microdomain arrangement of the SERCA-type Ca2+ pump (Ca2+-ATPase) in subplasmalemmal calcium stores of paramecium cells

We localized SERCA pumps to the inner region of alveolar sac membranes, facing the cell interior, by combining ultrastructural and biochemical methods. Immunogold labeling largely predominated in the inner alveolar sac region which displayed aggregates of intramembrane particles (IMPs). On image analysis, these represented oligomeric arrangements of approximately 8-nm large IMP subunits, suggesting formation of SERCA aggregates (as known from sarcoplasmic reticulum). We found not only monomers of typical molecular size ( approximately 106 kD) but also oligomeric forms on Western blots (using anti-SERCA antibodies, also against endogenous SERCA from alveolar sacs) and on electrophoresis gelautoradiographs of 32P-labeled phosphoenzyme intermediates. Selective enrichment of SERCA-pump molecules in the inner alveolar sac membrane region may eliminate Ca2+ after centripetal spread observed during exocytosis activation, while the plasmalemmal Ca2+ pump may maintain or reestablish [Ca2+] in the narrow subplasmalemmal space between the outer alveolar sac membrane region and the cell membrane. We show for the first time the microzonal arrangement of SERCA molecules in a Ca2+ store of a secretory system, an intensely discussed issue in stimulus-secretion coupling research.

Rearrangement of domain elements of the Ca-ATPase in cardiac sarcoplasmic reticulum membranes upon phospholamban phosphorylation

Phospholamban (PLB) is a major target of the beta-adrenergic cascade in the heart, and functions to modulate rate-limiting conformational transitions involving the transport activity of the Ca-ATPase. To investigate structural changes within the Ca-ATPase that result from the phosphorylation of PLB by cAMP-dependent protein kinase (PKA), we have covalently bound the long-lived phosphorescent probe erythrosin isothiocyanate (Er-ITC) to cytoplasmic sequences within the Ca-ATPase. Under these labeling conditions, the Ca-ATPase remains catalytically active, indicating that observed changes in rotational dynamics reflect normal conformational transitions. Two major Er-ITC labeling sites were identified using electrospray ionization mass spectrometry (ESI-MS), corresponding to Lys464 and Lys650, which are respectively located within the phosphorylation and nucleotide binding domains of the Ca-ATPase. Frequency-domain phosphorescence measurements of the rotational dynamics of Er-ITC bound to these cytoplasmic sequences within the Ca-ATPase permit the resolution of the dynamic structure of individual domain elements relative to the overall rotational motion of the entire Ca-ATPase polypeptide chain. We observe a significant decrease in the rotational dynamics of Er-ITC bound to the Ca-ATPase upon phosphorylation of PLB by PKA, as evidenced by an increase in the residual anisotropy. These results suggest that phosphorylation of PLB results in a structural reorientation of the phosphorylation or nucleotide binding domains with respect to the membrane normal. In contrast, calcium activation of the Ca-ATPase in the presence of dephosphorylated PLB results in no detectable change in the rotational dynamics of Er-ITC, suggesting that calcium binding and PLB phosphorylation have distinct effects on the conformation of the Ca-ATPase. We suggest that PLB functions to alter the efficiency of phosphoenyzme formation following calcium activation of the Ca-ATPase by modulating the spatial arrangement between ATP bound in the nucleotide binding domain and Asp351 in the phosphorylation domain.

NMR conformational study of the sixth transmembrane segment of sarcoplasmic reticulum Ca2+-ATPase

In current topological models, the sarcoplasmic reticulum Ca2+-ATPase contains 10 putative transmembrane spans (M1-M10), with spans M4/M5/M6 and probably M8 participating in the formation of the membranous calcium-binding sites. We describe here the conformational properties of a synthetic peptide fragment (E785-N810) encompassing the sixth transmembrane span (M6) of Ca2+-ATPase. Peptide M6 includes three residues (N796, T799, and D800) out of the six membranous residues critically involved in the ATPase calcium-binding sites. 2D-NMR experiments were performed on the M6 peptide selectively labeled with 15N and solubilized in dodecylphosphocholine micelles to mimic a membrane-like environment. Under these conditions, M6 adopts a helical structure in its N-terminal part, between residues I788 and T799, while its C-terminal part (G801-N810) remains disordered. Addition of 20% trifluoroethanol stabilizes the alpha-helical N-terminal segment of the peptide, and reveals the propensity of the C-terminal segment (G801-L807) to form also a helix. This second helix is located at the interface or in the aqueous environment outside the micelles, while the N-terminal helix is buried in the hydrophobic core of the micelles. Furthermore, the two helical segments of M6 are linked by a flexible hinge region containing residues T799 and D800. These conformational features may be related to the transient formation of a Schellman motif (L797VTDGL802) encoded in the M6 sequence, which probably acts as a C-cap of the N-terminal helix and induces a bend with respect to the helix axis. We propose a model illustrating two conformations of M6 and its insertion in the membrane. The presence of a flexible region within M6 would greatly facilitate concomitant participation of all three residues (N796, T799, and D800) believed to be involved in calcium complexation.