Structural perturbation of the transmembrane region interferes with calcium binding by the Ca2+ transport ATPase

Novel quercetin derivatives in treatment of peroxynitrite-oxidized SERCA1

Sarco/endoplasmic reticulum calcium ATP-ase (SERCA) is regulated by low concentrations of peroxynitrite and inhibited by high levels, as indicated in human diseases. We studied quercetin (Q) and its novel derivatives monochloropivaloylquercetin (MPQ) and chloronaphthoquinonequercetin (CHQ) as agents with expected preventive properties against peroxynitrite-induced SERCA impairment. Q and MPQ protected the SERCA1 against peroxynitrite induced activity decrease, while CHQ potentiated the inhibitory effect of peroxynitrite. Quercetin derivatives were found to be weaker antioxidants compared with Q, as indicated by their ability to scavenge peroxynitrite and prevent of SERCA1 carbonylation, both decreasing in the order (Q > MPQ > CHQ). Quantum-chemical values of theoretical parameter E HOMO also indicated lower antioxidant capacities for MPQ and CHQ. Prooxidant properties estimated by calculations of frontier molecular orbitals (E LUMO) correlated with experimentally determined SH-group decrease induced by the compounds studied. Both methods showed a decrease of prooxidant properties as follows: CHQ > MPQ > Q. In addition, experimentally measured half-wave potentials indicated stronger prooxidant properties of quercetin derivatives as compared to Q. More expressive alterations of conformation in the transmembrane region of SERCA1 induced by quercetin derivatives, as compared with Q, may at least partially correlate with their higher lipophilicities. The protective effects of Q and MPQ on different isoforms of SERCA activity may be useful in prevention and treatment of inflammation or muscle diseases. The inhibitory effect of CHQ on SERCA isoforms may be beneficial in therapeutic approaches aimed at anti-tumor treatment.

A structural overview of the plasma membrane Na+,K+-ATPase and H+-ATPase ion pumps

Plasma membrane ATPases are primary active transporters of cations that maintain steep concentration gradients. The ion gradients and membrane potentials derived from them form the basis for a range of essential cellular processes, in particular Na(+)-dependent and proton-dependent secondary transport systems that are responsible for uptake and extrusion of metabolites and other ions. The ion gradients are also both directly and indirectly used to control pH homeostasis and to regulate cell volume. The plasma membrane H(+)-ATPase maintains a proton gradient in plants and fungi and the Na(+),K(+)-ATPase maintains a Na(+) and K(+) gradient in animal cells. Structural information provides insight into the function of these two distinct but related P-type pumps.

pH and ligand binding modulate the strength of protein-protein interactions in the Ca(2+)-ATPase from sarcoplasmic reticulum membranes

The Ca(2+)-ATPase from sarcoplasmic reticulum (SR) membranes couples the Ca(2+) transport to ATP hydrolysis through phosphorylation in its cytoplasmic catalytic domain. Interactions between protein domains and the role of monomer-monomer interactions remain unclear. Here, we report a differential scanning calorimetric study of the thermal unfolding of this protein. In the pH range 6-8, thermal unfolding of the Ca(2+)-ATPase in glycogen phosphorylase-free SR membranes shows a major endothermic peak with a critical temperature midpoint ranging between 51 and 55 degrees C, depending on pH, Ca(2+), Mg(2+)-ADP and KCl concentrations. The enthalpy change of the overall unfolding process ranged between 250 and 300 kcal/mol of Ca(2+)-ATPase monomer. Thermal denaturation of the Ca(2+)-ATPase in SR membranes is well fitted to an irreversible process that can be rationalized in terms of a non-two state process, N (native)right harpoon over left harpoon I (intermediate)-->D (denatured). Thermodynamic analysis show that this protein has a compact structure, implying a tight structural interconnection between catalytic and Ca(2+) transport domains. The apparent cooperative unit, defined by the van 't Hoff enthalpy to the overall unfolding enthalpy ratio, increased from 1.1 at pH 6 to 1.8 at pH 8, showing that monomer-monomer interactions are stronger at weakly basic pH than at weakly acidic pH. While micromolar Ca(2+) concentrations had only a weak effect on the cooperativity of the unfolding process, this is clearly increased by millimolar Mg(2+)-ADP. In addition, high ionic strength lowered the apparent cooperative unit to approximately 1.0 in the pH range 6-8. Taken together, these results suggest that protein-protein interactions are altered by variables that modulate the catalytic activity of this enzyme.

Clotrimazole, an antimycotic drug, inhibits the sarcoplasmic reticulum calcium pump and contractile function in heart muscle

Clotrimazole (CLT), an antimycotic drug, has been shown to inhibit proliferation of normal and cancer cell lines and its systemic use as a new tool in the treatment of proliferative disorders is presently under scrutiny (Benzaquen, L. R., Brugnara, C., Byers, H. R., Gattoni-Celli, S., and Halperin, J. A. (1995) Nature Med. 1, 534-540). The action of CLT is thought to involve depletion of intracellular Ca2+ stores but the underlying mechanism has not been defined. The present study utilized membrane vesicles of rabbit cardiac sarcoplasmic reticulum (SR) to determine the mechanism by which CLT depletes intracellular Ca2+ stores. The results revealed a strong, concentration-dependent inhibitory action of CLT on the ATP-energized Ca2+ uptake activity of SR (50% inhibition with approximately 35 microM CLT). The inhibition was of rapid onset (manifested in <15 s), and was accompanied by a 7-fold decrease in the apparent affinity of the SR Ca2+-ATPase for Ca2+ and a minor decrement in the enzyme's apparent affinity toward ATP. Exposure of SR to CLT in the absence or presence of Ca2+ resulted in irreversible inhibition of Ca2+ uptake demonstrating that the Ca2+-bound and Ca2+-free conformations of the Ca2+-ATPase are CLT-sensitive. Introduction of CLT to the reaction medium subsequent to induction of enzyme turnover with Ca2+ and ATP resulted in instantaneous cessation of Ca2+ transport indicating that an intermediate enzyme species generated during turnover undergoes rapid inactivation by CLT. The inhibition of Ca2+ uptake by CLT was accompanied by inhibition of Ca2+-stimulated ATP hydrolysis and Ca2+-induced phosphoenzyme intermediate formation from ATP in the ATPase catalytic cycle. Phosphorylation of the Ca2+-deprived enzyme with Pi in the reverse direction of catalytic cycle and Ca2+ release from Ca2+-preloaded SR vesicles were unaffected by CLT. It is concluded that CLT depletes intracellular Ca2+ stores by inhibiting Ca2+ sequestration by the Ca2+-ATPase. The mechanism of ATPase inhibition involves a drug-induced alteration in the Ca2+-binding site(s) resulting in paralysis of the enzyme's catalytic and ion transport cycle. CLT (50 microM) caused marked depression of contractile function in isolated perfused, electrically paced rabbit heart preparations. The contractile function recovered gradually following withdrawal of CLT from the perfusate indicating the existence of mechanisms in the intact cell to inactivate, metabolize, or clear CLT from its target site.

Spectroscopic analysis of halothane binding to the plasma membrane Ca2+-ATPase

The intrinsic tryptophan (Trp) fluorescence of the plasma membrane Ca2+-ATPase (PMCA) is significantly quenched by halothane, a volatile anesthetic common in clinical practice. It has been proposed that halothane inhibition of the Ca2+-ATPase activity results from conformational changes following anesthetic binding in the enzyme. We have investigated whether the observed quenching reflects halothane binding to PMCA. We have shown that the quenching is dose dependent and saturable and can be fitted to a binding curve with an equilibrium constant K(Hal) = 2.1 mM, a concentration at which the anesthetic approximately half-maximally inhibits the Ca2+-ATPase activity. The relatively low sensitivity of halothane quenching of Trp fluorescence to the concentration of phosphatidylcholine and detergent in the PMCA preparation concurs with the quenching resulting from anesthetic binding in the PMCA molecule. Analysis of the Trp fluorescence quenching by acrylamide indicates that the Trp residues are not considerably exposed to the solvent (Stern-Volmer quenching constant of 2.9 M(-1)) and do not differ significantly in their accessibility to halothane. Other volatile anesthetics, diethyl ether and diisopropyl ether, reduce the quenching caused by halothane in a dose-dependent manner, suggesting halothane displacement from its binding site(s). These observations indicate that halothane quenching of intrinsic Trp fluorescence of PMCA results from anesthetic binding to the protein. The analysis, used as a complementary approach, provides new information to the still rudimentary understanding of the process of anesthetic interaction with membrane proteins.

ATPase gene transfer and mutational analysis of the cation translocation mechanism

The peptide segment interposed between cation binding and phosphorylation domains retains a high degree of homology in all cation transport ATPases. Mutational analysis and chimeric replacements of Ca2+ ATPase components with corresponding Na+,K(+)-ATPase components indicate that this segment is utilized by various cation ATPases as a common structural device for a long-range functional linkage of enzyme phosphorylation and cation transport. Vectorial displacement of bound cation is rendered possible by a transmembrane channel formed by four clustered helices (M4, M5, M6, and M8). Originating from the four helices, the oxygen functions of Glu309, Glu771, Thr799, Asp800, and Glu908 form a duplex Ca2+ binding site in the middle of the channel, while Lys297 seals the luminal end of the channel with its positively charged side chain. The perturbation triggered by enzyme phosphorylation is apparently transmitted through the linkage segment to produce rotational displacement of the M4 helix with minimal change of secondary structure. The cation binding site is thereby disrupted and the Lys297 side chain removed, permitting Ca2+ to dissociate in exchange for H+ and to flow through the luminal end of the channel.

Energy transduction and kinetic regulation by the peptide segment connecting phosphorylation and cation binding domains in transport ATPases

The sarcoplasmic reticulum ATPase segment (Thr316-Leu356) connecting the extramembranous phosphorylation domain to the preceding transmembrane helix M4 (which is an integral component of the Ca2+ binding domain) retains a high degree of sequence homology with other cation transport ATPases. Single, non conservative mutations of homologous residues in this segment produces enzyme inhibition (Zhang et al., 1995). We have now produced single and multiple mutations of non-homologous residues in this segment of the Ca2+ ATPase to match the corresponding residues of the Na+, K+ ATPase. We find that the main characteristics of the ATPase mechanism (i.e., Ca2+ dependent phosphoenzyme formation and thapsigargin sensitivity) are retained even when the entire 41-amino acid (Thr316-Leu356) segment of the Ca2+ ATPase is rendered identical to the corresponding segment of the Na+, K+ ATPase by sequential mutations of the 14 non-homologous amino acids. However, the phosphoenzyme turnover (likely rate limited by the "Ca2.E1-P-->Ca.E2-P transition") is progressively reduced if four or more Ca2+ ATPase residues are mutated to the corresponding residues of the Na+, K+ ATPase. The time course of enzyme inactivation by EGTA (likely rate limited by the "E1 to E2 transition") is also prolonged. Our findings suggest that an analogous peptide segment provides a functional linkage for energy transduction between phosphorylation and cation binding domains in various cation transport ATPase. However, its kinetic influence on rate-limiting conformational transitions is dependent on matching specific structures in each ATPase.

Short and long range functions of amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase. A mutational study

Mutational analysis of several amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase was performed by expressing wild type ATPase and 32 site-directed mutants in COS-1 cells followed by functional characterization of the microsomal fraction. Four different phenotype characteristics were observed in the mutants: (a) functions similar to those sustained by the wild type ATPase; (b) Ca2+ transport inhibited to a greater extent than ATPase hydrolytic activity; (c) inhibition of transport and hydrolytic activity in the presence of high levels of phosphorylated enzyme intermediate; and (d) total inhibition of ATP utilization by the enzyme while retaining the ability to form phosphoenzyme by utilization of P(i). Analysis of experimental observations and molecular models revealed short and long range functions of several amino acids within the transmembrane region. Short range functions include: (a) direct involvement of five amino acids in Ca2+ binding within a channel formed by clustered transmembrane helices M4, M5, M6, and M8; (b) roles of several amino acids in structural stabilization of the helical cluster for optimal channel function; and (c) a specific role of Lys297 in sealing the distal end of the channel, suggesting that the M4 helix rotates to allow vectorial flux of Ca2+ upon enzyme phosphorylation. Long range functions are related to the influence of several transmembrane amino acids on phosphorylation reactions with ATP or P(i), transmitted to the extramembranous region of the ATPase in the presence or in the absence of Ca2+.

Intrinsic fluorescence as a probe of structure-function relationships in Ca(2+)-transport ATPases

Applications of intrinsic fluorescence measurements in the study of Ca(2+)-transport ATPases are reviewed. Since the initial reports showing that the fluorescence emission was sensitive to Ca2+ binding, a substantial amount of work has focused on the use of both steady-state and time-resolved fluorescence spectroscopy to investigate structure-function relationships in sarcoplasmic reticulum and plasma membrane Ca(2+)-ATPases. These studies have revealed ligand-induced conformational changes, as well as provided information on protein-protein, protein-solvent and/or protein-lipid interactions in different functional states of these proteins. The main results of these studies, as well as possible future prospects are discussed.

How do Ca2+ ions pass through the sarcoplasmic reticulum membrane

We propose an overview of the mechanism of Ca2+ transport through the sarcoplasmic reticulum membrane via the Ca(2+)-ATPase. We describe cytoplasmic calcium binding, calcium occlusion in the membrane and lumenal calcium dissociation. A channel-like structure is discussed and related to structural data on the membranous domain of the Ca(2+)-ATPase.

Ca2+ binding and translocation by the sarcoplasmic reticulum ATPase: functional and structural considerations

Three experimental systems are described including sarcoplasmic reticulum (SR) vesicles, reconstituted proteoliposomes, and recombinant protein obtained by gene transfer and expression in foreign cells. It is shown that the Ca(2+) ATPase of sarcoplasmic reticulum (SR) includes an extramembranous globular head which is connected through a stalk to a membrane bound region. Cooperative binding of two calcium ions occurs sequentially, within a channel formed by four clustered helices within the membrane bound region. Destabilization of the helical cluster is produced following enzyme phosphorylation by ATP at the catalytic site in the extramembranous region. The affinity and orientation of the Ca2+ binding site are thereby changed, permitting vectorial dissociation of bound Ca2+ against a concentration gradient. A long range linkage between phosphorylation and Ca2+ binding sites is provided by an intervening peptide segment that retains high homology in cation transport ATPases, and whose function is highly sensitive to mutational perturbations.

Do transmembrane segments in proteolyzed sarcoplasmic reticulum Ca(2+)-ATPase retain their functional Ca2+ binding properties after removal of cytoplasmic fragments by proteinase K?

The present study was undertaken to investigate the Ca2+ binding properties of sarcoplasmic reticulum Ca(2+)-ATPase after removal of the cytoplasmic regions by treatment with proteinase K. One of the proteolysis cleavage sites (at the end of M6) was found unexpectedly close to the predicted membrane-water interphase, but otherwise the cleavage pattern was consistent with the presence of 10 transmembrane ATPase segments. C-terminal membranous peptides containing the putative transmembrane segments M7 to M10 accumulated after prolonged proteolysis, as well as large water-soluble fragments containing most of the phosphorylation and ATP-binding domain. Ca2+ binding was intact after cleavage of the polypeptide chain in the N-terminal region, but cuts at other locations disrupted the high affinity binding and sequential dissociation properties characteristic of native sarcoplasmic reticulum, leaving the translocation sites with only weak affinity for Ca2+. High affinity Ca2+ binding could only be maintained when proteolysis and subsequent manipulations took place in the presence of a Ca2+ concentration high enough to ensure permanent occupation of the binding sites with Ca2+. We conclude that in the absence of Ca2+, the complex of membrane-spanning segments in proteolyzed Ca(2+)-ATPase is labile, probably because of relatively free movement or rearrangement of individual segments. Our study, which is discussed in relation to results obtained on Na+,K(+)-ATPase and H+,K(+)-ATPase, emphasizes the importance of the cytosolic segments of the main polypeptide chain in exerting constraints on the intramembranous domain of a P-type ATPase.

Synthesis and interaction of fluorescent thapsigargin derivatives with the sarcoplasmic reticulum ATPase membrane-bound region

Fluorescent derivatives of thapsigargin (TG) were synthesized by replacing the C8-butanoyl chain with a dansyl (DTG) or eosin (ETG) moiety. DTG and ETG retain the inhibitory effect of TG on the sarcoplasmic reticulum (SR) ATPase, displaying a 2 and 10 microM Ki, respectively. Steady state and lifetime fluorescence measurements are consistent with energy transfer between tryptophanyl residues assigned to the ATPase membrane-bound region and DTG. This phenomenon exhibits saturation behavior, occurs in the presence of DTG concentrations producing ATPase inhibition, and is partially prevented by inhibitory concentrations of TG. Although long range conformational effects of TG binding affect the fluorescence properties of endogenous tryptophans as well as of a fluorescein 5'-isothiocyanate (FITC) label of the ATPase extramembranous region, no significant energy transfer was detected between DTG and the FITC label. It is concluded that the inhibitors partition within the membrane and the binding domain resides within or near the membrane-bound region of the ATPase.

Intracellular signaling through long-range linked functions in the Ca2+ transport ATPase

The Ca2+ transport ATPases of intracellular membranes exhibit an intracellular long-range functional linkage which is the basic mechanistic device for Ca2+ transport through ATP utilization. The functional linkage operates between a phosphorylation (catalytic) domain located in the extramembranous region, and a Ca2+ binding domain located in the membrane bound region of the enzyme. The two domains are separated by a distance of approximately 50 A, and are both affected by binding of a single molecule of the highly specific inhibitor, thapsigargin, to the enzyme. Functional and structural features are here described to explain the long-range linkage through the protein structure.

High sensitivity to site directed mutagenesis of the peptide segment connecting phosphorylation and Ca2+ binding domains in the Ca2+ transport ATPase

Nine residues (Leu321, Lys329, Asn330, Val333, Arg334, Leu336, Pro337, Val339 and Glu340), within the peptide segment intervening between the catalytic domain and the Ca2+ binding domain of the sarcoplasmic reticulum (SERCA 1) ATPase, were individually mutated to Ala. The mutated proteins were recovered in the microsomal fraction of COS-1 cells following transient expression, and exhibited inhibition of Ca2+ uptake and ATPase hydrolytic activity, while forming discernable levels of phosphorylated intermediate. Mutation of Glu340 to Gln (rather than to Ala) was much less effective, suggesting that the functional consequence of the mutation is related to structural perturbation, rather than loss of the acidic side chain. The high sensitivity of this peptide segment to single mutations suggests that its structural integrity is required for functional linkage of the phosphorylation and Ca2+ binding domains.

Inactivation of calcium uptake by EGTA is due to an irreversible thermotropic conformational change in the calcium binding domain of the Ca(2+)-ATPase

Calcium uptake by rabbit skeletal sarcoplasmic reticulum (SR) is inhibited with an effective inactivation temperature (TI) of 37 degrees C in EGTA with no effect on ATPase activity. Since the Ca-ATPase denatures at a much higher temperature (49 degrees C) in EGTA, this suggests that a small or localized conformational change of the Ca-ATPase at 37 degrees C results in inability to accumulate calcium by the SR. Using a fluorescent analogue of dicyclohexylcarbodiimide, N-cyclohexyl-N'-[4-(dimethylamino)-alpha-naphthyl]-carbodiimide (NCD-4), the region of the calcium binding sites of the SR Ca-ATPase was labeled. Steady-state and frequency-resolved fluorescence measurements were subsequently performed on the NCD-4-labeled Ca-ATPase. Site-specific information pertaining to the hydrophobicity and segmental flexibility of the region of the calcium binding sites was derived from the steady-state fluorescence intensity, lifetime, and rotational rate of the covalently bound NCD-4 label as a function of temperature (0-50 degrees C). A reversible transition at approximately 15 degrees C and an irreversible transition at approximately 35 degrees C were deduced from the measured fluorescence parameters. The low-temperature transition agrees with the previously observed break in the Arrhenius plot of ATPase activity of the native Ca-ATPase at 15-20 degrees C. The high-temperature transition conforms well with the conformational transition, resulting in uncoupling of Ca translocation from ATP hydrolysis as predicted from the irreversible inactivation of Ca uptake at 31-37 degrees C in 1 mM EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)