Is Ca2+-ATPase a water pump?

Peptide mapping and disulfide bond analysis of the cytoplasmic region of an intrinsic membrane protein by mass spectrometry

Intrinsic membrane proteins pose substantial obstacles to analysis by common analytical techniques due to their hydrophobic nature and solubilization requirements. This is the case for studies involving HPLC coupled to mass spectrometry. We have developed an HPLC/mass spectrometry approach to explore and map the peptide sequence of the SERCA1a Ca(2+)-ATPase from the sarcoplasmic reticulum an integral membrane protein of 110 kDa. After extensive proteolysis of the protein, the mass of the proteolytic fragments was analyzed by HPLC/mass spectrometry. Only part of the cytoplasmic fragments was recovered under nondenaturing conditions. On the other hand, peptide fragments obtained under denaturing conditions were found to cover nearly all the cytoplasmic region. Sarcoplasmic reticulum (SR) Ca(2+)-ATPase contains 24 cysteine residues, 18 of which are in the cytosolic or lumenal region of the protein. Peptides containing free cysteines were identified by a mass increase resulting from carboxyamidomethylation of the cysteines with iodoacetamide. Alkylation reactions were executed either before or after reduction of the peptide fragments by dithiothreitol. Analysis of the mass of the fragments indicates that no disulfide bonds exist in the cytoplasmic portion of SR Ca(2+)-ATPase.

Function of the N-acetyl-L-histidine system in the vertebrate eye. Evidence in support of a role as a molecular water pump

N-acetyl-L-histidine (NAH) is a major constituent of poikilotherm brain, eye, heart, and muscle, but for which there is no known function. NAH is characterized by high tissue concentrations, a high tissue/extracellular fluid (ECF) gradient, and by a continuous selective and regulated efflux into ECF. In the eye, there is a complete compartmentalization of the synthetic and hydrolytic enzymes, with synthesis of NAH from AcCoA and L-histidine (His) occurring in the lens, and its hydrolysis to acetate and His restricted to surrounding ocular fluids. Using 14C-isotopes, the cycling of NAH between lens and ocular fluids in a simple support medium consisting of NaCl (0.9%), Ca2+ (4 mEq/L) and D-glucose (5 mM) at pH 7.4 has previously been observed. In the present study, using the isolated lens of the goldfish eye, each of the components of that support medium has been individually varied in order to determine its effect on NAH release down its intercompartmental gradient. As a result of these and related studies, it is suggested that NAH may function as a metabolically recyclable gradient-driven molecular water pump. It is proposed that water influx or generation of metabolic water serves as the trigger mechanism to open a Ca-dependent gate for the release of NAH down its gradient, along with its associated water. Preliminary analyses suggest that in addition to its potential for multiple daily cycles, a strongly ionized hydrophilic molecule, such as NAH, may include a large power function as a result of its attraction to water, and it has been calculated that an aqua complex of each NAH molecule may have 33 dipole-dipole-associated water molecules as it passes into ECF. It is this unique combination of a capacity for multiple cycles per day, coupled with a large power function, that may allow for such an intracellular osmolyte to be present in relatively low concentration in comparison to total cellular osmolality, and yet to perform a large and important task with little expenditure of energy. With each NAH molecule recycled up to 10 times/d, and a power factor of 33, there could be 330 mmol of water transported/mmol of NAH each day. With typical NAH concentrations in brains of poikilothermic vertebrates of 5-10 mmol/kg, there is the potential for up to 3.3 mol (60 mL) of water to be removed each day/kg of brain, a value that represents about 8% of total brain water content. Dewatering of the released osmolyte would occur in two additional steps, consisting of its hydrolysis and the subsequent active uptake of its metabolites. It is also suggested that NAH is the archetype of several metabolically and structurally related cellular osmolytes found in both poikilotherms and homeotherms, for which there is similarly no known function, and these may form a family of cycling hydrophilic osmolytes that serve as molecular water pumps in a variety of tissues. These include the basic His containing derivatives: NAH, carnosine, anserine, ophidine, and homocarnosine, and the acidic aspartate derivatives: N-acetyl-L-aspartate (NAA) and N-acetyl-L-aspartylglutamate (NAAG). In each of these cases, the high intracellular/extracellular osmolyte gradient appears to be maintained by combining a hydrophilic protein amino acid with a nonprotein moiety to block its use in other intracellular metabolic pathways, and by blocking catabolism of the derivative by maintaining its hydrolytic enzyme in an extracytosolic membrane or extracellular compartment. Unlike other known water-regulating mechanisms, the proposed cellular system is unique in that as a water pump, it can function as a water regulator independently of extracellular solute composition or osmolality. Finally, based on the hypothesis developed, the NAH system would represent the first cellular water pump to be identified.

Proton ejection as a major feature of the Ca(2+)-pump

H+ ejection and Ca2+ uptake promoted by the sarcoplasmic reticulum (SR) Ca(2+)-pump are similarly stimulated by millimolar Mg2+. This cannot be assigned to enhanced Ca2+ uptake and H+ displacement from internal metal binding sites since: (1) loading SR vesicles with high Mg2+ concentrations does not impair H+ ejection; (2) loading SR vesicles with Mn2+ does not depress H+ ejection occurring during Mn2+ uptake; (3) H+ ejection occurs even when Ca2+ accumulation inside the vesicles is prevented with Ca2+ ionophores. It is concluded that the Ca(2+)-pump promotes an active Ca2+/H+ countertransport stimulated by Mg2+. Finally, a mechanism for Ca2+ translocation is proposed in basic physico-chemical terms.

Infrared spectroscopic signals arising from ligand binding and conformational changes in the catalytic cycle of sarcoplasmic reticulum calcium ATPase

Fourier transform infrared spectroscopy was used to investigate ligand binding and conformational changes in the Ca2(+)-ATPase of sarcoplasmic reticulum during the catalytic cycle. The ATPase reaction was started in the infrared sample by release of ATP from the inactive, photolabile ATP derivative P3-1-(2-nitro)phenylethyladenosine 5'-triphosphate (caged ATP). Absorption spectroscopy in the visible spectral region using the Ca2(+)-sensitive dye Antipyrylazo III ensured that the infrared samples were able to transport Ca2+ in spite of their low water content, which is required for mid-infrared measurements (1800-950 cm-1). Small, but characteristic and highly reproducible infrared absorbance changes were observed upon ATP release. These infrared absorbance changes exhibit different kinetic properties. Comparison with model compound infrared spectra indicates that they are related to photolysis of caged ATP, hydrolysis of ATP in consequence of ATPase activity and to molecular changes in the active ATPase. The absorbance changes due to alterations in the ATPase were observed mainly in the region of Amide I and Amide II protein absorbance and presumably reflect the molecular processes upon phosphoenzyme formation. Since the absorbance changes were small compared to the overall ATPase absorbance, no major rearrangement of ATPase conformation as the result of catalysis could be detected.

Structural dynamics of the Ca2(+)-ATPase of sarcoplasmic reticulum. Temperature profiles of fluorescence polarization and intramolecular energy transfer

The temperature dependence of fluorescence polarization and Förster-type resonance energy transfer (FRET) was analyzed in the Ca2(+)-ATPase of sarcoplasmic reticulum using protein tryptophan and site-specific fluorescence indicators such as 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulfonic acid (IAEDANS), fluorescein 5'-isothiocyanate (FITC), 2',3'-O-(2,4,3-trinitrophenyl)adenosine monophosphate (TNP-AMP) or lanthanides (Pr3+, Nd3+) as probes. The normalized energy transfer efficiency between AEDANS bound at cysteine-670 and -674 and FITC bound at lysine-515 increases with increasing temperature in the range of 10-37 degrees C, indicating the existence of a relatively flexible structure in the region of the ATPase molecule that links the AEDANS to the FITC site. These observations are consistent with the theory of Somogyi, Matko, Papp, Hevessy, Welch and Damjanovich (Biochemistry 23 (1984) 3403-3411) that thermally induced structural fluctuations increase the energy transfer. Structural fluctuations were also evident in the energy transfer between FITC linked to the nucleotide-binding domain and Nd3+ bound at the putative Ca2+ sites. By contrast the normalized energy transfer efficiency between AEDANS and Pr3+ was relatively insensitive to temperature, suggesting that the region between cysteine-670 and the putative Ca2+ site monitored by the AEDANS-Pr3+ pair is relatively rigid. A combination of the energy transfer data with the structural information derived from analysis of Ca2(+)-ATPase crystals yields a structural model, in which the location of the AEDANS-, FITC- and Ca2+ sites are tentatively identified.

Emerging views on the structure and dynamics of the Ca2(+)-ATPase in sarcoplasmic reticulum

The ATP-dependent Ca2+ transport in sarcoplasmic reticulum involves transitions between several structural states of the Ca2(+)-ATPase, that occur without major changes in the secondary structure. The rates of these transitions are modulated by the lipid environment and by interactions between ATPase molecules. Although the Ca2(+)-ATPase restricts the rotational mobility of a population of lipids, there is no evidence for specific interaction of the Ca2(+)-ATPase with phospholipids. Fluorescence polarization and energy transfer (FET) studies, using site specific fluorescent indicators, combined with crystallographic, immunological and chemical modification data, yielded a structural model of Ca2(+)-ATPase in which the binding sites of Ca2+ and ATP are tentatively identified. The temperature dependence of FET between fluorophores attached to different regions of the ATPase indicates the existence of 'rigid' and 'flexible' regions within the molecule characterized, by different degrees of thermally induced structural fluctuations.

Evidence of a calcium-induced structural change in the ATP-binding site of the sarcoplasmic-reticulum Ca2+-ATPase using terbium formycin triphosphate as an analogue of Mg-ATP

Terbium ions and terbium formycin triphosphate have been used to investigate the interactions between the cation and nucleotide binding sites of the sarcoplasmic reticulum Ca2+-ATPase. Three classes of Tb3+-binding sites have been found: a first class of low-affinity (Kd = 10 microM) corresponds to magnesium binding sites, located near a tryptophan residue of the protein; a second class of much higher affinity (less than 0.1 microM) corresponds to the calcium transport sites, their occupancy by terbium induces the E1 to E2 conformational change of the Ca2+-ATPase; a third class of sites is revealed by following the fluorescence transfer from formycin triphosphate (FTP) to terbium, evidencing that terbium ions can also bind into the nucleotide binding site at the same time as FTP. Substitution of H2O by D2O shows that Tb-FTP binding to the enzyme nucleotide site is associated with an important dehydration of the terbium ions associated with FTP. Two terbium ions, at least, bind to the Ca2+-ATPase in the close vicinity of FTP when this nucleotide is bound to the ATPase nucleotide site. Addition of calcium quenches the fluorescence signal of the terbium-FTP complex bound to the enzyme. Calcium concentration dependence shows that this effect is associated with the replacement of terbium by calcium in the transport sites, inducing the E2----E1 transconformation when calcium is bound. One interpretation of this fluorescence quenching is that the E1----E2 transition induces an important structural change in the nucleotide site. Another interpretation is that the high-affinity calcium sites are located very close to the Tb-FTP complex bound to the nucleotide site.

[Contraction of skeletal muscles: regulation of calcium intracellular movements]

The different membrane systems and proteins involved in the control of intracellular calcium movements in the skeletal muscle cell are described. These include the sarcoplasmic reticulum, that Ca(++)-ATPase sarcoplasmic reticular calcium pump, transverse tubules, calcium channels, and the ryanodine receptor protein. The significance of these systems is shown clearly in the myopathies, where the main errors involved do not concern the contractile system, but the command and control mechanisms.

Distances between functional sites in cardiac sarcoplasmic reticulum (Ca2+ +Mg2+)-ATPase. Inter-lanthanide energy transfer

The high-affinity Ca2+-binding sites of cardiac sarcoplasmic reticulum (Ca2+ +Mg2+)-ATPase have been probed using trivalent lanthanide ions. Non-radiative energy-transfer studies, using luminescent probe Eu3+ as a donor and Nd3+ or Pr3+ as acceptor, were carried out to estimate the distance between two high-affinity Ca2+-binding/transport sites. Eu3+ was excited directly with pulsed laser light and the energy-transfer efficiency to Nd3+ or Pr3+ was measured, under the conditions in which most donor-acceptor pairs occupied the high-affinity Ca2+ sites. The distance between two high-affinity Ca2+ sites is about 0.89 nm. In the presence of ATP the distance between the high-affinity sites is about 0.855 nm, whereas in the presence of adenosine 5'-[beta, gamma-methylene]triphosphate or adenosine 5'-[beta, gamma-imino]triphosphate the distance is about 0.895 nm. To estimate the distance between the high-affinity Ca2+ sites and ATP-binding/hydrolytic site, we have measured the energy-transfer efficiency between Eu3+ and Cr3+-ATP with Eu3+ at the high-affinity Ca2+ sites and Cr3+-ATP at the ATP-binding/hydrolytic site. Our results show that ATP-binding/hydrolytic site is separated by about 2.2 nm from each high-affinity Ca2+ site.

Gel to liquid crystalline phase transition promotes a conformational reorganization of Ca2+, Mg2+-ATPase from sarcoplasmic reticulum in dimyristoylphosphatidylcholine reconstituted systems

Sarcoplasmic reticulum Ca2+, Mg2+-ATPase has been reconstituted in membranes highly enriched in dimyristoylphosphatidylcholine. According to electron microscopy data these membranes form vesicles of an average diameter of 1000 +/- 200 A. These reconstituted membranes show hysteretic behavior in some physical-chemical properties, such as light scattering and fluorescence when labeled with iodoacetamidofluorescein and with N-iodoacetyl-N'-(5-sulfo-1-naphthyl) ethylenediamine. Hysteretic behavior in catalytic activity can also be inferred from the kinetic data presented in this paper, because the temperature dependence of the Ca2+, Mg2+-ATPase activity is altered by a mild thermal pretreatment of the samples. Furthermore, it was noticed that the Ca2+-dependent ATPase activity of these complexes, when assayed above the phase transition temperature (Tc) of the lipid matrix, showed a lag phase in the minute time scale range. On the basis of these findings, it is suggested that the gel-to-liquid crystalline phase transition of the lipid is able to shift the conformational equilibrium E----E* of Ca2+, Mg2+-ATPase. The fact that the -SH reactivity against 5,5'-dithio-bis-nitrobenzoic acid of these complexes is also altered by preincubation above Tc for several minutes also supports that lipid melting induces a conformational change in Ca2+, Mg2+-ATPase.

Effect of short-chain primary alcohols on fluidity and activity of sarcoplasmic reticulum membranes

Intramolecular excimer formation with the fluorescent probe 1,3-di(1-pyrenyl)propane, differential scanning calorimetry, and X-ray diffraction were used to assess the effect of ethanol, 1-butanol, and 1-hexanol on the bilayer organization in model membranes, sarcoplasmic reticulum (SR) lipids and native SR membranes. These alcohols have fluidizing effects on membranes and lower the main transition temperature of dimyristoylphosphatidylcholine (DMPC), but only 1-hexanol alters the cooperativity of the phase transition and significantly increases the thickness of DMPC bilayers. The interaction of the three alcohols with the SR Ca2+ pump was also investigated. Hydrolysis of ATP and coupled Ca2+ uptake are differently sensitive to the three alcohols. Whereas ethanol and 1-butanol inhibited the Ca2+ uptake, 1-hexanol stimulated it. Nevertheless, the energetic efficiency of the pump (Ca2+/ATP) is not significantly affected by ethanol or 1-hexanol, but uncoupling was observed with 1-butanol at high concentrations. The different effects of alcohols on the activity of SR membranes rule out an unitary mechanism of action on the basis of fluidity changes induced in the lipid bilayer. Depending on the chain length, the alcohols interact with the SR membranes in different domains, perturbing differently the Ca2+-pump activity.

Estimation of inter-binding-site distances in sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase using Eu(III) luminescence energy transfer

We have used several trivalent lanthanides as probes for the high-affinity Ca(II)-binding site of the Ca(II) + Mg(II)-ATPase of skeletal muscle sarcoplasmic reticulum. The luminescent probes Eu(III) and Tb(III) were excited directly with pulsed laser light and the energy transfer efficiencies to several lanthanide acceptors were measured, under conditions in which most donor-acceptor pair occupied high-affinity Ca(II) sites. We obtain an inter-ionic site distance of about 0.8-0.9 nm. Energy transfer measurements were also done with Eu(III) in at least one Ca(II) site and bidentate Cr-ATP complex at the ATP hydrolytic site. Quenching of Eu(III) luminescence by Cr-ATP was total under these conditions. We calculate an upper limit of 1.0 nm for the distance from the Ca(II) site(s) to the complexed Cr(III) ion at the hydrolytic site.

ATP synthesis catalyzed by the purified erythrocyte Ca-ATPase in the absence of calcium gradients

The Ca2+-transporting ATPase of erythrocytes was isolated by calmodulin affinity chromatography. The backward reaction of the ATPase was investigated. The phosphorylation of the solubilized enzyme by Pi required Mg and was inhibited by Ca and vanadate in the micromolar concentration range. Significant amounts of phosphoenzyme could be obtained only in a medium containing high dimethyl sulfoxide concentrations (greater than 25%) in order to diminish water activity at the phosphorylation site. The phosphoenzyme formed in this way could not phosphorylate ADP. However, upon addition of Ca2+ ions and dilution of dimethyl sulfoxide in the phosphorylated preparation (water activity jump), a highly reactive phosphoenzyme species was obtained which could transfer phosphate in nearly stoichiometric amounts to ADP to form ATP.

Cytochemical localization of ouabain-sensitive, K+ -dependent p-nitrophenylphosphatase and Ca++-stimulated adenosine triphosphatase activities in human parotid and submandibular glands

K+ -dependent p-nitrophenylphosphatase (pNPPase) and Ca++ -stimulated adenosine triphosphatase (ATPase) activities were studied in human parotid and submandibular glands using cytochemical methods at the ultrastructural level. In both glands, only the striated-duct epithelium showed K+ -pNPPase reaction product, thereby indicating the localization of Na+, K+ -ATPase. The precipitate was concentrated on the deep invaginations of the basolateral plasma membranes, in close association with their cytoplasmic surface. Ca++ -ATPase activity was also found on the basolateral plasma membranes, but two striking differences from the K+ -pNPPase distribution were observed: firstly, Ca++ -ATPase appeared in both acinar and ductal cells, and secondly, it was localized on the outer side of the plasma membranes.