Stoichiometry and mapping of the nucleotide sites in sarcoplasmic reticulum ATPase with the use of UTP

Mapping interactions between the Ca2+-ATPase and its substrate ATP with infrared spectroscopy

Infrared spectroscopy has been used to map substrate-protein interactions: the conformational changes of the sarcoplasmic reticulum Ca(2+)-ATPase upon nucleotide binding and ATPase phosphorylation were monitored using the substrate ATP and ATP analogues (2'-deoxy-ATP, 3'-deoxy-ATP, and inosine 5'-triphosphate), which were modified at specific functional groups of the substrate. Modifications to the 2'-OH, the 3'-OH, and the amino group of adenine reduce the extent of binding-induced conformational change of the ATPase, with particularly strong effects observed for the latter two. This demonstrates the structural sensitivity of the nucleotide-ATPase complex to individual interactions between nucleotide and ATPase. All groups studied are important for binding and interactions of a given ligand group with the ATPase depend on interactions of other ligand groups. Phosphorylation of the ATPase was observed for ITP and 2'-deoxy-ATP, but not for 3'-deoxy-ATP. There is no direct link between the extent of conformational change upon nucleotide binding and the rate of phosphorylation showing that the full extent of the ATP-induced conformational change is not mandatory for phosphorylation. As observed for the nucleotide-ATPase complex, the conformation of the first phosphorylated ATPase intermediate E1PCa(2) also depends on the nucleotide, indicating that ATPase states have a less uniform conformation than previously anticipated.

Substrate regulation of calcium binding in Ca2+-ATPase molecules of the sarcoplasmic reticulum. II. Effect of CTP, GTP, ITP, and UTP

To examine the effect of CTP, GTP, ITP, and UTP on calcium binding of Ca2+-ATPase molecules of the sarcoplasmic reticulum, the calcium dependence of the Ca2+-activated hydrolysis activities of these NTPs of the enzyme molecules was examined by comparison with that of calcium binding of the molecules in the absence of the NTPs at pH 7.40. In the sarcoplasmic reticulum membrane, CTP, GTP, and ITP did not affect the noncooperative (Hill value (n(H)) of approximately 1, apparent calcium affinity (K(0.5)) of 2-6 microm)) and cooperative (n(H) approximately 2, K(0.5) approximately 0.2 microm) calcium binding of the molecules, whereas UTP caused the molecules to highly cooperatively (n(H) approximately 4) bind calcium ions with a lowered K(0.5) of approximately 0.04 microm. When the enzyme molecules were solubilized with detergent, all of these NTPs reversibly degraded the calcium affinity of the molecule (from K(0.5) = 3-5 to >40 microm), although the effect of the NTPs on the negatively cooperative manner (n(H) approximately 0.5) of calcium binding was not experimentally obtained. Taking into account the first part of this study (Nakamura, J., Tajima, G., Sato, C., Furukohri, T., and Konishi, K. (2002) J. Biol. Chem. 277, 24180-24190) showing the improving effect of ATP on calcium binding of the membranous and solubilized molecules, the results show that ATP is the only intrinsic substrate for the enzyme molecule. This NTP regulation is discussed in terms of the oligomeric structure of the molecules.

Reaction of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole with the (Ca2+ + Mg2+)- ATPase protein of sarcoplasmic reticulum at low temperature

Modification of the (Ca2+ + Mg2+)-ATPase protein of rabbit skeletal sarcoplasmic reticulum (SR) with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, NBD-Cl, at 4 degrees C for 5 min caused a 63% loss of the Ca(2+)-dependent ATPase activity when 1 mol of the adenine analog was incorporated per 10(5) g of protein. At 25 degrees C, above the lipid phase transition, the extent of labeling was 3-fold higher although the Ca(2+)-ATPase activity was inhibited to the same extent. MgATP protected the ATPase activity at 4 degrees C and 25 degrees C but there was little change in the extent of labeling at 4 degrees C suggesting that changes in the fluidity of the lipid moiety made different sites on the ATPase protein accessible to the reagent. At 4 degrees C, addition of sodium deoxycholate enhanced the inactivation (6% ATPase activity remained) but the labeling of the SR-ATPase protein did not increase significantly. Incubation with MgATP prior to solubilization with deoxycholate resulted in the protection of the Ca(2+)-ATPase activity and only a small decrease in the labeling occurred. At 25 degrees C, a similar pattern was found with deoxycholate but the loss of ATPase activity was less dramatic and the extent of labeling by NBD-Cl was greater than that at 4 degrees C. MgATP induced changes in the conformation of the ATPase protein protecting essential cysteine residues while shifting the reaction of NBD-Cl with the ATPase protein to non-essential sites in the absence or presence of deoxycholate. An analysis of tryptic digests of the NBD-ATPase protein showed that MgATP shifted the labeling from the A2 subfragment to the A1 subfragment in the absence of deoxycholate and from the A1 subfragment to the A2 subfragment in the presence of deoxycholate. The reagent, NBD-Cl, can distinguish between different temperature dependent conformational states of the ATPase protein.

Effects of pH, Ca2+ and lanthanides on conformation of the sarcoplasmic reticulum Ca(2+)-ATPase catalytic site

The conformational changes at the ATP-catalytic site of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase have been studied by the fluorescence of the fluorescein 5-isothiocyanate (FITC) bound to the adenine subsite. The FITC-SR fluorescence parameters have been examined in the pH range 5.7-8.0 in the presence of EGTA, Ca2+ or Ln3+ (La3+, Pr3+, Nd3+, Tb3+, etc.). A quantitative method to calculate the equilibrium between the protein conformers is proposed on the basis of the fluorometric titration curve analysis. The distance Nd(3+)-FITC was estimated to be about 1 nm at pH 6-7 and 1.7 nm at pH 8 which can be interpreted as an increase of the distance between the nucleotide and phosphorylation domains of Ca(2+)-ATPase in alkaline media. These studies suggest that the ligand-stabilized E1-form of Ca(2+)-ATPase can exist in two conformational states with the closed and opened interdomain cleft in the pH range 5.7-8.0. The pH-dependence of the ratio of these states correlates with that of the E1----E2 equilibrium without ligands. These dependences were approximated by simple Henderson-Hasselbach equations with pK 7.0 +/- 0.1, i.e. the transition between two protein conformations is probably governed by one proton dissociation.

Inhibition and labeling of the Ca2(+)-ATPase from sarcoplasmic reticulum by periodate oxidized ATP

The analog of ATP obtained by oxidation of the ribose ring of ATP with periodate (oxATP) was used as a reagent for the inhibition and labeling of the Ca2(+)-ATPase purified from sarcoplasmic reticulum membranes. The substrate concentration dependence for hydrolysis showed a biphasic pattern for both ATP and oxATP as substrates. Preincubation of Ca2(+)-ATPase in the presence of 0.05 mM CaCl2, 5 mM MgCl2, 100 mM KCl and oxATP led to an irreversible inhibition. This inhibition occurred faster at alkaline pH. The presence of ADP, adenyl-5'-imidodiphosphate (AMP-PNP) or EGTA in the preincubation medium decreased the rate of inhibition. OxATP covalently labels the enzyme: the labeling was decreased by ADP. This ADP-protected labeling increased with time until it reached approx. 1 mol [3H]oxATP per mol ATPase. The rate of labeling of the ADP-protected group correlated with the rate of loss of ADP-protected activity. Trypsin digestion of oxATP-labeled ATPase followed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate showed that fragment A1 contained a high degree of label that is displaced by ADP. We propose that the A1 fragment is situated close to the ribose ring when the adenosine moiety of ATP is bound to the catalytic site of the Ca2(+)-ATPase.

Modification of the (Ca2+ + Mg2+)-ATPase protein of sarcoplasmic reticulum with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

The (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase (Ca2+-transporting), EC 3.6.1.38) protein of rabbit skeletal sarcoplasmic reticulum (SR) rapidly incorporated 2 mol of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) per 10(5) g of protein with little change in the Ca2+-dependent ATPase activity. When 2 additional mol of the reagent were bound the Ca2+-ATPase, activity was inhibited. The same pattern was found for modified intact SR and the Ca2+ uptake ability was inhibited. MgATP, CaATP and MgADP protected the Ca2+-ATPase activity concurrent with a decrease of about 1 mol of the NBD group per 10(5) g protein, but the Ca2+ uptake ability was not protected. Calcium alone had no effect on the modification. The modified ATPase protein or SR formed non-serial oligomers or aggregates, but the ATPase protein remained the predominant species present. In the presence of MgATP, oligomer formation was reduced partially but the major changes in the Ca2+-ATPase activity were due to the modification of the ATPase monomer. Thiolysis of the NBD-ATPase protein with dithiothreitol did not restore the Ca2+-ATPase activity, although more than 1 mol of the NBD group was removed from cysteine residues. Cysteine residues were modified in the NBD-ATPase protein or SR when the enzyme activity was inhibited. Trypsin digestion of NBD-SR or its ATPase protein released the A, B, A1, and A2 fragments. The A fragment and its subfragment A2 contained most of the label. Substrate MgATP protection studies showed that the A1 and A2 fragments were involved in maintaining the Ca2+-ATPase activity. Reagent-induced conformational changes of these fragments rather than direct active site group labeling accounted for the loss of ATPase activity.