Luminescence studies of Tb3+ bound to the high affinity sites of the Ca2+-ATPase of sarcoplasmic reticulum

Signal transduction, plasma membrane calcium movements, and pigment translocation in freshwater shrimp chromatophores

Crustacean color change results from the differential translocation of chromatophore pigments, regulated by neurosecretory peptides like red pigment concentrating hormone (RPCH) that, in the red ovarian chromatophores of the freshwater shrimp Macrobrachium olfersi, triggers pigment aggregation via increased cytosolic cGMP and Ca(2+) of both smooth endoplasmatic reticulum (SER) and extracellular origin. However, Ca(2+) movements during RPCH signaling and the mechanisms that regulate intracellular [Ca(2+)] are enigmatic. We investigate Ca(2+) transporters in the chromatophore plasma membrane and Ca(2+) movements that occur during RPCH signal transduction. Inhibition of the plasma membrane Ca(2+)-ATPase by La(3+) and indirect inhibition of the Na(+)/Ca(2+) exchanger by ouabain induce pigment aggregation, revealing a role for both in Ca(2+) extrusion. Ca(2+) channel blockade by La(3+) or Cd(2+) strongly inhibits slow-phase RPCH-triggered aggregation during which pigments disperse spontaneously. L-type Ca(2+) channel blockade by gabapentin markedly reduces rapid-phase translocation velocity; N- or P/Q-type blockade by ω-conotoxin MVIIC strongly inhibits RPCH-triggered aggregation and reduces velocity, effects revealing RPCH-signaled influx of extracellular Ca(2+). Plasma membrane depolarization, induced by increasing external K(+) from 5 to 50 mM, produces Ca(2+)-dependent pigment aggregation, whereas removal of K(+) from the perfusate causes pigment hyperdispersion, disclosing a clear correlation between membrane depolarization and pigment aggregation; K(+) channel blockade by Ba(2+) also partially inhibits RPCH action. We suggest that, during RPCH signal transduction, Ca(2+) released from the SER, together with K(+) channel closure, causes chromatophore membrane depolarization, leading to the opening of predominantly N- and/or P/Q-type voltage-gated Ca(2+) channels, and a Ca(2+)/cGMP cascade, resulting in pigment aggregation.

Pre-steady-state kinetic study of the mechanism of inhibition of the plasma membrane Ca(2+)-ATPase by lanthanum

Lanthanides are known to be effective inhibitors of the PMCa(2+)-ATPase. The effects of LaCl3 on the partial reactions that take place during ATP hydrolysis by the calcium-dependent ATPase from plasma membrane (PMCa(2+)-ATPase) were studied at 37 degrees C on fragmented intact membranes from pig red cells by means of a rapid chemical quenching technique. LaCl3 added before phosphorylation (K0.5 = 2.8 +/- 0.2 microM) raised the kapp of the E2-->E1 transition from 14 +/- 2 to 23 +/- 4 s-1. The effect was independent of Ca2+ and Mg2+, as if La3+ substituted for Mg2+ and/or Ca2+ in accelerating the formation of E1 with higher efficiency. At non-limiting conditions, LaCl3 doubled the apparent concentration of E1 in the enzyme at rest with Ca2+ and Mg2+. LaCl3 during phosphorylation (K0.5 near 20 microM) lowered the vo of the reaction from 300 +/- 20 to 60 +/- 7 pmol/mg of protein/s, a close rate to that in the absence of Mg2+. This effect was reversed by Mg2+ (and not by Ca2+), and the K0.5 for Mg2+ as activator of the phosphorylation reaction increased linearly with the concentration of LaCl3, suggesting that La3+ slowed phosphorylation by displacing Mg2+ from the activation site(s). If added before phosphorylation, LaCl3 lowered the kapp for decomposition of EP to 0.8 +/- 0.1 s-1, a value which is characteristic of phosphoenzyme without Mg2+. The K0.5 for this effect was 0.9 +/- 0.5 microM LaCl3 and increased linearly with the concentration of Mg2+. If added after phosphorylation, LaCl3 did not change the kapp of 90 +/- 7 s-1 of decomposition of EP, suggesting that La3+ displaced Mg2+ from the site whose occupation accelerates the shifting of E1P to E2P. In medium with 0.5 mM MgCl2, 2 microM LaCl3 lowered rapidly the rate of steady-state hydrolysis of ATP by the PMCa(2+)-ATPase to a value close to the rate of decomposition of EP made in medium with LaCl3. Increasing MgCl2 to 10 mM protected the PMCa(2+)-ATPase against inhibition during the first 10 min of incubation. Results show that combination of La3+ to the Mg2+ (and Ca2+) site(s) in the unphosphorylated PMCa(2+)-ATPase accelerates the E2-->E1 transition and inhibits the shifting E1P--> E2P. Since with less apparent affinity La3+ slowed but did not impede phosphorylation, it seems that the sharp slowing of the rate of transformation of E1P into E2P by displacement of Mg2+ was the cause of the high-affinity inhibition of the PMCa(2+)-ATPase by La3+.

Coexistence of high- and low-affinity Ca2+ binding sites of the sarcoplasmic reticulum calcium pump

We have recently shown [Mészáros, L. G., & Bak, J. (1992) Biochemistry 31, 1195-1200] that, during the rapid phase of Ca2+ uptake into sarcoplasmic reticulum (SR), internalization and binding of Ca2+ to the cytoplasmic high-affinity binding sites of the Ca2+ ATPase occur simultaneously, resulting in a transient supernumerary Ca/ATP stoichiometry. Here we address the question of whether the cytoplasmic high-affinity and the luminal low-affinity Ca2+ binding sites of the SR Ca2+ ATPase also coexist. SR vesicles were loaded with Ca2+ (0-10 mM), and then the kinetics of EP formation and decomposition as well as the maximum level of EP formed from radiolabeled ATP were determined at conditions which only allow single-cycle reactions to occur: empty or Ca-loaded SR vesicles (in micromolar extravesicular Ca2+) were either mixed with ATP plus millimolar EGTA or added in amounts that set a Ca2+ ATPase/ATP ratio of 80-85 at the initiation of the reaction. The rates of EP formation and decomposition were both significantly reduced in Ca-loaded, compared to empty (ionomycin-treated), vesicles. However, the value of EPmax was unaltered by Ca-loading, suggesting the existence of the enzyme intermediate, E.Ca2(cyt).Ca2(lum), i.e., the coexistence of the cytoplasmic and the luminal Ca2+ binding sites of the Ca-pump. These results suggest that the uphill transport of Ca2+ might not be based on an alternating relocation and conversion of the Ca2+ binding sites of the Ca2+ ATPase.

Functional characterization of lanthanide binding sites in the sarcoplasmic reticulum Ca(2+)-ATPase: do lanthanide ions bind to the calcium transport site?

Gd3+ binding sites on the purified Ca(2+)-ATPase of sarcoplasmic reticulum were characterized at 2 and 6 degrees C and pH 7.0 under conditions in which 45Ca2+ and 54Mn2+ specifically labeled the calcium transport site and the catalytic site of the enzyme, respectively. We detected several classes of Gd3+ binding sites that affected enzyme function: (a) Gd3+ exchanged with 54Mn2+ of the 54MnATP complex bound at the catalytic site. This permitted slow phosphorylation of the enzyme when two Ca2+ ions were bound at the transport site. The Gd3+ ion bound at the catalytic site inhibited decomposition of the ADP-sensitive phosphoenzyme. (b) High-affinity binding of Gd3+ to site(s) distinct from both the transport site and the catalytic site inhibited the decomposition of the ADP-sensitive phosphoenzyme. (c) Gd3+ enhanced 4-nitro-2,1,3-benzoxadiazole (NBD) fluorescence in NBD-modified enzyme by probably binding to the Mg2+ site that is distinct from both the transport site and the catalytic site. (d) Gd3+ inhibited high-affinity binding of 45Ca2+ to the transport site not by directly competing with Ca2+ for the transport site but by occupying site(s) other than the transport site. This conclusion was based mainly on the result of kinetic analysis of displacement of the enzyme-bound 45Ca2+ ions by Gd3+ and vice versa, and the inability of Gd3+ to phosphorylate the enzyme under conditions in which GdATP served as a substrate. These results strongly suggest that Ln3+ ions cannot be used as probes to structurally and functionally characterize the calcium transport site on the Ca(2+)-ATPase.

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.

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.

In vitro study of the binding of terbium to tryptophan by 1H-NMR and fluorescence spectroscopy

Terbium (Tb), a rare earth element having an ionic radius similar to that of Ca2+, is used as an antagonist of Ca2+. Recently, we found that when giant axon membrane of a loligo treated with Tb was excited at 290 nm, where tryptophan (Trp) gives absorption mechanism, emits fluorescence of Tb at 490 nm and 545 nm in addition to that of Trp at 345 nm. The present study was undertaken to locate on the Trp molecule the interaction between Trp and Tb by carrying out in vitro binding experiments with Tb and Trp and the molecular mechanism was analyzed by 1H-NMR spectroscopy and fluorescence spectroscopy. Fluorescence spectroscopy showed that Tb and Trp are combined at a molecular ratio close to 1:1. 1H-NMR spectroscopy revealed that Tb interacts with the amino group at the alpha-position and the imido group of the indole ring of Trp. Since all amino groups of Trp are involved in peptide linkages in vivo, it seems likely that the binding of Tb to Trp is undertaken by the imido group of the indole ring.

Characterization of (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum by laser-excited europium luminescence

The molecular environment of Ca2+ translocating sites of skeletal muscle sarcoplasmic reticulum (SR) (Ca2+ + Mg2+)-ATPase has been studied by pulsed-laser excited luminescence of Eu3+ used as a Ca2+ analogue. Interaction of Eu3+ with SR was characterized by investigating its effect on partial reactions of the Ca2+ transport cycle. In native SR vesicles, Eu3+ was found to inhibit Ca2+ binding, phosphoenzyme formation, ATP hydrolysis activity and Ca2+ uptake in parallel fashion. The non-specific binding of Eu3+ to acidic phospholipids associated with the enzyme was prevented by purifying (Ca2+ + Mg2+)-ATPase and exchanging the endogenous lipids with a neutral phospholipid, dioleoylglycerophosphocholine. The results demonstrate that the observed inhibition of Ca2+ transport by Eu3+ is due to its binding to Ca2+ translocating sites. The 7F0----5D0 transition of Eu3+ bound to these sites was monitored. The non-Lorentzian nature of the excitation profile and a double-exponential fluorescence decay revealed the heterogeneity of the two sites. Measurement of fluorescence decay rates in H2O/D2O mixture buffers further distinguished the sites. The number of water molecules in the first co-ordination sphere of Eu3+ bound at transport sites were found to be 4 and 1.5. Addition of ATP reduced these numbers to zero and 0.6. These data show that the calcium ions in translocating sites are well enclosed by protein ligands and are further occluded down to zero or one water molecule of solvation during the transport process.

Distances between the functional sites of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase and the lipid/water interface

Measurements of fluorescence energy transfer have been performed to determine the distance between the lipid-water interface and the ATP-binding site in the (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum. The calculated distance between the donor, FITC bound to the protein (nucleotide binding-site marker), and the acceptor, rhodamine-5'-isothiocyanyldipalmitoylphosphatidylethanolamine (RITC-DPPE) incorporated in the membrane, was in the range of 34-42 A. In addition the distance between the high affinity Ca2+-binding sites and the lipid/water interface has been calculated by luminescence energy transfer from Tb3+ bound to the Ca2+ sites to RITC-DPPE included in the membrane, and it was approx. 10 A.

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.

Evidence that the ATP binding site of sarcoplasmic reticulum CaATPase has a Mg(2+) ion binding sub-site

The CaATPase of skeletal muscle sarcoplasmic reticulum was specifically labeled in the ATP binding site with fluorescein isothiocyanate under gentle conditions (pH 7 X 5). Fluorescence energy transfer from the attached fluorescein to Nd3+ indicated that a cation binding site was about 1 X 0 nm away from the fluorescein. Thus it appears that the ATP site includes a cation binding site. At 25 degrees C in 0 X 5 M KCl, the association constants for Nd3+, Ca2+ and Mg2+ were 3 X 3 X 10(5) M-1, 84 M-1 and 35 M-1, respectively, making it possible that, in vivo, the site binds Mg2+.