Interaction of calcium and vanadate with fluorescein isothiocyanate labeled Ca2+-ATPase from sarcoplasmic reticulum: kinetics and equilibria

ANS Interacts with the Ca2+-ATPase Nucleotide Binding Site

The binding of 8-anilino-1-naphthalene sulfonate (ANS) to the nucleotide binding domain (N-domain) of the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) was studied. Molecular docking predicted two ANS binding modes (BMI and BMII) in the nucleotide binding site. The molecular interaction was confirmed as the fluorescence intensity of ANS was dramatically increased when in the presence of an engineered recombinant N-domain. Molecular dynamics simulation showed BMI (which occupies the ATP binding site) as the mode that is stable in solution. The above was confirmed by the absence of ANS fluorescence in the presence of a fluorescein isothiocyanate (FITC)-labeled N-domain. Further, the labeling of the N-domain with FITC was hindered by the presence of ANS, i.e., ANS was bound to the ATP binding site. Importantly, ANS displayed a higher affinity than ATP. In addition, ANS binding led to quenching the N-domain intrinsic fluorescence displaying a FRET pattern, which suggested the existence of a Trp-ANS FRET couple. Nonetheless, the chemical modification of the sole Trp residue with N-bromosuccinimide (NBS) discarded the existence of FRET and instead indicated structural rearrangements in the nucleotide binding site during ANS binding. Finally, Ca²⁺-ATPase kinetics in the presence of ANS showed a partial mixed-type inhibition. The Dixon plot showed the ANS-Ca²⁺-ATPase complex as catalytically active, hence supporting the existence of a functional dimeric Ca²⁺-ATPase in sarcoplasmic reticulum vesicles. ANS may be used as a molecular platform for the development of more effective inhibitors of Ca²⁺-ATPase and appears to be a new fluorescent probe for the nucleotide binding site. Graphical Abstract Molecular docking of ANS to the nucleotide binding site of Ca²⁺-ATPase. ANS fluorescence increase reveals molecular interaction.

Conformational changes produced by ATP binding to the plasma membrane calcium pump

The aim of this work was to study the plasma membrane calcium pump (PMCA) reaction cycle by characterizing conformational changes associated with calcium, ATP, and vanadate binding to purified PMCA. This was accomplished by studying the exposure of PMCA to surrounding phospholipids by measuring the incorporation of the photoactivatable phosphatidylcholine analog 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine to the protein. ATP could bind to the different vanadate-bound states of the enzyme either in the presence or in the absence of Ca(2+) with high apparent affinity. Conformational movements of the ATP binding domain were determined using the fluorescent analog 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate. To assess the conformational behavior of the Ca(2+) binding domain, we also studied the occlusion of Ca(2+), both in the presence and in the absence of ATP and with or without vanadate. Results show the existence of occluded species in the presence of vanadate and/or ATP. This allowed the development of a model that describes the transport of Ca(2+) and its relation with ATP hydrolysis. This is the first approach that uses a conformational study to describe the PMCA P-type ATPase reaction cycle, adding important features to the classical E1-E2 model devised using kinetics methodology only.

Distinct natures of beryllium fluoride-bound, aluminum fluoride-bound, and magnesium fluoride-bound stable analogues of an ADP-insensitive phosphoenzyme intermediate of sarcoplasmic reticulum Ca2+-ATPase: changes in catalytic and transport sites during phosphoenzyme hydrolysis

The structural natures of stable analogues for the ADP-insensitive phosphoenzyme (E2P) of Ca(2+)-ATPase formed in sarcoplasmic reticulum vesicles, i.e. the enzymes with bound beryllium fluoride (BeF.E2), bound aluminum fluoride (AlF.E2), and bound magnesium fluoride (MgF.E2), were explored and compared with those of actual E2P formed from P(i) without Ca(2+). Changes in trinitrophenyl-AMP fluorescence revealed that the catalytic site is strongly hydrophobic in BeF.E2 as in E2P but hydrophilic in MgF.E2 and AlF.E2; yet, the three cytoplasmic domains are compactly organized in these states. Thapsigargin, which was shown in the crystal structure to fix the transmembrane helices and, thus, the postulated Ca(2+) release pathway to lumen in a closed state, largely reduced the tryptophan fluorescence in BeF.E2 as in E2P, but only very slightly (hence, the release pathway is likely closed without thapsigargin) in MgF.E2 and AlF.E2 as in dephosphorylated enzyme. Consistently, the completely suppressed Ca(2+)-ATPase activity in BeF-treated vesicles was rapidly restored in the presence of ionophore A23187 but not in its absence by incubation with Ca(2+) (over several millimolar concentrations) at pH 6, and, therefore, lumenal Ca(2+) is accessible to reactivate the enzyme. In contrast, no or only very slow restoration was observed with vesicles treated with MgF and AlF even with A23187. BeF.E2 thus has the features very similar to those characteristic of the E2P ground state, although AlF.E2 and MgF.E2 most likely mimic the transition or product state for the E2P hydrolysis, during which the hydrophobic nature around the phosphorylation site is lost and the Ca(2+) release pathway is closed. The change in hydrophobic nature is probably associated with the change in phosphate geometry from the covalently bound tetrahedral ground state (BeF(3)(-)) to trigonal bipyramidal transition state (AlF(3) or AlF(4)(-)) and further to tetrahedral product state (MgF(4)(2-)), and such change likely rearranges transmembrane helices to prevent access and leakage of lumenal Ca(2+).

Calcium accumulation by organelles within Myxicola axoplasm

1. 45Ca2+ accumulation into inulin-inaccessible compartments within cytoplasm from the giant axon of Myxicola infundibulum was measured as a function of free calcium, pH, and time. Accumulation reached a maximum after 1 h and remained stable for at least 3 h. 2. At 0.5, 5, and 50 microM [Ca2+], in the presence of 1 mM ATP or 5 mM succinate, steady-state calcium uptake had a bell-shaped dependence on pH with a maximum near pH 7. Uptake was abolished by the proton uncoupling reagent carbonyl cyanide p-trifluoromethoxy-phenylhydrazone (FCCP, 4 micrograms ml-1). 3. Uptake of the membrane permeant cation, [14C]-tetraphenylphosphonium (TPP+), also had a bell-shaped dependence on pH with a maximum pH approximately 7, suggesting a pH dependence of the electrical potential of a membrane enclosed cytoplasmic compartment. Cyanide (2 mM) inhibited TPP+ uptake. 4. Inositol 1,4,5-trisphosphate (IP3, 10 microM), reduced steady-state calcium accumulation by 20-22% at 0.5 microM free calcium, pH 7 (P < 0.01, n = 16) and at 5 microM free calcium, pH 8 (P < 0.0005, n = 35). No effects of IP3 were found at other pH or calcium concentrations. 5. Neither guanosine 5'-triphosphate (GTP) nor inositol 1,3,4,5-tetrakisphosphate (IP4) had an effect on calcium uptake (5 microM [Ca2+], pH 8). 6. At 0.5 microM free calcium; vanadate (10 microM) inhibited 20-30%, of the 45Ca2+ accumulation, thapsigargin (33 nM) inhibited 20-30%, and cyanide (2 mM) plus oligomycin B (2 micrograms ml-1), or valinomycin (1 microM), inhibited 70-80%. The fraction of uptake sensitive to thapsigargin fell as the free calcium increased; however, the sensitivity of uptake to cyanide plus oligomycin B was approximately 80% for 0.5, 5.0, and 50 microM [Ca2+]. 7. Thapsigargin had no additional inhibiting effect in the presence of cyanide plus oligomycin B. IP3 had no effect in the presence of cyanide plus oligomycin B or other mitochondrial inhibitors. 8. Results suggest the presence of both mitochondrial (70-80%) and non-mitochondrial (20-30%) calcium pools in this system (at 0.5-5.0 microM Ca2+). The apparent non-mitochondrial uptake (sensitive to thapsigargin, or IP3) is not detectable in the presence of mitochondrial inhibitors. We interpret these results as evidence of functional communication between mitochondrial and non-mitochondrial calcium stores.

Inhibition of tonoplast ATPase from etiolated mung bean seedlings by fluorescein 5'-isothiocyanate

Fluorescein 5'-isothiocyanate (FITC) was used to modify the lysine residue in the active site of tonoplast H(+)-ATPase from etiolated mung-bean (Vigna radiata L.) seedlings. FITC caused marked inactivation of the enzyme activities of both membrane-bound and soluble ATPase and its associated H+ translocation. The SDS/PAGE pattern revealed that the FITC-binding site was in the large (A) subunit of ATPase. Inhibition could be substantially prevented by its physiological substrate ATP, pyrophosphate and nucleotides in the decreasing order: ATP greater than pyrophosphate greater than ADP greater than AMP greater than GTP greater than CTP greater than UTP. The mode of inhibition by FITC was competitive with respect to ATP. Loss of ATPase activity followed pseudo-first-order kinetics with a Ki of 0.33 mM, a minimum inactivation half-time of 110 s, and a first-order rate constant of 0.244 s-1. A double-logarithmic plot of apparent rate constant versus FITC concentration gave a slope of 0.913, indicating that inactivation results from reaction of at least one lysine residue at the catalytic site of the large subunit. Labelling studies indicated that the incorporation of approx. 1 mol of FITC/mol of ATPase is sufficient to inhibit ATPase completely. The enhancement and blue shift of emission maxima of FITC after modification of ATPase indicated that the labelled lysine residue was located in a relatively hydrophobic domain.

Vanadate inhibition of ATP and p-nitrophenyl phosphate hydrolysis in the fragmented sarcoplasmic reticulum

The vanadate inhibition of the Ca(2+)-ATPase activity was analysed both in intact sarcoplasmic reticulum vesicles and in the presence of low concentrations of Tween 20, using ATP and p-nitrophenyl phosphate as substrates. The saturation of the internal low-affinity calcium-binding sites protects the enzyme against vanadate inhibition, because: (1) p-nitrophenyl phosphate hydrolysis is not inhibited by vanadate in intact vesicles, but inhibition developed after solubilization with detergents; (2) the vanadate inhibition of the p-nitrophenyl phosphate hydrolysis in solubilized preparations is prevented by free Ca2+ concentrations higher than 10(-3) M and vanadate competes with calcium (10(-5)-10(-3) M); and (3) the vanadate inhibition of ATP hydrolysis is decreased with an increase in vesicular Ca2+ concentration. The presence of magnesium ions is indispensable for the vanadate effect. The vanadate inhibition is non-competitive with respect to Mg-p-nitrophenyl phosphate and uncompetitive with respect to Mg-ATP. However, in the presence of dimethyl sulfoxide, which facilitates phosphorylation of the enzyme, the inhibition is converted to a competitive one with respect to a substrate. The results suggest, that in the process of enzyme operation vanadate interacts with the unliganded E form of Ca(2+)-ATPase, occupying probably an intermediate position between the E2 and E1 forms, with the formation of an E2 Van complex, that imposes the inhibition on the Ca(2+)-ATPase activity.

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.

Vanadate-catalyzed, conformationally specific photocleavage of the Ca2(+)-ATPase of sarcoplasmic reticulum

Vanadate-sensitized photocleavage of the Ca2(+)-ATPase of rabbit sarcoplasmic reticulum was observed upon illumination of sarcoplasmic reticulum vesicles or the purified Ca2(+)-ATPase by ultraviolet light in the presence of 1 mM monovanadate or decavanadate. The site of the photocleavage is influenced by the Ca2+ concentration of the medium. When the [Ca2+] is maintained below 10 nM by EGTA, the vanadate-catalyzed photocleavage yields fragments of approximately equal to 87 and approximately equal to 22 kDa, while in the presence of 2-20 mM Ca, polypeptides of 71 and 38 kDa are obtained as the principal cleavage products. These observations indicate that the site of the vanadate-catalyzed photocleavage is altered by changes in the conformation of Ca2(+)-ATPase. Selective tryptic proteolysis, at Arg-505-Ala-506, combined with covalent labeling of Lys-515 by fluorescein 5'-isothiocyanate and with the use of anti-ATPase antibodies of defined specificity, permitted the tentative allocation of the sites of photocleavage to the A fragment near the T2 cleavage site in the absence of Ca2+, and to the B fragment between Lys-515 and Asp-659 in the presence of 2-20 mM Ca2+. The loss of ATPase activity during illumination is accelerated by calcium in the presence of vanadate. The vanadate-catalyzed photocleavage in the presence of Ca2+ is consistent with the existence of an ATPase-Ca2(+)-vanadate complex (Markus et al. (1989) Biochemistry 28, 793-799).