Phosphorylation-dependent changes in the spatial relationship between Ca-ATPase polypeptide chains in sarcoplasmic reticulum membranes

Phosphorylated phospholamban stabilizes a compact conformation of the cardiac calcium-ATPase

The sarcoendoplasmic reticulum calcium ATPase (SERCA) plays a key role in cardiac calcium handling and is considered a high-value target for the treatment of heart failure. SERCA undergoes conformational changes as it harnesses the chemical energy of ATP for active transport. X-ray crystallography has provided insight into SERCA structural substates, but it is not known how well these static snapshots describe in vivo conformational dynamics. The goals of this work were to quantify the direction and magnitude of SERCA motions as the pump performs work in live cardiac myocytes, and to identify structural determinants of SERCA regulation by phospholamban. We measured intramolecular fluorescence resonance energy transfer (FRET) between fluorescent proteins fused to SERCA cytoplasmic domains. We detected four discrete structural substates for SERCA expressed in cardiac muscle cells. The relative populations of these discrete states oscillated with electrical pacing. Low FRET states were most populated in low Ca (diastole), and were indicative of an open, disordered structure for SERCA in the E2 (Ca-free) enzymatic substate. High FRET states increased with Ca (systole), suggesting rigidly closed conformations for the E1 (Ca-bound) enzymatic substates. Notably, a special compact E1 state was observed after treatment with β-adrenergic agonist or with coexpression of phosphomimetic mutants of phospholamban. The data suggest that SERCA calcium binding induces the pump to undergo a transition from an open, dynamic conformation to a closed, ordered structure. Phosphorylated phospholamban stabilizes a unique conformation of SERCA that is characterized by a compact architecture.

Interdomain fluorescence resonance energy transfer in SERCA probed by cyan-fluorescent protein fused to the actuator domain

We have used a biosynthetically incorporated fluorescent probe to monitor domain movements involved in ion transport by the sarcoendoplasmic reticulum Ca-ATPase (SERCA) from rabbit fast-twitch skeletal muscle. X-ray crystal structures suggest that the nucleotide-binding (N) and actuator (A) domains of SERCA move apart by several nanometers upon Ca binding. To test this hypothesis, cDNA constructs were created to fuse cyan-fluorescent protein (CFP) to the N terminus of SERCA (A domain). This CFP-SERCA fluorescent fusion protein retained activity when expressed in Sf21 insect cells using the baculovirus system. Fluorescence resonance energy transfer (FRET) was used to monitor the A-N interdomain distance for CFP-SERCA selectively labeled with fluorescein isothiocyanate (FITC) at Lys 515 in the N domain. At low [Ca (2+)] (E2 biochemical state), the measured FRET efficiency between CFP (donor in A domain) and FITC (acceptor in N domain) was 0.34 +/- 0.03, indicating a mean distance of 61.6 +/- 2.0 A between probes on the two domains. An increase of [Ca (2+)] to 0.1 mM (E1-Ca biochemical state) decreased the FRET efficiency by 0.06 +/- 0.03, indicating an increase in the mean distance by 3.0 +/- 1.2 A. Quantitative molecular modeling of dual-labeled SERCA, including an accurate calculation of the orientation factor, shows that the FRET data observed in the absence of Ca is consistent with the E2 crystal structure, but the increase in distance (decrease in FRET) induced by Ca is much less than predicted by the E1 crystal structure. We conclude that the E1 crystal structure does not reflect the predominant structure of SERCA under physiological conditions in a functional membrane bilayer.

Critical role of Val-304 in conformational transitions that allow Ca2+ occlusion and phosphoenzyme turnover in the Ca2+ transport ATPase

Site-directed mutations were produced in the distal segments of the Ca(2+)-ATPase (SERCA) transmembrane region. Mutations of Arg-290 (M3-M4 loop), Lys-958, and Thr-960 (M9 - M10 loop) had minor effects on ATPase activity and Ca(2+) transport. On the other hand, Val-304 (M4) mutations to Ile, Thr, Lys, Ala, or Glu inhibited transport by 90-95% while reducing ATP hydrolysis by 83% (Ile, Thr, and Lys), 56% (Ala), or 45% (Glu). Val-304 participates in Ca(2+) coordination with its main-chain carbonyl oxygen, and this function is not expected to be altered by mutations of its side chain. In fact, despite turnover inhibition, the Ca(2+) concentration dependence of residual ATPase activity remained unchanged in Val-304 mutants. However, the rates (but not the final levels) of phosphoenzyme formation, as well the rates of its hydrolytic cleavage, were reduced in proportion to the ATPase activity. Furthermore, with the Val-304 --> Glu mutant, which retained the highest residual ATPase activity, it was possible to show that occlusion of bound Ca(2+) was also impaired, thereby explaining the stronger inhibition of Ca(2+) transport relative to ATPase activity. The effects of Val-304 mutations on phosphoenzyme turnover are attributed to interference with mechanical links that couple movements of transmembrane segments and headpiece domains. The effects of thermal activation energy on reaction rates are thereby reduced. Furthermore, inadequate occlusion of bound Ca(2+) following utilization of ATP in Val-304 side-chain mutations is attributed to inadequate stabilization of the Glu-309 side chain and consequent defect of its gating function.

Phospholamban Inhibits Ca-ATPase Conformational Changes Involving the E2 Intermediate

We have used steady-state fluorescence spectroscopy in combination with enzyme kinetic assays to test the hypothesis that phospholamban (PLB) stabilizes the Ca-ATPase in the E2 intermediate state. The cardiac muscle Ca-ATPase (SERCA2a) isoform was expressed either alone or coexpressed with PLB in High-Five insect cells and was isolated as insect cell microsomes. Fluorescence studies of the Ca-ATPase covalently labeled with the probe 5-(2-((iodoacetyl)amino)ethyl)aminonaphthalene-1-sulfonic acid showed that PLB decreased the amplitude of the Ca-ATPase E2 --> E1 conformational transition by 45 +/- 3% and shifted the [Ca2+] dependence of the transition to higher Ca2+ levels (DeltaKCa = 230 nM), similar to the effect of PLB on Ca-ATPase activity. Similarly, PLB decreased the amplitude of Ca-ATPase phosphorylation by inorganic phosphate (Pi) by 55 +/- 2% and decreased slightly the affinity for Pi (DeltaK0.5 = 70 microM). However, PLB did not affect the Ca2+-dependent inhibition of Ca-ATPase phosphorylation by Pi. Finally, PLB decreased Ca-ATPase sensitivity to vanadate, increasing the IC50 value by 300 nM. The results suggest that PLB binding to Ca-ATPase stabilizes the enzyme in a conformation distinct from E2, decreasing the number of enzymes in the E2 state capable of undergoing ligand-dependent conformational changes involving the Ca-free E2 intermediate. The inability of conformation-specific ligands to fully convert this E2-like state into E1 or E2 implies that these states are not in a simple equilibrium relationship.

Intermolecular conformational coupling and free energy exchange enhance the catalytic efficiency of cardiac muscle SERCA2a following the relief of phospholamban inhibition

Activation of cardiac muscle sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) by beta1-agonists involves cAMP- and PKA-dependent phosphorylation of phospholamban (PLB), which relieves the inhibitory effects of PLB on SERCA2a. To investigate the mechanism of SERCA2a activation, we compared the kinetic properties of SERCA2a expressed with (+) and without (-) PLB in High Five insect cell microsomes to those of SERCA1 and SERCA2a in native skeletal and cardiac muscle SR. Both native SERCA1 and expressed SERCA2a without PLB exhibited high-affinity (10-50 microM) activation of pre-steady-state catalytic site dephosphorylation by ATP, steady-state accumulation of the ADP-sensitive phosphoenzyme (E1P), and a rapid phase of EGTA-induced phosphoenzyme (E2P) hydrolysis. In contrast, SERCA2a in native cardiac SR vesicles and expressed SERCA2a with PLB lacked the high-affinity activation by ATP and the rapid phase of E2P hydrolysis, and exhibited low steady-state levels of E1P. The results indicate that the kinetic differences in Ca2+ transport between skeletal and cardiac SR are due to the presence of phospholamban in cardiac SR, and not due to isoform-dependent differences between SERCA1 and SERCA2a. Therefore, the results are discussed in terms of a model in which PLB interferes with SERCA2a oligomeric interactions, which are important for the mechanism of Ca2+ transport in skeletal muscle SERCA1 [Mahaney, J. E., Thomas, D. D., and Froehlich, J. P. (2004) Biochemistry 43, 4400-4416]. We propose that intermolecular coupling of SERCA2a molecules during catalytic cycling is obligatory for the changes in Ca2+ transport activity that accompany the relief of PLB inhibition of the cardiac SR Ca2+-ATPase.

The time-dependent distribution of phosphorylated intermediates in native sarcoplasmic reticulum Ca2+-ATPase from skeletal muscle is not compatible with a linear kinetic model

Quenched-flow mixing was used to characterize the kinetic behavior of the intermediate reactions of the skeletal muscle sarcoplasmic reticulum (SR) Ca-ATPase (SERCA1) at 2 and 21 degrees C. At 2 degrees C, phosphorylation of SR Ca-ATPase with 100 microM ATP labeled one-half of the catalytic sites with a biphasic time dependence [Mahaney, J. E., Froehlich, J. P., and Thomas, D. D. (1995) Biochemistry 34, 4864-4879]. Chasing the phosphoenzyme (EP) with 1.66 mM ADP 10 ms after the start of phosphorylation revealed mostly ADP-insensitive E2P (95% of EP(total)), consistent with its rapid formation from ADP-sensitive E1P. The consecutive relationship of the phosphorylated intermediates predicts a decrease in the proportion of E1P ([E1P]/[EP(total)]) with increasing phosphorylation time. Instead, after 10 ms the proportion of E1P increased and that of E2P decreased until they reached a constant 1:1 stoichiometry ([E1P]:[E2P] approximately 1). At 21 degrees C, phosphorylation displayed a transient overshoot associated with an inorganic phosphate (P(i)) burst, reflecting increased turnover of E2P at the higher temperature. The P(i) burst exceeded the decay of the EP overshoot, suggesting that rephosphorylation of the enzyme occurs before the recycling step (E2 --> E1). This behavior and the reversed order of accumulation of phosphorylated intermediates at 2 degrees C are not compatible with the conventional linear consecutive reaction mechanism: E1 + ATP --> E1.ATP --> E1P + ADP --> E2P --> E2.P(i) --> E1 + P(i). Solubilization of the Ca-ATPase into monomers using the nonionic detergent C(12)E(8) gave a pattern of phosphorylation in which E1P and E2P behave like consecutive intermediates. Kinetic modeling of the C(12)E(8)-solubilized SR Ca-ATPase showed that it behaves according to the conventional Ca-ATPase reaction mechanism, consistent with monomeric catalytic function. We conclude that the nonconforming features of native SERCA1 arise from oligomeric protein conformational interactions that constrain the subunits to a staggered or out-of-phase mode of operation.

Calcium activation of the Ca-ATPase enhances conformational heterogeneity between nucleotide binding and phosphorylation domains

High-resolution crystal structures obtained in two conformations of the Ca-ATPase suggest that a large-scale rigid-body domain reorientation of approximately 50 degrees involving the nucleotide-binding (N) domain is required to permit the transfer of the gamma-phosphoryl group of ATP to Asp(351) in the phosphorylation (P) domain during coupled calcium transport. However, variability observed in the orientations of the N domain relative to the P domain in the different crystal structures of the Ca-ATPase following calcium activation and the structures of other P-type ATPases suggests the presence of conformational heterogeneity in solution, which may be modulated by contact interactions within the crystal. Therefore, to address the extent of conformational heterogeneity between these domains in solution, we have used fluorescence resonance energy transfer to measure the spatial separation and conformational heterogeneity between donor (i.e., 5-[[2-[(iodoacetyl)amino]ethyl]amino]naphthalene-1-sulfonic acid) and acceptor (i.e., fluorescein 5-isothiocyanate) chromophores covalently bound to the P and N domains, respectively, within the Ca-ATPase stabilized in different enzymatic states associated with the transport cycle. In comparison to the unliganded enzyme, the spatial separation and conformational heterogeneity between these domains are unaffected by enzyme phosphorylation. However, calcium activation results in a 3.4 A increase in the average spatial separation, from 29.4 to 32.8 A, in good agreement with the 4.3 A increase in the distance estimated from high-resolution structures where these sites are respectively separated by 31.6 A (1IWO.pdb) and 35.9 A (1EUL.pdb). Thus, the crystal structures accurately reflect the average solution structures of the Ca-ATPase. These results suggest that the approximation of cytoplasmic domains accompanying calcium transport, as observed from crystal structures, occurs in solution within the context of large amplitude domain motions important for catalysis. We suggest that these domain motions enhance the rates of substrate (ATP) access and product (ADP) egress and the probability of a productive juxtaposition of the gamma-phosphoryl moiety of ATP with Asp(351) on the phosphorylation domain to facilitate enzyme phosphorylation and calcium transport.

Gastric H/K-ATPase liberates two moles of Pi from one mole of phosphoenzyme formed from a high-affinity ATP binding site and one mole of enzyme-bound ATP at the low-affinity site during cross-talk between catalytic subunits

The maximum amount of acid-stable phosphoenzyme (E32P)/mol of alpha chain of pig gastric H/K-ATPase from [gamma-32P]ATP (K(1/2) = 0.5 microM) was found to be approximately 0.5, which was half of that formed from 32P(i) (K(1/2) = 0.22 mM). The maximum 32P binding for the enzyme during turnover in the presence of [gamma-32P]ATP or [alpha-32P]ATP was due to 0.5 mol of E32P + 0.5 mol of an acid-labile enzyme-bound [gamma-32P]ATP (EATP) or 0.5 mol of an acid-labile enzyme-bound [alpha-32P]ATP, respectively. The K(1/2) for EATP formation in both cases was 0.12 approximately 0.14 mM. The turnover number of the enzyme (i.e., the H+-ATPase activity/(EP + EATP)) was very close to the apparent rate constants for EP breakdown and P(i) liberation, both of which decreased with increasing concentrations of ATP. The ratio of the amount of P(i) liberated to that of EP that disappeared increased from 1 to approximately 2 with increasing concentrations of ATP (i.e., equal amounts of EP and EATP exist, both of which release phosphate in the presence of high concentrations of ATP). This represents the first direct evidence, for the case of a P-type ATPase, in which 2 mol of P(i) liberation occurs simultaneously from 1 mol of EP for half of the enzyme molecules and 1 mol of EATP for the other half during ATP hydrolysis. Each catalytic alpha chain is involved in cross-talk, thus maintaining half-site phosphorylation and half-site ATP binding which are induced by high- and low-affinity ATP binding, respectively, in the presence of Mg2+.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Plausible stoichiometry of the interacting nucleotide-binding sites in the Ca(2+)-ATPase from sarcoplasmic reticulum membranes

The Ca(2+),Mg(2+)-ATPase from sarcoplasmic reticulum couples ATP hydrolysis to Ca(2+) transport toward the lumen of the muscular vesicular system. Combined structural and functional studies suggest that the Ca(2+) binding sites are formed by six amino acids of the same polypeptide and that cation translocation may take place through a channel inside a monomer of the ATPase. However, calorimetric, fluorescent, and kinetic studies suggest that the ATPase may assemble into functional oligomers of as yet unknown stoichiometry. We have addressed this question and attempted to determine the ATPase stoichiometry using a biophysical approach based on the analysis of the ATPase inhibition by fluorescein 5'-isothiocyanate in the presence of increasing ATP concentrations. For native SR membranes, our inhibition data are well described by a model consisting of two interacting nucleotide-binding sites per oligomer. This stoichiometry was disrupted in detergent C(12)E(8)-solubilized ATPase. Thus, these findings suggest that interacting nucleotide binding sites of the ATPase may appear as dimers, and imply that interactions of the globular cytoplasmic domains would play a modulatory role of the protein enzymatic activity.

Enhanced rotational dynamics of the phosphorylation domain of the Ca-ATPase upon calcium activation

We have used labeling conditions that permit the specific and covalent attachment of erythrosin isothiocyanate (Er-ITC) to Lys464 within the phosphorylation domain of the Ca-ATPase in skeletal sarcoplasmic reticulum membranes. These labeling conditions do not interfere with high-affinity ATP binding, phosphoenzyme formation, or phosphoenzyme hydrolysis [Huang, S., Negash, S., and Squier, T. C. (1998) Biochemistry 37, 6949-6957]. Thus, we can use frequency-domain phosphorescence spectroscopy to measure the rotational dynamics of the Ca-ATPase stabilized in different enzymatic states corresponding to the absence of bound ligands (E), calcium activation (E x Ca2), the presence of bound nucleotide (E x ATP), and formation of phosphoenzyme (E-P). We resolve three rotational correlation times corresponding to (i) a large-amplitude domain motion of the phosphorylation domain (phi1 approximately 5 +/- 1 micros), (ii) overall protein rotational motion with respect to the membrane normal (phi2 approximately 50 +/- 10 micros), and (iii) the rotational motion of the SR vesicles (phi3 approximately 1.1 +/- 0.4 ms). No differences are observed in the rotational dynamics of E, E x ATP, or E-P, indicating that phosphoenzyme formation or nucleotide binding result in no global structural changes involving the phosphorylation domain. In contrast, calcium activation enhances the amplitude of motion of the phosphorylation domain. These observed calcium-dependent changes in rotational dynamics result from structural changes within a single Ca-ATPase polypeptide chain, since protein-protein interactions do not change upon calcium binding. Thus, calcium binding induces concerted domain motions within a single Ca-ATPase polypeptide chain that may play a critical role in facilitating substrate binding and utilization.

Phosphorylation of phospholamban by cAMP-dependent protein kinase enhances interactions between Ca-ATPase polypeptide chains in cardiac sarcoplasmic reticulum membranes

We have used spin-label EPR spectroscopy to examine possible alterations in protein-protein interactions that accompany the activation of the cardiac sarcoplasmic reticulum (SR) Ca-ATPase following the phosphorylation of phospholamban (PLB). Using a radioactive derivative of a maleimide spin label (MSL), we have developed conditions for the selective spin-labeling of the Ca-ATPase in both native cardiac and skeletal sarcoplasmic reticulum membranes. The rotational dynamics of the cardiac and skeletal Ca-ATPase isoforms in native SR membranes were measured using saturation transfer EPR. We report that the phosphorylation of PLB in cardiac SR results in a (1.8 +/- 0.2)-fold reduction in the overall rotational mobility of the Ca-ATPase. The alteration in the rotational dynamics of the Ca-ATPase is the direct result of the phosphorylation of PLB, and is not related to the phosphorylation of the Ca-ATPase or any other SR proteins since no alteration in the ST-EPR spectrum is observed as a result of conditions that phosphorylate the cardiac Ca-ATPase with ATP. Neither do the use of conditions that activate the Ca-ATPase in cardiac SR result in the alteration of the rotational dynamics or catalytic properties of the Ca-ATPase in skeletal SR where PLB is not expressed. Measurements of the rotational dynamics of stearic acid spin labels (SASL) incorporated into cardiac SR membranes with a nitroxide at the 5- and 12-positions using conventional EPR indicate that there is virtually no difference in the lipid acyl chain dynamics in cardiac SR membranes upon the phosphorylation of PLB. These results indicate that the decrease in the rotational dynamics of the Ca-ATPase in cardiac SR membranes associated with the phosphorylation of PLB is related to enhanced interactions between individual Ca-ATPase polypeptide chains due to (i) an alteration in the spatial arrangement of cardiac Ca-ATPase polypeptide chains within a defined oligomeric state or (ii) increased protein-protein associations. We suggest that altered interactions between Ca-ATPase polypeptide chains and PLB serves to modulate the activation barrier associated with calcium activation of the Ca-ATPase in cardiac SR membranes.

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.

Interactions of phosphorylation and dimerizing domains of the alpha-subunits of Na+/K(+)-ATPase

Chemical cross-linking studies are among a number of experimental approaches that have suggested the functional significance of higher association states of alpha,beta-protomers of Na+/K(+)-ATPase. Formation of the phosphointermediate of the enzyme on Asp369 of the alpha-subunit is known to induce oxidative cross-linking of the alpha-subunits catalyzed by Cu(2+)-phenanthroline. To localize the phosphorylation-induced alpha,alpha-interface, we cleaved alpha at Arg438-Ala439 by controlled proteolysis and exposed the partially cleaved enzyme to the cross-linking reagent. In addition to the alpha,alpha-dimer, two other phosphorylation-induced cross-linked products were obtained. Using gel electrophoretic resolution of the cross-linked 32P-labeled enzyme, N-terminal analyses of the products, and their reactivities with sequence-specific antibodies, the two products were identified as a homodimer of the C-terminal 64-kDa fragment of alpha and a heterodimer of alpha and the 64-kDa peptide. The latter dimer was also obtained when the cross-linked alpha,alpha-dimer was formed first and then subjected to proteolysis. The findings localize the dimerizing domain to the C-terminal side of Ala439 and indicate that intersubunit proximities of dimerizing domains are regulated by phosphorylation-dephosphorylation of Asp369 during the reaction cycle of the enzyme.

E1/E2 type cation transport ATPases: evidence for transient associations between protomers

E1/E2 type cation transport ATPases are known to exist in different conformeric states. Recent evidence characterizing these conformers in membrane is reviewed. A consensus view is proposed in which E2 conformers tend to form oligomeric complexes by lateral association between monomeric protomers and E1 conformers exhibit the opposite behaviour. It is suggested that transient associations between monomers during cation pump cycles may be a common feature of the ion translocation mechanism under physiological conditions.

Localization of the hinge region of the Ca(2+)-ATPase of sarcoplasmic reticulum using resonance energy transfer

The Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum can be labelled at Cys-670 and Cys-674 with 5-[[2-[(iodoacetyl) amino]ethyl]amino]naphthalene-1-sulphonic acid (IAEDANS). Resonance energy transfer has been used to measure the distance between Cys-670/Cys-674 and Glu-439 labelled with 5-(bromomethyl)fluorescein as 40 A. The height of Cys-670/Cys-674 above the phospholipid/water interface has been measured by resonance energy transfer between IAEDANS-labelled ATPase and fluorescein-labelled phosphatidylethanolamine as 54 A. This locates the hinge region of the ATPase close to the mouth of the pore observed in the cytoplasmic region of the ATPase in electron micrographs. No significant changes in these distances can be detected by resonance energy transfer on binding Ca2+ or vanadate. The height of the IAEDANS label above the phospholipid/water interface is the same for bilayers of dimyristoleoylphosphatidylcholine and dioleoylphosphatidylcholine. Conformation changes on the Ca(2+)-ATPase appear to be localised to small regions of the ATPase.

Chemical modification of the Ca(2+)-ATPase of rabbit skeletal muscle sarcoplasmic reticulum: identification of sites labeled with aryl isothiocyanates and thiol-directed conformational probes

The Ca(2+)-ATPase protein of rabbit skeletal muscle sarcoplasmic reticulum is a single polypeptide chain of 1001 amino-acid residues. Among these residues are 24 Cys, 9 of which have previously been shown to be accessible to one or more thiol-specific reagents. Many studies on the structure and function of this Ca(2+)-ATPase have made use of sulfhydryl-directed, conformationally-sensitive probes, but the labeling sites for these probes have been directly identified in only a few cases, causing uncertainty in the interpretation of results. In the present work, we have investigated the Ca(2+)-ATPase labeling sites for three thiol-directed spectroscopic probes: fluorescein 5'-maleimide (Fmal), 4-dimethylaminophenyl-azo phenyl-4'-maleimide (DABmal), and 4-dimethylaminophenylazophenyl-4'-iodoacetamide (DABIA). Labeled Ca(2+)-ATPase was digested exhaustively with trypsin, and labeled peptides were purified and sequenced in order to identify the labeled Cys residues. Our results do not support the widely held assumptions that Cys-344 and Cys-364 are the most reactive residues with maleimide-based reagents, while Cys-670 and Cys-674 react most rapidly with iodoacetamide derivatives. We found instead that Fmal reacted most rapidly with Cys-471, followed by Cys-364, and more slowly with Cys-498, -525, -614 and -636. DABmal reacted most rapidly with Cys-364, followed by Cys-614, and more slowly with Cys-471, -498, -636 and -670. Cys-344 was not labeled by either Fmal or DABmal. DABIA reacted with the same six Cys residues, including Cys-670, as were labeled with DABmal, but in much lower yield. There was no evidence for labeling of Cys-674 with DABIA. The high reactivity of Fmal, but not the more hydrophobic DABmal, with Cys-471 is of interest because of previous studies suggesting that the accessibility of Cys-471 is influenced by ATP and that fluorescein derivatives bind to a hydrophobic pocket in the ATP binding site. Another derivative, fluorescein-5'-isothiocyanate (FITC), is thought to label the catalytic site of the Ca(2+)-ATPase and has been widely used as a conformational probe in structure-function studies on this and related proteins. We reinvestigated the chemical modification of the Ca(2+)-ATPase by FITC and 4-dimethyl-aminophenyl-4'-isothiocyanate (DABITC). Incorporation of stoichiometric amounts of FITC resulted in a nearly complete loss of ATPase activity. Labeling and inactivation of the Ca(2+)-ATPase by FITC did not occur in the presence of ATP. DABITC was less reactive than FITC, and did not inactivate the Ca(2+)-ATPase to any significant extent.(ABSTRACT TRUNCATED AT 400 WORDS)

Intramolecular distances within the Ca(2+)-ATPase from sarcoplasmic reticulum as estimated through fluorescence energy transfer between probes

Fluorescence energy transfer measurements have been carried out to estimate intramolecular distances between probes bound to Ca(2+)-transporting ATPase (Ca(2+)-ATPase) as well as distances between these probes and the phospholipid headgroup. The nucleotide binding site was monitored by using 1,N6-ethenoadenosine 5'-triphosphate, a fluorescent analogue of ATP, and also by labelling Lys515 with fluorescein 5'-isothiocyanate. Three different cysteine residues were individually labelled using the following probes: 5-[(2-iodoacetyl)aminoethyl]amino-naphthalene-1-sulfonic acid (I-AEDANS), 7-chloro-4-nitro-2,1,3-benzoxadiazole (NBD-Cl) and fluorescent maleimides. The surface of the membrane was labelled by reconstitution with fluorescent phospholipids (fluorescein and rhodamine derivatives). We found a distance of 4.1 nm from the nucleotide binding site to NBD (at Cys344), and the same distance to fluorescent maleimides (at Cys364). The AEDANS label (at Cys670,672) was found separated 3.5 nm from NBD, 4.4 nm from fluorescent maleimides, and 3.9 nm from the lipid matrix. The NBD label was 3.2 nm apart from fluorescent maleimides and 2.2 nm from the lipid matrix. Finally, fluorescent maleimides were found to be located 4.2 nm above the membrane surface. All these distances agree with a molecular model in which NBD is located in the stalk portion of the Ca(2+)-ATPase, near the surface of the membrane, and the rest of the probes are above it, in the globular domain of the protein.