The cytoplasmic loop located between transmembrane segments 6 and 7 controls activation by Ca2+ of sarcoplasmic reticulum Ca2+-ATPase

Deciphering the Mechanism of Inhibition of SERCA1a by Sarcolipin Using Molecular Simulations

SERCA1a is an ATPase calcium pump that transports Ca²⁺ from the cytoplasm to the sarco/endoplasmic reticulum lumen. Sarcolipin (SLN), a transmembrane peptide, regulates the activity of SERCA1a by decreasing its Ca²⁺ transport rate, but its mechanism of action is still not well-understood. To decipher this mechanism, we have performed normal mode analysis in the all-atom model, with the SERCA1a-SLN complex, or the isolated SERCA1a, embedded in an explicit membrane. The comparison of the results allowed us to provide an explanation at the atomic level for the action of SLN that is in good agreement with experimental observations. In our analyses, the presence of SLN locally perturbs the TM6 transmembrane helix and as a consequence modifies the position of D800, one of the key metal-chelating residues. Additionally, it reduces the flexibility of the gating residues, V304, and E309 in TM4, at the entrance of the Ca²⁺ binding sites, which would decrease the affinity for Ca²⁺. Unexpectedly, SLN has also an effect on the ATP binding site more than 35 Å away, due to the straightening of TM5, a long helix considered as the spine of the protein. The straightening of TM5 modifies the structure of the P-N linker that sits above it, and which comprises the ³⁵¹DKTG³⁵⁴ conserved motif, resulting in an increase of the distance between ATP and the phosphorylation site. As a consequence, the turn-over rate could be affected. All this gives SERCA1a the propensity to go toward a Ca²⁺ low-affinity E2-like state in the presence of SLN and toward a Ca²⁺ high-affinity E1-like state in the absence of SLN. In addition to a general mechanism of inhibition of SERCA1a regulatory peptides, this study also provides an insight into the conformational transition between the E2 and E1 states.

The SERCA residue Glu340 mediates interdomain communication that guides Ca2+ transport

We present a crystal structure, functional data, and molecular dynamics (MD) simulations of the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) mutant E340A. The mutation slows Ca²⁺-binding kinetics, and the structural differences between wild type and E340A indicate that the mutation disrupts a central interdomain “communication hub” governing Ca²⁺ binding/dissociation. MD simulations reveal altered dynamics in regions mediating Ca²⁺ occlusion, a critical step in SERCA’s alternating access mechanism. The mutation stabilizes a more occluded state of the Ca²⁺ sites. The strict conservation of Glu340 among P-type ATPases is the result of its critical role in interdomain communication between the cytosolic headpiece and the transmembrane domain, ensuring a delicate balance between dynamics of ion binding, occlusion, and release—key steps in the transport process.

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) is a P-type ATPase that transports Ca²⁺ from the cytosol into the sarco(endo)plasmic reticulum (SR/ER) lumen, driven by ATP. This primary transport activity depends on tight coupling between movements of the transmembrane helices forming the two Ca²⁺-binding sites and the cytosolic headpiece mediating ATP hydrolysis. We have addressed the molecular basis for this intramolecular communication by analyzing the structure and functional properties of the SERCA mutant E340A. The mutated Glu340 residue is strictly conserved among the P-type ATPase family of membrane transporters and is located at a seemingly strategic position at the interface between the phosphorylation domain and the cytosolic ends of 5 of SERCA’s 10 transmembrane helices. The mutant displays a marked slowing of the Ca²⁺-binding kinetics, and its crystal structure in the presence of Ca²⁺ and ATP analog reveals a rotated headpiece, altered connectivity between the cytosolic domains, and an altered hydrogen bonding pattern around residue 340. Supported by molecular dynamics simulations, we conclude that the E340A mutation causes a stabilization of the Ca²⁺ sites in a more occluded state, hence displaying slowed dynamics. This finding underpins a crucial role of Glu340 in interdomain communication between the headpiece and the Ca²⁺-binding transmembrane region.

SERCA mutant E309Q binds two Ca(2+) ions but adopts a catalytically incompetent conformation

The sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) couples ATP hydrolysis to transport of Ca(2+). This directed energy transfer requires cross-talk between the two Ca(2+) sites and the phosphorylation site over 50 Å distance. We have addressed the mechano-structural basis for this intramolecular signal by analysing the structure and the functional properties of SERCA mutant E309Q. Glu(309) contributes to Ca(2+) coordination at site II, and a consensus has been that E309Q only binds Ca(2+) at site I. The crystal structure of E309Q in the presence of Ca(2+) and an ATP analogue, however, reveals two occupied Ca(2+) sites of a non-catalytic Ca2E1 state. Ca(2+) is bound with micromolar affinity by both Ca(2+) sites in E309Q, but without cooperativity. The Ca(2+)-bound mutant does phosphorylate from ATP, but at a very low maximal rate. Phosphorylation depends on the correct positioning of the A-domain, requiring a shift of transmembrane segment M1 into an 'up and kinked position'. This transition is impaired in the E309Q mutant, most likely due to a lack of charge neutralization and altered hydrogen binding capacities at Ca(2+) site II.

Tracing cytoplasmic Ca(2+) ion and water access points in the Ca(2+)-ATPase

Sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) transports two Ca(2+) ions across the membrane of the sarco(endo)plasmic reticulum against the concentration gradient, harvesting the required energy by hydrolyzing one ATP molecule during each transport cycle. Although SERCA is one of the best structurally characterized membrane transporters, it is still largely unknown how the transported Ca(2+) ions reach their transmembrane binding sites in SERCA from the cytoplasmic side. Here, we performed extended all-atom molecular dynamics simulations of SERCA. The calculated electrostatic potential of the protein reveals a putative mechanism by which cations may be attracted to and bind to the Ca(2+)-free state of the transporter. Additional molecular dynamics simulations performed on a Ca(2+)-bound state of SERCA reveal a water-filled pathway that may be used by the Ca(2+) ions to reach their buried binding sites from the cytoplasm. Finally, several residues that are involved in attracting and guiding the cations toward the possible entry channel are identified. The results point to a single Ca(2+) entry site close to the kinked part of the first transmembrane helix, in a region loaded with negatively charged residues. From this point, a water pathway outlines a putative Ca(2+) translocation pathway toward the transmembrane ion-binding sites.

Transmembrane helix 11 is a genuine regulator of the endoplasmic reticulum Ca2+ pump and acts as a functional parallel of β-subunit on α-Na+,K+-ATPase

The housekeeping sarco(endo)plasmic reticulum Ca(2+) ATPase SERCA2b transports Ca(2+) across the endoplasmic reticulum membrane maintaining a vital Ca(2+) gradient. Compared with the muscle-specific isoforms SERCA2a and SERCA1a, SERCA2b houses an 11th transmembrane segment (TM11) and a short luminal extension (LE) at its C terminus (2b-tail). The 2b-tail imposes a 2-fold higher apparent Ca(2+) affinity and lower V(max). Previously, we assumed that LE is the sole functional region of the 2b-tail and that TM11 is a passive element providing an additional membrane passage. However, here we show that peptides corresponding to the TM11 region specifically modulate the activity of the homologous SERCA1a in co-reconstituted proteoliposomes and mimic the 2b-tail effect (i.e. lower V(max) and higher Ca(2+) affinity). Using truncated 2b-tail variants we document that TM11 regulates SERCA1a independently from LE, confirming that TM11 is a second, previously unrecognized functional region of the 2b-tail. A phylogenetic analysis further indicates that TM11 is the oldest and most conserved feature of the 2b-tail, found in the SERCA pump of all Bilateria, whereas LE is only present in Nematoda and vertebrates. Considering remarkable similarities with the Na(+),K(+)-ATPase α-β interaction, we now propose a model for interaction of TM11 with TM7 and TM10 in the anchoring subdomain of the Ca(2+) pump. This model involves a TM11-induced helix bending of TM7. In conclusion, more than just a passive structural feature, TM11 acts as a genuine regulator of Ca(2+) transport through interaction with the pump.

A diversity of SERCA Ca2+ pump inhibitors

The SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) is probably the most extensively studied membrane protein transporter. There is a vast array of diverse inhibitors for the Ca2+ pump, and many have proved significant in helping to elucidate both the mechanism of transport and gaining conformational structures. Some SERCA inhibitors such as thapsigargin have been used extensively as pharmacological tools to probe the roles of Ca2+ stores in Ca2+ signalling processes. Furthermore, some inhibitors have been implicated in the cause of diseases associated with endocrine disruption by environmental pollutants, whereas others are being developed as potential anticancer agents. The present review therefore aims to highlight some of the wide range of chemically diverse inhibitors that are known, their mechanisms of action and their binding location on the Ca2+ ATPase. Additionally, some ideas for the future development of more useful isoform-specific inhibitors and anticancer drugs are presented.

The sarcoplasmic Ca2+-ATPase: design of a perfect chemi-osmotic pump

The sarcoplasmic (SERCA 1a) Ca2+-ATPase is a membrane protein abundantly present in skeletal muscles where it functions as an indispensable component of the excitation-contraction coupling, being at the expense of ATP hydrolysis involved in Ca2+/H+ exchange with a high thermodynamic efficiency across the sarcoplasmic reticulum membrane. The transporter serves as a prototype of a whole family of cation transporters, the P-type ATPases, which in addition to Ca2+ transporting proteins count Na+, K+-ATPase and H+, K+-, proton- and heavy metal transporting ATPases as prominent members. The ability in recent years to produce and analyze at atomic (2·3-3 Å) resolution 3D-crystals of Ca2+-transport intermediates of SERCA 1a has meant a breakthrough in our understanding of the structural aspects of the transport mechanism. We describe here the detailed construction of the ATPase in terms of one membraneous and three cytosolic domains held together by a central core that mediates coupling between Ca2+-transport and ATP hydrolysis. During turnover, the pump is present in two different conformational states, E1 and E2, with a preference for the binding of Ca2+ and H+, respectively. We discuss how phosphorylated and non-phosphorylated forms of these conformational states with cytosolic, occluded or luminally exposed cation-binding sites are able to convert the chemical energy derived from ATP hydrolysis into an electrochemical gradient of Ca2+ across the sarcoplasmic reticulum membrane. In conjunction with these basic reactions which serve as a structural framework for the transport function of other P-type ATPases as well, we also review the role of the lipid phase and the regulatory and thermodynamic aspects of the transport mechanism.

A structural overview of the plasma membrane Na+,K+-ATPase and H+-ATPase ion pumps

Plasma membrane ATPases are primary active transporters of cations that maintain steep concentration gradients. The ion gradients and membrane potentials derived from them form the basis for a range of essential cellular processes, in particular Na(+)-dependent and proton-dependent secondary transport systems that are responsible for uptake and extrusion of metabolites and other ions. The ion gradients are also both directly and indirectly used to control pH homeostasis and to regulate cell volume. The plasma membrane H(+)-ATPase maintains a proton gradient in plants and fungi and the Na(+),K(+)-ATPase maintains a Na(+) and K(+) gradient in animal cells. Structural information provides insight into the function of these two distinct but related P-type pumps.

Molecular dynamics simulation exploration of cooperative migration mechanism of calcium ions in sarcoplasmic reticulum Ca2+-ATPase

Calcium ATPase is a member of the P-type ATPase, and it pumps calcium ions from the cytoplasm into the reticulum against a concentration gradient. Several X-ray structures of different conformations have been solved in recent years, providing basis for elucidating the active transport mechanism of Ca2+ ions. In this work, molecular dynamics (MD) simulations were performed at atomic level to investigate the dynamical process of calcium ions moving from the outer mouth of the protein to their binding sites. Five initial locations of Ca2+ ions were considered, and the simulations lasted for 2 or 6 ns, respectively. Specific pathways leading to the binding sites and large structural rearrangements around binding sites caused by uptake of calcium ions were identified. A cooperative binding mechanism was observed from our simulation. Firstly, the first Ca2+ ion binds to site I, and then, the second Ca2+ ion approaches. The interactions between the second Ca2+ and the residues around site I disturb the binding state of site I and weaken its binding ability for the first bound Ca2+. Because of the electrostatic repulsion of the second Ca2+ and the electrostatic attraction of site II, the first bound Ca2+ shifts from site I to site II. Concertedly, the second Ca2+ binds to site I, forming a binding state with two Ca2+ ions, one at site I and the other at site II. Both of Glu908 and Asp800 coordinate with the two Ca2+ ions simultaneously during the concerted binding process, which is believed to be the hinge to achieve the concerted binding. In our simulations, four amino acid residues that serve as the channel to link the outer mouth and the binding sites during the binding process were recognized, namely Tyr837, Tyr763, Asn911, and Ser767. The analyses regarding the activity of the proteins via mutations of some key residues also supported our cooperative mechanism.

Role of the charged amino acid residues in the cytoplasmic loop between putative transmembrane segments 6 and 7 of Na+-ATPase of an alkaliphilic bacterium, Exiguobacterium aurantiacum

ATPase activity of the membrane-bound Na(+)-ATPase of an alkaliphilic bacterium, Exiguobacterium aurantiacum, was measured in various concentrations of NaCl. Hill plot analysis showed a Hill number of 1.7 with 5.2 mM as the K(0.5) value for Na(+). When the site-directed mutagenesis of seven charged amino acid residues in the cytoplasmic loop (L6/7) between putative transmembrane segments 6 and 7 of the enzyme was conducted, all the mutated enzymes exhibited Hill numbers close to that of the wild-type enzyme (WT). When reconstituted with lecithin, all the mutants exhibited Na(+)-transport activity. While alanine substitution for several residues gave some significant effects on the enzyme function, the most remarkable effect was observed in the substitution for Glu-733. The K(0.5) value of E733A for Na(+) was 83.2 mM. The mutant exhibited only 8.5% of the ATPase activity and 54.0% of the energy-coupling efficiency for Na(+) transport as compared with those of WT, respectively. Drastic decreases of apparent affinity for Na(+) and energy efficiency of ion transport were also observed in E733K and E733T, respectively.

Use of glycerol-containing media to study the intrinsic fluorescence properties of detergent-solubilized native or expressed SERCA1a

Rapid irreversible inactivation of Ca (2+)-free states of detergent-solubilized SERCA1a (sarco-endoplasmic reticulum calcium ATPase 1a) has so far prevented the use of Trp fluorescence for functional characterization of this ATPase after its solubilization in various detergents. Here we show that using 20-40% glycerol for protection makes this fluorescence characterization possible. Most of the ligand-induced Trp fluorescence changes previously demonstrated to occur for SERCA1a embedded in native sarcoplasmic reticulum membranes were observed in the combined presence of glycerol and detergent, although the results greatly depended on the detergent used, namely, octaethylene glycol mono- n-dodecyl ether (C 12E 8) or dodecyl maltoside (DDM). In particular, at pH 6, we found a C 12E 8-dependent unexpectedly huge reduction in SERCA1a affinity for Ca (2+). We suggest that a major reason for the different effects of the two detergents is that high concentrations of C 12E 8, but not of DDM, slow down the E2 to E1 transition in solubilized and delipidated SERCA1a. Independently of the characterization of the specific effects of various detergents on SR vesicles, our results open the way to functional characterization by Trp fluorescence of heterologously expressed and purified mutants of SERCA1a in the presence of detergent, without their preliminary reconstitution into liposomes. As an example, we used the E309Q mutant to demonstrate our previous suspicion that Ca (2+) binding to Site I of SERCA1a in fact slightly reduces Trp fluorescence, and consequently that the rise in this fluorescence generally observed when two Ca (2+) ions bind to WT SERCA1a mainly reflects Ca (2+) binding at Site II of SERCA1a.

The widely utilized brominated flame retardant tetrabromobisphenol A (TBBPA) is a potent inhibitor of the SERCA Ca2+ pump

TBBPA (tetrabromobisphenol A) is currently the most widely used type of BFR (brominated flame retardant) employed to reduce the combustibility of a large variety of electronic and other manufactured products. Recent studies have indicated that BFRs, including TBBPA, are bio-accumulating within animal and humans. BFRs including TBBPA have also been shown to be cytotoxic and potentially endocrine-disrupting to a variety of cells in culture. Furthermore, TBBPA has specifically been shown to cause disruption of Ca2+ homoeostasis within cells, which may be the underlying cause of its cytotoxicity. In this study, we have demonstrated that TBBPA is a potent non-isoform-specific inhibitor of the SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) (apparent K(i) 0.46-2.3 microM), thus we propose that TBBPA inhibition of SERCA contributes in some degree to Ca2+ signalling disruption. TBBPA binds directly to the SERCA without the need to partition into the phospholipid bilayer. From activity results and Ca2+-induced conformational results, it appears that the major effect of TBBPA is to decrease the SERCA affinity for Ca2+ (increasing the K(d) from approx. 1 microM to 30 microM in the presence of 10 microM TBBPA). Low concentrations of TBBPA can quench the tryptophan fluorescence of the SERCA and this quenching can be reversed by BHQ [2,5-di-(t-butyl)-1,4-hydroquinone] and 4-n-nonylphenol, but not thapsigargin, indicating that TBBPA and BHQ may be binding to similar regions in the SERCA.

Electrogenic steps of the SR Ca-ATPase enzymatic cycle and the effect of curcumin

Sarcoplasmic reticulum (SR) vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps in the presence of calcium ions. The resulting capacitive current transients are compared with those calculated on the basis of the enzymatic cycle of the calcium pump. This comparison provides information on the kinetics of the E(2)-E(1) conformational change and on its pH dependence. The alteration in the current transients following ATP concentration jumps in the presence of curcumin is examined. In particular, curcumin decreases the rate of slippage of the Ca-ATPase, and at concentrations above 10 microM reduces calcium transport by this pump.

A724A polymorphism of sarco(endo)plasmic reticulum Ca2+-ATPase 2 (SERCA2) in hypertensive patients

BACKGROUND

Impaired function of calcium ion transporter sarco(endo)plasmic reticulum Ca2+-ATPase2 (SERCA2), encoded by ATP2A2 gene, was observed in hypertension. The aim of this study was to screen for mutations in the ATP2A2 gene, in hypertensive patients compared to healthy controls.

METHODS

The frequency of a novel mutation in exon 15 of ATP2A2, coding SERCA2, was studied in 107 hypertensive patients and a control group of 50 healthy volunteers. 24-h ambulatory blood-pressure monitoring (ABPM) was carried out. ATP2A2 genotyping was performed by denaturing HPLC and sequencing.

RESULTS

In exon 15 of the ATP2A2 gene, a novel c.2171G>A polymorphism was identified that does not change the amino acid sequence. The frequency of the A allele was significantly higher in normotensive controls than in hypertensive patients (p=0.017). GA genotype carriers demonstrated a tendency towards lower blood pressure values in the doctor's office (p=0.367 systolic, p=0.439 diastolic blood pressure) and measured by ABPM.

CONCLUSIONS

Our results suggest a protective role of the A724A (c.2171G>A) polymorphism of ATP2A2 in subjects without hypertension.

ATP-binding to P-type ATPases as revealed by biochemical, spectroscopic, and crystallographic experiments

P-type ATPases form a large family of cation translocating ATPases. Recent progress in crystallography yielded several high-resolution structures of Ca(2+)-ATPase from sarco(endo)plasmic reticulum (SERCA) in various conformations. They could elucidate the conformational changes of the enzyme, which are necessary for the translocation of cations, or the mechanism that explains how the nucleotide binding is coupled to the cation transport. However, crystals of proteins are usually obtained only under conditions that significantly differ from the physiological ones and with ligands that are incompatible with the enzyme function, and both of these factors can inevitably influence the enzyme structure. Biochemical (such as mutagenesis, cleavage, and labeling) or spectroscopic experiments can yield only limited structural information, but this information could be considered relevant, because measurement can be performed under physiological conditions and with true ligands. However, interpretation of some biochemical or spectroscopic data could be difficult without precise knowledge of the structure. Thus, only a combination of both these approaches can extract the relevant information and identify artifacts. Briefly, there is good agreement between crystallographic and other experimental data concerning the overall shape of the molecule and the movement of cytoplasmic domains. On the contrary, the E1-AMPPCP crystallographic structure is, in details, in severe conflict with numerous spectroscopic experiments and probably does not represent the physiological state. Notably, the E1-ADP-AlF(4) structure is almost identical to the E1-AMPPCP, again suggesting that the structure is primarily determined by the crystal-growth conditions. The physiological relevance of the E2 and E2-P structures is also questionable, because the crystals were prepared in the presence of thapsigargin, which is known to be a very efficient inhibitor of SERCA. Thus, probably only crystals of E1-2Ca conformation could reflect some physiological state. Combination of biochemical, spectroscopic, and crystallographic data revealed amino acids that are responsible for the interaction with the nucleotide. High sequence homology of the P-type ATPases in the cytoplasmic domains enables prediction of the ATP-interacting amino acids also for other P-type ATPases.

Conformational changes in sarcoplasmic reticulum Ca(2+)-ATPase mutants: effect of mutations either at Ca(2+)-binding site II or at tryptophan 552 in the cytosolic domain

By analyzing, after expression in yeast and purification, the intrinsic fluorescence properties of point mutants of rabbit Ca(2+)-ATPase (SERCA1a) with alterations to amino acid residues in Ca(2+)-binding site I (E(771)), site II (E(309)), in both sites (D(800)), or in the nucleotide-binding domain (W(552)), we were able to follow the conformational changes associated with various steps in the ATPase catalytic cycle. Whereas Ca(2+) binding to purified wild-type (WT) ATPase in the absence of ATP leads to the rise in Trp fluorescence expected for the so-called E2 --> E1Ca(2) transition, the Ca(2+)-induced fluorescence rise is dramatically reduced for the E(309)Q mutant. As this purified E(309)Q mutant retains the ability to bind Ca(2+) at site I (but not at site II), we tentatively conclude that the protein reorganization induced by Ca(2+) binding at site II makes the major contribution to the overall Trp fluorescence changes observed upon Ca(2+) binding to both sites. Judging from the fluorescence response of W(552)F, similar to that of WT, these changes appear to be primarily due to membranous tryptophans, not to W(552). The same holds for the fluorescence rise observed upon phosphorylation from P(i) (the so-called E2 --> E2P transition). As for WT ATPase, Mg(2+) binding in the absence of Ca(2+) affects the fluorescence of the E(309)Q mutant, suggesting that this Mg(2+)-dependent fluorescence rise does not reflect binding of Mg(2+) to Ca(2+) sites; instead, Mg(2+) probably binds close to the catalytic site, or perhaps near transmembrane span M3, at a location recently revealed by Fe(2+)-catalyzed oxidative cleavage. Mutation of W(552) hardly affects ATP-induced fluorescence changes in the absence of Ca(2+), which are therefore mostly due to membranous Trp residues, demonstrating long-range communication between the nucleotide-binding domain and the membranous domain.

Kinetics of Ca2+ binding to the SR Ca-ATPase in the E1 state

The time-resolved kinetics of Ca2+ binding to the SR Ca-ATPase in the E1 state was investigated by Ca(2+)-concentration jump experiments. Ca2+ was released by an ultraviolet-light flash from caged calcium, and charge movements in the membrane domain of the ion pumps were detected by the fluorescent styryl dye 2BITC. The partial reaction (H3E1 <-->) E1 <--> CaE1 <--> Ca2E1 can be characterized by two time constants, tau1 and tau2, both of which are not significantly Ca(2+)-concentration-dependent and only weakly pH-dependent at pH < 7.5. Both time constants differ by a factor of approximately 50 (4.7 vs. 200 ms). The weak substrate-dependence indicates that the rate-limiting process is not related to Ca2+ migration through the access channel and ion binding to the binding sites but to conformational rearrangements preceding the ion movements. The high activation energy obtained for both processes, 42.3 kJ mol(-1) and 60.3 kJ mol(-1) at pH 7.2, support this concept. Transient binding of Ca ions to the loop L67 and a movement of the Ca-loaded loop are discussed as a mechanism that facilitates the entrance of both Ca ions into the access channel to the ion-binding sites.

Mapping the BKCa channel's "Ca2+ bowl": side-chains essential for Ca2+ sensing

There is controversy over whether Ca²⁺ binds to the BK_Ca channel's intracellular domain or its integral-membrane domain and over whether or not mutations that reduce the channel's Ca²⁺ sensitivity act at the point of Ca²⁺ coordination. One region in the intracellular domain that has been implicated in Ca²⁺ sensing is the “Ca²⁺ bowl”. This region contains many acidic residues, and large Ca²⁺-bowl mutations eliminate Ca²⁺ sensing through what appears to be one type of high-affinity Ca²⁺-binding site. Here, through site-directed mutagenesis we have mapped the residues in the Ca²⁺ bowl that are most important for Ca²⁺ sensing. We find acidic residues, D898 and D900, to be essential, and we find them essential as well for Ca²⁺ binding to a fusion protein that contains a portion of the BK_Ca channel's intracellular domain. Thus, much of our data supports the conclusion that Ca²⁺ binds to the BK_Ca channel's intracellular domain, and they define the Ca²⁺ bowl's essential Ca²⁺-sensing motif. Overall, however, we have found that the relationship between mutations that disrupt Ca²⁺ sensing and those that disrupt Ca²⁺ binding is not as strong as we had expected, a result that raises the possibility that, when examined by gel-overlay, the Ca²⁺ bowl may be in a nonnative conformation.

Functional consequences of alterations to Thr247, Pro248, Glu340, Asp813, Arg819, and Arg822 at the interfaces between domain P, M3, and L6-7 of sarcoplasmic reticulum Ca2+-ATPase. Roles in Ca2+ interaction and phosphoenzyme processing

Point mutants with alterations to amino acid residues Thr(247), Pro(248), Glu(340), Asp(813), Arg(819), and Arg(822) of sarcoplasmic reticulum Ca(2+)-ATPase were analyzed by transient kinetic measurements. In the Ca(2+)-ATPase crystal structures, most of these residues participate in a hydrogen-bonding network between the phosphorylation domain (domain P), the third transmembrane helix (M3), and the cytoplasmic loop connecting the sixth and the seventh transmembrane helices (L6-7). In several of the mutants, a pronounced phosphorylation "overshoot" was observed upon reaction of the Ca(2+)-bound enzyme with ATP, because of accumulation of dephosphoenzyme at steady state. Mutations of Glu(340) and its partners, Thr(247) and Arg(822), in the bonding network markedly slowed the Ca(2+) binding transition (E2 --> E1 --> Ca(2)E1) as well as Ca(2+) dissociation from Ca(2+) site II back toward the cytosol but did not affect the apparent affinity for vanadate. These mutations may have caused a slowing, in both directions, of the conformational change associated directly with Ca(2+) interaction at Ca(2+) site II. Because mutation of Asp(813) inhibited the Ca(2+) binding transition, but not Ca(2+) dissociation, and increased the apparent affinity for vanadate, the effect on the Ca(2+) binding transition seems in this case to be exerted by slowing the E2 --> E1 conformational change. Because the rate was not significantly enhanced by a 10-fold increase of the Ca(2+) concentration, the slowing is not the consequence of reduced affinity of any pre-binding site for Ca(2+). Furthermore, the mutations interfered in specific ways with the phosphoenzyme processing steps of the transport cycle; the transition from ADP-sensitive phosphoenzyme to ADP-insensitive phosphoenzyme (Ca(2)E1P --> E2P) was accelerated by mutations perturbing the interactions mediated by Glu(340) and Asp(813) and inhibited by mutation of Pro(248), and mutations of Thr(247) induced charge-specific changes of the rate of dephosphorylation of E2P.

The role of loop 6/7 in folding and functional performance of Na,K-ATPase

Alanine substitutions were made for 15 amino acids in the cytoplasmic loop between transmembrane helices 6 and 7 (L6/7) of the human alpha(1)-subunit of Na,K-ATPase. Most mutations reduced Na,K-ATPase activity by less than 50%; however, the mutations R834A, R837A, and R848A reduced Na,K-ATPase activity by 75, 89, and 66%, respectively. Steady-state phosphoenzyme formation from ATP was reduced in mutants R834A, R837A, and R848A, and R837A also had a faster E(2)P --> E(2) dephosphorylation rate compared with the wild-type enzyme. Effects of L6/7 mutations on the phosphorylation domain of the protein were also demonstrated by (18)O exchange, which showed that intrinsic rate constants for P(i) binding and/or reaction with the protein were altered. Although most L6/7 mutations had no effect on the interaction of Na(+) or K(+) with Na,K-ATPase, the E825A, E828A, R834A, and R837A mutations reduced the apparent affinity of the enzyme for both Na(+) and K(+) by 1.5-3-fold. 1-Bromo-2,4,6-tris(methylisothiouronium)benzene (Br-TITU(3+)), a competitive antagonist of Rb(+) and Na(+) occlusion, was used to test whether charged residues in L6/7 are involved in binding monovalent cations and cation antagonists. Br-TITU(3+) inhibited ouabain binding to wild type Na,K-ATPase with an IC(50) of 30 microM. Ouabain binding to the E825A, E828A, R834A, or R837A mutants was still inhibited by Br-TITU(3+), indicating that Br-TITU(3+) does not bind to charged residues in L6/7. This observation makes it unlikely that L6/7 functions as a cytoplasmic cation binding site in Na,K-ATPase, and together with the effects of L6/7 mutations on phosphate interactions with the enzyme suggests that L6/7 is important in stabilizing the phosphorylation domain and its relationship to the ion binding sites of the protein.

Transconformations of the SERCA1 Ca-ATPase: a normal mode study

The transport of Ca(2+) by Ca-ATPase across the sarcoplasmic reticulum membrane is accompanied by several transconformations of the protein. Relying on the already established functional importance of low-frequency modes in dynamics of proteins, we report here a normal mode analysis of the Ca(2+)-ATPase based on the crystallographic structures of the E1Ca(2) and E2TG forms. The lowest-frequency modes reveal that the N and A(+Nter) domains undergo the largest amplitude movements. The dynamical domain analysis performed with the DomainFinder program suggests that they behave as rigid bodies, unlike the highly flexible P domain. We highlight two types of movements of the transmembrane helices: i), a concerted movement around an axis perpendicular to the membrane which "twists open" the lumenal side of the protein and ii), an individual translational and rotational mobility which is of lower amplitude for the helices hosting the calcium binding sites. Among all modes calculated for E1Ca, only three are enough to describe the transition to E2TG; the associated movements involve almost exclusively the A and N domains, reflecting the closure of the cytoplasmic headpiece and high displacement of the L7-8 lumenal loop. Subsequently, we discuss the potential contribution of the remaining low-frequency normal modes to the transconformations occurring within the overall calcium transport cycle.

Involvement of the L6-7 loop in SERCA1a Ca2+-ATPase activation by Ca2+ (or Sr2+) and ATP

Wild-type (WT) and the double mutant D813A,D818A (ADA) of the L6-7 loop of SERCA1a were expressed in yeast, purified, and reconstituted into lipids. This allowed us to functionally study these ATPases by both kinetic and spectroscopic means, and to solve previous discrepancies in the published literature about both experimental facts and interpretation concerning the role of this loop in P-type ATPases. We show that in a solubilized state, the ADA mutant experiences a dramatic decrease of its calcium-dependent ATPase activity. On the contrary, reconstituted in a lipid environment, it displays an almost unaltered maximal calcium-dependent ATPase activity at high (millimolar) ATP, with an apparent affinity for Ca(2+) altered only moderately (3-fold). In the absence of ATP, the true affinity of ADA for Ca(2+) is, however, more significantly reduced (20-30-fold) compared with WT, as judged from intrinsic (Trp) or extrinsic (fluorescence isothiocyanate) fluorescence experiments. At low ATP, transient kinetics experiments reveal an overshoot in the ADA phosphorylation level primarily arising from the slowing down of the transition between the nonphosphorylated "E2" and "Ca(2)E1" forms of ADA. At high ATP, this slowing down is only partially compensated for, as ADA turnover remains more sensitive to orthovanadate than WT turnover. ADA ATPase also proved to have a reduced affinity for ATP in studies performed under equilibrium conditions in the absence of Ca(2+), highlighting the long range interactions between L6-7 and the nucleotide-binding site. We propose that these mutations in L6-7 could affect protonation-dependent winding and unwinding events in the nearby M6 transmembrane segment.

Functional Properties of Sarcoplasmic Reticulum Ca2+-ATPase after Proteolytic Cleavage at Leu119-Lys120, Close to the A-domain

By measuring the phosphorylation levels of individual proteolytic fragments of SERCA1a separated by electrophoresis after their phosphorylation, we were able to study the catalytic properties of a p95C-p14N complex arising from SERCA1a cleavage by proteinase K between Leu(119) and Lys(120), in the loop linking the A-domain with the second transmembrane segment. ATP hydrolysis by the complex was very strongly inhibited, although ATP-dependent phosphorylation and the conversion of the ADP-sensitive E1P form to E2P still occurred at appreciable rates. However, the rate of subsequent dephosphorylation of E2P was inhibited to a dramatic extent, and this was also the case for the rate of "backdoor" formation of E2P from E2 and P(i). E2P formation from E2 at equilibrium nevertheless indicated little change in the apparent affinity for P(i) or Mg(2+), while binding of orthovanadate was weaker. The p95C-p14N complex also had a slightly reduced affinity for Ca(2+) and exhibited a reduced rate for its Ca(2+)-dependent transition from E2 to Ca(2)E1. Thus, disruption of the N-terminal link of the A-domain with the transmembrane region seems to shift the conformational equilibria of Ca(2+)-ATPase from the E1/E1P toward the E2/E2P states and to increase the activation energy for dephosphorylation of Ca(2+)-ATPase, reviving the old idea of the A-domain being a phosphatase domain as part of the transduction machinery.

Evidence for tryptophan residues in the cation transport path of the Na(+),K(+)-ATPase

A family of aryl isothiouronium derivatives was designed as probes for cation binding sites of Na(+),K(+)-ATPase. Previous work showed that 1-bromo-2,4,6-tris(methylisothiouronium)benzene (Br-TITU) acts as a competitive blocker of Na(+) or K(+) occlusion. In addition to a high-affinity cytoplasmic site (K(D) < 1 microM), a low-affinity site (K(D) approximately 10 microM) was detected, presumably extracellular. Here we describe properties of Br-TITU as a blocker at the extracellular surface. In human red blood cells Br-TITU inhibits ouabain-sensitive Na(+) transport (K(D) approximately 30 microM) in a manner antagonistic with respect to extracellular Na(+). In addition, Br-TITU impairs K(+)-stimulated dephosphorylation and Rb(+) occlusion from phosphorylated enzyme of renal Na(+),K(+)-ATPase, consistent with binding to an extracellular site. Incubation of renal Na(+),K(+)-ATPase with Br-TITU at pH 9 irreversibly inactivates Na(+),K(+)-ATPase activity and Rb(+) occlusion. Rb(+) or Na(+) ions protect. Preincubation of Br-TITU with red cells in a K(+)-free medium at pH 9 irreversibly inactivates ouabain-sensitive (22)Na(+) efflux, showing that inactivation occurs at an extracellular site. K(+), Cs(+), and Li(+) ions protect against this effect, but the apparent affinity for K(+), Cs(+), or Li(+) is similar (K(D) approximately 5 mM) despite their different affinities for external activation of the Na(+) pump. Br-TITU quenches tryptophan fluorescence of renal Na(+),K(+)-ATPase or of digested "19 kDa membranes". After incubation at pH 9 irreversible loss of tryptophan fluorescence is observed and Rb(+) or Na(+) ions protect. The Br-TITU appears to interact strongly with tryptophan residue(s) within the lipid or at the extracellular membrane-water interface and interfere with cation occlusion and Na(+),K(+)-ATPase activity.

Structure and mechanism of Na,K-ATPase: functional sites and their interactions

The cell membrane Na,K-ATPase is a member of the P-type family of active cation transport proteins. Recently the molecular structure of the related sarcoplasmic reticulum Ca-ATPase in an E1 conformation has been determined at 2.6 A resolution. Furthermore, theoretical models of the Ca-ATPase in E2 conformations are available. As a result of these developments, these structural data have allowed construction of homology models that address the central questions of mechanism of active cation transport by all P-type cation pumps. This review relates recent evidence on functional sites of Na,K-ATPase for the substrate (ATP), the essential cofactor (Mg(2+) ions), and the transported cations (Na(+) and K(+)) to the molecular structure. The essential elements of the Ca-ATPase structure, including 10 transmembrane helices and well-defined N, P, and A cytoplasmic domains, are common to all PII-type pumps such as Na,K-ATPase and H,K-ATPases. However, for Na,K-ATPase and H,K-ATPase, which consist of both alpha- and beta-subunits, there may be some detailed differences in regions of subunit interactions. Mutagenesis, proteolytic cleavage, and transition metal-catalyzed oxidative cleavages are providing much evidence about residues involved in binding of Na(+), K(+), ATP, and Mg(2+) ions and changes accompanying E1-E2 or E1-P-E2-P conformational transitions. We discuss this evidence in relation to N, P, and A cytoplasmic domain interactions, and long-range interactions between the active site and the Na(+) and K(+) sites in the transmembrane segments, for the different steps of the catalytic cycle.

Involvement of the cytoplasmic loop L6-7 in the entry mechanism for transport of Ca2+ through the sarcoplasmic reticulum Ca2+-ATPase

We have found that despite a markedly low calcium affinity the D813A/D818A mutant is capable, after complexation with Cr.ATP, of occluding Ca(2+) to the same extent (1-2 Ca(2+) per ATPase monomer) as wild- type ATPase. The inherent ability of the synthetic L6-7 loop peptide to bind Ca(2+) was demonstrated with murexide and mass spectrometry. NMR analysis indicated the formation of specific 1:1 cation complexes of the peptide with calcium and lanthanum with coordination by all three aspartate residues D813/D815/D818 that resulted in an altered conformation of the peptide chain. Overall our observations suggest that, in addition to mediating contact between the intramembranous Ca(2+) binding sites and the cytosolic phosphorylation site as previously suggested, the L6-7 loop, in a preceding step, participates in the formation of an entrance port important for lodging Ca(2+) at a high-affinity binding site inside the membrane.

Site-directed mutagenesis of amino acids in the cytoplasmic loop 6/7 of Na,K-ATPase

The loop between transmembrane helices 6 and 7 (L6/7) of P-type ATPases has been suggested to be important for the functional linkage of ion binding and enzyme phosphorylation or to be a site of initial cation binding. To investigate the role of L6/7 in Na,K-ATPase, alanine substitutions were made for charged and conserved residues in L6/7 of the human alpha1 subunit and the proteins were expressed in yeast for analysis. All mutants except the triple mutant E825A/E828A/D830A bound ouabain. Although the equilibrium dissociation constant for ouabain binding by most mutants was similar to the wild-type value, the K(d) of R837A for ouabain binding was approximately 15-fold higher than the wild-type K(d). (18)O exchange measurements indicated that the apparent affinity of this mutant for Pi was reduced about 3-fold. The concentration dependence of KCl inhibition of ouabain binding or of NaCl inhibition of ouabain binding revealed 2-4-fold changes in the apparent affinity for cations in the E825A, E828A, and R837A mutants. The E825A and E828A mutants lost the ability to bind ouabain after extraction with 0.1% SDS or after brief heating, indicating that these mutations affected the stability of the enzyme. The ATPase activity of the other mutants was measured after extraction of crude yeast membranes with 0.1% SDS. For all mutants except R834A, R837A, and R848A, the activity was at least 50% of wild-type activity.

Interaction of an aromatic dibromoisothiouronium derivative with the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum

Isothiouronium compounds [Hoving, S., Bar-Shimon, M., Tijmes, J. J., Goldshleger, R., Tal, D. M., and Karlish, S. J. (1995) J. Biol. Chem. 270, 29788-29793] act as high-affinity competitive antagonists for Na(+) and K(+) (Rb(+)) on the renal Na(+)/K(+)-ATPase where they favor the E1 conformation. We have now characterized the effects of 1,3-dibromo-2,4,6-tris(methylisothiouronium)benzene (Br(2)-TITU) on the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum. Br(2)-TITU inhibited the Ca(2+)-ATPase, both transport and catalytic activity, with a K(0.5) of 5-15 microM. Maximum inhibition was at 10 min with t(0.5) of 3-5 min. Br(2)-TITU, 100 microM, quenched Trp autofluorescence by 80%, but the residual signal still responded to Ca(2+) binding. Maximum quenching of fluorescence was at pH 9.0. Total E-P levels, during the steady state of turnover of the Ca(2+)-ATPase, were increased from 0.5 to 5.8 nmol x mg(-1) by Br(2)-TITU at pH 6.8. Trinitrophenyl-ATP (TNP-ATP) superfluorescence, which monitors hydrophobicity of the ATP site, was increased 3-4-fold, suggesting that Br(2)-TITU favors an "E2"-like state. Fluorescence was also increased 3-5-fold when E-P was induced with P(i) plus EGTA. Br(2)-TITU increased the rate constants of induction of superfluorescence with ATP plus Ca(2+) from 0.32 to 0.69 s(-1) and with P(i) plus EGTA from 0.84 to 7.45 s(-1). Br(2)-TITU also decreased rate constants for "off" reactions from 2.9 to 0.66 s(-1) and from 10.9 to 0.73 s(-1) for the ATP and P(i) reactions, respectively. Br(2)-TITU, which competitively inhibits the Na(+)/K(+)-ATPase, has a novel effect on the Ca(2+)-ATPase. It promotes accumulation of E2-P species due to increased rate of formation and decreased rate of hydrolysis and quenches tryptophan autofluorescence. Br(2)-TITU could be a useful inhibitor to probe intermediate reactions of the Ca(2+)-ATPase that link catalysis with Ca(2+) translocation.

Calcium binding to the transmembrane domain of the sarcoplasmic reticulum Ca2+-ATPase: insights from molecular modeling

Sarcoplasmic reticulum Ca(2+)- ATPase pumps Ca(2+) ions from muscle cells to the sarcoplasmic reticulum. Here we use molecular dynamics and electrostatic modeling to investigate structural and dynamical features of key intermediates in the Ca(2+) binding process of the protein. Structural models of the protein (containing either two, one, or no calcium ions in the transmembrane domain) are constructed based on the X-ray structure by Toyoshima et al. (Nature 2000;405:647-655). The protein is embedded in a water/octane bilayer, which mimics the water/membrane environment. Our calculations provide information on the hydration of the two Ca(2+) ions, not emerging from the X-ray structure. Furthermore, they indicate that uptake of the metal ions causes large structural rearrangements of the metal binding sites. In addition, they suggest that the two ions reach their binding sites via two specific pathways. Finally, they allow identification of residues in the outer mouth of the protein that might interact with the Ca(2+) ions during the binding process.

Modeling the three-dimensional structure of H+-ATPase of Neurospora crassa

Homology modeling in combination with transmembrane topology predictions are used to build the atomic model of Neurospora crassa plasma membrane H+-ATPase, using as template the 2.6 A crystal structure of rabbit sarcoplasmic reticulum Ca2+-ATPase [Toyoshima, C., Nakasako, M., Nomura, H. & Ogawa, H. (2000) Nature 405, 647-655]. Comparison of the two calcium-binding sites in the crystal structure of Ca2+-ATPase with the equivalent region in the H+-ATPase model shows that the latter is devoid of most of the negatively charged groups required to bind the cations, suggesting a different role for this region. Using the built model, a pathway for proton transport is then proposed from computed locations of internal polar cavities, large enough to contain at least one water molecule. As a control, the same approach is applied to the high-resolution crystal structure of halorhodopsin and the proton pump bacteriorhodopsin. This revealed a striking correspondence between the positions of internal polar cavities, those of crystallographic water molecules and, in the case of bacteriorhodopsin, the residues mediating proton translocation. In our H+-ATPase model, most of these cavities are in contact with residues previously shown to affect coupling of proton translocation to ATP hydrolysis. A string of six polar cavities identified in the cytoplasmic domain, the most accurate part of the model, suggests a proton entry path starting close to the phosphorylation site. Strikingly, members of the haloacid dehalogenase superfamily, which are close structural homologs of this domain but do not share the same function, display only one polar cavity in the vicinity of the conserved catalytic Asp residue.

Roles of Asp54 and Asp213 in Ca2+ utilization by soluble human CD39/ecto-nucleotidase

Soluble human CD39 (solCD39) rapidly metabolizes nucleotides, especially ADP released from activated platelets, thereby inhibiting further platelet activation and recruitment. Using alanine substitution mutagenesis, we established a functional role for aspartates D54 and D213 in solCD39. Kinetic analyses of D54A and D213A indicated decreased K(m)s of the mutants, compared to wild type, for the cofactor calcium and for the substrates ADP and ATP. These decreases in calcium and nucleotide affinity of the mutants were accompanied by increases in their rate of catalysis. The decreased affinity of the mutants for calcium was responsible for their diminished ability to reverse platelet aggregation in plasma anticoagulated with citrate, a known calcium chelator. Their ADPase activity in the presence of citrated plasma was also decreased, although this could be overcome with excess calcium. Thus, aspartates 54 and 213 are involved in calcium utilization and potentially involved in cation coordination with substrate in the catalytic pocket of solCD39.

Inhibition of SERCA Ca2+ pumps by 2-aminoethoxydiphenyl borate (2-APB). 2-APB reduces both Ca2+ binding and phosphoryl transfer from ATP, by interfering with the pathway leading to the Ca2+-binding sites

2-Aminoethoxydiphenyl Borate (2-APB) has been extensively used recently as a membrane permeable modulator of inositol-1,4,5-trisphosphate-sensitive Ca2+ channels and store-operated Ca2+ entry. Here, we report that 2-APB is also an inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ pumps, and additionally increases ion leakage across the phospholipid bilayer. Therefore, we advise caution in the interpretation of results when used in Ca2+ signalling experiments. The inhibition of 2-APB on the SERCA Ca2+ pumps is isoform-dependent, with SERCA 2B being more sensitive than SERCA 1A (IC50 values for inhibition being 325 and 725 micro m, respectively, measured at pH 7.2). The Ca2+-ATPase is also more potently inhibited at lower pH (IC50 = 70 micro m for SERCA1A at pH 6). 2-APB decreases the affinity for Ca2+ binding to the ATPase by more than 20-fold, and also inhibits phosphoryl transfer from ATP (by 35%), without inhibiting nucleotide binding. Activity studies performed using mutant Ca2+-ATPases show that Tyr837 is critical for the inhibition of activity by 2-APB. Molecular modeling studies of 2-APB binding to the Ca2+ ATPase identified two potential binding sites close to this residue, near or between transmembrane helices M3, M4, M5 and M7. The binding of 2-APB to these sites could influence the movement of the loop between M6 and M7 (L6-7), and reduce access of Ca2+ to their binding sites.

Involvement of the cytoplasmic loop L6-7 in the entry mechanism for transport of Ca2+ through the sarcoplasmic reticulum Ca2+-ATPase

We previously found that mutants of conserved aspartate residues of sarcoplasmic reticulum Ca(2+)-ATPase in the cytosolic loop, connecting transmembrane segments M6 and M7 (L6-7 loop), exhibit a strongly reduced sensitivity toward Ca(2+) activation of the transport process. In this study, yeast membranes, expressing wild type and mutant Ca(2+)-ATPases, were reacted with Cr small middle dotATP and tested for their ability to occlude (45)Ca(2+) by HPLC analysis, after cation resin and C(12)E(8) treatment. We found that the D813A/D818A mutant that displays markedly low calcium affinity was capable of occluding Ca(2+) to the same extent as wild type ATPase. Using NMR and mass spectrometry we have analyzed the conformational properties of the synthetic L6-7 loop and demonstrated the formation of specific 1:1 cation complexes of the peptide with calcium and lanthanum. All three aspartate Asp(813)/Asp(815)/Asp(818) were required to coordinate the trivalent lanthanide ion. Overall these observations suggest a dual function of the loop: in addition to mediating contact between the intramembranous Ca(2+)-binding sites and the cytosolic phosphorylation site (Zhang, Z., Lewis, D., Sumbilla, C., Inesi G., and Toyoshima, C. (2001) J. Biol. Chem. 276, 15232-15239), the L6-7 loop, in a preceding step, participates in the formation of an entrance port, before subsequent high affinity binding of Ca(2+) inside the membrane.

Overproduction in yeast and rapid and efficient purification of the rabbit SERCA1a Ca(2+)-ATPase

Large amounts of heterologous C-terminally his-tagged SERCA1a Ca(2+)-ATPase were expressed in yeast using a galactose-regulated promoter and purified by Ni(2+) affinity chromatography followed by Reactive red chromatography. Optimizing the number of galactose inductions and increasing the amount of Gal4p transcription factor improved expression. Lowering the temperature from 28 degrees C to 18 degrees C during expression enhanced the recovery of solubilized and active Ca(2+)-ATPase. In these conditions, a 4 l yeast culture produced 100 mg of Ca(2+)-ATPase, 60 and 22 mg being pelleted with the heavy and light membrane fractions respectively, representing 7 and 1.7% of total proteins. The Ca(2+)-ATPase expressed in light membranes was 100% solubilized with L-alpha-lysophosphatidylcholine (LPC), 50% with n-dodecyl beta-D-maltoside (DM) and 25% with octaethylene glycol mono-n-dodecyl ether (C(12)E(8)). Compared to LPC, DM preserved specific activity of the solubilized Ca(2+)-ATPase during the chromatographic steps. Starting from 1/6 (3.8 mg) of the total amount of Ca(2+)-ATPase expressed in light membranes, 800 microg could be routinely purified to 50% purity by metal affinity chromatography and then 200 microg to 70% with Reactive red chromatography. The purified Ca(2+)-ATPase displayed the same K(m) for calcium and ATP as the native enzyme but a reduced specific activity ranging from 4.5 to 7.3 micromol ATP hydrolyzed/min/mg Ca(2+)-ATPase. It was stable and active for several days at 4 degrees C or after removal of DM with Bio-beads and storage at -80 degrees C.

Kinetics of the Ca(2+), H(+), and Mg(2+) interaction with the ion-binding sites of the SR Ca-ATPase

Electrochromic styryl dyes were used to investigate mutually antagonistic effects of Ca(2+) and H(+) on binding of the other ion in the E(1) and P-E(2) states of the SR Ca-ATPase. On the cytoplasmic side of the protein in the absence of Mg(2+) a strictly competitive binding sequence, H(2)E(1) <==> HE(1) <==> E(1) <==> CaE(1) <==> Ca(2)E(1), was found with two Ca(2+) ions bound cooperatively. The apparent equilibrium dissociation constants were in the order of K(1/2)(2 Ca) = 34 nM, K(1/2)(H) = 1 nM and K(1/2)(H(2)) = 1.32 microM. Up to 2 Mg(2+) ions were also able to enter the binding sites electrogenically and to compete with the transported substrate ions (K(1/2)(Mg) = 165 microM, K(1/2)(Mg(2)) = 7.4 mM). In the P-E(2) state, with binding sites facing the lumen of the sarcoplasmatic reticulum, the measured concentration dependence of Ca(2+) and H(+) binding could be described satisfactorily only with a branched reaction scheme in which a mixed state, P-E(2)CaH, exists. From numerical simulations, equilibrium dissociation constants could be determined for Ca(2+) (0.4 mM and 25 mM) and H(+) (2 microM and 10 microM). These simulations reproduced all observed antagonistic concentration dependences. The comparison of the dielectric ion binding in the E(1) and P-E(2) conformations indicates that the transition between both conformations is accompanied by a shift of their (dielectric) position.

Role of conserved TGDGVND-loop in Mg2+ binding, phosphorylation, and energy transfer in Na,K-ATPase

In the P-domain, the 369-DKTGTLT and the 709-GDGVNDSPALKK segment are highly conserved during evolution of P-type E1-E2-ATPase pumps irrespective of their cation specificities. The focus of this article is on evaluation of the role of the amino acid residues in the P domain of the alpha subunit of Na,K-ATPase for the E1P[3Na]--> E2P[2Na] conversion, the K+-activated dephosphorylation, and the transmission of these changes to and from the cation binding sites. Mutations of residues in the TGDGVND loop show that Asp710 is essential, and Asn713 is important, for Mg2+ binding and formation of the high-energy MgE1P[3Na] intermediate. In contrast Asp710 and Asp713 do not contribute to Mg2+ binding in the E2P-ouabain complex. Transition to E2P thus involves a shift of Mg2+ coordination away from Asp710 and Asn713 and the two residues become more important for K+-activated hydrolysis of the acyl phosphate bond at Asp369. Transmission of structural changes between the P-domain and cation sites in the membrane domain is evaluated in light of the protein structure, and the information from proteolytic or metal-catalyzed cleavage and mutagenesis studies.

Structural organization and energy transduction mechanism of Na+,K+-ATPase studied with transition metal-catalyzed oxidative cleavage

This chapter describes contributions of transition metal-catalyzed oxidative cleavage of Na+,K+-ATPase to our understanding of structure-function relations. In the presence of ascorbate/H2O2, specific cleavages are catalyzed by the bound metal and because more than one peptide bond close to the metal can be cleaved, this technique reveals proximity of the different cleavage positions within the native structure. Specific cleavages are catalyzed by Fe2+ bound at the cytoplasmic surface or by complexes of ATP-Fe2+, which directs the Fe2+ to the normal ATP-Mg2+ site. Fe2+- and ATP-Fe2+-catalyzed cleavages reveal large conformation-dependent changes in interactions between cytoplasmic domains, involving conserved cytoplasmic sequences, and a change of ligation of Mg2+ ions between E1P and E2P, which may be crucial in facilitating hydrolysis of E2P. The pattern of domain interactions in E1 and E2 conformations, and role of Mg2+ ions, may be common to all P-type pumps. Specific cleavages can also be catalyzed by Cu2+ ions, bound at the extracellular surfaces, or a hydrophobic Cu2+-diphenyl phenanthroline (DPP) complex, which directs the Cu2+ to the membrane-water interface. Cu2+ or Cu2+-DPP-catalyzed cleavages are providing information on alpha/beta subunit interactions and spatial organization of transmembrane segments. Transition metal-catalyzed cleavage could be widely used to investigate other P-type pumps and membrane proteins and, especially, ATP binding proteins.

Phospholamban domain IB forms an interaction site with the loop between transmembrane helices M6 and M7 of sarco(endo)plasmic reticulum Ca2+ ATPases

Transmembrane helix M6 of the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) has been shown to form a site of interaction with phospholamban (PLN). Site-directed mutagenesis was carried out in the cytoplasmic loop (L67) between M6 and M7 in SERCA1a to detect other SERCA-PLN binding sites. Mutants N810A, D813A, and R822A had diminished ability to interact functionally with PLN, but only D813A and R822A had reduced physical interaction with PLN. PLN mutants R25A, Q26A, N27A, L28A, Q29A, and N30A had enhanced physical interaction with wild-type (wt) SERCA1a, but physical interaction of these PLN mutants with SERCA1a mutants D813A and R822A was reduced about 2.5 fold (range 1.44-2.82). Exceptions were the interactions of PLN N27A and N30A with SERCA1a D813A, which were reduced by 7.3- and 5.8-fold, respectively. A superinhibitory PLN deletion mutant, PLNDelta21-29, had strong physical interactions with SERCA1a and with SERCA1a mutant D813A. Physical interactions with SERCA1a and mutant D813A were sharply diminished, however, for the PLN deletion mutant, PLNDelta21-30, lacking PLN N30. Physical interactions between SERCA1a and a PLN-cytochrome b(5) chimera containing PLN residues 1-29 were much stronger than those between a PLN-cytochrome b(5) chimera containing PLN residues 1-21 and lacking N27. These results suggest that a SERCA1-PLN interaction site occurs between L67 of SERCA1a and domain IB of PLN, which involves SERCA1a D813 and PLN N27 and N30.

Importance of transmembrane segment M3 of the sarcoplasmic reticulum Ca2+-ATPase for control of the gateway to the Ca2+ sites

The specific functional roles of various parts of the third transmembrane segment (M3) of the sarcoplasmic reticulum Ca(2+)-ATPase were examined by functionally characterizing a series of mutants with multiple or single substitutions of M3 residues. Steady-state and transient kinetic measurements, assisted by computer simulation of the time and Ca(2+) dependences of the phosphorylation level, were used to study the partial reaction steps of the enzyme cycle, including the binding and dissociation of Ca(2+) at the high affinity cytoplasmically facing sites. The mutation Lys-Leu-Asp-Glu(255) --> Glu-Ile-Glu-His resulted in a conspicuous increase in the rate of Ca(2+) dissociation as well as a displacement of the major conformational equilibria of the phosphoenzyme and dephosphoenzyme forms. The point mutant Phe(256) --> Ala also showed an increased rate of Ca(2+) dissociation, whereas a conspicuous decrease both in the rate of Ca(2+) dissociation and in the rate of Ca(2+) binding was found for the mutant Gly-Glu-Gln-Leu(260) --> Ile-His-Leu-Ile. These findings suggest that the NH(2)-terminal half of M3 is involved in control of the gateway to the Ca(2+) sites. The main effect of two mutations to the COOH-terminal half of M3, Ser-Lys-Val-Ile-Ser(265) --> Thr-Gly-Val-Ala-Val and Leu-Ile-Cys-Val-Ala-Val-Trp-Leu-Ile(274) --> Phe-Leu-Gly-Val-Ser-Phe-Phe-Ile-Leu, was a block of the dephosphorylation.

What the structure of a calcium pump tells us about its mechanism

The report of the crystal structure of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum in its Ca(2+)-bound form [Toyoshima, Nakasako and Ogawa (2000) Nature (London) 405, 647-655] provides an opportunity to interpret much kinetic and mutagenic data on the ATPase in structural terms. There are no large channels leading from the cytoplasmic surface to the pair of high-affinity Ca(2+) binding sites within the transmembrane region. One possible access pathway involves the charged residues in transmembrane alpha-helix M1, with a Ca(2+) ion passing through the first site to reach the second site. The Ca(2+)-ATPase also contains a pair of binding sites for Ca(2+) that are exposed to the lumen. In the four-site model for transport, phosphorylation of the ATPase leads to transfer of the two bound Ca(2+) ions from the cytoplasmic to the lumenal pair of sites. In the alternating four-site model for transport, phosphorylation leads to release of the bound Ca(2+) ions directly from the cytoplasmic pair of sites, linked to closure of the pair of lumenal binding sites. The lumenal pair of sites could involve a cluster of conserved acidic residues in the loop between M1 and M2. Since there is no obvious pathway from the high-affinity sites to the lumenal surface of the membrane, transport of Ca(2+) ions must involve a significant change in the packing of the transmembrane alpha-helices. The link between the phosphorylation domain and the pair of high-affinity Ca(2+) binding sites is probably provided by two small helices, P1 and P2, in the phosphorylation domain, which contact the loop between transmembrane alpha-helices M6 and M7.

SERCA1 truncated proteins unable to pump calcium reduce the endoplasmic reticulum calcium concentration and induce apoptosis

By pumping calcium from the cytosol to the ER, sarco/endoplasmic reticulum calcium ATPases (SERCAs) play a major role in the control of calcium signaling. We describe two SERCA1 splice variants (S1Ts) characterized by exon 4 and/or exon 11 splicing, encoding COOH terminally truncated proteins, having only one of the seven calcium-binding residues, and thus unable to pump calcium. As shown by semiquantitative RT-PCR, S1T transcripts are differentially expressed in several adult and fetal human tissues, but not in skeletal muscle and heart. S1T proteins expression was detected by Western blot in nontransfected cell lines. In transiently transfected cells, S1T homodimers were revealed by Western blot using mildly denaturing conditions. S1T proteins were shown, by confocal scanning microscopy, to colocalize with endogenous SERCA2b into the ER membrane. Using ER-targeted aequorin (erAEQ), we have found that S1T proteins reduce ER calcium and reverse elevation of ER calcium loading induced by SERCA1 and SERCA2b. Our results also show that SERCA1 variants increase ER calcium leakage and are consistent with the hypothesis of a cation channel formed by S1T homodimers. Finally, when overexpressed in liver-derived cells, S1T proteins significantly induce apoptosis. These data reveal a further mechanism modulating Ca(2+) accumulation into the ER of nonmuscle cells and highlight the relevance of S1T proteins to the control of apoptosis.

The role of the M6-M7 loop (L67) in stabilization of the phosphorylation and Ca(2+) binding domains of the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA)

The amino acid sequence (L67) intervening between the M6 and M7 transmembrane segments of the Ca(2+) transport ATPase was subjected to mutational analysis. Mutation of Pro(820) to Ala interferes with protein expression even though transcription occurs at normal levels. Single mutations of Lys(819) or Arg(822) to Ala, Phe, or Glu allow good expression, but produce strong inhibition of ATPase activity. The main defect produced by these mutations is strong interference with enzyme phosphorylation by ATP in the presence of Ca(2+), and also by P(i) in the absence of Ca(2+). The Lys(819) and Arg(822) mutants undergo slight and moderate reduction of Ca(2+) binding affinity, respectively. Reduction of overall steady state ATPase velocity is then due to inhibition of phosphorylated intermediate formation. On the other hand, a cluster of conservative mutations of Asp(813), Asp(815), and Asp(818) to Asn interferes strongly with enzyme activation by Ca(2+) binding and formation of phosphorylated enzyme intermediate by utilization of ATP. Enzyme phosphorylation by Pi in the absence of Ca(2+) undergoes slight or no inhibition by the triple aspartate mutation. Therefore, the triple mutation interferes mainly with the calcium-dependent activation of the ATPase. The effect of the triple mutation can be to a large extent reproduced by single mutation of Asp(813) (but not of Asp(815) or Asp(818)) to Asn. Functional and structural analysis of the experimental data demonstrates that the L67 loop plays an important role in protein folding and function. This role is sustained by linking the cytosolic catalytic domain and the transmembrane Ca(2+) binding domain through a network of hydrogen bonds.

Detailed characterization of the cooperative mechanism of Ca(2+) binding and catalytic activation in the Ca(2+) transport (SERCA) ATPase

Expression of heterologous SERCA1a ATPase in Cos-1 cells was optimized to yield levels that account for 10-15% of the microsomal protein, as revealed by protein staining on electrophoretic gels. This high level of expression significantly improved our characterization of mutants, including direct measurements of Ca(2+) binding by the ATPase in the absence of ATP, and measurements of various enzyme functions in the presence of ATP or P(i). Mutational analysis distinguished two groups of amino acids within the transmembrane domain: The first group includes Glu771 (M5), Thr799 (M6), Asp800 (M6), and Glu908 (M8), whose individual mutations totally inhibit binding of the two Ca(2+) required for activation of one ATPase molecule. The second group includes Glu309 (M4) and Asn796 (M6), whose individual or combined mutations inhibit binding of only one and the same Ca(2+). The effects of mutations of these amino acids were interpreted in the light of recent information on the ATPase high-resolution structure, explaining the mechanism of Ca(2+) binding and catalytic activation in terms of two cooperative sites. The Glu771, Thr799, and Asp800 side chains contribute prominently to site 1, together with less prominent contributions by Asn768 and Glu908. The Glu309, Asn796, and Asp800 side chains, as well as the Ala305 (and possibly Val304 and Ile307) carbonyl oxygen, contribute to site 2. Sequential binding begins with Ca(2+) occupancy of site 1, followed by transition to a conformation (E') sensitive to Ca(2+) inhibition of enzyme phosphorylation by P(i), but still unable to utilize ATP. The E' conformation accepts the second Ca(2+) on site 2, producing then a conformation (E' ') which is able to utilize ATP. Mutations of residues (Asp813 and Asp818) in the M6/M7 loop reduce Ca(2+) affinity and catalytic turnover, suggesting a strong influence of this loop on the correct positioning of the M6 helix. Mutation of Asp351 (at the catalytic site within the cytosolic domain) produces total inhibition of ATP utilization and enzyme phosphorylation by P(i), without a significant effect on Ca(2+) binding.

Site-directed mutagenesis of human endothelial cell ecto-ADPase/soluble CD39: requirement of glutamate 174 and serine 218 for enzyme activity and inhibition of platelet recruitment

Endothelial cell CD39/ecto-ADPase plays a major role in vascular homeostasis. It rapidly metabolizes ADP released from stimulated platelets, thereby preventing further platelet activation and recruitment. We recently developed a recombinant, soluble form of human CD39, solCD39, with enzymatic and biological properties identical to CD39. To identify amino acids essential for enzymatic/biological activity, we performed site-directed mutagenesis within the four highly conserved apyrase regions of solCD39. Mutation of glutamate 174 to alanine (E174A) and serine 218 to alanine (S218A) resulted in complete and approximately 90% loss of solCD39 enzymatic activity, respectively. Furthermore, compared to wild-type, S57A exhibited a 2-fold increase in ADPase activity without change in ATPase activity, while the tyrosine 127 to alanine (Y127A) mutant lost 50-60% of both ADPase and ATPase activity. The ADPase activity of wild-type solCD39 and each mutant, except for R135A, was greater with calcium as the required divalent cation than with magnesium, but for ATPase activity generally no such preference was observed. Y127A demonstrated the highest calcium/magnesium ADPase activity ratio, 2.8-fold higher than that of wild-type, even though its enzyme activity was greatly reduced. SolCD39 mutants were further characterized by correlating enzymatic with biological activity in an in vitro platelet aggregation system. Each solCD39 mutant was similar to wild-type in reversing platelet aggregation, except for E174A and S218A. E174A, completely devoid of enzymatic activity, failed to inhibit platelet responsiveness, as anticipated. S218A, with 91% loss of ADPase activity, could still reverse platelet aggregation, albeit much less effectively than wild-type solCD39. Thus, glutamate 174 and serine 218 are essential for both the enzymatic and biological activity of solCD39.

Entrance port for Na(+) and K(+) ions on Na(+),K(+)-ATPase in the cytoplasmic loop between trans-membrane segments M6 and M7 of the alpha subunit. Proximity Of the cytoplasmic segment of the beta subunit

Based on the following observations we propose that the cytoplasmic loop between trans-membrane segments M6 and M7 (L6/7) of the alpha subunit of Na(+),K(+)-ATPase acts as an entrance port for Na(+) and K(+) ions. 1) In defined conditions chymotrypsin specifically cleaves L6/7 in the M5/M6 fragment of 19-kDa membranes, produced by extensive proteolysis of Na(+),K(+)-ATPase, and in parallel inactivates Rb(+) occlusion. 2) Dissociation of the M5/M6 fragment from 19-kDa membranes is prevented either by occluded cations or by competitive antagonists such as Ca(2+), Mg(2+), La(3+), p-xylylene bisguanidinium and m-xylylene bisguanidinium, or 1-bromo-2,4, 6-tris(methylisothiouronium)benzene and 1,3-dibromo-2,4,6-tris (methylisothiouronium)benzene (Br(2)-TITU(3+)). 3) Ca(2+) ions raise electrophoretic mobility of the M5/M6 fragment but not that of the other fragments of the alpha subunit. It appears that negatively charged residues in L6/7 recognize either Na(+) or K(+) ions or the competitive cation antagonists. Na(+) and K(+) ions are then occluded within trans-membrane segments and can be transported, whereas the cation antagonists are not occluded and block transport at the entrance port. The cytoplasmic segment of the beta subunit appears to be close to or contributes to the entrance port, as inferred from the following observations. 1) Specific chymotryptic cleavage of the 16-kDa fragment of the beta subunit to 15-kDa at 20 degrees C (Shainskaya, A., and Karlish, S. J. D. (1996) J. Biol. Chem. 271, 10309-10316) markedly reduces affinity for Br(2)-TITU(3+) and for Na(+) ions, detected by Na(+) occlusion assays or electrogenic Na(+) binding, whereas Rb(+) occlusion is unchanged. 2) Na(+) ions specifically protect the 16-kDa fragment against this chymotryptic cleavage.

The importance of carboxyl groups on the lumenal side of the membrane for the function of the Ca(2+)-ATPase of sarcoplasmic reticulum

The conventional model for transport of Ca(2+) by the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR) involves a pair of binding sites for Ca(2+) that change upon phosphorylation of the ATPase from being high affinity and exposed to the cytoplasm to being low affinity and exposed to the lumen. However, a number of recent experiments suggest that in fact transport involves two separate pairs of binding sites for Ca(2+), one pair exposed to the cytoplasmic side and the other pair exposed to the lumenal side. Here we show that the carbodiimide 1-ethyl-3-[3-(dimethylamino)-propyl] carbodiimide (EDC) is membrane-impermeable, and we use EDC to distinguish between cytoplasmic and lumenal sites of reaction. Modification of the Ca(2+)-ATPase in sealed SR vesicles with EDC leads to loss of ATPase activity without modification of the pair of high affinity Ca(2+)-binding sites. Modification of the purified ATPase in unsealed membrane fragments was faster than modification in SR vesicles, suggesting the presence of more quickly reacting lumenal sites. This was confirmed in experiments measuring EDC modification of the ATPase reconstituted randomly into sealed lipid vesicles. Modification of sites on the lumenal face of the ATPase led to loss of the Ca(2+)-induced increase in phosphorylation by P(i). It is concluded that carboxyl groups on the lumenal side of the ATPase are involved in Ca(2+) binding to the lumenal side of the ATPase and that modification of these sites leads to loss of ATPase activity. The presence of MgATP or MgADP leads to faster inhibition of the ATPase by EDC in unsealed membrane fragments than in sealed vesicles, suggesting that binding of MgATP or MgADP to the ATPase leads to a conformational change on the lumenal side of the membrane.

NMR conformational study of the sixth transmembrane segment of sarcoplasmic reticulum Ca2+-ATPase

In current topological models, the sarcoplasmic reticulum Ca2+-ATPase contains 10 putative transmembrane spans (M1-M10), with spans M4/M5/M6 and probably M8 participating in the formation of the membranous calcium-binding sites. We describe here the conformational properties of a synthetic peptide fragment (E785-N810) encompassing the sixth transmembrane span (M6) of Ca2+-ATPase. Peptide M6 includes three residues (N796, T799, and D800) out of the six membranous residues critically involved in the ATPase calcium-binding sites. 2D-NMR experiments were performed on the M6 peptide selectively labeled with 15N and solubilized in dodecylphosphocholine micelles to mimic a membrane-like environment. Under these conditions, M6 adopts a helical structure in its N-terminal part, between residues I788 and T799, while its C-terminal part (G801-N810) remains disordered. Addition of 20% trifluoroethanol stabilizes the alpha-helical N-terminal segment of the peptide, and reveals the propensity of the C-terminal segment (G801-L807) to form also a helix. This second helix is located at the interface or in the aqueous environment outside the micelles, while the N-terminal helix is buried in the hydrophobic core of the micelles. Furthermore, the two helical segments of M6 are linked by a flexible hinge region containing residues T799 and D800. These conformational features may be related to the transient formation of a Schellman motif (L797VTDGL802) encoded in the M6 sequence, which probably acts as a C-cap of the N-terminal helix and induces a bend with respect to the helix axis. We propose a model illustrating two conformations of M6 and its insertion in the membrane. The presence of a flexible region within M6 would greatly facilitate concomitant participation of all three residues (N796, T799, and D800) believed to be involved in calcium complexation.