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

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.

Crystal structure of the calcium pump with a bound ATP analogue

P-type ATPases are ATP-powered ion pumps that establish ion concentration gradients across cell and organelle membranes. Here, we describe the crystal structure of the Ca2+ pump of skeletal muscle sarcoplasmic reticulum, a representative member of the P-type ATPase superfamily, with an ATP analogue, a Mg2+ and two Ca2+ ions in the respective binding sites. In this state, the ATP analogue reorganizes the three cytoplasmic domains (A, N and P), which are widely separated without nucleotide, by directly bridging the N and P domains. The structure of the P-domain itself is altered by the binding of the ATP analogue and Mg2+. As a result, the A-domain is tilted so that one of the transmembrane helices moves to lock the cytoplasmic gate of the transmembrane Ca2+-binding sites. This appears to be the mechanism for occluding the bound Ca2+ ions, before releasing them into the lumen of the sarcoplasmic reticulum.

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.

Ca2+ occlusion and gating function of Glu309 in the ADP-fluoroaluminate analog of the Ca2+-ATPase phosphoenzyme intermediate

In the absence of ATP the sarcoplasmic reticulum ATPase (SERCA) binds two Ca(2+) with high affinity. The two bound Ca(2+) rapidly undergo reverse dissociation upon addition of EGTA, but can be distinguished by isotopic exchange indicating fast exchange at a superficial site (site II), and retardation of exchange at a deeper site (site I) by occupancy of site II. Site II mutations that allow high affinity binding to site I, but only low affinity binding to site II, show that retardation of isotopic exchange requires higher Ca(2+) concentrations with the N796A mutant, and is not observed with the E309Q mutant even at millimolar Ca(2+). Fluoroaluminate forms a complex at the catalytic site yielding stable analogs of the phosphoenzyme intermediate, with properties similar to E2-P or E1-P.Ca(2). Mutational analysis indicates that Asp(351), Lys(352), Thr(353), Asp(703), Asn(706), Asp(707), Thr(625), and Lys(684) participate in stabilization of fluoroaluminate and Mg(2+) at the phosphorylation site. In the presence of fluoroaluminate and Ca(2+), ADP (or AMP-PCP) favors formation of a stable ADP.E1-P.Ca(2) analog. This produces strong occlusion of Ca(2+) bound to both sites (I and II), whereby dissociation occurs very slowly even following addition of EGTA. Occlusion by fluoraluminate and ADP is not observed with the E309Q mutant, suggesting a gating function of Glu(309) at the mouth of a binding cavity with a single path of entry. This phenomenon corresponds to the earliest step of the catalytic cycle following utilization of ATP. Experiments on limited proteolysis reveal that a long range conformational change, involving displacement of headpiece domains and transmembrane helices, plays a mechanistic role.

Mutational analysis of the ATP2A2 gene in two Darier disease families with intrafamilial variability

BACKGROUND

Darier disease (DD), an autosomal dominant genodermatosis characterized by warty papules and plaques over seborrhoeic areas, is caused by mutations in the ATP2A2 gene, which encodes the sarco/endoplasmic reticulum Ca(2+) ATPase type 2 isoform (SERCA2). While markedly different clinical severity within DD-affected family members is known, the pathomechanism has not been elucidated.

OBJECTIVES

Based on the hypothesis that multiple ATP2A2 mutations might contribute to the pathomechanism, we have analysed two DD families in which the clinical severity differs markedly within a single pedigree, and, as controls, eight DD families without differing clinical severity.

METHODS

All the exons and intron-exon borders of ATP2A2 were directly sequenced from the genomic DNA extracted from all the subjects.

RESULTS

We identified the heterozygous mutations, G233R in pedigree 1 and C318R in pedigree 2, respectively, whereas no other ATP2A2 mutations in any of severely affected individuals were found. In eight DD pedigrees as control, we have found M1V, N39D, L180R, A838P and 2170 insertion G in each of five pedigrees, but no mutation was found in three DD pedigrees.

CONCLUSIONS

Our results together with previous data indicate that the distribution of mutations is scattered over the entire ATP2A2 without any, as yet, discernible 'hotspots'. The mutations in pedigrees 1 and 2 with intrafamiliar clinical differences occurred around the Ca(2+)-binding sites on SERCA2, which might be associated with differences in clinical severity. These variations in ATP2A2 mutations alone cannot account for the clinical heterogeneity within DD pedigrees.

Structural basis of ion pumping by Ca2+-ATPase of sarcoplasmic reticulum

The structures of the Ca(2+)-ATPase (SERCA1a) have been determined for five different states by X-ray crystallography. Detailed comparison of the structures in the Ca(2+)-bound form and unbound (but thapsigargin-bound) form reveals that very large rearrangements of the transmembrane helices take place accompanying Ca2+ dissociation and binding and that they are mechanically linked with equally large movements of the cytoplasmic domains. The meanings of the rearrangements of the transmembrane helices and those of the cytoplasmic domains, and the mechanistic roles of the phosphorylation are now becoming clear.

Substrate-induced conformational fit and headpiece closure in the Ca2+ATPase (SERCA)

Protection of the Ca2+ATPase (SERCA) from proteinase K digestion has been observed following the addition of Ca2+, Mg2+, and nucleotide and interpreted as a substrate-dependent conformational change (1). The protected digestion site is located on the loop connecting the A domain and the M3 transmembrane helix. We studied by mutational analysis the protective effect of AMP-PCP, an ATP analog that is not utilized for enzyme phosphorylation. We found that the nucleotide protective effect is interfered with by single mutations of Arg-560 and Glu-439 in the N domain and Lys-352, Lys-684, Thr-353, Asp-703, and Asp-707 in the P domain. This is consistent with a transition from the open to the compact configuration of the ATPase headpiece and approximation of the N and P domains by interactions with the nucleotide adenosine and phosphate moieties, respectively. The A domain-M3 loop is consequently involved. Protection by nucleotide substrate increased following the mutations of Asp-351 (the residue undergoing phosphorylation by ATP) and neighboring Asn-706 to Ala, underlying the importance of side chain specificity in positioning the nucleotide terminal phosphate and limiting the stability of the substrate-enzyme complex. Protection is not observed when AMP-PCP is added in the absence of Ca2+ or following mutations (E771Q or N796A) that interfere with Ca2+ binding. Therefore, nucleotide binds to the Ca2+-activated enzyme in the open headpiece conformation and the consequent approximation of the N and P domains occurs while the transmembrane domain is still in the Ca2+-bound conformation. Mg2+ is not required for the protective effect of nucleotide, even though it is specifically required for the subsequent catalytic reactions.

Molecular characterisation of the smooth endoplasmic reticulum Ca(2+)-ATPase of Porcellio scaber and its expression in sternal epithelia during the moult cycle

The anterior sternal epithelial cells of the terrestrial isopod Porcellio scaber transport large amounts of calcium during the formation and resorption of intermittent calcium carbonate deposits. Recent investigations on epithelia involved in mineralisation processes suggest a role of the smooth endoplasmic reticulum Ca(2+)-ATPase (SERCA) in transcellular calcium transport. We present the first molecular characterisation of a SERCA within a crustacean mineralising epithelium. We cloned the SERCA from a cDNA library of the anterior sternal epithelium and used in situ hybridisation to compare the expression of the SERCA mRNA between three different moulting stages. The full-length SERCA cDNA has an open reading frame of 3006 nucleotides. The deduced 1002 amino-acid polypeptide has a predicted molecular mass of 109.7 kDa and 87% identity to the SERCA of Procambarus clarkii axial muscle isoform. In situ hybridisation confirmed expression within the anterior sternal epithelium and revealed an increase in SERCA mRNA abundance from the non-transporting, early premoult stage to the calcium transporting, late premoult and intramoult stage. The results support previous indications of a contribution by the smooth endoplasmic reticulum to transcellular calcium transport and suggest a transcriptional regulation of SERCA activity.

NMR studies of the fifth transmembrane segment of sarcoplasmic reticulum Ca2+-ATPase reveals a hinge close to the Ca2+-ligating residues

Two recent X-ray structures have tremendously increased the understanding of the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) and related proteins. Both structures show the fifth transmembrane span (M5) as a single continuous alpha-helix. The inherent structural and dynamic features of this span (Lys758-Glu785) were studied in isolation in sodium dodecyl sulfate (SDS) micelles using liquid-state nuclear magnetic resonance (NMR) spectroscopy. We find that a flexible region (Ile765-Asn768) is interrupting the alpha-helix. The location of the flexible region near the Ca(2+) binding residues Asn768 and Glu771 suggests that together with a similar region in M6 it has a hinge function that may be important for cooperative Ca(2+) binding and occlusion.

Characterization of Ca2+ ATPase residues involved in substrate and cation binding

The role of amino acid residues involved in substrate and cation binding was investigated in complementary experiments on Fe(2+)-catalyzed oxidation and cleavage, limited digestion with proteinase K, and mutational analysis. Cleavage at Ser346 was produced by Fe(2+) in the presence of substrate (ATP or AMP-PNP) and Ca(2+), and was attributed to Fe(2+) bound to a Mg(2+) site near Ser346 and neighboring Glu696. Ca(2+)- and ATP-dependent oxidation of the Thr441 side chain was also observed and attributed to Fe(2+) substituting for Mg(2+) in the Mg(2+)-ATP complex bound to the N domain. Mutation of Arg560 or Glu439 within the N domain interfered with nucleotide-dependent ATPase resistance to digestion with proteinase K. Furthermore, mutation of Lys352, Lys684, Thr353, Asp703, or Asp707 within the P domain produced similar interference, consistent with a role of these residues in substrate stabilization at the catalytic site. In a third group of experiments, equilibrium isotherms were obtained with Asn796Ala and Glu309Gln mutants, demonstrating non-cooperative binding of one Ca(2+) per ATPase, as opposed to cooperative binding of two Ca(2+) by WT enzyme. No high-affinity binding by Asp800Asn, Glu771Gln, and Thr799Ala mutants was detected. It was also demonstrated that the conformational transitions involved in enzyme activation and interconversion of Ca(2+) binding and phosphorylation energy, are triggered by Ca(2+) binding to site II and stabilization of Glu309 (M4) and N796 (M6).

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.

Homology modeling of the cation binding sites of Na+K+-ATPase

Homology modeling of the alpha-subunit of Na+K+-ATPase, a representative member of P-type ion transporting ATPases, was carried out to identify the cation (three Na+ and two K+) binding sites in the transmembrane region, based on the two atomic models of Ca2+-ATPase (Ca2+-bound form for Na+, unbound form for K+). A search for potential cation binding sites throughout the atomic models involved calculation of the valence expected from the disposition of oxygen atoms in the model, including water molecules. This search identified three positions for Na+ and two for K+ at which high affinity for the respective cation is expected. In the models presented, Na+- and K+-binding sites are formed at different levels with respect to the membrane, by rearrangements of the transmembrane helices. These rearrangements ensure that release of one type of cation coordinates with the binding of the other. Cations of different radii are accommodated by the use of amino acid residues located on different faces of the helices. Our models readily explain many mutational and biochemical results, including different binding stoichiometry and affinities for Na+ and K+.

Cooperative setting for long-range linkage of Ca(2+) binding and ATP synthesis in the Ca(2+) ATPase

High-affinity and cooperative binding of two Ca(2+) per ATPase (SERCA) occurs within the membrane-bound region of the enzyme. Direct measurements of binding at various Ca(2+) concentrations demonstrate that site-directed mutations within this region interfere selectively with Ca(2+) occupancy of either one or both binding sites and with the cooperative character of the binding isotherms. A transition associated with high affinity and cooperative binding of the second Ca(2+) and the engagement of N796 and E309 are both required to form a phosphoenzyme intermediate with ATP in the forward direction of the cycle and also to form ATP from phosphoenzyme intermediate and ADP in the reverse direction of the cycle. This transition, defined by equilibrium and kinetic characterization of the partial reactions of the enzyme cycle, extends from transmembrane helices to the catalytic site through a long-range linkage and is the mechanistic device for interconversion of binding and phosphorylation potentials.

Importance of Glu(282) in transmembrane segment M3 of the Na(+),K(+)-ATPase for control of cation interaction and conformational changes

Glu(282) located in the NH(2)-terminal part of transmembrane helix M3 of the Na(+),K(+)-ATPase was replaced by alanine, glycine, leucine, lysine, aspartate, or glutamine, and the effects of the mutations on the overall and partial reactions of the enzyme were analyzed. The mutations affected at least 3 important functions of the Na(+),K(+)-ATPase: (i) the conformational transitions between E(1) and E(2) forms of dephospho- and phosphoenzyme, (ii) Na(+) binding at the cytoplasmically facing sites of E(1), and (iii) long-range interaction controlling dephosphorylation. In mutants Glu(282) --> Lys and Glu(282) --> Asp, the E(1) form was favored during ATP hydrolysis, whereas the E(2) form was favored in Glu(282) --> Ala and Glu(282) --> Gly. Regardless of the change of conformational equilibrium, all the mutants displayed a reduced apparent affinity for Na(+), at least 3-fold for Glu(282) --> Lys and Glu(282) --> Asp, suggesting a direct effect on the Na(+) binding properties of E(1). Glu(282) --> Ala and Glu(282) --> Gly exhibited an extraordinary high rate of ATP hydrolysis in the mere presence of Na(+) without K(+) ("Na(+)-ATPase activity"), because of an increased rate of dephosphorylation of E(2)P. These results are in accordance with the hypothesis that Glu(282) is involved in the communication between the cation binding pocket and the catalytic site and in control of the cytoplasmic entry pathway for Na(+).

Cell volume homeostasis: ionic and nonionic mechanisms. The sodium pump in the emergence of animal cells

Plant cells and bacterial cells are surrounded by a massive polysaccharide wall, which constrains their high internal osmotic pressure (tens of atmospheres). Animal cells, in contrast, are in osmotic equilibrium with their environment, have no restraining surround, and can take on a variety of shapes and can change these from moment to moment. This osmotic balance is achieved, in the first place, by the action of the energy-consuming sodium pump, one of the P-type ATPase transport protein family, members of which are found also in bacteria. The pump's action brings about a transmembranal electrochemical gradient of sodium ions, harnessed in a range of transport systems which couple the dissipation of this gradient to establishing a gradient of the coupled substrate. These transport systems include many which are responsible for short-term regulation of the cell's volume in response to acute changes of their osmotic balance. Thus, the primary role of the sodium pump as a regulator of cell volume has been built upon to provide the basis for an enormous variety of physiological functions.

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.

Structural changes in the calcium pump accompanying the dissociation of calcium

In skeletal muscle, calcium ions are transported (pumped) against a concentration gradient from the cytoplasm into the sarcoplasmic reticulum, an intracellular organelle. This causes muscle cells to relax after cytosolic calcium increases during excitation. The Ca(2+) ATPase that carries out this pumping is a representative P-type ion-transporting ATPase. Here we describe the structure of this ion pump at 3.1 A resolution in a Ca(2+)-free (E2) state, and compare it with that determined previously for the Ca(2+)-bound (E1Ca(2+)) state. The structure of the enzyme stabilized by thapsigargin, a potent inhibitor, shows large conformation differences from that in E1Ca(2+). Three cytoplasmic domains gather to form a single headpiece, and six of the ten transmembrane helices exhibit large-scale rearrangements. These rearrangements ensure the release of calcium ions into the lumen of sarcoplasmic reticulum and, on the cytoplasmic side, create a pathway for entry of new calcium ions.

Close proximity between residue 30 of phospholamban and cysteine 318 of the cardiac Ca2+ pump revealed by intermolecular thiol cross-linking

Phospholamban (PLB) is a 52-amino acid inhibitor of the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum (SERCA2a), which acts by decreasing the apparent affinity of the enzyme for Ca(2+). To localize binding sites of SERCA2a for PLB, we performed Cys-scanning mutagenesis of PLB, co-expressed the PLB mutants with SERCA2a in insect cell microsomes, and tested for cross-linking of the mutated PLB molecules to SERCA2a using 1,6-bismaleimidohexane, a 10-A-long, homobifunctional thiol cross-linking agent. Of several mutants tested, only PLB with a Cys replacement at position 30 (N30C-PLB) cross-linked to SERCA2a. Cross-linking occurred specifically and with high efficiency. The process was abolished by micromolar Ca(2+) or by an anti-PLB monoclonal antibody and was inhibited 50% by phosphorylation of PLB by cAMP-dependent protein kinase. The SERCA2a inhibitors thapsigargin and cyclopiazonic acid also completely prevented cross-linking. The two essential requirements for cross-linking of N30C-PLB to SERCA2a were a Ca(2+)-free enzyme and, unexpectedly, a micromolar concentration of ATP or ADP, demonstrating that N30C-PLB cross-links preferentially to the nucleotide-bound, E2 state of SERCA2a. Sequencing of a purified proteolytic fragment in combination with SERCA2a mutagenesis identified Cys(318) of SERCA2a as the sole amino acid cross-linked to N30C-PLB. The proximity of residue 30 of PLB to Cys(318) of SERCA2a suggests that PLB may interfere with Ca(2+) activation of SERCA2a by a protein interaction occurring near transmembrane helix M4.

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.

Purification of heterologous sarcoplasmic reticulum Ca2+-ATPase Serca1a allowing phosphoenzyme and Ca2+-affinity measurements

We describe here a protocol to prepare milligrams of active and stable heterologous sarcoplasmic reticulum Ca(2+)-ATPase (Serca1a). Serca1a was tagged with 6 histidines at its C-terminal end and overexpressed using the baculovirus-Sf9 system. In a first trial, Serca1a accounted for 24% of membrane proteins, 95% of which were inactive. Glucose in the culture medium reduced the production of Serca1a to 3 to 5% of membrane proteins and all Serca1a was active. Seventy-five percent of active Serca1a was solubilized by C(12)E(8) in the presence of phosphatidylcholine under conditions avoiding denaturation. Purification by Ni(2+)-nitrilo-triacetic acid affinity chromatography was tried, but only 3% of active Serca1a remained bound to the column, as if the His-tag were not accessible. Yields of 43% were reached by purification on reactive red 120 columns when eluting with 2 M NaCl. The purity was about 25% and Serca1a was stable for at least 1 week at 0 degrees C. Typically, 500 ml of culture medium produced 3 mg of active Serca1a and 1 mg of purified active Serca1a allowing measurements of phosphoenzyme (2 nmol/mg) or Ca(2+) affinity (2 microM at pH 7).

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.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

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.