Functional consequences of alterations to Glu309, Glu771, and Asp800 in the Ca(2+)-ATPase of sarcoplasmic reticulum

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.

A Darier disease mutation relieves kinetic constraints imposed by the tail of sarco(endo)plasmic reticulum Ca2+-ATPase 2b

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) 2b isoform possesses an extended C terminus (SERCA2b tail) forming an 11th transmembrane (TM) helix, which slows conformational changes of the Ca²⁺-pump reaction cycle. Here, we report that a Darier disease (DD) mutation of SERCA2b that changes a glutamate to a lysine in the cytoplasmic loop between TM8 and TM9 (E917K) relieves these kinetic constraints. We analyzed the effects of this mutation on the overall reaction and the individual partial reactions of the Ca²⁺ pump compared with the corresponding mutations of the SERCA2a and SERCA1a isoforms, lacking the SERCA2b tail. In addition to a reduced affinity for Ca²⁺, caused by the mutation in all three isoforms examined, we observed a unique enhancing effect on the turnover rates of ATPase activity and Ca²⁺ transport for the SERCA2b E917K mutation. This relief of kinetic constraints contrasted with inhibitory effects observed for the corresponding SERCA2a and SERCA1a (E918K) mutations. These observations indicated that the E917K/E918K mutations affect the rate-limiting conformational change in isoform-specific ways and that the SERCA2b mutation perturbs the interactions of TM11 with other SERCA2b regions. Mutational analysis of an arginine in TM7 that interacts with the glutamate in SERCA1a crystal structures suggested that in wildtype SERCA2b, the corresponding arginine (Arg-835) may be involved in mediating the conformational restriction by TM11. Moreover, the E917K mutation may disturb TM11 through the cytoplasmic loop between TM10 and TM11. In conclusion, our findings have identified structural elements of importance for the kinetic constraints imposed by TM11.

Microsecond molecular dynamics simulations of Mg²⁺- and K⁺-bound E1 intermediate states of the calcium pump

We have performed microsecond molecular dynamics (MD) simulations to characterize the structural dynamics of cation-bound E1 intermediate states of the calcium pump (sarcoendoplasmic reticulum Ca²⁺-ATPase, SERCA) in atomic detail, including a lipid bilayer with aqueous solution on both sides. X-ray crystallography with 40 mM Mg²⁺ in the absence of Ca²⁺ has shown that SERCA adopts an E1 structure with transmembrane Ca²⁺-binding sites I and II exposed to the cytosol, stabilized by a single Mg²⁺ bound to a hybrid binding site I'. This Mg²⁺-bound E1 intermediate state, designated E1•Mg²⁺, is proposed to constitute a functional SERCA intermediate that catalyzes the transition from E2 to E1•2Ca²⁺ by facilitating H⁺/Ca²⁺ exchange. To test this hypothesis, we performed two independent MD simulations based on the E1•Mg²⁺ crystal structure, starting in the presence or absence of initially-bound Mg²⁺. Both simulations were performed for 1 µs in a solution containing 100 mM K⁺ and 5 mM Mg²⁺ in the absence of Ca²⁺, mimicking muscle cytosol during relaxation. In the presence of initially-bound Mg²⁺, SERCA site I' maintained Mg²⁺ binding during the entire MD trajectory, and the cytosolic headpiece maintained a semi-open structure. In the absence of initially-bound Mg²⁺, two K⁺ ions rapidly bound to sites I and I' and stayed loosely bound during most of the simulation, while the cytosolic headpiece shifted gradually to a more open structure. Thus MD simulations predict that both E1•Mg²⁺ and E•2K+ intermediate states of SERCA are populated in solution in the absence of Ca²⁺, with the more open 2K+-bound state being more abundant at physiological ion concentrations. We propose that the E1•2K⁺ state acts as a functional intermediate that facilitates the E2 to E1•2Ca²⁺ transition through two mechanisms: by pre-organizing transport sites for Ca²⁺ binding, and by partially opening the cytosolic headpiece prior to Ca²⁺ activation of nucleotide binding.

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.

Sarcolipin protein interaction with sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) is distinct from phospholamban protein, and only sarcolipin can promote uncoupling of the SERCA pump

Sarco(endo)plasmic reticulum Ca(2+)ATPase (SERCA) pump activity is modulated by phospholamban (PLB) and sarcolipin (SLN) in cardiac and skeletal muscle. Recent data suggest that SLN could play a role in muscle thermogenesis by promoting uncoupling of the SERCA pump (Lee, A.G. (2002) Curr. Opin. Struct. Biol. 12, 547-554 and Bal, N. C., Maurya, S. K., Sopariwala, D. H., Sahoo, S. K., Gupta, S. C., Shaikh, S. A., Pant, M., Rowland, L. A., Bombardier, E., Goonasekera, S. A., Tupling, A. R., Molkentin, J. D., and Periasamy, M. (2012) Nat. Med. 18, 1575-1579), but the mechanistic details are unknown. To better define how binding of SLN to SERCA promotes uncoupling of SERCA, we compared SLN and SERCA1 interaction with that of PLB in detail. The homo-bifunctional cross-linker (1,6-bismaleimidohexane) was employed to detect dynamic protein interaction during the SERCA cycle. Our studies reveal that SLN differs significantly from PLB: 1) SLN primarily affects the Vmax of SERCA-mediated Ca(2+) uptake but not the pump affinity for Ca(2+); 2) SLN can bind to SERCA in the presence of high Ca(2+), but PLB can only interact to the ATP-bound Ca(2+)-free E2 state; and 3) unlike PLB, SLN interacts with SERCA throughout the kinetic cycle and promotes uncoupling of the SERCA pump. Using SERCA transmembrane mutants, we additionally show that PLB and SLN can bind to the same groove but interact with a different set of residues on SERCA. These data collectively suggest that SLN is functionally distinct from PLB; its ability to interact with SERCA in the presence of Ca(2+) causes uncoupling of the SERCA pump and increased heat production.

Calcium and copper transport ATPases: analogies and diversities in transduction and signaling mechanisms

The calcium transport ATPase and the copper transport ATPase are members of the P-ATPase family and retain an analogous catalytic mechanism for ATP utilization, including intermediate phosphoryl transfer to a conserved aspartyl residue, vectorial displacement of bound cation, and final hydrolytic cleavage of Pi. Both ATPases undergo protein conformational changes concomitant with catalytic events. Yet, the two ATPases are prototypes of different features with regard to transduction and signaling mechanisms. The calcium ATPase resides stably on membranes delimiting cellular compartments, acquires free Ca²⁺ with high affinity on one side of the membrane, and releases the bound Ca²⁺ on the other side of the membrane to yield a high free Ca²⁺ gradient. These features are a basic requirement for cellular Ca²⁺ signaling mechanisms. On the other hand, the copper ATPase acquires copper through exchange with donor proteins, and undergoes intracellular trafficking to deliver copper to acceptor proteins. In addition to the cation transport site and the conserved aspartate undergoing catalytic phosphorylation, the copper ATPase has copper binding regulatory sites on a unique N-terminal protein extension, and has also serine residues undergoing kinase assisted phosphorylation. These additional features are involved in the mechanism of copper ATPase intracellular trafficking which is required to deliver copper to plasma membranes for extrusion, and to the trans-Golgi network for incorporation into metalloproteins. Isoform specific glyocosylation contributes to stabilization of ATP7A copper ATPase in plasma membranes.

Glutamate 90 at the luminal ion gate of sarcoplasmic reticulum Ca2+-ATPase is critical for Ca(2+) binding on both sides of the membrane

The roles of Ser(72), Glu(90), and Lys(297) at the luminal ends of transmembrane helices M1, M2, and M4 of sarcoplasmic reticulum Ca(2+)-ATPase were examined by transient and steady-state kinetic analysis of mutants. The dependence on the luminal Ca(2+) concentration of phosphorylation by P(i) ("Ca(2+) gradient-dependent E2P formation") showed a reduction of the apparent affinity for luminal Ca(2+) in mutants with alanine or leucine replacement of Glu(90), whereas arginine replacement of Glu(90) or Ser(72) allowed E2P formation from P(i) even at luminal Ca(2+) concentrations much too small to support phosphorylation in wild type. The latter mutants further displayed a blocked dephosphorylation of E2P and an increased rate of conversion of the ADP-sensitive E1P phosphoenzyme intermediate to ADP-insensitive E2P as well as insensitivity of the E2.BeF(3)(-) complex to luminal Ca(2+). Altogether, these findings, supported by structural modeling, indicate that the E2P intermediate is stabilized in the mutants with arginine replacement of Glu(90) or Ser(72), because the positive charge of the arginine side chain mimics Ca(2+) occupying a luminally exposed low affinity Ca(2+) site of E2P, thus identifying an essential locus (a "leaving site") on the luminal Ca(2+) exit pathway. Mutants with alanine or leucine replacement of Glu(90) further displayed a marked slowing of the Ca(2+) binding transition as well as slowing of the dissociation of Ca(2+) from Ca(2)E1 back toward the cytoplasm, thus demonstrating that Glu(90) is also critical for the function of the cytoplasmically exposed Ca(2+) sites on the opposite side of the membrane relative to where Glu(90) is located.

Ca2+ binding to site I of the cardiac Ca2+ pump is sufficient to dissociate phospholamban

Phospholamban (PLB) inhibits the activity of SERCA2a, the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum, by decreasing the apparent affinity of the enzyme for Ca(2+). Recent cross-linking studies have suggested that PLB binding and Ca(2+) binding to SERCA2a are mutually exclusive. PLB binds to the E2 conformation of the Ca(2+)-ATPase, preventing formation of E1, the conformation that binds two Ca(2+) (at sites I and II) with high affinity and is required for ATP hydrolysis. Here we determined whether Ca(2+) binding to site I, site II, or both sites is sufficient to dissociate PLB from the Ca(2+) pump. Seven SERCA2a mutants with amino acid substitutions at Ca(2+)-binding site I (E770Q, T798A, and E907Q), site II (E309Q and N795A), or both sites (D799N and E309Q/E770Q) were made, and the effects of Ca(2+) on N30C-PLB cross-linking to Lys(328) of SERCA2a were measured. In agreement with earlier reports with the skeletal muscle Ca(2+)-ATPase, none of the SERCA2a mutants (except E907Q) hydrolyzed ATP in the presence of Ca(2+); however, all were phosphorylatable by P(i) to form E2P. Ca(2+) inhibition of E2P formation was observed only in SERCA2a mutants retaining site I. In cross-linking assays, strong cross-linking between N30C-PLB and each Ca(2+)-ATPase mutant was observed in the absence of Ca(2+). Importantly, however, micromolar Ca(2+) inhibited PLB cross-linking only to mutants retaining a functional Ca(2+)-binding site I. The dynamic equilibrium between Ca(2+) pumps and N30C-PLB was retained by all mutants, demonstrating normal regulation of cross-linking by ATP, thapsigargin, and anti-PLB antibody. From these results we conclude that site I is the key Ca(2+)-binding site regulating the physical association between PLB and SERCA2a.

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.

Side-chain protonation and mobility in the sarcoplasmic reticulum Ca2+-ATPase: implications for proton countertransport and Ca2+ release

Protonation of acidic residues in the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA 1a) was studied by multiconformation continuum electrostatic calculations in the Ca(2+)-bound state Ca(2)E1, in the Ca(2+)-free state E2(TG) with bound thapsigargin, and in the E2P (ADP-insensitive phosphoenzyme) analog state with MgF(4)(2-) E2(TG+MgF(4)(2-)). Around physiological pH, all acidic Ca(2+) ligands (Glu(309), Glu(771), Asp(800), and Glu(908)) were unprotonated in Ca(2)E1; in E2(TG) and E2(TG+MgF(4)(2-)) Glu(771), Asp(800), and Glu(908) were protonated. Glu(771) and Glu(908) had calculated pK(a) values larger than 14 in E2(TG) and E2(TG+MgF(4)(2-)), whereas Asp(800) titrated with calculated pK(a) values near 7.5. Glu(309) had very different pK(a) values in the Ca(2+)-free states: 8.4 in E2(TG+MgF(4)(2-)) and 4.7 in E2(TG) because of a different local backbone conformation. This indicates that Glu(309) can switch between a high and a low pK(a) mode, depending on the local backbone conformation. Protonated Glu(309) occupied predominantly two main, very differently orientated side-chain conformations in E2(TG+MgF(4)(2-)): one oriented inward toward the other Ca(2+) ligands and one oriented outward toward a protein channel that seems to be in contact with the cytoplasm. Upon deprotonation, Glu(309) adopted completely the outwardly orientated side-chain conformation. The contact of Glu(309) with the cytoplasm in E2(TG+MgF(4)(2-)) makes this residue unlikely to bind lumenal protons. Instead it might serve as a proton shuttle between Ca(2+)-binding site I and the cytoplasm. Glu(771), Asp(800), and Glu(908) are proposed to take part in proton countertransport.

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

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

Conformational fluctuations of the Ca2+-ATPase in the native membrane environment. Effects of pH, temperature, catalytic substrates, and thapsigargin

Digestion with proteinase K or trypsin yields complementary information on conformational transitions of the Ca(2+)-ATPase (SERCA) in the native membrane environment. Distinct digestion patterns are obtained with proteinase K, revealing interconversion of E1 and E2 or E1 approximately P and E2-P states. The pH dependence of digestion patterns shows that, in the presence of Mg(2+), conversion of E2 to E1 pattern occurs (even when Ca(2+) is absent) as H(+) dissociates from acidic residues. Mutational analysis demonstrates that the Glu(309) and Glu(771) acidic residues (empty Ca(2+)-binding sites I and II) are required for stabilization of E2. Glu(309) ionization is most important to yield E1. However, a further transition produced by Ca(2+) binding to E1 (i.e. E1.2Ca(2+)) is still needed for catalytic activation. Following ATP utilization, H(+)/Ca(2+) exchange is involved in the transition from the E1 approximately P.2Ca(2+) to the E2-P pattern, whereby alkaline pH will limit this conformational transition. Complementary experiments on digestion with trypsin exhibit high temperature dependence, indicating that, in the E1 and E2 ground states, the ATPase conformation undergoes strong fluctuations related to internal protein dynamics. The fluctuations are tightly constrained by ATP binding and phosphoenzyme formation, and this constraint must be overcome by thermal activation and substrate-free energy to allow enzyme turnover. In fact, a substantial portion of ATP free energy is utilized for conformational work related to the E1 approximately P.2Ca(2+) to E2-P transition, thereby disrupting high affinity binding and allowing luminal diffusion of Ca(2+). The E2 state and luminal path closure follow removal of conformational constraint by phosphate.

Protonation and hydrogen bonding of Ca2+ site residues in the E2P phosphoenzyme intermediate of sarcoplasmic reticulum Ca2+-ATPase studied by a combination of infrared spectroscopy and electrostatic calculations

Protonation of the Ca(2+) ligands of the SR Ca(2+)-ATPase (SERCA1a) was studied by a combination of rapid scan FTIR spectroscopy and electrostatic calculations. With FTIR spectroscopy, we investigated the pH dependence of C=O bands of the Ca(2+)-free phosphoenzyme (E2P) and obtained direct experimental evidence for the protonation of carboxyl groups upon Ca(2+) release. At least three of the infrared signals from protonated carboxyl groups of E2P are pH dependent with pK(a) values near 8.3: a band at 1758 cm(-1) characteristic of nonhydrogen-bonded carbonyl groups, a shoulder at 1720 cm(-1), and part of a band at 1710 cm(-1), both characteristic of hydrogen-bonded carbonyl groups. The bands are thus assigned to H(+) binding residues, some of which are involved in H(+) countertransport. At pH 9, bands at 1743 and 1710 cm(-1) remain which we do not attribute to Ca(2+)/H(+) exchange. We also obtained evidence for a pH-dependent conformational change in beta-sheet or turn structures of the ATPase. With MCCE on the E2P analog E2(TG+MgF(4)(2-)), we assigned infrared bands to specific residues and analyzed whether or not the carbonyl groups of the acidic Ca(2+) ligands are hydrogen bonded. The carbonyl groups of Glu(771), Asp(800), and Glu(908) were found to be hydrogen bonded and will thus contribute to the lower wave number bands. The carbonyl group of some side-chain conformations of Asp(800) is left without a hydrogen-bonding partner; they will therefore contribute to the higher wave number band.

Pre-steady state electrogenic events of Ca2+/H+ exchange and transport by the Ca2+-ATPase

Native or recombinant SERCA (sarco(endo)plasmic reticulum Ca(2+) ATPase) was adsorbed on a solid supported membrane and then activated with Ca(2+) and ATP concentration jumps through rapid solution exchange. The resulting electrogenic events were recorded as electrical currents flowing along the external circuit. Current transients were observed following Ca(2+) jumps in the absence of ATP and following ATP jumps in the presence of Ca(2+). The related charge movements are attributed to Ca(2+) reaching its binding sites in the ground state of the enzyme (E(1)) and to its vectorial release from the enzyme phosphorylated by ATP (E(2)P). The Ca(2+) concentration and pH dependence as well as the time frames of the observed current transients are consistent with equilibrium and pre-steady state biochemical measurements of sequential steps within a single enzymatic cycle. Numerical integration of the current transients recorded at various pH values reveal partial charge compensation by H(+) in exchange for Ca(2+) at acidic (but not at alkaline) pH. Most interestingly, charge movements induced by Ca(2+) and ATP vary over different pH ranges, as the protonation probability of residues involved in Ca(2+)/H(+) exchange is lower in the E(1) than in the E(2)P state. Our single cycle measurements demonstrate that this difference contributes directly to the reduction of Ca(2+) affinity produced by ATP utilization and results in the countertransport of two Ca(2+) and two H(+) within each ATPase cycle at pH 7.0. The effects of site-directed mutations indicate that Glu-771 and Asp-800, within the Ca(2+) binding domain, are involved in the observed Ca(2+)/H(+) exchange.

SERCA2a in heart failure: role and therapeutic prospects

Ca(2+) is a key molecule controlling several cellular processes, from fertilization to cell death, in all cell types. In excitable and contracting cells, such as cardiac myocytes, Ca(2+) controls muscle contractility. The spatial and temporal segregation of Ca(2+) concentrations are central to maintain its concentration gradients across the cells and the cellular compartments for proper function. SERCA2a is a cornerstone molecule for maintaining a balanced concentration of Ca(2+) during the cardiac cycle, since it controls the transport of Ca(2+) to the sarcoplasmic reticulum (SR) during relaxation. Alterations of the activity of this pump have been widely investigated, emphasizing its central role in the control of Ca(2+) homeostasis and consequently in the pathogenesis of the contractile defect seen with heart failure. This review focuses on the molecular characteristics of the pump, its role during the cardiac cycle and the prospects derived from the manipulation of SERCA2a for heart failure treatment.

Modulatory and catalytic modes of ATP binding by the calcium pump

We present crystal structures of the calcium-free E2 state of the sarcoplasmic reticulum Ca2+ -ATPase, stabilized by the inhibitor thapsigargin and the ATP analog AMPPCP. The structures allow us to describe the ATP binding site in a modulatory mode uncoupled from the Asp351 phosphorylation site. The Glu439 side chain interacts with AMPPCP via an Mg2+ ion in accordance with previous Fe2+ -cleavage studies implicating this residue in the ATPase cycle and in magnesium binding. Functional data on Ca2+ mediated activation indicate that the crystallized state represents an initial stage of ATP modulated deprotonation of E2, preceding the binding of Ca2+ ions in the membrane from the cytoplasmic side. We propose a mechanism of Ca2+ activation of phosphorylation leading directly from the compact E2-ATP form to the Ca2E1-ATP state. In addition, a role of Glu439 in ATP modulation of other steps of the functional cycle is suggested.

Cloning, sequencing and functional expression in Escherichia coli of the gene for a P-type Na(+)-ATPase of a facultatively anaerobic alkaliphile, Exiguobacterium aurantiacum

Cloning and sequencing of the gene encoding a P-type Na(+)-ATPase of a facultatively anaerobic alkaliphile, Exiguobacterium aurantiacum, were conducted. The structural gene was composed of 2628 nucleotides. The deduced amino acid sequence (876 amino acid residues; Mr, 96,664) suggested that the enzyme possesses 10 membrane-spanning regions. When the amino acid sequences of the four putative membrane regions, M4, M5, M6 and M8, of BL77/1 ATPase were aligned with those of fungal Na(+)-ATPase, Na(+)/K(+)-ATPase, H(+)-ATPases and sarcoplasmic reticulum Ca(2+)-ATPase, it exhibited the highest homology with Ca(2+)-ATPase except M5 region. By the transformation of Escherichia coli with the expression vector (pQE30) containing the ATPase gene, the enzyme was functionally expressed in E. coli membranes.

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.

The fourth transmembrane segment of the Na,K-ATPase alpha subunit: a systematic mutagenesis study

The Na,K-ATPase is a major ion-motive ATPase of the P-type family responsible for many aspects of cellular homeostasis. To determine the structure of the pathway for cations across the transmembrane portion of the Na,K-ATPase, we mutated 24 residues of the fourth transmembrane segment into cysteine and studied their function and accessibility by exposure to the sulfhydryl reagent 2-aminoethyl-methanethiosulfonate. Accessibility was also examined after treatment with palytoxin, which transforms the Na,K-pump into a cation channel. Of the 24 tested cysteine mutants, seven had no or a much reduced transport function. In particular cysteine mutants of the highly conserved "PEG" motif had a strongly reduced activity. However, most of the non-functional mutants could still be transformed by palytoxin as well as all of the functional mutants. Accessibility, determined as a 2-aminoethyl-methanethiosulfonate-induced reduction of the transport activity or as inhibition of the membrane conductance after palytoxin treatment, was observed for the following positions: Phe(323), Ile(322), Gly(326), Ala(330), Pro(333), Glu(334), and Gly(335). In accordance with a structural model of the Na,K-ATPase obtained by homology modeling with the two published structures of sarcoplasmic and endoplasmic reticulum calcium ATPase (Protein Data Bank codes 1EUL and 1IWO), the results suggest the presence of a cation pathway along the side of the fourth transmembrane segment that faces the space between transmembrane segments 5 and 6. The phenylalanine residue in position 323 has a critical position at the outer mouth of the cation pathway. The residues thought to form the cation binding site II ((333)PEGL) are also part of the accessible wall of the cation pathway opened by palytoxin through the Na,K-pump.

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.

Dissection of the functional differences between sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) 1 and 2 isoforms and characterization of Darier disease (SERCA2) mutants by steady-state and transient kinetic analyses

Steady-state and rapid kinetic studies were conducted to functionally characterize the overall and partial reactions of the Ca2+ transport cycle mediated by the human sarco(endo)plasmic reticulum Ca2+-ATPase 2 (SERCA2) isoforms, SERCA2a and SERCA2b, and 10 Darier disease (DD) mutants upon heterologous expression in HEK-293 cells. SERCA2b displayed a 10-fold decrease in the rate of Ca2+ dissociation from E1Ca2 relative to SERCA2a (i.e. SERCA2b enzyme manifests true high affinity at cytosolic Ca2+ sites) and a lower rate of dephosphorylation. These fundamental kinetic differences explain the increased apparent affinity for activation by cytosolic Ca2+ and the reduced catalytic turnover rate in SERCA2b. Relative to SERCA1a, both SERCA2 isoforms displayed a 2-fold decrease of the rate of E2 to E1Ca2 transition. Furthermore, seven DD mutants were expressed at similar levels as wild type. The expression level was 2-fold reduced for Gly23 --> Glu and Ser920 --> Tyr and 10-fold reduced for Gly749 --> Arg. Uncoupling between Ca2+ translocation and ATP hydrolysis and/or changes in the rates of partial reactions account for lack of function for 7 of 10 mutants: Gly23 --> Glu (uncoupling), Ser186 --> Phe, Pro602 --> Leu, and Asp702 --> Asn (block of E1 approximately P(Ca2) to E2-P transition), Cys318 --> Arg (uncoupling and 3-fold reduction of E2-P to E2 transition rate), and Thr357 --> Lys and Gly769 --> Arg (lack of phosphorylation). A 2-fold decrease in the E1 approximately P(Ca2) to E2-P transition rate is responsible for the 2-fold decrease in activity for Pro895 --> Leu. Ser920 --> Tyr is a unique DD mutant showing an enhanced molecular Ca2+ transport activity relative to wild-type SERCA2b. In this case, the disease may be a consequence of the low expression level and/or reduction of Ca2+ affinity and sensitivity to inhibition by lumenal Ca2+.

Effect of Hailey-Hailey Disease mutations on the function of a new variant of human secretory pathway Ca2+/Mn2+-ATPase (hSPCA1)

ATP2C1, encoding the human secretory pathway Ca2+/Mn2+ ATPase (hSPCA1), was recently identified as the defective gene in Hailey-Hailey Disease (HHD), an autosomal dominant skin disorder characterized by persistent blisters and erosions. To investigate the underlying cause of HHD, we have analyzed the changes in expression level and function of hSPCA1 caused by mutations found in HHD patients. Mutations were introduced into hSPCA1d, a novel splice variant expressed in keratinocytes, described here for the first time. Encoded by the full-length of optional exons 27 and 28, hSPCA1d was longer than previously identified splice variants. The protein competitively transported Ca2+ and Mn2+ with equally high affinity into the Golgi of COS-1 cells. Ca2+- and Mn2+-dependent phosphoenzyme intermediate formation in forward (ATP-fuelled) and reverse (Pi-fuelled) directions was also demonstrated. HHD mutant proteins L341P, C344Y, C411R, T570I, and G789R showed low levels of expression, despite normal levels of mRNA and correct targeting to the Golgi, suggesting instability or abnormal folding of the mutated hSPCA1 polypeptides. P201L had little effect on the enzymatic cycle, whereas I580V caused a block in the E1 approximately P --> E2-P conformational transition. D742Y and G309C were devoid of Ca2+- and Mn2+-dependent phosphoenzyme formation from ATP. The capacity to phosphorylate from Pi was retained in these mutants but with a loss of sensitivity to both Ca2+ and Mn2+ in D742Y and a preferential loss of sensitivity to Mn2+ in G309C. These results highlight the crucial role played by Asp-742 in the architecture of the hSPCA1 ion-binding site and reveal a role for Gly-309 in Mn2+ transport selectivity.

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.

Mutagenesis of residues involved in control of the Ca2+ entry pathway and conformational changes associated with Ca2+ binding in the SR Ca2+-ATPase

Rapid kinetic measurements were used to study the rate of Ca(2+) dissociation from the high-affinity Ca(2+) sites of the dephosphoenzyme (i.e., from the E(1)Ca(2) form toward the cytoplasmic side) as well as the rate of Ca(2+) binding with associated conformational changes (E(2) --> E(1)Ca(2) transition) in the wild type and mutants of the sarcoplasmic reticulum Ca(2+)-ATPase expressed in mammalian cells. Cluster mutations as well as single mutations in transmembrane segment M3 resulted in conspicuous effects on the rate of Ca(2+) migration. Furthermore, mutation of Asp(59) in transmembrane segment M1 to arginine exerted a profound effect on Ca(2+) interaction. The data demonstrate an important role for M3 residues in control of the Ca(2+) entry pathway and provide functional evidence in support of a close relationship between this pathway and the water-accessible channel leading between transmembrane segments M1 and M3 in the thapsigargin stabilized E(2) structure. In addition, rapid kinetic measurements demonstrated that the hydrogen bond network involving Asp(813) of loop L6-7 and Lys(758) of M5 is important for the E(2) --> E(1)Ca(2) transition.

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.

Hailey-Hailey disease: molecular and clinical characterization of novel mutations in the ATP2C1 gene

Hailey-Hailey disease is an autosomal dominant skin disorder characterized by suprabasal cell separation (acantholysis) of the epidermis. Mutations in ATP2C1, the gene encoding a novel, P-type Ca2+-transport ATPase, were recently found to cause Hailey-Hailey disease. In this study, we used conformation-sensitive gel electrophoresis to screen all 28 translated exons of ATP2C1 in 24 Hailey-Hailey disease families and three sporadic cases with the disorder. We identified 22 different mutations, 18 of which have not previously been reported, in 25 probands. The novel mutations comprise three nonsense, six insertion/deletion, three splice-site, and six missense mutations and are distributed throughout the ATP2C1 gene. Six mutations were found in multiple families investigated here or in our previous study. Haplotype analysis revealed that two of these are recurrent mutations that have not been inherited from a common ancestor. Comparison between genotype and phenotype in 23 families failed to yield any clear correlation between the nature of the mutation and clinical features of Hailey-Hailey disease. The extensive interfamilial and intrafamilial phenotypic variability observed suggests that modifying genes and/or environmental factors may greatly influence the clinical features of this disease.

Thapsigargin and dimethyl sulfoxide activate medium P(i)<-->HOH oxygen exchange catalyzed by sarcoplasmic reticulum Ca2+-ATPase

Thapsigargin is a potent inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase. It binds the Ca(2+)-free E2 conformation in the picomolar range, supposedly resulting in a largely catalytically inactive species. We now find that thapsigargin has little effect on medium P(i) <--> HOH oxygen exchange and that this activity is greatly stimulated (up to 30-fold) in the presence of 30% (v/v) Me(2)SO. Assuming a simple two-step mechanism, we have evaluated the effect of thapsigargin and Me(2)SO on the four rate constants governing the reaction of P(i) with Ca(2+)-ATPase. The principal effect of thapsigargin alone is to stimulate EP hydrolysis (k(-2)), whereas that of Me(2)SO is to greatly retard P(i) dissociation (k(-1)), accounting for its well known effect on increasing the apparent affinity for P(i). These effects persist when the agents are used in combination and substantially account for the activated oxygen exchange (v(exchange) = k(-2)[EP]). Kinetic simulations show that the overall rate constant for the formation of EP is very fast (approximately 300 s(-1)) when the exchange is maximal. Thapsigargin greatly stabilizes Ca(2+)-ATPase against denaturation in detergent in the absence of Ca(2+), as revealed by glutaraldehyde cross-linking, suggesting that the membrane helices lock together. It seems that the reactions at the phosphorylation site, associated with the activated exchange reaction, are occurring without much movement of the transport site helices, and we suggest that they may be associated solely with an occluded H+ state.

Site-Directed Mutagenesis of Cation Coordinating Residues in the Gastric H,K-ATPase

Site-mutations were introduced into putative cation binding site 1 of the H,K-ATPase at glu-797, thr-825, and glu-938. The side chain oxygen of each was not essential but the mutations produced different activation and inhibition kinetics. Site mutations thr-825 (ala, leu) and glu-938 (ala, gln) modestly decreased the apparent affinity to K+, while glu-797 (gln) was equivalent to wild type. As expected of competitive inhibition, mutations of thr-825 and glu-938 that decreased the apparent affinity for K+ also increased the apparent affinity for SCH28080. This is consistent with the participation of thr-825 and glu-938 in a cation binding domain. The sidechain geometry, but not the sidechain charge of glu-797, is essential to ATPase function as the site mutant glu-797 (gly) inactivated the H,K-ATPase, while glu-797 (gln) was active but the apparent affinity to SCH 28080 was decreased by four-fold. Lys-793, a unique residue of the H,K-ATPase, was essential for ATPase function. Since this residue is adjacent to site 1, the result suggests that charge pairing between lys-793 and residues at or near this site may be essential to ATPase function.

Single amino acid mutations in transmembrane domain 5 confer to the plasma membrane Ca2+ pump properties typical of the Ca2+ pump of endo(sarco)plasmic reticulum

Conserved residues in some of the transmembrane domains are proposed to mediate ion translocation by P-type pumps. The plasma membrane Ca(2+) pump (PMCA) lacks 2 of these residues in transmembrane domains (TM) 5 and 8. In particular, a glutamic acid (Glu-771) residue in TM5, which is proposed to be involved in the binding and transport of Ca(2+) by the sarcoplasmic reticulum Ca(2+) pump (SERCA), is replaced by an alanine (Ala-854) in the PMCA pump. Ala-854 has been mutated to Glu, Asp, or Gln; Glu-975 in TM8, which is an Ala in the SERCA pump, has been mutated to Gln, Asp, or Ala. The mutants have been expressed in three cell systems, with or without the help of viruses. When expressed in large amounts in Sf9 cells, the mutated pumps were isolated and analyzed in the purified state. Two of the three TM8 mutants were correctly delivered to the plasma membrane and were active. All the TM5 mutants were retained in the endoplasmic reticulum; two of them (A854Q and A854E) retained activity. Their properties (La(3+) sensitivity and decay of the phosphorylated intermediate, higher cooperativity of Ca(2+) binding with a Hill's coefficient approaching 2) differed from those of the expressed wild type PMCA pump, and resembled those of the SERCA pump.

Stalk segment 4 of the yeast plasma membrane H+-ATPase. Mutational evidence for a role in the E1-E2 conformational change

In the P(2)-type ATPases, there is growing evidence that four alpha-helical stalk segments connect the cytoplasmic part of the molecule, responsible for ATP binding and hydrolysis, to the membrane-embedded part that mediates cation transport. The present study has focused on stalk segment 4, which displays a significant degree of sequence conservation among P(2)-ATPases. When site-directed mutants in this region of the yeast plasma membrane H(+)-ATPase were constructed and expressed in secretory vesicles, more than half of the amino acid substitutions led to a severalfold decrease in the rate of ATP hydrolysis, although they had little or no effect on the coupling between hydrolysis and transport. Strikingly, mutant ATPases bearing single substitutions of 13 consecutive residues from Ile-359 through Gly-371 were highly resistant to inorganic orthovanadate, with IC(50) values at least 10-fold above those seen in the wild-type enzyme. Most of the same mutants also displayed a significant reduction in the K(m) for MgATP and an increase in the pH optimum for ATP hydrolysis. Taken together, these changes in kinetic behavior point to a shift in equilibrium from the E(2) conformation of the ATPase toward the E(1) conformation. The residues from Ile-359 through Gly-371 would occupy three full turns of an alpha-helix, suggesting that this portion of stalk segment 4 may provide a conformationally active link between catalytic sites in the cytoplasm and cation-binding sites in the membrane.

Functional role of charged residues in the transmembrane segments of the yeast plasma membrane H+-ATPase

As defined by hydropathy analysis, the membrane-spanning segments of the yeast plasma membrane H(+)-ATPase contain seven negatively charged amino acids (Asp and Glu) and four positively charged amino acids (Arg and His). To explore the functional role of these residues, site-directed mutants at all 11 positions and at Glu-288, located near the cytoplasmic end of M3, have been constructed and expressed in yeast secretory vesicles. Substitutions at four of the positions (Glu-129, Glu-288, Asp-833, and Arg-857) had no significant effect on ATP hydrolysis or ATP-dependent proton pumping, substitutions at five additional positions (Arg-695, His-701, Asp-730, Asp-739, and Arg-811) led to misfolding of the ATPase and blockage at an early stage of biogenesis, and substitutions of Asp-143 allowed measurable biogenesis but nearly abolished ATP hydrolysis and proton transport. Of greatest interest were mutations of Glu-703 in M5 and Glu-803 in M8, which altered the apparent coupling between hydrolysis and transport. Three Glu-703 mutants (E703Q, E703L, E703D) showed significantly reduced pumping over a wide range of hydrolysis values and thus appeared to be partially uncoupled. At Glu-803, by contrast, one mutant (E803N) was almost completely uncoupled, while another (E803Q) pumped protons at an enhanced rate relative to the rate of ATP hydrolysis. Both Glu-703 and Glu-803 occupy positions at which amino acid substitutions have been shown to affect transport by mammalian P-ATPases. Taken together, the results provide growing evidence that residues in membrane segments 5 and 8 of the P-ATPases contribute to the cation transport pathway and that the fundamental mechanism of transport has been conserved throughout the group.

Molecular aspects of higher plant P-type Ca(2+)-ATPases

Recent genomic data in the model plant Arabidopsis thaliana reveal the existence of at least 11 Ca(2+)-ATPase genes, and an analysis of expressed sequence tags suggests that the number of calcium pumps in this organism might be even higher. A phylogenetic analysis shows that 11 Ca(2+)-ATPases clearly form distinct groups, type IIA (or ECA for ER-type Ca(2+)-ATPase) and type IIB (ACA for autoinhibited Ca(2+)-ATPase). While plant IIB calcium pumps characterized so far are localized to internal membranes, their animal homologues are exclusively found in the plasma membrane. However, Arabidopsis type IIB calcium pump isoforms ACA8, ACA9 and ACA10 form a separate outgroup and, based on the high molecular masses of the encoded proteins, are good candidates for plasma membrane bound Ca(2+)-ATPases. All known plant type IIB calcium ATPases seem to employ an N-terminal calmodulin-binding autoinhibitor. Therefore it appears that the activity of type IIB Ca(2+)-ATPases in plants and animals is controlled by N-terminal and C-terminal autoinhibitory domains, respectively. Possible functions of plant calcium pumps are described and - beside second messenger functions directly linked to calcium homeostasis - new data on a putative involvement in secretory and salt stress functions are discussed.

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

During active cation transport, sarcoplasmic reticulum Ca2+-ATPase, like other P-type ATPases, undergoes major conformational changes, some of which are dependent on Ca2+ binding to high affinity transport sites. We here report that, in addition to previously described residues of the transmembrane region (Clarke, D. M., Loo, T. W., Inesi, G., and MacLennan, D. H. (1989) Nature 339, 476-478), the region located in the cytosolic L6-7 loop connecting transmembrane segments M6 and M7 has a definite influence on the sensitivity of the Ca2+-ATPase to Ca2+, i.e. on the affinity of the ATPase for Ca2+. Cluster mutation of aspartic residues in this loop results in a strong reduction of the affinity for Ca2+, as shown by the Ca2+ dependence of ATPase phosphorylation from either ATP or Pi. The reduction in Ca2+ affinity for phosphorylation from Pi is observed both at acidic and neutral pH, suggesting that these mutations interfere with binding of the first Ca2+, as proposed for some of the intramembranous residues essential for Ca2+ binding (Andersen, J. P. (1995) Biosci. Rep. 15, 243-261). Treatment of the mutated Ca2+-ATPase with proteinase K, in the absence or presence of various Ca2+ concentrations, leads to Ca2+-dependent changes in the proteolytic degradation pattern similar to those in the wild type but observed only at higher Ca2+ concentrations. This implies that these effects are not due to changes in the conformational state of Ca2+-free ATPase but that changes affecting the proteolytic digestion pattern require higher Ca2+ concentrations. We conclude that aspartic residues in the L6-7 loop might interact with Ca2+ during the initial steps of Ca2+ binding.

Site-directed disulfide mapping of helices M4 and M6 in the Ca2+ binding domain of SERCA1a, the Ca2+ ATPase of fast twitch skeletal muscle sarcoplasmic reticulum

In an attempt to define the spatial relationships among SERCA1a transmembrane helices M4, M5, M6, and M8, involved in Ca2+ binding, all six cysteine residues were removed from predicted transmembrane sequences by substitution with Ser or Ala. The cysteine-depleted protein retained 44% of wild type Ca2+ transport activity. Pairs of cysteine residues were then reintroduced to determine whether their juxtaposition would result in the formation of disulfide cross-links between transmembrane helices. In initial studies designed to map the juxtaposition of Ca2+ binding residues, Cys was substituted for Glu309 or Gly310 in transmembrane sequence M4, in combination with the substitution of Cys for Glu771 in M5; for Asn796, Thr799, or Asp800 in M6; or for Glu908 in M8. These double mutants all retained the capacity to form a phosphoenzyme intermediate from Pi (but not from ATP in the presence of Ca2+), and in all but mutants E309C/N796C and G310C/N796C, phosphoenzyme formation was insensitive to 100 microM Ca2+. These results support the view that both Glu309 and Asn796 contribute to Ca2+ binding site II, which is not required for conversion of E2, the substrate for Pi phosphorylation, to E1. Cross-linking in mutants E309C/N796C and G310C/D800C established reference points for the orientation of M4 and M6 relative to each other and provided the basis for the prediction of potential additional cross-links. Strong links were formed with the pairs T317C/A804C and T317C/L807C near the cytoplasmic ends of the two helices and with A305C/L792C and A305C/L793C near the lumenal ends. These combined results support the conclusion that M4 and M6 form a right-handed coiled-coil structure that forms part of the pathway of Ca2+ translocation. In addition to providing a possible explanation for the mutation sensitivity of several pairs of residues in these helices, the proposed association of M4 and M6 supports a new model for the orientation of the two Ca2+ binding sites among transmembrane helices M4, M5, and M6.

Scanning mutagenesis reveals a similar pattern of mutation sensitivity in transmembrane sequences M4, M5, and M6, but not in M8, of the Ca2+-ATPase of sarcoplasmic reticulum (SERCA1a)

Scanning mutagenesis was performed on all amino acids in transmembrane sequences M5, M6, and M8, which, together with M4, make up the Ca2+ binding domain of the Ca2+-ATPase of sarcoplasmic reticulum (SERCA1a). When these transmembrane sequences were displayed on a helical net, examination of the effects of 101 novel point mutations and 95 prior mutations carried out on 92 transmembrane amino acids revealed "patches" of sensitivity to mutation in M4, M5, and M6 but not in M8. The patches of mutation-sensitive residues spanned 6 of the 7 tiers of the helical net and covered about 240 degrees at their widest point in tiers 3 or 4 and 140 degrees in tiers 2 and 5. A contiguous column of mutation-insensitive hydrophobic amino acids was found in M4 and M6 and in tiers 4 to 7 of M5. A six-residue motif, (E/D)GLPA(T/V) in tiers 3 and 4 of M4 and M6 with Ca2+-binding residues Glu309 and Asp800 as the first residue, was highlighted by mutation sensitivity. Elements of the motif could also be discerned in M5, but reading in the C-terminal to N-terminal direction. Mutation sensitivity in tier 5 of M4 mirrored mutation sensitivity of tier 5 in M6, although the amino acid sequences were not similar. The motif or its counterpart was found in a region in M4, M5, and M6 that is made up of tiny or small amino acids but is bounded by tiers with a larger percentage of bulky amino acids. Tiers 3, 4, and 5 of M4, M5, and M6 contain Ca2+ binding and affinity mutations, E1P to E2P block mutations and E2P dephosphorylation mutations, indicating an important role for these central tiers in Ca2+ binding and in the conformational changes that accompany Ca2+ translocation. Analysis of M8 revealed only a single mutation-sensitive residue, the Ca2+-binding amino acid, Glu908. This residue and a mutation-insensitive residue, Ala912, were the only vestiges of the motif that was found in M4 and M6. Additional mutations to Glu908 provided further evidence for its role in Ca2+ binding. Since mutation of M8 failed to identify residues involved in blocking conformational changes or altering Ca2+ affinity, it is apparent that M8 plays a peripheral role in Ca2+ binding and translocation in comparison with M4, M5, and M6.

Role of negatively charged residues in the fifth and sixth transmembrane domains of the catalytic subunit of gastric H+,K+-ATPase

The role of six negatively charged residues located in or around the fifth and sixth transmembrane domain of the catalytic subunit of gastric H+,K+-ATPase, which are conserved in P-type ATPases, was investigated by site-directed mutagenesis of each of these residues. The acid residues were converted into their corresponding acid amides. Sf9 cells were used as the expression system using a baculovirus with coding sequences for the alpha- and beta-subunits of H+,K+-ATPase behind two different promoters. Both subunits of all mutants were expressed like the wild type enzyme in intracellular membranes of Sf9 cells as indicated by Western blotting experiments, an enzyme-linked immunosorbent assay, and confocal laser scan microscopy studies. The mutants D824N, E834Q, E837Q, and D839N showed no 3-(cyanomethyl)-2-methyl-8(phenylmethoxy)-imidazo[1, 2a]pyridine (SCH 28080)-sensitive ATP dependent phosphorylation capacity. Mutants E795Q and E820Q formed a phosphorylated intermediate, which, like the wild type enzyme, was hydroxylamine-sensitive, indicating that an acylphosphate was formed. Formation of the phosphorylated intermediate from the E795Q mutant was similarly inhibited by K+ (I50 = 0.4 mM) and SCH 28080 (I50 = 10 nM) as the wild type enzyme, when the membranes were preincubated with these ligands before phosphorylation. The dephosphorylation reaction was K+-sensitive, whereas ADP had hardly any effect. Formation of the phosphorylated intermediate of mutant E820Q was much less sensitive toward K+ (I50 = 4.5 mM) and SCH 28080 (I50 = 1.7 microM) than the wild type enzyme. The dephosphorylation reaction of this intermediate was not stimulated by either K+ or ADP. In contrast to the wild type enzyme and mutant E795Q, mutant E820Q did not show any K+-stimulated ATPase activity. These findings indicate that residue Glu820 might be involved in K+ binding and transition to the E2 form of gastric H+,K+-ATPase.

Alanine-scanning mutagenesis along membrane segment 4 of the yeast plasma membrane H+-ATPase. Effects on structure and function

Membrane segment 4 of P-type cation pumps has been suggested to play a critical role in the coupling of ATP hydrolysis to ion translocation. In this study, structure-function relationships in M4 of the yeast (Saccharomyces cerevisiae) plasma membrane H+-ATPase have been explored by alanine-scanning mutagenesis. Mutant enzymes were expressed behind an inducible heat-shock promoter in yeast secretory vesicles, as described previously (Nakamoto, R. K., Rao, R. , and Slayman, C. W. (1991) J. Biol. Chem. 266, 7940-7949). One substitution (I329A) led to arrest of the enzyme at an early stage of biogenesis, and three others (G333A, L338A, G349A) reduced ATP hydrolysis to near-background levels. The remaining 26 mutants were expressed well enough in secretory vesicles (44-121% of wild type) and had sufficient ATPase activity (16-123% of wild type) to be characterized in detail. When acridine orange fluorescence quenching was used to measure rates of ATP-dependent proton pumping over a range of ATP concentrations, only minor changes were seen. In kinetic studies, however, seven of the mutant enzymes (I331A, I332A, V334A, V336A, V341A, V342A, and M346A) were resistant to vanadate inhibition, and three of them (I332A, V336A, and V341A) also had a decreased Km and increased pH optimum for ATP hydrolysis. Limited trypsinolysis was used to probe the structure of two different Val-336 substitutions, V336A, described above, and V336R, which displayed little or no ATPase activity. Both were cleaved at a relatively normal rate to give a pattern of fragments essentially identical to that seen with the wild-type enzyme. However, while vanadate, ADP, and ATP were able to protect the wild-type and V336A enzymes against trypsinolysis, the V336R ATPase was protected only by ADP and ATP. Taken together, the data suggest that key residues in the M4 segment may help to communicate the E1-E2 conformational change to ion-binding sites in the membrane.

Site-directed mutagenesis studies of energy coupling in the sarcoplasmic reticulum Ca(2+)-ATPase

Site-directed mutagenesis studies identifying residues important to energy transduction in the sarcoplasmic reticulum Ca(2+)-ATPase are reviewed. Mutations blocking the crucial E1P to E2P transition are located in the small and the large cytoplasmic domains, in the stalk segment S4 linking transmembrane segment M4 with the catalytic site, as well as in transmembrane segments M4 and M8. Mutations that block the dephosphorylation of the E2P phosphoenzyme intermediate are located in transmembrane segments M4, M5, and M6, i.e., in the same domain as the Ca(2+)-binding sites. Removal of the sidechain of Tyr763 located at the boundary between transmembrane segment M5 and the corresponding stalk segment S5 linking M5 with the catalytic site leads to uncoupling of ATP hydrolysis from Ca2+ uptake. Uncoupling may be due to efflux through the Ca(2+)-ATPase of Ca2+ that has been transported, and may thus be caused by a defective gating process in the late part of the catalytic cycle. A nearby located residue Lys758 is also involved in energy coupling, since its substitution with Ile activates dephosphorylation at high pH and slows the E2 to E1 transition.

Critical effects on catalytic function produced by amino acid substitutions at Asp804 and Asp808 of the alpha1 isoform of Na,K-ATPase

At two intramembrane carboxyl-containing amino acids of the sheep alpha1 isoform of Na,K-ATPase (Asp804 and Asp808), both charge-conserving (Asp to Glu) and charge-deleting (Asp to Asn, Leu and Ala) replacements were made and the altered enzymes studied. Nucleotide changes encoding the amino acid substitutions were placed in a cDNA encoding a ouabain-resistant enzyme (sheep alpha1 RD) and the encoded enzymes were expressed in ouabain-sensitive HeLa cells. Transfections with cDNAs carrying all Asp804 substitutions, along with those carrying Asp808Ala, Asp808Asn, and Asp808Leu replacements failed to confer ouabain resistance to the cells, indicating critical roles for Asp804 and Asp808. Only the expression of the Asp808Glu enzyme produced ouabain-resistant HeLa cells, demonstrating that the altered protein was functional. When the inactive proteins Asp804Ala and Asp808Ala were expressed using an alternative selection system (the protein carrying the amino acid substitution was the ouabain-sensitive wild-type sheep alpha1 Na,K-ATPase, which was expressed in ouabain-resistant 3T3 cells), intact cells were able to bind extracellular ouabain with high affinity (Kd = 1-30 nM), indicating that the inactive proteins were synthesized and folded properly in the plasma membrane. The results demonstrate that carboxyl side chains at positions 804 and 808 are critical for enzyme catalytic function.

Short and long range functions of amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase. A mutational study

Mutational analysis of several amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase was performed by expressing wild type ATPase and 32 site-directed mutants in COS-1 cells followed by functional characterization of the microsomal fraction. Four different phenotype characteristics were observed in the mutants: (a) functions similar to those sustained by the wild type ATPase; (b) Ca2+ transport inhibited to a greater extent than ATPase hydrolytic activity; (c) inhibition of transport and hydrolytic activity in the presence of high levels of phosphorylated enzyme intermediate; and (d) total inhibition of ATP utilization by the enzyme while retaining the ability to form phosphoenzyme by utilization of P(i). Analysis of experimental observations and molecular models revealed short and long range functions of several amino acids within the transmembrane region. Short range functions include: (a) direct involvement of five amino acids in Ca2+ binding within a channel formed by clustered transmembrane helices M4, M5, M6, and M8; (b) roles of several amino acids in structural stabilization of the helical cluster for optimal channel function; and (c) a specific role of Lys297 in sealing the distal end of the channel, suggesting that the M4 helix rotates to allow vectorial flux of Ca2+ upon enzyme phosphorylation. Long range functions are related to the influence of several transmembrane amino acids on phosphorylation reactions with ATP or P(i), transmitted to the extramembranous region of the ATPase in the presence or in the absence of Ca2+.

Dissection of the functional domains of the sarcoplasmic reticulum Ca(2+)-ATPase by site-directed mutagenesis

The results of site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase are reviewed. More than 250 different point mutants have been expressed in cell culture and analysed by a panel of functional assays. Thereby, 40-50 important amino acid residues have been pinpointed, and the mutants have been assigned to functional classes: the Ca(2+)-affinity mutants, the phosphorylation-negative mutants, the ATP-affinity mutants, the E1P mutants, the E2P mutants, and the uncoupled mutants. Moreover, regions important to the specific inhibition by thapsigargin have been identified by analysis of Ca(2+)-ATPase/Na+,K(+)-ATPase chimeric constructs.

Glutamic acid 327 in the sheep alpha 1 isoform of Na+,K(+)-ATPase stabilizes a K(+)-induced conformational change

By combining the tools of site-directed mutagenesis and [3H]ouabain binding, the functional role of glutamic acid 327 in the fourth transmembrane domain of the sheep alpha 1 isoform of Na+,K(+)-ATPase was examined with respect to its interactions with ouabain, Na+,K+,Mg2+, and inorganic phosphate. Using site-directed mutagenesis, this glutamic acid was substituted with alanine, aspartic acid, glutamine, and leucine. The mutant proteins were constructed in a sheep alpha 1 protein background such that [3H]ouabain binding could be utilized as a highly specific probe of the exogenous protein expressed in NIH 3T3 cells. Na+ competition of [3H]ouabain binding to the mutant forms of Na+,K(+)-ATPase revealed only slight alterations in their affinities for Na+ and in their abilities to undergo Na(+)-induced conformational changes which inhibit ouabain binding. In contrast, K+ competition of [3H]ouabain binding to all four mutant forms of Na+,K(+)-ATPase displayed severely altered interactions between these proteins and K+. Interestingly, [3H]ouabain binding to the mutant E327Q was not inhibited by the presence of K+. This mutant was previously reported to be functionally able to support cation transport with a 5-fold reduced K0.5 for K(+)-dependent ATPase activity (Jewell-Motz, E. A., and Lingrel, J.B. (1993) Biochemistry 32, 13523-13530; Vilsen, B. (1993) Biochemistry 32, 13340-13349). Thus, it appears that this glutamic acid in the fourth transmembrane domain may be important for stabilizing a K(+)-induced conformation within the catalytic cycle of Na+,K(+)-ATPase that is not rate-limiting in the overall ATPase cycle but that displays a greatly reduced affinity for ouabain.

Structure-function relationships of cation translocation by Ca(2+)- and Na+, K(+)-ATPases studied by site-directed mutagenesis

Site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase have pinpointed five amino acid residues that are essential to Ca2+ occlusion, and these residues have been assigned to different parts of a Ca2+ binding pocket with channel-like structure. Three of the homologous Na+, K(+)-ATPase residues have been shown to be important for binding of cytoplasmic Na+ at transport sites. In addition, three of the above mentioned Ca(2+)-ATPase residues appear to participate in the countertransport of H+, and two of the Na+, K(+)-ATPase residues to participate in the countertransport of K+. Residues involved in energy transducing conformational changes have also been identified by mutagenesis. In the Ca(2+)-ATPase, ATP hydrolysis is uncoupled from Ca2+ transport following mutation of a tyrosine residue located at the top of transmembrane segment M5. This tyrosine, present also in the Na+, K(+)-ATPase, may play a critical role in closing the gate to a transmembrane channel.

Functional consequences of alterations to amino acids at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum. Mutation Tyr763-->Gly uncouples ATP hydrolysis from Ca2+ transport

The roles of the hydrophobic side chains of residues Phe760, Ile761, Tyr763, Leu764, and Ile765 located at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum were analyzed by site-directed mutagenesis. Substitution of Tyr763 with glycine resulted in a new phenotypic variant of the Ca(2+)-ATPase that catalyzed a high rate of Ca(2+)-activated ATP hydrolysis without net accumulation of Ca2+ in the microsomal vesicles. The ATPase activity of the Tyr763-->Gly mutant displayed characteristics similar to the ATPase activity of the wild-type enzyme measured in the presence of calcium ionophore, and the mutant was able to form the ADP-insensitive phosphoenzyme intermediate. Mutants Phe760-->Gly, Ile761-->Gly, Leu764-->Gly, and Ile765-->Gly were able to accumulate Ca2+. In mutants Leu764-->Gly and Ile765-->Gly, the turnover rate was low due to inhibition of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate. On the other hand, mutant Leu764-->Lys dephosphorylated rapidly. Mutants Phe760-->Gly and Leu764-->Lys displayed apparent Ca2+ affinities that were reduced two and three orders of magnitude, respectively, relative to that of the wild-type.