Critical role of Glu40-Ser48 loop linking actuator domain and first transmembrane helix of Ca2+-ATPase in Ca2+ deocclusion and release from ADP-insensitive phosphoenzyme

The tilting motion of the central core reveals the transport mechanism of the sarco/endoplasmic reticulum Ca2+-ATPase

The sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) transports two Ca2+ ions per ATP hydrolyzed from the cytoplasm to the lumen. However, how the ATP hydrolysis remotely drives the Ca2+ transport is unclear. In the SERCA1a crystal structures, the ATP hydrolysis is accompanied by the notably increasing tilting angle of the central core (CC) and the Ca2+ transport, and the CC tilting angle dramatically decreases in the E2 to E1 transition. We demonstrated that the significantly increasing tilting motion of the CC drove the Ca2+ release in the molecular dynamics simulation of the R836A variant, and the dramatic spontaneous decrease in the CC tilting angle of the E2 state triggers the restart of the SERCA1a's transport cycle. The repulsion between the phosphorylated D351 and the phosphate groups in ADP triggers the release of ADP from the SERCA1a headpiece. We proposed a novel SERCA transport mechanism in which ATP hydrolysis drives a significant tilting motion of the CC, which drives Ca2+ transport and the A domain rotational motion in the E1P-ADP-2Ca2+ to E2P transition. The dramatic spontaneous decrease in the CC tilting angle of the E2 state drives the restart of the transport cycle.

Multiple sub-state structures of SERCA2b reveal conformational overlap at transition steps during the catalytic cycle

Sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) pumps Ca²⁺ into the endoplasmic reticulum (ER). Herein, we present cryo-electron microscopy (EM) structures of three intermediates of SERCA2b: Ca²⁺-bound phosphorylated (E1P·2Ca²⁺) and Ca²⁺-unbound dephosphorylated (E2·Pi) intermediates and another between the E2P and E2·Pi states. Our cryo-EM analysis demonstrates that the E1P·2Ca²⁺ state exists in low abundance and preferentially transitions to an E2P-like structure by releasing Ca²⁺ and that the Ca²⁺ release gate subsequently undergoes stepwise closure during the dephosphorylation processes. Importantly, each intermediate adopts multiple sub-state structures including those like the next one in the catalytic series, indicating conformational overlap at transition steps, as further substantiated by atomistic molecular dynamic simulations of SERCA2b in a lipid bilayer. The present findings provide insight into how enzymes accelerate catalytic cycles.

Structural basis of ion uptake in copper-transporting P1B-type ATPases

Copper is essential for living cells, yet toxic at elevated concentrations. Class 1B P-type (P1B-) ATPases are present in all kingdoms of life, facilitating cellular export of transition metals including copper. P-type ATPases follow an alternating access mechanism, with inward-facing E1 and outward-facing E2 conformations. Nevertheless, no structural information on E1 states is available for P1B-ATPases, hampering mechanistic understanding. Here, we present structures that reach 2.7 Å resolution of a copper-specific P1B-ATPase in an E1 conformation, with complementing data and analyses. Our efforts reveal a domain arrangement that generates space for interaction with ion donating chaperones, and suggest a direct Cu+ transfer to the transmembrane core. A methionine serves a key role by assisting the release of the chaperone-bound ion and forming a cargo entry site together with the cysteines of the CPC signature motif. Collectively, the findings provide insights into P1B-mediated transport, likely applicable also to human P1B-members.

Functional characterization of Legionella pneumophila Cu+ transport ATPase. The activation by Cu+ and ATP

Cu⁺-ATPases are integral membrane proteins belonging to the IB subfamily of the P-type ATPases that couple Cu⁺ transport to the hydrolysis of ATP. As some structural and functional particularities arise for Cu⁺-ATPases, several authors suggest that some of the reaction steps of the Albers-Post model postulated for other P-ATPases may be different. In this work we describe a functional characterization of Legionella pneumophila Cu⁺-ATPase (LpCopA), the first P_IB-ATPase whose structure was determined by X-ray crystallography. Cu⁺-ATPase activity of the enzyme presents a maximum at ∼37 °C and pH 6.6-6.8. Phospholipids enhance LpCopA Cu⁺-ATPase activity in a non-essential mode where optimal activity is achieved at an asolectin molar fraction of 0.15 and an amphiphile-protein ratio of ~30,000. As described for other P-ATPases, Mg²⁺ acts as an essential activator. Furthermore, Cu⁺-ATPase activity dependence on [Cu⁺] and [ATP] can both be described by a sum of two hyperbolic functions. Based on that, and the [Cu⁺] and [ATP] dependencies of the best fitting parameters of the hyperbolae pointed above, we propose a minimal reaction scheme for the catalytic mechanism that shares the basic reaction steps of the Albers-Post model for P-type ATPases. The reaction scheme postulated contemplates two different binding affinities for a single ATP (apparent affinities of 0.66 and 550 μM at [Cu⁺] → ∞) and binding of at least 2 Cu⁺ with different affinities as well (apparent affinities of 1.4 and 102.5 μM at [ATP] → ∞).

Angle change of the A-domain in a single SERCA1a molecule detected by defocused orientation imaging

The sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) transports Ca²⁺ ions across the membrane coupled with ATP hydrolysis. Crystal structures of ligand-stabilized molecules indicate that the movement of actuator (A) domain plays a crucial role in Ca²⁺ translocation. However, the actual structural movements during the transitions between intermediates remain uncertain, in particular, the structure of E2PCa₂ has not been solved. Here, the angle of the A-domain was measured by defocused orientation imaging using isotropic total internal reflection fluorescence microscopy. A single SERCA1a molecule, labeled with fluorophore ReAsH on the A-domain in fixed orientation, was embedded in a nanodisc, and stabilized on Ni–NTA glass. Activation with ATP and Ca²⁺ caused angle changes of the fluorophore and therefore the A-domain, motions lost by inhibitor, thapsigargin. Our high-speed set-up captured the motion during EP isomerization, and suggests that the A-domain rapidly rotates back and forth from an E1PCa₂ position to a position close to the E2P state. This is the first report of the detection in the movement of the A-domain as an angle change. Our method provides a powerful tool to investigate the conformational change of a membrane protein in real-time.

Structural Basis of Substrate-Independent Phosphorylation in a P4-ATPase Lipid Flippase

P4-ATPases define a eukaryotic subfamily of the P-type ATPases, and are responsible for the transverse flip of specific lipids from the extracellular or luminal leaflet to the cytosolic leaflet of cell membranes. The enzymatic cycle of P-type ATPases is divided into autophosphorylation and dephosphorylation half-reactions. Unlike most other P-type ATPases, P4-ATPases transport their substrate during dephosphorylation only, i.e. the phosphorylation half-reaction is not associated with transport. To study the structural basis of the distinct mechanisms of P4-ATPases, we have determined cryo-EM structures of Drs2p-Cdc50p from Saccharomyces cerevisiae covering multiple intermediates of the cycle. We identify several structural motifs specific to Drs2p and P4-ATPases in general that decrease movements and flexibility of domains as compared to other P-type ATPases such as Na⁺/K⁺-ATPase or Ca²⁺-ATPase. These motifs include the linkers that connect the transmembrane region to the actuator (A) domain, which is responsible for dephosphorylation. Additionally, mutation of Tyr380, which interacts with conserved Asp340 of the distinct DGET dephosphorylation loop of P4-ATPases, highlights a functional role of these P4-ATPase specific motifs in the A-domain. Finally, the transmembrane (TM) domain, responsible for transport, also undergoes less extensive conformational changes, which is ensured both by a longer segment connecting TM helix 4 with the phosphorylation site, and possible stabilization by the auxiliary subunit Cdc50p. Collectively these adaptions in P4-ATPases are responsible for phosphorylation becoming transport-independent.

Elusive Intermediate State Key in the Conversion of ATP Hydrolysis into Useful Work Driving the Ca2+ Pump SERCA

A key event in the ATP-driven transport cycle of the calcium pump sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) occurs when autophosphorylation of the pump with two bound ions Ca²⁺ triggers a large conformational change that opens a gate on the luminal side of the membrane allowing the release of the ions. It is believed that this conformational transition proceeds through a two-step mechanism, with an initial rearrangement of the three cytoplasmic domains of the pump responsible for ATP binding and hydrolysis followed by the opening of the gate toward the luminal side in the transmembrane region. Here, molecular dynamics computation of the free energy landscapes associated with this transition show how, in response to phosphorylation, the cytoplasmic domains are partially reconfigured into an intermediate state on the path toward the E2 state with a closed luminal gate. It is suggested that the free energy associated with this conformational reorganization must subsequently be used to drive the opening of the gate on the luminal side.

Structure and Mechanism of P-Type ATPase Ion Pumps

P-type ATPases are found in all kingdoms of life and constitute a wide range of cation transporters, primarily for H⁺, Na⁺, K⁺, Ca²⁺, and transition metal ions such as Cu(I), Zn(II), and Cd(II). They have been studied through a wide range of techniques, and research has gained very significant insight on their transport mechanism and regulation. Here, we review the structure, function, and dynamics of P2-ATPases including Ca²⁺-ATPases and Na,K-ATPase. We highlight mechanisms of functional transitions that are associated with ion exchange on either side of the membrane and how the functional cycle is regulated by interaction partners, autoregulatory domains, and off-cycle states. Finally, we discuss future perspectives based on emerging techniques and insights.

Allosteric modulation of the sarcoplasmic reticulum Ca2+ ATPase by thapsigargin via decoupling of functional motions

The sarcoplasmic reticulum Ca2+-ATPase (SERCA) is a widely studied member of the large family of phosphorylation(P)-type ATPase membrane transporters. Ligands and nucleotide binding naturally modulate the conformational space of P-type ATPases through allosteric inter-domain communications. Whereas many inhibitory ATPase ligands act by directly blocking substrate uptake or release, SERCA is a target for thapsigargin (TG), a plant-derived natural product that allosterically inhibits the transport cycle. While thapsigargin's inhibitory effects on SERCA have been widely studied experimentally, the molecular mechanisms underlying these remain incompletely understood. Here, we apply modelling and molecular simulations to probe the effects of TG binding to the major functional states along SERCA's reaction cycle. Our results provide insight into the atomic-level details of the conformational changes induced by TG binding to SERCA, and suggest mechanisms for its effect. Since other P-type ATPases share closely related reaction cycles, our data suggests that similar modulators might exist for these.

Dynamics of P-type ATPase transport revealed by single-molecule FRET

Phosphorylation-type (P-type) ATPases are ubiquitous primary transporters that pump cations across cell membranes through the formation and breakdown of a phosphoenzyme intermediate. Structural investigations suggest that the transport mechanism is defined by conformational changes in the cytoplasmic domains of the protein that are allosterically coupled to transmembrane helices so as to expose ion binding sites to alternate sides of the membrane. Here, we have used single-molecule fluorescence resonance energy transfer to directly observe conformational changes associated with the functional transitions in the Listeria monocytogenes Ca²⁺-ATPase (LMCA1), an orthologue of eukaryotic Ca²⁺-ATPases. We identify key intermediates with no known crystal structures and show that Ca²⁺ efflux by LMCA1 is rate-limited by phosphoenzyme formation. The transport process involves reversible steps and an irreversible step that follows release of ADP and extracellular release of Ca²⁺.

Nanodisc-based kinetic assays reveal distinct effects of phospholipid headgroups on the phosphoenzyme transition of sarcoplasmic reticulum Ca2+-ATPase

Sarco(endo)plasmic reticulum Ca²⁺-ATPase catalyzes ATP-driven Ca²⁺ transport from the cytoplasm to the lumen and is critical for a range of cell functions, including muscle relaxation. Here, we investigated the effects of the headgroups of the 1-palmitoyl-2-oleoyl glycerophospholipids phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylglycerol (PG) on sarcoplasmic reticulum (SR) Ca²⁺-ATPase embedded into a nanodisc, a lipid-bilayer construct harboring the specific lipid. We found that Ca²⁺-ATPase activity in a PC bilayer is comparable with that of SR vesicles and is suppressed in the other phospholipids, especially in PS. Ca²⁺ affinity at the high-affinity transport sites in PC was similar to that of SR vesicles, but 2-3-fold reduced in PE and PS. Ca²⁺ on- and off-rates in the non-phosphorylated ATPase were markedly reduced in PS. Rate-limiting phosphoenzyme (EP) conformational transition in 0.1 m KCl was as rapid in PC as in SR vesicles, but slowed in other phospholipids, especially in PS. Using kinetic plots of the logarithm of rate versus the square of mean activity coefficient of solutes in 0.1-1 m KCl, we noted that PC is optimal for the EP transition, but PG and especially PS had markedly unfavorable electrostatic effects, and PE exhibited a strong non-electrostatic restriction. Thus, the major SR membrane lipid PC is optimal for all steps and, unlike the other headgroups, contributes favorable electrostatics and non-electrostatic elements during the EP transition. Our analyses further revealed that the surface charge of the lipid bilayer directly modulates the transition rate.

Membrane Perturbation of ADP-insensitive Phosphoenzyme of Ca2+-ATPase Modifies Gathering of Transmembrane Helix M2 with Cytoplasmic Domains and Luminal Gating

Ca2+ transport by sarcoplasmic reticulum Ca2+-ATPase involves ATP-dependent phosphorylation of a catalytic aspartic acid residue. The key process, luminal Ca2+ release occurs upon phosphoenzyme isomerization, abbreviated as E1PCa2 (reactive to ADP regenerating ATP and with two occluded Ca2+ at transport sites) → E2P (insensitive to ADP and after Ca2+ release). The isomerization involves gathering of cytoplasmic actuator and phosphorylation domains with second transmembrane helix (M2), and is epitomized by protection of a Leu119-proteinase K (prtK) cleavage site on M2. Ca2+ binding to the luminal transport sites of E2P, producing E2PCa2 before Ca2+-release exposes the prtK-site. Here we explore E2P structure to further elucidate luminal gating mechanism and effect of membrane perturbation. We find that ground state E2P becomes cleavable at Leu119 in a non-solubilizing concentration of detergent C12E8 at pH 7.4, indicating a shift towards a more E2PCa2-like state. Cleavage is accelerated by Mg2+ binding to luminal transport sites and blocked by their protonation at pH 6.0. Results indicate that possible disruption of phospholipid-protein interactions strongly favors an E2P species with looser head domain interactions at M2 and responsive to specific ligand binding at the transport sites, likely an early flexible intermediate in the development towards ground state E2P.

Glycine 105 as Pivot for a Critical Knee-like Joint between Cytoplasmic and Transmembrane Segments of the Second Transmembrane Helix in Ca2+-ATPase

The cytoplasmic actuator domain of the sarco(endo)plasmic reticulum Ca²⁺-ATPase undergoes large rotational movements that influence the distant transmembrane transport sites, and a long second transmembrane helix (M2) connected with this domain plays critical roles in transmitting motions between the cytoplasmic catalytic domains and transport sites. Here we explore possible structural roles of Gly¹⁰⁵ between the cytoplasmic (M2c) and transmembrane (M2m) segments of M2 by introducing mutations that limit/increase conformational freedom. Alanine substitution G105A markedly retards isomerization of the phosphoenzyme intermediate (E1PCa₂ → E2PCa₂ → E2P + 2Ca²⁺), and disrupts Ca²⁺ occlusion in E1PCa₂ and E2PCa₂ at the transport sites uncoupling ATP hydrolysis and Ca²⁺ transport. In contrast, this substitution accelerates the ATPase activation (E2 → E1Ca₂). Introducing a glycine by substituting another residue on M2 in the G105A mutant (i.e. "G-shift substitution") identifies the glycine positions required for proper Ca²⁺ handling and kinetics in each step. All wild-type kinetic properties, including coupled transport, are fully restored in the G-shift substitution at position 112 (G105A/A112G) located on the same side of the M2c helix as Gly¹⁰⁵ facing M4/phosphorylation domain. Results demonstrate that Gly¹⁰⁵ functions as a flexible knee-like joint during the Ca²⁺ transport cycle, so that cytoplasmic domain motions can bend and strain M2 in the correct direction or straighten the helix for proper gating and coupling of Ca²⁺ transport and ATP hydrolysis.

CHARMM Force-Fields with Modified Polyphosphate Parameters Allow Stable Simulation of the ATP-Bound Structure of Ca(2+)-ATPase

Adenosine triphosphate (ATP) is an indispensable energy source in cells. In a wide variety of biological phenomena like glycolysis, muscle contraction/relaxation, and active ion transport, chemical energy released from ATP hydrolysis is converted to mechanical forces to bring about large-scale conformational changes in proteins. Investigation of structure-function relationships in these proteins by molecular dynamics (MD) simulations requires modeling of ATP in solution and ATP bound to proteins with accurate force-field parameters. In this study, we derived new force-field parameters for the triphosphate moiety of ATP based on the high-precision quantum calculations of methyl triphosphate. We tested our new parameters on membrane-embedded sarcoplasmic reticulum Ca(2+)-ATPase and four soluble proteins. The ATP-bound structure of Ca(2+)-ATPase remains stable during MD simulations, contrary to the outcome in shorter simulations using original parameters. Similar results were obtained with the four ATP-bound soluble proteins. The new force-field parameters were also tested by investigating the range of conformations sampled during replica-exchange MD simulations of ATP in explicit water. Modified parameters allowed a much wider range of conformational sampling compared with the bias toward extended forms with original parameters. A diverse range of structures agrees with the broad distribution of ATP conformations in proteins deposited in the Protein Data Bank. These simulations suggest that the modified parameters will be useful in studies of ATP in solution and of the many ATP-utilizing proteins.

Assembly of a Tyr122 Hydrophobic Cluster in Sarcoplasmic Reticulum Ca2+-ATPase Synchronizes Ca2+ Affinity Reduction and Release with Phosphoenzyme Isomerization

The mechanism whereby events in and around the catalytic site/head of Ca(2+)-ATPase effect Ca(2+) release to the lumen from the transmembrane helices remains elusive. We developed a method to determine deoccluded bound Ca(2+) by taking advantage of its rapid occlusion upon formation of E1PCa2 and of stabilization afforded by a high concentration of Ca(2+). The assay is applicable to minute amounts of Ca(2+)-ATPase expressed in COS-1 cells. It was validated by measuring the Ca(2+) binding properties of unphosphorylated Ca(2+)-ATPase. The method was then applied to the isomerization of the phosphorylated intermediate associated with the Ca(2+) release process E1PCa2 → E2PCa2 → E2P + 2Ca(2+). In the wild type, Ca(2+) release occurs concomitantly with EP isomerization fitting with rate-limiting isomerization (E1PCa2 → E2PCa2) followed by very rapid Ca(2+) release. In contrast, with alanine mutants of Leu(119) and Tyr(122) on the cytoplasmic part of the second transmembrane helix (M2) and Ile(179) on the A domain, Ca(2+) release in 10 μm Ca(2+) lags EP isomerization, indicating the presence of a transient E2P state with bound Ca(2+). The results suggest that these residues function in Ca(2+) affinity reduction in E2P, likely via a structural rearrangement at the cytoplasmic part of M2 and a resulting association with the A and P domains, therefore leading to Ca(2+) release.

The N Terminus of Sarcolipin Plays an Important Role in Uncoupling Sarco-endoplasmic Reticulum Ca2+-ATPase (SERCA) ATP Hydrolysis from Ca2+ Transport

The sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) is responsible for intracellular Ca(2+) homeostasis. SERCA activity in muscle can be regulated by phospholamban (PLB), an affinity modulator, and sarcolipin (SLN), an uncoupler. Although PLB gets dislodged from Ca(2+)-bound SERCA, SLN continues to bind SERCA throughout its kinetic cycle and promotes uncoupling of Ca(2+) transport from ATP hydrolysis. To determine the structural regions of SLN that mediate uncoupling of SERCA, we employed mutagenesis and generated chimeras of PLB and SLN. In this study we demonstrate that deletion of SLN N-terminal residues (2)ERSTQ leads to loss of the uncoupling function even though the truncated peptide can target and constitutively bind SERCA. Furthermore, molecular dynamics simulations of SLN and SERCA interaction showed a rearrangement of SERCA residues that is altered when the SLN N terminus is deleted. Interestingly, transfer of the PLB cytosolic domain to the SLN transmembrane (TM) and luminal tail causes the chimeric protein to lose SLN-like function. Further introduction of the PLB TM region into this chimera resulted in conversion to full PLB-like function. We also found that swapping PLB N and C termini with those from SLN caused the resulting chimera to acquire SLN-like function. Swapping the C terminus alone was not sufficient for this conversion. These results suggest that domains can be switched between SLN and PLB without losing the ability to regulate SERCA activity; however, the resulting chimeras acquire functions different from the parent molecules. Importantly, our studies highlight that the N termini of SLN and PLB influence their respective unique functions.

Second transmembrane helix (M2) and long range coupling in Ca²⁺-ATPase

The actuator (A) domain of sarco(endo)plasmic reticulum Ca(2+)-ATPase not only plays a catalytic role but also undergoes large rotational movements that influence the distant transport sites through connections with transmembrane helices M1 and M2. Here we explore the importance of long helix M2 and its junction with the A domain by disrupting the helix structure and elongating with insertions of five glycine residues. Insertions into the membrane region of M2 and the top junctional segment impair Ca(2+) transport despite reasonable ATPase activity, indicating that they are uncoupled. These mutants fail to occlude Ca(2+). Those at the top segment also exhibited accelerated phosphoenzyme isomerization E1P → E2P. Insertions into the middle of M2 markedly accelerate E2P hydrolysis and cause strong resistance to inhibition by luminal Ca(2+). Insertions along almost the entire M2 region inhibit the dephosphorylated enzyme transition E2 → E1. The results pinpoint which parts of M2 control cytoplasm gating and which are critical for luminal gating at each stage in the transport cycle and suggest that proper gate function requires appropriate interactions, tension, and/or rigidity in the M2 region at appropriate times for coupling with A domain movements and catalysis.

Hyperactivation of the human plasma membrane Ca2+ pump PMCA h4xb by mutation of Glu99 to Lys

The transport of calcium to the extracellular space carried out by plasma membrane Ca(2+) pumps (PMCAs) is essential for maintaining low Ca(2+) concentrations in the cytosol of eukaryotic cells. The activity of PMCAs is controlled by autoinhibition. Autoinhibition is relieved by the binding of Ca(2+)-calmodulin to the calmodulin-binding autoinhibitory sequence, which in the human PMCA is located in the C-terminal segment and results in a PMCA of high maximal velocity of transport and high affinity for Ca(2+). Autoinhibition involves the intramolecular interaction between the autoinhibitory domain and a not well defined region of the molecule near the catalytic site. Here we show that the fusion of GFP to the C terminus of the h4xb PMCA causes partial loss of autoinhibition by specifically increasing the Vmax. Mutation of residue Glu(99) to Lys in the cytosolic portion of the M1 transmembrane helix at the other end of the molecule brought the Vmax of the h4xb PMCA to near that of the calmodulin-activated enzyme without increasing the apparent affinity for Ca(2+). Altogether, the results suggest that the autoinhibitory interaction of the extreme C-terminal segment of the h4 PMCA is disturbed by changes of negatively charged residues of the N-terminal region. This would be consistent with a recently proposed model of an autoinhibited form of the plant ACA8 pump, although some differences are noted.

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

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

Roles of long-range electrostatic domain interactions and K+ in phosphoenzyme transition of Ca2+-ATPase

Sarcoplasmic reticulum Ca(2+)-ATPase couples the motions and rearrangements of three cytoplasmic domains (A, P, and N) with Ca(2+) transport. We explored the role of electrostatic force in the domain dynamics in a rate-limiting phosphoenzyme (EP) transition by a systematic approach combining electrostatic screening with salts, computer analysis of electric fields in crystal structures, and mutations. Low KCl concentration activated and increasing salt above 0.1 m inhibited the EP transition. A plot of the logarithm of the transition rate versus the square of the mean activity coefficient of the protein gave a linear relationship allowing division of the activation energy into an electrostatic component and a non-electrostatic component in which the screenable electrostatic forces are shielded by salt. Results show that the structural change in the transition is sterically restricted, but that strong electrostatic forces, when K(+) is specifically bound at the P domain, come into play to accelerate the reaction. Electric field analysis revealed long-range electrostatic interactions between the N and P domains around their hinge. Mutations of the residues directly involved and other charged residues at the hinge disrupted in parallel the electric field and the structural transition. Favorable electrostatics evidently provides a low energy path for the critical N domain motion toward the P domain, overcoming steric restriction. The systematic approach employed here is, in general, a powerful tool for understanding the structural mechanisms of enzymes.

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.

Uncoupling of sarcoplasmic reticulum Ca²⁺-ATPase by N-arachidonoyl dopamine. Members of the endocannabinoid family as thermogenic drugs

BACKGROUND AND PURPOSE

The sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) plays a role in thermogenesis. The exogenous compound capsaicin increased SERCA-mediated ATP hydrolysis not coupled to Ca²⁺ transport. Here, we have sought to identify endogenous compounds that may function as SERCA uncoupling agents.

EXPERIMENTAL APPROACH

Using isolated SR vesicles from rabbits, we have screened for endogenous compounds that uncouple SERCA. We have also studied their ability to deplete cytoplasmic ATP from human skeletal muscle cells in culture.

KEY RESULTS

Studies on SR vesicles showed that the endogenous lipid metabolite N-arachidonoyl dopamine (NADA) was a potent stimulator of SERCA uncoupling. NADA stabilized an E₁-like pump conformation that had a lower dephosphorylation rate, low affinity for Ca²⁺ at the luminal sites and a specific proteinase K cleavage pattern involving protection of the C-terminal p83C fragment from further cleavage. Moreover, we found a significantly decreased cytoplasmic ATP levels following treatment of skeletal muscle cells with 100 nM NADA. This effect was dependent on the presence of glucose and abolished by pretreatment with the specific SERCA inhibitor thapsigargin, regardless of the presence of glucose.

CONCLUSIONS AND IMPLICATIONS

NADA is an endogenous molecule that may function as SERCA uncoupling agent in vivo. Members of the endocannabinoid family exert concerted actions on several Ca²⁺-handling proteins. Uncoupling of SERCA by exogenous compounds could be a novel post-mitochondrial strategy for reduction of cellular ATP levels. In addition, signalling networks leading to SERCA uncoupling can be explored to study the importance of this ion pump in pathophysiological conditions related to metabolism.

Substrate-induced conformational changes in sarcoplasmic reticulum Ca2+-ATPase probed by surface modification using diethylpyrocarbonate with mass spectrometry

We have identified 15 residues from the surface of sarcoplasmic reticulum Ca(2+)-pump ATPase, by mass spectrometry using diethylpyrocarbonate modification. The reactivity of 9 residues remained high under all the conditions. The reactivity of Lys-515 at the nucleotide site was severely inhibited by ATP, whereas that of Lys-158 in the A-domain decreased by one-half and increased by five-fold in the presence of Ca(2+) and MgF(4), respectively. These are well explained by solvent accessibility, pK(a) and nearby hydrophobicity of the reactive atom on the basis of the atomic structure. However, the reactivity of 4 residues near the interface among A-, N- and P-domain suggested larger conformational changes of these domains in membrane upon binding of Ca(2+) (Lys-436), ATP (Lys-158) and MgF(4) (His-5, -190, Lys-436).

The role of domain: domain interactions versus domain: water interactions in the coarse-grained simulations of the E1P to E2P transitions in Ca-ATPase (SERCA)

SERCA is an important model system for understanding the molecular details of conformational change in membrane transport systems. This reflects the large number of solved X-ray structures and the equally large database of mutations that have been assayed. In this computational study, we provide a molecular dynamics description of the conformational changes during the E1P → E2P transitions. This set of states further changes with insertion mutants in the A-M3 linker region. These mutants were experimentally shown to lead to significant shifts in rates between the E1P → E2P states. Using the population shift framework and dynamic importance sampling method along with coarse-grained representations of the protein, lipid, and water, we suggest why these changes are found. The calculations sample on intermediates and suggest that changes in interactions, individual helix interactions, and water behavior are key elements in the molecular compositions that underlie shifts in kinetics. In particular, as the insertion length grows, it attracts more water and disrupts domain interactions, creating changes as well at the sites of key helix interactions between the A-Domain and the P-Domain. This provides a conceptual picture that aids understanding of the experimental results.

Crystal structure of a copper-transporting PIB-type ATPase

Heavy-metal homeostasis and detoxification is crucial for cell viability. P-type ATPases of the class IB (PIB) are essential in these processes, actively extruding heavy metals from the cytoplasm of cells. Here we present the structure of a PIB-ATPase, a Legionella pneumophila CopA Cu(+)-ATPase, in a copper-free form, as determined by X-ray crystallography at 3.2 Å resolution. The structure indicates a three-stage copper transport pathway involving several conserved residues. A PIB-specific transmembrane helix kinks at a double-glycine motif displaying an amphipathic helix that lines a putative copper entry point at the intracellular interface. Comparisons to Ca(2+)-ATPase suggest an ATPase-coupled copper release mechanism from the binding sites in the membrane via an extracellular exit site. The structure also provides a framework to analyse missense mutations in the human ATP7A and ATP7B proteins associated with Menkes' and Wilson's diseases.

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

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

Modulatory ATP binding affinity in intermediate states of E2P dephosphorylation of sarcoplasmic reticulum Ca2+-ATPase

The mechanism of ATP modulation of E2P dephosphorylation of sarcoplasmic reticulum Ca(2+)-ATPase wild type and mutant forms was examined in nucleotide binding studies of states analogous to the various intermediates of the dephosphorylation reaction, obtained by binding of metal fluorides, vanadate, or thapsigargin. Wild type Ca(2+)-ATPase displays an ATP affinity of 4 μM for the E2P ground state analog, 1 μM for the E2P transition state and product state analogs, and 11 μM for the E2 dephosphoenzyme. Hence, ATP binding stabilizes the transition and product states relative to the ground state, thereby explaining the accelerating effect of ATP on dephosphorylation. Replacement of Phe(487) (N-domain) with serine, Arg(560) (N-domain) with leucine, or Arg(174) (A-domain) with alanine or glutamate reduces ATP affinity in all E2/E2P intermediate states. Alanine substitution of Ile(188) (A-domain) increases the ATP affinity, although ATP acceleration of dephosphorylation is disrupted, thus indicating that the critical role of Ile(188) in ATP modulation is mechanistically based rather than being associated with the binding of nucleotide. Mutants with alanine replacement of Lys(205) (A-domain) or Glu(439) (N-domain) exhibit an anomalous inhibition by ATP of E2P dephosphorylation, due to ATP binding increasing the stability of the E2P ground state relative to the transition state. The ATP affinity of Ca(2)E2P, stabilized by inserting four glycines in the A-M1 linker, is similar to that of the E2P ground state, but the Ca(2+)-free E1 state of this mutant exhibits 3 orders of magnitude reduction of ATP affinity.

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

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

Ca2+ release to lumen from ADP-sensitive phosphoenzyme E1PCa2 without bound K+ of sarcoplasmic reticulum Ca2+-ATPase

During Ca(2+) transport by sarcoplasmic reticulum Ca(2+)-ATPase, the conformation change of ADP-sensitive phosphoenzyme (E1PCa(2)) to ADP-insensitive phosphoenzyme (E2PCa(2)) is followed by rapid Ca(2+) release into the lumen. Here, we find that in the absence of K(+), Ca(2+) release occurs considerably faster than E1PCa(2) to E2PCa(2) conformation change. Therefore, the lumenal Ca(2+) release pathway is open to some extent in the K(+)-free E1PCa(2) structure. The Ca(2+) affinity of this E1P is as high as that of the unphosphorylated ATPase (E1), indicating the Ca(2+) binding sites are not disrupted. Thus, bound K(+) stabilizes the E1PCa(2) structure with occluded Ca(2+), keeping the Ca(2+) pathway to the lumen closed. We found previously (Yamasaki, K., Wang, G., Daiho, T., Danko, S., and Suzuki, H. (2008) J. Biol. Chem. 283, 29144-29155) that the K(+) bound in E2P reduces the Ca(2+) affinity essential for achieving the high physiological Ca(2+) gradient and to fully open the lumenal Ca(2+) gate for rapid Ca(2+) release (E2PCa(2) → E2P + 2Ca(2+)). These findings show that bound K(+) is critical for stabilizing both E1PCa(2) and E2P structures, thereby contributing to the structural changes that efficiently couple phosphoenzyme processing and Ca(2+) handling.

In and out of the cation pumps: P-type ATPase structure revisited

Active transport across membranes is a crucial requirement for life. P-type ATPases build up electrochemical gradients at the expense of ATP by forming and splitting a covalent phosphoenzyme intermediate, coupled to conformational changes in the transmembrane section where the ions are translocated. The marked increment during the last three years in the number of crystal structures of P-type ATPases has greatly improved our understanding of the similarities and differences of pumps with different ion specificities, since the structures of the Ca2+-ATPase, the Na+,K+-ATPase and the H+-ATPase can now be compared directly. Mechanisms for ion gating, charge neutralization and backflow prevention are starting to emerge from comparative structural analysis; and in combination with functional studies of mutated pumps this provides a framework for speculating on how the ions are bound and released as well as on how specificity is achieved.

Stable structural analog of Ca2+-ATPase ADP-insensitive phosphoenzyme with occluded Ca2+ formed by elongation of A-domain/M1'-linker and beryllium fluoride binding

We have developed a stable analog for the ADP-insensitive phosphoenzyme intermediate with two occluded Ca(2+) at the transport sites (E2PCa(2)) of sarcoplasmic reticulum Ca(2+)-ATPase. This is normally a transient intermediate state during phosphoenzyme isomerization from the ADP-sensitive to ADP-insensitive form and Ca(2+) deocclusion/release to the lumen; E1PCa(2) --> E2PCa(2) --> E2P + 2Ca(2+). Stabilization was achieved by elongation of the Glu(40)-Ser(48) loop linking the Actuator domain and M1 (1st transmembrane helix) with four glycine insertions at Gly(46)/Lys(47) and by binding of beryllium fluoride (BeF(x)) to the phosphorylation site of the Ca(2+)-bound ATPase (E1Ca(2)). The complex E2Ca(2)xBeF(3)(-) was also produced by lumenal Ca(2+) binding to E2xBeF(3)(-) (E2P ground state analog) of the elongated linker mutant. The complex was stable for at least 1 week at 25 degrees C. Only BeF(x), but not AlF(x) or MgF(x), produced the E2PCa(2) structural analog. Complex formation required binding of Mg(2+), Mn(2+), or Ca(2+) at the catalytic Mg(2+) site. Results reveal that the phosphorylation product E1PCa(2) and the E2P ground state (but not the transition states) become competent to produce the E2PCa(2) transient state during forward and reverse phosphoenzyme isomerization. Thus, isomerization and lumenal Ca(2+) release processes are strictly coupled with the formation of the acylphosphate covalent bond at the catalytic site. Results also demonstrate the critical structural roles of the Glu(40)-Ser(48) linker and of Mg(2+) at the catalytic site in these processes.

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.

[Mechanism of ca(2+) pump as revealed by mutations, development of stable analogs of phosphorylated intermediates, and their structural analyses]

Sarco(endo)plasmic reticulum Ca(2+)-ATPase is a representative member of P-type cation transporting ATPases and catalyzes Ca(2+) transport coupled with ATP hydrolysis. The ATPase possesses three cytoplasmic domains (N, P, and A) and ten transmembrane helices (M1-M10). Ca(2+) binding at the transport sites in the transmembrane domain activates the ATPase and then the catalytic aspartate is auto-phosphorylated to form the phosphorylated intermediate (EP). Structural and functional studies have shown that, during the isomerization of EP in the Ca(2+) transport cycle, large motions of the three cytoplasmic domains take place, which then rearranges the transmembrane helices thereby destroying the Ca(2+) binding sites, opening the lumenal gate, and thus releasing the Ca(2+) into lumen. Stable structural analogues for the Ca(2+)-occluded and -released states of phosphorylated intermediates and for the transition and product states of the phosphorylation and dephosphorylation reactions were developed for biochemical and atomic-level structural studies to reveal the coupled changes in the catalytic and transport sites. Mutation studies identified the residues and structural regions essential for the structural changes and Ca(2+) transport function. Genetic dysfunction of Ca(2+)-ATPase causes various isoform-specific diseases. In this manuscript, recent understanding of the Ca-ATPase will be reviewed.

What can be learned about the function of a single protein from its various X-ray structures: the example of the sarcoplasmic calcium pump

Improvements in the handling of membrane proteins for crystallization, combined with better synchrotron sources for X-ray diffraction analysis, are leading to clarification of the structural details of an ever increasing number of membrane transporters and receptors. Here we describe how this development has resulted in the elucidation at atomic resolution of a large number of structures of the sarcoplasmic Ca(2+)-ATPase (SERCA1a) present in skeletal muscle. The structures corresponding to the various intermediary states have been obtained after stabilization with structural analogues of ATP and of metal fluorides as mimicks of inorganic phosphate. From these results it is possible, in accordance with previous biochemical and molecular biology data, to give a detailed structural description of both ATP hydrolysis and Ca(2+) transport through the membrane, to serve as the starting point for a fuller understanding of the pump mechanism and, in future studies, on the regulatory role of this ubiquitous intracellular Ca(2+)-ATPase in cellular Ca(2+) metabolism in normal and pathological conditions.

Roles of interaction between actuator and nucleotide binding domains of sarco(endo)plasmic reticulum Ca(2+)-ATPase as revealed by single and swap mutational analyses of serine 186 and glutamate 439

Roles of hydrogen bonding interaction between Ser(186) of the actuator (A) domain and Glu(439) of nucleotide binding (N) domain seen in the structures of ADP-insensitive phosphorylated intermediate (E2P) of sarco(endo)plasmic reticulum Ca(2+)-ATPase were explored by their double alanine substitution S186A/E439A, swap substitution S186E/E439S, and each of these single substitutions. All the mutants except the swap mutant S186E/E439S showed markedly reduced Ca(2+)-ATPase activity, and S186E/E439S restored completely the wild-type activity. In all the mutants except S186E/E439S, the isomerization of ADP-sensitive phosphorylated intermediate (E1P) to E2P was markedly retarded, and the E2P hydrolysis was largely accelerated, whereas S186E/E439S restored almost the wild-type rates. Results showed that the Ser(186)-Glu(439) hydrogen bond stabilizes the E2P ground state structure. The modulatory ATP binding at sub-mm approximately mm range largely accelerated the EP isomerization in all the alanine mutants and E439S. In S186E, this acceleration as well as the acceleration of the ATPase activity was almost completely abolished, whereas the swap mutation S186E/E439S restored the modulatory ATP acceleration with a much higher ATP affinity than the wild type. Results indicated that Ser(186) and Glu(439) are closely located to the modulatory ATP binding site for the EP isomerization, and that their hydrogen bond fixes their side chain configurations thereby adjusts properly the modulatory ATP affinity to respond to the cellular ATP level.

Formation of the stable structural analog of ADP-sensitive phosphoenzyme of Ca2+-ATPase with occluded Ca2+ by beryllium fluoride: structural changes during phosphorylation and isomerization

As a stable analog for ADP-sensitive phosphorylated intermediate of sarcoplasmic reticulum Ca(2+)-ATPase E1PCa(2).Mg, a complex of E1Ca(2).BeF(x), was successfully developed by addition of beryllium fluoride and Mg(2+) to the Ca(2+)-bound state, E1Ca(2). In E1Ca(2).BeF(x), most probably E1Ca(2).BeF(3)(-), two Ca(2+) are occluded at high affinity transport sites, its formation required Mg(2+) binding at the catalytic site, and ADP decomposed it to E1Ca(2), as in E1PCa(2).Mg. Organization of cytoplasmic domains in E1Ca(2).BeF(x) was revealed to be intermediate between those in E1Ca(2).AlF(4)(-) ADP (transition state of E1PCa(2) formation) and E2.BeF(3)(-).(ADP-insensitive phosphorylated intermediate E2P.Mg). Trinitrophenyl-AMP (TNP-AMP) formed a very fluorescent (superfluorescent) complex with E1Ca(2).BeF(x) in contrast to no superfluorescence of TNP-AMP bound to E1Ca(2).AlF(x). E1Ca(2).BeF(x) with bound TNP-AMP slowly decayed to E1Ca(2), being distinct from the superfluorescent complex of TNP-AMP with E2.BeF(3)(-), which was stable. Tryptophan fluorescence revealed that the transmembrane structure of E1Ca(2).BeF(x) mimics E1PCa(2).Mg, and between those of E1Ca(2).AlF(4)(-).ADP and E2.BeF(3)(-). E1Ca(2).BeF(x) at low 50-100 microm Ca(2+) was converted slowly to E2.BeF(3)(-) releasing Ca(2+), mimicking E1PCa(2).Mg --> E2P.Mg + 2Ca(2+). Ca(2+) replacement of Mg(2+) at the catalytic site at approximately millimolar high Ca(2+) decomposed E1Ca(2).BeF(x) to E1Ca(2). Notably, E1Ca(2).BeF(x) was perfectly stabilized for at least 12 days by 0.7 mm lumenal Ca(2+) with 15 mm Mg(2+). Also, stable E1Ca(2).BeF(x) was produced from E2.BeF(3)(-) at 0.7 mm lumenal Ca(2+) by binding two Ca(2+) to lumenally oriented low affinity transport sites, as mimicking the reverse conversion E2P. Mg + 2Ca(2+) --> E1PCa(2).Mg.

The length of the A-M3 linker is a crucial determinant of the rate of the Ca2+ transport cycle of sarcoplasmic reticulum Ca2+-ATPase

Ion translocation by the sarcoplasmic reticulum Ca(2+)-ATPase depends on large movements of the A-domain, but the driving forces have yet to be defined. The A-domain is connected to the ion-binding membranous part of the protein through linker regions. We have determined the functional consequences of changing the length of the linker between the A-domain and transmembrane helix M3 ("A-M3 linker") by insertion and deletion mutagenesis at two sites. It was feasible to insert as many as 41 residues (polyglycine and glycine-proline loops) in the flexible region of the linker without loss of the ability to react with Ca(2+) and ATP and to form the phosphorylated Ca(2)E1P intermediate, but the rate of the energy-transducing conformational transition to E2P was reduced by >80%. Insertion of a smaller number of residues gave effects gradually increasing with the length of the insertion. Deletion of two residues at the same site, but not replacement with glycine, gave a similar reduction as the longest insertion. Insertion of one or three residues in another part of the A-M3 linker that forms an alpha-helix ("A3 helix") in E2/E2P conformations had even more profound effects on the ability of the enzyme to form E2P. These results demonstrate the importance of the length of the A-M3 linker and of the position and integrity of the A3 helix for stabilization of E2P and suggest that, during the normal enzyme cycle, strain of the A-M3 linker could contribute to destabilize the Ca(2)E1P state and thereby to drive the transition to E2P.

Critical interaction of actuator domain residues arginine 174, isoleucine 188, and lysine 205 with modulatory nucleotide in sarcoplasmic reticulum Ca2+-ATPase

ATP plays dual roles in the reaction cycle of the sarcoplasmic reticulum Ca2+-ATPase by acting as the phosphorylating substrate as well as in nonphosphorylating (modulatory) modes accelerating conformational transitions of the enzyme cycle. Here we have examined the involvement of actuator domain residues Arg174, Ile188, Lys204, and Lys205 by mutagenesis. Alanine mutations to these residues had little effect on the interaction of the Ca2E1 state with nucleotide or on the HnE 2 to Ca2E1 transition of the dephosphoenzyme. The phosphoenzyme processing steps, Ca2E1P to E2P and E2P dephosphorylation, and their stimulation by MgATP/ATP were markedly affected by mutations to Arg174, Ile188, and Lys205. Replacement of Ile188 with alanine abolished nucleotide modulation of dephosphorylation but not the modulation of the Ca2E1P to E2P transition. Mutation to Arg174 interfered with nucleotide modulation of either of the phosphoenzyme processing steps, indicating a significant overlap between the modulatory nucleotide-binding sites involved. Mutation to Lys205 enhanced the rates of the phosphoenzyme processing steps in the absence of nucleotide and disrupted the nucleotide modulation of the Ca2E1P to E2P transition. Remarkably, the mutants with alterations to Lys205 showed an anomalous inhibition by ATP of the dephosphorylation, and in the alanine mutant the affinity for the inhibition by ATP was indistinguishable from that for stimulation by ATP of the wild type. Hence, the actuator domain is an important player in the function of ATP as modulator of phosphoenzyme processing, with Arg174, Ile188, and Lys205 all being critically involved, although in different ways. The data support a variable site model for the modulatory effects with the nucleotide binding somewhat differently in each of the conformational states occurring during the transport cycle.

Roles of Tyr122-hydrophobic cluster and K+ binding in Ca2+ -releasing process of ADP-insensitive phosphoenzyme of sarcoplasmic reticulum Ca2+ -ATPase

Tyr(122)-hydrophobic cluster (Y122-HC) is an interaction network formed by the top part of the second transmembrane helix and the cytoplasmic actuator and phosphorylation domains of sarcoplasmic reticulum Ca(2+)-ATPase. We have previously found that Y122-HC plays critical roles in the processing of ADP-insensitive phosphoenzyme (E2P) after its formation by the isomerization from ADP-sensitive phosphoenzyme (E1PCa(2)) (Wang, G., Yamasaki, K., Daiho, T., and Suzuki, H. (2005) J. Biol. Chem. 280, 26508-26516). Here, we further explored kinetic properties of the alanine-substitution mutants of Y122-HC to examine roles of Y122-HC for Ca(2+) release process in E2P. In the steady state, the amount of E2P decreased so that of E1PCa(2) increased with increasing lumenal Ca(2+) concentration in the mutants with K(0.5) 110-320 microm at pH 7.3. These lumenal Ca(2+) affinities in E2P agreed with those estimated from the forward and lumenal Ca(2+)-induced reverse kinetics of the E1PCa(2)-E2P isomerization. K(0.5) of the wild type in the kinetics was estimated to be 1.5 mM. Thus, E2P of the mutants possesses significantly higher affinities for lumenal Ca(2+) than that of the wild type. The kinetics further indicated that the rates of lumenal Ca(2+) access and binding to the transport sites of E2P were substantially slowed by the mutations. Therefore, the proper formation of Y122-HC and resulting compactly organized structure are critical for both decreasing Ca(2+) affinity and opening the lumenal gate, thus for Ca(2+) release from E2PCa(2). Interestingly, when K(+) was omitted from the medium of the wild type, the properties of the wild type became similar to those of Y122-HC mutants. K(+) binding likely functions via producing the compactly organized structure, in this sense, similarly to Y122-HC.

Capsaicin stimulates uncoupled ATP hydrolysis by the sarcoplasmic reticulum calcium pump

In muscle cells the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA) couples the free energy of ATP hydrolysis to pump Ca(2+) ions from the cytoplasm to the SR lumen. In addition, SERCA plays a key role in non-shivering thermogenesis through uncoupled reactions, where ATP hydrolysis takes place without active Ca(2+) translocation. Capsaicin (CPS) is a naturally occurring vanilloid, the consumption of which is linked with increased metabolic rate and core body temperature. Here we document the stimulation by CPS of the Ca(2+)-dependent ATP hydrolysis by SERCA without effects on Ca(2+) accumulation. The stimulation by CPS was significantly dependent on the presence of a Ca(2+) gradient across the SR membrane. ATP activation assays showed that the drug reduced the nucleotide affinity at the catalytic site, whereas the affinity at the regulatory site increased. Several biochemical analyses indicated that CPS stabilizes an ADP-insensitive E(2)P-related conformation that dephosphorylates at a higher rate than the control enzyme. Under conditions where uncoupled SERCA was specifically inhibited by the treatment with fluoride, low temperatures, or dimethyl sulfoxide, CPS had no stimulatory effect on ATP hydrolysis by SERCA. It is concluded that CPS stabilizes a SERCA sub-conformation where Ca(2+) is released from the phosphorylated intermediate to the cytoplasm instead of the SR lumen, increasing ATP hydrolysis not coupled with Ca(2+) transport. To the best of our knowledge CPS is the first natural drug that augments uncoupled SERCA, presumably resulting in thermogenesis. The role of CPS as a SERCA modulator is discussed.

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

Ca2+-ATPase of muscle sarcoplasmic reticulum is an ATP-powered Ca2+-pump that establishes a >10,000-fold concentration gradient across the membrane. Its crystal structures have been determined for nine different states that cover nearly the entire reaction cycle. Presented here is a brief structural account of the ion pumping process, which is achieved by a series of very large domain rearrangements.

The structural basis of calcium transport by the calcium pump

The sarcoplasmic reticulum Ca2+-ATPase, a P-type ATPase, has a critical role in muscle function and metabolism. Here we present functional studies and three new crystal structures of the rabbit skeletal muscle Ca2+-ATPase, representing the phosphoenzyme intermediates associated with Ca2+ binding, Ca2+ translocation and dephosphorylation, that are based on complexes with a functional ATP analogue, beryllium fluoride and aluminium fluoride, respectively. The structures complete the cycle of nucleotide binding and cation transport of Ca2+-ATPase. Phosphorylation of the enzyme triggers the onset of a conformational change that leads to the opening of a luminal exit pathway defined by the transmembrane segments M1 through M6, which represent the canonical membrane domain of P-type pumps. Ca2+ release is promoted by translocation of the M4 helix, exposing Glu 309, Glu 771 and Asn 796 to the lumen. The mechanism explains how P-type ATPases are able to form the steep electrochemical gradients required for key functions in eukaryotic cells.

How processing of aspartylphosphate is coupled to lumenal gating of the ion pathway in the calcium pump

Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum is the best-studied member of the P-type or E1/E2 type ion transporting ATPases. It has been crystallized in seven different states that cover nearly the entire reaction cycle. Here we describe the structure of this ATPase complexed with phosphate analogs BeF(3)(-) and AlF(4)(-) in the absence of Ca(2+), which correspond to the E2P ground state and E2 approximately P transition state, respectively. The luminal gate is open with BeF(3)(-) and closed with AlF(4)(-). These and the E1 approximately P.ADP analog crystal structures show that a two-step rotation of the cytoplasmic A-domain opens and closes the luminal gate through the movements of the M1-M4 transmembrane helices. There are several conformational switches coupled to the rotation, and the one in the cytoplasmic part of M2 has critical importance. In the second step of rotation, positioning of one water molecule couples the hydrolysis of aspartylphosphate to closing of the gate.