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

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.

Electrostatic interactions between single arginine and phospholipids modulate physiological properties of sarcoplasmic reticulum Ca2+-ATPase

Arg324 of sarcoplasmic reticulum Ca²⁺-ATPase forms electrostatic interactions with the phosphate moiety of phospholipids in most reaction states, and a hydrogen bond with Tyr122 in other states. Using site-directed mutagenesis, we explored the functional roles of Arg324 interactions, especially those with lipids, which at first glance might seem too weak to modulate the function of such a large membrane protein. The hydrogen bond forms transiently and facilitates Ca²⁺ binding from the cytoplasmic side. The contributions of the electrostatic interactions to the reaction steps were quantified using a rate vs activity coefficient plot. We found that the interaction between Arg324 and lipids decreases the affinity for luminal Ca²⁺. The transformation rate of the phosphoenzyme intermediate is facilitated by the electrostatic interactions, and the function of these interactions depends not only on the type but also on the composition of the phospholipids. The properties observed in microsomes could not be reproduced with any single phospholipid, but with a mixture of phospholipids that mimics the native membrane. These results suggest the importance of swapping of the lipid partners of different headgroups in the reaction step. This study shows that Arg324 plays a role in the reaction cycle via complex intra-protein and protein-lipid interactions.

Structural and energetic analysis of metastable intermediate states in the E1P-E2P transition of Ca2+-ATPase

Ion pumps (or P-type ATPases) are membrane proteins, which transport ions through biological membranes against a concentration gradient, a function essential for many biological processes, such as muscle contraction, neurotransmission, and metabolism. Molecular mechanisms underlying active ion transport by ion pumps have been investigated by biochemical experiments and high-resolution structure analyses. Here, the transition of sarcoplasmic reticulum Ca²⁺-ATPase upon dissociation of Ca²⁺ is investigated using atomistic molecular dynamics simulations. We find intermediate structures along the pathway are stabilized by transient interactions between A- and P-domains as well as lipid molecules in the transmembrane helices.

Sarcoplasmic reticulum (SR) Ca²⁺-ATPase transports two Ca²⁺ ions from the cytoplasm to the SR lumen against a large concentration gradient. X-ray crystallography has revealed the atomic structures of the protein before and after the dissociation of Ca²⁺, while biochemical studies have suggested the existence of intermediate states in the transition between E1P⋅ADP⋅2Ca²⁺ and E2P. Here, we explore the pathway and free energy profile of the transition using atomistic molecular dynamics simulations with the mean-force string method and umbrella sampling. The simulations suggest that a series of structural changes accompany the ordered dissociation of ADP, the A-domain rotation, and the rearrangement of the transmembrane (TM) helices. The luminal gate then opens to release Ca²⁺ ions toward the SR lumen. Intermediate structures on the pathway are stabilized by transient sidechain interactions between the A- and P-domains. Lipid molecules between TM helices play a key role in the stabilization. Free energy profiles of the transition assuming different protonation states suggest rapid exchanges between Ca²⁺ ions and protons when the Ca²⁺ ions are released toward the SR lumen.

Thermodynamics of Cation Binding to the Sarcoendoplasmic Reticulum Calcium ATPase Pump and Impacts on Enzyme Function

Sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) is a transmembrane pump that plays an important role in transporting calcium into the sarcoplasmic reticulum (SR). While calcium (Ca²⁺) binds SERCA with micromolar affinity, magnesium (Mg²⁺) and potassium (K⁺) also compete with Ca²⁺ binding. However, the molecular bases for these competing ions' influence on the SERCA function and the selectivity of the pump for Ca²⁺ are not well-established. We therefore used in silico methods to resolve molecular determinants of cation binding in the canonical site I and II Ca²⁺ binding sites via (1) triplicate molecular dynamics (MD) simulations of Mg²⁺, Ca²⁺, and K⁺-bound SERCA, (2) mean spherical approximation (MSA) theory to score the affinity and selectivity of cation binding to the MD-resolved structures, and (3) state models of SERCA turnover informed from MSA-derived affinity data. Our key findings are that (a) coordination at sites I and II is optimized for Ca²⁺ and to a lesser extent for Mg²⁺ and K⁺, as determined by MD-derived cation-amino acid oxygen and bound water configurations, (b) the impaired coordination and high desolvation cost for Mg²⁺ precludes favorable Mg²⁺ binding relative to Ca²⁺, while K⁺ has limited capacity to bind site I, and (c) Mg²⁺ most likely acts as inhibitor and K⁺ as intermediate in SERCA's reaction cycle, based on a best-fit state model of SERCA turnover. These findings provide a quantitative basis for SERCA function that leverages molecular-scale thermodynamic data and rationalizes enzyme activity across broad ranges of K⁺, Ca²⁺, and Mg²⁺ concentrations.

Conformational Transitions and Alternating-Access Mechanism in the Sarcoplasmic Reticulum Calcium Pump

Ion pumps are integral membrane proteins responsible for transporting ions against concentration gradients across biological membranes. Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), a member of the P-type ATPases family, transports two calcium ions per hydrolyzed ATP molecule via an "alternating-access" mechanism. High-resolution crystallographic structures provide invaluable insight on the structural mechanism of the ion pumping process. However, to understand the molecular details of how ATP hydrolysis is coupled to calcium transport, it is necessary to gain knowledge about the conformational transition pathways connecting the crystallographically resolved conformations. Large-scale transitions in SERCA occur at time-scales beyond the current reach of unbiased molecular dynamics simulations. Here, we overcome this challenge by employing the string method, which represents a transition pathway as a chainofstates linking two conformational endpoints. Using a multiscale methodology, we have determined all-atom transition pathways for three main conformational transitions responsible for the alternating-access mechanism. The present pathways provide a clear chronology and ordering of the key events underlying the active transport of calcium ions by SERCA. Important conclusions are that the conformational transition that leads to occlusion with bound ATP and calcium is highly concerted and cooperative, the phosphorylation of Asp351 causes areorganization of the cytoplasmic domains that subsequently drives the opening of the luminal gate, and thereclosing of luminal gate induces a shift in the cytoplasmic domains that subsequently enables the dephosphorylation of Asp351-P. Formation of transient residue-residue contacts along the conformational transitions predicted by the computations provide an experimental route to test the general validity of the computational pathways.

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.

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.

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.

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.

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 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.

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.

[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.

Crystal structure of A3B3 complex of V-ATPase from Thermus thermophilus

Vacuolar-type ATPases (V-ATPases) exist in various cellular membranes of many organisms to regulate physiological processes by controlling the acidic environment. Here, we have determined the crystal structure of the A(3)B(3) subcomplex of V-ATPase at 2.8 A resolution. The overall construction of the A(3)B(3) subcomplex is significantly different from that of the alpha(3)beta(3) sub-domain in F(o)F(1)-ATP synthase, because of the presence of a protruding 'bulge' domain feature in the catalytic A subunits. The A(3)B(3) subcomplex structure provides the first molecular insight at the catalytic and non-catalytic interfaces, which was not possible in the structures of the separate subunits alone. Specifically, in the non-catalytic interface, the B subunit seems to be incapable of binding ATP, which is a marked difference from the situation indicated by the structure of the F(o)F(1)-ATP synthase. In the catalytic interface, our mutational analysis, on the basis of the A(3)B(3) structure, has highlighted the presence of a cluster composed of key hydrophobic residues, which are essential for ATP hydrolysis by V-ATPases.

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.