The mechanism of Ca2+ transport by sarco(endo)plasmic reticulum Ca2+-ATPases

Mass spectrometry in aging research

This review covers the application of mass spectrometric techniques to aging research. Modern proteomic strategies will be discussed as well as the targeted analysis of specific proteins for the correlation of post-translational modifications with protein function. Selected examples will show both the power and also current limitations of the respective techniques. Experimental results and strategies are discussed in view of current theories of the aging process.

Crystallization of a mammalian membrane protein overexpressed in Saccharomyces cerevisiae

The Ca2+-ATPase SERCA1a (sarcoplasmic-endoplasmic reticulum Ca2+-ATPase isoform 1a) from rabbit has been overexpressed in Saccharomyces cerevisiae. This membrane protein was purified by avidin agarose affinity chromatography based on natural biotinylation in the expression host, followed by HPLC gel filtration. Both the functional and structural properties of the overexpressed protein validate the method. Thus, calcium-dependent ATPase activity and calcium transport are essentially intact after reconstitution in proteoliposomes. Moreover, the recombinant protein crystallizes in a form that is isomorphous to the native SERCA1a protein from rabbit, and the diffraction properties are similar. This represents a successful crystallization of a mammalian membrane protein derived from a heterologous expression system, and it opens the way for the study of mutant forms of SERCA1a.

Functional implications of modifying RyR-activating peptides for membrane permeability

1. Our aim was to determine whether lipoamino acid conjugation of peptides that are high-affinity activators of ryanodine receptor (RyR) channels would (a) render the peptides membrane permeable, (b) alter their structure or (a) reduce their activity. The peptides correspond to the A region of the II-III loop of the skeletal dihydropyridine receptor. 2. The lipoamino acid conjugation increased the apparent permeability of the peptide across the Caco-2 cell monolayer by up to approximately 20-fold. 3. Nuclear magnetic resonance showed that the alpha-helical structure of critical basic residues, required for optimal activation of RyRs, was retained after conjugation. 4. The conjugated peptides were more effective in enhancing resting Ca2+ release, Ca2+-induced Ca2+ release and caffeine-induced Ca2+ release from isolated sarcoplasmic reticulum (SR) than their unconjugated counterparts, and significantly enhanced caffeine-induced Ca2+ release from mechanically skinned extensor digitorum longus (EDL) fibres. 5. The effect of both conjugated and unconjugated peptides on Ca2+ release from skeletal SR was 30-fold greater than their effect on either cardiac Ca2+ release or on the Ca2+ Mg2+ ATPase. 6. A small and very low affinity effect of the peptide in slowing Ca2+ uptake by the Ca2+, Mg2+ ATPase was exacerbated by lipoamino acid conjugation in both isolated SR and in skinned EDL fibres. 7. The results show that lipoamino acid conjugation of A region peptides increases their membrane permeability without impairing their structure or efficacy in activating skeletal and cardiac RyRs.

Altered mRNA splicing of the skeletal muscle ryanodine receptor and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a debilitating multisystemic disorder caused by a CTG repeat expansion in the DMPK gene. Aberrant splicing of several genes has been reported to contribute to some symptoms of DM1, but the cause of muscle weakness in DM1 and elevated Ca2+ concentrations in cultured DM muscle cells is unknown. Here, we investigated the alternative splicing of mRNAs of two major proteins of the sarcoplasmic reticulum, the ryanodine receptor 1 (RyR1) and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) 1 or 2. The fetal variants, ASI(-) of RyR1 which lacks residue 3481-3485, and SERCA1b which differs at the C-terminal were significantly increased in skeletal muscles from DM1 patients and the transgenic mouse model of DM1 (HSA(LR)). In addition, a novel variant of SERCA2 was significantly decreased in DM1 patients. The total amount of mRNA for RyR1, SERCA1 and SERCA2 in DM1 and the expression levels of their proteins in HSA(LR) mice were not significantly different. However, heterologous expression of ASI(-) in cultured cells showed decreased affinity for [3H]ryanodine but similar Ca2+ dependency, and decreased channel activity in single-channel recording when compared with wild-type (WT) RyR1. In support of this, RyR1-knockout myotubes expressing ASI(-) exhibited a decreased incidence of Ca2+ oscillations during caffeine exposure compared with that observed for myotubes expressing WT-RyR1. We suggest that aberrant splicing of RyR1 and SERCA1 mRNAs might contribute to impaired Ca2+ homeostasis in DM1 muscle.

Electrogenic glutamate transporters in the CNS: molecular mechanism, pre-steady-state kinetics, and their impact on synaptic signaling

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The spatiotemporal profile of the glutamate concentration in the synapse is critical for excitatory synaptic signalling. The control of this spatiotemporal concentration profile requires the presence of large numbers of synaptically localized glutamate transporters that remove pre-synaptically released glutamate by uptake into neurons and adjacent glia cells. These glutamate transporters are electrogenic and utilize energy stored in the transmembrane potential and the Na+/K+-ion concentration gradients to accumulate glutamate in the cell. This review focuses on the kinetic and electrogenic properties of glutamate transporters, as well as on the molecular mechanism of transport. Recent results are discussed that demonstrate the multistep nature of the transporter reaction cycle. Results from pre-steady-state kinetic experiments suggest that at least four of the individual transporter reaction steps are electrogenic, including reactions associated with the glutamate-dependent transporter halfcycle. Furthermore, the kinetic similarities and differences between some of the glutamate transporter subtypes and splice variants are discussed. A molecular mechanism of glutamate transport is presented that accounts for most of the available kinetic data. Finally, we discuss how synaptic glutamate transporters impact on glutamate receptor activity and how transporters may shape excitatory synaptic transmission.

Oscillatory activity of P-type membrane adenosine triphosphatases: a kinetic model

A kinetic model for membrane P-type adenosine triphosphatases is considered, the main application being to the erythrocyte Ca2+-ATPase. It is shown that a simple modification of the known catalytic mechanism of the ATPase by addition of a self-inhibition step and the steady calcium influx leads to damped oscillations in the system discussed. In this way, the model can explain the kinetic experimental results obtained for the purified enzyme in solution as well as for the enzyme incorporated into liposome membranes. The estimated kinetic parameters are close to the experimental ones. Alternative changes in time, demonstrated by the kinetic model for the conformational enzyme states, E(1 )and E(2), confirm the model of two alternatively functioning gates in the ion pumping Ca2+-ATPase.

Critical hydrophobic interactions between phosphorylation and actuator domains of Ca2+-ATPase for hydrolysis of phosphorylated intermediate

Functional roles of seven hydrophobic residues on the interface between the actuator (A) and phosphorylation (P) domains of sarcoplasmic reticulum Ca2+-ATPase were explored by alanine and serine substitutions. The residues examined were Ile179/Leu180/Ile232 on the A domain, Val705/Val726 on the P domain, and Leu119/Tyr122 on the loop linking the A domain and M2 (the second transmembrane helix). These residues gather to form a hydrophobic cluster around Tyr122 in the crystal structures of Ca2+-ATPase in Ca2+-unbound E2 (unphosphorylated) and E2P (phosphorylated) states but are far apart in those of Ca2+-bound E1 (unphosphorylated) and E1P (phosphorylated) states. The substitution-effects were also compared with those of Ile235 on the A domain/M3 linker and those of T181GE of the A domain, since they are in the immediate vicinity of the Tyr122-cluster. All these substitutions almost completely inhibited ATPase activity without inhibiting Ca2+-activated E1P formation from ATP. Substitutions of Ile235 and T181GE blocked the E1P to E2P transition, whereas those in the Tyr122-cluster blocked the subsequent E2P hydrolysis. Substitutions of Ile235 and Glu183 also blocked EP hydrolysis. Results indicate that the Tyr122-cluster is formed during the E1P to E2P transition to configure the catalytic site and position Glu183 properly for hydrolyzing the acylphosphate. Ile235 on the A domain/M3 linker likely forms hydrophobic interactions with the A domain and thereby allowing the strain of this linker to be utilized for large motions of the A domain during these processes. The Tyr122-cluster, Ile235, and T181GE thus seem to have different roles and are critical in the successive events in processing phosphorylated intermediates to transport Ca2+.

A mutation in serca underlies motility dysfunction in accordion zebrafish

Zebrafish acquire the ability for fast swimming early in development. The motility mutant accordion (acc) undergoes exaggerated and prolonged contractions on both sides of the body, interfering with the acquisition of patterned swimming responses. Our whole cell recordings from muscle indicate that the defect is not manifested in neuromuscular transmission. However, imaging of skeletal muscle of larval acc reveals greatly prolonged calcium transients and associated contractions in response to depolarization. Positional cloning of acc identified a serca mutation as the cause of the acc phenotype. SERCA is a sarcoplasmic reticulum transmembrane protein in skeletal muscle that mediates calcium re-uptake from the myoplasm. The mutation in SERCA, a serine to phenylalanine substitution, is likely to result in compromised protein function that accounts for the observed phenotype. Indeed, direct evidence that mutant SERCA causes the motility dysfunction was provided by the finding that wild type fish injected with an antisense morpholino directed against serca, exhibited accordion-like contractions and impaired swimming. We conclude that the motility dysfunction in embryonic and larval accordion zebrafish stems directly from defective calcium transport in skeletal muscle rather than defective CNS drive.

Calcium pump disorders of the skin

The causes of Darier disease (DD) and Hailey-Hailey disease (HHD) have eluded clinicians and scientists for more than 60 years. DD is characterized by loss of adhesion between suprabasal epidermal cells associated with abnormal keratinization, while loss of epidermal cell-to-cell adhesion is predominant in HHD. The genes for both conditions have recently been identified using candidate positional cloning approaches. The gene for DD (ATP2A2) encodes a calcium transport ATPase of the sarco (endo)plasmic reticulum (SERCA2) Verboomen et al. [1992: Biochem J 286(Pt 2):591-595], while the gene for HHD (ATP2C1) codes for a secretory pathway for calcium and manganese transport ATPase of the Golgi apparatus (SPCA1) Hu et al. [2000: Nat Genet 24:61-65]. These results have provided completely new insights into the role of calcium and/or manganese in maintaining skin integrity. Although the precise disease mechanisms remain to be understood, these discoveries open a new field in research for the understanding and the treatment of these distressing disorders.

Anti-oxidant gene expression imbalance, aging and Down syndrome

The expression of copper zinc superoxide dismutase (SOD1), manganese superoxide dismutase (SOD2), glutathione peroxidase (GPx), and catalase (CAT) genes have been detected in human skin fibroblast cells for 2 year normal child (control), 50 year old normal male and female and a 1 year old Down Syndrome (DS) male and female with established trisomy karyotype using the RT-PCR technique. Differential expression of these genes is quantified individually against a beta-Actin gene that has been employed as an internal control. The immunoblotting of cell lysate proteins with polyclonal antibodies exhibit SOD1 (16 kD), SOD2 (40 kD), GPx (23 and 92 kD), CAT (64 kD), and Actin (43 kD) as translational products. The results demonstrate that the enhancement in the level of mRNAs encoding SOD1 in DS male and female, as well as aged male and female are 51, 21, 31 and 50% respectively compared to the normal child (control). In SOD2, DS male and female display higher (176%) and lower (26%) levels of expression whereas aged male and female exhibit enhanced levels of expression (66 and 119%) respectively compared to the control. This study demonstrates that DS affects the female less than the male whereas in the aging process, the female is more prone to oxidative damage than the male. These results not only indicate that the level of GPx mRNA is constant except in DS male, which shows a downward regulation but that even CAT mRNA is upward regulated in aged as well as in DS males and females. These disproportionate changes in anti-oxidant genes, which are incapable of coping with over expressed genes, may contribute towards the aging process, dementia and Down syndrome.

HSP70 Binds to the Fast-twitch Skeletal Muscle Sarco(endo)plasmic Reticulum Ca2+-ATPase (SERCA1a) and Prevents Thermal Inactivation

This study examined whether HSP70 could bind to and protect against thermal inactivation of SERCA1a, the SERCA isoform expressed in adult fast-twitch skeletal muscle. Sarcoplasmic reticulum vesicles prepared from rat gastrocnemius muscle were incubated with purified HSP70 at both 37 and 41 degrees C for either 30, 60, or 120 min. Maximal SERCA1a activity (micromol/g protein/min) in the absence of HSP70 was reduced progressively with time, with greater reductions occurring at 41 degrees C compared with 37 degrees C. HSP70 protected against thermal inactivation of SERCA1a activity at 37 degrees C but not at 41 degrees C and only at 30 and 60 min but not at 120 min. HSP70 also protected against reductions in binding capacity for fluorescein isothiocyanate, a fluorescent probe that binds to Lys515 in the nucleotide binding domain of SERCA, at 30 and 60 min but not at 120 min, an effect that was independent of temperature. HEK-293 cells were co-transfected with cDNAs encoding rabbit SERCA1a and human HSP-EYFP and subjected to 40 degrees C for 1 h. Immunohistochemistry revealed nearly complete co-localization of SERCA1a with HSP70 under these conditions. Co-immunoprecipitation showed physical interaction between HSP70 and SERCA1a under all thermal conditions both in vitro and in HEK-293 cells. Modeling showed that the fluorescein isothiocyanate-binding site of intact SERCA1a in the E2 form lies in its close proximity to a potential interaction site between SERCA1a and HSP70. These results indicate that HSP70 can bind to SERCA1a and, depending on the severity of heat stress, protect SERCA1a function by stabilizing the nucleotide binding domain.

HIP/PAP, a C-type lectin overexpressed in hepatocellular carcinoma, binds the RII alpha regulatory subunit of cAMP-dependent protein kinase and alters the cAMP-dependent protein kinase signalling

HIP/PAP is a C-type lectin overexpressed in hepatocellular carcinoma (HCC). Pleiotropic biological activities have been ascribed to this protein, but little is known about the function of HIP/PAP in the liver. In this study, therefore, we searched for proteins interacting with HIP/PAP by screening a HCC cDNA expression library. We have identified the RII alpha regulatory subunit of cAMP-dependent protein kinase (PKA) as a partner of HIP/PAP. HIP/PAP and RII alpha were coimmunoprecipitated in HIP/PAP expressing cells. The biological relevance of the interaction between these proteins was established by demonstrating, using fractionation methods, that they are located in a same subcellular compartment. Indeed, though HIP/PAP is a protein secreted via the Golgi apparatus we showed that a fraction of HIP/PAP escaped the secretory apparatus and was recovered in the cytosol. Basal PKA activity was increased in HIP/PAP expressing cells, suggesting that HIP/PAP may alter PKA signalling. Indeed, we showed, using a thymidine kinase-luciferase reporter plasmid in which a cAMP responsive element was inserted upstream of the thymidine kinase promoter, that luciferase activity was enhanced in HIP/PAP expressing cells. Thus our findings suggest a novel mechanism for the biological activity of the HIP/PAP lectin.

Genetic Distance in Housekeeping Genes Between Plasmodium falciparum and Plasmodium reichenowi and Within P. falciparum

The time to the most recent common ancestor of the extant populations of Plasmodium falciparum is controversial. The controversy primarily stems from the limited availability of sequences from Plasmodium reichenowi, a chimpanzee malaria parasite closely related to P. falciparum. Since the rate of nucleotide substitution differs in different loci and DNA regions, the estimation of genetic distance between P. falciparum and P. reichenowi should be performed using orthologous sequences that are evolving neutrally. Here, we obtained full-length sequences of two housekeeping genes, sarcoplasmic and endoplasmic reticulum Ca2+ -ATPase (serca) and lactate dehydrogenase (ldh), from 11 isolates of P. falciparum and 1 isolate of P. reichenowi and estimate the interspecific genetic distance (divergence) between the two species and intraspecific genetic distance (polymorphism) within P. falciparum. Interspecific distance and intraspecific distance at synonymous sites of interspecies-conserved regions of serca and ldh were 0.0672 +/- 0.0088 and 0.0011 +/- 0.0007, respectively, using the Nei and Gojobori method. Based on the ratio of interspecific distance to intraspecific distance, the time to the most recent common ancestor of P. falciparum was estimated to be (8.30 +/- 5.40) x 10(4) and (11.62 +/- 7.56) x 10(4) years ago, assuming the divergence time of the two parasite species to be 5 and 7 million years ago, respectively.

Anti-apoptotic protein Bcl-2 interacts with and destabilizes the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA)

The anti-apoptotic effect of Bcl-2 is well established, but the detailed mechanisms are unknown. In the present study, we show in vitro a direct interaction of Bcl-2 with the rat skeletal muscle SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase), leading to destabilization and inactivation of the protein. Recombinant human Bcl-2D21, a truncated form of Bcl-2 with a deletion of 21 residues at the C-terminal membrane-anchoring region, was expressed and affinity-purified as a glutathione S-transferase fusion protein. Bcl-2D21 co-immunoprecipitated and specifically interacted with SERCA in an in vitro-binding assay. The original level of Bcl-2 in sarcoplasmic reticulum vesicles was very low, i.e. hardly detectable by immunoblotting with specific antibodies. The addition of Bcl-2D21 to the sarcoplasmic reticulum resulted in the inhibition of the Ca2+-ATPase activity dependent on the Bcl-2D21/SERCA molar ratio and incubation time. A complete inactivation of SERCA was observed after 2.5 h of incubation at approx. 2:1 molar ratio of Bcl-2D21 to SERCA. In contrast, Bcl-2D21 did not significantly change the activity of the plasma-membrane Ca2+-ATPase. The redox state of the single Cys158 residue in Bcl-2D21 and the presence of GSH did not affect SERCA inhibition. The interaction of Bcl-2D21 with SERCA resulted in a conformational transition of SERCA, assessed through a Bcl-2-dependent increase in SERCA thiols available for the labelling with a fluorescent reagent. This partial unfolding of SERCA did not lead to a higher sensitivity of SERCA towards oxidative inactivation. Our results suggest that the direct interaction of Bcl-2 with SERCA may be involved in the regulation of apoptotic processes in vivo through modulation of cytoplasmic and/or endoplasmic reticulum calcium levels required for the execution of apoptosis.

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

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

accordion, a zebrafish behavioral mutant, has a muscle relaxation defect due to a mutation in the ATPase Ca2+ pump SERCA1

When wild-type zebrafish embryos are touched at 24 hours post-fertilization (hpf), they typically perform two rapid alternating coils of the tail. By contrast, accordion (acc) mutants fail to coil their tails normally but contract the bilateral trunk muscles simultaneously to shorten the trunk, resulting in a pronounced dorsal bend. Electrophysiological recordings from muscles showed that the output from the central nervous system is normal in mutants, suggesting a defect in muscles is responsible. In fact, relaxation in acc muscle is significantly slower than normal. In vivo imaging of muscle Ca2+ transients revealed that cytosolic Ca2+ decay was significantly slower in acc muscle. Thus, it appears that the mutant behavior is caused by a muscle relaxation defect due to the impairment of Ca2+ re-uptake. Indeed, acc mutants carry a mutation in atp2a1 gene that encodes the sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA1), a Ca2+ pump found in the muscle sarcoplasmic reticulum (SR) that is responsible for pumping Ca2+ from the cytosol back to the SR. As SERCA1 mutations in humans lead to Brody disease, an exercise-induced muscle relaxation disorder, zebrafish accordion mutants could be a useful animal model for this condition.

Darier’s disease: from dyskeratosis to endoplasmic reticulum calcium ATPase deficiency

The skin is the body's largest organ and has an essential barrier protective function against physical, chemical, and pathogen aggressions and prevents fluid loss. The outer layer of the skin, known as the epidermis, plays a key role in this protection, through a tightly regulated differentiation programme from basal keratinocytes to the stratum corneum at the skin surface. During this process, keratinocytes from the base of the epidermis undergo major morphological and functional changes during their migration through the spinous and granular layers, to become terminally differentiated corneocytes which will be shed from the skin's surface. The role of extracellular Ca2+ in cell-to-cell adhesion and in epidermal differentiation was known to be important, but the identification of the sarco/endoplasmic reticulum Ca2+ transport ATPase (ATP2A2) as the defective gene in a rare genetic skin disease known as Darier's disease, came as a surprise and shed light on the key role of Ca2+ signaling in the homeostasis of the epidermis.

Calcium dyshomeostasis in beta-amyloid and tau-bearing skeletal myotubes

The relative scarcity of inclusion-affected muscle cells or markers of cell death in inclusion body myositis (IBM) is in distinction to the specific and early intracellular deposition of several Alzheimer's Disease (AD)-related proteins. The current study examined the possible correlation between myotube beta-amyloid and/or Tau accumulations and a widespread mishandling of intracellular muscle calcium concentration that could potentially account for the unrelenting weakness in affected patients. Cultured myogenic cells (C(2)C(12)) expressed beta-amyloid-42 (Abeta(42)) and fetal Tau peptides, as human transgenes encoded by herpes simplex virus, either individually or concurrently. Co-expression of Abeta(42) in C(2)C(12) myotubes resulted in hyperphosphorylation of Tau protein that was not observed when Tau was expressed alone. Resting calcium concentration and agonist-induced RyR-mediated Ca(2+) release were examined using calcium-specific microelectrodes and Fluo-4 epifluorescence, respectively. Co-expression of Abeta(42) and Tau cooperatively elevated basal levels of myoplasmic-free calcium, an effect that was accompanied by depolarization of the plasma membrane. Sarcoplasmic reticulum (SR) calcium release, induced by KCl depolarization, was not affected by Abeta(42) or Tau. In contrast, expression of Abeta(42), Tau, or Abeta(42) together with Tau resulted in enhanced sensitivity of ryanodine receptors to activation by caffeine. Notably, expression of beta-amyloid, alone, was sufficient to result in an increased sensitivity to direct activation by caffeine. Current results indicate that amyloid proteins cooperate to raise resting calcium levels and that these effects are associated with a passive SR Ca(2+) leak and Tau hyperphosphorylation in skeletal muscle.

The plasma membrane Ca2+pump from proximal kidney tubules is exclusively localized and active in caveolae

Plasma membrane Ca2+-ATPase is involved in the fine-tuned regulation of intracellular Ca2+. In this study, the presence of Ca2+-ATPase in caveolae from kidney basolateral membranes was investigated. With the use of a discontinuous sucrose gradient, we show that Ca2+-ATPase is exclusively located and fully active in caveolin-containing microdomains. Treatment with methyl-beta-cyclodextrin--a cholesterol chelator--leads to a spreading of both caveolin and completely inactive Ca2+-ATPase toward high-density fractions. These data support the view that Ca2+ fluxes mediated by Ca2+-ATPase in kidney epithelial cells occur only in caveolae, being strictly dependent on the integrity of these microdomains.

Probing nucleotide-binding effects on backbone dynamics and folding of the nucleotide-binding domain of the sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase

In muscle cells, SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase) plays a key role in restoring cytoplasmic Ca2+ levels to resting concentrations after transient surges caused by excitation-coupling cycles. The mechanism by which Ca2+ is translocated to the lumen of the ER (endoplasmic reticulum) involves major conformational rearrangements among the three cytoplasmic domains: actuator (A), nucleotide-binding (N) and phosphorylation (P) domains; and within the transmembrane Ca2+-binding domain of SERCA. CD, fluorescence spectroscopy and NMR spectroscopy were used in the present study to probe the conformation and stability of the isolated N domain of SERCA (SERCA-N), in the presence and absence of AMP-PNP (adenosine 5'-[beta,gamma-imido]triphosphate). CD and tryptophan fluorescence spectroscopy results established that the effects of nucleotide binding were not readily manifested on the global fold and structural stability of SERCA-N. 15N-backbone-relaxation experiments revealed site-specific changes in backbone dynamics that converge on the central beta-sheet domain. Nucleotide binding produced diverse effects on dynamics, with enhanced mobility observed for Ile369, Cys420, Arg467, Asp568, Phe593 and Gly598, whereas rigidifying effects were found for Ser383, Leu419, Thr484 and Thr532. These results demonstrate that the overall fold and backbone motional properties of SERCA-N remained essentially the same in the presence of AMP-PNP, yet revealing evidence for internal counter-balancing effects on backbone dynamics upon binding the nucleotide, which propagate through the central beta-sheet.

Crystal structure of the calcium pump with a bound ATP analogue

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

Enrichment of endoplasmic reticulum with cholesterol inhibits sarcoplasmic-endoplasmic reticulum calcium ATPase-2b activity in parallel with increased order of membrane lipids: implications for depletion of endoplasmic reticulum calcium stores and apoptosis in cholesterol-loaded macrophages

Macrophages in advanced atherosclerotic lesions accumulate large amounts of unesterified, or "free," cholesterol (FC). FC accumulation induces macrophage apoptosis, which likely contributes to plaque destabilization. Apoptosis is triggered by the enrichment of the endoplasmic reticulum (ER) with FC, resulting in depletion of ER calcium stores, and induction of the unfolded protein response. To explain the mechanism of ER calcium depletion, we hypothesized that FC enrichment of the normally cholesterol-poor ER membrane inhibits the macrophage ER calcium pump, sarcoplasmic-endoplasmic reticulum calcium ATPase-2b (SERCA2b). FC enrichment of ER membranes to a level similar to that occurring in vivo inhibited both the ATPase activity and calcium sequestration function of SERCA2b. Enrichment of ER with ent-cholesterol or 14:0-18:0 phosphatidylcholine, which possess the membrane-ordering properties of cholesterol, also inhibited SERCA2b. Moreover, at various levels of FC enrichment of ER membranes, there was a very close correlation between increasing membrane lipid order, as monitored by 16-doxyl-phosphatidycholine electron spin resonance, and SERCA2b inhibition. In view of these data, we speculate that SERCA2b, a conformationally active protein with 11 membrane-spanning regions, loses function due to decreased conformational freedom in FC-ordered membranes. This biophysical model may underlie the critical connection between excess cholesterol, unfolded protein response induction, macrophage death, and plaque destabilization in advanced atherosclerosis.

Distinct types of abnormality in kinetic properties of three Darier disease-causing sarco(endo)plasmic reticulum Ca2+-ATPase mutants that exhibit normal expression and high Ca2+ transport activity

The possible functional abnormalities in three different Darier disease-causing Ca(2+)-ATPase (SERCA2b) mutants, Ile(274) --> Val at the lumenal end of M3, Leu(321) --> Phe on the cytoplasmic part of M4, and Met(719) --> Ile in P domain, were explored, because they exhibited nearly normal expression and localization in COS-1 cells and the high ATPase and coupled Ca(2+) transport activities that were essentially identical (L321F) or slightly lower (I274V by approximately 35% and M719I by approximately 30%) as compared with those of the wild type. These mutations happened to be in Japanese patients found previously by us. Kinetic analyses revealed that each of the mutants possesses distinct types of abnormalities; M719I and L321F possess the 2-3-fold reduced affinity for cytoplasmic Ca(2+), whereas I274V possesses the normal high affinity. L321F exhibited also the remarkably reduced sensitivity to the feedback inhibition of the transport cycle by accumulated lumenal Ca(2+), as demonstrated with the effect of Ca(2+) ionophore on ATPase activity and more specifically with the effects of Ca(2+) (up to 50 mm) on the decay of phosphoenzyme intermediates. The results on I274V and M719I suggest that the physiological requirement for Ca(2+) homeostasis in keratinocytes to avoid haploinsufficiency is very strict, probably much more than considered previously. The insensitivity to lumenal Ca(2+) in L321F likely brings the lumenal Ca(2+) to an abnormally elevated level. The three mutants with their distinctively altered kinetic properties will thus likely cause different types of perturbation of intracellular Ca(2+) homeostasis, but nevertheless all types of perturbation result in Darier disease. It might be possible that the observed unique feature of L321F could possibly be associated with the specific symptoms in the pedigree with this mutation, neuropsychiatric disorder, and behavior problems. The results also provided further insight into the global nature of conformational changes of SERCAs for ATP-driven Ca(2+) transport.

Calsequestrin and the calcium release channel of skeletal and cardiac muscle

Calsequestrin is by far the most abundant Ca(2+)-binding protein in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. It allows the Ca2+ required for contraction to be stored at total concentrations of up to 20mM, while the free Ca2+ concentration remains at approximately 1mM. This storage capacity confers upon muscle the ability to contract frequently with minimal run-down in tension. Calsequestrin is highly acidic, containing up to 50 Ca(2+)-binding sites, which are formed simply by clustering of two or more acidic residues. The Kd for Ca2+ binding is between 1 and 100 microM, depending on the isoform, species and the presence of other cations. Calsequestrin monomers have a molecular mass of approximately 40 kDa and contain approximately 400 residues. The monomer contains three domains each with a compact alpha-helical/beta-sheet thioredoxin fold which is stable in the presence of Ca2+. The protein polymerises when Ca2+ concentrations approach 1mM. The polymer is anchored at one end to ryanodine receptor (RyR) Ca2+ release channels either via the intrinsic membrane proteins triadin and junctin or by binding directly to the RyR. It is becoming clear that calsequestrin has several functions in the lumen of the SR in addition to its well-recognised role as a Ca2+ buffer. Firstly, it is a luminal regulator of RyR activity. When triadin and junctin are present, calsequestrin maximally inhibits the Ca2+ release channel when the free Ca2+ concentration in the SR lumen is 1mM. The inhibition is relieved when the Ca2+ concentration alters, either because of small changes in the conformation of calsequestrin or its dissociation from the junctional face membrane. These changes in calsequestrin's association with the RyR amplify the direct effects of luminal Ca2+ concentration on RyR activity. In addition, calsequestrin activates purified RyRs lacking triadin and junctin. Further roles for calsequestrin are indicated by the kinase activity of the protein, its thioredoxin-like structure and its influence over store operated Ca2+ entry. Clearly, calsequestrin plays a major role in calcium homeostasis that extends well beyond its ability to buffer Ca2+ ions.

The cation/Ca(2+) exchanger superfamily: phylogenetic analysis and structural implications

Cation/Ca(2+) exchangers are an essential component of Ca(2+) signaling pathways and function to transport cytosolic Ca(2+) across membranes against its electrochemical gradient by utilizing the downhill gradients of other cation species such as H(+), Na(+), or K(+). The cation/Ca(2+) exchanger superfamily is composed of H(+)/Ca(2+) exchangers and Na(+)/Ca(2+) exchangers, which have been investigated extensively in both plant cells and animal cells. Recently, information from completely sequenced genomes of bacteria, archaea, and eukaryotes has revealed the presence of genes that encode homologues of cation/Ca(2+) exchangers in many organisms in which the role of these exchangers has not been clearly demonstrated. In this study, we report a comprehensive sequence alignment and the first phylogenetic analysis of the cation/Ca(2+) exchanger superfamily of 147 sequences. The results present a framework for structure-function relationships of cation/Ca(2+) exchangers, suggesting unique signature motifs of conserved residues that may underlie divergent functional properties. Construction of a phylogenetic tree with inclusion of cation/Ca(2+) exchangers with known functional properties defines five protein families and the evolutionary relationships between the members. Based on this analysis, the cation/Ca(2+) exchanger superfamily is classified into the YRBG, CAX, NCX, and NCKX families and a newly recognized family, designated CCX. These findings will provide guides for future studies concerning structures, functions, and evolutionary origins of the cation/Ca(2+) exchangers.

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

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

Effects of prolonged exercise and recovery on sarcoplasmic reticulum Ca2+ cycling properties in rat muscle homogenates

AIM

To examine the effects of exercise and exercise plus active and passive recovery on sarcoplasmic reticulum (SR) Ca2+-handling properties.

METHODS

Crude muscle homogenates were prepared from adult rat gastrocnemius muscle from two experiments. In one experiment, the muscle was extracted immediately after prolonged treadmill running (RUN), after a 45 min period of reduced exercise intensity (RUN+) following RUN and compared with controls (CON). In the second experiment, muscle was extracted during passive recovery following the same run protocol at 10 min (REC10), 25 min (REC25) and 45 min (REC45) and compared with CON.

RESULTS

Sarcoplasmic reticulum Ca2+-uptake was 31% higher (P < 0.05) in RUN+ compared with CON and RUN. Higher values (P < 0.05) were also found in REC25 (48%) and REC45 (50%) compared with CON. Maximal Ca2+-ATPase was increased by 23% (P < 0.05) in RUN+ compared with CON and RUN and by 65-68% (P < 0.05) in REC25 and REC45 compared with CON. A higher (P < 0.05) Hill coefficient for Ca2+-ATPase activity was observed in RUN+ (2.3 +/- 0.2) compared with CON (1.7 +/- 0.2) or RUN (1.6 +/- 0.2), but not for any REC conditions. In addition, the coupling ratio (Ca2+-uptake/Ca2+-ATPase activity) was higher (P < 0.05) in RUN+ (2.2 +/- 0.10) compared with CON (1.9 +/- 0.05) and RUN (1.9 +/- 0.08).

CONCLUSIONS

It is concluded that in crude homogenates, SR Ca2+-uptake and Ca2+-ATPase activity are elevated in recovery following prolonged running and that the elevation in these properties is more pronounced during passive compared with active recovery.

Calcium pumps and keratinocytes: lessons from Darier's disease and Hailey-Hailey disease

Darier's disease and Hailey-Hailey disease are autosomal dominantly inherited skin disorders in which desmosomal adhesion between keratinocytes is abnormal. ATP2A2 and ATP2C1 have been identified as the causative genes for Darier's disease and Hailey-Hailey disease, respectively. ATP2A2 encodes the sarco(endo)plasmic reticulum Ca(2+)-ATPase isoform 2 (SERCA2) pump, while ATP2C1 encodes a secretory pathway Ca(2+)/Mn(2+)-ATPase (SPCA1) found in the Golgi apparatus. We review recent work into the function of these pumps in human keratinocytes and discuss how mutations in these genes might cause these diseases by altering the formation or stability of desmosomes.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

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

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

Electron Paramagnetic Resonance Reveals a Large-Scale Conformational Change in the Cytoplasmic Domain of Phospholamban upon Binding to the Sarcoplasmic Reticulum Ca-ATPase

We used EPR spectroscopy to probe directly the interaction between phospholamban (PLB) and its regulatory target, the sarcoplasmic reticulum Ca-ATPase (SERCA). Synthetic monomeric PLB was prepared with a single cytoplasmic cysteine at residue 11, which was then spin labeled. PLB was reconstituted into membranes in the presence or absence of SERCA, and spin label mobility and accessibility were measured. The spin label was quite rotationally mobile in the absence of SERCA, but became more restricted in the presence of SERCA. SERCA also decreased the dependence of spin label mobility on PLB concentration in the membrane, indicating that SERCA reduces PLB-PLB interactions. The spin label MTSSL, attached to Cys11 on PLB by a disulfide bond, was stable at position 11 in the absence of SERCA. In the presence of SERCA, the spin label was released and a covalent bond was formed between PLB and SERCA, indicating direct interaction of one or more SERCA cysteine residues with Cys11 on PLB. The accessibility of the PLB-bound spin label IPSL to paramagnetic agents, localized in different phases of the membrane, indicates that SERCA greatly reduces the level of interaction of the spin label with the membrane surface. We propose that the cytoplasmic domain of PLB associates with the lipid surface, and that association with SERCA induces a major conformational change in PLB in which the cytoplasmic domain is drawn away from the lipid surface by SERCA.

Importance of Transmembrane Segment M1 of the Sarcoplasmic Reticulum Ca2+-ATPase in Ca2+ Occlusion and Phosphoenzyme Processing

The functional consequences of a series of point mutations in transmembrane segment M1 of sarcoplasmic reticulum Ca2+-ATPase were analyzed in steady-state and transient kinetic experiments examining the partial reaction steps involved in Ca2+ interaction and phosphoenzyme turnover. Arginine or leucine substitution of Glu51, Glu55, or Glu58, located in the N-terminal third of M1, did not affect these functions. Arginine or leucine substitution of Asp59, located right at the bend of M1 seen in the crystal structure of the thapsigargin-bound form, caused a 10-fold increase of the rate of Ca2+ dissociation toward the cytoplasmic side. Mutation of Leu60 to alanine or proline and of Val62 to alanine also enhanced Ca2+ dissociation, whereas an 11-fold reduction of the rate of Ca2+ dissociation was observed upon alanine substitution of Leu65, thus providing evidence for a relation of the middle part of M1 to a gating mechanism controlling the dissociation of occluded Ca2+ from its membranous binding sites. Moreover, phosphoenzyme processing was affected by some of the latter mutations, in particular leucine substitution of Asp59, and alanine substitution of Leu65 accelerated the transition to ADP-insensitive phosphoenzyme and blocked its dephosphorylation, thus demonstrating that this part of M1, besides being important in Ca2+ interaction, furthermore, is a critical element in the long range signaling between the transmembrane domain and the cytoplasmic catalytic site.

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

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

8Br-cGMP mediates relaxation of tracheal smooth muscle through PKA

In this study, guinea pig tracheal smooth muscle pre-contracted with histamine was relaxed by the addition of 100microM 8Br-cGMP, a non-hydrolyzable and cell-permeable analog for cGMP. This effect was not sensitive to cGMP-dependent protein kinase (PKG) inhibitors, whereas it was partially blocked by cAMP-dependent protein kinase (PKA) inhibitors. The relaxation observed was also reverted up to 50+/-8.5% by iberiotoxin, a selective inhibitor of large conductance, calcium-activated potassium channels (BK(Ca)). Our results indicate that there exists a crosstalk mechanism between cAMP and cGMP signaling pathways which lead to relaxation of guinea pig tracheal smooth muscle and also that BK(Ca) channels are involved to a certain extent in this phenomenon.

Glutamate-183 in the conserved TGES motif of domain A of sarcoplasmic reticulum Ca2+-ATPase assists in catalysis of E2/E2P partial reactions

The recently determined crystal structures of the sarcoplasmic reticulum Ca(2+)-ATPase show that in the E(1)Ca(2) form, domain A is almost isolated from the other cytoplasmic domains, P and N, whereas in E(2), domain A has approached domains P and N, with E183 of the highly conserved P-type ATPase signature sequence TGES in domain A now being close to the phosphorylated aspartate in domain P, thus raising the question whether E183 acquires a catalytic role in E(2) and E(2)P conformations. This study compares the partial reactions of mutant E183A and wild-type Ca(2+)-ATPase, using transient and steady-state kinetic measurements. It is demonstrated that dephosphorylation of the E(2)P phosphoenzyme intermediate, as well as reverse phosphorylation of E(2) with P(i), is severely inhibited in the mutant. Furthermore, the apparent affinity of E(2) for the phosphoryl transition state analog vanadate is reduced by three orders of magnitude, consistent with a destabilization of the transition state complex, and the mutant displays reduced apparent affinity for P(i) in the E(2) form. The E(1)Ca(2) conformation, on the other hand, shows normal phosphorylation with ATP and normal Ca(2+) binding properties, and the rates of the conformational transitions E(1)PCa(2) --> E(2)P and E(2) --> E(1)Ca(2) are only 2- to 3-fold reduced, relative to wild type. These results, which likely can be generalized to other P-type ATPases, indicate that E183 is critical for the phosphatase function of E(2) and E(2)P, possibly interacting with the phosphoryl group or attacking water in the transition state complex, but is of little functional importance in E(1) and E(1)P.

Diabetes increases formation of advanced glycation end products on Sarco(endo)plasmic reticulum Ca2+-ATPase

Prolongation of relaxation is a hallmark of diabetic cardiomyopathy. Most studies attribute this defect to decreases in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) expression and SERCA2a-to-phospholamban (PLB) ratio. Since its turnover rate is slow, SERCA2a is susceptible to posttranslational modifications during diabetes. These modifications could in turn compromise conformational rearrangements needed to translocate calcium ions, also leading to a decrease in SERCA2a activity. In the present study one such modification was investigated, namely advanced glycation end products (AGEs). Hearts from 8-week streptozotocin-induced diabetic (8D) rats showed typical slowing in relaxation, confirming cardiomyopathy. Hearts from 8D animals also expressed lower levels of SERCA2a protein and higher levels of PLB. Analysis of matrix-assisted laser desorption/ionization time-of-flight mass data files from trypsin-digested SERCA2a revealed several cytosolic SERCA2a peptides from 8D modified by single noncrosslinking AGEs. Crosslinked AGEs were also found. Lysine residues within actuator and phosphorylation domains were cross-linked to arginine residues within the nucleotide binding domain via pentosidine AGEs. Two weeks of insulin-treatment initiated after 6 weeks of diabetes attenuated these changes. These data demonstrate for the first time that AGEs are formed on SERCA2a during diabetes, suggesting a novel mechanism by which cardiac relaxation can be slowed during diabetes.

Distinct natures of beryllium fluoride-bound, aluminum fluoride-bound, and magnesium fluoride-bound stable analogues of an ADP-insensitive phosphoenzyme intermediate of sarcoplasmic reticulum Ca2+-ATPase: changes in catalytic and transport sites during phosphoenzyme hydrolysis

The structural natures of stable analogues for the ADP-insensitive phosphoenzyme (E2P) of Ca(2+)-ATPase formed in sarcoplasmic reticulum vesicles, i.e. the enzymes with bound beryllium fluoride (BeF.E2), bound aluminum fluoride (AlF.E2), and bound magnesium fluoride (MgF.E2), were explored and compared with those of actual E2P formed from P(i) without Ca(2+). Changes in trinitrophenyl-AMP fluorescence revealed that the catalytic site is strongly hydrophobic in BeF.E2 as in E2P but hydrophilic in MgF.E2 and AlF.E2; yet, the three cytoplasmic domains are compactly organized in these states. Thapsigargin, which was shown in the crystal structure to fix the transmembrane helices and, thus, the postulated Ca(2+) release pathway to lumen in a closed state, largely reduced the tryptophan fluorescence in BeF.E2 as in E2P, but only very slightly (hence, the release pathway is likely closed without thapsigargin) in MgF.E2 and AlF.E2 as in dephosphorylated enzyme. Consistently, the completely suppressed Ca(2+)-ATPase activity in BeF-treated vesicles was rapidly restored in the presence of ionophore A23187 but not in its absence by incubation with Ca(2+) (over several millimolar concentrations) at pH 6, and, therefore, lumenal Ca(2+) is accessible to reactivate the enzyme. In contrast, no or only very slow restoration was observed with vesicles treated with MgF and AlF even with A23187. BeF.E2 thus has the features very similar to those characteristic of the E2P ground state, although AlF.E2 and MgF.E2 most likely mimic the transition or product state for the E2P hydrolysis, during which the hydrophobic nature around the phosphorylation site is lost and the Ca(2+) release pathway is closed. The change in hydrophobic nature is probably associated with the change in phosphate geometry from the covalently bound tetrahedral ground state (BeF(3)(-)) to trigonal bipyramidal transition state (AlF(3) or AlF(4)(-)) and further to tetrahedral product state (MgF(4)(2-)), and such change likely rearranges transmembrane helices to prevent access and leakage of lumenal Ca(2+).

A 100 kDa vanadate and lanzoprazole-sensitive ATPase from Streptococcus mutans membrane

The cariogenic potential of Streptococcus mutans is due to the production of organic acids derived from energy metabolism, which implies the need of mechanisms for the organism to tolerate this acidic environment. The F(1)F(o)-ATPase is generally considered as the main enzyme responsible for cytoplasmic proton extrusion, but mutations that resulted in a 50% reduction in F(1)F(o)-ATPase activity in S. mutans still allowed the micro-organism to grow and extrude acid, keeping the intracellular pH one pH unit above the extracellular ambient. This finding suggests the existence of other enzymatic (or cellular) mechanisms that keep the cytosolic pH neutral during micro-organism growth. This paper describes a membrane protein in S. mutans, with a molecular weight of 100 kDa, which exhibits ATPase activity inhibited by classic inhibitors of P-type ATPases (orthovanadate) and H(+),K(+)-ATPase (lanzoprazole), has an optimum pH comparable to other H(+)-ATPases and undergoes phosphorylation during the catalytic reaction, like that of H(+)-ATPases described in yeast and plant plasma membrane. Together, these results strongly suggest that the enzyme we describe here is a P-type H(+)-ATPase or H(+),ion-ATPase that can act in association with F(1)F(o)-ATPase during the growth of the S. mutans.

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

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

Spatial and dynamic interactions between phospholamban and the canine cardiac Ca2+ pump revealed with use of heterobifunctional cross-linking agents

Heterobifunctional thiol to amine cross-linking agents were used to gain new insights on the dynamics and conformational factors governing the interaction between the cardiac Ca2+ pump (SERCA2a) and phospholamban (PLB). PLB is a small protein inhibitor of SERCA2a that reduces enzyme affinity for Ca2+ and thereby regulates cardiac contractility. We found that the PLB monomer with Asn27 or Asn30 changed to Cys (N27C-PLB or N30C-PLB) cross-linked to lysine of SERCA2a within seconds with > or =80% efficiency. Optimal cross-linking occurred at spacer chain lengths of 10 and 15 A for N27C and N30C, respectively. The rapid time course of cross-linking indicated that neither dissociation of PLB pentamers nor binding of PLB monomers to SERCA2a was rate-limiting. Cross-linking occurred only to the E2 (Ca2+-free) conformation of SERCA2a, was strongly favored by nucleotide binding to this state, and was completely inhibited by thapsigargin. Protein sequencing in combination with mutagenesis identified of Lys328 of SERCA2a as the target of cross-linking. A three-dimensional map of interacting residues indicated that the cross-linking distances were entirely compatible with the 10-A distance recently determined between N30C of PLB and Cys318 of SERCA2a. In contrast, Lys3 of PLB did not cross-link to any Lys (or Cys) of SERCA2a, suggesting that previous three-dimensional models that constrain Lys3 near residues 397-400 of thapsigargin-inhibited SERCA2a should be viewed with caution. Furthermore, although earlier models of PLB.SERCA2a are based on thapsigargin-bound SERCA, our results suggest that the nucleotide-bound, E2 conformation is substantially different and represents the key conformational state for interacting with PLB.

Multiple and distinct effects of mutations of Tyr122, Glu123, Arg324, and Arg334 involved in interactions between the top part of second and fourth transmembrane helices in sarcoplasmic reticulum Ca2+-ATPase: changes in cytoplasmic domain organization during isometric transition of phosphoenzyme intermediate and subsequent Ca2+ release

We explored, by mutational substitutions and kinetic analysis, possible roles of the four residues involved in the hydrogen-bonding or ionic interactions found in the Ca2+-bound structure of sarcoplasmic reticulum Ca2+-ATPase, Tyr(122)-Arg(324), and Glu(123)-Arg(334) at the top part of second transmembrane helix (M2) connected to the A domain and fourth transmembrane helix (M4) in the P domain. The observed substitution effects indicated that Glu(123), Arg(334), and Tyr(122) contributed to the rapid transition between the Ca2+-unbound and bound states of the unphosphorylated enzyme. Results further showed the more profound inhibitory effects of the substitutions in the M4/P domain (Arg(324) and Arg(334)) upon the isomeric transition of phosphorylated intermediate (EP) (loss of ADP sensitivity) and those in M2/A domain (Tyr(122) and Glu(123)) upon the subsequent processing and hydrolysis of EP. The observed distinct effects suggest that the interactions seen in the Ca2+-bound structure are not functionally important but indicate that Arg(334) with its positive charge and Tyr(122) with its aromatic ring are critically important for the above distinct steps. On the basis of the available structural information, the results strongly suggest that Arg(334) moves downward and forms new interactions with M2 (likely Asn(111)); it thus contributes to the inclination of the M4/P domain toward the M2/A domain, which is crucial for the appropriate gathering between the P domain and the largely rotated A domain to cause the loss of ADP sensitivity. On the other hand, Tyr(122) most likely functions in the subsequent Ca2+-releasing step to produce hydrophobic interactions at the A-P domain interface formed upon their gathering and thus to produce the Ca2+-released form of EP. During the Ca2+-transport cycle, the four residues seem to change interaction partners and thus contribute to the coordinated movements of the cytoplasmic and transmembrane domains.

Electrophysiological analysis of the mutated Na,K-ATPase cation binding pocket

Na,K-ATPase mediates net electrogenic transport by extruding three Na+ ions and importing two K+ ions across the plasma membrane during each reaction cycle. We mutated putative cation coordinating amino acids in transmembrane hairpin M5-M6 of rat Na,K-ATPase: Asp776 (Gln, Asp, Ala), Glu779 (Asp, Gln, Ala), Asp804 (Glu, Asn, Ala), and Asp808 (Glu, Asn, Ala). Electrogenic cation transport properties of these 12 mutants were analyzed in two-electrode voltage-clamp experiments on Xenopus laevis oocytes by measuring the voltage dependence of K+-stimulated stationary currents and pre-steady-state currents under electrogenic Na+/Na+ exchange conditions. Whereas mutants D804N, D804A, and D808A hardly showed any Na+/K+ pump currents, the other constructs could be classified according to the [K+] and voltage dependence of their stationary currents; mutants N776A and E779Q behaved similarly to the wild-type enzyme. Mutants E779D, E779A, D808E, and D808N had in common a decreased apparent affinity for extracellular K+. Mutants N776Q, N776D, and D804E showed large deviations from the wild-type behavior; the currents generated by mutant N776D showed weaker voltage dependence, and the current-voltage curves of mutants N776Q and D804E exhibited a negative slope. The apparent rate constants determined from transient Na+/Na+ exchange currents are rather voltage-independent and at potentials above -60 mV faster than the wild type. Thus, the characteristic voltage-dependent increase of the rate constants at hyperpolarizing potentials is almost absent in these mutants. Accordingly, dislocating the carboxamide or carboxyl group of Asn776 and Asp804, respectively, decreases the extracellular Na+ affinity.

Deletions of any single residues in Glu40-Ser48 loop connecting a domain and the first transmembrane helix of sarcoplasmic reticulum Ca(2+)-ATPase result in almost complete inhibition of conformational transition and hydrolysis of phosphoenzyme intermediate

Possible roles of the Glu40-Ser48 loop connecting A domain and the first transmembrane helix (M1) in sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) were explored by mutagenesis. Deletions of any single residues in this loop caused almost complete loss of Ca(2+)-ATPase activity, while their substitutions had no or only slight effects. Single deletions or substitutions in the adjacent N- and C-terminal regions of the loop (His32-Asn39 and Leu49-Ile54) had no or only slight effects except two specific substitutions of Asn39 found in SERCA2b in Darier's disease pedigrees. All the single deletion mutants for the Glu40-Ser48 loop and the specific Asn39 mutants formed phosphoenzyme intermediate (EP) from ATP, but their isomeric transition from ADP-sensitive EP (E1P) to ADP-insensitive EP (E2P) was almost completely or strongly inhibited. Hydrolysis of E2P formed from Pi was also dramatically slowed in these deletion mutants. On the other hand, the rates of the Ca(2+)-induced enzyme activation and subsequent E1P formation from ATP were not altered by the deletions and substitutions. The results indicate that the Glu40-Ser48 loop, with its appropriate length (but not with specific residues) and with its appropriate junction to A domain, is a critical element for the E1P to E2P transition and formation of the proper structure of E2P, therefore, most likely for the large rotational movement of A domain and resulting in its association with P and N domains. Results further suggest that the loop functions to coordinate this movement of A domain and the unique motion of M1 during the E1P to E2P transition.

Mutations in the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase isoform cause Darier's disease

Darier's disease is an autosomal dominantly inherited skin disorder, characterized by loss of adhesion between epidermal cells and abnormal keratinization. ATP2A2 encoding the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA)2 has been identified as the defective gene in Darier's disease. All mutations previously reported occur in the region of ATP2A2 encoding both SERCA2a and SERCA2b isoforms. These isoforms result from alternative splicing of exon 20, with SERCA2b being the major isoform expressed in the epidermis. In this report, we studied a family affected with Darier's disease and identified a deletion (2993delTG) in a region of exon 20 of ATP2A2, which is specific for SERCA2b. This heterozygous mutation predicts a frameshift with a premature termination codon (PTC+32aa) in the eleventh transmembrane domain of SERCA2b. It segregates with the disease phenotype in the family members tested, and functional analysis shows a drastic reduction of the expression of the mutated protein in comparison with the wild-type SERCA2b. Our result suggests that the mutated allele causes the disease phenotype through loss of function of SERCA2b isoform. This finding indicates that SERCA2b plays a key role in the biology of the epidermis, and its defects are sufficient to cause Darier's disease.

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

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

A Japanese case of segmental Darier's disease caused by mosaicism for the ATP2A2 mutation

Darier's disease is an autosomal dominant skin disorder that is characterized by multiple keratotic papules, focal loss of adhesion and abnormal keratinization. Mutations in the ATP2A2 gene encoding sarco/endoplasmic reticulum calcium pumping ATPase type 2 have been identified as the molecular basis of Darier's disease. Segmental Darier's disease is a rare type of Darier's disease in which there is characteristic localization of the keratotic papules in a linear pattern following Blaschko's lines. In this study we examined ATP2A2 mutations in a Japanese patient with segmental Darier's disease. The samples from affected skin, unaffected skin and peripheral leucocytes were subjected to polymerase chain reaction (PCR). Direct sequencing of the PCR products was performed. Sequence analysis revealed that the patient had 160A-->G substitution mutation which predicts I54V. This novel mutation was present in the affected skin, but not in the unaffected skin or peripheral leucocytes. This is the first report of segmental Darier's disease caused by mosaicism for an ATP2A2 mutation in Japan.