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Nakamura J, Maruyama Y, Tajima G, Komeiji Y, Suwa M, Sato C. Ca 2+-ATPase Molecules as a Calcium-Sensitive Membrane-Endoskeleton of Sarcoplasmic Reticulum. Int J Mol Sci 2021; 22:ijms22052624. [PMID: 33807779 PMCID: PMC7961605 DOI: 10.3390/ijms22052624] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2020] [Revised: 02/13/2021] [Accepted: 02/24/2021] [Indexed: 11/16/2022] Open
Abstract
The Ca2+-transport ATPase of sarcoplasmic reticulum (SR) is an integral, transmembrane protein. It sequesters cytoplasmic calcium ions released from SR during muscle contraction, and causes muscle relaxation. Based on negative staining and transmission electron microscopy of SR vesicles isolated from rabbit skeletal muscle, we propose that the ATPase molecules might also be a calcium-sensitive membrane-endoskeleton. Under conditions when the ATPase molecules scarcely transport Ca2+, i.e., in the presence of ATP and ≤ 0.9 nM Ca2+, some of the ATPase particles on the SR vesicle surface gathered to form tetramers. The tetramers crystallized into a cylindrical helical array in some vesicles and probably resulted in the elongated protrusion that extended from some round SRs. As the Ca2+ concentration increased to 0.2 µM, i.e., under conditions when the transporter molecules fully carry out their activities, the ATPase crystal arrays disappeared, but the SR protrusions remained. In the absence of ATP, almost all of the SR vesicles were round and no crystal arrays were evident, independent of the calcium concentration. This suggests that ATP induced crystallization at low Ca2+ concentrations. From the observed morphological changes, the role of the proposed ATPase membrane-endoskeleton is discussed in the context of calcium regulation during muscle contraction.
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Affiliation(s)
- Jun Nakamura
- Health and Medical Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan; (Y.M.); (Y.K.)
- Correspondence: (J.N.); (C.S.)
| | - Yuusuke Maruyama
- Health and Medical Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan; (Y.M.); (Y.K.)
| | - Genichi Tajima
- Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai, Miyagi 980-8576, Japan;
| | - Yuto Komeiji
- Health and Medical Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan; (Y.M.); (Y.K.)
| | - Makiko Suwa
- Biological Science Course, Graduate School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuou-ku, Sagamihara, Kanagawa 252-5258, Japan;
| | - Chikara Sato
- Health and Medical Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan; (Y.M.); (Y.K.)
- Correspondence: (J.N.); (C.S.)
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The dimeric form of Ca2+-ATPase is involved in Ca2+ transport in the sarcoplasmic reticulum. Biochem J 2008; 414:357-61. [DOI: 10.1042/bj20071701] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
To identify the functional unit of Ca2+-ATPase in the sarcoplasmic reticulum, we assessed Ca2+-transport activities occurring on sarcoplasmic reticulum membranes with different combinations of active and inactive Ca2+-ATPase molecules. We prepared heterodimers, consisting of a native Ca2+-ATPase molecule and a Ca2+-ATPase molecule inactivated by FITC labelling, by fusing vesicles loaded with each type of Ca2+-ATPase. The heterodimers exhibited neither Ca2+ transport nor ATP hydrolysis, suggesting that Ca2+ transport by the Ca2+-ATPase requires an interaction between functional Ca2+-ATPase monomers. This finding implies that the functional unit of the Ca2+-ATPase is a dimer.
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Vieyra A, Mintz E, Lowe J, Guillain F. Ca2+ binding to sarcoplasmic reticulum ATPase phosphorylated by Pi reveals four thapsigargin-sensitive Ca2+ sites in the presence of ADP. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2005; 1667:103-13. [PMID: 15581845 DOI: 10.1016/j.bbamem.2004.09.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/14/2003] [Revised: 08/06/2004] [Accepted: 09/13/2004] [Indexed: 10/26/2022]
Abstract
Sarcoplasmic reticulum (SR) Ca2+-ATPase was phosphorylated by Pi at pH 8.0 in the presence of dimethyl sulfoxide (Me2SO). Under these conditions, it was possible to measure transient 45Ca2+ binding to the phosphoenzyme. Binding reached 1.2 Ca2+ per phosphoenzyme (E-PCax) within 10 min in 30% Me2SO, 20 mM MgCl2 and 0.1 mM Pi and the phosphoenzyme only decreased by 23% during this period. This Ca2+ binding was abolished by thapsigargin, showing that it is associated with functional sites of the Ca2+-ATPase. At 40% Me2SO, simultaneous addition of Ca2+ and ADP increased Ca2+ binding up to almost four Ca2+ per phosphoenzyme (ADPE-PCay), revealing a species bearing simultaneously four Ca2+ sites. Both E-PCax and ADPE-PCay were further identified as distinct species by (2',3'-O-2(2,4,6-trinitrophenyl)adenosine 5'-triphosphate) fluorescence, which revealed long-range modifications in the Ca2+-transport sites induced by ADP binding to E-P. In addition, E-PCax was shown to be a functional intermediate of the cycle leading to ATP synthesis provided that Me2SO was diluted. These findings indicate that more than two functional Ca2+-sites exist on the functional Ca2+-ATPase unit, and that the additional sites become accessible upon ADP addition. This is compatible with a four-site model of the SR Ca2+-ATPase allowing simultaneous binding of Ca2+ at lumenal and cytosolic sites. The stoichiometries for Ca2+ binding found here could either be interpreted as binding of four Ca2+ on a Ca2+-ATPase monomer considered as the functional unit or as binding of two Ca2+ per monomer of a functional dimer.
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Affiliation(s)
- Adalberto Vieyra
- Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, 21941-590 Rio de Janeiro, Brasil
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Zhao Y, Fan X, Yang F, Zhang X. Gangliosides modulate the activity of the plasma membrane Ca(2+)-ATPase from porcine brain synaptosomes. Arch Biochem Biophys 2004; 427:204-12. [PMID: 15196995 DOI: 10.1016/j.abb.2004.04.009] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2004] [Revised: 04/14/2004] [Indexed: 11/18/2022]
Abstract
We systematically examined the effects of gangliosides on the plasma membrane Ca(2+)-ATPase (PMCA) from porcine brain synaptosomes. Our results showed that GD1b (two sialic acid residues) stimulated the activity, GM1 (one sialic acid residue) slightly reduced the activity, while asialo-GM1 (no sialic acid residue) markedly inhibited it, suggesting that sialic acid residues of gangliosides are important in the modulation of the PMCA. We also examined the oligosaccharide effects by using GM1, GM2, and GM3 whose only difference was in the length of their oligosaccharide chain. GM1, GM2, and GM3 reduced the enzyme activities, whereas GM2 and GM3 were potent inhibitors. Gangliosides affect both affinity for Ca(2+) and the Vmax of enzyme. It was observed that GD1b and GM2 increased the affinity of the enzyme for Ca(2+). GD1b, GM2 affected the Vmax with an increase of GD1b, but decreases of GM2. The study of the affinity for ATP and the Vmax of enzyme in the presence of gangliosides showed that GD1b and GM2 had little effect on the ATP binding to the enzyme, but the Vmax was apparently changed. Moreover, the effects of gangliosides are additive to that of calmodulin, suggesting that the modulation of PMCA by gangliosides should be through a different mechanism. The conformational changes induced by gangliosides were probed by fluorescence quenching. We found that fluorescent quenchers (I(-) and Cs(+)) with opposite charges had different accessibility to the IAEDANS binding to the PMCA in the presence of gangliosides. An apparent red shift (25nm) with increased maximum of fluorescence spectrum was also observed in the presence of GD1b.
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Affiliation(s)
- Yongfang Zhao
- National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China
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Teramachi S, Imagawa T, Kaya S, Taniguchi K. Replacement of several single amino acid side chains exposed to the inside of the ATP-binding pocket induces different extents of affinity change in the high and low affinity ATP-binding sites of rat Na/K-ATPase. J Biol Chem 2002; 277:37394-400. [PMID: 12138102 DOI: 10.1074/jbc.m204772200] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To investigate the relationship between the high and the low affinity ATP-binding site, which appears during the Na(+)/K(+)-ATPase reaction, four amino acids were mutated, the side chains of which are exposed to inside of the ATP-binding pocket. Six mutants, F475Y, K480A, K480E, K501A, K501E, and R544A, where the numbers correspond to the pig Na(+)/K(+)-ATPase alpha-chain, were expressed in HeLa cells. The apparent affinities were determined by high affinity ATP-dependent phosphorylation and by the low affinity activation of Na(+)/K(+)-ATPase or low affinity ATP inhibition of K(+)-para-nitrophenylphosphatase (pNPPase). For the mutants K480A and K501A, little affinity change was detected for either the high affinity or the low affinity effect. In contrast, the other four mutants reduced both apparent affinities. Strikingly, R544A had a 30-fold greater effect on the high affinity ATP site than the low affinity site. For the F475Y mutant, it is likely that there was a greater effect on the low affinity site than the high affinity site, but for both F475Y and K480E the affinity for the low affinity ATP effect was reduced so much that it was not possible to estimate a K(0.5). However, both the affinities for the K480E were reduced to approximately 1/20. The turnover number of the Na(+)/K(+)-ATPase and the apparent affinity for Na(+) and pNPP was reduced slightly or not at all for these mutants, but the turnover number of K(+)-pNPPase and the apparent affinity for K(+) were increased. These and other data suggest the presence of only one ATP-binding site, which can change its conformation to accept ATP with a high and low affinity. The requirement of Arg-544 and possibly Lys-501 is more important in forming a high affinity ATP binding conformation, and Phe-475 and possibly Lys-480 are more important in forming the low affinity ATP binding conformation.
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Affiliation(s)
- Satomi Teramachi
- Department of Biological Chemistry, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
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Nakamura J, Tajima G, Sato C. Substrate regulation of calcium binding in Ca2+-ATPase molecules of the sarcoplasmic reticulum. II. Effect of CTP, GTP, ITP, and UTP. J Biol Chem 2002; 277:24191-6. [PMID: 11976322 DOI: 10.1074/jbc.m111836200] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To examine the effect of CTP, GTP, ITP, and UTP on calcium binding of Ca2+-ATPase molecules of the sarcoplasmic reticulum, the calcium dependence of the Ca2+-activated hydrolysis activities of these NTPs of the enzyme molecules was examined by comparison with that of calcium binding of the molecules in the absence of the NTPs at pH 7.40. In the sarcoplasmic reticulum membrane, CTP, GTP, and ITP did not affect the noncooperative (Hill value (n(H)) of approximately 1, apparent calcium affinity (K(0.5)) of 2-6 microm)) and cooperative (n(H) approximately 2, K(0.5) approximately 0.2 microm) calcium binding of the molecules, whereas UTP caused the molecules to highly cooperatively (n(H) approximately 4) bind calcium ions with a lowered K(0.5) of approximately 0.04 microm. When the enzyme molecules were solubilized with detergent, all of these NTPs reversibly degraded the calcium affinity of the molecule (from K(0.5) = 3-5 to >40 microm), although the effect of the NTPs on the negatively cooperative manner (n(H) approximately 0.5) of calcium binding was not experimentally obtained. Taking into account the first part of this study (Nakamura, J., Tajima, G., Sato, C., Furukohri, T., and Konishi, K. (2002) J. Biol. Chem. 277, 24180-24190) showing the improving effect of ATP on calcium binding of the membranous and solubilized molecules, the results show that ATP is the only intrinsic substrate for the enzyme molecule. This NTP regulation is discussed in terms of the oligomeric structure of the molecules.
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Affiliation(s)
- Jun Nakamura
- Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8578, Japan.
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Nakamura J, Tajima G, Sato C, Furukohri T, Konishi K. Substrate regulation of calcium binding in Ca2+-ATPase molecules of the sarcoplasmic reticulum. I. Effect of ATP. J Biol Chem 2002; 277:24180-90. [PMID: 11976321 DOI: 10.1074/jbc.m111834200] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The effect of ATP on calcium binding of the Ca2+-ATPase of the sarcoplasmic reticulum has not been clarified. By comparing the calcium dependence of the ATPase activity and of phosphorylation of the ATPase molecules with that of calcium binding in the absence of ATP, we show the existence of two types of regulatory site of the enzyme molecules at which ATP binding variously improves the calcium binding performance of the molecules depending on the aggregation state of the molecules and pH; the two regulatory sites bind ATP at submillimolar (0.25 mm) and millimolar (5 mm) ATP, respectively. The results are discussed based on a model of two conformational variants (A and B forms) of the chemically equivalent ATPase molecules (Nakamura, J., and Furukohri, T. (1994) J. Biol. Chem. 269, 30818-30821). For example, in the sarcoplasmic reticulum membrane at pH 7.40, submillimolar ATP converted the calcium binding manner of the A form from noncooperative (Hill number (n(H)) of approximately 1) to cooperative (n(H) approximately 2), concurrent with a decrease in the apparent calcium affinity (K(0.5)) from 2-6 to 0.1-0.3 microm. The binding of the A form became almost the same as that of the B form (n(H) approximately 2, K(0.5) approximately 0.2 microm), which was not affected by ATP. Millimolar ATP further decreased the K(0.5) of the cooperative binding of the two forms to approximately 0.05 microm. Regulation of the calcium binding performance by ATP is discussed in terms of monomeric and oligomeric pathway models.
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Affiliation(s)
- Jun Nakamura
- Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8578, Japan.
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Majima E, Ishida M, Miki S, Shinohara Y, Terada H. Specific labeling of the bovine heart mitochondrial phosphate carrier with fluorescein 5-isothiocyanate: roles of Lys185 and putative adenine nucleotide recognition site in phosphate transport. J Biol Chem 2001; 276:9792-9. [PMID: 11133984 DOI: 10.1074/jbc.m007222200] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The amine/SH-modifying fluorescein 5-isothiocyanate (FITC) specifically labeled Lys(185) in the putative membrane-spanning region of the phosphate carrier from both the cytosolic and matrix sides of bovine heart mitochondria at 0 degrees C and pH 7.2, and the labeling inhibited the phosphate transport. Nonmodifying fluorescein derivatives having similar structural features to those of ADP and ATP (Majima, E., Yamaguchi, N., Chuman, H., Shinohara, Y., Ishida, M., Goto, S., and Terada, H. (1998) Biochemistry 37, 424-432) inhibited the specific FITC labeling and phosphate transport, but the nonfluorescein phenylisothiocyanate did not inhibit FITC labeling, suggesting that there is a region recognizing the adenine nucleotides in the phosphate carrier and that this region is closely associated with the transport activity. The phosphate transport inhibitor pyridoxal 5'-phosphate inhibited the specific FITC labeling, possibly due to competitive modification of Lys(185). In addition, FITC inhibited the ADP transport and specific labeling of the ADP/ATP carrier with the fluorescein SH reagent eosin 5-maleimide. Based on these results, we discuss the structural features of the phosphate carrier in relation to its transport activity.
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Affiliation(s)
- E Majima
- Faculty of Pharmaceutical Sciences, University of Tokushima, Shomachi-1, Tokushima 770-8505, Japan
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9
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Yang SJ, Jiang SS, Van RC, Hsiao YY, Pan R. A lysine residue involved in the inhibition of vacuolar H(+)-pyrophosphatase by fluorescein 5'-isothiocyanate. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1460:375-83. [PMID: 11106777 DOI: 10.1016/s0005-2728(00)00203-6] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
Vacuolar proton pumping pyrophosphatase (H(+)-PPase; EC 3.6.1.1) plays a central role in the electrogenic translocation of protons from cytosol to the vacuole lumen at the expense of PP(i) hydrolysis. A fluorescent probe, fluorescein 5'-isothiocyanate (FITC), was used to modify a lysine residue of vacuolar H(+)-PPase. The enzymatic activity and its associated H(+) translocation of vacuolar H(+)-PPase were markedly decreased by FITC in a concentration-dependent manner. The inhibition of enzymatic activity followed pseudo-first-order rate kinetics. A double-logarithmic plot of the apparent reaction rate constant against FITC concentration yielded a straight line with a slope of 0.89, suggesting that the alteration of a single lysine residue on the enzyme is sufficient to inhibit vacuolar H(+)-PPase. Changes in K(m) but not V(max) values of vacuolar H(+)-PPase as inhibited by FITC were obtained, indicating that the labeling caused a modification in affinity of the enzyme to its substrate. FITC inhibition of vacuolar H(+)-PPase could be protected by its physiological substrate, Mg(2+)-PP(i). These results indicate that FITC might specifically compete with the substrate at the active site and the FITC-labeled lysine residue locates probably in or near the catalytic domain of the enzyme. The enhancement of fluorescence intensity and the blue shift of the emission maximum of FITC after modification of vacuolar H(+)-PPase suggest that the FITC-labeled lysine residue is located in a relatively hydrophobic region.
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Affiliation(s)
- S J Yang
- Department of Radiological Technology, Chungtai Institute of Health Sciences and Technology, Taichung 40605, Taiwan, ROC
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Merino JM, Gutiérrez-Merino C, Henao F. Plausible stoichiometry of the interacting nucleotide-binding sites in the Ca(2+)-ATPase from sarcoplasmic reticulum membranes. Arch Biochem Biophys 1999; 368:298-302. [PMID: 10441381 DOI: 10.1006/abbi.1999.1317] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The Ca(2+),Mg(2+)-ATPase from sarcoplasmic reticulum couples ATP hydrolysis to Ca(2+) transport toward the lumen of the muscular vesicular system. Combined structural and functional studies suggest that the Ca(2+) binding sites are formed by six amino acids of the same polypeptide and that cation translocation may take place through a channel inside a monomer of the ATPase. However, calorimetric, fluorescent, and kinetic studies suggest that the ATPase may assemble into functional oligomers of as yet unknown stoichiometry. We have addressed this question and attempted to determine the ATPase stoichiometry using a biophysical approach based on the analysis of the ATPase inhibition by fluorescein 5'-isothiocyanate in the presence of increasing ATP concentrations. For native SR membranes, our inhibition data are well described by a model consisting of two interacting nucleotide-binding sites per oligomer. This stoichiometry was disrupted in detergent C(12)E(8)-solubilized ATPase. Thus, these findings suggest that interacting nucleotide binding sites of the ATPase may appear as dimers, and imply that interactions of the globular cytoplasmic domains would play a modulatory role of the protein enzymatic activity.
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Affiliation(s)
- J M Merino
- Facultad de Ciencias, Universidad de Extremadura, Badajoz, 06080, Spain.
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Zhang X, Min X, Yang F. Conformational basis of the phospholipid requirement for the activity of SR Ca(2+)-ATPase. Chem Phys Lipids 1998; 97:55-64. [PMID: 10081149 DOI: 10.1016/s0009-3084(98)00092-9] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The delipidated sarcoplasmic reticulum (SR) Ca(2+)-ATPase was reconstituted into proteoliposomes containing different phospholipids. The result demonstrated the necessity of phosphatidylcholine (PC) for optimal ATPase activity and phosphatidylethanolamine (PE) for the optimal calcium transport activity. Fluorescence intensity of Fluorescein 5-isothiocyanate (FITC)-labeled enzyme at Lys515 as well as the measurement of the distance between 5-((2-[(iodoacetyl) amino] ethyl) amino)naphthalene-1-sulphonic acid (IAEDANS) label sites (Cys674/670) and Pr3+ demonstrated a conformational change of cytoplasmic domain, consequently, leading to the variation of the enzyme function with the proteoliposomes composition. Both the intrinsic fluorescence of Trp and its dynamic quenching by HB decreased with increasing PE content, revealing the conformational change of transmembrane domain. Time-resolved fluorescence study characterized three classes of Trp residues, which showed distinctive variation with the change in phospholipid composition. The phospholipid headgroup size caused the conformational change of SR Ca(2+)-ATPase, subsequent the ATPase activity and Ca2+ uptake.
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Affiliation(s)
- X Zhang
- National Laboratory of Biomacromolecules, Institute of Biophysics, Academia Sinica, People's Republic of China
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12
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Snajdrova L, Xu A, Narayanan N. Clotrimazole, an antimycotic drug, inhibits the sarcoplasmic reticulum calcium pump and contractile function in heart muscle. J Biol Chem 1998; 273:28032-9. [PMID: 9774419 DOI: 10.1074/jbc.273.43.28032] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Clotrimazole (CLT), an antimycotic drug, has been shown to inhibit proliferation of normal and cancer cell lines and its systemic use as a new tool in the treatment of proliferative disorders is presently under scrutiny (Benzaquen, L. R., Brugnara, C., Byers, H. R., Gattoni-Celli, S., and Halperin, J. A. (1995) Nature Med. 1, 534-540). The action of CLT is thought to involve depletion of intracellular Ca2+ stores but the underlying mechanism has not been defined. The present study utilized membrane vesicles of rabbit cardiac sarcoplasmic reticulum (SR) to determine the mechanism by which CLT depletes intracellular Ca2+ stores. The results revealed a strong, concentration-dependent inhibitory action of CLT on the ATP-energized Ca2+ uptake activity of SR (50% inhibition with approximately 35 microM CLT). The inhibition was of rapid onset (manifested in <15 s), and was accompanied by a 7-fold decrease in the apparent affinity of the SR Ca2+-ATPase for Ca2+ and a minor decrement in the enzyme's apparent affinity toward ATP. Exposure of SR to CLT in the absence or presence of Ca2+ resulted in irreversible inhibition of Ca2+ uptake demonstrating that the Ca2+-bound and Ca2+-free conformations of the Ca2+-ATPase are CLT-sensitive. Introduction of CLT to the reaction medium subsequent to induction of enzyme turnover with Ca2+ and ATP resulted in instantaneous cessation of Ca2+ transport indicating that an intermediate enzyme species generated during turnover undergoes rapid inactivation by CLT. The inhibition of Ca2+ uptake by CLT was accompanied by inhibition of Ca2+-stimulated ATP hydrolysis and Ca2+-induced phosphoenzyme intermediate formation from ATP in the ATPase catalytic cycle. Phosphorylation of the Ca2+-deprived enzyme with Pi in the reverse direction of catalytic cycle and Ca2+ release from Ca2+-preloaded SR vesicles were unaffected by CLT. It is concluded that CLT depletes intracellular Ca2+ stores by inhibiting Ca2+ sequestration by the Ca2+-ATPase. The mechanism of ATPase inhibition involves a drug-induced alteration in the Ca2+-binding site(s) resulting in paralysis of the enzyme's catalytic and ion transport cycle. CLT (50 microM) caused marked depression of contractile function in isolated perfused, electrically paced rabbit heart preparations. The contractile function recovered gradually following withdrawal of CLT from the perfusate indicating the existence of mechanisms in the intact cell to inactivate, metabolize, or clear CLT from its target site.
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Affiliation(s)
- L Snajdrova
- Department of Physiology, University of Western Ontario, London, Ontario N6A 5C1, Canada
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Feher JJ, Waybright TD, Fine ML. Comparison of sarcoplasmic reticulum capabilities in toadfish (Opsanus tau) sonic muscle and rat fast twitch muscle. J Muscle Res Cell Motil 1998; 19:661-74. [PMID: 9742450 DOI: 10.1023/a:1005333215172] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The sonic muscle of the oyster toadfish, Opsanus tau, can produce unfused contractions at 300 Hz. Electron microscopy shows a great abundance of the Sarcoplasmic reticulum (SR) in this muscle, but no functional characterization of the capabilities of the SR has been reported. We measured the oxalate-supported Ca2+ uptake rate and capacities of homogenates of toadfish sonic muscle and rat extensor digitorum longus (EDL) muscle, and estimated the number of pump units by titration with thapsigargin, a high-affinity, specific inhibitor of the SR Ca-ATPase. The Ca2+ uptake rate averaged 70.9 +/- 9.5 mumol min -1 per g tissue for the toad fish sonic muscle, and 73.5 +/- 3.7 mumol min -1 g-1 for rat EDL. The capacity for Ca2+ -oxalate uptake was 161 +/- 20 mumol g -1 and 33 +/- 2 mumol g -1 for toadfish sonic muscle and rat EDL, respectively. Thus, the rates of Ca2+ uptake were similar in the two muscles, but the toadfish sonic muscle had about five times the capacity of the rat EDL. The number of pumps as estimated by thapsigargin titration was 68 +/- 4 nmol of Ca-ATPase per g tissue in the toadfish, and 42 +/- 5 nmol Ca-ATPase per g tissue in the rat EDL. The turnover number, defined as the Ca2+ uptake divided by the number of pumps, was 1065 +/- 150 min -1 for toadfish and 1786 +/- 230 min -1 for rat EDL (p < 0.05) at 37 degrees C. The Ca2+ uptake rate of toadfish sonic muscle at 22 degree C, a typical temperature for calling toadfish, averaged 42 +/- 1% of its rate at 37 degree C. At these operating temperatures, the toadfish SR is likely to be slower than the rat fast-twitch SR, yet the toadfish sonic muscle supports more rapid contractions. One explanation for this is that the voluminous SR provides activator Ca2+ for contraction, but the abundant parvalbumin plays a major role in relaxation.
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Affiliation(s)
- J J Feher
- Department of Physiology, Virginia Commonwealth University, Richmond 23298-0551, USA.
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Nakamura J, Tajima G. Independence of two conformations of sarcoplasmic reticulum Ca2+-ATPase molecules in hydrolyzing acetyl phosphate. A two-pair model of the ATPase structural unit. J Biol Chem 1997; 272:19290-4. [PMID: 9235924 DOI: 10.1074/jbc.272.31.19290] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
The sarcoplasmic reticulum Ca2+-ATPase molecules have been shown to exist in two conformations (A and B) that result from intermolecular interaction of ATPase molecules (Nakamura, J., and Tajima, G. (1995) J. Biol. Chem. 270, 17350-17354). The A form binds two calcium ions noncooperatively, whereas the B form binds the calcium ions cooperatively. Here, we examined the independence of these two forms in the calcium-activated hydrolysis of acetyl phosphate (AcP) under asynchronous and synchronous conditions of their E1-E2 transitions at 0-5 and 25 degrees C. Irrespective of their synchronism and temperature, the two forms hydrolyzed AcP due to calcium that was bound to each of the forms, indicating the independence of the two forms in hydrolyzing AcP. Taking into account the monomer-dimer transition of the ATPase molecules on the sarcoplasmic reticulum membrane accompanying E1-E2 transition of the molecules (Dux, L., Taylor, K. A., Ting-Beall, H. P., and Martonosi, A. (1985) J. Biol. Chem. 260, 11730-11743), the two types of molecules seem to independently carry out such monomer-dimer transition of the same type of molecules. Two pairs, each consisting of the same type of molecules, are suggested to be the structural unit of the ATPase molecules.
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Affiliation(s)
- J Nakamura
- Biological Institute, Graduate School of Science, Tohoku University, Aoba-yama, Aoba-ku, Sendai, Miyagi 980-77, Japan.
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