Plant cells contain calsequestrin

Calsequestrin: a well-known but curious protein in skeletal muscle

Calsequestrin (CASQ) was discovered in rabbit skeletal muscle tissues in 1971 and has been considered simply a passive Ca²⁺-buffering protein in the sarcoplasmic reticulum (SR) that provides Ca²⁺ ions for various Ca²⁺ signals. For the past three decades, physiologists, biochemists, and structural biologists have examined the roles of the skeletal muscle type of CASQ (CASQ1) in skeletal muscle and revealed that CASQ1 has various important functions as (1) a major Ca²⁺-buffering protein to maintain the SR with a suitable amount of Ca²⁺ at each moment, (2) a dynamic Ca²⁺ sensor in the SR that regulates Ca²⁺ release from the SR to the cytosol, (3) a structural regulator for the proper formation of terminal cisternae, (4) a reverse-directional regulator of extracellular Ca²⁺ entries, and (5) a cause of human skeletal muscle diseases. This review is focused on understanding these functions of CASQ1 in the physiological or pathophysiological status of skeletal muscle.

Calsequestrin, a key protein in striated muscle health and disease

Calsequestrin (CASQ) is the most abundant Ca²⁺ binding protein localized in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. The genome of vertebrates contains two genes, CASQ1 and CASQ2. CASQ1 and CASQ2 have a high level of homology, but show specific patterns of expression. Fast-twitch skeletal muscle fibers express only CASQ1, both CASQ1 and CASQ2 are present in slow-twitch skeletal muscle fibers, while CASQ2 is the only protein present in cardiomyocytes. Depending on the intraluminal SR Ca²⁺ levels, CASQ monomers assemble to form large polymers, which increase their Ca²⁺ binding ability. CASQ interacts with triadin and junctin, two additional SR proteins which contribute to localize CASQ to the junctional region of the SR (j-SR) and also modulate CASQ ability to polymerize into large macromolecular complexes. In addition to its ability to bind Ca²⁺ in the SR, CASQ appears also to be able to contribute to regulation of Ca²⁺ homeostasis in muscle cells. Both CASQ1 and CASQ2 are able to either activate and inhibit the ryanodine receptors (RyRs) calcium release channels, likely through their interactions with junctin and triadin. Additional evidence indicates that CASQ1 contributes to regulate the mechanism of store operated calcium entry in skeletal muscle via a direct interaction with the Stromal Interaction Molecule 1 (STIM1). Mutations in CASQ2 and CASQ1 have been identified, respectively, in patients with catecholamine-induced polymorphic ventricular tachycardia and in patients with some forms of myopathy. This review will highlight recent developments in understanding CASQ1 and CASQ2 in health and diseases.

Calsequestrin distribution, structure and function, its role in normal and pathological situations and the effect of thyroid hormones

Calsequestrin is the main calcium binding protein of the sarcoplasmic reticulum, serving as an important regulator of Ca(2+). In mammalian muscles, it exists as a skeletal isoform found in fast- and slow-twitch skeletal muscles and a cardiac isoform expressed in the heart and slow-twitch muscles. Recently, many excellent reviews that summarised in great detail various aspects of the calsequestrin structure, localisation or function both in skeletal and cardiac muscle have appeared. The present review focuses on skeletal muscle: information on cardiac tissue is given, where differences between both tissues are functionally important. The article reviews the known multiple roles of calsequestrin including pathology in order to introduce this topic to the broader scientific community and to stimulate an interest in this protein. Newly we describe our results on the effect of thyroid hormones on skeletal and cardiac calsequestrin expression and discuss them in the context of available literary data on this topic.

Development of a procedure for the sequential extraction of substances binding trace elements in plant biomass

This work investigates how the amounts of some important substances in a plant, and their behaviour inside the plant, depend on the levels of stress placed on the plant. To this end, model plant spinach (Spinacia oleracea L.) was cultivated on soil treated with sewage sludge. The sewage sludge contained various trace elements (As, Cd, Cu, Zn), and the uptake of these trace elements placed the plant under stress. Following this, a sequential extraction procedure was employed to determine the levels and distributions of trace elements within the most important groups of compounds present in the spinach plants. Since the usual five-step sequential extraction procedure provides only general information on the distributions of elements within individual groups of organic compounds, due to the wide range of organic compounds within the individual fractions, this scheme was extended and improved through the addition of two solvent extraction steps-a butanol step (between the ethyl acetate and methanol solvent steps) and an H(2)O step (after the methanol+H(2)O solvent step). The distributions and levels of the trace elements within the main groups of compounds in spinach biomass was investigated using this new seven step sequential extraction (water free solvents: petroleum ether (A) --> ethyl acetate (B) --> butanol (C) --> methanol (D) --> water solvents: methanol+H(2)O (1+1; v/v) (E) --> H(2)O (F) --> methanol+H(2)O+HCl (49.3+49.3+1.4; v/v/v) (G)). The isolated fractions were characterized using IR spectroscopy and the trace element contents were determined in the individual fractions. Lipophilic compounds with low contents of Cd, Cu and Zn were separated in the first two fractions (A, B). Compounds with higher As contents (11.5-12.8% of total content) were also extracted in the second fraction, B. These two fractions formed the smallest portion of the isolated fractions. Low molecular compounds from secondary metabolism and polar lipids were separated in the third (C) and fourth (D) fractions, and high molecular compounds (mainly polypeptides and proteins) separated in the fifth and sixth fractions (E, F). The addition of the H(2)O solvent step was particularly useful for separating compounds that have a significant impact on trace element bounds. The methanol fraction was dominant for all treatments, and a significant decrease in the spinach biomass separated in this fraction was observed when the soil was treated with sewage sludge. Most of the As (35.5-38.8% of total content), Cu (45.0-51.6%) and Zn (39.8-47.2%) was also determined in this fraction. The G fraction (obtained after acid hydrolysis) contained polar compounds. Most of the Cd was also found in this fraction, as was a significant amount of Zn. Non-extractable residues formed the last fraction (polysaccharides, proteins).

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 calcium-binding activity of a vacuole-associated, dehydrin-like protein is regulated by phosphorylation

A vacuole membrane-associated calcium-binding protein with an apparent mass of 45 kD was purified from celery (Apium graveolens). This protein, VCaB45, is enriched in highly vacuolate tissues and is located within the lumen of vacuoles. Antigenically related proteins are present in many dicotyledonous plants. VCaB45 contains significant amino acid identity with the dehydrin family signature motif, is antigenically related to dehydrins, and has a variety of biochemical properties similar to dehydrins. VCaB45 migrates anomalously in sodium dodecyl sulfate-polyacrylamide gel electrophoresis having an apparent molecular mass of 45 kD. The true mass as determined by matrix-assisted laser-desorption ionization time of flight was 16.45 kD. VCaB45 has two characteristic dissociation constants for calcium of 0.22 +/- 0.142 mM and 0.64 +/- 0.08 mM, and has an estimated 24.7 +/- 11.7 calcium-binding sites per protein. The calcium-binding properties of VCaB45 are modulated by phosphorylation; the phosphorylated protein binds up to 100-fold more calcium than the dephosphorylated protein. VCaB45 is an "in vitro" substrate of casein kinase II (a ubiquitous eukaryotic kinase), the phosphorylation resulting in a partial activation of calcium-binding activity. The vacuole localization, calcium binding, and phosphorylation of VCaB45 suggest potential functions.

CtCdc55p and CtHa13p: two putative regulatory proteins from Candida tropicalis with long acidic domains

The salt-tolerance gene HAL3 from Saccharomyces cerevisiae encodes a novel regulatory protein (Hal3p) which modulates the expression of the ENA1 sodium-extrusion ATPase (Ferrando et al., Mol. Cell. Biol. vol. 15, 1995, pp. 5470-5481). Hal3p contains an essential acidic domain rich in aspartates at its carboxyl terminus. We have isolated two cross-hybridizing genes from a genomic library of Candida tropicalis. One of the genes (CtHAL3) is a true homolog of HAL3 and it partially complements the salt sensitivity of a S. cerevisiae hal3 mutant. The activity of CtHAL3 was equivalent to that of an open reading frame (YKL088w) identified by genome sequencing of S. cerevisiae and with homology to HAL3. The other cross-hybridizing gene (CtCDC55) is a CDC55 homolog, encoding a protein with an internal acidic domain not present in the S. cerevisiae CDC55 product. Cdc55p is a regulatory subunit of protein phosphatase 2A and CtCDC55 complements the cold sensitivity of a S. cerevisiae cdc55 mutant. The presence of acidic domains in different putative regulatory proteins may suggest a role for this type of domain in molecular interactions.

Organization of calsequestrin-positive sarcoplasmic reticulum in rat cardiomyocytes in culture

The sarcoplasmic reticulum (SR) regulates the levels of cytoplasmic free Ca2+ ions in muscle cells. Calsequestrin is a major Ca(2+)-storing protein and is localized at special sites in the SR. To investigate the development of calsequestrin-positive SR and its interaction with the cytoskeleton, we examined the distribution of calsequestrin in cultured cardiomyocytes from newborn rats by immunofluorescence with anticalsequestrin and antitubulin antibodies and rhodamine-phalloidin. In frozen sections of neonatal rat heart, anticalsequestrin immunostaining was apparent as cross-striations at Z-lines. When newborn cardiomyocytes were isolated, calsequestrin-positive SR was disorganized and was apparent as small vesicles beneath the sarcolemma, whereas myofibrils accumulated in the center of the cells. As the cells spread in culture, calsequestrin-positive vesicles spread to the periphery of the cytoplasm, becoming associated with the developing myofibrils. In mature cells, calsequestrin was closely associated with myofibrils, showing cross-striations at the Z-lines. Double-labeling using anticalsequestrin and antitubulin antibodies demonstrated that the distribution of calsequestrin-positive structures was similar to that of the microtubular arrays. When the microtubules were depolymerized by nocodazole at an early stage, the extension of the SR to the cell periphery was inhibited. In mature cardiomyocytes, nocodazole appeared not to affect the distribution of the SR. These results indicate that the calsequestrin-positive SR in cardiomyocytes is organized at the proper sites of myofibrils during myofibrillogenesis and that the microtubules might serve as tracts for the transport of components of the SR.

Calcium and signal transduction in plants

Environmental and hormonal signals control diverse physiological processes in plants. The mechanisms by which plant cells perceive and transduce these signals are poorly understood. Understanding biochemical and molecular events involved in signal transduction pathways has become one of the most active areas of plant research. Research during the last 15 years has established that Ca2+ acts as a messenger in transducing external signals. The evidence in support of Ca2+ as a messenger is unequivocal and fulfills all the requirements of a messenger. The role of Ca2+ becomes even more important because it is the only messenger known so far in plants. Since our last review on the Ca2+ messenger system in 1987, there has been tremendous progress in elucidating various aspects of Ca(2+) -signaling pathways in plants. These include demonstration of signal-induced changes in cytosolic Ca2+, calmodulin and calmodulin-like proteins, identification of different Ca2+ channels, characterization of Ca(2+) -dependent protein kinases (CDPKs) both at the biochemical and molecular levels, evidence for the presence of calmodulin-dependent protein kinases, and increased evidence in support of the role of inositol phospholipids in the Ca(2+) -signaling system. Despite the progress in Ca2+ research in plants, it is still in its infancy and much more needs to be done to understand the precise mechanisms by which Ca2+ regulates a wide variety of physiological processes. The purpose of this review is to summarize some of these recent developments in Ca2+ research as it relates to signal transduction in plants.

Differences in the subcellular localization of calreticulin and organellar Ca(2+)-ATPase in neurons

It has become clear that calcium is an important mediator in the transduction of signals due to ligand binding to cell surface receptors. Cytosolic calcium is typically maintained at low levels in both muscle and non-muscle cells and intracellular sequestering of calcium appears to be important in this process. The identification of intracellular calcium pools has been the subject of much recent study, and it has been proposed that such pools would contain three components: a calcium-activated pump or Ca(2+)-ATPase, a calcium channel such as the inositol trisphosphate receptor or ryanodine receptor, and a high-capacity calcium-binding protein such as calsequestrin or calreticulin. We report here on the localization of two components, the organellar Ca(2+)-ATPase (SERCA) and calreticulin, in neuronal tissues. Using immunofluorescence and subcellular fractionation, we have found that for the most part, these two proteins do not co-localize in neuron cell bodies, dendrites, or axons; but may co-localize at the axon terminal.

Widespread tissue distribution of rabbit calreticulin, a non-muscle functional analogue of calsequestrin

Calreticulin was identified in a variety of rabbit tissues by Western blot analysis. Indirect immunofluorescence studies on cultured cells or frozen sections from the corresponding tissues revealed that the protein was distributed to the endoplasmic reticulum or sarcoplasmic reticulum. Calreticulin was found to be an abundant calcium-binding protein in non-muscle and smooth muscle cells and a constituent calcium-binding protein in cardiac and skeletal muscle. From the immunoblot data, calreticulin may exist as an isoform in rabbit neural retina. The present study establishes the ubiquity of calreticulin in intracellular calcium binding.

Isolation of a calreticulin-like calcium binding protein from bovine brain

Intracellular calcium levels are stringently regulated in all cells. The nature of this regulation is incompletely understood, but recent evidence indicates that the endoplasmic reticulum plays an important role in sequestering intracellular calcium. Using methods for isolating both calsequestrin and calreticulin, we have isolated a 58 kDa, high capacity calcium binding protein that exists in microsomes that shift their density in an oxalate-mediated density shift assay. This protein which we call CBP-58 bears similarities to the endoplasmic reticulum protein, calreticulin, in that it has a pI of 4.7 containing approximately 30% glutamate and aspartate, has a high capacity for calcium, and stains blue with the carbocyanine dye, 'Stains-all'. Peptide, amino acid, nucleotide and immunochemical analyses reveal further similarities between CBP-58 and calreticulin, but also some marked differences. Its tissue distribution suggests it is highly enriched in brain versus other tissues. We believe that CBP-58 is a calreticulin-like protein and that differences in the amino acid composition and sequences may reflect species diversity in calreticulin.

Identification and immunolocalization of calreticulin in pancreatic cells: no evidence for "calciosomes"

In the present study, we have shown that calreticulin is a major Ca(2+)-sequestering protein in pancreatic microsomes. This protein is a peripheral membrane protein and could be extracted from the microsomal membrane with carbonate buffer at pH 11.4. Calreticulin was identified in the membrane fractions by immunoblotting with a specific antibody, by a 45Ca2+ overlay technique, and by NH2-terminal amino acid analysis of the purified protein. Immunocytochemical localization of calreticulin in pancreatic acinar cells and pancreatic fibroblasts showed that the protein is localized to the ER membranes in these cells. We were unable to detect calsequestrin or any calsequestrin-like proteins in the pancreas and found no evidence for the existence of large numbers of specialized, calreticulin-containing vesicles which could be an equivalent of the calsequestrin-containing calciosomes previously reported in this tissue. Purified pancreatic calreticulin binds Ca2+ with both a low and a high capacity (approximately 1 mol of Ca2+/mol of protein and approximately 20-23 mol of Ca2+/mol of protein). The concentrations of Ca2+ required for half-maximal saturation of the low and high capacity sites were approximately 4-6 microM and approximately 1.5 mM, respectively. We conclude that calreticulin, which is confined to the lumen of the ER, plays a major role in Ca2+ storage in pancreatic cells.

Frog cardiac calsequestrin. Identification, characterization, and subcellular distribution in two structurally distinct regions of peripheral sarcoplasmic reticulum in frog ventricular myocardium

Calsequestrin is a calcium-binding protein known to sequester calcium accumulated in the sarcoplasmic reticulum (SR) of muscle cells during relaxation. In the present study, we used affinity-purified antibodies to chicken cardiac calsequestrin to identify a 60,000-Da calsequestrin in frog myocardium. Like previously identified cardiac calsequestrins, it is enriched in cardiac microsomes, it is enriched by biochemical procedures previously used to purify cardiac and skeletal calsequestrins, and it exhibits a pH-dependent shift in its apparent Mr on a two-dimensional gel system. Finally, the NH2-terminal amino acid sequence of this 60,000-Da immunoreactive protein purified by fast protein liquid chromatography was identical to that of rabbit skeletal and canine cardiac calsequestrin. Thus, we conclude that this protein corresponds to the calsequestrin isoform in frog ventricular muscle. Frog calsequestrin was localized in discrete foci present at the periphery but absent from the central regions of frog ventricular myocytes as determined by immunofluorescence labeling. Immunoelectron microscopic labeling demonstrated that calsequestrin was confined to the lumen of two structurally distinct regions of the SR, where it was localized in the subsarcolemmal region of the myofibers. One of these appeared to correspond to the terminal SR previously reported to be closely apposed to the sarcolemma of frog myofibers. The other region, although close to the sarcolemma, was not physically joined to it and appeared to correspond to corbular SR. It generally is believed that frog cardiac SR does not provide activator Ca2+ required for excitation-contraction coupling. However, the identification of a calsequestrin isoform very similar to mammalian cardiac calsequestrin that is confined to specialized regions of frog cardiac SR lends support to the idea that frog cardiac SR has the ability to store Ca2+ and thus function in some capacity in frog cardiac muscle contraction.

Identification and developmental expression of a chicken calsequestrin homolog

Calsequestrin (CAL) is a calcium-binding protein whose primary function is thought to involve sequestration of calcium in the muscle sarcoplasmic reticulum (SR). Little is known about the mechanisms regulating CAL expression, or about the role of this protein in muscle development. In addition, CAL may regulate calcium localization in some nonmuscle cells. We have identified an avian calsequestrin homolog. The predicted amino acid sequence of the avian CAL, first described as a laminin binding protein, and named aspartactin, is 70-80% identical to mammalian CAL sequences. We have used affinity-purified antibodies and cDNA probes to investigate expression in developing and adult chicken tissues. In adult chickens, the avian CAL homolog was expressed in slow and fast twitch skeletal muscle as well as in cardiac muscle. Surprisingly high levels of CAL protein were also detected in cerebellum. During development, CAL mRNA and protein were detected in Embryonic Day 5 (E-5) limb primordia, well before the initiation of myoblast fusion. In leg skeletal muscle, CAL protein and mRNA increase approximately 10-fold from E-8 to E-18 with a time course that just precedes myoblast fusion. This early expression pattern was also observed in cultured chicken pectoral myoblasts, and appears to be regulated at the level of mRNA abundance. The developmental profile of CAL expression is compared to that of other muscle proteins and possible additional functions of CAL are discussed.

Characterization of high-capacity low-affinity calcium binding protein of liver endoplasmic reticulum: calsequestrin-like and divergent properties

It had been previously demonstrated that endoplasmic reticulum membranes from rat hepatocytes contain a major calsequestrin-like protein, on account of electrophoretic and Stains All-staining properties (Damiani et al., J. Biol. Chem. 263, 340-343). Here we show that a Ca2+-binding protein sharing characteristics in size and biochemical properties with this protein is likewise present in the isolated endoplasmic reticulum from human liver. Human calsequestrin-like protein was characterized as 62 kDa, highly acidic protein (pl 4.5), using an extraction procedure from whole tissue, followed by DEAE-Cellulose chromatography, that was originally developed for purification of skeletal muscle and cardiac calsequestrin. Liver calsequestrin-like protein bound Ca2+ at low affinity (Kd = 4 mM) and in high amounts (Bmax = 1600 nmol Ca2+/mg of protein), as determined by equilibrium dialysis, but differed strikingly from skeletal muscle calsequestrin for the lack of binding to phenyl-Sepharose resin in the absence of Ca2+, and of changes in intrinsic fluorescence upon binding of Ca2+. Thus, these results suggest that liver 62 kDa protein, in spite of its calsequestrin-like Ca2+-binding properties, does not contain a Ca2+-regulated hydrophobic site, which is a specific structural feature of the calsequestrin-class of Ca2+-binding proteins.