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

Identification of ferredoxin II as a major calcium binding protein in the nitrogen-fixing symbiotic bacterium Mesorhizobium loti

BACKGROUND

Legumes establish with rhizobial bacteria a nitrogen-fixing symbiosis which is of the utmost importance for both plant nutrition and a sustainable agriculture. Calcium is known to act as a key intracellular messenger in the perception of symbiotic signals by both the host plant and the microbial partner. Regulation of intracellular free Ca(2+) concentration, which is a fundamental prerequisite for any Ca(2+)-based signalling system, is accomplished by complex mechanisms including Ca(2+) binding proteins acting as Ca(2+) buffers. In this work we investigated the occurrence of Ca(2+) binding proteins in Mesorhizobium loti, the specific symbiotic partner of the model legume Lotus japonicus.

RESULTS

A soluble, low molecular weight protein was found to share several biochemical features with the eukaryotic Ca(2+)-binding proteins calsequestrin and calreticulin, such as Stains-all blue staining on SDS-PAGE, an acidic isoelectric point and a Ca(2+)-dependent shift of electrophoretic mobility. The protein was purified to homogeneity by an ammonium sulfate precipitation procedure followed by anion-exchange chromatography on DEAE-Cellulose and electroendosmotic preparative electrophoresis. The Ca(2+) binding ability of the M. loti protein was demonstrated by (45)Ca(2+)-overlay assays. ESI-Q-TOF MS/MS analyses of the peptides generated after digestion with either trypsin or endoproteinase AspN identified the rhizobial protein as ferredoxin II and confirmed the presence of Ca(2+) adducts.

CONCLUSIONS

The present data indicate that ferredoxin II is a major Ca(2+) binding protein in M. loti that may participate in Ca(2+) homeostasis and suggest an evolutionarily ancient origin for protein-based Ca(2+) regulatory systems.

Delayed activation of the store-operated calcium current induced by calreticulin overexpression in RBL-1 cells

Calreticulin (CRT) is a high-capacity, low-affinity Ca2+-binding protein located in the lumen of the endoplasmic reticulum (ER) of all eukaryotic cells investigated so far. Its high level of conservation among different species suggests that it serves functions fundamental to cell survival. The role originally proposed for CRT, i.e., the main Ca2+ buffer of the ER, has been obscured or even casted by its implication in processes as diverse as gene expression, protein folding, and cell adhesion. In this work we seek the role of CRT in Ca2+ storing and signaling by evaluating its effects on the kinetics and amplitude of the store-operated Ca2+ current (ICRAC). We show that, in the rat basophilic leukemia cell line RBL-1, overexpression of CRT, but not of its mutant lacking the high-capacity Ca2+-binding domain, markedly retards the ICRAC development, however, only when store depletion is slower than the rate of current activation. On the contrary, when store depletion is rapid and complete, overexpression of CRT has no effect. The present results are compatible with a major Ca2+-buffering role of CRT within the ER but exclude a direct, or indirect, role of this protein on the mechanism of ICRAC activation.

Functional conservation of calreticulin in Euglena gracilis

Calreticulin is the major high capacity, low affinity Ca2+ binding protein localized within the endoplasmic reticulum. It functions as a reservoir for triggered release of Ca2+ by the endoplasmic reticulum and is thus integral to eukaryotic signal transduction pathways involving Ca2+ as a second messenger. The early branching photosynthetic protist Euglena gracilis is shown to possess calreticulin as its major high capacity Ca2+ binding protein. The protein was purified, microsequenced and cloned. Like its homologues from higher eukaryotes, calreticulin from Euglena possesses a short signal peptide for endoplasmic reticulum import and the C-terminal retention signal KDEL, indicating that these components of the eukaryotic protein routing apparatus were functional in their present form prior to divergence of the euglenozoan lineage. A gene phylogeny for calreticulin and calnexin sequences in the context of eukaryotic homologues indicates i) that these Ca2+ binding endoplasmic reticulum proteins descend from a gene duplication that occurred in the earliest stages of eukaryotic evolution and furthermore ii) that Euglenozoa express the calreticulin protein of the kinetoplastid (trypanosomes and their relatives) lineage, rather than that of the eukaryotic chlorophyte which gave rise to Euglena's plastids. Evidence for conservation of endoplasmic reticulum routing and Ca2+ binding function of calreticulin from Euglena traces the functional history of Ca2+ second messenger signal transduction pathways deep into eukaryotic evolution.

Overexpression of calreticulin increases the Ca2+ capacity of rapidly exchanging Ca2+ stores and reveals aspects of their lumenal microenvironment and function

A molecularly tagged form of calreticulin (CR), a low affinity-high capacity Ca2+ binding protein that resides in the ER lumen, was transiently transfected into HeLa cells to specifically modify the Ca2+ buffering capacity of the intracellular Ca2+ stores. Fluorescence and confocal microscope immunocytochemistry revealed the tagged protein to be expressed by over 40% of the cells and to overlap in its distribution the endogenous CR yielding a delicate cytoplasmic network, i.e., the typical pattern of ER. In contrast, no signal was observed associated with the plasmalemma (marked by ConA) and within the nucleus. One- and two-dimensional Western blots revealed the transfected to exceed the endogenous CR of approximately 3.5-fold and to maintain its Ca2+ binding ability, whereas the expression of other ER proteins was unchanged. Ca2+ homeostasis in the transfected cells was investigated by three parallel approaches: (a) 45Ca equilibrium loading of cell populations; (b) [Ca2+]c measurement with fura-2 followed by quantitative immunocytochemistry of single cells and iii) [Ca2+]c measurement of cell population upon cotransfection with the Ca(2+)-sensitive photoprotein, aequorin. The three approaches revealed different aspects of Ca2+ homeostasis, yielding results which were largely complementary. In particular, the following conclusions were established: (a) both endogenous and transfected CR participate in Ca2+ buffering within the IP3-sensitive, rapidly exchanging, Ca2+ stores; the other pools of the cells were in contrast unaffected by CR transfection; (b) the Ca2+ capacity of the stores is not the main limiting factor of individual IP3-mediated Ca2+ release responses triggered by receptor agonists; (c) in control cells, the contribution of CR to Ca2+ buffering within the IP3-sensitive stores accounts for approximately 45% of the total, the rest being probably contributed by the other lumenal (and also membrane) Ca2+ binding proteins; (d) the free [Ca2+] within the lumen of the IP3-sensitive stores, revealed by the degree of Ca2+ binding to the transfected CR protein, amounts to values in (or approaching) the millimolar range; and (e) Ca2+ influx across the plasmalemma activated by depletion of the stores is directly dependent on the lumenal [Ca2+].

Characterization of a calsequestrin-like protein from sea-urchin eggs

Following our studies on the identification of a calsequestrin-like protein (CSLP) from sea-urchin eggs [Oberdorf, Lebeche, Head & Kaminer (1988) J. Biol Chem. 263, 6806-6809], we have characterized its Ca(2+)-binding properties and identified it as a glycoprotein. The molecule binds 23 mol of Ca2+/mol of protein, as determined by equilibrium dialysis. This is in the range reported for cardiac calsequestrin but is about half the binding capacity of striated muscle calsequestrin. The affinities of the CSLP for Ca2+ are decreased by increasing KCl concentrations (20-250 mM) and the presence of Mg2+ (3 mM) in the medium: the half-maximal binding values varied from 1.62 to 5.77 mM. Hill coefficients indicated mild co-operativity in the Ca2+ binding. Ca2+ (1-8 mM)-induced u.v. difference spectra and intrinsic fluorescence changes suggest a net exposure of aromatic residues to an aqueous environment. C.d. measurements showed minor Ca(2+)-induced changes in alpha-helical and beta-sheet content of less than 10%. These spectral changes are distinctly different from those found in muscle calsequestrin. Immunoblotting studies showed that the CSLP is distinct from calreticulin, a low-affinity Ca(2+)-binding protein.

Calreticulin

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Purification and characterization of calsequestrin from chicken cerebellum

Chicken cerebellum microsomal fractions contain a protein tentatively identified as calsequestrin (CS) (Volpe et al., Neuron 5, 713-721, 1990). Here we report, for the first time, the purification of cerebellum CS from whole tissue homogenate by DEAE-Cellulose chromatography and Ca(2+)-dependent elution from phenyl-Sepharose. The purified cerebellum CS displays the shift and increase in intrinsic fluorescence characteristic of skeletal muscle CS, and is shown to be a high-capacity, low-affinity Ca2+ binding protein (Kd = 1 mM).

The identity of the calcium-storing, inositol 1,4,5-trisphosphate-sensitive organelle in non-muscle cells: calciosome, endoplasmic reticulum ... or both?

Although the initial phase of receptor-mediated Ca2+ signaling, involving Ca2+ release from intracellular stores by inositol 1,4,5-trisphosphate, is relatively well characterized, the nature of the organelle releasing Ca2+ is a controversial subject. At issue is the question of whether Ca2+ is released from the endoplasmic reticulum, or from a more specialized organelle called the 'calciosome'. In this review, we attempt to analyse the arguments for and against these two views, and attempt to reconcile some of the apparently conflicting findings by proposing a hypothetical model of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool.

Frog brain expresses a 60 KDa Ca2+ binding protein similar to mammalian calreticulin

The present report was undertaken in an effort to characterize the nature of Ca2+ binding protein(s) in the central nervous system of less evolved vertebrates. In particular we investigated whether the brain microsomal fraction of Rana esculenta expresses calsequestrin, calreticulin and/or other related Ca2+ binding protein(s). We found that a 60 KDa protein having an NH2-terminal amino acid sequence similar to mammalian calreticulin is the major microsomal Ca2(+)-binding protein.

Calreticulin is a candidate for a calsequestrin-like function in Ca2(+)-storage compartments (calciosomes) of liver and brain

In a search for the non-muscle equivalent of calsequestrin (the low-affinity high-capacity Ca2(+)-binding protein responsible for Ca2+ storage within the terminal cisternae of the sarcoplasmic reticulum), acidic proteins were extracted from rat liver and brain microsomal preparations and purified by column chromatography. No calsequestrin was observed in these extracts, but the N-terminal amino acid sequence of the major Ca2(+)-binding protein of the liver microsomal fraction was determined and found to correspond to that of calreticulin. This protein was found to bind approx. 50 mol of Ca2+/mol of protein, with low affinity (average Kd approx. 1.0 mM). A monoclonal antibody, C6, raised against skeletal-muscle calsequestrin cross-reacted with calreticulin in SDS/PAGE immunoblots, but polyclonal antibodies reacted with native calreticulin only weakly, or not at all, after SDS denaturation. Immuno-gold decoration of liver ultrathin cryosections with affinity-purified antibodies against liver calreticulin revealed luminal labelling of vacuolar profiles indistinguishable from calciosomes, the subcellular structures previously identified by the use of anti-calsequestrin antibodies. We conclude that calreticulin is the Ca2(+)-binding protein segregated within the calciosome lumen, previously described as being calsequestrin-like. Because of its properties and intraluminal location, calreticulin might play a critical role in Ca2+ storage and release in non-muscle cells, similar to that played by calsequestrin in the muscle sarcoplasmic reticulum.