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.

Molecular cloning, functional expression and tissue distribution of the cDNA encoding frog skeletal muscle calsequestrin

We have cloned, sequenced and expressed the cDNA encoding frog skeletal muscle calsequestrin. The processed frog calsequestrin is 398 residues long, with an Mr of 45941 (unglycosylated form), and exhibits 77% sequence similarity with its rabbit counterpart. Consensus sequences for glycosylation and phosphorylation of the protein were conserved. Compared with rabbit calsequestrin, the mature amphibian protein has peculiar structural properties, which include (i) a higher content of negatively charged residues (142 versus 109), and (ii) a striking repeat sequence at the C-terminal region of 44 aspartic acid residues. Furthermore, this is the first report on the expression of calsequestrin cDNA in COS-1 cells; the expressed protein exhibited an Mr and antigenic properties which were indistinguishable from those of the native protein. In addition, it was capable of binding 45Ca in a ligand overlay. Northern blot analysis of frog skeletal muscle, liver, heart and brain RNA showed that the protein is mainly expressed in skeletal muscle. The high density of negative charges at the C-terminus might constitute high-capacity low-affinity Ca(2+)-binding sites, which may account for the higher Ca(2+)-binding capacity of frog calsequestrin compared with other members of the calsequestrin family (56 mol/mol versus 40-44 mol/mol of protein).

Altered properties of calsequestrin and the ryanodine receptor in the cardiac sarcoplasmic reticulum of hibernating mammals

A novel isoform of calsequestrin was identified in sarcoplasmic reticulum vesicles from myocardial tissue of two species of hibernating ground squirrel. The protein was identified as calsequestrin by its cross-reactivity with antibodies raised against bovine cardiac calsequestrin, its pH-sensitive mobility in sodium dodecylsulphate-polyacrylamide gels, staining blue with the cationic carbocyanine dye 'Stains-All', binding peroxidase-conjugated concanavalin A, its endoglycosidase F sensitivity. Its NH2-terminal amino acid sequence is similar, but not identical, to that already determined for cardiac calsequestrin. Some of the biochemical properties of this protein distinguish it from the other mammalian isoforms. It has a unique electrophoretic mobility in both alkaline and neutral sodium dodecylsulphate-polyacrylamide gel electrophoresis, it appears to have a molecular weight approximately 7% greater than that of cardiac calsequestrin from other mammalian species, and its glycosylation pattern differs. This novel form of calsequestrin is expressed in cardiac SR vesicles which possess an abnormally high number of Ca2(+)-release channel/ryanodine receptor molecules. This ryanodine receptor also shows an altered Ca2(+)-sensitivity of ryanodine binding. The divergent biophysical properties of this novel form of cardiac calsequestrin, together with the apparently atypical ryanodine receptors in the cardiac sarcoplasmic reticulum membranes may have some functional significance in the adaptive mechanisms which allow the heart to function despite the severely reduced body temperatures (to approx. 0 degree C) encountered during hibernation.

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.

Purification and characterization of a calsequestrin-like calcium-binding protein from carp (Cyprinus carpio) sarcoplasmic reticulum

1. A calsequestrin-like calcium-binding protein was purified from carp sarcoplasmic reticulum by column chromatographies using DEAE-cellulose and Butyl-Toyopearl 650S. 2. The mol. wt was estimated to be 50 kDa, which was larger than that of rabbit calsequestrin (42 kDa). 3. Carp calsequestrin-like protein bound Ca2+ with a higher affinity (apparent Kd = 400 microM) and lower capacity (25 mol/mol) compared with rabbit calsequestrin (1 mM and 40-50 mol/mol, respectively). 4. Anti-carp calsequestrin-like protein rabbit antiserum reacted with rabbit calsequestrin in immunoblotting analysis. 5. Carp calsequestrin-like protein was rich in acidic amino acids, as was rabbit calsequestrin.

Coexpression of two isoforms of calsequestrin in rabbit slow-twitch muscle

The cardiac and fast-twitch skeletal muscle forms of the Ca2(+)-binding protein calsequestrin (CS) are the products of two different genes, both of which are transcribed in slow-twitch skeletal muscle, though at much different rates (Scott et al., 1988., Fliegel et al., 1989). We have investigated this problem more closely at the protein level, on isolated terminal cisternae (TC) of the sarcoplasmic reticulum (SR) of rabbit slow-twitch muscle, and following purification of two distinct forms of CS from whole tissue by DEAE-Cellulose chromatography and CA2(+)-dependent elution from phenyl-Sepharose. Two electrophoretically (apparent molecular mass of 64 kDa and 54 kDa, respectively), and antigenically distinct forms of CS, here shown to be related to the fast-twitch skeletal muscle and to cardiac-type isoform of CS, respectively, colocalize to junctional TC of slow-twitch muscle. The cardiac-type isoform that is expressed in slow-twitch muscle accounts for about 25% of total CS present in isolated TC, it binds Ca2+ as effectively as the major CS form, using a 45Ca-overlay technique, and it shares extensive similarities with dog cardiac CS, not only in size and antigenically, but also in pl, as well as in the DEAE-elution characteristics. No difference in behaviour with phenyl-Sepharose resin were observed between the two CS isoforms from slow-twitch muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Calsequestrin, a component of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store of chicken cerebellum

The presence and distribution of calsequestrin (CS), Ca2+ pump, and inositol 1,4,5-trisphosphate (IP3) receptor were investigated biochemically and immunologically in microsomal (P3) fractions isolated from chicken cerebrum and cerebellum. Two different batches of polyclonal antibodies specific for chicken skeletal muscle CS identified a Ca2+ binding, CS-like protein that was extremely enriched in cerebellum P3 fractions and absent from all cerebrum fractions. The cerebellum CS-like protein was deemed authentic CS because the N-terminal amino acid domain and peptide mapping were identical to those of skeletal muscle CS in the same species. CS was detected in striated muscles and cerebellum only. Cerebellum P3 fractions were also found to be considerably enriched in Ca2+ pump and IP3 receptor compared with the homologous cerebrum fractions, as judged by measurements of Ca2+ uptake, Ca2(+)-ATPase activity, IP3-induced Ca2+ release, and [3H]IP3 binding, respectively. Cerebellum microsomal fractions therefore appear to contain membrane fragments endowed with Ca2+ pump, IP3 receptor, and CS, i.e., three key components of a Ca2+ storage organelle.

Intralumenal sarcoplasmic reticulum Ca(2+)-binding proteins

The sarcoplasmic reticulum (SR) controls the level of intracellular Ca2+ in cardiac and skeletal muscle by storing and releasing Ca2+. A set of intralumenal SR Ca(2+)-binding proteins has been identified that may serve important roles in SR Ca2+ storage and mobilization. The most prominent of these SR proteins, calsequestrin, is discretely localized to junctional SR. Other intralumenal proteins are more widely distributed throughout the SR. All of these intralumenal SR Ca(2+)-binding proteins are acidic, stain blue with dye Stains-All, and appear to be substrates for casein kinase II. The biochemistry and cell biology of lumenal SR proteins may conform to a paradigm now emerging from the study of endoplasmic reticulum proteins.