Calcium-binding proteins in the parathyroid gland

Mitochondrial ATP synthase F1-beta-subunit is a calcium-binding protein

Mitochondrial ATP synthase is responsive to changes in cytosolic calcium concentration, but the regulatory mechanisms are unclear. Here we identified a major 52 kDa calcium-binding protein in rat enamel cells as the mitochondrial ATP synthase F1-beta-subunit. The F1-beta-subunit behaved as a low affinity and moderate capacity calcium-binding protein during comparative 45Ca overlay analyses. Equivalent behavior was shown by the F1-beta-subunit from rat liver mitochondria, but not by the homologous F1-alpha-subunit, supporting the specificity of calcium binding. Evidence that the catalytic F1-beta-subunit binds calcium specifically introduces new mechanistic possibilities for regulating ATP synthase, and thereby coordinating ATP production with demand for ATP-fuelled calcium pump activity.

Abundant calcium homeostasis machinery in rat dental enamel cells. Up-regulation of calcium store proteins during enamel mineralization implicates the endoplasmic reticulum in calcium transcytosis

Enamel cells handle large amounts of calcium, particularly during the developmental phase (termed maturation) when dental enamel is hypermineralized. The extent of intracellular calcium burden, and the nature of calcium homeostasis machinery used to accommodate it, are largely unknown. Here, the calcium-binding capacity of enamel cell cytosol was found to increase during development, in parallel with the putative transcellular flux of calcium. At maturation, the abundance of calcium-binding proteins in enamel cells exceeded that in brain and other established calcium-oriented tissues, which implies a large calcium burden. A search for likely cytosolic calcium transporters revealed only one high-affinity calcium-binding protein (12 kDa, distinguished from alpha-parvalbumin) that was up-regulated during maturation, but its low abundance (0.02% of soluble protein) precluded a major calcium transport or cytoprotective role. Two low-affinity calcium-binding proteins up-regulated during maturation (by 1.8-fold and 2.1-fold respectively) were identified as calreticulin and endoplasmin, both residents of the endoplasmic reticulum. Together, calreticulin and endoplasmin constituted an exceptionally high proportion (5%) of soluble protein during maturation, which gives an inferred calcium capacity 67-fold higher than that of the principal cytosolic calcium-binding protein. 28-kDa calbindin. Evidence that endoplasmin expression varied inversely with serum calcium concentration, and that the inositol trisphosphate receptor also was highly expressed during maturation, supported the novel hypothesis that non-mitochondrial calcium stores play a major role in transcellular calcium transport.

IN CONCLUSION

(a) enamel cells contain a general high abundance of calcium homeostasis proteins, consistent with a heavy intracellular calcium burden; (b) the expression pattern (phenotype) of calcium-binding proteins varies with enamel cell function; (c) enamel cells appear to contain unusually large non-mitochondrial calcium stores; (d) contrary to the prevailing view that calcium passes mainly through the cytosol of calcium-transporting cells, the findings imply a route through the endoplasmic reticulum. This study gives novel information about how a highly calcium-oriented tissue avoids calcium toxicity, and provides a new focus for investigations into the mechanisms of transcellular calcium transport.

Calcium-induced aggregation of neuroendocrine protein 7B2 in vitro and its modulation by ATP

To study the behavior of the neuroendocrine polypeptide 7B2 in the presence of calcium, various fragments of this molecule were produced in Escherichia coli as fusion proteins to glutathione S-transferase (GST). Addition of millimolar concentrations of Ca2+ to purified preparations of hybrid molecules carrying the N-terminal segment of 7B2 induced precipitation in a manner dependent on protein and cation concentrations. This precipitation occurred at pH 7.5 but not at pH 5.2. It was augmented by 4 and 8 mM ATP, and reduced by 12 and 24 mM ATP. ADP had a similar but weaker effect. Calcium failed to cause precipitation of GST alone or of GST fused to the C-terminal peptide 7B2(156-186). However, when the latter protein was mixed with a GST protein carrying a short fragment of the N-terminal region of 7B2, both proteins were precipitated by calcium. Except for the pH dependence, the behavior of 7B2 fusion proteins in the presence of calcium and adenosine nucleotides are reminiscent of those exhibited by chromogranins and secretogranins, which, like 7B2, are acidic proteins found in the secretory granules of a variety of neuroendocrine cells. As suggested for other granins, this property may underlie the segregation of 7B2 fragments into secretory granules.

Effects of calcium on recombinant bovine chromogranin A

Bovine chromogranin A, the acidic calcium-binding protein characteristic of endocrine secretory vesicles, has been expressed in Escherichia coli using the pET3a vector system under T7 polymerase control. The expressed protein is located in the bacterial cytosol and can be purified from bacterial proteins by a heat treatment step, followed by gel filtration, anion-exchange, and reversed-phase chromatography. The purified recombinant chromogranin A has an apparent M(r) of ca. 72,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in spite of its 432-amino acid polypeptide chain, consistent with observations on natural chromogranin A. The primary structure has been confirmed by mass spectral analysis of tryptic peptides, by Edman degradation of the intact protein, and by immunoreactivity with sequence-specific antibodies. Analysis by circular dichroism spectroscopy shows pH- and concentration-dependent spectra. The spectra are Ca2(+)-dependent from 5 to 40 microM.

Binding and aggregation of pro-atrial natriuretic factor by calcium

Analysis of atrial secretory granule content by sodium dodecyl sulfate-gel electrophoresis followed by a 45Ca2+ overlay assay indicates that a 17,000 protein binds 45Ca2+. This protein, which can be immunostained by atrial natriuretic factor (ANF) antiserum, corresponds to proANF. Ca2+ binding is proportional to the amount of proANF and pH dependent. Generation of ANF-(1-98) by thrombin digestion of proANF does not affect Ca2+ binding. Blocking the carboxyl groups of proANF and the use of NH2-terminal fragments bearing those carboxyl groups demonstrated that the Ca(2+)-interaction site is probably located within the highly acidic portion (11-30) of the propeptide. Ca2+ binding to proANF induces its aggregation that can be verified by sedimentation. ProANF aggregation is Ca2+ dependent, being optimal at 10 mM, partially pH dependent, and greatly increased by high concentrations of proANF. However, because of its relatively low-binding affinity, Ca2+ can be substituted by other divalent cations such as Sr2+, Ba2+, or Mg2+. The high level of Ca2+ in atrial secretory granules and the aggregation of proANF in the presence of Ca2+ suggest a possible involvement of these physicochemical properties in the condensed state of the matrix of secretory granules. Indeed, detergent solubilization of the membrane of the secretory granules in presence of Ca2+ resulted only in a partial dissolution of the dense core matrix. We therefore postulate that, in the Golgi complex, proANF and Ca2+ associate to form a condensed aggregate that helps package secretory material into secretory vesicles.

Microprobe analysis of maturation-related elemental changes in rat parotid secretory granules

Electron probe X-ray microanalysis was use to quantitate the elemental and mass changes that take place during the secretory granule maturation process. A single injection of isoproterenol stimulated the depletion of secretory granules from rat parotid acinar cells. Granules at different stages of maturation were analyzed as they reaccumulated within the cells over time. Dry mass measurements revealed that secretory material becomes concentrated about twofold within maturing granules. Nearly all of the increase in mass concentration could be attributed to a reduction in water space. Data are presented that indicate that Na, K, Cl, and water all efflux from secretory granules during maturation. In contrast, granule S content is positively correlated with maturation. Hence, significant changes in granule elemental and water contents occur during the maturation process.

A chromogranin A-derived peptide differentially regulates the secretion of calcitonin gene products

We studied the regulation of the secretion of CT and CGRP by chromogranin A (CgA)-derived peptides in a human lung tumor cell line. The amino-terminal peptide of CgA, CgA1-40, stimulated the secretion of CGRP and inhibited the secretion of CT; both effects occurred in a dose-dependent manner. These studies demonstrate a differential effect of a CgA-derived peptide on two products of the CT gene, CT itself and CGRP. CgA may be processed at its multiple dibasic sites to peptides that specifically regulate the secretion of its coresident hormones, in this case two calcitonin gene products that are present in the same secretory vesicle. This novel mechanism represents a new pathway for the control of calcium regulating hormones.

Distinct patterns of chromogranin A-related species can be demonstrated in endocrine cells

We have studied the pattern of chromogranin A (CgA)-related species in different human endocrine cells that produce CgA and also express the calcitonin gene. Antibodies against CgA peptides that span its linear sequence were used in Western analysis of cell lines derived from medullary thyroid carcinoma (MTC), small cell lung cancers (SCLC), epidermoid cell lung cancer (ECLC) and a pulmonary carcinoid tumor (CRND). Each of the cell lines demonstrated a distinct pattern of CgA-related species. Gel filtration studies also revealed multiple and different forms of immunoreactive CgA in the cell lines. Although proteolysis may contribute to our results, these observations suggest that native CgA is processed to smaller species in a tissue-specific pattern by different endocrine cells. More conclusive studies, however, are necessary to establish that cell processing leads to the specific CgA moieties that we have observed.

Interaction of calcium with porcine adrenal chromogranin A (secretory protein-I) and chromogranin B (secretogranin I)

Secretory granules of endocrine cells contain one or more of the acidic secretory proteins chromogranin A (secretory protein-I), chromogranin B (secretogranin I), and secretogranin II (chromogranin C). It has been proposed that these proteins play a role in the packaging of secretory products. In the present study, lysates of purified porcine adrenal chromaffin granules containing chromogranins A and B and a putative chromogranin B fragment bound calcium and formed aggregates in the presence of 10-20 mM calcium at pH 5-6 and at 100 mM or less KCl, NaCl, or norepinephrine. The precipitates contained virtually all of the chromogranin B and the chromogranin B fragment and about one-third of the chromogranin A. The aggregates did not form or were dissociated at the pH and salt concentration of the extracellular fluid. Calcium precipitated purified chromogranin A and chromogranin B from pure solution to the same extent as from the granule lysates. Parathormone, added to the lysates, was incorporated in the precipitates, whereas the acidic secretory protein ovalbumin and norepinephrine were not. These findings suggest that secretory protein-I and secretogranin can exist in situ as aggregates that may include selected secretory products.