title: Ca2(+)-induced conformational change and aggregation of chromogranin A.
abstract: Chromogranin A, the most abundant protein in bovine adrenal chromaffin granules, bound calmodulin in a Ca2(+)-dependent manner, and the calmodulin-binding property was utilized to purify chromogranin A. Chromogranin A has been described in the past as a "random-coil polypeptide" with little alpha-helix or beta-sheet conformation. However, circular dichroism measurements with pure, native chromogranin A revealed relatively high alpha-helical contents (40% at the intravesicular pH of 5.5). Fluorescence studies confirmed previous observations that chromogranin A binds Ca2+ with low affinity. Considering the high concentration of Ca2+ in the secretory vesicle, the effect of Ca2+ on the secondary structure and self-association of chromogranin A was examined. Ca2+ induced a decrease of alpha-helicity of chromogranin A from 40 to 30% at pH 5.5. In contrast, at pH 7.5 the same amount of Ca2+ increased alpha-helicity of the protein from 25 to 40%. Boiling of the adrenal extract, a commonly used purification procedure for chromogranin A, resulted in the isolation of conformationally distinct chromogranin A molecule. Unlike secretory protein-I of the parathyroid gland (Gorr, S.-V., Dean, W. L., Radley, T. L., and Cohn, D. V. (1988) Bone Mineral 4, 17-25), chromogranin A aggregated rapidly in the presence of Ca2+. The extent and rate of aggregation were highly dependent on Ca2+ concentration. Although both the rate and extent of aggregation at pH 7.5 were much lower than those at pH 5.5, aggregation of chromogranin A proceeded at both pH's. In this respect, chromogranin A differs from human chromogranin C which was shown by Gerdes et al. (Gerdes, H.-H., Rosa, P., Phillips, E., Baeuerle, P. A., Frank, R., Argos, P., and Huttner, W. B. (1989) J. Biol. Chem. 264, 12009-12015) to aggregate at pH 5.2 but not at pH 7.4.
authors.affiliation: Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75235.
authors.forename: S H
authors.initials: SH
authors.lastname: Yoo
authors.forename: J P
authors.initials: JP
authors.lastname: Albanesi
mesh_terms: D000313:Adrenal Medulla / Q000378:metabolism; D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002846:Chromatography, Affinity; D002852:Chromatography, Ion Exchange; D053379:Chromogranin A; D002864:Chromogranins / Q000302:isolation & purification / Q000378:metabolism*; D002942:Circular Dichroism; D015180:Electrophoresis, Gel, Two-Dimensional; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013997:Time Factors
chemical_list: C506150:CHGA protein, human; D053379:Chromogranin A; D002864:Chromogranins; D046911:Macromolecular Substances; D009419:Nerve Tissue Proteins; D002118:Calcium