Ca2(+)-induced conformational change and aggregation of chromogranin A.

Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75235.

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.

PMID: 2387861 [PubMed - indexed for MEDLINE]