NATURE OF THE PH-INDUCED CONFORMATIONAL-CHANGES AND EXPOSURE OF THE C-TERMINAL REGION OF CHROMOGRANIN-A

Chromogranin A is known to undergo pH induced conformational changes, and the difference in conformation is supposed to be responsible for t he difference in Ca2+ binding property. To gain insight regarding the overall structure and the nature of pH-induced conformational changes of chromogranin A, limited trypsin digestions were carried out at pH 5 .5 and pH 7.5. The resulting fragments were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the amino acid sequen ces of the tryptic fragments were determined. From these analyses it w as shown that the chromogranin A structure consists of an N-terminal c ompact core region and a rather loosely organized C-terminal region an d that the change of pH from 7.5 to 5.5 loosened the overall structure of chromogranin A, exposing the C-terminal region. Since the conserve d C-terminal region (residues 407-431) was shown to exist in monomer-d imer and monomer-tetramer equilibria at pH 7.5 and 5.5, respectively, the conformational changes of the region at pH 7.5 and 5.5 were studie d by circular dichroism spectroscopy using a synthetic peptide represe nting the conserved C-terminal region. When the pH was changed from 7. 5 to 5.5, the coil structure of the C-terminal peptide decreased with an accompanying increase of alpha-helicity.