It was shown that chromogranin A structure consists of a N-terminal compact core region and a rather loosely organized C-terminal region and that the change of Ph from 7.5 to 5.5 loosened the overall structure of chromogranin A. Our study by analytical ultracentrifugation indicated dimeric and tetrameric forms of chromogranin A at the near physiological Ph of 7.5 and at the intravesicular Ph of 5.5, respectively. Whether chromogranin A forms dimer or tetrameres appears to be determined by the Ph-dependent conformational state of the protein. The Ph-dependent conformational changes have been shown by circular dichroism spectroscopy and trypsin digestion analysis. The above results, in conjunction with the observation that chromogranin A bound to the vesicle membrane at Ph 5.5 and released from it at Ph 7.5, suggest that chromogranin A may exist in an ordered structure in the intravesicular matrix. Formation of a highly ordered structural arrangement by chromogranin A inside the vesicle might have multiple functional implications. First, the formation of tetrameres from dimers as the Ph vesicle becomes more acidic during vesicle biogenesis may initiate a protein sorting process, segregating the putative intravesicular proteins from other proteins in the trans-Golgi network. Second, the increasing tetramer formation may accompany an increased binding of the tetrameric CGA to the putative vesicle membrane, thereby enabling the vesicle membrane sorting process. Third, the highly ordered structure of chromogranin A in the vesicle may be directly tied to its role as the Ca2+binding protein of the major inositol 1,4,5-trisphosphate-sensitive Ca2+ store of chromaffin cells.
