Chromogranins A and B have been shown to aggregate in a low pH and high calcium environment, the condition found in the trans-Golgi network where secretory vesicles are formed. Moreover, chromogranins A and B, as well as several other secretory vesicle matrix proteins, have recently been shown to bind to the vesicle membrane at the intravesicular pH of 5.5 and to be released from it at a near physiological pH of 7.5. The pH- and Ca2+-dependent aggregation and interaction of chromogranins with the vesicle membrane are considered essential in vesicle biogenesis. Therefore, to gain further insight into how vesicle matrix proteins find their way into the secretory vesicles, the pH- and Ca2+-dependent aggregation and vesicle membrane binding properties of the vesicle matrix proteins were studied. It was found that most of the vesicle matrix proteins aggregated in the presence of Ca2+ at the intravesicular pH of 5.5. Furthermore, most of the vesicle matrix proteins bound not only to the vesicle membrane but also to chromogranin A (CGA) at pH 5.5. Purified chromogranin B (CGB) was also shown to interact with CGA at pH 5.5. Hence, the abundance of chromogranins A and B in secretory vesicles and their low pH- and high calcium-dependent aggregation property, combined with their ability to interact with both the vesicle matrix proteins and the vesicle membrane, chromogranins A and B are proposed to play essential roles in the selective aggregation and sorting of potential vesicle matrix proteins to the immature secretory vesicles of the regulated secretory pathway. Moreover, two molecules of CGA bound to two molecules of CGB at pH 5.5 and one molecule of CGA bound to one molecule of CGB at pH 7.5. In view of the interaction of tetrameric CGA with tetrameric inositol 1,4,5- trisphosphate (IP3) receptor and the existence of heterotetrameric IP3 receptor in the cell, the heterotetramer formation by CGA and CGB not only raises the possibility of interaction between the heterotetrameric chromogranin and heterotetrameric IP3 receptor but also appears to reflect their important roles in the cell.
