pH-dependent association of chromogranin A with secretory vesicle membrane and a putative membrane binding region of chromogranin A.

Chromogranin A is a low-affinity, high-capacity Ca2+ binding protein, postulated to be responsible for the Ca2+ buffering role of secretory vesicles, and has been found only in the soluble portions of the vesicular proteins. Contrary to the generally accepted notion of chromogranin A existing as a soluble matrix protein, chromogranin A bound to the secretory vesicle membrane at the intravesicular pH of 5.5 and freed from the membrane when the pH was raised to a more physiological pH of 7.5. Trypsin digestion studies of the vesicle membrane suggested that chromogranin A interacts with the protein component(s) on the intravesicular side of the membrane. Furthermore, in a study using 14 synthetic chromogranin A peptides which represent various portions of chromogranin A, a segment in the N-terminal region (residues 18-37) was shown to bind to the vesicle membrane in a pH-dependent manner. The pH-dependent vesicle membrane binding property of chromogranin A appears to be of fundamental physiological importance with regard to the potential roles of chromogranin A in secretory vesicle biogenesis, particularly in segregating secretory vesicle membranes from others in the trans-Golgi network, and also in transmitting extravesicular signals such as inositol 1,4,5-trisphosphate or inositol 1,3,4,5-tetrakisphosphate for Ca2+ release or uptake to the inside of vesicles.