Subcellular distribution of chromogranins A and B in bovine adrenal chromaffin cells

The major secretory granule proteins chromogranins A (CGA) and B (CGB) have recently been shown to play critical roles in inositol 1,4,5-trisphosphate-dependent intracellular Ca2+ mobilizations. We determined here the subcellular distribution of CGA and CGB based on 3D-images of chromaffin cells, and found that ∼95% of cellular CGA was present in secretory granules while ∼5% was in the endoplasmic reticulum (ER), whereas ∼57% of cellular CGB was in secretory granules while ∼24% and ∼19% were in the ER and nucleus, respectively. These results suggest that chromogranins are at the center of intracellular Ca2+ homeostasis in secretory cells.