WIDESPREAD ASSOCIATION OF ANNEXIN-II WITH INTRACELLULAR MEMBRANE SYSTEMS AND INVOLVEMENT OF ANNEXIN-II IN GRANULE-GRANULE FUSION IN ADDITION TO GRANULE PLASMA-MEMBRANE FUSION IN ANTERIOR-PITUITARY-CELLS

The subcellular localization in anterior pituitary secretory cells of annexin II, one of the Ca2+-dependent phospholipid-binding proteins, w as examined by immunohistochemistry and immunoelectron microscopy. Ann exin II was associated with the plasma membrane, the membranes of secr etory granules and cytoplasmic organelles, such as rough endoplasmic r eticulum, mitochondria and vesicles, and with the nuclear envelope. An nexin II was frequently detected at the contact sites of secretory gra nules with other granules and with the plasma membrane. The anterior p ituitary and adrenal medulla were treated with Clostridium perfringens enterotoxin, which induces Ca2+ influx, and examined under an electro n microscope. The anterior pituitary cells showed multigranular exocyt osis, i.e. multiple fusions of secretory granules with each other and with the plasma membrane, but adrenal chromaffin cells, which lack ann exin II on the granule membranes, never showed granule-granule fusion and only single granule exocytosis. From these results, we conclude th at, in anterior pituitary secretory cells, annexin II is involved in g ranule-granule fusion in addition to granule-plasma membrane fusion. ( C) 1998 Chapman & Hall.