Fig. 1.

Small G proteins as regulators of the steps by which secretory proteins move through the secretory pathway from rough endoplasmic reticulum (RER) to enzyme release by exocytosis. TGN, trans-Golgi network. Small G proteins are listed in blue and shown next to steps where they have been identified. Other proteins identified on zymogen granules (ZGs), which may play a role in secretion, are listed in black. Right: mature ZGs pass through the actin filaments of the terminal web with the aid of RhoA and Rac1, are attached to the membrane by tethering proteins, and then dock and fuse with the apical membrane to release their contents. These terminal steps involve the participation of SNARE proteins. At the time of fusion, the granules also become coated with actin, and at least some of the G proteins are released.