Fig. 2.

The secretory pathway and vesicular trafficking. Protein enters the secretory pathway at the endoplasmic reticulum (ER) and is trafficked in COPII‐coated vesicular structures to the intermediate compartment (ERGIC/VTC), from which COPI‐coated vesicles carry it to the cis face of the Golgi. Cargo protein (C) continues along the secretory pathway through the Golgi apparatus to the trans‐Golgi network (TGN). Retention signals in ER resident proteins (R) ensure they undergo retrograde trafficking from the Golgi in COPI vesicles. Retrograde transport of Golgi enzymes that may have escaped their resident cisternae also occurs in COPI‐coated vesicles. At the distal face of the Golgi the TGN ensures the correct targeting of proteins, either constitutively or regulated to the plasma membrane, or to intracellular membrane compartments such as proteolytic or secretory lysosomes.