Coordinated regulation of bidirectional COPI transport at the Golgi by cdc42

The Golgi complex plays a central role in the intracellular sorting of secretory proteins (1,2). Anterograde transport through the Golgi has been explained by the movement of Golgi cisternae, known as cisternal maturation (3–5). Because this explanation is now appreciated to be incomplete (6), interest has developed in understanding tubules that connect the Golgi cisternae (7–9). Here, we find that the Coat Protein I (COPI) complex sorts anterograde cargoes into these tubules. Moreover, the small GTPase cdc42 regulates bidirectional Golgi transport by targeting the dual functions of COPI in cargo sorting and carrier formation. Cdc42 also directly imparts membrane curvature in promoting COPI tubule formation. Our findings further reveal that COPI tubular transport complements cisternal maturation in explaining how anterograde Golgi transport is achieved, and that bidirectional COPI transport is modulated by environmental cues through cdc42.