Coordinated regulation of bidirectional COPI transport at the Golgi

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. In addition to its known role in generating vesicles 10, we recently discovered that COPI also generates tubules that connect the Golgi stacks 9. However, whether these tubules act passively or actively in cargo transport has been unclear. Active transport involves coat proteins binding to cargoes. Thus, we initially examined whether COPI binds to a temperature-sensitive form of the vesicular stomatitis virus G protein (referred hereon as VSVG), which has been widely used to track anterograde Golgi transport 6–9. We found that coatomer, the core components of the COPI complex 10, binds directly to the cytoplasmic tail of VSVG (Fig 1a and Extended data Figure. 1a). Further defining this binding, we found that coatomer binds to the membrane proximal region of the VSVG tail (Fig 1b and Extended data Figure. 1b), and then identified residues within this region critical for binding by coatomer (Fig 1c and Extended data Figure. 1a). Address correspondence to: Victor Hsu, Tel: 617-525-1103, [email protected]. AUTHOR CONTRIBUTIONS SYP, JSY, and ABS performed the experiments. VWH, SYP, RJS, and ABS designed the experiments and wrote the paper. The authors declare no competing financial interests. HHS Public Access Author manuscript Nature. Author manuscript; available in PMC 2015 November 28. Published in final edited form as: Nature. 2015 May 28; 521(7553): 529–532. doi:10.1038/nature14457. A uhor M anscript