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. Author manuscript; available in PMC: 2020 Aug 1.
Published in final edited form as: Curr Opin Cell Biol. 2019 Mar 18;59:8–15. doi: 10.1016/j.ceb.2019.02.004

Figure 3. Model for how a PS flippase (Drs2-Cdc50) contributes to the polyubiquitin- and COPI-dependent recycling of an exocytic SNARE.

Figure 3.

Snc1 is a t-SNARE that functions in the fusion of exocytic carriers with the plasma membrane. After endocytosis, Snc1 recycles through the endosomal system back to the Golgi for re-packaging into exocytic carriers. This recycling process requires Drs2-Cdc50, Rcy1, the ArfGAP Gcs1, COPI, and modification of Snc1 by K63-linked polyubiquitin. Rcy1 binds to a C-terminal regulatory domain on Drs2 and likely stimulates flippase activity. PS flip to the cytosolic leaflet increases membrane curvature and negative charge to recruit Gcs1, which binds to both COPI and Snc1. In addition, the twin β propellers of α and β’ COP bind directly to K63-linked polyubiquitin and this interaction is essential for Snc1 recycling.