Fig. 2.

The budding of exocytic vesicles presumably depends on the association of sterol with sphingolipids into microdomains and the flip-flop of PS catalyzed by Drs2. By sterol/PI4P exchange, Osh4 would supply the trans-Golgi with sterol (made in the ER) while removing PI4P (then transported back to the ER and hydrolyzed). Sterol import might be coordinated with the flippase activity of Drs2, repressed by Osh4 via PI4P removal. The exposition of PS on the cytosolic side of the trans-Golgi can in turn downregulate Osh4. Once released, vesicles dock to the PM prior to fusion. Ypt32, a Rab protein that promotes vesicle formation, also initiates a cascade of events leading to the docking process. The GTP-bound form of Ypt32 associated with PI4P recruits Sec2. Then Sec2 activates the Rab Sec4p that in turn recruits Sec15, an exocyst subunit. Osh4 extracts PI4P and the lack of PI4P triggers a conformational change in Sec2 that gains avidity for Sec15. The formation of Sec2-Sec4-Sec15 complexes makes vesicles competent for docking. Simultaneously, due to its exchange activity, Osh4 likely reinforces the enrichment of vesicles in sterol. All these elements suggest that Osh4 synchronizes the build-up of sterol in vesicles with key steps of their life cycle, thereby ensuring an appropriate lipid composition and organization of the PM during its expansion