Osh4p is needed to reduce the level of phosphatidylinositol-4-phosphate on secretory vesicles as they mature.
Bottom Line: Phosphatidylinositol-4-phosphate (PI4P) is produced on both the Golgi and the plasma membrane.We show here that in yeast the oxysterol-binding proteins Osh1-Osh7 are collectively needed to maintain the normal distribution of PI4P and that Osh4p is critical in this function.This reduction in PI4P is necessary for a switch in the regulation of the Sec4p exchange protein, Sec2p, from an interaction with the upstream Rab, Ypt31/32, to an interaction with a downstream Sec4p effector, Sec15p.
Affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093.Show MeSH
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Mentions: PI4P inhibits the interaction of Sec15p with Sec2p and thereby promotes the interaction of Sec2p with the competing ligand, Ypt32p (Mizuno-Yamasaki et al., 2010). We carried out fluorescence microscopy experiments to ascertain whether Osh4p functions as an upstream factor regulating the localization of Ypt32p and Sec15p, perhaps through its effects on the PI4P levels of secretory vesicles. First, we examined the localization of GFP-Ypt32 in wild-type and osh4Δ cells. As a control, we also included sac1Δ and inp52Δ inp53Δ cells, which are defective in PI4P turnover and may have increased PI4P levels on secretory vesicles. In wild-type cells, the majority of Ypt32p localized to the Golgi, with a pool of Ypt32p observed at the tips of small-budded cells; however, it was rarely observed at polarized secretory sites in large-budded cells (Figure 5, A and C). Strikingly, we observed a dramatic increase of GFP-Ypt32 at polarized secretory sites in large buds of osh4Δ cells (Figure 5A). Similar results were observed in the sac1Δ cells and inp52Δ inp53Δ cells (Figure 5A), suggesting that Osh4p may regulate the localization of Ypt32p by controlling PI4P turnover on secretory vesicles.
Affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093.