Figure 8.

Model for regulation of PtdIns(3,5)P₂ synthesis and degradation. PtdIns is phosphorylated by the PtdIns 3-kinase Vps34 to generate PtdIns(3)P, which is essential for vacuolar protein sorting through interactions with FYVE and PX domain-containing proteins. PtdIns(3)P can be further phosphorylated by the PtdIns(3)P 5-kinase Fab1 to produce PtdIns(3,5)P₂. Fab1 kinase activity is regulated by the upstream activator Vac7. The generation of PtdIns(3,5)P₂ is essential for the maintenance of vacuolar size control and homeostasis through interactions with unknown effectors. Fig4, through its Sac1 polyphosphoinositide phosphatase domain, functions with Sj12 and Sjl3, in regulating PtdIns(3,5)P₂ levels by mediating the turnover of this lipid.