Figure 12.
The pathways for biosynthesis of PtdIns(4)P and PtdIns(4,5)P2 in yeast, indicating cellular role(s) for each phosphoinositide produced. The Stt4p- or Pik1p-dependent phosphorylation of PtdIns leads to the production of distinct pools of PtdIns(4)P. Arrows indicate the proposed roles for each phosphorylated lipid product. Stt4p-dependent PtdIns(4)P is required for maintenance of vacuole morphology and the actin cytoskeleton, whereas Pik1p-dependent PtdIns(4)P is essential for secretory vesicle formation and Golgi/endosome maintenance. Both Stt4 and Pik1 kinase activities appear to be required for generating the cellular pool of PtdIns(4,5)P2. Although we propose that Stt4p-dependent generation of PtdIns(4,5)P2 is required for actin polarization, a role for Pik1p-dependent PtdIns(4,5)P2 synthesis is not yet apparent.