PI4KIII is required for cortical integrity and cell polarity during Drosophila oogenesis. Julie Tan^1,2, Jason Burgess^1,2, Karen Oh^3,4, David Hipfner^3,4, Julie Brill^1,2. 1) Program in Cell Biology, Hosp Sick Children, Toronto, ON, Canada; 2) Dept of Molecular Genetics, University of Toronto, Toronto, ON, Canada; 3) Epithelial Cell Biology, Institut de Recherches Cliniques de Montreal, Montreal, QC, Canada; 4) Department of Medicine, University of Montreal, Montreal, QC, Canada.

   Phosphoinositides are lipids that provide a molecular link for membrane interactions with cellular machinery. Phosphatidylinositol (PI) 4-kinases (PI4Ks) catalyze the conversion of PI to PI 4-phosphate (PI4P), a cellular effector that recruits trafficking proteins and serves as a precursor to other essential phosphoinositides. To investigate the function of phosphoinositides during development, we generated a deletion in the gene encoding the Drosophila melanogaster type III PI4K (PI4KIII). We find that PI4KIII is required for production of plasma membrane phosphoinositides that are crucial for membrane trafficking, actin organization and cell polarity during oogenesis. Female germ cells mutant for PI4KIII lose cortical integrity and are impaired in activation of the PI 4,5-bisphosphate [PI(4,5)P₂]-binding cytoskeletal-membrane crosslinker Moesin. Titration of PI(4,5)P₂ using the pleckstrin homology domain of phospholipase C partially recapitulates PI4KIII phenotypes, indicating that PI(4,5)P₂ is the main phosphoinositide effector downstream of PI4KIII. These effects are specific to PI4KIII, as they are not produced in egg chambers mutant for either or both of the two remaining PI4Ks, PI4KIII/fwd and PI4KII. Instead, fwd and PI4KII mutant germ cells exhibit altered Golgi distribution, whereas this is unaffected in PI4KIII mutant cells. Furthermore, membrane defects in fwd, PI4KII, and fwd PI4KII double mutant cells appears morphologically distinct from that in PI4KIII cells. Thus, as in yeast and mammalian cells, different Drosophila PI4Ks appear to have organelle-specific roles and produce functionally distinct pools of PI4P.