Investigating the Role of PI4P in Lysosome-related Organelle Biogenesis in the Drosophila Eye. Lauren M. Del Bel^1,2, Ronit Wilk¹, Jason Burgess^1,2, Gordon Polevoy¹, Ho-Chun Wei¹, Julie Brill^1,2. 1) Cell Biology, The Hospital for Sick Children, Toronto, Ontario, Canada; 2) Department of Molecular Genetics, University of Toronto, Toronto, Ontario, Canada.

   Phosphatidylinositol (PI) 4-phosphate (PI4P) is an essential membrane lipid within cells. PI4P is mainly present at the Golgi where it recruits regulators of intracellular trafficking. In Drosophila melanogaster, the type II PI 4-kinase (PI4KII) is one of the kinases responsible for generating PI4P at the Golgi, while a single phosphatase (Sac1) keeps PI4P levels in check. By generating Drosophila mutants, our lab has discovered that PI4KII and Sac1 together control a pool of PI4P that is critical for eye pigmentation. We have found that sac1 and PI4KII mutants have reduced eye pigment levels, due to an altered number and distribution of eye pigment granules. Drosophila eye pigment granules are a type of lysosome-related organelle (LRO), which is a specialized membrane-bound compartment within specific cell-types. LROs are generated by specialized protein sorting and membrane trafficking, which is carried out by intracellular trafficking complexes, such as the clathrin Adaptor Protein complex 3 (AP-3). We have found that PI4KII and sac1 mutants genetically interact with other trafficking complex mutants, such as those affecting AP-3. Indeed, we have found that Sac1 and PI4KII are required for proper distribution of the AP-3 subunit Garnet in the Drosophila retina. This evidence suggests a novel role for PI4P in pigment granule formation.