Regulation of Lipid Homeostasis by the TBC Protein dTBC1D22 Via Modulation of the Small GTPase Rab40

Abstract

Lipid homeostasis is essential to cells. However, its regulation is not well understood. Using forward genetic screen, we demonstrated that the loss of CG5745, an essential gene in Drosophila that encodes a Tre2–Bub2–Cdc16 (TBC) domain-containing protein, results in the lipid droplet accumulation in multiple tissues. We named this gene as dTBC1D22 based on its homology with the mammalian genes TBC1D22A and TBC1D22B. The TBC domain-containing proteins are potential Rab GTPase activating proteins (GAPs). We observed that the GAP activity of dTBC1D22 was critical for its function in lipid metabolism. Additionally, dTBC1D22 interacted with the small GTPase Rab40 and exhibited Rab40 GAP activity. Overexpression of either GTP-binding-mimic or GDP-binding-mimic form of Rab40 resulted in lipid droplet accumulation. We observed that the Rab40 mutant flies, which were generated in this study, were defective in lipid mobilization. The lipid depletion induced by overexpression of Brummer, a triglyceride lipase, was dependent on Rab40. Under well-fed conditions Rab40 was enriched on the lipid droplets, whereas under starvation conditions it was translocated to the Golgi apparatus. Rab40 mutant flies exhibited decreased lipophagy and small size of autolysosomal structures, which may be due to the defective Golgi functions. Finally, we demonstrated that Rab40 physically interacts with Lamp1 and that Rab40 was required for the distribution of Lamp1 during starvation. We proposed that dTBC1D22 functions as a GAP for Rab40 to regulate lipid homeostasis by modulating lipophagy.Lipid homeostasis is essential to cells. However, its regulation is not well understood. Using forward genetic screen, we demonstrated that the loss of CG5745, an essential gene in Drosophila that encodes a Tre2–Bub2–Cdc16 (TBC) domain-containing protein, results in the lipid droplet accumulation in multiple tissues. We named this gene as dTBC1D22 based on its homology with the mammalian genes TBC1D22A and TBC1D22B. The TBC domain-containing proteins are potential Rab GTPase activating proteins (GAPs). We observed that the GAP activity of dTBC1D22 was critical for its function in lipid metabolism. Additionally, dTBC1D22 interacted with the small GTPase Rab40 and exhibited Rab40 GAP activity. Overexpression of either GTP-binding-mimic or GDP-binding-mimic form of Rab40 resulted in lipid droplet accumulation. We observed that the Rab40 mutant flies, which were generated in this study, were defective in lipid mobilization. The lipid depletion induced by overexpression of Brummer, a triglyceride lipase, was dependent on Rab40. Under well-fed conditions Rab40 was enriched on the lipid droplets, whereas under starvation conditions it was translocated to the Golgi apparatus. Rab40 mutant flies exhibited decreased lipophagy and small size of autolysosomal structures, which may be due to the defective Golgi functions. Finally, we demonstrated that Rab40 physically interacts with Lamp1 and that Rab40 was required for the distribution of Lamp1 during starvation. We proposed that dTBC1D22 functions as a GAP for Rab40 to regulate lipid homeostasis by modulating lipophagy.