Rapamycin inhibits trypanosome cell growth by preventing TOR complex 2 formation

Target of rapamycin (TOR) kinases control cell growth through two functionally distinct multiprotein complexes. TOR complex 1 (TORC1) controls temporal cell growth and is sensitive to rapamycin, whereas TOR complex 2 (TORC2) is rapamycin resistant and regulates spatial cell growth. Here, we identified two TOR orthologues, TbTOR1 and TbTOR2, in the protozoan parasite Trypanosoma brucei, as well as orthologues of the well-known TORC1 and TORC2 partners, KOG1/raptor and AVO3/rictor. TbTOR proteins differ in their functions, subcellular localization, and rapamycin sensitivity. TbTOR1 controls cell growth by regulating cell cycle, nucleolus structure, and protein synthesis, whereas TbTOR2 coordinates cell polarization and cytokinesis. Rapamycin treatment of bloodstream trypanosomes resulted in a pronounced reduction of cell proliferation, with an EC₅₀ of 152 nM. Unique for a eukaryote, we observed that rapamycin acted exclusively by preventing TORC2 formation, with no effect on TORC1. Our findings on TOR signaling in this protozoan, which is located in a distal position in the eukaryotic cell lineage, highlight the clinical possibilities of rapamycin derivates and provide valuable insights into understanding rapamycin-mediated inhibition of TORC2.