Every eukaryotic cell contains two distinct multisubunit protein kinase complexes that each contain a TOR (target of rapamycin) protein as the catalytic subunit. These ensembles, designated TORC1 and TORC2, serve as nutrient and stress sensors, signal integrators, and regulators of cell growth and homeostasis, but they differ in their composition, localization, and function. TORC1, activated on the cytosolic surface of the vacuole (or, in mammalian cells, on the cytosolic surface of the lysosome), promotes biosynthesis and suppresses autophagy. TORC2, located primarily at the plasma membrane (PM), maintains the proper levels and bilayer distribution of all PM components (sphingolipids, glycerophospholipids, sterols, and integral membrane proteins), which are needed for the membrane expansion that accompanies cell growth and division and for combating insults to PM integrity. This review summarizes our current understanding of the assembly, structural features, subcellular distribution, and function and regulation of TORC2, obtained largely through studies conducted with Saccharomyces cerevisiae.
Keywords: PM; endosomes; phosphoinositides; phosphorylation; plasma membrane; protein kinase; subunits.