Structural basis for the association of the redox-sensitive TOR FATC domain with membrane-mimetic micelles

The target of rapamycin (TOR) is a conserved eukaryotic ser/thr kinase that regulates cellular growth in response to the nutrient and energy state. TOR signaling plays an important role for the development of diseases such as cancer, obesity, and diabetes and for different redox-sensitive processes (hypoxia, apoptosis, ageing). Because TOR has been detected at different cellular membranes and in the nucleus, its localization may influence the specific signaling readout. In order to better understand how TOR can associate with different membranes, the lipid-binding properties of the redox-sensitive yeast TOR1 FATC domain (y1fatc) have been characterized by solution NMR spectroscopy. Binding studies with different lipids indicate that y1fatc interacts specifically with a membrane-mimetic environment but appears not to recognize a specific lipid headgroup. In both, the structures of oxidized and reduced micelle-bound y1fatc, residues I2456 to W2470 of the lipid-binding motif form a hydrophobic bulb that has a rim of charged residues. The diffusion constants for both micelle-bound states are consistent with the rotational correlation times from the analysis of the 15N-relaxation data. Based on the Kd-values, the oxidized form (Kd ~ 0.31 mM) binds DPC micelles slightly tighter than the reduced form (Kd ~ 1.86 mM). Binding studies with y1fatc in which one or both tryptophans (W2466, W2470) were replaced by alanine suggest that these residues are important for the exact positioning in the membrane and that the other aromatic (H2462, Y2463, F2469) and aliphatic residues (I2456, L2459, I2464, P2468) in the lipid-binding motif contribute significantly to the affinity.