Cell Biology

Raptor Forms a Nutrient-Sensitive Complex with mTOR

Science's STKE  30 Jul 2002:
Vol. 2002, Issue 143, pp. tw277-TW277
DOI: 10.1126/stke.2002.143.tw277

The regulation of cell growth by nutrient status and mitochondrial function involves the target of rapamycin (TOR) pathway. Rapamycin is an immunosuppressive and anticancer drug that binds to its receptor FKBP12, which interacts with TOR to inhibit its activity. So far, the mechanism by which the kinase activity of TOR is regulated by nutrients has been elusive, because isolated TOR did not show any change in activity no matter what the nutrient status of the cells from which it was isolated. Two groups, Kim et al. and Hara et al. identified Raptor as a binding partner for TOR that may be the key to nutrient and mitochondrial status regulation of TOR activity. TOR phosphorylates the p70 ribosomal S6 kinase (S6K) and the inhibitor of translation initiation eIF-4E binding protein 1(4E-BP1) to promote cell growth. The two groups both used protein purification techniques that limited the exposure of the complex to detergents to identify Raptor. Kim et al. inhibited the synthesis of Raptor by RNA interference in cultured cells and found that the cells were smaller and phenotypically identical to cells depleted of mTOR (mammalian TOR). In addition, phosphorylation of S6K in response to increasing concentrations of leucine was compromised in Raptor-depleted cells compared with the S6K phosphorylation in control cells. The Raptor-TOR complex was sensitive to nutrient and mitochondrial status, with a tighter association between the proteins upon nutrient deprivation or inhibition of mitochondrial function. The isolated complex containing tightly bound Raptor exhibited decreased kinase activity toward S6K. In vivo, Raptor and TOR appear to be in a complex at all times, regardless of nutrient status, but the conformation of the complex appears to be regulated by nutrient status. Hara et al. focused on the activity of TOR toward 4E-BP1 and found that phosphorylation of 4E-BP1 by TOR required the presence of Raptor and was lost if TOR was isolated under conditions in which Raptor dissociated. Hara et al. reported that Raptor interacted with both TOR and the TOR substrates S6K and 4E-BP1. Using beads coupled to 7-methyl-guanosine triphosphate to mimic the cap of mRNA, Hara et al. showed that Raptor, 4E-BP1, and eIF-4E all were copurified and that the Raptor interaction was highest under conditions in which 4E-BP1 was unphosphorylated. Inhibition of Raptor expression in Caenorhabditis elegans produced the same phenotypes as those observed for worms in which TOR was inhibited. Thus, in mammalian cells and in the model organism C. elegans, Raptor is involved in controlling the TOR pathway.

D.-H. Kim, D. D. Sarbassov, S. M. Ali, J. E. King, R. R. Latek, H. Erdjument-Bromage, P. Tempst, D. M. Sabatini, mTOR interacts with Raptor to form a nutrient-sensitive complex that signals to the cell growth machinery. Cell 110, 163-175 (2002). [Online Journal]

K. Hara, Y. Maruki, X. Long, K. Yoshino, N. Oshiro, S. Hidayat, C. Tokunaga, J. Avruch, K. Yonezawa, Raptor, a binding partner of target of rapamycin (TOR), mediates TOR action. Cell 110, 177-189 (2002). [Online Journal]