The target of rapamycin (TOR) is a highly conserved protein kinase and a central controller of cell growth. In budding yeast, TOR is found in structurally and functionally distinct protein complexes: TORC1 and TORC2. A mammalian counterpart of TORC1 (mTORC1) has been described, but it is not known whether TORC2 is conserved in mammals. Here, we report that a mammalian counterpart of TORC2 (mTORC2) also exists. mTORC2 contains mTOR, mLST8 and mAVO3, but not raptor. Like yeast TORC2, mTORC2 is rapamycin insensitive and seems to function upstream of Rho GTPases to regulate the actin cytoskeleton. mTORC2 is not upstream of the mTORC1 effector S6K. Thus, two distinct TOR complexes constitute a primordial signalling network conserved in eukaryotic evolution to control the fundamental process of cell growth.

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The tumor suppressor p27Kip1 is an inhibitor of cyclin/cyclin-dependent kinase (CDK) complexes and plays a crucial role in cell cycle regulation. However, p27Kip1 also has cell cycle-independent functions. Indeed, we find that p27Kip1 regulates cell migration, as p27Kip1-null fibroblasts exhibit a dramatic decrease in motility compared with wild-type cells. The regulation of motility by p27Kip1 is independent of its cell-cycle regulatory functions, as re-expression of both wild-type p27Kip1 and a mutant p27Kip1 (p27CK–) that cannot bind to cyclins and CDKs rescues migration of p27–/– cells. p27–/– cells have increased numbers of actin stress fibers and focal adhesions. This is reminiscent of cells in which the Rho pathway is activated. Indeed, active RhoA levels were increased in cells lacking p27Kip1. Moreover, inhibition of ROCK, a downstream effector of Rho, was able to rescue the migration defect of p27–/– cells in response to growth factors. Finally, we found that p27Kip1 binds to RhoA, thereby inhibiting RhoA activation by interfering with the interaction between RhoA and its activators, the guanine–nucleotide exchange factors (GEFs). Together, the data suggest a novel role for p27Kip1 in regulating cell migration via modulation of the Rho pathway.