The serine and threonine kinase mammalian target of rapamycin (mTOR), which acts as a component of two different complexes (mTORC1 and mTORC2) that have distinct functions and downstream targets, coordinates signals from growth factors with information about nutrient and energy status to regulate cell size and proliferation. Using cells derived from transgenic mice in which the Rho-family small guanosine triphosphatase (GTPase) Rac1 could be conditionally deleted, Saci et al. determined that loss of Rac1 led to a decrease in cell size [assessed in mouse embryonic fibroblasts and T and B lymphocytes]. Further analysis revealed that Rac1 deletion or knockdown led to a decrease in the serum-stimulated phosphorylation of both mTORC1 and mTORC2 targets. Phosphoinositide 3-kinase (PI3K)–Akt signaling can activate mTORC1; however, pharmacological analysis indicated that the effects of Rac1 on phosphorylation of mTORC1 targets were independent of PI3K; moreover, pharmacological and mutational analyses indicated that they were also independent of Rac1 binding to GTP. Immunofluorescence analysis revealed colocalization of Rac1 with mTOR and components of both mTORC1 (Raptor) and mTORC2 (Rictor and Sin1) and indicated that Rac1 loss led to mTOR mislocalization. A combination of mutational analysis, immunoprecipitation, and GST pull-down assays indicated that Rac1 associated with mTOR, as well as with Raptor and Rictor, and that its interaction with both complexes was independent of its GTP-binding status. Deletion of Rac2, which differs from Rac1 only in the C terminus, failed to affect cell size. Rac2 showed little interaction with mTORC proteins compared with Rac1, an observation echoed by analyses with biotinylated peptides corresponding to the C-terminal 10 amino acids of Rac1 or Rac2. The Rac1 peptide bound to mTOR that had been purified after disruption of the mTORC1 and mTORC2 complexes. The authors thus conclude that Rac1 binds directly to mTOR, thereby directing its localization and allowing it to participate in the regulation of both mTORC1 and mTORC2 activity.
A. Saci, L. C. Cantley, C. L. Carpenter, Rac1 regulates the activity of mTORC1 and mTORC2 and controls cellular size. Mol. Cell 42, 50–61 (2011). [Online Journal]