Revealing the Molecular Wiring Of The TSC-mTOR Signaling Network

The signaling network surrounding the TSC1-TSC2 complex regulates cell metabolism, growth, proliferation, and survival, and its misregulation has been implicated in both cancer and metabolic diseases. The TSC1-TSC2 complex controls downstream processes through its regulation of the protein kinase mTOR which functions within two distinct complexes, mTOR complex 1 and 2 (mTORC1 and 2). mTORC1 is inhibited by the TSC1-TSC2 complex, while conversely, mTORC2 activity is promoted. Although this network is well studied, there are several outstanding mechanistic questions concerning how the TSC1-TSC2 complex regulates the mTOR complexes. To better understand how mTORC2 is regulated we identified phosphorylation sites on its essential component Rictor and found that one site is phosphorylated by S6K1 downstream of mTORC1. This phosphorylation event constitutes an mTORC1-dependent negative feedback loop that inhibits the ability of mTORC2 to signal to Akt in response to insulin. In a broader effort to uncover regulatory mechanisms of the TSC1-TSC2 complex, we searched for proteins that interact with the complex with high specificity and stoichiometry. We discovered that TBC1D7 tightly associates with the TSC1-TSC2 complex in diverse tissues and our data indicate it is a third component of the complex. Further, RNAi knockdowns of TBC1D7 affect mTOR signaling in a manner qualitatively similar to knockdowns of TSC1 or TSC2 suggesting that this protein may be a missing link in the regulation of the mTOR complexes by the TSC1-TSC2 complex. A more complete understanding of regulatory mechanisms within the TSC-mTOR network will facilitate the development of rational therapies for cancer and metabolic diseases.