Evolution of growth factor signaling to the TSC complex to regulate mTORC1

The mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) integrates signals from factors that both stimulate (exogenous growth factors) and are essential for (intracellular nutrients and energy) cellular growth. Activation of the protein kinase mTOR within mTORC1 results in the phosphorylation of downstream substrates that collectively stimulate biomass accumulation to drive cell growth. Many upstream signals, especially growth factors, regulate mTORC1 by inducing the phosphorylation of the tuberous sclerosis complex 2 (TSC2) subunit of the TSC protein complex, a conserved brake on mTORC1 activation and its promotion of cell growth. Cryo-electron microscopy studies of the TSC protein complex have revealed that this phosphoregulation of TSC2 occurs almost exclusively on residues in loops that are outside of the evolutionarily conserved core structural elements and that did not resolve in these structures. These phosphorylation-rich unstructured loops evolved with metazoans, suggesting that the regulation of mTORC1 by diverse growth factors likely evolved with the emergence of complex body plans and diverse cell types to coordinate cell growth and metabolism within and across distinct tissues. Unlike the core structure of TSC2, these loops lack disease-associated missense mutations. These features suggest that the regulatory loops on TSC2 are more amenable to evolutionary changes that enable diverse signals to converge on the TSC protein complex to regulate mTORC1.