The expanding TOR signaling network

Cell growth (increase in cell mass or size) is tightly coupled to nutrient availability, growth factors and the energy status of the cell. The target of rapamycin (TOR) integrates all three inputs to control cell growth. The discovery of upstream regulators of TOR (AMPK, the TSC1-TSC2 complex and Rheb) has provided new insights into the mechanism by which TOR integrates its various inputs. A recent finding in flies reveals that TOR controls not only growth of the cell in which it resides (cell-autonomous growth) but also the growth of distant cells, thereby determining organ and organism size in addition to the size of isolated cells. In yeast and mammals, the identification of two structurally and functionally distinct multiprotein TOR complexes (TORC1 and TORC2) has provided a molecular basis for the complexity of TOR signaling. Furthermore, TOR has emerged as a regulator of growth-related processes such as development, aging and the response to hypoxia. Thus, TOR is part of an intra- and inter-cellular signaling network with a remarkably broad role in eukaryotic biology.