The target of rapamycin (TOR) is a conserved Ser/Thr kinase that regulates cell growth and metabolism in response to environmental cues. Here, highlighting contributions from studies in model organisms, we review mammalian TOR complexes and the signaling branches they mediate. TOR is part of two distinct multiprotein complexes, TOR complex 1 (TORC1), which is sensitive to rapamycin, and TORC2, which is not. The physiological consequences of mammalian TORC1 dysregulation suggest that inhibitors of mammalian TOR may be useful in the treatment of cancer, cardiovascular disease, autoimmunity, and metabolic disorders.

The target of rapamycin (TOR) proteins in Saccharomyces cerevisiae, TOR1 and TOR2, redundantly regulate growth in a rapamycin-sensitive manner. TOR2 additionally regulates polarization of the actin cytoskeleton in a rapamycin-insensitive manner. We describe two functionally distinct TOR complexes. TOR Complex 1 (TORC1) contains TOR1 or TOR2, KOG1 (YHR186c), and LST8. TORC2 contains TOR2, AVO1 (YOL078w), AVO2 (YMR068w), AVO3 (YER093c), and LST8. FKBP-rapamycin binds TORC1, and TORC1 disruption mimics rapamycin treatment, suggesting that TORC1 mediates the rapamycin-sensitive, TOR-shared pathway. FKBP-rapamycin fails to bind TORC2, and TORC2 disruption causes an actin defect, suggesting that TORC2 mediates the rapamycin-insensitive, TOR2-unique pathway. Thus, the distinct TOR complexes account for the diversity, specificity, and selective rapamycin inhibition of TOR signaling. TORC1 and possibly TORC2 are conserved from yeast to man.

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.

Cell growth (increase in cell mass) and cell proliferation (increase in cell number) are distinct yet coupled processes that go hand-in-hand to give rise to an organ, organism, or tumor. Cyclin-dependent kinase(s) is the central regulator of cell proliferation. Is there an equivalent regulator for cell growth? Recent findings reveal that the target of rapamycin TOR controls an unusually abundant and diverse set of readouts all of which are important for cell growth, suggesting that this conserved kinase is such a central regulator.

FK506 and rapamycin are related immunosuppressive compounds that block helper T cell activation by interfering with signal transduction. In vitro, both drugs bind and inhibit the FK506-binding protein (FKBP) proline rotamase. Saccharomyces cerevisiae cells treated with rapamycin irreversibly arrested in the G1 phase of the cell cycle. An FKBP-rapamycin complex is concluded to be the toxic agent because (i) strains that lack FKBP proline rotamase, encoded by FPR1, were viable and fully resistant to rapamycin and (ii) FK506 antagonized rapamycin toxicity in vivo. Mutations that conferred rapamycin resistance altered conserved residues in FKBP that are critical for drug binding. Two genes other than FPR1, named TOR1 and TOR2, that participate in rapamycin toxicity were identified. Nonallelic noncomplementation between FPR1, TOR1, and TOR2 alleles suggests that the products of these genes may interact as subunits of a protein complex. Such a complex may mediate nuclear entry of signals required for progression through the cell cycle.