Target of rapamycin (TOR): balancing the opposing forces of protein synthesis and degradation.

Mitogenic and nutritional signals must be integrated for a cell to grow. The target of rapamycin (TOR) is emerging as an effector for signals which indicate to the cell whether the external environment is conducive for growth. Use of the immunosuppressant rapamycin, a bacterial macrolide, has been instructive in identifying potential signaling components downstream of TOR, leading to the observation that both protein synthesis and turnover are under TOR control. The central issues concerning TOR are the identification of the proliferative and anti-proliferative signals which mediate its function and the mechanisms by which these signals are transduced to downstream molecules.