Elucidating TOR signaling and rapamycin action: lessons from Saccharomyces cerevisiae

Abstract

TOR (target of rapamycin) is a phosphatidylinositol kinase-related protein kinase that controls cell growth in response to nutrients. Rapamycin is an immunosuppressive and anticancer drug that acts by inhibiting TOR. The modes of action of TOR and rapamycin are remarkably conserved from S. cerevisiae to humans. The current understanding of TOR and rapamycin is derived largely from studies with S. cerevisiae. In this review, we discuss the contributions made by S. cerevisiae to understanding rapamycin action and TOR function.

FIG. 1.

Rapamycin-FKBP complex binds and inhibits TOR. (A) Chemical structure of rapamycin. The TOR- and FKBP-interacting regions of rapamycin (30) are indicated by dashed lines. (B) TOR is active in the presence of nutrients and inactive upon nutrient limitation or FKBP-rapamycin (Rap) binding.

FIG. 2.

Structure of the TOR kinases. Functional domains conserved in TOR proteins are depicted, including the N-terminal HEAT repeats, the central FAT domain, and the C-terminal FKBP-rapamycin binding (FRB), kinase, and FATC domains. aa, amino acids.

FIG. 3.

TOR controls phosphatases in S. cerevisiae. Under good nutrient (nitrogen) conditions, TOR inhibits the phosphatase SIT4 by promoting the association of SIT4 with TAP42. Two different models have been proposed for the mechanism by which TOR controls the SIT4-TAP42 complex. Jiang and Broach (81) proposed that TOR controls the interaction between SIT4 and TAP42 by phosphorylating TAP42 directly (indicated by a dashed arrow). Jacinto et al. (79) suggested that the association of SIT4 and TAP42 is controlled primarily by the TAP42 interactor TIP41. TOR may phosphorylate and inactivate TIP41 by an unknown mechanism. Dephosphorylated TIP41 positively regulates SIT4 by binding and inhibiting TAP42. The association of TIP41 with TAP42 enhances SIT4 phosphatase activity, allowing free SIT4 subunits to associate with SAPs and activate target phosphoproteins such as NPR1 and GLN3. Dephosphorylation and activation of TIP41 are mediated by SIT4, indicating that TIP41 is part of a feedback loop that amplifies SIT4 activity. Arrows indicate activation; bars indicate inhibition. Adapted from Jacinto et al. (79). Rap, rapamycin.

FIG. 4.

TOR inhibits autophagy in S. cerevisiae. Under good nutrient conditions, TOR inhibits autophagy by promoting phosphorylation (P) of APG13 and thereby preventing the formation of an APG1-APG13 complex, which is essential for the induction of autophagy. Inactivation of TOR by rapamycin (Rap) treatment or nutrient deprivation results in rapid dephosphorylation of APG13. Dephosphorylated APG13 associates with APG1, and the active APG13-APG1 complex induces autophagy. Arrows indicate activation; bars indicate inhibition.

FIG. 5.

TOR prevents nuclear accumulation of the nitrogen-regulated transcription activator GLN3 via TAP42-mediated inhibition of the phosphatase SIT4. Under good nitrogen conditions, GLN3 is phosphorylated and retained in the cytoplasm by URE2. Upon nitrogen starvation or rapamycin treatment, SIT4 is released from TAP42 and activated. Activated SIT4 dephosphorylates the GATA transcription factor GLN3. Dephosphorylated GLN3 dissociates from URE2 and translocates into the nucleus, where it activates transcription of target genes. Arrows indicate activation; bars indicate inhibition.