The target of rapamycin (TOR) is a conserved Ser/Thr kinase that controls cell growth by activating an array of anabolic processes including protein synthesis, transcription and ribosome biogenesis, and by inhibiting catabolic processes such as mRNA degradation and autophagy. Control of autophagy by TOR occurs primarily at the induction step, and involves activation of the ATG1 kinase, a conserved component of the autophagic machinery. A substantial number of genes participating in autophagy have been originally identified in yeast. Most of these genes have mammalian homologues and many have apparent homologues in plants, indicating that autophagy is conserved among eukaryotes. The recent identification of TOR as a key element in cell growth control in plants and algae opens the way for future studies to investigate whether this signaling pathway may also control autophagy in photosynthetic organisms.

The macrolide rapamycin specifically binds the 12-kD FK506-binding protein (FKBP12), and this complex potently inhibits the target of rapamycin (TOR) kinase. The identification of TOR in Arabidopsis (Arabidopsis thaliana) revealed that TOR is conserved in photosynthetic eukaryotes. However, research on TOR signaling in plants has been hampered by the natural resistance of plants to rapamycin. Here, we report TOR inactivation by rapamycin treatment in a photosynthetic organism. We identified and characterized TOR and FKBP12 homologs in the unicellular green alga Chlamydomonas reinhardtii. Whereas growth of wild-type Chlamydomonas cells is sensitive to rapamycin, cells lacking FKBP12 are fully resistant to the drug, indicating that this protein mediates rapamycin action to inhibit cell growth. Unlike its plant homolog, Chlamydomonas FKBP12 exhibits high affinity to rapamycin in vivo, which was increased by mutation of conserved residues in the drugbinding pocket. Furthermore, pull-down assays demonstrated that TOR binds FKBP12 in the presence of rapamycin. Finally, rapamycin treatment resulted in a pronounced increase of vacuole size that resembled autophagic-like processes. Thus, our findings suggest that Chlamydomonas cell growth is positively controlled by a conserved TOR kinase and establish this unicellular alga as a useful model system for studying TOR signaling in photosynthetic eukaryotes.The macrolide antibiotic rapamycin is a product of the bacterium Streptomyces hygroscopicus. Rapamycin was originally identified as a potent antifungal agent (Vezina et al., 1975) and much later was found to exhibit immunosuppressive activity due to its capacity to block the growth and proliferation of T cells (Schreiber, 1992;Sigal and Dumont, 1992). More recently, rapamycin has been found to display anticancer properties (Bjornsti and Houghton, 2004).Rapamycin inhibits cell growth in many types of cells, including the budding yeast Saccharomyces cerevisiae. Studies performed in yeasts uncovered the unique mechanism of action of rapamycin. Both receptor and functional target were initially identified in S. cerevisiae (Heitman et al., 1991). Rapamycin first binds the 12-kD FK506-binding protein (FKBP12) and this complex inhibits the target of rapamycin (TOR) Ser/Thr kinase. FKBP12 is a member of the FK506-and rapamycin-binding protein (FKBP) family. This group of proteins, together with the cyclosporin A receptors, is collectively referred to as immunophilins because of their ability to tightly bind the immunosuppressive drugs rapamycin, FK506, or cyclosporin A (Schreiber, 1991;Fruman et al., 1994). Similar to other immunophilins, FKBP12 has peptidyl prolyl cis/trans isomerase activity that is involved in protein-folding processes. Studies have identified FKBP12 in bacteria, fungi, animals, plants (for review, see Schreiber, 1991;Fruman et al., 1994;He et al., 2004), and more recently in the green alga Chlamydomonas reinhardtii (Vallon, 2005). However, the physiological function of this protein is still poorly understood....