Rapamycin as a Tool

No plant species tested is sensitive to rapamycin (Menand et al. 2002). The soil dwelling nematode C. elegans, among others, is also rapamycin insensitive (Jia et al. 2004). Rapamycin sensitivity may be generally weak among organisms associated with soil.

In yeast three-hybrid assays and in vitro, AtTOR and rapamycin form a complex with human FKBP12 but not with AtFKBP12 (Menand et al. 2002; Robaglia et al. 2004; Mahfouz et al. 2006). This result indicates that Ara-bidopsis insensitivity to rapamycin is due to the failure of AtFKBP12 to form a TORC1-inhibiting complex with rapamycin rather than an insensitivity of AtTOR to rapamycin-FKBP12 complexes.

Unlike land plants, the unicellular green alga Chlamydomonas shows ra-pamycin sensitivity (Menand et al. 2002; Crespo et al. 2005). Growth is slowed but not arrested completely as in budding yeast. Disruption of the Chlamydomonas FKBP12 locus suppresses rapamycin sensitivity. Sophisticated analysis of Chlamydomonas FKBP12 revealed that the rapamycin-binding pocket in this protein makes fewer hydrogen bonds with rapamycin than human or yeast FKBP12. Complementing rapamycin-insensitive mutants with FKBP alleles engineered to increase the number of bonds with rapamycin yields lines that are more sensitive to rapamycin than are wildtype cells (Crespo et al. 2005).

From the work above one can conclude the following. Rapamycin requires FKBP to exert its effect on cell growth in the single-celled algal model Chlamydomonas as it does in yeast and mammals. Arabidopsis TOR binds ra-pamycin; its insensitivity to rapamycin treatment may be due to the failure of AtFKBP12 to form a TOR-inhibitory complex with rapamycin (the failure of plants to take up rapamycin has not been excluded). Thus, transgenic Ara-bidopsis lines expressing mutant FKBP alleles engineered to bind rapamycin, as was done in Chlamydomonas, may provide a means of hypersensitizing Arabidopsis to rapamycin, opening up an avenue to studying TOR signaling which has been so productive in other systems.

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