Abstract
BackgroundThe eukaryotic TOR pathway controls translation, growth and the cell cycle in response to environmental signals such as nutrients or growth-stimulating factors. The TOR protein kinase can be inactivated by the antibiotic rapamycin following the formation of a ternary complex between TOR, rapamycin and FKBP12 proteins. The TOR protein is also found in higher plants despite the fact that they are rapamycin insensitive. Previous findings using the yeast two hybrid system suggest that the FKBP12 plant homolog is unable to form a complex with rapamycin and TOR, while the FRB domain of plant TOR is still able to bind to heterologous FKBP12 in the presence of rapamycin. The resistance to rapamycin is therefore limiting the molecular dissection of the TOR pathway in higher plants.ResultsHere we show that none of the FKBPs from the model plant Arabidopsis (AtFKBPs) is able to form a ternary complex with the FRB domain of AtTOR in the presence of rapamycin in a two hybrid system. An antibody has been raised against the AtTOR protein and binding of recombinant yeast ScFKBP12 to native Arabidopsis TOR in the presence of rapamycin was demonstrated in pull-down experiments. Transgenic lines expressing ScFKBP12 were produced and were found to display a rapamycin-dependent reduction of the primary root growth and a lowered accumulation of high molecular weight polysomes.ConclusionThese results further strengthen the idea that plant resistance to rapamycin evolved as a consequence of mutations in plant FKBP proteins. The production of rapamycin-sensitive plants through the expression of the ScFKBP12 protein illustrates the conservation of the TOR pathway in eukaryotes. Since AtTOR null mutants were found to be embryo lethal [1], transgenic ScFKBP12 plants will provide an useful tool for the post-embryonic study of plant TOR functions. This work also establish for the first time a link between TOR activity and translation in plant cells
Highlights
The eukaryotic TOR pathway controls translation, growth and the cell cycle in response to environmental signals such as nutrients or growth-stimulating factors
Comparison of Arabidopsis FKBP sequences shows that the closest relatives of ScFKBP12 are AtFKBP12, AtFKBP15-1, AtFKBP15-2 and the first FRB (FK506 and Rapamycin Binding) domain of AtFKBP62 (Fig. 1A)
This is the case of Tyr26, Asp 38 and Gln 54 which are absent from AtFKBP12 but exists in AtFKBP15-1, AtFKBP15-2 and AtFKBP62
Summary
The eukaryotic TOR pathway controls translation, growth and the cell cycle in response to environmental signals such as nutrients or growth-stimulating factors. Studies in yeast and animal cells have shown that TOR acts positively on the activity of the eIF4F translation initiation complex and on the transcription of ribosomal RNA and protein genes promoting growth in nutrient sufficient conditions [5,6,7]. Rapamycin leads to the formation of a ternary complex by binding simultaneously to the FRB [FKP12 and Rapamycin Binding] domain of TOR and to the ScFKBP12 protein [14]. ScFKBP12 is a peptidylprolyl isomerase that was originally identified as the cytosolic receptor for the immunosuppressive drugs FK506 and rapamycin [15] This ternary complex is inactivating the TOR kinase activity in a specific manner since no other cellular targets of rapamycin are known [16]. RAPTOR, a member of one of the two TOR [TORC1] complexes, is supposed to recruit the various TOR substrates [18,19,20]
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