A Heat-Sensitive Arabidopsis thalianaKinase Substitutes for Human p70s6k Function In Vivo

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Abstract
In mammalian cells, mitogen-induced phosphorylation of ribosomal protein S6 by p70s6k has been implicated in the selective translational upregulation of 5′TOP mRNAs. We demonstrate here that the homologous Arabidopsis thaliana protein, AtS6k2, ectopically expressed in human 293 cells or isolated from plant cells, phosphorylates specifically mammalian and plant S6 at 25°C but not at 37°C. When Arabidopsis suspension culture cells are shifted from 25 to 37°C, the kinase becomes rapidly inactivated, consistent with the observation that heat shock abrogates S6 phosphorylation in plants. Treatment with potato acid phosphatase reduced the specific activity of immunoprecipitated AtS6k2 threefold, an effect which was blocked in the presence of 4-nitrophenyl phosphate. In quiescent mammalian cells, AtS6k2 is activated by serum stimulation, a response which is abolished by the fungal metabolite wortmannin but is resistant to rapamycin. Treatment of mammalian cells with rapamycin abolishes in vivo S6 phosphorylation by p70s6k; however, ectopic expression of AtS6k2 rescues the rapamycin block. Collectively, the data demonstrate that AtS6k2 is the functional plant homolog of mammalian p70s6k and identify a new signalling pathway in plants.