Abstract
TARGET OF RAPAMYCIN (TOR) kinase controls many cellular functions in eukaryotic cells in response to stress and nutrient availability and was shown to be essential for embryonic development in
Arabidopsis
thaliana.
We demonstrated that
Arabidopsis
RAPTOR1 (a TOR regulatory protein) interacts with the HEAT repeats of TOR and that RAPTOR1 regulates the activity of S6 kinase (S6K) in response to osmotic stress. RAPTOR1 also interacts in vivo with
Arabidopsis
S6K1, a putative substrate for TOR. S6K1 fused to green fluorescent protein and immunoprecipitated from tobacco (
Nicotiana tabacum
) leaves after transient expression was active in phosphorylating the
Arabidopsis
ribosomal S6 protein. The catalytic domain of S6K1 could be phosphorylated by
Arabidopsis
3-phosphoinositide-dependent protein kinase-1 (PDK1), indicating the involvement of PDK1 in the regulation of S6K. The S6K1 activity was sensitive to osmotic stress, while PDK1 activity was not affected. However, S6K1 sensitivity to osmotic stress was relieved by co-overexpression of RAPTOR1. Overall, these observations demonstrated the existence of a functional TOR kinase pathway in plants. However,
Arabidopsis
seedlings do not respond to normal physiological levels of rapamycin, which appears to be due its inability to bind to the
Arabidopsis
homolog of FKBP12, a protein that is essential for the binding of rapamycin with TOR. Replacement of the
Arabidopsis
FKBP12 with the human FKBP12 allowed rapamycin-dependent interaction with TOR. Since homozygous mutation in TOR is lethal, it suggests that this pathway is essential for integrating the stress signals into the growth regulation.