title: Arabidopsis TARGET OF RAPAMYCIN interacts with RAPTOR, which regulates the activity of S6 kinase in response to osmotic stress signals.
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.
authors.affiliation: Department of Molecular Genetics and Plant Biotechnology Center, The Ohio State University, Columbus, Ohio 43210, USA.
authors.forename: Magdy M
authors.initials: MM
authors.lastname: Mahfouz
authors.forename: Sunghan
authors.initials: S
authors.lastname: Kim
authors.forename: Ashton J
authors.initials: AJ
authors.lastname: Delauney
authors.forename: Desh Pal S
authors.initials: DP
authors.lastname: Verma
mesh_terms: D064413:3-Phosphoinositide-Dependent Protein Kinases; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D005091:Exons / Q000235:genetics; D015870:Gene Expression; D018506:Gene Expression Regulation, Plant; D008969:Molecular Sequence Data; D009997:Osmotic Pressure; D019869:Phosphatidylinositol 3-Kinases; D010766:Phosphorylation; D030821:Plants, Genetically Modified / Q000033:anatomy & histology; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D019893:Ribosomal Protein S6 Kinases / Q000737:chemistry / Q000378:metabolism*; D015398:Signal Transduction; D022061:Tacrolimus Binding Protein 1A / Q000378:metabolism
chemical_list: D064413:3-Phosphoinositide-Dependent Protein Kinases; D029681:Arabidopsis Proteins; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D019893:Ribosomal Protein S6 Kinases; D022061:Tacrolimus Binding Protein 1A; C496442:Raptor1 protein, Arabidopsis; C512171:TOR protein, Arabidopsis; C578562:PDPK1 protein, human