Autoregulation and homodimerization are involved in the activation of the plant steroid receptor BRI1.
The leucine-rich-repeat receptor serine/threonine kinase, BRI1, is a cell-surface receptor for brassinosteroids (BRs), the steroid hormones of plants, yet its activation mechanism is unknown. Here, we report a unique autoregulatory mechanism of BRI1 activation. Removal of BRI1's C terminus leads to a hypersensitive receptor, indicated by suppression of dwarfism of BR-deficient and BR-perception mutants and by enhanced BR signaling as a result of elevated phosphorylation of BRI1. Several sites in the C-terminal region can be phosphorylated in vitro, and transgenic Arabidopsis expressing BRI1 mutated at these sites demonstrates an essential role of phosphorylation in BRI1 activation. BRI1 is a ligand-independent homo-oligomer, as evidenced by the transphosphorylation of BRI1 kinase in vitro, the dominant-negative effect of a kinase-inactive BRI1 in transgenic Arabidopsis, and coimmunoprecipitation experiments. Our results support a BRI1-activation model that involves inhibition of kinase activity by its C-terminal domain, which is relieved upon ligand binding to the extracellular domain.
Wang
Xuelu
X
Howard Hughes Medical Institute and Plant Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, California 92037, USA.
Li
Xiaoqing
X
Meisenhelder
Jill
J
Hunter
Tony
T
Yoshida
Shigeo
S
Asami
Tadao
T
Chory
Joanne
J
D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000378:metabolism*; D015153:Blotting, Western / Q000379:methods; D002462:Cell Membrane / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018506:Gene Expression Regulation, Plant / Q000187:drug effects / Q000502:physiology; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006706:Homeostasis / Q000502:physiology*; D047468:Immunoprecipitation / Q000379:methods; D066298:In Vitro Techniques; D008954:Models, Biological; D016296:Mutagenesis / Q000502:physiology; D009154:Mutation; D009842:Oligopeptides; D010449:Peptide Mapping / Q000379:methods; D010455:Peptides / Q000378:metabolism; D010641:Phenotype; D010766:Phosphorylation; D010937:Plant Growth Regulators / Q000502:physiology*; D030821:Plants, Genetically Modified; D011494:Protein Kinases / Q000737:chemistry / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D013260:Steroids, Heterocyclic / Q000737:chemistry / Q000378:metabolism*
D029681:Arabidopsis Proteins; D004791:Enzyme Inhibitors; D009842:Oligopeptides; D010455:Peptides; D010937:Plant Growth Regulators; D013260:Steroids, Heterocyclic; D049452:Green Fluorescent Proteins; C085706:FLAG peptide; D011494:Protein Kinases; C108270:BRI1 protein, Arabidopsis