Autoregulation and homodimerization are involved in the activation of the plant steroid receptor BRI1.
Wang X, Li X, Meisenhelder J, Hunter T, Yoshida S, Asami T, Chory J.
Howard Hughes Medical Institute and Plant Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, California 92037, USA.
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.
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PMID: 15935775 [PubMed - indexed for MEDLINE]