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 brassinosterolds (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 homooligomer, as evidenced by the transphosphorylation of BRI1 kinase in vitro, the dominant-negative effect of a kinase-inactive BRI1 in transgenic Arabidopsis, and colmmunoprecipitation experiments. Our results support a BFI1-activation model that involves inhibition of kinase activity by its C-terminal domain, which is relieved upon ligand binding to the extracellular domain.