Autoregulation and Homodimerization Are Involved in the Activation of the Plant Steroid Receptor BRI1

Abstract

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.

The brassinosteroid signal transduction pathway

“…The second hypothesis is supported by a recent study from Joanne Chory's lab, which provided evidence that receptor activation involves ligand induced conformation changes in the BRI1 kinase [29]. In this study, deletion of the C-terminal 40 amino acids (CT) of BRI1 was shown to increase receptor activity, as indicated by increased phosphorylation of BRI1 and growth response in transgenic plants expressing such truncated BRI1 receptor [29].…”

“…Although the model of Wang et al [29] is consistent with most of the observations, many questions remain to be answered. For example, the possibility that activation of BRI1 kinase requires dimerization with BAK1 has yet to be tested experimentally.…”

“…In this study, deletion of the C-terminal 40 amino acids (CT) of BRI1 was shown to increase receptor activity, as indicated by increased phosphorylation of BRI1 and growth response in transgenic plants expressing such truncated BRI1 receptor [29]. The inhibitory function of the CT region appears to be reduced by phosphorylation, because the kinase activity was increased by mutation of several Ser/Thr residues in this region to Asp, which likely mimics phosphorylation [29], and reduced by mutations of Ser/Thr to Ala, which prevent phosphorylation [21]. BRI1 was also shown to interact with itself in vivo and the interaction appears to be increased by BR, suggesting that BR binding stabilizes the BRI1 homodimer.…”

“…BRI1 was also shown to interact with itself in vivo and the interaction appears to be increased by BR, suggesting that BR binding stabilizes the BRI1 homodimer. Based on these observations, it was proposed that homodimerization is involved in BRI1 kinase activation [29]. In the absence of ligand, BRI1 exists as a preformed inactive homo-oligomer, and its kinase activity is maintained at a basal level via trans-or cis-repression by its CT domain.…”

“…In the absence of ligand, BRI1 exists as a preformed inactive homo-oligomer, and its kinase activity is maintained at a basal level via trans-or cis-repression by its CT domain. Steroid binding to the extracellular domain of BRI1 induces a conformational change in the kinase domain and allows trans-phosphorylation of the CT region, which likely enhances BRI1's kinase activity and promotes further phosphorylation of BRI1, thereby providing binding sites for BAK1 [29].…”