Current Model of BR Signal Transduction.
The active BR, brassinolide (BL), binds to the extracellular domain of the BRI1 receptor kinase, promoting a basal BRI1 kinase activity that phosphorylates the negative regulator BKI1 on Y211, releasing it from the membrane and allowing BRI1 to associate with BAK1 or its homologs BKK1 and SERK1. BRI1 and BAK1 transphosphorylate each other on specific residues, which enhances the signaling capacity of BRI1, leading to phosphorylation of BSK1 and its release from the receptor complex. Activated BSK1 associates with and activates BSU1, which dephosphorylates the BIN2 kinase on Tyr-200, inactivating it. The unphosphorylated forms of BES1 and BZR1 transcription factors then accumulate (with aid of PP2A) and bind to the promoters of BR-regulated target genes, eliciting a specific physiological response such as cell elongation. Positive regulators of the pathway are shown in black, with negative regulators in red. For simplicity, the nuclear or cytoplasmic localization of BSU1, BIN2, BES1, and BZR1 are not shown in this model but are discussed in more detail in the text.