Brassinosteroid signal perception and relay to BZR1/BES1 transcription factors

Brassinosteroid (BR) signalling converts perception of a steroid hormone at the cell surface into a change in nuclear gene expression through a chain of phosphorylation and dephosphorylation steps. In the absence of BR, the receptor kinase BRI1 is held inactive by its cytoplasmic inhibitor BKI1, and the constitutively active GSK3-like kinase BIN2 phosphorylates the BZR1 and BES1 transcription factors, which inhibits their DNA binding, promotes 14-3-3 binding and cytoplasmic retention, and targets them for degradation. Brassinolide binds the extracellular LRR domain of BRI1, which recruits the coreceptor kinase BAK1; transphosphorylation activates BRI1, which phosphorylates BKI1 (releasing it from the membrane) and the receptor-like cytoplasmic kinases BSK1 (and CDG1). Phosphorylated BSK1 binds the Kelch-repeat phosphatase BSU1, which (after CDG1 phosphorylation) dephosphorylates the conserved pTyr200 of BIN2 and inactivates it. With BIN2 off, PP2A dephosphorylates BZR1/BES1, which accumulate in the nucleus and bind BRRE and E-box elements to repress BR biosynthetic genes (feedback) and activate or repress thousands of growth-related genes. BR biosynthesis, receptor endocytosis/turnover, and downstream transcriptional networks are outside this module's boundary.

Grounded on Arabidopsis thaliana proteins; each step has paralogous families (BRI1/BRL1/BRL3, SERK1-5, BSK1-12, BSU1/BSL1-3, BIN2/BIL1/BIL2 and other group-II GSK3s, BZR1/BES1/BEH1-4), so the single named members are exemplars, not an exhaustive list. CDG1/CDL1 (parallel RLCK relay to BSU1), PP2A (dephosphorylation of BZR1), KIB1 (BR-induced degradation of BIN2) and 14-3-3 proteins (cytoplasmic retention of phosphorylated BZR1/BES1) are described in the module description but not modelled as parts because their gene reviews are not part of this batch. BSK1 catalytic activity is debated (BSK8 structures suggest pseudokinase behaviour), and BSK1 promotes BSU1 activity in vivo without a demonstrated direct effect in vitro; the BSK1 -> BSU1 connection is therefore a positive regulatory link, not an asserted enzymatic activation. BRI1 annoton uses its reviewed core function (signaling receptor activity); a brassinosteroid-binding-specific receptor MF could be considered on the BRI1 review. No production GO-CAM in gocams/index.tsv contains these Arabidopsis proteins.