Brassinosteroid insensitive-1

BRI1 is an integral membrane protein. On the extracellular side of the membrane lies a series of 25 leucine-rich repeats (LRRs). The LRR domain forms a horseshoe shape. An atypical LRR within this domain acts as the brassinosteroid binding site. Next to the LRR domain there is a single-pass transmembrane section. The intracellular domain of BRI1 functions as a kinase and it is this domain triggers the phosphorylation cascade that results in changes of gene expression.^[5]

In the absence of brassinosteroid, BRI1 is held in an inactive state by another protein, BRI1 kinase inhibitor 1 (BKI1).^[6] When brassinosteroid binds to BRI1, it reduces the stability of the BRI1:BKI1 complex and promotes the binding of BRI1 to another membrane protein, BRI1-associated receptor kinase 1 (BAK1).^[7] In the BRI1:BAK1 complex, both BRI1 and BAK1 make contact with the brassinosteroid molecule and for this reason they are considered a co-receptor.^[5][7] BRI1 and BAK1 sequentially phosphorylate each other in their kinase domains, which results in the activation of BRI1. The activated kinase domain of BRI1 phosphorylates several receptor-like cytoplasmic kinases (RLCKs), notably the brassinosteroid signalling kinase (BSK) and constituitive differential growth 1 (CDG1) families. The RLCKs transduce this signal to downstream components, which ultimately results in the activation or de-activation of several transcription factors.^[5]