Brassinosteroid receptor activation

Brassinosteroids are hormones that are important for plant growth, development and immune responses. They are sensed by the transmembrane receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1) when they bind to its extracellular Leu-rich repeat (LRR) domain. Brassinosteroid binding to BRI1 is known to induce conformational changes that create a platform in BRI1 for co-receptor binding. Somatic embryogenesis receptor kinases (SERKs; which are also receptor kinases with LRR ectodomains) have been implicated in mediating brassinosteroid signalling, and their size and membrane localization makes them BRI1 co-receptor candidates. This study reveals that BRI1 and SERK1 interact to activate BRI1 signalling. The authors solved the crystal structure of a ternary complex formed by BRI1, the LRR ectodomain of SERK1 and the hormone at 3.3 Å resolution. Their findings indicate that the SERK1 LRR domain forms an integral part of the ligand-binding pocket of the receptor and thus that BRI1 and SERK1 interact in a ligand-dependent manner. Therefore, the activation of the cytoplasmic kinase domain of BRI1, the underlying mechanism of which was unclear, involves BRI1–SERK1 heterodimerization following extracellular hormone binding at the cell surface.