Molecular Mechanism for Plant Steroid Receptor Activation by Somatic Embryogenesis Co-Receptor Kinases

Brassinosteroids (BRs) control plant growth and development. BRs are sensed by the leucine-rich repeat (LRR) domain of the membrane receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1), but it is unknown how steroid binding at the cell surface activates the cytoplasmic kinase domain of the receptor. A family of somatic embryogenesis receptor kinases (SERKs) has been genetically implicated in mediating early BR-signaling events. Here, we present evidence for a direct and steroid-dependent interaction between the BRI1 and SERK1 LRR domains by analysis of their complex crystal structure at 3.3 A resolution. We show that the SERK1 LRR domain is involved in steroid sensing and, through receptor-co-receptor heteromerisation, in the activation of the BRI1 signaling pathway. Our work rationalizes the effects of known mis-sense mutations in BRI1 and in SERKs and the targeting mechanism of BRI1 receptor antagonists.