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Steroid Receptor Signaling
Plant brassinosteroids signal to diverse pathways in plant physiology. These steroid hormones are perceived at the cell surface, where they bind to the receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1). Santiago et al. (p. 889, published online 8 August) now show that somatic embryogenesis receptor kinase 1 (SERK1) complexes with BRI1. Together, these receptor kinases form the steroid binding site, with the hormone acting as a “molecular glue” that stabilizes the interaction. Hormone-induced heteromerization of BRI1 with SERK1 leads to the activation of the cytoplasmic signaling cascade, triggering plant growth and differentiation.
Brassinosteroids, which control plant growth and development, 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 brassinosteroid signaling events. We found a direct and steroid-dependent interaction between the BRI1 and SERK1 LRR domains by analysis of their complex crystal structure at 3.3 angstrom resolution. We show that the SERK1 LRR domain is involved in steroid sensing and, through receptor–co-receptor heteromerization, in the activation of the BRI1 signaling pathway. Our work reveals how known missense mutations in BRI1 and in SERKs modulate brassinosteroid signaling and the targeting mechanism of BRI1 receptor antagonists.