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Summary  Brassinosteroids, which control plant growth and development, are sensed by the membrane receptor kinase  BRASSINOSTEROID INSENSITIVE  1 ( BRI 1). Brassinosteroid binding to the  BRI 1 leucine‐rich repeat ( LRR ) domain induces heteromerisation with a  SOMATIC EMBRYOGENESIS RECEPTOR KINASE  ( SERK )‐family co‐receptor. This process allows the cytoplasmic kinase domains of  BRI 1 and  SERK  to interact,  trans ‐phosphorylate and activate each other. Here we report crystal structures of the  BRI 1 kinase domain in its activated form and in complex with nucleotides.  BRI 1 has structural features reminiscent of both serine/threonine and tyrosine kinases, providing insight into the evolution of dual‐specificity kinases in plants. Phosphorylation of Thr1039, Ser1042 and Ser1044 causes formation of a catalytically competent activation loop. Mapping previously identified serine/threonine and tyrosine phosphorylation sites onto the structure, we analyse their contribution to brassinosteroid signaling. The location of known genetic missense alleles provide detailed insight into the  BRI 1 kinase mechanism, while our analyses are inconsistent with a previously reported guanylate cyclase activity. We identify a protein interaction surface on the C‐terminal lobe of the kinase and demonstrate that the isolated  BRI 1,  SERK 2 and  SERK 3 cytoplasmic segments form homodimers in solution and have a weak tendency to heteromerise. We propose a model in which heterodimerisation of the  BRI 1 and  SERK  ectodomains brings their cytoplasmic kinase domains in a catalytically competent arrangement, an interaction that can be modulated by the  BRI 1 inhibitor protein  BKI 1.

Crystal structures of the phosphorylated BRI 1 kinase domain and implications for brassinosteroid signal initiation