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

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.