Brassinosteroids, which control plant growth and development, are sensed by the membrane receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1). Brassinosteroid binding to the BRI1 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 BRI1 and SERK to interact, trans-phosphorylate and activate each other. Here we report crystal structures of the BRI1 kinase domain in its activated form and in complex with nucleotides. BRI1 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 BRI1 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 BRI1, SERK2 and SERK3 cytoplasmic segments form homodimers in solution and have a weak tendency to heteromerise. We propose a model in which heterodimerisation of the BRI1 and SERK ectodomains brings their cytoplasmic kinase domains in a catalytically competent arrangement, an interaction that can be modulated by the BRI1 inhibitor protein BKI1.

Background: UDP-GlcNAc-2 epimerase/ManNAc kinase is the key enzyme for sialic acid biosynthesis. Results: The crystal structure of ManNAc kinase in complex with its substrate ManNAc has been determined at 1.64 Å resolution. Conclusion: Insights into the mechanism of ManNAc phosphorylation have been gained from the substrate-bound structure. Significance: The new structural information presented here offers a basis for designing potential inhibitors of sialic acid biosynthesis.

Background: Triphosphate tunnel metalloenzymes carry out diverse enzymatic reactions.Results: Two metal co-factors are identified involved in substrate binding and in catalysis.Conclusion: A unified catalytic mechanism is proposed and biochemically investigated.Significance: The functional diversity of TTM enzymes is rationalized by a common mechanism that allows very different substrates to be bound and processed.