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Title
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Crystal structures of the phosphorylated BRI1 kinase domain and implications for brassinosteroid signal initiation.
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Authors
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D.Bojar,
J.Martinez,
J.Santiago,
V.Rybin,
R.Bayliss,
M.Hothorn.
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Ref.
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Plant J, 2014,
78,
31-43.
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PubMed id
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Abstract
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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.
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