Sequential transphosphorylation of the BRI1/BAK1 receptor kinase complex impacts early events in brassinosteroid signaling

Dev Cell. 2008 Aug;15(2):220-35. doi: 10.1016/j.devcel.2008.06.011.


Brassinosteroids (BRs) regulate plant development through a signal transduction pathway involving the BRI1 and BAK1 transmembrane receptor kinases. The detailed molecular mechanisms of phosphorylation, kinase activation, and oligomerization of the BRI1/BAK1 complex in response to BRs are uncertain. We demonstrate that BR-dependent activation of BRI1 precedes association with BAK1 in planta, and that BRI1 positively regulates BAK1 phosphorylation levels in vivo. BRI1 transphosphorylates BAK1 in vitro on specific kinase-domain residues critical for BAK1 function. BAK1 also transphosphorylates BRI1, thereby quantitatively increasing BRI1 kinase activity toward a specific substrate. We propose a sequential transphosphorylation model in which BRI1 controls signaling specificity by direct BR binding followed by substrate phosphorylation. The coreceptor BAK1 is then activated by BRI1-dependent transphosphorylation and subsequently enhances signaling output through reciprocal BRI1 transphosphorylation. This model suggests both conservation and distinct differences between the molecular mechanisms regulating phosphorylation-dependent kinase activation in plant and animal receptor kinases.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Arabidopsis / drug effects
  • Arabidopsis / enzymology*
  • Arabidopsis Proteins / chemistry
  • Arabidopsis Proteins / metabolism*
  • Brassinosteroids
  • Cholestanols / metabolism*
  • Cholestanols / pharmacology
  • Chromatography, Liquid
  • Flagellin / metabolism
  • Mass Spectrometry
  • Models, Biological
  • Molecular Sequence Data
  • Mutation / genetics
  • Phosphorylation / drug effects
  • Protein Kinases / chemistry
  • Protein Kinases / metabolism*
  • Protein Serine-Threonine Kinases / chemistry
  • Protein Serine-Threonine Kinases / metabolism*
  • Protein Structure, Tertiary
  • Signal Transduction* / drug effects
  • Steroids, Heterocyclic / metabolism*
  • Steroids, Heterocyclic / pharmacology
  • Substrate Specificity / drug effects


  • Arabidopsis Proteins
  • Brassinosteroids
  • Cholestanols
  • Steroids, Heterocyclic
  • Flagellin
  • Protein Kinases
  • BAK1 protein, Arabidopsis
  • BRI1 protein, Arabidopsis
  • Protein Serine-Threonine Kinases
  • brassinolide