title: Sequential transphosphorylation of the BRI1/BAK1 receptor kinase complex impacts early events in brassinosteroid signaling.
abstract: 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.
authors.affiliation: Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695, USA.
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Wang
authors.forename: Uma
authors.initials: U
authors.lastname: Kota
authors.forename: Kai
authors.initials: K
authors.lastname: He
authors.forename: Kevin
authors.initials: K
authors.lastname: Blackburn
authors.forename: Jia
authors.initials: J
authors.lastname: Li
authors.forename: Michael B
authors.initials: MB
authors.lastname: Goshe
authors.forename: Steven C
authors.initials: SC
authors.lastname: Huber
authors.forename: Steven D
authors.initials: SD
authors.lastname: Clouse
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000187:drug effects / Q000201:enzymology*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000378:metabolism*; D060406:Brassinosteroids; D002777:Cholestanols / Q000378:metabolism* / Q000494:pharmacology; D002853:Chromatography, Liquid; D005408:Flagellin / Q000378:metabolism; D013058:Mass Spectrometry; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D010766:Phosphorylation / Q000187:drug effects; D011494:Protein Kinases / Q000737:chemistry / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D015398:Signal Transduction / Q000187:drug effects; D013260:Steroids, Heterocyclic / Q000378:metabolism* / Q000494:pharmacology; D013379:Substrate Specificity / Q000187:drug effects
chemical_list: D029681:Arabidopsis Proteins; D060406:Brassinosteroids; D002777:Cholestanols; D013260:Steroids, Heterocyclic; D005408:Flagellin; D011494:Protein Kinases; C463660:BAK1 protein, Arabidopsis; C108270:BRI1 protein, Arabidopsis; D017346:Protein Serine-Threonine Kinases; C023623:brassinolide