title: Structural basis of steroid hormone perception by the receptor kinase BRI1.
abstract: Polyhydroxylated steroids are regulators of body shape and size in higher organisms. In metazoans, intracellular receptors recognize these molecules. Plants, however, perceive steroids at membranes, using the membrane-integral receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1). Here we report the structure of the Arabidopsis thaliana BRI1 ligand-binding domain, determined by X-ray diffraction at 2.5 Å resolution. We find a superhelix of 25 twisted leucine-rich repeats (LRRs), an architecture that is strikingly different from the assembly of LRRs in animal Toll-like receptors. A 70-amino-acid island domain between LRRs 21 and 22 folds back into the interior of the superhelix to create a surface pocket for binding the plant hormone brassinolide. Known loss- and gain-of-function mutations map closely to the hormone-binding site. We propose that steroid binding to BRI1 generates a docking platform for a co-receptor that is required for receptor activation. Our findings provide insight into the activation mechanism of this highly expanded family of plant receptors that have essential roles in hormone, developmental and innate immunity signalling.
authors.affiliation: Plant Biology Laboratory, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, California 92037, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Hothorn
authors.forename: Youssef
authors.initials: Y
authors.lastname: Belkhadir
authors.forename: Marlene
authors.initials: M
authors.lastname: Dreux
authors.forename: Tsegaye
authors.initials: T
authors.lastname: Dabi
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Noel
authors.forename: Ian A
authors.initials: IA
authors.lastname: Wilson
authors.forename: Joanne
authors.initials: J
authors.lastname: Chory
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000737:chemistry* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites; D060406:Brassinosteroids; D002777:Cholestanols / Q000737:chemistry / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004789:Enzyme Activation; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010937:Plant Growth Regulators / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D011494:Protein Kinases / Q000737:chemistry* / Q000378:metabolism*; D055503:Protein Multimerization; D017434:Protein Structure, Tertiary; D013260:Steroids, Heterocyclic / Q000737:chemistry / Q000378:metabolism*; D013329:Structure-Activity Relationship
chemical_list: D029681:Arabidopsis Proteins; D060406:Brassinosteroids; D002777:Cholestanols; D010937:Plant Growth Regulators; D013260:Steroids, Heterocyclic; D011494:Protein Kinases; C108270:BRI1 protein, Arabidopsis; C023623:brassinolide