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Binding of brassinosteroids to the extracellular domain of plant receptor kinase BRI1


Both animals and plants use steroids as signalling molecules during growth and development. Animal steroids are principally recognized by members of the nuclear receptor superfamily of transcription factors1. In plants, BRI1, a leucine-rich repeat (LRR) receptor kinase localized to the plasma membrane, is a critical component of a receptor complex for brassinosteroids2,3. Here, we present the first evidence for direct binding of active brassinosteroids to BRI1 using a biotin-tagged photoaffinity castasterone (BPCS), a biosynthetic precursor of brassinolide (the most active of the brassinosteroids). Binding studies using BPCS, 3H-labelled brassinolide and recombinant BRI1 fragments show that the minimal binding domain for brassinosteroids consists of a 70-amino acid island domain (ID) located between LRR21 and LRR22 in the extracellular domain of BRI1, together with the carboxy-terminal flanking LRR (ID-LRR22). Our results demonstrate that brassinosteroids bind directly to the 94 amino acids comprising ID-LRR22 in the extracellular domain of BRI1, and define a new binding domain for steroid hormones.

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Figure 1: Biotin-tagged photoaffinity castasterone (BPCS) specifically binds to BRI1–GFP.
Figure 2: ID-LRR22 in the extracellular domain of BRI1 is sufficient for BPCS binding.
Figure 3: Specific binding of BPCS with ID-LRRs from BRL1 and BRL3.


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We thank S. Richard and J. Noel for discussions, and Y. Zhao and S. Mora-Garcı´a for reading the manuscript and providing critical comments. This work was funded by grants from the USDA and HFSP to J.C., by a Grant-in-Aid for Young Scientist (A) to T.K. and Grant-in-Aid for scientific research (C) to H.S. from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and by an HFSP fellowship to A.C.D. J.C. is an Investigator of the Howard Hughes Medical Institute.

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Correspondence to Joanne Chory.

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Supplementary information

Supplementary Figure S1

Preparation of biotin-tagged photoaffinity castasterone (BPCS). (PDF 341 kb)

Supplementary Figure S1 Legend

Text to accompany the above Supplementary Figure. (PDF 50 kb)

Supplementary Figure S2

Competition for [3H]-labelled BL binding to membrane fractions of BRI1–GFP plants by BL and BPCS. Includes legend for Supplementary Figure S2. (PDF 301 kb)

Supplementary Figure S3

3H-labelled BL binding in membrane fractions from bri1-119. Includes legend for Supplementary Figure S3. (PDF 241 kb)

Supplementary Figure S4

3H–BL finding is unaltered in a bak1 null mutant. Includes legend for Supplementary Figure S4. (PDF 61 kb)

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Kinoshita, T., Caño-Delgado, A., Seto, H. et al. Binding of brassinosteroids to the extracellular domain of plant receptor kinase BRI1. Nature 433, 167–171 (2005).

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