1. Howard Hughes Medical Institute and Plant Biology Laboratory, The Salk Institute for Biological Studies, 10010 N. Torrey Pines Road, La Jolla, California 92037, USA
2. Department of Biology, Faculty of Science, Kyushu University, Ropponmatsu, Fukuoka 810-8560, Japan
3. Plant Functions Lab, RIKEN, Wako-shi, Saitama 351-0198, Japan
4. Plant Science Center, RIKEN, Suehirocho, Tsurumi, Yokohama, Kanagawa 230-0045, Japan
5. These authors contributed equally to this work

Correspondence to: Joanne Chory¹ Correspondence and requests for materials should be addressed to J.C. (Email: chory@salk.edu).

Abstract

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 brassinosteroids^{2, 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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