Abstract
Both animals and plants use steroids as signaling molecules for their growth and development. Animal steroids are mainly perceived by members of the nuclear receptor superfamily of transcription factors. In plants, BRI1, a plasma membrane-localized leucine-rich repeat (LRR) receptor kinase, is a critical component of a receptor complex for brassinosteroids (BRs). To date, no evidence has shown direct binding of BRs to BRI1. Here, we present evidence for direct binding of active BRs to BRI1 using a biotin-tagged photoaffinity castasterone, (BPCS), a biosynthetic precursor of brassinolide. Binding studies using BPCS and recombinant BRI1 fragments reveal that the minimal binding domain for BRs is a 70 amino acid island domain (ID) localized between LRR21 and LRR22 in the extracellular domain, together with its C-terminal flanking LRR, (ID-LRR22). Our results demonstrate that BRs bind directly to the 94 amino acids comprising ID-LRR22 in the extracellular domain of BRI1.
