Abstract
A plasma membrane-localized leucine-rich repeat (LRR) receptor-lile kinase, BRI1, is a critical component of a receptor complex for a plant hormone, brassinosteroid (BR). However, no evidence had shown the direct binding of BR to BRI1. To analyze the binding mechanism, we prepared biotin-tagged photoaffinity castasterone (BPCS) having a photo-reactive phenyldiazirine and a biotin for the nonradioactive detection of photo-labeled targets. By photoaffnity labeling using BPCS, we clarified that BR binds directly to the 94 amino acids comprising ID-LRR22 in the extracellular domain of BRI1, showing that BRI1 is a BR-receptor [Kinoshita et al. (2005) Nature 410, 167-171]. Photoaffnity labeling is useful and reliable for the identification of the ligand-target protein and the structural investigation of its binding site. We discuss the potential of photoaffinity labeling techniques in investigation of binding mechanisms of plant hormones and their receptors in the context of our success in analysis of BR-BRI1 binding mechanism.
