Brassinosteroids are plant steroid hormones that control a variety of processes such as stem elongation and stress resistance. Brassinolide (BL), the most active brassinosteroid in the plant Arabidopsis thaliana, binds to a cell surface receptor with serine-threonine kinase activity called Brassinosteroid insensitive 1 (BRI1). This interaction induces BRI1 phosphorylation, but the mechanism by which this modification occurs has not been clear. Wang et al. expressed full-length and truncated mutant forms of BRI1 in plants that were defective in sensing BR and exhibited a dwarf phenotype. Expression of a receptor lacking the C terminus (CT) of its intracellular domain rescued the growth defects of the dwarf plants to a greater extent than expression of the full-length receptor, which suggests that the CT negatively regulates receptor activity. Absence of the CT also increased BRI1 signaling, determined by monitoring the modification of BES1, a downstream regulator of BR-responsive gene expression. Lack of the CT also enhanced the kinase activity of the intracellular domain in vitro. The authors also used phosphopeptide mapping to identify multiple putative phosphorylation sites within the CT. Mutation of these residues to Asp, which can mimic phosphorylation, increased BRI1 activity in plants, which indicates that phosphorylation plays a role in BRI1 function. The authors also suggest that BRI1 forms a homodimer and propose that upon binding to BL, a conformational change in the receptor's intracellular domain could facilitate transphosphorylation of the CT domains. This would release autoinhibition of the receptor and allow further autophosphorylation. The homodimer could then associate with BAK1, another membrane receptor that has been implicated in BRI1 signaling.
X. Wang, X. Li, J. Meisenhelder, T. Hunter, S. Yoshida, T. Asami, J. Chory, Autoregulation and homodimerization are involved in the activation of the plant steroid receptor BRI1. Devel. Cell 8, 855-865 (2005). [PubMed]