Reports

Plants have membrane-integral receptor kinases that sense diverse extra-cellular signals, ranging from steroid hormones (1) to peptides (2) and proteins (3). The steroid receptor kinase BRI1, which controls plant growth and development (4), binds the hormone brassinolide (BL) with its LRR ectodomain (5–8). BL binding to BRI1 causes ordering of a ~70-residue island domain that together with the LRR core may form a docking platform for a co-receptor protein (7, 8). Based on the size of the envisioned interaction surface and its proximity to the membrane, likely co-receptor candidates are the SERK family of plant receptor ki-nases. SERK1 and SERK3 are genetic components of the BR signaling pathway (9–12). BRI1 and SERK3 associate in a ligand-dependent man-ner (13), and this interaction can be disrupted by the BRI1 inhibitor pro-tein BKI1 (14). The BRI1 and SERK3 kinase domains transphosphorylate each other upon steroid sensing (13). In order to study the interaction between BRI1 and SERK-family proteins, we first characterized the bri1sud1 gain-of-function allele (15) in its molecular detail. A 2.5 Å crystal structure of bri1sud1 (table S1) identi-fies the Gly643 to Glu point-mutation to be involved in the stabilization of the island domain that we previously suggested to be crucial for the interaction with the co-receptor (8) (Fig. 1A). Glu643 in bri1sud1 contrib-utes to a hydrogen-bonding network in the island domain β-sheet, which is part of the hormone binding pocket (green in Fig. 1A). The bri1sud1 – BL complex indicates that bri1sud1 cannot signal independent of ligand, but rather stabilizes the hormone-bound state of the receptor. Consistent with this observation, we find that bri1sud1 plants are only partially re-sistant to the brassinosteroid biosynthesis inhibitor brassinazole (16) (Fig. 1B). We next produced the isolated SERK1 ectodomain (residues 24-213) by secreted expression in insect cells and obtained crystals after mutat-ing two of the five N-glycosylation sites (Asn115Asp, Asn163Gln). A 1.5 Å crystal structure of SERK1 (table S1) reveals the presence of 5 LRRs and the canonical N- and C-terminal capping motifs found in plant LRR proteins (7, 8, 17) (Fig. 1C). The size of the SERK1 LRR domain is in good agreement with the size of the envisioned docking platform i