Structural insight into brassinosteroid perception by BRI1

Abstract

Brassinosteroids (BRs) are essential phytohormones that play crucial roles in plant growth and development. Perception of BRs requires an active complex of brassinosteroid-insensitive 1 (BRI1) and BRI1-associated kinase 1 (BAK1). Recognized by the extracellular leucine-rich repeat (LRR) domain of BRI1, BRs induce a phosphorylation-mediated cascade to regulate gene expression. Here we present the crystal structures of BRI1-LRR in free and brassinolide (BL)-bound forms. BRI1-LRR exists as a monomer in crystals and solution independent of BL. It comprises a helical solenoid structure that accommodates a separate insertion domain at its concave surface. Sandwiched between them, BL binds to a hydrophobicity-dominating surface groove on BRI1-LRR. BL recognition by BRI1-LRR is through an induced-fit mechanism involving stabilization of two inter-domain loops that creates a pronounced non-polar surface groove for the hormone binding. Together, our results define the molecular mechanisms by which BRI1 recognizes BRs and provide insight into BR-induced BRI1 activation.

Crystal structure of an LRR protein with two solenoids

“…The non-LRR regions from several LRR proteins including BRI1 have been shown to be important for ligand recognition [5,6,13]. However, our crystal structure raises a cautionary note on functional annotations for the non-LRR region of an LRR protein.…”

“…Compared with the canonical ones, these atypical LRR proteins are much less well-characterized structurally. BRI1 is the only atypical LRR protein whose LRR domain structure has been reported [5,6]. Despite that its superhelical structure is markedly different from those of the typical LRR proteins, the interrupted LRRs in BRI1 still pack continuously, forming a single solenoid structure as observed in all the other known LRR structures.…”

“…It is widely believed that LRR proteins adopt a horse shoe-like structure. Although LRR structures with deviations from the canonical structure have been reported [5,6,11,12], the continuous packing of the LRRs in these structures still results in the formation of a single solenoid. Our study shows a unique LRR structure containing two LRR domains, providing experimental evidence for a previous prediction that a non-LRR region-containing LRR protein can contain two solenoids [7].…”

“…We expressed TMK1-LRR (residues 1~469) in insect cells, purified the protein as previously described [5] and solved its crystal structure with molecular replacement. The structure was refined to a resolution of 1.55 Å with R factor of 16.9% and R free of 19.0% (Supplementary information, Table S1).…”

“…In stark contrast to the non-LRR region in BRI1-LRR that contacts the concave inner surface of BRI1-LRR [5,6], the non-LRR region of TMK1-LRR interacts with the spine of the N-LRR ( Figure 1A and 1B). Gln292 is located at the center of the interface and forms bidentate hydrogen bonds with the carbonyl LETTER TO THE EDITOR oxygen of Trp345 (water-mediated) and the main-chain nitrogen of Gly269.…”