Brassinosteroids,a group of plant steroid hormones,reg-ulate many aspects of plant growth and development.We and other have previously solved the crystal structures of BRI1(LRR)in complex with brassinolide,the most active brassinosteroid identifi ed thus far.Although these studies provide a structural basis for the recognition of brassi-nolide by its receptor BRI1,it still remains poorly under-stood how the hormone differentiates among its con-served receptors.Here we present the crystal structure of the BRI1 homolog BRL1 in complex with brassinolide.The structure shows that subtle differences around the brassinolide binding site can generate a striking effect on its recognition by the BRI1 family of receptors.Structural comparison of BRL1 and BRI1 in their brassinolide-bound forms reveals the molecular basis for differential binding of brassinolide to its different receptors,which can be used for more effi cient design of plant growth regulators for agricultural practice.On the basis of our structural studies and others’data,we also suggest possible mech-anisms for the activation of BRI1 family receptors.
Leucine rich repeat(LRR)domain,characterized by a repetitive sequence pattern rich in leucine residues,is a universal protein-protein interaction motif present in all life forms.LRR repeats interrupted by sequences of 30 70 residues(termed island domain,ID)have been found in some plant LRR receptor-like kinases(RLKs)and animal Toll-like receptors(TLR7-9).Recent studies provide insight into how a single ID is structurally integrated into an LRR protein.However,structural information on an LRR protein with two IDs is lacking.The receptor-like protein kinase 2(RPK2)is an LRR-RLK and has important roles in controlling plant growth and development by perception and transduction of hormone signal.Here we present the crystal structure of the extracellular LRR domain of RPK2(RPK2-LRR)containing two IDs from Arabidopsis.The structure reveals that both of the IDs are helical and located at the central region of the single RPK2-LRR solenoid.One of them binds to the inner surface of the solenoid,whereas the other one mainly interacts with the lateral side.Unexpectedly,a long loop immediately following the N-terminal capping domain of RPK2-LRR is presented toward and sandwiched between the two IDs,further stabilizing their embedding to the LRR solenoid.A potential ligand binding site formed by the two IDs and the solenoid is located at the C-terminal side of RPK2-LRR.The structural information of RPK2-LRR broadens our understanding toward the large family of LRR proteins and provides insight into RPK2-mediated signaling.
