Structural basis for differential recognition of brassinolide by its receptors

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.