Receptor with a dual specificity kinase activity acting on both serine/threonine- and tyrosine-containing substrates. Binds brassinolide. Regulates, in response to brassinosteroid binding, a signaling cascade involved in plant development. May be involved in cell growth and vascular differentiation. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Predominantly expressed in vascular tissues. Expressed only during postembryonic development with a very discrete pattern of expression, preferentially in the two protophloem cell files at the elongation zone of the root. The expression in these two cell files attenuates as the phloem cells differentiate in the upper root. In cotyledons and leaves, it is expressed in phloem cells, starting at the cotyledons and shoot apex, moving toward the basal part of the leaves, where the expression is weak. Expressed in the secondary and tertiary veins and in the upper part of the cotyledons and leaves. Weakly or not expressed in the inflorescence stems. Has some complementary expression with BRL1. Contains two pairs of conservatively spaced Cys (Cys pair 1 and 2) possibly involved in forming some heterodimers. A 70 amino acid island between the 18th and the 19th LRR is essential for the binding of brassinosteroids. Autophosphorylated on Tyr and Thr residues. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.