Receptor with a serine/threonine-protein kinase activity, which may transduce extracellular spatial and temporal signals into downstream cell differentiation responses in provascular and procambial cells. In contrast to BRI1, BRL1 and BRL3, it does not bind brassinolide. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with TTL3. Expressed in provascular and procambial sites throughout plant development. Expressed throughout globe- to heart-staged embryos. Then, it is restricted to procambial cells by the late torpedo stage, and this pattern persists throughout the duration of embryo development. After germination, it is expressed not only in procambial cells throughout the plant but also in all lateral organ primordia before the onset of vascularization. Contains two pairs of conservatively spaced Cys (Cys pair 1 and 2) possibly involved in forming some heterodimers. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.