Negative regulator of brassinosteroid signaling. When associated to the membrane, limits the interaction of BRI1 with BAK1 by binding to the kinase-inactive form of BRI1. Interacts (via C-terminus) with BRI1 (via kinase domain). Once phosphorylated following brassinosteroid treatment, displaced from the plasma membrane into the cytosol where it is inactive. Expressed in leaves, petioles, shoot apices, hypocotyls, roots and flowers. The c-terminal part (253-337) is necessary and sufficient for the interaction with BRI1. Phosphorylated on Tyr-211 in response to brassinosteroid perception, leading to its inactivation: once phosphorylated, displaced into the cytosol where it is inactive. BRI1 is not required for BKI1 association with the plasma membrane, but the BRI1 kinase activity is necessary for the membrane release.