Plants own a large number of plasma membrane receptor kinases (RKs) to control growth and development. Plant RKs share a common domain organization with the animal membrane receptors and similarly undergo endocytosis. Ligand-triggered endocytosis of RKs, also known as receptormediated endocytosis, is a common mechanism for signaling attenuation in animals. In this project we will use the best-studied RK in Arabidopsis, the receptor for brassinosteroid hormones BRASSINOSTEROID INSENSITIVE 1 (BRI1), as a model system to address signaling attenuation in plants. Whereas, BRI1 activation is well understood, a crucial question still to be addressed is how BRI1 is downregulated. Our laboratory has established that clathrin-dependent mechanisms regulate the removal of activated BRI1 from the plasma membrane but is unclear when, where and if BRI1 is dephosphorylated. In this project we will address the dynamics of BRI1 dephosphorylation following activation. Next, we will perform a more in depth analysis of how phosphorylation regulates endocytosis of BRI1 and if it determines the interaction with selected endocytic components required for internalization. The endocytic machinery of BRI1 will be identified by using proteomics approaches. These studies will establish the role of endocytosis in BRI1 downregulation and it will provide new insights into receptor-mediated signaling control in plants.