Phosphorylation-dependent routing of RLP44 towards brassinosteroid or phytosulfokine signalling

Plants rely on a complex network of cell surface receptors to integrate developmental and environmental cues into behaviour adapted to the conditions. The largest group of these receptors, leucine-rich repeat receptor-like kinases, form a complex interaction network that is modulated and extended by receptor-like proteins. This raises the question of how specific outputs can be generated when receptor proteins are engaged in a plethora of promiscuous interactions. RECEPTOR-LIKE PROTEIN 44 (RLP44) acts to promote both brassinosteroid and phytosulfokine signalling, which orchestrate a wide variety of cellular responses. However, it is unclear how these activities are coordinated. Here, we show that RLP44 is phosphorylated in its highly conserved C-terminal cytosolic tail and that this post-translational modification governs its subcellular localization. RLP44 variants in which phosphorylation is blocked enter endocytosis prematurely, leading to an almost entirely intracellular localization, whereas phospho-mimicking or ectopic phosphorylation results in preferential RLP44 localization at the plasma membrane. Phosphorylation of the C-terminus is essential for brassinosteroid-associated functions of RLP44. In contrast, RLP44’s role in phytosulfokine signalling is not affected by its phospho-status. Detailed mutational analysis suggests that phospho-charge, rather than modification of individual amino acids determines routing of RLP44 to its target receptor complexes, providing a framework to understand how a common component of different receptor complexes can get specifically engaged in a particular signalling pathway.