LYK4 is a component of a tripartite chitin receptor complex in Arabidopsis thaliana

LysM receptor-like kinases (LYKs) of Arabidopsis thaliana (namely LYK1, LYK4 and LYK5) play a major role in chitin perception and immunity against pathogenic fungi. Chitin-induced heterodimerization of LYK1 and LYK5 has been previously reported, but protein interaction partners of LYK4 have not yet been identified. In this study, by analysing mutants we confirmed a role of LYK4 in chitin perception, and found that the ectodomain of LYK4 homodimerizes and also interacts with the ectodomain of LYK5 in vitro. Pull-down experiments with proteins expressed in protoplasts indicated LYK4-LYK4 and LY4-LYK5 interactions in planta. When protoplasts were treated with chitoheptaose or chitin, a protein complex was immunoprecipitated that appeared to be composed of LYK1, LYK4, and LYK5. Similar experiments with proteins expressed in lyk mutant plants suggested that elicitor treatment induced a physical interaction between LYK1 and LYK5 but not between LYK1 and LYK4. Bimolecular fluorescence complementation experiments substantiated these findings. Overall, our data suggest that LYK4 functions as a LYK5-associated co-receptor or scaffold protein that enhances chitin-induced signaling in Arabidopsis.