Chitin-Induced Dimerization Activates a Plant Immune Receptor

Dissecting Chitin Binding The chitin in fungal cells walls serves as a trigger to initiate plant defenses against pathogenic fungi. Arabidopsis detects these signals through a cell surface chitin receptor whose intracellular kinase domain initiates a signaling cascade in response to chitin that activates the plant's response to infection. Liu et al. (p. 1160) have now solved the crystal structure of the Arabidopsis chitin receptor AtCERK1. The results show how chitin binds to the receptor and suggest that the biological response requires dimerisation of the receptor when it binds a chitin oligomer at least seven or eight subunits long. Structural analysis shows how fungus-derived chitin dimerizes its receptor on target plants and triggers defense responses. Pattern recognition receptors confer plant resistance to pathogen infection by recognizing the conserved pathogen-associated molecular patterns. The cell surface receptor chitin elicitor receptor kinase 1 of Arabidopsis (AtCERK1) directly binds chitin through its lysine motif (LysM)–containing ectodomain (AtCERK1-ECD) to activate immune responses. The crystal structure that we solved of an AtCERK1-ECD complexed with a chitin pentamer reveals that their interaction is primarily mediated by a LysM and three chitin residues. By acting as a bivalent ligand, a chitin octamer induces AtCERK1-ECD dimerization that is inhibited by shorter chitin oligomers. A mutation attenuating chitin-induced AtCERK1-ECD dimerization or formation of nonproductive AtCERK1 dimer by overexpression of AtCERK1-ECD compromises AtCERK1-mediated signaling in plant cells. Together, our data support the notion that chitin-induced AtCERK1 dimerization is critical for its activation.