How plant lysin motif receptors get activated: lessons learned from structural biology.

Lysin motif (LysM) receptor kinases are unique to plants and serve important functions in plant-microbe interactions. These proteins recognize microbe-derived N-acetylglucosamine (NAG)-containing ligands, but the molecular mode of ligand perception and of receptor activation has remained unknown. The three-dimensional structure of the LysM receptor kinase CERK1 (chitin elicitor receptor kinase 1) from Arabidopsis thaliana has been reported. CERK1 binds NAG oligomers derived from chitin-the major constituent of fungal cell walls-and mediates immunity to fungal infection. The crystal structure of CERK1 complexed with a NAG pentamer revealed that three NAG moieties attach tightly to one of three lysin motifs within the CERK1 ectodomain. Receptor activation and immune signaling requires, however, ligand-induced CERK1 homodimerization. By acting as bivalent ligands, NAG octamers stabilize CERK1 dimers, providing a structural explanation for why the immunogenic activity of NAG oligomers is restricted to fragments larger than those required for receptor binding. Because CERK1 might serve as a paradigm for the functionality of a whole class of plant LysM proteins, insight into its mode of action will direct future research on these receptors.