Abstract
While host immune receptors detect pathogen-associated molecular patterns to activate immunity, pathogens attempt to deregulate host immunity through secreted effectors. Fungi employ LysM effectors to prevent recognition of cell wall-derived chitin by host immune receptors, although the mechanism to compete for chitin binding remained unclear. Structural analysis of the LysM effector Ecp6 of the fungal tomato pathogen Cladosporium fulvum reveals a novel mechanism for chitin binding, mediated by intrachain LysM dimerization, leading to a chitin-binding groove that is deeply buried in the effector protein. This composite binding site involves two of the three LysMs of Ecp6 and mediates chitin binding with ultra-high (pM) affinity. Intriguingly, the remaining singular LysM domain of Ecp6 binds chitin with low micromolar affinity but can nevertheless still perturb chitin-triggered immunity. Conceivably, the perturbation by this LysM domain is not established through chitin sequestration but possibly through interference with the host immune receptor complex. DOI:http://dx.doi.org/10.7554/eLife.00790.001.
Highlights
Fungi constitute an evolutionarily and ecologically diverse group of microorganisms
Structural analysis of the lysin motifs (LysMs) effector Ecp6 of the fungal tomato pathogen Cladosporium fulvum reveals a novel mechanism for chitin binding, mediated by intrachain LysM dimerization, leading to a chitin-binding groove that is deeply buried in the effector protein
Our structural and biochemical analysis of the LysM effector Ecp6 has unveiled a novel mechanism for chitin binding that evolved in fungi, in which the concerted action of two LysM domains results in sequestration of a chitin oligomer with ultra-high affinity
Summary
Fungi constitute an evolutionarily and ecologically diverse group of microorganisms. most species are saprophytic, many are causative agents of disease and include plant pathogens that cause considerable yield losses in agricultural crops worldwide (Skamnioti and Gurr, 2009; Oliver, 2012). An N-acetyl-D-glucosamine (GlcNAc) homopolymer, is the primary structural component of fungal cell walls and is recognized as a PAMP by plant cell surface receptors that contain extracellular lysin motifs (LysMs) (Felix et al, 1993; Shibuya et al, 1993; Shibuya et al, 1996; Kombrink et al, 2011). Arabidopsis thaliana CERK1 binds chitin and is required for chitintriggered immunity (Miya et al, 2007; Wan et al, 2008; Iizasa et al, 2010; Petutschnig et al, 2010). No homologs of rice CEBiP could be implicated in chitin-triggered immunity against fungal infection in Arabidopsis (Shinya et al, 2012; Wan et al, 2012). LysM2 of AtCERK1 binds three GlcNAc residues of a longer chitin oligomer in a shallow groove on the surface of the protein, with both
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