Molecular Mechanism for Fungal Cell Wall Recognition by Rice Chitin Receptor OsCEBiP

Abstract

Chitin is the major component of fungal cell wall and serves as a molecular pattern that can be recognized by the receptor OsCEBiP in rice, a lysine motif (LysM) receptor-like protein (RLP), to trigger immune responses. The molecular mechanisms underlying chitin recognition remain elusive. Here we report the crystal structures of the ectodomain of OsCEBiP (OsCEBiP-ECD) in free and chitin-bound forms. The structures reveal that OsCEBiP-ECD contains three tandem LysMs followed by a novel structure fold of cysteine-rich domain. The structures showed that chitin binding induces no striking conformational changes in OsCEBiP. Structural comparison among N-acetylglucosamine (NAG) oligomer-bound LysMs revealed a highly conserved recognition mechanism, which is expected to facilitate study of other LysM-containing proteins for their NAG binding. Modeling study showed that chitin induces OsCEBiP homodimerization in a "sliding mode". Our data provide insights into rice chitin receptor-mediated immunity triggered by fungal cell wall.

The Structural Basis of Ligand Perception and Signal Activation by Receptor Kinases

“…In the AtCERK1 crystal structure, (NAG) 5 binds in an extended conformation and makes extensive hydrogen bond interactions, mainly with backbone atoms in the AtCERK1 LysM2 binding site, several of which are mediated by water molecules (77) (Figure 3c). A highly similar assembly of three LysM domains is also present in OsCEBiP, with LysM2 again providing the chitin-binding site (76) (Figure 3b). In addition to its cloverleaf-shaped LysM assembly, OsCEBiP features a C-terminal antiparallel β-domain of unknown function (cysteine-rich domain; see Figure 3b,d ) (76).…”

“…A highly similar assembly of three LysM domains is also present in OsCEBiP, with LysM2 again providing the chitin-binding site (76) (Figure 3b). In addition to its cloverleaf-shaped LysM assembly, OsCEBiP features a C-terminal antiparallel β-domain of unknown function (cysteine-rich domain; see Figure 3b,d ) (76). It is unclear whether the remaining LysM domains in AtCERK1 and OsCEBiP also bind chitin or other NAG-containing ligands, as reported for symbiotic plant LysM-RKs (79) and for fungal effector proteins (29).…”

“…It is unclear whether the remaining LysM domains in AtCERK1 and OsCEBiP also bind chitin or other NAG-containing ligands, as reported for symbiotic plant LysM-RKs (79) and for fungal effector proteins (29). It also remains to be determined what types of carbohydrates are sensed by OsCERK1, which does not detectably bind chitin oligomers in vitro (76). All plant LysM domains show a high degree of structural conservation with bacterial LysM proteins, such as the bacterial endopeptidase NlpC/P60 (134) (Figure 3e).…”

“…However, recent evidence also supports heterodimerization models of AtCERK1 with the high-affinity chitin RK AtLYK5 (18) (Figure 4c). The roles of LysM-containing RLPs such as OsCEBiP remain to be clarified; although OsCEBiP interacts with chitin tetramers with micromolar affinity via its LysM2 domain, whether OsCEBiP by itself can undergo ligand-dependent dimerization is not known (44,76). In planta experiments have suggested that LysM-RLPs (such as OsCEBiP) or LysM-RKs with cytoplasmic pseudokinase modules may interact with active LysM-RKs in larger signaling complexes, the significance of which remains to be characterized (18,107) (Figure 4b).…”

Receptor Kinases in Plant-Pathogen Interactions: More Than Pattern Recognition

“…However, this does not explain why shorter chitin oligomers do not induce dimerization. Whether CEBiP possesses significantly different affinities toward short and long chitin oligomers remains controversial, in part because different fragments of CEBiP-ECD were used in different studies (Hayafune et al, 2014;S. Liu et al, 2016).…”