Structural basis of OsCERK1-mediated signal activation and transduction in rice immunity and symbiosis.

Chitin elicitor receptor kinase 1 (OsCERK1) from Oryza sativa plays a central role in coordinating symbiotic and immune responses by recognizing fungal chitin fragments of different lengths. Although the extracellular domain of OsCERK1 has been well characterized with respect to chitin recognition, the structural basis underlying intracellular activation and downstream signal transduction remains poorly understood. In this study, we determined nine crystal structures of truncated OsCERK1 cytoplasmic kinase domains in either phosphorylated or dephosphorylated states, resolved in the absence of nucleotide or in complex with ADP, ATP, or the ATP analog AMP-PNP. Structural analyses reveal that OsCERK1 consistently adopts an intermediate kinase conformation characterized by an αC helix-out and DFG-in configuration, regardless of phosphorylation status or nucleotide binding. Functional assays further demonstrate that both the symbiotic receptor OsMYR1 and the receptor-like cytoplasmic kinase OsRLCK185 act as substrates that allosterically enhance OsCERK1 activity, with OsMYR1 exerting a stronger activation effect. This enhancement correlates with substrate-binding affinity, whereas phosphorylation does not uniformly increase substrate association. Notably, intermolecular autophosphorylation of OsCERK1 markedly elevates its catalytic activity. In addition, T479, T484, and Y492 within the activation segment are identified as critical residues required for OsCERK1 catalytic activity and substrate phosphorylation. Collectively, these findings support a dual regulatory model in which autophosphorylation enhances catalytic capacity, while substrate binding promotes allosteric activation. This study provides structural insight into OsCERK1 activation and establishes a mechanistic framework for receptor-like kinase-mediated signaling in plant immunity and symbiosis.