Disulfide-bond-driven neck-ring-like conformation mediates SCREW recognition in plant immunity.

Secreted cysteine-rich peptides (CRPs) are vital plant signalling molecules, yet how their essential intramolecular disulfide bonds structurally mediate receptor complex activation is poorly understood. Here we present the crystal structure of an Arabidopsis immune complex comprising the CRP SMALL PHYTOCYTOKINES REGULATING DEFENSE AND WATER LOSS (SCREW), the receptor PLANT SCREW UNRESPONSIVE RECEPTOR (NUT) and the coreceptor BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1). Unlike typical multi-disulfide CRPs with compact folds, SCREW maintains a flexible loop constrained into a neck-ring-like conformation through stabilization by a single disulfide bond and a critical proline residue. In the complex, SCREW's carboxy-terminal cyclic region inserts between NUT and BAK1, burying a large surface area on BAK1. Disrupting this neck-ring-like conformation or its key interfaces abolishes complex assembly and downstream signalling. This assembly mechanism is conserved in rapeseed and probably among other dicots. Our work reveals a distinct disulfide-dependent conformation, critical for receptor activation and potentially common among two-cysteine CRPs.