Single Disulfide Bond Sculpts SCREW Peptide for Plant Immune Receptor Activation

Plants defend themselves not only with hard physical barriers but with an elaborate chemical language built from tiny signalling peptides. When pathogens attack, plant cells secrete small cysteine-rich peptides, or CRPs, into the extracellular space, where they act as phytocytokines—immune-modulating messengers that rally neighbouring cells into a coordinated defensive state. Yet despite decades of study, a fundamental question has remained stubbornly unresolved: how exactly do these miniature peptides, folded and locked into shape by internal disulfide bridges, physically trigger the membrane-bound receptor complexes that launch the immune response? A new study published in Nature Plants now provides the most detailed answer to date, revealing an entirely unexpected structural mechanism in which a single disulfide bond forces a peptide into a ring-like configuration that is essential for immune signalling.

The research, led by Zhiyun Wang, Lihao Wan, Siqi Tang, Xiao Yu and Shutong Xu of Huazhong Agricultural University in Wuhan, China, focused on a signalling peptide called SCREW—short for SMALL PHYTOCYTOKINES REGULATING DEFENSE AND WATER LOSS. SCREW belongs to a recently identified family of phytocytokines that helps plants balance two competing priorities: sealing stomata and mounting chemical defences during infection, while still regulating water loss and growth. Previous work had shown that SCREW is perceived at the cell surface by a receptor kinase known as NUT, or PLANT SCREW UNRESPONSIVE RECEPTOR, which partners with a shared co-receptor called BAK1, a central hub of plant innate immunity. But the molecular choreography by which SCREW brings these two receptor proteins together had never been visualized.

To capture the interaction in atomic detail, the team used X-ray crystallography to solve the three-dimensional structure of a ternary immune complex from the model plant Arabidopsis thaliana, comprising the extracellular domains of NUT and BAK1 bound to the SCREW peptide. The resulting crystal structure, deposited in the Protein Data Bank, exposes for the first time the precise geometry of the ligand-receptor-coreceptor assembly—and it delivered a surprise. Unlike most well-characterized cysteine-rich peptides, which contain multiple disulfide bonds that fold them into compact, rigid structures, SCREW carries only two conserved cysteine residues forming a single intramolecular disulfide bond. That lone bond, together with a strategically placed proline residue, constrains the peptide’s flexible loop into what the researchers describe as a neck-ring-like or noose-like conformation.

This ring architecture turns out to be far more than a structural curiosity. Within the complex, the carboxy-terminal cyclic region of SCREW wedges itself into the junction between NUT and BAK1, wedging the two receptor ectodomains together while burying an unusually large contact surface on BAK1. In effect, the peptide acts as a molecular clasp: its ringed C-terminus is gripped by the receptor, while the same region simultaneously makes extensive interactions with the co-receptor, nucleating the assembly of the active signalling complex. Mutating the disulfide-forming cysteines, disrupting the critical proline, or altering any of the key interface residues abolished both complex formation in vitro and downstream immune signalling in planta, including MAP kinase activation—demonstrating that the neck-ring conformation is not merely permissive but strictly required for receptor activation.

The finding challenges a long-standing assumption in the field. Because CRPs with multiple disulfide bonds—such as defensins, EPF stomatal peptides, RALF peptides and pollen-attracting LURE peptides—fold into well-defined compact folds that are recognized as intact structural units, researchers have largely assumed that all CRP ligands operate this way. The new structure shows that a two-cysteine CRP can instead rely on a single disulfide to create a constrained loop that functions as a recognition epitope in its own right. The team’s biochemical analyses, including thiol-labeling assays of both recombinant and synthetic SCREW peptides, confirmed that the disulfide bond is formed in the extracellular environment, and surface plasmon resonance experiments quantified how each mutation erodes binding affinity, often eliminating detectable interaction altogether.

Structural comparisons with previously solved receptor complexes sharpened the picture. When the researchers superposed their NUT-SCREW-BAK1 structure onto related complexes such as HAESA-IDA-SERK1, HSL1-IDL1-SERK1, MIK2-SCOOP12-BAK1, PXY-CLE41-SERK2 and FLS2-flg22-BAK1, the overall architecture resembled the canonical mode in which a linear peptide bridges a receptor and a SERK-family co-receptor. But SCREW departs sharply from the mode seen in other receptor-CRP complexes, such as ERL1-EPF1-TMM or PRK6-LURE1.2, where multi-disulfide peptides bind through their folded cores. SCREW thus occupies an intriguing middle ground—structurally a CRP, but functionally behaving much like a linear peptide whose conformational constraint is imposed by disulfide chemistry rather than encoded in a compact fold.

Conservation across species adds an evolutionary dimension to the work. The team showed that the same assembly mechanism operates in rapeseed (Brassica napus), an economically vital crop, where BnSCREW1 induces heterodimerization of BnNUT and BnBAK1 in the same ring-mediated manner. Sequence alignments spanning the Brassicaceae, Solanaceae, Fabaceae and Poaceae families revealed that the two disulfide-forming cysteines and the structural proline are strictly conserved, and AlphaFold3-predicted models of NUT-SCREW-BAK1 complexes from pepper, potato, barrel medic and common bean reproduced the same ternary architecture with high confidence. The authors conclude that this disulfide-dependent recognition mechanism is probably widespread among dicot plants, suggesting it represents a general design principle for two-cysteine CRP signalling.

The implications extend well beyond basic structural biology. BAK1 is a co-receptor shared by dozens of immune and developmental receptor kinases, and understanding how different ligands engage it illuminates how plants achieve specificity with a limited molecular toolkit. Because SCREW signalling regulates both stomatal immunity and water loss, dissecting its activation mechanism at the atomic level could inform strategies for engineering disease-resistant crops that maintain yield under pathogen pressure and drought. The detailed map of the NUT-SCREW-BAK1 interfaces—now verifiable down to individual residues—provides a rational template for designing peptide analogues or screening for small molecules that tune this signalling axis in agriculturally important species.

The study also offers methodological lessons for the broader receptor-kinase community. The researchers had to engineer a functional quadruple-mutant version of the NUT ectodomain to obtain diffraction-quality crystals, carefully verifying by SPR that the engineered protein retained wild-type binding behaviour. Their structural comparisons drew on a decade of crystallographic work on plant receptor complexes, and their hybrid approach—combining crystallography, SPR kinetics, pull-down assays, size-exclusion chromatography, MAP kinase readouts and AlphaFold3 modelling—illustrates how modern structural biology integrates complementary techniques to resolve transient extracellular assemblies. What emerges is a vivid picture of plant immune signalling at the atomic scale: a tiny peptide, shaped like a molecular ring by one covalent bond and one proline, snapping two receptor proteins together to sound the alarm. It is a reminder that in biology, the smallest molecules often carry the heaviest structural burdens—and that a single disulfide bond can be the difference between silence and a full-blown immune response.

Subject of Research: Structural mechanism of SCREW peptide recognition by the NUT-BAK1 receptor complex in plant immunity

Article Title: Disulfide-bond-driven neck-ring-like conformation mediates SCREW recognition in plant immunity

Article References: Wang, Z., Wan, L., Tang, S., Wang, X., Yang, Y., Wu, H., Zhang, S., Yu, X., & Xu, S. (2026). Disulfide-bond-driven neck-ring-like conformation mediates SCREW recognition in plant immunity. Nature Plants, 12(9), 1756-1768. https://doi.org/10.1038/s41477-026-02377-7

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02377-7

Keywords: plant immunity, SCREW peptide, cysteine-rich peptides, disulfide bond, receptor kinase, BAK1 co-receptor, crystal structure, phytocytokines, Arabidopsis thaliana, X-ray crystallography, Brassica napus, peptide signalling