The ligands in question, the SCREW phytocytokines, belong to a growing family of small secreted peptides that act as immune signals. Phytocytokines, a term that has gained currency over the past several years, function much like the cytokines of the animal immune system: they are released at sites of infection or damage and amplify or modulate the defensive responses of surrounding tissue. What makes the SCREW family particularly intriguing is its architecture. Rather than being simple linear peptides, SCREW peptides are cyclic, their N- and C-termini stitched together by a disulfide bond that locks the molecule into a constrained loop. That constraint is not a decorative feature; as the new structures reveal, it is the very thing the receptor reads.

The receptor on the receiving end is NUT, a leucine-rich repeat receptor kinase, or LRR-RK. These receptors form one of the largest families of cell-surface signaling proteins in plants, each built from a curved extracellular scaffold of leucine-rich repeats that presents an interaction surface to the extracellular milieu, a single membrane-spanning helix, and an intracellular kinase domain that relays the binding event into the cytoplasm. Decades of work, crystallized in influential reviews of the field, have established the canonical logic of this family: a ligand binds to the extracellular LRR domain, and that binding event creates or exposes a docking site for a co-receptor, most often a member of the SERK family of somatic embryogenesis receptor-like kinases. The ligand thus acts as a molecular clamp, gluing the receptor and co-receptor together and allowing their intracellular kinase domains to trans-phosphorylate one another and launch the signaling cascade.

The classic example of this logic is the brassinosteroid system, in which the steroid hormone brassinolide nestles into a pocket formed between the receptor BRI1 and the co-receptor BAK1, itself a SERK. Peptide systems such as the bacterial-flagellin receptor FLS2 follow a broadly similar script: the flg22 peptide lies along the LRR solenoid of FLS2, and its C-terminal tail recruits BAK1. In each of these cases, the ligand is a comparatively extended molecule, and the co-receptor docks onto a surface that only exists once the ligand is in place. The structural biology of the past decade has made this ‘induced proximity’ model the default expectation for LRR-RK signaling, so much so that deviations from it are automatically newsworthy.

SCREW and NUT deviate. The three new studies, from independent teams led respectively by Wang and colleagues, Jiménez-Sandoval and colleagues, and Wei and colleagues, converged on the same central finding: NUT perceives the disulfide-locked loop of the SCREW peptide in a manner that is mechanistically distinct from the canonical clamp model. The cyclic nature of the ligand, imposed by the disulfide bridge that closes its backbone into a ring, means that SCREW does not present the kind of extended, linear epitope that flg22 offers to FLS2. Instead, the constrained loop presents a three-dimensional surface, and NUT reads that surface with a specificity that depends on the integrity of the cyclization itself. Break the disulfide, and the recognition logic collapses.

This distinction matters because it changes what the receptor is actually measuring. A linear peptide ligand is, in structural terms, a sequence read out in one dimension: the receptor contacts successive residues along an extended chain, and binding energy accumulates additively along that chain. A disulfide-locked cyclic peptide is a shape read out in three dimensions: the receptor contacts a pre-organized conformation whose stability is guaranteed by the covalent bridge. The new structures show that NUT’s extracellular domain cradles the SCREW loop in a way that exploits this pre-organization, contacting the constrained backbone and the side chains it positions. The result is a recognition event that is exquisitely sensitive to the ligand’s tertiary structure, not merely its primary sequence.

The second half of the mechanism concerns co-receptor recruitment. Here, too, the studies reveal a departure from the textbook picture. NUT, once engaged with SCREW, recruits a SERK co-receptor to complete the signaling-competent complex, confirming that the SERK family’s role as universal signaling partners extends to this new ligand class. But the geometry of the recruitment differs from that observed in the canonical systems. The figure accompanying the News & Views commentary by Kira Gysel contrasts ‘two modes of co-receptor recruitment by LRR-RK ligands’, and the contrast is the analytical heart of the story: the same family of co-receptors can be harnessed by structurally unrelated ligand-presenting mechanisms, expanding the functional repertoire of the receptor kinase superfamily without requiring new co-receptor hardware.

That economy is worth pausing on. Plants encode dozens of LRR-RKs and a small number of SERK co-receptors, and the combinatorial potential of the system has long fascinated biologists. If a single SERK can be recruited by a steroid nestled in a binding pocket, by a linear peptide lying along an LRR groove, and now by a disulfide-locked cyclic loop presented by NUT, then the co-receptor is best understood not as a specific reader of any one ligand but as a general-purpose signal transducer that is activated whenever the right kind of receptor–ligand complex presents the appropriate docking surface. The SCREW–NUT structures demonstrate that the ‘appropriate docking surface’ can be generated in more ways than the field had imagined, which has implications for how researchers go about hunting for the receptors of the many orphan phytocytokines still awaiting assignment.

The convergence of three independent groups on the same mechanism is itself scientifically meaningful. Structural biology of membrane receptors is notoriously sensitive to construct design, crystallization conditions and the choice of whether to work with isolated extracellular domains or full-length proteins, and disagreements between groups have historically been as common as agreements. That Wang and colleagues, Jiménez-Sandoval and colleagues, and Wei and colleagues, working separately, arrived at mutually consistent pictures of SCREW recognition and SERK recruitment gives the field unusually high confidence in the mechanism. The Nature Plants commentary that accompanies the papers frames them as a single advance for precisely this reason: the redundancy of the evidence is the evidence.

What remains open is the biology downstream of the binding event. The structural work explains how SCREW finds its NUT, but the physiological consequences of that encounter, the precise immune outputs the complex triggers, the pathogens or wounds against which it protects, and the way the pathway is tuned by other receptors and co-receptors in the membrane, will occupy the next phase of research. There is also the question of evolutionary breadth: whether disulfide-locked cyclic phytocytokines and their loop-reading receptors represent a widespread signaling module across plant species or a specialized innovation of particular lineages. Either answer would be interesting. What the three studies have already secured is a structural principle: plant cell-surface receptors are not confined to reading linear peptide text, and the covalent architecture of a ligand can be as informative as its sequence. For a family of receptors long studied through the lens of a handful of canonical complexes, SCREW and NUT have loosened the screw on the field’s assumptions, and the coming years will show how many more NUTs are waiting for their ligands to be found.

Subject of Research: Structural mechanism of cyclic peptide perception by the LRR receptor kinase NUT and its SCREW phytocytokine ligands

Article Title: How SCREW finds its NUT

Article References: Gysel, K. (2026). How SCREW finds its NUT. Nature Plants, 12(9), 1671-1672. https://doi.org/10.1038/s41477-026-02405-6

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02405-6

Keywords: SCREW, NUT receptor, phytocytokines, LRR receptor kinase, SERK co-receptor, disulfide-locked loop, cyclic peptide, plant signaling, structural biology, plant immunity, peptide perception, Nature Plants

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