Plant cells live in a world of whispers. Unable to move, they coordinate growth, immunity and development through a dense conversation of small secreted peptides that drift through the apoplast and dock onto receptor proteins embedded in the plasma membrane. Deciphering the grammar of this conversation is one of the central tasks of modern plant molecular biology, and few chapters illustrate the field’s progress better than the story of SCREW and its cognate receptor NUT. Three complementary structural studies, published together in Nature Plants, have now resolved how this receptor–ligand pair recognizes one another at atomic resolution, and in doing so they have uncovered a mode of peptide perception that does not fit comfortably into any of the established templates.
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
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Drew Townsend. (September 24, 2026). Locked and Loaded: Structural Studies Reveal How the SCREW Peptide Engages Its NUT Receptor. Scienmag. https://scienmag.com/locked-and-loaded-structural-studies-reveal-how-the-screw-peptide-engages-its-nut-receptor/
Drew Townsend. “Locked and Loaded: Structural Studies Reveal How the SCREW Peptide Engages Its NUT Receptor.” Scienmag, 24 September 2026, https://scienmag.com/locked-and-loaded-structural-studies-reveal-how-the-screw-peptide-engages-its-nut-receptor/. Accessed 24 September 2026.
Drew Townsend. “Locked and Loaded: Structural Studies Reveal How the SCREW Peptide Engages Its NUT Receptor.” Scienmag. September 24, 2026. https://scienmag.com/locked-and-loaded-structural-studies-reveal-how-the-screw-peptide-engages-its-nut-receptor/
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Tags: atomic resolution of peptide recognitioncyclic peptidedisulfide-locked loopLRR receptor kinaseNature Plantsnon-canonical peptide recognition modesNUT receptorNUT receptor structural studiespeptide perceptionphytocytokinesphytocytokines in plant immunityplant immune response modulationplant immune signaling peptidesplant immunityplant peptide perception mechanismsPlant peptide signalingplant receptor-ligand recognitionPlant signalingSCREWSCREW peptide receptor interactionsecreted plant signaling peptidesSERK co-receptorstructural biologystructural biology of plant receptor complexes
