# Jacalin-like lectin domain
The jacalin-like lectin domain (JRL domain) is a carbohydrate-binding protein domain of about 18 kDa that adopts a β-prism fold and is named after jacalin, the T-antigen disaccharide-binding lectin first isolated from the seeds of jackfruit (*Artocarpus integrifolia*).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) Domains of this family occur in plants, in eukaryotes outside the plant kingdom, and in some [prokaryotes](https://www.edgepedia.org/prokaryote), and in plants they are frequently joined to unrelated partner domains in chimeric proteins involved in development and defense.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/)
| Key fact | Detail |
|---|---|
| Prototype | Jacalin, a T-antigen (Galβ1,3GalNAc) disaccharide-binding lectin from jackfruit (*Artocarpus integrifolia*) seeds[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) |
| Fold | β-prism I fold formed by three Greek key motifs, with the monosaccharide binding site formed by three loops at one extremity of the prism[3](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/jacalin) |
| Plant subgroups | Galactose-binding JRLs in the vacuole, reported mainly within the Moraceae; mannose-binding JRLs in the nucleocytoplasmic compartment, widespread in Viridiplantae[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) |
| Specificity | Jacalin binds galactose and also mannose; the affinity of Me-α-Man is 20 times weaker than that of Me-α-Gal[4](https://www.rcsb.org/structure/1WS4) |
| Chimeric defense proteins | JRL plus dirigent domain proteins such as OsJAC1, TaJRLL1, TaVER2, TaHfr-1, and TaJA1 occur in monocots and mediate fungal resistance[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/) |
| Beyond plants | 47 putative jacalin genes identified across 11 fish species, grouped into 4 phylogenetic groups[5](https://pubmed.ncbi.nlm.nih.gov/27514782/) |
| Pathogen side | Oomycete *Sclerospora graminicola* secretes JRL proteins that act as apoplastic virulence effectors suppressing plant defense[6](https://pubmed.ncbi.nlm.nih.gov/35257477/) |
## Structure and fold
All jacalin domains, whether or not the mature protein is proteolytically cleaved, share the same β-prism fold formed by three Greek key motifs.[3](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/jacalin) In jacalin itself and the related *Maclura pomifera* agglutinin, a 20-residue β-chain and a 133-residue α-chain form three bundles of four antiparallel β-strands arranged around a pseudo-symmetry axis, and the single monosaccharide binding site is formed by three loops at one end of the prism.[3](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/jacalin) Native jacalin is a tetramer of about 65 kDa with clear 4-fold symmetry.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC1133028/?page=1)
The domain tolerates structural variation. The relatively large carbohydrate-binding site accommodates monosaccharides with different hydroxyl conformations, and crystallography of a jacalin–α-methyl-mannose complex at 2 Å resolution showed that the β-prism is a flexible scaffold that can support different binding specificities within a single lectin.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC1222559/) [Quaternary](https://www.edgepedia.org/quaternary) associations across the family vary widely: jacalin and artocarpin are tetramers, Heltuba from [Jerusalem artichoke](https://www.edgepedia.org/jerusalem-artichoke) forms an octahedral ring, Calsepa from hedge bindweed is dimeric, and the *Parkia platycephala* lectin carries three tandem jacalin domains in a hexahedral dimer.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3394770/)
## Carbohydrate specificity
Jacalin was long described as specific for the T-antigen disaccharide Galβ1,3GalNAc, lactose, and galactose, but surface plasmon resonance and the 2 Å crystal structure of a methyl-α-mannose complex demonstrated that it also binds mannose and oligomannosides, as well as glucose, N-acetylneuraminic acid, and N-acetylmuramic acid.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC1222559/) Later calorimetric and crystallographic work quantified this promiscuity: the binding affinity of Me-α-Man is 20 times weaker than that of Me-α-Gal, glucose shows no measurable binding, and the results indicate that jacalin is specific to O-glycans while its affinity for N-glycans is extremely weak or non-existent.[4](https://www.rcsb.org/structure/1WS4)
The two plant subgroups differ in specificity and location. Galactose-binding JRLs reside in the vacuole and have been reported mainly within the Moraceae, whereas mannose-binding JRLs occupy the nucleocytoplasmic compartment and are widespread in Viridiplantae.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) An evolutionary event in the Moraceae lineage, involving insertion of a signal peptide and vacuolar targeting sequences into a gene for a cytoplasmic mannose-specific JRL, gave rise to the small galactose-specific subfamily.[10](https://febs.onlinelibrary.wiley.com/doi/10.1016/S0014-5793%2800%2901801-9) N-terminal structure can also regulate specificity: the sweet potato lectin ipomoelin binds Me-Man, Me-Glc, and Me-Gal with similar binding constants, but removing its two extra N-terminal β-strands abolishes Me-Gal binding while leaving mannose and glucose binding intact.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3394770/)
## Distribution and evolution
The jacalin family is one of twelve carbohydrate-binding domain families classified in plants on the basis of sequence similarity and evolutionary relationships.[11](https://www.nature.com/articles/s41598-018-29439-x) Among plant lectin families it shows rapid expansion: the highest gene birth rate observed was in the Jacalin family, at 30 genes per 100 million years in the lineage leading to *Arabidopsis*, and in *Arabidopsis* the Jacalin family is the largest lectin family.[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC2846932/) Phylogenetic analysis of jacalin domains from *Arabidopsis*, soybean, cucumber, and rice distinguished nine clusters, with sequences from one species grouping together more closely than sequences of the same domain architecture from different species, a pattern distinct from several other lectin domain families.[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC5485960/)
The domain is not restricted to plants. A similar domain has been reported in eukaryotes outside the plant kingdom and in some prokaryotes.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) In fishes, a survey of 11 species identified 47 putative jacalin genes divided into 4 phylogenetic groups, with conserved gene organization within each group; some fishes carry eleven jacalin genes while others have only one or none, and intragenic recombination played a key role in the family's evolution.[5](https://pubmed.ncbi.nlm.nih.gov/27514782/)
## Chimeric proteins and plant defense
In plants, chimeric proteins combining one or more jacalin domains with an unrelated domain are widely distributed, and partner domains range from kinases to toxins.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/) Lectin receptor-like kinases, which pair a lectin binding domain with a kinase action domain, are one such class; a second class pairs the JRL domain with a dirigent domain, as in TaVER2, TaHfr-1, TaJA1, and the rice protein OsJAC1.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/) Proteins with this JRL plus dirigent architecture occur exclusively within monocotyledonous plants and show an unusually high range of family member expansion compared with other JRL fusion proteins.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/)
**Targeted relocation.** The JRL domain of OsJAC1 is responsible for relocating the protein toward the site of pathogen attack, most likely by binding oligosaccharide signatures typical of the infection process; the domain is predicted to bind mannose-containing oligosaccharides, while the dirigent domain might function in stereoselective coupling of monolignols.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/) • [14](https://pubmed.ncbi.nlm.nih.gov/26708413/) Rice, wheat, and barley plants overexpressing OsJAC1 are resistant against important fungal pathogens, and OsJAC1 has been described as a representative of a novel type of resistance protein derived from a plant lineage-specific gene fusion event.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/) • [14](https://pubmed.ncbi.nlm.nih.gov/26708413/)
**Hormone-linked defense.** The wheat protein TaJRLL1, a mannose-specific JRL with two jacalin-like lectin domains expressed mainly in stem and spike, is induced by pathogen infection and by phytohormone treatments. Silencing TaJRLL1 increased susceptibility to *Fusarium graminearum* and *Blumeria graminis*, while expression in *Arabidopsis* increased resistance to *F. graminearum* and *Botrytis cinerea* and raised salicylic acid and jasmonic acid levels, linking the protein to SA- and JA-dependent defense signaling.[15](https://pmc.ncbi.nlm.nih.gov/articles/PMC3223046/) Not all JRL defenses run through hormones: the *Arabidopsis* protein JAX1 confers resistance to potex viruses by inhibiting viral RNA accumulation independent of hormone signaling.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/) A proposed additional function is decoy activity, in which the JRL domain helps alert the plant to attacking pathogens.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/)
## Use by pathogens
The domain also appears on the pathogen side of the interaction. *Sclerospora graminicola*, the downy mildew oomycete of foxtail millet, produces SgJRLs consisting of a jacalin-like lectin domain and an N-terminal secretion signal, with expression induced during infection. Heterologous expression of three SgJRLs in *Nicotiana benthamiana* enhanced the virulence of *Phytophthora palmivora* on the same leaves, and one of them, SG06536, localized to the apoplastic space and suppressed INF1-mediated induction of defense-related genes, indicating that JRLs can serve as apoplastic effectors that contribute to pathogenicity by suppressing plant defense responses.[6](https://pubmed.ncbi.nlm.nih.gov/35257477/)
## The prototype lectin and its applications
Jacalin itself remains a widely used biochemical tool because of its high specificity for the tumor-associated T-antigen, and it unfolds at about 60 °C, with carbohydrate binding stabilizing the protein structure.[16](https://pubmed.ncbi.nlm.nih.gov/38232886/) Documented biological activities include specific binding to the tumor-associated T-antigen, inhibition of HIV through binding to specific regions of the virus, potent and selective stimulation of distinct [T cell](https://www.edgepedia.org/t-cell) functions, and specific recognition of immunoglobulin A1 from human serum.[18](https://doi.org/10.1093/glycob/cwh140) Processing of the precursor into its two chains is not required for sugar binding but appears to be required for full affinity, since a recombinant single-chain jacalin folds normally yet shows reduced saccharide affinity.[17](https://pmc.ncbi.nlm.nih.gov/articles/PMC2287297/)
## References
1. [Lectin domains at the frontiers of plant defense, Frontiers in Plant Science (2014)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4131498/)
2. [Esch L., Schaffrath U. An Update on Jacalin-Like Lectins and Their Role in Plant Defense, Int. J. Mol. Sci. (2017)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5536079/)
3. [Jacalin - an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/jacalin)
4. [RCSB PDB 1WS4: Crystal structure of Jacalin–Me-alpha-Mannose complex: Promiscuity vs Specificity](https://www.rcsb.org/structure/1WS4)
5. [Evolutionary analysis of the jacalin-related lectin family genes in 11 fishes, Fish & Shellfish Immunology (2016)](https://pubmed.ncbi.nlm.nih.gov/27514782/)
6. [A jacalin-like lectin domain-containing protein of Sclerospora graminicola acts as an apoplastic virulence effector, Molecular Plant Pathology (2022)](https://pubmed.ncbi.nlm.nih.gov/35257477/)
7. [Structural and electron-microscopic studies of jacalin from jackfruit show that this lectin is a 65 kDa tetramer](https://pmc.ncbi.nlm.nih.gov/articles/PMC1133028/?page=1)
8. [Structural basis for the unusual carbohydrate-binding specificity of jacalin towards galactose and mannose](https://pmc.ncbi.nlm.nih.gov/articles/PMC1222559/)
9. [Ipomoelin, a Jacalin-Related Lectin with a Compact Tetrameric Association, PLoS ONE (2012)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3394770/)
10. [Peumans W.J. et al. The galactose-binding and mannose-binding jacalin-related lectins are located in different sub-cellular compartments, FEBS Letters (2000)](https://febs.onlinelibrary.wiley.com/doi/10.1016/S0014-5793%2800%2901801-9)
11. [Biochemical and structural characterization of a mannose binding jacalin-related lectin from pineapple stem, Scientific Reports (2018)](https://www.nature.com/articles/s41598-018-29439-x)
12. [Evolutionary history and stress regulation of the lectin superfamily in higher plants, BMC Evolutionary Biology (2010)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2846932/)
13. [Comparative Study of Lectin Domains in Model Species: New Insights into Evolutionary Dynamics, Int. J. Mol. Sci. (2017)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5485960/)
14. [Polarized Defense Against Fungal Pathogens Is Mediated by the Jacalin-Related Lectin Domain of Modular Poaceae-Specific Proteins, Plant Physiology (2016)](https://pubmed.ncbi.nlm.nih.gov/26708413/)
15. [A jacalin-related lectin-like gene in wheat is a component of the plant defence system, Journal of Experimental Botany (2011)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3223046/)
16. [Effect of temperature and pH on the structure and stability of tumor-specific lectin jacalin (2024)](https://pubmed.ncbi.nlm.nih.gov/38232886/)
17. [Studies on recombinant single chain Jacalin lectin reveal reduced affinity for saccharides despite normal folding](https://pmc.ncbi.nlm.nih.gov/articles/PMC2287297/)
18. [A database analysis of jacalin-like lectins: sequence–structure–function relationships, Glycobiology (Oxford University Press)](https://doi.org/10.1093/glycob/cwh140)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Protein families and complexes › Conserved protein domains and motifs › Lectin and carbohydrate-binding domains*
*Initially written Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license