Abstract
Many proteins are induced in the plant defense response to biotic stress or mechanical wounding. One group is lectins. Ipomoelin (IPO) is one of the wound-inducible proteins of sweet potato (Ipomoea batatas cv. Tainung 57) and is a Jacalin-related lectin (JRL). In this study, we resolved the crystal structures of IPO in its apo form and in complex with carbohydrates such as methyl α-D-mannopyranoside (Me-Man), methyl α-D-glucopyranoside (Me-Glc), and methyl α-D-galactopyranoside (Me-Gal) in different space groups. The packing diagrams indicated that IPO might represent a compact tetrameric association in the JRL family. The protomer of IPO showed a canonical β-prism fold with 12 strands of β-sheets but with 2 additional short β-strands at the N terminus. A truncated IPO (ΔN10IPO) by removing the 2 short β-strands of the N terminus was used to reveal its role in a tetrameric association. Gel filtration chromatography confirmed IPO as a tetrameric form in solution. Isothermal titration calorimetry determined the binding constants (KA) of IPO and ΔN10IPO against various carbohydrates. IPO could bind to Me-Man, Me-Glc, and Me-Gal with similar binding constants. In contrast, ΔN10IPO showed high binding ability to Me-Man and Me-Glc but could not bind to Me-Gal. Our structural and functional analysis of IPO revealed that its compact tetrameric association and carbohydrate binding polyspecificity could be regulated by the 2 additional N-terminal β-strands. The versatile carbohydrate binding properties of IPO might play a role in plant defense.
Highlights
Plant defense is a complicated mechanism in response to mechanical wounding, herbivore and microorganism attack
In the packing diagram for apo IPO, we observed a tetrameric association with an additional monomer in an asymmetric unit (Figure 1A)
The additional monomer could form a tetrameric association with the other 3 neighboring molecules, which were generated by symmetric operations (-X, Y, -Z), (X, -Y, -Z), and (-X-1, -Y, Z)
Summary
Plant defense is a complicated mechanism in response to mechanical wounding, herbivore and microorganism attack. One group of wound-inducible proteins is lectin, the carbohydrate binding protein [3,4]. Plant lectins are involved in the plant defense mechanism because of carbohydrate binding properties [5,6,7,8,9]. Plant lectins show resistance to digestive enzymes and can bind selectively to the carbohydrate moieties of gut epithelial cells to interfere in nutrient digestion and absorption [12], so they could be a natural insecticide. Plant lectins have been used for blood typing and immunological assay. Plant lectins have long been reported as potential inhibitors of viruses [13,14,15,16,17]
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