Ipomoelin (IPO), expressed in the leaves of sweet potato (Ipomoea batatas cv. Tainung 57), was found easily inducible by wounding and methyl jasmonate [29, 30] . Previous study showed that IPO can agglutinate human blood and bind to different carbohydrates, such as methyl a-D-mannopyranoside (Me-Man), methyl a-Dglucopyranoside (Me-Glc), mannose, glucose and galactose [10] . In this study, we resolved the crystal structures of IPO in the apo form and in complex with Me-Man, Me-Glc and methyl a-Dgalactopyranoside (Me-Gal) to reveal the different quaternary associations of IPO and its binding pocket for carbohydrates. A The packing diagram of apo IPO with 5 molecules in green. Four of 5 molecules form a tetramer and the 5th molecule can form another tetramer by the red one, the yellow one and the blue one in the center. The molecules in red, yellow and blue are generated by symmetric operations (-X, Y, -Z), (X, -Y, -Z), and (-X-1, -Y, Z). (B) The resolved IPO-Me-Man complex in green is 2 molecules in an asymmetric unit. However, the other 2 molecules in red are generated by the symmetric operation (X, -Y, -Z) to form a tetramer in the center. (C) The resolved IPO-Me-Glc complex is a tetrameric form, and (D) the IPO-Me-Gal complex is also a tetramer. All packing diagrams reveal its tetrameric nature. doi:10.1371/journal.pone.0040618.g001 The carbohydrate binding pockets are indicated by green mesh. Tetrameric IPO is represented by monomer A in blue, monomer B in purple, monomer C in light blue, and monomer D in pink. The symmetric axis is represented by a black ellipse in the center of tetramer. (C) Structure-based multiple sequence alignment of Jacalin family. Five homologs were selected for sequence comparison from resolved protein structures: Ipomoelin-tetramer from Ipomoea batatas (PDB: 3R52); Calsepa-dimer from Calystegia sepium (PDB: 1OUW); Banlec-dimer from Musa acuminate (PDB: 2BMZ); Jacalin-tetramer from Artocarpus hirsutus (PDB: 1TOQ); Parkia-hexamer from Parkia platycephala (PDB: 1ZGS); and Heltuba-octomer from Helianthus tuberosus (PDB: 1C3K). Positions of identical conserved residues are shown in white on dark grey background, and regions of similarly conserved residues in light grey are boxed. Representation of secondary structure elements and numbering above the alignment is based on the IPO structure. The secondary structure elements below the alignment are based on the Heltuba truncated IPO (DN10IPO) was prepared to reveal its role in tetrameric association in solution by gel filtration chromatography. In addition, the carbohydrate binding constants of IPO and DN10IPO were determined by isothermal titration calorimetry (ITC). DN10IPO showed a recovered mannose/glucose-specific lectin. Structural and functional analysis identified IPO as a member of the JRL family but with a different tetrameric association. The N-terminus of IPO plays a critical role in regulating broad carbohydrate binding.