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Protein Sci. 2017 Aug;26(8):1574-1583. doi: 10.1002/pro.3185. Epub 2017 May 12.

Carbohydrate recognition by the rhamnose-binding lectin SUL-I with a novel three-domain structure isolated from the venom of globiferous pedicellariae of the flower sea urchin Toxopneustes pileolus.

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Biomolecular Chemistry Laboratory, Graduate School of Engineering, Nagasaki University, 852-8521, Japan.
Division of Biochemistry, Faculty of Fisheries, Nagasaki University, 852-8521, Japan.
Biotechnology Research Institute for Drug Discovery, National Institute of Advanced Industrial Science and Technology, Tsukuba, 305-8568, Japan.
Center for Technical Support, Faculty of Science and Technology, Tokushima University, 770-8506, Japan.
Laboratory of Pharmacology, Faculty of Nursing, Shikoku University, Tokushima, 771-1192, Japan.


The globiferous pedicellariae of the venomous sea urchin Toxopneustes pileolus contains several biologically active proteins. We have cloned the cDNA of one of the toxin components, SUL-I, which is a rhamnose-binding lectin (RBL) that acts as a mitogen through binding to carbohydrate chains on target cells. Recombinant SUL-I (rSUL-I) was produced in Escherichia coli cells, and its carbohydrate-binding specificity was examined with the glycoconjugate microarray analysis, which suggested that potential target carbohydrate structures are galactose-terminated N-glycans. rSUL-I exhibited mitogenic activity for murine splenocyte cells and toxicity against Vero cells. The three-dimensional structure of the rSUL-I/l-rhamnose complex was determined by X-ray crystallographic analysis at a 1.8 Å resolution. The overall structure of rSUL-I is composed of three distinctive domains with a folding structure similar to those of CSL3, a RBL from chum salmon (Oncorhynchus keta) eggs. The bound l-rhamnose molecules are mainly recognized by rSUL-I through hydrogen bonds between its 2-, 3-, and 4-hydroxy groups and Asp, Asn, and Glu residues in the binding sites, while Tyr and Ser residues participate in the recognition mechanism. It was also inferred that SUL-I may form a dimer in solution based on the molecular size estimated via dynamic light scattering as well as possible contact regions in its crystal structure.


X-ray crystallographic analysis; lectin; rhamnose; sea urchin; toxin

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