Characteristic recognition of N-acetylgalactosamine by an invertebrate C-lype lectin, CEL-I, revealed by X-ray crystallographic analysis

CEL-I is a C-type lectin, purified from the sea cucumber Cucumaria echinata, that shows a high specificity for N-acetylgalactosamine ( GalNAc). We determined the crystal structures of CEL-I and its complex with GalNAc at 2.0 and 1.7 Angstrom resolution, respectively. CEL-I forms a disulfide-linked homodimer and contains two intramolecular disulfide bonds, although it lacks one intramolecular disulfide bond that is widely conserved among various C-type carbohydrate recognition domains (CRDs). Although the sequence similarity of CEL-I with other C-type CRDs is low, the overall folding of CEL-I was quite similar to those of other C-type CRDs. The structure of the complex with GalNAc revealed that the basic recognition mode of GalNAc was very similar to that for the GalNAc-binding mutant of the mannose-binding protein. However, the acetamido group of GalNAc appeared to be recognized more strongly by the combination of hydrogen bonds to Arg(115) and van der Waals interaction with Gln(70). Mutational analyses, in which Gln(70) and/or Arg(115) were replaced by alanine, confirmed that these residues contributed to GalNAc recognition in a cooperative manner.