Abstract
A novel galactose-specific lectin, AJLec (18.5 kDa), was isolated from the sea anemone, Anthopleura japonica. AJLec was characterized using the hemagglutination assay, isothermal titration calorimetry (ITC), and glycoconjugate microarray analysis and we found that AJLec has a specificity for galactose monomers and β-linked terminal galactose residues in complex carbohydrates, but not for N-acetylgalactosamine (GalNAc), which is commonly recognized by galactose-binding lectins. The primary structure of AJLec did not show homology with known lectins, and a crystal structural analysis also revealed a unique homodimeric structure. The crystal structure of AJLec complexed with lactose was solved by measuring the sulfur single-wavelength anomalous diffraction (S-SAD) phasing with an in-house Cu Kα source method. This analysis revealed that the galactose residue in lactose was recognized via its O2, O3, and O4 hydroxyl groups and ring oxygen by calcium coordination and two hydrogen bonds with residues in the carbohydrate-binding site, which demonstrated strict specificity for the β-linked terminal galactose in this lectin.
Highlights
Lectins are ubiquitously distributed in various tissues and body fluids of living organisms
A single peak appeared after elution with Tris-buffered saline (TBS) containing 100 mM lactose (Supplementary Fig. S1A)
AJLec was purified from A. japonica and identified as a Ca2+-dependent and galactose-specific lectin (18.5 kDa) forming a disulfide-bonded dimer
Summary
We purified a novel galactose-binding lectin named AJLec from the sea anemone Anthopleura japonica. Lectins from sea anemone species have not been identified previously. Characterization of AJLec revealed strict specificity for galactose monomers and β-linked terminal galactoses in complex carbohydrates. Its amino acid sequence does not show any similarity with those of known lectins. Analysis of AJLec complexed with lactose revealed its binding mechanism and a structural basis for its specificity
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