Carbohydrate-binding ability of a recombinant protein containing the DM9 motif from Drosophila melanogaster

Proteins containing DM9 motifs, which were originally identified in the Drosophila melanogaster genome, are widely distributed in various organisms and are assumed to be involved in their innate immune response. In this study, we produced a recombinant protein of CG13321 (rCG13321) from D. melanogaster, which consists of four DM9 motifs, in Escherichia coli cells. In affinity chromatography using a mannose-immobilized column, rCG13321 exhibited mannose-binding ability and was separated into high-affinity and low-affinity fractions, named HA and LA, respectively, based on its binding ability to the column. In addition to having a higher affinity for the column, HA exhibited self-oligomerization ability, suggesting slight differences in tertiary structure. Both LA and HA showed hemagglutinating activity and were able to agglutinate an oligomannose-containing dendrimer, indicating that they have multiple carbohydrate-binding sites. Glycan array analysis suggested that rCG13321 primarily recognizes d-mannose and d-rhamnose through hydrogen bonding with the 2-, 3- and 4-hydroxy groups. Isothermal titration calorimetry demonstrated that rCG13321 has a comparable affinity to typical lectins. These findings suggest that CG13321 functions as a carbohydrate-binding protein or lectin that recognizes mannose and related carbohydrate-containing molecules on the surface of foreign organisms as a pattern recognition molecule.