Purification and characterisation of a humoral opsonin, with specificity for D-galactose, in the colonial ascidian Botryllus schlosseri

A humoral agglutinin from the hemolysate of the colonial ascidian Botryllus schlosseri was purified by affinity chromatography. This agglutinin does not require metal cations for its activity and is specific for derivatives of D-galactose. On SDS-PAGE analysis, it was resolved in two bands, of 17 and 19 kDa in reducing conditions and 15 and 16 kDa in non-reducing conditions. This behavior is due to the establishment of disulfide bridges between the thiols of cysteine, well-represented in the molecule as revealed by amino acid analysis. The latter also indicated high percentages of hydrophilic residues, probably involved in sugar recognition. The lectin is an opsonin, as it increases both the phagocytic index and the number of phagocytized yeast cells. The hypothesis that this Botryllus agglutinin belongs to the galectin family of lectins is discussed.