Purification and characterization 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 n ot require metal cations for its activity and is specific for derivatives o f D-galactose. On SDS-PAGE analysis, it was resolved in two bands, of 17 an d 19 kDa in reducing conditions and 15 and 16 kDa in non-reducing condition s. This behavior is due to the establishment of disulfide bridges between t he thiols of cysteine, well-represented in the molecule as revealed by amin o 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 y east cells. The hypothesis that this Botryllus agglutinin belongs to the ga lectin family of lectins is discussed. (C) 1999 Elsevier Science Inc. All r ights reserved.