Opsonin-like molecule found in coelomic fluid of a sea cucumber, Holothuria leucospilota

Opsonins, a group of serum proteins present in vertebrates, are known to bind to the surface of many kinds of pathogens and tag them as targets for phagocytosis. Opsonin-like molecules have been suggested to exist in several invertebrates. In this paper, we report the discovery of a new opsonin-like molecule in the coelomic fluid of the sea cucumber Holothuria leucospilota. Opsonic activity in cell-free coelomic fluid was characterized by sodium dodecyl sulfate-polyacrylalmide gel electrophoresis, two-dimensional electrophoresis and gel filtration. Phagocytic uptake of yeast cells and latex beads by amoebocytes was quantified by microscopic analysis. It was found that a 20 kdalton protein bound to and enhanced phagocytosis of heat-killed yeast cells. The results of Western blot analysis suggest that both amoebocytes and spherule cells produce this opsonin-like molecule.