A large calcium-binding protein associated with the larval spicules of the sea urchin embryo

A tissue-specific, high molecular weight, calcium-binding protein from the sea urchin embryo is described. This protein, designated as CBP 180, has a molecular weight of 180,000 under reducing conditions, and is extractable with 1% Triton X-100. It accumulates rapidly during development, starting roughly at the onset of spiculogenesis. When embryos are cultured in the presence of inhibitors of spicule formation, such as tunicamycin and zinc ions, accumulation of CBP180 is depressed or stopped. By immunofluorescence technique and by using an antibody specifically generated against this protein, CBP180 is mainly localized in primary mesenchyme cells and spicular syncytium of the pluteus larva. Little or none is detectable in ectoderm, endoderm or blastocoelar extracellular matrix. These results suggest that the protein is involved in calcium sequestration in the differentiation of larval spicules.