A protein with a novel calcium-binding domain associated with calcareous corpuscles in Echinococcus granulosus

A novel intracellular calcium-binding protein from Echinococcus granulosus is described in this work. A cDNA was isolated from a lambdagt11 protoscolex expression library and the deduced amino acid sequence has at least fifteen sequentially repeated twelve-residue repeats that resemble the calcium-binding loop of EF-hands; however, the dodecamer motif has no flanking helices. The cDNA was expressed in Escherichia coli using the pGEX vector, and a recombinant fusion protein (EgCaBP1-GST) was obtained. The recombinant fusion protein binds calcium when assayed with 45Ca. It is possible that the calcium-binding motifs present a secondary structure similar to the parallel beta roll structure described for an alkaline protease from Pseudomonas aeruginosa. A native protein of more than 300 kDa was recognized by an anti-EgCaBP1 monoclonal antibody by Western-blot analysis. Immunohistochemistry using a pool of anti-EgCaBP1-GST mouse sera demonstrated a strong association of the protein with calcareous corpuscles. The possible role of this protein and that of the calcareous corpuscles in the protoscolex are discussed.