Molecular characterization of a calcium-binding protein SjCa8 from Schistosoma japonicum

A full-length cDNA encoding a cercarial stage-specifically expressed 8-kDa calcium-binding protein (SjCa8) was isolated from Schistosoma japonicum cercarial cDNA library using microarray screen. The putative gene coding for SjCa8 is of 371 bp with an open reading frame of 69 amino acid (aa). The deduced aa sequence showed 83% identity with the Schistosoma mansoni 8-kDa CaBP, 47% identity with Clonorchis sinensis calcium-binding protein, and 38% identity with Fasciola hepatica putative calcium-binding protein. Also, it shares more than 30% identity with the calmodulin of many different species, while the most significant similarity between them lies around the two calcium-binding loop regions. There are two potential sites for phosphorylation and one potential site for N-myristoylation in the sequence. The SjCa8 has also been predicted to contain a single pair of EF-hand Ca2+-binding domain. The recombinant SjCa8 (rSjCa8) protein expressed and purified from E. coli has been demonstrated to possess the calcium-binding activity. Immune serum from UV-attenuated S. japonicum cercariae-immunized rabbit detected rSjCa8 by Western blot assay, while the sera from S. japonicum naturally infected rabbit and normal rabbit could not. These findings may contribute to the development of an effective vaccine against schistosomiasis.