Cloning of a 21.7-kDa vaccine-dominant antigen gene of Schistosoma mansoni reveals an EF hand-like motif.

Several cDNA clones encoding a 21.7-kDa antigen (Sm21.7) were detected from a Schistosoma mansoni sporocyst cDNA expression library using irradiated cercaria-vaccinated rabbit serum. The antigen was designated 'vaccine dominant' because parasite-derived Sm21.7 was recognised preferentially by mouse vaccine sera compared with mouse infection sera. The cDNA and corresponding gDNA sequences showed 64% identity at the nucleotide level and 47% identity at the amino acid level with the sequence of a previously described S. mansoni tegumental antigen, sma22.6. Whereas sma22.6 mRNA occurs almost exclusively in the adult worm, Sm21.7 mRNA was equally abundant in the sporocyst, schistosomular and adult stages. Both Sm21.7 and sma22.6 sequences reveal a motif strongly homologous to the EF hand calcium binding domain but lacking the invariant glycine in the calcium binding loop. The disruptive nature of the glutamine which in Sm21.7 replaces the glycine explains why the motif is non-functional, as shown by the inability of Sm21.7 to bind calcium.