Characterization of the Babesia gibsoni P18 as a homologue of thrombospondin related adhesive protein

Thrombospondin related adhesive proteins (TRAPs) are well conserved among several apicomplexans. In this study, we reported the identification of the Babesia gibsoni P18, designated by our group previously, as a homologue of TRAP and renamed the P18 as the B. gibsoni TRAP (BgTRAP). The amino acid sequence of BgTRAP consists of several typical regions, including a signal peptide, a vonWillebrand factor A domain, a thrombospondin type 1 domain, a transmembrane region, and a cytoplasmic C-terminus. The B. gibsoni infected dog serum recognized recombinant BgTRAP expressed in E. coli by Western blotting. The antiserum against recombinant BgTRAP recognized an 80 kDa protein in the lysate of infected erythrocytes (RBCs), which was detectable in the micronemal area of the parasites by confocal microscopic observation. The BgTRAP showed a bivalent cation-independent binding to canine RBC, and the specific antiserum was found to inhibit the growth of B. gibsoni in the infected severe combined immune deficiency mice given canine RBC. These results suggest that the BgTRAP is a new member of TRAP family identified from the merozoites of B. gibsoni and functionally important in merozoite invasion; this protein may be useful as a vaccine candidate against canine B. gibsoni infection.