GENETIC AND ANTIGENIC CHARACTERIZATION OF BABESIA-BOVIS MEROZOITE SPHERICAL BODY PROTEIN BB-1

A Babesia bovis merozoite protein, Bb-1, was localized by immunoelectr on microscopy to an apical organelle known as the spherical body. This unique structure appears to be analogous to dense granules of other a picomplexan protozoa. Similar to previously described dense granule pr oteins of Plasmodium spp., Bb-1 is secreted during or just after invas ion of host erythrocytes and becomes associated with the cytoplasmic f ace of the infected cell. The amino terminal sequence of Bb-1 contains a predicted signal peptide and is similar to the amino terminus of an other spherical body protein (BvVA1/225) which is also translocated to the erythrocyte membrane. Importantly, these two spherical body prote ins are the major components of a protective fraction of B. bovis anti gen. There is marked conservation of Bb-1 amino acid sequences and B-l ymphocyte epitopes among geographic strains. However, a divergent Bb-1 allele (Bv80) in Australia strains encodes six regions of amino acid polymorphism, including a region of tetrapeptide repeats in the C-term inal half of the polypeptide. Two of the polymorphic regions map to pr eviously defined Th1 epitopes on Bb-1.