Characterization of the gene encoding a 60-kilodalton Babesia bovis merozoite protein with conserved and surface exposed epitopes.

A clone expressing a surface exposed, conserved epitope of a 60-kDa merozoite polypeptide was identified in a cDNA library constructed from a cloned Mexico strain of Babesia bovis. Sequencing of the 1.9-kb insert (pBv60) revealed an open reading frame encoding a 65-kDa polypeptide with a signal peptide and a tandemly repeated region. Monoclonal antibody 23/56.156, which binds a surface exposed epitope on the native polypeptide, specifically immunoprecipitated [35S]methionine-labeled polypeptides ranging from 60-30 kDa from pBv60 directed transcription and translation. Antibodies raised in rabbits against recombinant polypeptide reacted with the live merozoite surface in a polar immunofluorescence pattern, immunoprecipitated the native 60-kDa polypeptide, and were used to deplete the polypeptide by adsorption from a preparation of native [35S]methionine-labeled merozoite antigen. Restriction enzyme analysis indicated a single gene copy and the absence of introns. Hybridization demonstrated the presence of the gene in Mexico, Australia 'L', and Texas strains of B. bovis, but not in Babesia bigemina. A slightly different hybridization pattern was present in uncloned Australia 'L' B. bovis, indicating sequence diversity in the Bv60 gene among isolates. Cloning and structural analysis of pBv60 provides a source of defined antigen for determining the role of conserved merozoite surface epitopes in protective immunity against babesiosis.