TheBabesia bovisMerozoite Surface Antigen 2 Locus Contains Four Tandemly Arranged and Expressed Genes Encoding Immunologically Distinct Proteins

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Abstract
Members of the variable merozoite surface antigen (vmsa) gene family ofBabesia bovisencode membrane proteins involved in erythrocyte invasion. In this study, we have identified and sequenced the complete 8.3-kb genomic locus containingmsa-2, a member of thevmsafamily, in the biologically cloned Mexico Mo7 strain. Four tandemly arranged copies ofmsa-2-related genes were found in the locus. The four genes, designatedmsa-2a1(which corresponds to the originally describedmsa-2gene),msa-2a2,msa-2b, andmsa-2c, were shown to be transcribed and expressed and encode proteins with open reading frames ranging in size from 266 (MSA-2c) to 317 (MSA-2a1) amino acids. MSA-2a1and -2a2are the most closely related of the four proteins (90% identity), differing by (i) the number of 24-amino-acid repeats that comprise a surface-exposed B-cell epitope and (ii) the presence of a 32-amino-acid area of recombination between MSA-2a2and -2b. In contrast,msa-2cis most closely related to the previously describedbabr 0.8gene in Australia strains ofB. bovis. Comparison of MSA-2 proteins in the Argentina R1A strain ofB. boviswith the Mexico Mo7 clone revealed a relatively high degree of conservation (83.6, 69.4, 79.1, and 88.7% amino acid identity for MSA-2a1, -2a2, -2b, and -2c, respectively), in contrast to the extensive MSA-1 sequence variation (52% identity) between the same two strains. Postinfection bovine immune serum contains antibodies that bound to each of the recombinant MSA-2 proteins. Blocking assays demonstrated the presence of unique B-cell epitopes in MSA-2a1, -2b, and -2c. The results support the evolution of themsa-2locus through at least two gene duplications, with selection for multiple related but antigenically distinct merozoite surface proteins.

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