Submitted to: Infection and Immunity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2002
Publication Date: N/A
Interpretive Summary: In this study we identified and sequenced the complete 8.3- kb genomic locus containing four members of msa-2, a family of genes encoding for surface antigens expressed in Babesia bovis merozoites. We demonstrated that all four msa-2 genes (termed msa-2a1, msa-2a2, msa-2b and msa-2c) are transcribed and translated in B. bovis merozoites, and postinfection bovine serum contains antibodies that bound to each of the recombinant MSA-2 proteins. Sequence comparisons of the MSA-2 proteins between the biologically cloned Mexico strain Mo7 and an Argentina strain show a relatively high degree of sequence conservation. The newly identified proteins might be of importance for the development of novel diagnostic vaccine tools and subunit vaccines.
Technical Abstract: Members of the variable merozoite surface antigen (vmsa) gene family of Babesia bovis encode membrane proteins involved in erythrocyte invasion. In this study, we have identified and sequenced the complete 8.3-kb genomic locus containing msa-2, a member of the vmsa family, in the biologically cloned Mexico Mo7 strain. Four tandemly arranged copies of msa-2-related genes were found in the locus. The four genes, designated msa-2a1 (which corresponds to the originally described msa-2 gene), msa-2a2, msa-2b, and msa-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-2a1 and -2a2 are 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-2a2 and -2b. In contrast, msa-2c is most closely related to the previously described babr 0.8 gene in Australia strains of B. bovis. Comparison of MSA-2 proteins in the Argentina R1A strain of B. bovis with 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 the msa-2 locus through at least two gene duplications, with selection for multiple related but antigenically distinct merozoite surface proteins.