Smorf gene family in the hemoparasite Babesia bovis

Lucas M. Ferreri

The smorf multigene family is the second largest identified in the Babesia bovis genome. All 44 variant smorf genes are found associated with ves1 genes, which comprise the largest B. bovis gene family. VESA proteins encoded by ves genes are exported to the infected erythrocyte surface where they mediate cytoadhesion and through antigenic variation contribute to immune evasion. The function of smorf gene products is unknown. This study further characterized the general structure of smorf genes, and compared the gene repertoire and transcriptional profile among strains, both in vitro and in vivo. Sequence analysis of the T2Bo and Mo7 strains of B. bovis identified additional smorf genes in each strain, and demonstrated that the smorf gene family repertoire varies among strains, with both conserved and unique genes in both strains. Both semi-conserved and hypervariable regions are consistently present in all members of the gene family. A large hypervariable insertion defines 23 of the 44 SMORF proteins. Primer pairs specific for individual smorf genes were used to identify smorf transcripts from cultured infected erythrocytes and from parasites located in the brain. Up to 32 genes were transcribed in both the uncloned T2Bo strain parasites sequestered in brain capillaries, and a recently cloned Mo7 parasite line grown in vitro. SMORF peptide specific antiserum recognized multiple SMORF proteins of predicted sizes in immunoblots of cultured parasites, indicating translation of multiple smorf transcripts in the parasite population. SMORF specific antiserum labeled merozoites in a punctate pattern, suggesting an organellar association, and identified SMORF protein in the infected erythrocyte cytoplasm and membrane. Surface exposed expression of SMORF epitopes similar to VESA proteins could not be demonstrated. The role of SMORF proteins in parasite pathogenesis and persistence is unknown. However, results indicate that the smorf multigene family is even larger than expected, and is under selective pressure to express multiple variant genes concurrently within a parasite population, including within the mammalian host.

Smorf gene family in the hemoparasite Babesia bovis

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