Abstract

BackgroundThe spherical body is a distinct organelle only existing in Babesia and Theileria. Spherical body proteins (SBPs) are secreted from spherical bodies and incorporated into the cytoplasm of infected erythrocytes during invasion and post-invasion stages. Four different SBP homologues (SBP1, SBP2, SBP3 and SBP4) have been identified in Babesia bovis and Babesia bigemina. So far, there has been no report available about the identification of SBPs in Babesia orientalis.MethodsThe SBP3-like in B. orientalis (BoSBP3-like) was cloned, sequenced, characterized and compared to the SBP3 sequences of B. bovis and B. bigemina by bioinformatics analyses. The BoSBP3-like gene was truncated into three fragments: BoSBP3-like-1 (915 bp), BoSBP3-like-2 (1311 bp) and BoSBP3-like-3 (1011 bp), which were amplified and cloned into the expression vector pET-28a and expressed as three truncated recombinant (His-fusion) proteins. The immunogenicity, native forms and localization of BoSBP3-like were identified by western blot and indirect immunofluorescence assay (IFA).ResultsThe BoSBP3-like gene was intronless with an open reading frame (ORF) of 3237 bp, encoded a 1079 amino acid polypeptide with a predicted size of 135 kDa, and contained a cysteine-rich region, three dispersing FAINT domains and a signal peptide (1–16 aa) at the N-terminus. The amino acid sequence of BoSBP3-like was 61.6 and 35.0% identical to that of B. bovis and B. bigemina, respectively. BoSBP3-like was identified as 135 kDa in the parasite lysate by rabbit antiserum against the truncated recombinant BoSBP3-like-1 (rBoSBP3-like-1). Three specific bands corresponding to rBoSBP3-like-1 (1–305 aa, 43 kDa), rBoSBP3-like-2 (306–742 aa, 58 kDa) and rBoSBP3-like-3 (743–1079 aa, 52 kDa) were detected by reaction with serum from B. orientalis-infected buffalo. The BoSBP3-like was not only localized in the spherical body of B. orientalis but also in the cytoplasm of infected erythrocytes of buffalo as puncta-like protein specks at both single and paired parasite development stages.ConclusionsThrough secretion into the cytoplasm of infected erythrocytes, BoSBP3-like may play a significant role in adaptation, interaction, and modification related to the host environment to benefit the growth and survival of Babesia. BoSBP3-like could react with the serum from B. orientalis-infected buffalo, but not healthy buffalo, implicating that BoSBP3-like is highly antigenic and may serve as a candidate diagnostic antigen for the detection of B. orientalis.

Highlights

  • The spherical body is a distinct organelle only existing in Babesia and Theileria

  • Through secretion into the cytoplasm of infected erythrocytes, BoSBP3-like may play a significant role in adaptation, interaction, and modification related to the host environment to benefit the growth and survival of Babesia

  • A 3237 bp fragment was obtained by cloning and sequencing the entire sequence coding for amino acids (CDS) of BoSBP3-like from Genomic DNA (gDNA) and complementary DNA (cDNA) (Fig. 1)

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Summary

Introduction

The spherical body is a distinct organelle only existing in Babesia and Theileria. Spherical body proteins (SBPs) are secreted from spherical bodies and incorporated into the cytoplasm of infected erythrocytes during invasion and post-invasion stages. Several spherical body specific proteins (SBP1, SBP2, SBP3 and SBP4) have been only identified and characterized in Babesia bovis and Babesia bigemina [6,7,8,9,10]. All of these spherical body proteins are found localized either in the cytoplasm or cytoplasmic face of the infected red blood cells (iRBC). Despite so many differences between spherical bodies and dense granules, the released proteins may play the same roles during parasite invasion, nutrient intake, waste elimination and adaptation to the host environment

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