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Babesia caballi is a tick-borne hemoprotozoan parasite with a life cycle that alternates between an ixodid tick host, and mammalian hosts such as horses, in which it causes economically important diseases worldwide.1 It is an obligatory intraerythrocytic equine parasite belonging to the Apicomplexa. Although members of the Apicomplexa infect different host and cell types, they have similar host cell invasion processes.2 Apicomplexa parasites invade their host cells using molecules located at the cell surface and in apical secretory organelles. These organelles are localized at the anterior end of the invasive stages and are named micronemes, rhoptries, and dense granules.2–4 For intraerythrocytic Plasmodium spp., when an extracellular merozoite enters an erythrocyte, it forms an initial reversible attachment that leads to reorientation of the merozoite to bring the anterior apical pole in contact with the plasma membrane of the erythrocyte.2 A tight junction is formed through which the parasite invades the erythrocyte.
The adaptation of B. caballi at different stages of its development within host cells and in the invasive process may involve heat shock or stress proteins. The ubiquitous 70 kD heat shock protein (HSP70) family comprises a diverse group of proteins found in a large number of different organisms.5,6 The HSP70 family performs an essential molecular chaperone role for the intracellular trafficking of proteins and has other diverse cellular functions.7–9 The immunoglobulin heavy chain binding protein (BiP) is a member of the HSP70 family of molecular chaperones in eukaryotic cells, and is located in the endoplasmic reticulum (ER).10,11 It is an abundant and essential protein involved in polypeptide translocation, and it also assists in the folding and assembly of newly synthesized secreted or membrane proteins.11 However, BiP has not yet been characterized in B. caballi. Therefore, we studied the complete cDNA sequence of a novel BiP gene isolated from B. caballi to characterize the BiP gene and its product.
United States Department of Agriculture strains of B. caballi were maintained in purified horse erythrocytes in continuous cultures as previously described.12,13 A B. caballi merozoite cDNA library constructed in the 
ZAP II (Stratagene, La Jolla, CA) was screened with anti-B. caballi mouse serum according to the method of according to the method of Ikadai and others.14,15 Phagemids were excised from the clones and sequencing of the DNA insert of the pBluescript SK (+) plasmid was performed on both strands using the Dye Terminator Cycle Sequencing Kit (Applied Biosystems, Foster City, CA) with six primers: T3 (5'-AATTAACCCTCAC-TAAAGGG-3'), T7 (5'-GTAATACGACTCACTATAGG-GC-3'), F1 (5'-CGAAATGGGAAACCGTATCA-3'), F2 (5'-AACATCCTGGTGTACGATCT-3'), F3 (5'-CCCCA-AGATCAGGAAAATGA-3'), and F4 (5'-GAAGCGCAA-CATCGTCATTA-3'). Electrophoresis was carried out on an ABI PRISM 310 DNA sequencer (Applied Biosystems). The sequencing analysis was performed using the computer program GENETYX-MAC version 10.1 (Software Development, Tokyo, Japan).
Several positive clones were obtained and two cDNA clones showed the BiP homolog sequence (GenBank accession no. AB159783). Analysis of the cDNA insert sequence showed that the constructed 2,206 nucleotide fragment encoded BiP with a single open reading frame (ORF) of 1,962 nucleotides starting with methionine at position 189. The ORF encoded a polypeptide of 654 amino acid residues with a size of 72.1 kD. Comparison of the deduced amino acid sequence was performed using the GenBank database and the FASTA program (European Molecular Biology Organization Institute-European Bioinformatics Institute, Heidelberg, Germany). The B. caballi ORF encoded a protein of 654 amino acids that showed 64.3% identity with BiP of Toxoplasma gondii16 (GenBank accession no. AF110397), 64.8% identity with BiP of Eimeria tenella17 (GenBank accession no. Z66492), 62.9% identity with the heat shock protein of Plasmodium falciparum18 (GenBank accession no. X69121), 56.1% identity with BiP of Trypanosoma brucei19 (GenBank accession no. L14477), 55.5% identity with BiP of Saccharomyces cerevisiae20 (GenBank accession no. M31006), and 62.3% identity with BiP of Rattus norvegicus21 (GenBank accession no. M14050).
Using the algorithm described by von Heijne,22 we predicted that the B. caballi BiP signal sequence was the first 18 N-terminal amino acids (1MYAKKLVTALVTFLFGQA18) of the peptide. The C-terminal peptides Ser-Asp-Glu-Leu (651SDEL654) of the putative B. caballi BiP may function as an anchor to the ER. In T. gondii and E. tenella, the ER-retention signals are C-terminal peptides composed of Lys-Asp-Glu-Leu (KDEL) and His-Asp-Glu-Leu (HDEL), respectively.16,17,23–25 Moreover, the C-terminal peptides in P. falciparum are SDEL.18,26 The results establish the generality of the XDEL targeting signal throughout the broad range of eukaryotic phylogenetics. Presumably, this conservation extends to the mechanism that mediates ER localization.
DNA was extracted from B. caballi and horse blood by a standard method.27 Babesia caballi genomic DNA was amplified by a polymerase chain reaction (PCR) with oligonucleotide primers bipF (5'-AAAGTGTGTGTTGTGCAGAC-3') and bipR (5'-ATTAGACTGGCTTACAGCTC-3'). The positions of the two primers on the cDNA were nucleotides 70–89 and 2,164–2,145, respectively. The resulting DNA fragment was approximately 2,100 nucleotides. Moreover, horse genomic DNA was not amplified by the PCR with these two primers. The amplified DNA was cloned into a pCR 2.1-TOPO vector using the TOPO TA Cloning Kit (Invitrogen, Carlsbad, CA). The plasmid containing the gene was isolated and subjected to DNA sequence analysis. The completed DNA sequence of the BiP gene was analyzed and contained a single intron of 36 nucleotides (Figure 1A
).
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The Sf9 insect cells were infected with the recombinant baculovirus AcBiP in protein-free Sf-900 medium (Invitrogen) at a multiplicity of infection of 5 plaque-forming units/cell. At four days post-infection, infected cells (SfBiP) were harvested and washed three times with cold phosphate-buffered saline (PBS). Infected cells (5 x 106) in Freud’s complete adjuvant (Difco Laboratories, Detroit, MI) were injected intraperitoneally into seven-week old BALB/c mice. The same antigen in Freud’s incomplete adjuvant (Difco Laboratories) was injected intraperitoneally into the mice on day 14 and day 28. Sera were collected from immunized mice 10 days after the last immunization.
Development of monoclonal antibody (MAb) 3B2 for B. caballi BiP was conducted in this study as described previously.14 Hybridoma supernatants were screened by an indirect immunofluorescence test (IFAT). A mouse MAb isotyping kit (Amersham Bioscience, Branchburg, NJ) was used to classify MAb 3B2 as an IgM antibody. The Sf9 cells infected with AcBiP were analyzed by sodium dodecyl sulfate–polyacrylamide gel electrophoresis and Western blotting14 to determine whether the SfBiP protein was expressed. Cell lysate and culture media were tested using Western blotting with MAb 3B2. A single band of SfBiP protein was observed in the cell lysate, and the molecular mass of the SfBiP protein was the same as that of native B. caballi 72 kD protein by western blotting (Figure 1B
). This indicates that the ORF observed in the BiP gene was complete. In contrast, no band was detected in the culture medium or uninfected Sf9 cells. Moreover, antibodies against SfBiP, which were obtained from mice, recognized only the 72 kD native protein, as observed with MAb 3B2. These results indicate that the antibodies against SfBiP and MAb 3B2 reacted with the same B. caballi 72 kD protein.
The localization of BiP was examined using thin films of B. caballi-infected erythrocytes and extracellular merozoites. Thin blood smear films of cultured B. caballi- infected erythrocytes were fixed in cold methanol:acetone (1:1) for 20 minutes and incubated in undiluted culture supernatant containing MAb 3B2 or mouse antibodies against SfBiP at 37°C for one hour. Slides were washed with PBS for 10 minutes and incubated with fluorescein-conjugated goat anti-mouse IgM plus IgG plus IgA (heavy and light chains) (Southern Biotechnology, Birmingham, AL) with 5 µg/mL of Hoechst 33258 (Polysciences, Warrington, PA) at 37°C for one hour. The slides were then washed with PBS for 10 minutes and mounted in 90% glycerol for microscopic observation. Different patterns of reactivity with MAb 3B2 were observed in the cold methanol:acetone–fixed preparations of B. caballi in the IFAT (Figure 2). Monoclonal antibody 3B2 reacted strongly with extracellular merozoites, but did not react with the early intraerythrocytic stage and horse erythrocytes. Detailed observation showed that the reactivity of the MAb against pear-shaped forms was markedly irregular, with either no reactivity or reactivity with one or two brightly fluorescent pear-shaped forms (two parasites) (Figure 2). This result suggests that the maturation of merozoites after binary fission may not be synchronous, and that the rate of maturation may differ among individual merozoites.
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In conclusion, we report the complete cDNA sequence of a novel BiP gene isolated from B. caballi in this study. The high degree of homology with BiP proteins from other species suggests that the B. caballi BiP is localized in the ER of this protozoan, where it may play an important role in polypeptide translocation into and through the ER by ensuring that secretory or membrane proteins are correctly folded and assembled.
Received March 11, 2005. Accepted for publication July 21, 2005.
Financial support: This study was supported by Grants-in-Aid for Scientific Research and Young Scientists from the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Japan Society for the Promotion of Science, and the Kitasato University Research Grant for Young Researchers.
* Address correspondence to Hiromi Ikadai, Department of Veterinary Parasitology, School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Aomori 034-8628, Japan. E-mail: ikadai{at}vmas.kitasato-u.ac.jp 
Authors’ addresses: Hiromi Ikadai, Yumi Takamatsu, Ryoko Takashiro, Ayaka Segawa, Noboru Kudo, and Takashi Oyamada, Department of Veterinary Parasitology, School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Aomori 034-8628, Japan. Ikuo Igarashi, National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080-8555, Japan.
Reprint requests: Hiromi Ikadai, Department of Veterinary Parasitology, School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Aomori 034-8628, Japan, Telephone: 81-176-23-4371, Fax: 81-176-25-0165, E-mail: ikadai{at}vmas.kitasato-u.ac.jp.
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