Echinococcus granulosus antigen B hydrophobic ligand binding properties.

Antigen B (AgB), an immunodominant component of the cestode parasite Echinococcus granulosus, presents homology to and shares apparent structural similarities with helix-rich hydrophobic ligand binding proteins (HLBPs) from other cestodes. In order to investigate the fatty acid binding properties of AgB, two of its subunit components (rAgB8/1 and rAgB8/2) were expressed in Escherichia coli and purified, and the native antigen was purified from the hydatid cyst fluid by affinity chromatography using a monoclonal antibody raised against rAgB8/1. The interaction of the purified native and recombinant proteins with the fluorescent ligands DAUDA, ANS, DACA and 16-AP was investigated. The palmitic acid derived fluorescent ligand, 16-AP, showed the greatest enhancement in fluorescence when bound to native AgB or to its recombinant subunits, and the dissociation constants for 16-AP binding were determined. Surprisingly, in contrast to HLBPs from other cestodes, interactions with other fatty acids, including palmitic acid, caused an increase in fluorescence instead of competing with 16-AP. Our results suggest that AgB might have evolved different functions in the binding of hydrophobic compounds, dependent on cestode environment.

Proteomic analysis of the larval stage of the parasite Echinococcus granulosus: causative agent of cystic hydatid disease.

We describe the preparation of Echinococcus granulosus metacestode protein extracts for two-dimensional electrophoresis (2-DE). Protoscoleces and hydatid fluid were prepared by precipitation using trichloroacetic acid (TCA) to remove nonprotein contaminants. Compared to the untreated control, TCA precipitation improved the 2-DE gel profile of the protoscoleces proteins. Comparison of 2-DE gels from insoluble and soluble fractions of the protoscoleces protein extract showed that most proteins are insoluble after lysis by sonication. Host serum proteins, especially albumin and globulins, caused horizontal streaking problems on the hydatid fluid 2-DE gels due to their high content in this sample. Even after the preparation of a hydatid fluid parasite enriched fraction, the high amount of bovine serum albumin and globulins made parasite-specific proteins difficult to detect by 2-DE. Despite the absence of an E. granulosus genome sequencing or expressed sequence tag (EST) projects, it was possible to identify 15 prominent protein spots from a whole protein protoscoleces 2-DE gel by peptide mass fingerprinting. These include actins, tropomyosin, paramyosin, thioredoxin reductase, antigen P-29, cyclophilin, and the heat shock proteins hsp70 and hsp20. This work demonstrates that 2-DE and PMF are important tools to identify proteins from the hydatid fluid and protoscoleces and for the comparative analysis of cysts from different hosts or between active and resting cysts.