A tegument-specific venom allergen-like protein of Clonorchis sinensis

Venom allergen-like (VAL) proteins, members of the SCP/TAPS (sperm coating protein/Tpx-1/Ag5/PR-1/Sc7) protein superfamily, have been reported from several parasitic helminths. As little is known about their biological functions, a VAL protein of the Chinese liver fluke Clonorchis sinensis was cloned and characterized. A complementary DNA (cDNA) encoding a 25-kDa protein was identified from an EST database of C. sinensis. A BLAST search revealed that the protein shares 46% sequence identity with Schistosoma mansoni VAL 13 protein, and thus, the protein was named CsVAL13. Multiple sequence alignment indicated that the SCP/TAPS domain of CsVAL13 shares 39-46% sequence identity with VAL proteins from parasitic helminths. His and Tyr, which help to stabilize protein structure, were highly conserved across the VAL protein sequences. Phylogenetic analysis showed that the SCP/TAPS domain of the CsVAL13 sequence clusters together with other group 2 VAL protein sequences. In the homology-modeled structure of CsVAL13, an α-β-α sandwich and residues for a putative active site were highly conserved. Immune sera were obtained from BALB/c mice immunized with the recombinant CsVAL13 protein. Immunohistochemical localization using the immune sera revealed that CsVAL13 was distributed mainly in the tegument and intrauterine eggs of adult C. sinensis. These findings suggest that CsVAL13 may be involved in host-parasite interactions and immune stimulation on the surrounding host environments.