Venom allergen-like protein 28 in Clonorchis sinensis: four epitopes on its surface and the potential role of Cys124 for its conformational stability.

Venom allergen-like (VAL) proteins are important to host-parasite interactions. We previously demonstrated that a Clonorchis sinensis VAL (CsVAL) protein-derived synthetic peptide suppresses allergic and inflammatory responses. However, little is known regarding the physicochemical and antigenic properties of CsVAL proteins. Here, we identified a novel 194 amino acid VAL protein, named C. sinensis VAL 28 (CsVAL28), and characterized its functional motifs and structural details as a new member of the CAP superfamily. Unlike members of the Schistosoma mansoni VAL (SmVAL) family, CsVAL28 has a single CAP1 motif and six highly conserved disulfide bond-forming cysteines. Tertiary models of wild-type CsVAL28 and mutants were built using SmVAL4 as template via homology modeling. Normal mode analysis predicted that disulfide bond breaking by mutation of cysteine 124 to serine would greatly affect protein mobility. Four major immunoreactive linear epitopes were identified in the surface-exposed region or its vicinity via epitope mapping, using sera from clonorchiasis patients and healthy controls. Our findings provide in-depth knowledge on the structure-function properties of VAL proteins and may help determine highly antigenic regions for developing new diagnostic approaches.