Abstract
Thelohanellus kitauei, is a member of obligate parasitic myxozoans, which causes intestinal giant-cystic disease of common carp (Cyprinus carpio) and has resulted in significant economic losses in carp farms. Cystatin secreted by parasites can regulate the immune response of host to facilitate parasite's survival. In this study, the secretory TK-cystatin gene, encoding a protein of 120 amino acid residues (13.65 kDa), was cloned from T. kitauei genome. Phylogenetic analysis showed that TK-cystatin gene is closely related to the cystatin-A from Hydra vulgaris. Multiple sequence alignment revealed that TK-cystatin had three conserved motifs: N-terminal G19G20, Q73VVAG77, and C-terminal L102P103. Molecular docking between TK-cystatin and three cysteine proteases showed a lower binding energy (- 13 KJ/mol) with cathepsin L whereas a higher binding energy (- 8.6 KJ/mol) with cathepsin B. TK-cystatin gene was expressed in Escherichia coli. Activity assays revealed that TK-cystatin has stronger inhibitory activity on endopeptidases (papain and cathepsin L) and weaker inhibitory activity on exopeptidase (cathepsin B). TK-cystatin was stable under the condition of acidity or alkalinity or below 57 °C. This study laid a foundation for the design and development of the anti-T. kitauei vaccine in carp culture in the future.