Molecular characterization, expression and functional analysis of cystatin C in Japanese flounder (Paralichthys olivaceus).

Cystatins are natural tight-binding reversible inhibitors of cysteine proteases found in a wide arrange of organisms. Studies have shown that cystatins play important roles under both physiological and pathological conditions in mammals. However, much less is known about fish cystatins. In this study, we described the identification and analysis of the gene encoding cystatin C in Japanese flounder (Paralichthys olivaceus). This gene had a high homology with the sequence of cystatin C in many fish species and had a signal peptide and three conserved functional sites. The results of qRT-PCR showed that the gene was highly expressed in the liver. Lipopolysaccharide, peptidoglycan and polyinosinic-polycytidylic acid all increased its expression after stimulation. Functional analysis showed that the recombinant P. olivaceus cystatin C purified from Escherichia coli had cysteine protease inhibitory activity and could inhibit bacterial growth by binding to bacteria. Meanwhile, rPocystatin C could up-regulate the expression of cytokines tumor necrosis factor α and interleukin 10. These results indicated that cystatin C of P. olivaceus might be considered to have the similar immunomodulatory function to mammalian cystatin.