Characterization of an intracellular cystatin homolog from the tick Haemaphysalis longicornis

Cystatins are tight-binding inhibitors of papain-like cysteine proteases. On the basis of amino acid sequencing, cystatins can be subdivided into three closely related families, family 1, family 2 and family 3. Among them, only family 1 cystatins are intracellular. In this report, a gene encoding family 1 cystatin from tick Haemaphysalis longicornis (Hlcyst-1) has been identified. Its full-length cDNA is 437 bp, including an intact ORF encoding an expected protein with 98 amino acids. Sequence analysis demonstrated that it has significant homology with the known family 1 cystatin. The recombinant protein was expressed in a GST-fused soluble form in Escherichia coli, and its inhibitory activity against papain, cathepsin L, and cathepsin B was identified by fluorogenic substrate analysis. This is the first report of an intracellular cystatin homolog from the tick H. longicornis.