Molecular cloning and gene expression analysis of cystatin C-like proteins in spinyhead croaker Collichthys lucidus

Cystatins are natural tight-binding reversible inhibitors of cysteine proteases. In this study, a cDNA library was constructed from Collichthys lucidus using the SMART technique. A complete cDNA sequence with high identity to the conserved sequence of the cystatin C gene was cloned from the library using EST analysis and rapid amplification of cDNA ends (RACE), then subjected to further investigation. The full-length cDNA of cystatin C from C. lucidus (Clcys) was 699 bp long, including a 5'-terminal untranslated region (5'-UTR) of 52 bp, a 3'-UTR of 290 bp, and an open-reading frame of 357 bp. The gene encoded a polypeptide of 118 amino acids, constituting a predicted molecular weight of 12.875 kDa and a theoretical isoelectric point of 8.81. The amino acid sequence of Clcys possessed typical features of type II cystatins and had the highest identity with cystatin C of Pseudosciaena crocea (89%); therefore, it clustered with the cystatin C group in the UPGMA phylogenetic tree. Quantitative real-time reverse transcription analysis revealed that the highest expression was found in the kidney, followed by the liver, heart, and testis, with the lowest expression in muscle. Interestingly, Clcys had relatively low identity with cystatin C genes from other fish and mammals, and its expression pattern did not possess features of a housekeeping gene. Based on these findings, we suspect that the classification of cystatins in fish is somewhat confusing, and the identification of more cystatin gene sequences is needed before a definite conclusion can be drawn.