Characterization and function analysis of a Kazal-type serine proteinase inhibitor in the red claw crayfish Cherax quadricarinatus

Kazal-type serine proteinase inhibitors (KPIs) function in physiological and immunological processes requiring proteinase action. In the present study, the first Cherax quadricarinatus KPI gene (designated CqKPI) was identified and characterized. The open reading frame of CqKPI contains 405 nucleotides and encodes a protein of 134 amino acids. CqKPI has two Kazal domains comprising 44 amino acid residues with the conserved amino acid sequence C-X3-C-X7-C-X6-Y-X3-C-X6-C-X12-C. Each Kazal domain has six conserved cysteine residues, which can form a structural conformation of three pairs of disulfide bonds stabilizing the Kazal domain. CqKPI exhibited high similarity with previously identified KPIs from crayfish hemocytes. The results of tissue distribution showed that CqKPI had the highest expression level in hemocytes, and this was in agreement with phylogenic relationships. Recombinant CqKPI (rCqKPI) was heterologously expressed in Escherichia coli and purified for further study. The proteinase inhibition assays suggested that rCqKPI could potently inhibit elastase and weakly inhibit trypsin, subtilisin A, and proteinase K, but not α-chymotrypsin. It can firmly bind to Bacillus hwajinpoensis, Staphylococcus aureus, and Vibrio parahaemolyticus, with weak binding to Candida albicans. In addition, CqKPI inhibited bacterial secretory proteinase activity and inhibited the growth of B. hwajinpoensis and C. albicans. These data suggest that CqKPI might be involved in anti-bacterial immunity, acting as an inhibitor of the proteinase cascade in the resistance to invasion of pathogens.