High-level soluble expression of the functional peptide derived from the C-terminal domain of the sea cucumber lysozyme and analysis of its antimicrobial activity
Background The sea cucumber lysozyme belongs to the family of invertebrate lysozymes and is thought to be a key defense factor in protecting aquaculture animals against bacterial infection. Recently, evidence was found that the sea cucumber lysozyme exerts broad spectrum antimicrobial action in vitro against Gram-negative and Gram-positive bacteria, and it also has more potent antimicrobial activity independent of its enzymatic activity. To explore the antimicrobial role of this non-enzymatic lysozyme and model its structure to novel antimicrobial peptides, the peptide from the C-terminal amino acid residues 70-146 of the sea cucumber lysozyme in Stichopus japonicus (SjLys-C) was heterologously expressed in Escherichia coli Rosetta(DE3)pLysS. Results The fusion protein system led to over-expression of the soluble and highly stable product, an approximate 26 kDa recombinant SjLys-C protein (rSjLys-C). The present study showed that rSjLys-C displayed strong antimicrobial activity against the tested Gram-positive and Gram-negative bacteria. In particular, the heat-treated rSjLys-C exhibited more inhibitive activity than the native rSjLys-C. The structural analysis of SjLys-C showed that it is a typical hydrophilic peptide and contains a helix-loop-helix motif. The modeling of SjLys-C molecular structures at different temperatures revealed that the tertiary structure of SjLys-C at 100°C underwent a conformational change which is favorable for enhancing antimicrobial activity. Conclusion These results indicate that the expressed rSjLys-C is a highly soluble product and has a strong antimicrobial activity. Therefore, gaining a large quantity of biologically active rSjLys-C will be used for further biochemical and structural studies and provide a potential use in aquaculture and medicine.