cgMolluscidin, a novel dibasic residue repeat rich antimicrobial peptide, purified from the gill of the Pacific oyster, Crassostrea gigas

• A novel antimicrobial peptide, cgMolluscidin, was isolated and cloned from the gill of the Pacific oyster.
• cgMolluscidin was constitutively expressed as a mature form in a non-tissue-specific manner.
• cgMolluscidin showed broad and potent antimicrobial spectrum without hemolysis.
• cgMolluscidin mRNA was up-regulated by the injection of Vibrio sp.
• cgMolluscidin play an important role in the innate defenses of the Pacific oyser.

A 5.5 kDa antimicrobial peptide consisting of 55 amino acids, cgMolluscidin, was purified from the acidified gill extract of the Pacific oyster, Crassostrea gigas, by ion-exchange and C18 reversed-phase high performance liquid chromatography. By comparing the N-terminal amino acid sequences and the molecular weight of this peptide with those of other known antimicrobial peptides, it has been revealed that this peptide had no homology with any known peptides. cgMolluscidin showed potent antimicrobial activity against both Gram-positive bacteria, including Bacillus subtilis, Micrococcus luteus, and Staphylococcus aureus (minimal effective concentrations [MECs]; 1.3–31.3 μg/mL), and Gram-negative bacteria, including Escherichia coli, Salmonella enterica, and Vibrio parahaemolyticus ([MECs]; 0.4–2.3 μg/mL), without hemolytic activity. However, cgMolluscidin did not show any significant activity against Candida albicans. The deduced amino acid sequence of the cgMolluscidin showed no hit in public protein databases, while the nucleotide sequence had a 99% homology (E value = 0) with only the unknown ESTs sequenced by C. gigas EST project. Tissue distribution of the cgMolluscidin mRNA suggests that it is constitutively expressed as a mature form in a non-tissue-specific manner. The cgMolluscidin mRNA expression level was significantly up-regulated at 12 h (2.8-fold) post injection with Vibrio sp. This peptide is highly basic and contains several dibasic residue repeats including Lysine–Lysine or Lysine–Arginine in the sequence, but may not form an ordered structure. These results suggest that cgMolluscidin might be an oyster-specific novel antimicrobial peptide.