Molecular characterization of a novel antimicrobial peptide from Mytilus coruscus

Antimicrobial peptides (AMPs) are components of the innate immune responses that form the first line of host defense against pathogens. Marine mussels can produce a surprising abundance of cysteine-rich AMPs pertaining to the defensin, myticin, mytilin and mytimycin families, particularly in the circulating hemocytes. In the current study, we purified and characterized a novel cysteine-rich peptide with remarkable antibacterial activity from Mytilus coruscus and designated with myticusin-1, a 104-amino acid long polypeptide including 10 cysteine residues forming an unusual cysteine pattern. Antimicrobial assays demonstrated that myticusin-1 exhibited stronger anti-microbial properties against Gram-positive bacteria more than Gram-negative bacteria and fungus. Furthermore, myticusin-1 caused significant morphological alterations in both Sarcina luteus and Escherichia coli as shown by transmission electron microscopy (TEM). The cDNA of myticusin-1 was cloned and sequenced from the hemocytes cDNA library of M. coruscus. The mRNA transcripts of myticusin-1 are mainly detected in hemocyte, which indicates that myticusin-1 are specifically synthesized and stored in circulating hemocytes. The expression level of myticusin-1 in hemocytes was up-regulated and reached the highest level at 36 h after S. luteus challenge, which was 20-fold increase compared to that of the control group. These results indicated that myticusin-1 was involved in the host immune response against bacterial infection and might contribute to the clearance of invading bacteria.