Myticin is a cysteine-rich peptide produced in three isoforms, A, B and C, by Mytilus galloprovincialis (Mediterranean mussel). Isoforms A and B show antibacterial activity against Gram-positive bacteria, while isoform C is additionally active against the fungus Fusarium oxysporum and bacterium Escherichia coli (streptomycin resistant strain D31). Myticin-prepro is the precursor peptide.
The mature molecule, named myticin, consists of 40 residues, with four intramolecular disulphide bridges and a cysteine array in the primary structure different from that of previously characterised cysteine-rich antimicrobial peptides. The first 20 amino acids are a putative signal peptide, and the antimicrobial peptide sequence is a 36-residue C-terminal extension. Such a structure suggests that myticins are synthesised as prepro-proteins that are then processed by various proteolytic events before storage in the haemocytes as the active peptide. Myticin precursors are expressed mainly in the haemocytes.