Myticin, a novel cysteine-rich antimicrobial peptide isolated from haemocytes and plasma of the mussel Mytilus galloprovincialis

We report here the isolation of two isoforms of a novel cysteine-rich pepti de from haemocytes (isoform A of 4.438 Da and B of 4.562 Da) and plasma (is oform A) of the mussel, Mytilus galloprovincialis. The two molecules displa y antibacterial activity against gram-positive bacteria, whereas only isofo rm B is active against the fungus Fusarium oxysporum and a gram-negative ba cteria Escherichia coli D31. Complete peptide sequences were determined by a combination of Edman degradation, mass spectrometry and cDNA cloning usin g a haemocyte cDNA library. The mature molecules, named myticins, comprise 40 residues with four intramolecular disulfide bridges and a cysteine array in the primary structure different to that of the previously characterized cysteine-rich antimicrobial peptides. Sequence analysis of the cloned cDNA s revealed that myticin precursors consist of 96 amino acids with a putativ e signal peptide of 20 amino acids, the antimicrobial peptide sequence and a 36-residue C-terminal extension. This structure suggests that myticins ar e synthesized as preproproteins and then processed by various proteolytic e vents before storage of the active peptide in the haemocytes. Myticin precu rsors are expressed mainly in the haemocytes as revealed by Northern blot a nalysis.