A member of the arthropod defensin family from edible Mediterranean mussels (Mytilus galloprovincialis)

Plasma from the mussel Mytilus galloprovincialis previously immunized by injecting them with bacteria contains several bactericidal proteins. One protein, MGD-1, was purified by reverse-phase HPLC of supernatant from acidified cell-free hemolymph. Its biological activity is directed against both gram-positive and gram-negative bacteria but it is not cytotoxic towards human erythrocytes nor protozoa. As determined by mass spectrometry, the molecular mass of MGD-1 is 4418 Da. Primary-structure analysis revealed 38 amino acids including 8 cysteines and a modified amino acid residue in position 28. Computer searches unambiguously recognized the signature of an arthropod defensin, but the presence of two extra cysteines and of one modified amino acid suggest that it is a previously unknown member of that family.