Involvement of mytilins in mussel antimicrobial defense

Four cationic peptides were purified from mussel (Mytilus galloprovincialis ) hemocytes, A combination of Edman degradation and mass spectrometry of pl asma revealed (i) a previously characterized molecule, mytilin B (Charlet, M., Chernysh, S., Philippe, H., Hetrut, C., Hoffmann, J., and Bulet, P. (19 96) J. Biol. Chem. 271, 21808-21813) and (ii) three new isoforms, mytilin C , D, and G1. The four molecules exhibited complementary antimicrobial prope rties. The cDNA sequence coding for the mytilin B precursor was obtained fr om a hemocyte cDNA library, This precursor contains a putative signal pepti de of 22 residues, a processing peptide sequence of 34 amino acids, and a C -terminal extension of 48 residues rich in acidic residues. Distribution of mytilin B mRNA and of the corresponding peptide in various mussel tissues revealed that mytilins are synthesized and stared in a specific hemocyte su btype. Furthermore, in an experimental model of infection, we showed (i) a recruitment of hemocytes containing mytilins toward the injection site with in hours following bacterial challenge, (ii) that mytilins probably play a prominent role in Billing intracellular bacteria after phagocytosis, and (i i) later an increase of mytilin-like material occurred in the plasma sugges ting a secondary systemic role.