Mussel defensins are synthesised and processed in granulocytes then released into the plasma after bacterial challenge

MGD1 (Mytilus galloprovincialis defensin 1), a new member of the arthropod defensin family, is a 4 kDa antibacterial peptide previously isolated from the plasma of Mediterranean mussels, We report here the presence of MGD1 in the organelle-rich fraction of hemocytes and the cDNA sequence correspondi ng to MGD1 and one new isoform mRNA: MGD2, Sequence analysis indicated that MGDs are synthesised as precursors consisting of a putative signal peptide of 21 residues, the active peptide of 39 amino acids and a 21 residue carb oxyl-terminal extension, rich in acidic amino acids. Localisation of the tr anscripts by northern blot revealed that the precursors are abundantly expr essed in hemocytes, Immunocytochemistry at both the optical and ultrastruct ural levels showed that defensins (i) are predominantly located in vesicles of a granulocyte subclass of hemocytes containing small granules, (ii) are also found in large clear granules of another granulocyte subclass, and (i ii) that MGD immune reactivity existed in granular structures of enterocyte s, Finally, we revealed that bacterial challenge triggered a plasmatic incr ease of MGD1 concentration and gave evidence of the simultaneous release of the peptides from the hemocytes.