Big DefensinApBD1 from the scallopArgopecten purpuratusis an antimicrobial peptide which entraps bacteria through nanonets formation

Antimicrobial peptides (AMPs) are ancient innate immune components. Big defensins is a family of AMPs found in a restricted number of animal phyla, in particular mollusks where they have highly diversified. Big defensins are composed of a highly hydrophobic N-terminal region and a C-terminal β-defensin-like region, stabilized by three disulfide bridges. They have been shown to be active against both Gram-positive, Gram-negative bacteria and fungi. Antimicrobial aggregates called nanonets entrapping bacteria have been recently described as the mechanism of action of the Cg -BigDef1 from the oyster Crassostrea gigas . Specifically, the N-terminal domain of Cg -BigDef1 was identified as responsible of nanonet formation. In order to determine whether nanonets are specific to oyster Cg -BigDef1 or common to other big defensins outside this species, we assessed the potential entrapping of bacteria through nanonets of the big defensin from the scallop Argopecten purpuratus , namely Ap BD1. Recombinant Ap BD1 was produced as a fusion polypeptide which carried a N-terminal His6 tag, with a thrombin cleavage site before the mature peptide sequence and an unfolded C-terminal domain by mutating the last Cys to Arg. Activity of r Ap BD1 was assessed against the gram-positive bacteria Staphylococcus aureus SG511. r Ap BD1 inhibited bacterial growth. Moreover, strong immune staining of r Ap BD1 in numerous areas surrounding bacteria was observed. Overall, results suggest that r Ap BD1 entrap bacteria in peptide aggregates similar to those reported to Cg -BigDef1. This study demonstrates the conservation of nanonet formation across big defensins and supports further a role for the N-terminal domain in this conserved process.