Abstract: Big defensins are cysteine-rich cationic antimicrobial peptides (AMPs) formed by serially connecting the N-terminal hydrophobic domain with the C-terminal β-defensin-like domain. These domains have different antibacterial activities. Moreover, big defensins have conserved salt-stable antimicrobial activity and the N-terminal domain can drive the self-assembly of big defensins into nanonetworks that entrap and kill bacteria. So far, big defensins have been mainly found in the innate immune system of invertebrate animals and amphioxus that is a primitive chordate holding a key phylogenetic position bridging from invertebrates to vertebrates. Moreover, the C-terminal domain of big defensins has similar structure and bioactivity to the β-defensin of vertebrates, which has aroused interest in investigating the biological evolution and function of defensins and the evolution of the immune system in species related to defensins. Based on the progress that has been made in the identification and characterization of big defensins in recent years, herein we present an integrated review of the molecular structures and evolution of big defensins, together with their antibacterial mechanism and expression regulation as well as their application prospects in the fields of aquaculture and aquatic product safety and in the development of new antimicrobial drugs. This review will provide a reference in the application of big defensins.

Key words: big defensins; invertebrates; disulfide bond; expression; antibacterial activity