Identification and functional characterisation of a novel antimicrobial peptide from the housefly, <i>Musca domestica</i>

Antimicrobial peptides (AMPs) are increasingly being recognised as promising alternatives to conventional antibiotics due to their distinctive antimicrobial mechanisms and reduced likelihood of inducing drug resistance. Insects represent a significant source of AMPs. In this study, a potential AMP gene, MdAMP5, was identified based on its strong immunoinducibility and the presence of a signal peptide, and an amphipathic α-helix in the encoded protein. MdAMP5 encoded a 50-amino acid precursor protein with an N-terminal 22-amino acid signal peptide. The calculated molecular mass of the mature protein was 2.92 kDa, with an estimated isoelectric point of 6.23. Structural analyses revealed that the N-terminus of mature MdAMP5 contained an irregularly coiled segment, while the C-terminus featured an amphipathic α-helix with a glycine-lysine residue at the end. Furthermore, the MdAMP5 gene was successfully expressed in the yeast Pichia pastoris, and the recombinant MdAMP5 (rMdAMP5) protein exhibited effective and broad-spectrum antimicrobial activity against both Gram-positive and Gram-negative bacteria in vitro and in vivo. Treatment with rMdAMP5 resulted in significant changes in bacterial morphology, including cell lysis and deformation of bacteriophages. In conclusion, this study identified and successfully expressed a novel AMP that showed low cytotoxicity to mammalian cells and high selectivity towards bacterial cells. This research offers a new candidate for therapeutic drug development, and enhances the understanding of the mechanism and application of AMPs.