Structural Insights into the Mode of Action of the Peptide Antibiotic Copsin

Defensins make up
a class of cysteine-rich antimicrobial peptides,
expressed by virtually all eukaryotes as part of their innate immune
response. Because of their unique mode of action and rapid killing
of pathogenic microbes, defensins are considered promising alternatives
to clinically applied antibiotics. Copsin is a defensin-like peptide,
previously identified in the mushroom Coprinopsis cinerea. It exerts its activity against a range of Gram-positive bacteria
by binding to the peptidoglycan precursor lipid II and prevention
of proper cell wall formation. In this study, we present a new workflow
for the generation, production, and activity-driven selection of copsin
derivatives, based on their expression in Pichia pastoris. One hundred fifty-two single-amino acid mutants and combinations
thereof allowed the identification of k-copsin, a peptide variant
exhibiting significantly enhanced activity against Bacillus
subtilis and Staphylococcus aureus. Furthermore,
we performed in silico characterizations of membrane
interactions of copsin and k-copsin, in the presence and absence of
lipid II. The molecular dynamics data highlighted a high variability
in lipid II binding, with a preference for the MurNAc moiety with
47 and 35% of the total contacts for copsin and k-copsin, respectively.
Mutated amino acids were located in loop regions of k-copsin and shown
to be crucial in the perturbation of the bacterial membrane. These
structural studies provide a better understanding of how defensins
can be developed toward antibacterial therapies less prone to resistance
issues.