The intramolecular disulfide-stapled structure of laterosporulin, a class IId bacteriocin, conceals a human defensin-like structural module.

The growing emergence of antibiotic-resistant bacteria has led to the exploration of naturally occurring defense peptides as antimicrobials. In this study, we found that laterosporulin (LS), a class IId bacteriocin, effectively kills active and nonmultiplying cells of both Gram-positive and Gram-negative bacteria. Fluorescence and electron microscopy suggest that growth inhibition occurs because of increased membrane permeability. The crystal structure of LS at 2.0 angstrom resolution reveals an all-beta conformation of this peptide, with four beta-strands forming a twisted beta-sheet. All six intrinsic cysteines are intramolecularly disulfide-bonded, with two disulfides constraining the N terminus of the peptide and the third disulfide crosslinking the extreme C terminus, resulting in the formation of a closed structure. The significance of disulfides in maintaining the in-solution peptide structure was confirmed by CD and fluorescence analyses. Despite a low overall sequence similarity, LS has disulfide connectivity [C-I-C-V, C-II-C-IV, and C-III-C-VI] like that of beta-defensins and a striking architectural similarity with alpha-defensins. Therefore LS presents a missing link between bacteriocins and mammalian defensins, and is also a potential antimicrobial lead, in particular against nonmultiplying bacteria.