The C-terminal Domain of Pediocin-like Antimicrobial Peptides (Class IIa Bacteriocins) Is Involved in Specific Recognition of the C-terminal Part of Cognate Immunity Proteins and in Determining the Antimicrobial Spectrum

The pediocin-like bacteriocins contain two domains: a cationic N-terminal β-sheet domain that mediates binding of the bacteriocin to the target cell surface and a more hydrophobic C-terminal hairpin-like domain that penetrates into the hydrophobic part of the target cell membrane. The two domains are joined by a hinge, which enables movement of the domains relative to each other. In this study, 12 different hybrid bacteriocins were constructed by exchanging domains between 5 different bacteriocins. The hybrid bacteriocins were by and large highly potent (i.e. similar potencies as the parental bacteriocins) when constructed such that the recombination point was in the hinge region, indicating that the two domains function independently. The use of optimal recombination points was, however, crucial. Shifting the recombination point just one residue from the hinge could reduce the activity of the hybrid by 3–4 orders of magnitude. Most interestingly, the active hybrids displayed target cell specificities similar to those of the parental bacteriocin from which their membrane-penetrating C-terminal hairpin domain was derived. The results also indicate that the negatively charged aspartate reside in the hinge of most pediocin-like bacteriocins interacts with the C-terminal hairpin domain, perhaps by interacting with the positively charged residue that is present at one of the last three positions in the C-terminal end of most pediocin-like bacteriocins. Bacteria that produce pediocin-like bacteriocins also produce a cognate immunity protein that protects the producer from being killed by its own bacteriocin. Four different active hybrid immunity proteins constructed by exchanging regions between three different immunity proteins were tested for their ability to confer immunity to the hybrid bacteriocins. The results showed that the C-terminal half of the immunity proteins contains a region that directly or indirectly specifically recognizes the membrane-penetrating C-terminal hairpin domain of pediocin-like bacteriocins. The implications these results have on how pediocin-like bacteriocins and their immunity proteins interact with cellular specificity determinants (for instance a putative bacteriocin receptor) are discussed.