Gram-positive lactobacilli produce bacteriocins to kill closely-related competitor species. To protect themselves from the bacteriocidal activity of this molecule they co-express an immunity protein (for discussion of this operon see Bacteriocin_IIc pfam10439). The immunity protein structure is a soluble, cytoplasmic, antiparallel four alpha-helical globular bundle with a fifth, more flexible and more divergent C-terminal helical hair-pin. The C-terminal hair-pin recognizes the C-terminus of the producer bacteriocin and this interaction is sufficient to dis-orient the bacteriocin within the membrane and close up the permeabilising pore that on its own the bacteriocin creates. These immunity proteins interact in the same way with other bacteriocins, family Bacteriocin_II, pfam01721. Since many enterococci can produce more than one bacteriocin it seems likely that the whole operon can be carried on transferable plasmids.