Gram-positive lactobacilli produce bacteriocins to kill closely-related competitor species [1]. To protect themselves from the bacteriocidal activity of this molecule they co-express an immunity protein (for discussion of this operon see Bacteriocin_IIc Pfam:PF10439). 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 [2]. The C-terminal hair-pin recognises 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 [3]. These immunity proteins interact in the same way with other bacteriocins, family Bacteriocin_II, Pfam:PF01721. Since many enterococci can produce more than one bacteriocin it seems likely that the whole operon can be carried on transferable plasmids [4].

1: 1.6-Angstroms crystal structure of EntA-im. A bacterial immunity protein conferring immunity to the antimicrobial activity of the pediocin-like bacteriocin enterocin A. Johnsen L, Dalhus B, Leiros I, Nissen-Meyer J; J Biol Chem. 2005;280:19045-19050. PMID:15753083

2: Comparative studies of immunity proteins of pediocin-like bacteriocins. Fimland G, Eijsink VG, Nissen-Meyer J; Microbiology. 2002;148:3661-3670. PMID:12427956

3: Molecular dynamics simulation study of interaction between a class IIa bacteriocin and its immunity protein. Soliman W, Bhattacharjee S, Kaur K; Biochim Biophys Acta. 2007;1774:1002-1013. PMID:17586105

4: 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. Johnsen L, Fimland G, Nissen-Meyer J; J Biol Chem. 2005;280:9243-9250. PMID:15611086