1.C.21 The Lacticin 481 (Lacticin 481) Family
Many organisms synthesize proteins (or peptides) which are degraded to relatively small hydrophobic or amphipathic, bioactive peptides. These peptides exhibit antibiotic, fungicidal, virucidal, hemolytic and/or tumoricidal activities by interacting with membranes and forming transmembrane channels that allow the free flow of electrolytes, metabolites and water across the phospholipid bilayers. Most of these peptides appear to function in biological warfare. There are many designations given to these bioactive peptides. They include the magainins, cecropins, melittins, defensins, bacteriocidins, etc. The proteins in each family within this functional superfamily are homologous, but they exhibit little or no significant sequence similarity with members of the other families. Thus, each family may have evolved independently. However, certain common structural features observed between members of distinct families suggest that at least some of these families share a common ancestry.
The generalized transport reaction catalyzed by channel-forming amphipathic peptides is:
small solutes, electrolytes and water (in) small solutes, electrolytes and water (out).
Bacteriocins are bacterially produced peptide antibiotics with the ability to kill a limited range of bacteria, usually but not always those that are closely related to the producer bacterium. Many of them exhibit structural features typical of members of the eukaryotic channel-forming amphipathic peptides. That is, they are usually synthesized as small precursor proteins or peptides which are processed with proteolytic elimination of their N-terminal leader sequences, and the resultant mature peptides form one, two or more putative amphipathic transmembrane α-helical spanners (TMSs). For those with two TMSs, a characteristic hinge region that separates the two putative transmembrane segments is usually observed. A similar structural arrangement occurs in the two-TMS Cecropin A proteins (TC #1.C.17).
Many bacteriocins are encoded in operons that also encode an immunity protein and an ABC transport system (TC #3.A.1) with a protease domain at the N-terminus. The ABC systems export the bacteriocins while the protease domains cleave the N-terminal leader sequence. A few bacteriocins are exported by the type II general secretory pathway rather than by ABC-type export systems. In some cases, expression of the bacteriocin-encoding operon is induced by a bacteriocin-like peptide which acts in conjunction with a two component sensor kinase-response regulator to effect induction.
Class I lantibiotic bacteriocins are small membrane-active channel-forming peptides of less than 5 kDa. They contain the unusual amino acids lanthionine and β-methyl lanthionine, as well as dehydrated residues. One member of family 1.C.22 (TC #1.C.22.1.2) is the thiol-activated peptide, Lactococcin B, included in Class IIc by Klaenhammer (1993).
Many bacteriocins have been identified in addition to those tabulated in the TC system, but those listed are among the best characterized, with respect to evidence for channel formation in target bacterial membranes. Class III and IV bacteriocins (Klaenhammer, 1993) are large heat-labile proteins that function by mechanisms unrelated to those of the bacteriocins listed here.
Mutacin II (Mutacin-2) of 53 aas. It dissipates the pmf and the H+ gradient and interfers with energy metabolism (Chikindas et al. 1995).
Mutacin-2 of Streptococcus mutans
Nukacin ISK-1 of 57 aas (Okuda et al., 2008). It is active on Gram-positive bacteria, including Lactobacillus sakei, Leuconostoc mesenteroides and Pediococcus
pentosaceus. The bactericidal activity is based on
depolarization of energized bacterial cytoplasmic membranes, initiated
by the formation of aqueous transmembrane pores (Aso et al. 2004). It is processed and secreted by NukT (TC# 3.A.1.111.7) (Zheng et al. 2017).
Nukacin ISK-1 of Staphylococcus warneri (Q9KWM4)
Cyclic bacteriocin, Group II, Butyrivibriocin ARIO (BviA; 80 aas)
BviA of Butyrivibrio fibrisolvens (Q99Q15)
Salivaricin 9 (SivA; 56 aas; 1 or 2 TMSs) (Wescombe et al., 2011)
SivA of Strepococcus salivarius (Q09I51)
Macedocin, McdA1, a pore-forming lantibiotic of 53 aas
Macedocin of Streptococcus macedonicus
Putative lantibiotic bacteriocin precursor of 71 aas (van Heel et al. 2013).
Bacteriocin of Streptococcus pneumoniae
Lichenicidin prepeptide, LanA of 68 aas
Lichenicidin of Bacillus licheniformis
Lantibiotic, mersacidin, of 69 aas
Mersacidin of Bacillus halodurans
Two component Lacticin 3147 (Ltnα of 59 aas and Ltnβ of 65 aas (Draper et al. 2015). Lacticin 3147 and other lantibiotics target Lipid II to inhibit cell wall synthesis, and then form pores in the membrane (Biswas and Biswas 2014). They target a large number of bacteria, and several mechanisms of pore-formation have been proposed (Draper et al. 2015).
Lactincin 3147 of Streptococcus mutans
Uncharacterized protein of 55 aas
UP of Clostridium saccharobutylicum
Lantibiotic, mersacadin, MrsA, of 58 aa
MrsA of Bacillus subtilis
Uncharacterized protein of 72 aas and 1 TMS. Shows sequence similarity with members of both lantibiotic families, 1.C.21 and 1.C.60.
UP of Lentibacillus amyloliquefaciens