Bacteriocins are antimicrobial peptides and proteins which are produced by bacteria. The activity of those that are secreted by lactic acid bacteria is generally related to the formation of pores in the cytoplasmic membrane of their target organisms, eventually leading to cell death. This report addresses the initial steps involved in the mode of action of class lla bacteriocins which share a highly conserved N-terminal 'YGIMGV motif. We report the complete primary amino acid sequence of mundticin, a novel class lla bacteriocin of 43 amino acids (Mr 4287.2), produced by Enterococcus mundtii. This bacteriocin was demonstrated to permeate the cytoplasmic membrane of sensitive organisms. Furthermore, we have used mundticin as a prototype of class lla bacteriocins to study the initial steps involved in the pore-forming activity of these compounds by computer modeling. A three-dimensional representation of the molecular hydrophobicity potential around lipid-associating helices revealed an asymmetric distribution of hydrophobic residues along the axis of an a-helix in the central region of mundticin, and the same was found for homologous class lla bacteriocins. The predicted angle of insertion of these a-helical regions in a simulated hydrophobic-hydrophilic interface varied from 30 to 50°. Based on these predictions, we believe that the oblique orientation of these regions in bacteriocins results in the destabilization of the phospholipid bilayer and may facilitate the insertion and/or aggregation of monomers into functional pores in the cytoplasmic membrane.