Mode of Action of the Antimicrobial Peptide Indolicidin

Abstract

Indolicidin is a cationic antimicrobial peptide isolated from bovine neutrophils. It consists of only 13 amino acids, has the highest tryptophan content of any known protein, and is amidated at the carboxyl terminus in nature. By circular dichroism spectroscopy a weak poly-L-proline II extended helix structure was observed that became substantially more pronounced upon interaction with liposomes. Indolicidin bound purified surface lipopolysaccharide with high affinity and permeabilized the outer membrane of Escherichia coli to the small hydrophobic molecule 1-N-phenylnapthylamine (M r 200), results consistent with indolicidin crossing the outer membrane via the self-promoted uptake pathway. The methyl esterification of indolicidin's carboxyl terminus increased its activity for Gram-negative and Gram-positive bacteria. In Gram-negative bacteria this was associated with an increased binding to lipopolysaccharide and increased permeabilization of the outer membrane. The cytoplasmic membrane was the site of action of indolicidin as assayed in E. coli by the unmasking of cytoplasmic ␤-galactosidase due to membrane permeabilization. The mechanism for this activity was shown to be the ability of the peptide to cause an increase in the transmembrane current of planar lipid bilayers. This current increase was activated by transmembrane potentials in excess of ؊70 to ؊80 mV. Consistent with this, there was a substantial decrease in indolicidin-mediated bacterial killing and permeabilization of the cytoplasmic membrane of E. coli that had been pretreated with the uncoupler carbonyl cyanidem-chlorophenyl hydrazone. In planar bilayers, indolicidin induced the formation of discrete channels, which ranged in conductance from 0.05-0.15 nS. Thus despite the small size and unique composition of indolicidin, it was capable of killing Gram-negative bacteria by crossing the outer membrane and causing disruption of the cytoplasmic membrane by channel formation.During the past decade a multitude of antimicrobial cationic peptides have been isolated from a wide range of animal, plant, and bacterial species (1). These antimicrobial peptides can be subdivided by composition and secondary structure into four major groups. One group, including cecropins (2) and magainins (3), exhibit an ␣-helical structure in lipid membranes. Such peptides are often unordered in solution. A second group includes those, such as the defensins, that adopt an antiparallel ␤-sheet structure containing one or more disulfide bonds (4).The third group comprises those peptides forming looped structures containing one or more disulfide bonds such as bactenecin (5). The fourth group involves peptides that contain a high percentage of specific amino acids such as the proline-/arginine-rich bovine peptides, Bac5 and Bac7 (6), and the porcine peptide PR-39 (7). The majority of these well characterized antimicrobial peptides range in size from 30 to well over 100 residues in length and consist of a wide range of different amino acids. The mode of action of the majority o...

Alpha-helical cationic antimicrobial peptides: relationships of structure and function

“…To determine the underlying mechanism, many studies analyzed the activities of diastereomers that possess an amphipathic structure, assuming that they play similar roles as native AMPs composed by L-amino acids only (Zasloff, 1987;Mor et al, 1991;Falla et al, 1996;Shai and Oren, 1996).…”

“…Amidation of the C-terminal carboxyl increases antimicrobial activity It has been shown that amidation of the C-terminal carboxyl group is able to improve the antimicrobial activity of AMPs (Falla et al, 1996;Oren and Shai, 1996;Jia et al, 2000;Konno et al, 2007). When the structures of the AMP dermaseptin S3 and its amidated analog were compared, Shalev et al found that the amidated dermaseptin has an induced and/or stabilized α-helical conformation, and thus, is more rigid and extended than the nonamidated analog (Shalev et al, 2002).…”

Cationic Antimicrobial Peptides

“…This process is followed by inhibition of intracellular processes, such as protein synthesis [45], DNA replication [46], or by targeting mitochondria and causing efflux of ATP [5]. Support for these mechanisms of action comes from investigations of BUF [46], histatins [47], the P-rich antimicrobial peptide indolicidin [48], and others [4].…”

Antimicrobial peptides of Lactobacillus salivarius K4 isolated from chicken intestine

“…The fourth group comprises extended peptides with one or two amino acids predominating [7][8][9] . Well characterized antimicrobial peptides have a size varying from 30 to over 100 residues and consist of a wide range of different amino acids 10 . The mode of action of these peptides can be categorized into two mechanisms: transmembrane pore-forming mechanisms and modes of intracellular killing 11,12 .…”