Tryptophan-rich antimicrobial peptides: comparative properties and membrane interactions

Effect of ionic strength on the organization and dynamics of tryptophan residues in erythroid spectrin: A fluorescence approach

The ionic strength of the medium plays an important role in the structure and conformation of erythroid spectrin. The spectrin dimer is a flexible rod at physiological ionic strength. However, lower ionic strength results in elongation and rigidification (stiffening) of spectrin as shown earlier by electron microscopy and hydrodynamic studies. The ionic strength induced structural transition does not involve any specific secondary structural changes. In this article, we have used a combination of fluorescence spectroscopic approaches that include red edge excitation shift (REES), fluorescence quenching, time-resolved fluorescence measurements, and chemical modification of the spectrin tryptophans to assess the environment and dynamics of tryptophan residues of spectrin under different ionic strength conditions. Our results show that while REES, fluorescence anisotropy, lifetime, and chemical modification of spectrin tryptophans remain unaltered in low and high ionic strength conditions, quenching of tryptophan fluorescence by the aqueous quencher acrylamide (but not the hydrophobic quencher trichloroethanol) and resonance energy transfer to a dansyl-labeled fatty acid show differences in tryptophan environment. These results, which report tertiary structural changes in spectrin upon change in ionic strength, are relevant in understanding the molecular details underlying the conformational flexibility of spectrin.

Antimicrobial peptides: New candidates in the fight against bacterial infections

Antimicrobial peptides (AMPs) of innate origin are agents of the most ancient form of defense systems. They can be found in a wide variety of species ranging from bacteria through insects to humans. Through the course of evolution, host organisms developed arsenals of AMPs that protect them against a large variety of invading pathogens including both Gram-negative and Gram-positive bacteria. At a time of increasing bacterial resistance, AMPs have been the focus of investigation in a number of laboratories worldwide. Although recent studies show that some of the peptides are likely to have intracellular targets, the vast majority of AMPs appear to act by permeabilization of the bacterial cell membrane. Their activity and selectivity are governed by the physicochemical parameters of the peptide chains as well as the properties of the membrane system itself. In this review, we will summarize some of the recent developments that provide us with a better understanding of the mode of action of this unique family of antibacterial agents. Particular attention will be given to the determinants of AMP-lipid bilayer interactions as well as to the different pore formation mechanisms. The emphasis will be on linear AMPs but representatives of cysteine-bridged AMPs will also be discussed.

Factors contributing to the potency of antimicrobial cationic peptides from the N-terminal region of human lactoferrin

This study investigated the antimicrobial activities of peptides derived from the N-terminal region of human lactoferrin, and examined the contributions of individual residues to the activity of the most potent peptide. Two regions of antimicrobial activity were identified, the first corresponding to a weakly active peptide, HLP-9, comprising residues 1-9, and a second corresponding to a more potent peptide, HLP-10, comprising residues 18-26 and containing the hexapeptide motif, FQWQRN. Inhibitory studies on peptides from the first region confirm the importance of tryptophan residues in enhancing and broadening peptide activity. Inhibitory studies with glycine-substituted homologues of the more potent peptide showed that F21/G and R25/G substitutions resulted in a major reduction or complete loss of activity, while increased peptide cationicity or flexibility had little effect. Our findings demonstrate that F21 and R25 are critical determinants of potency for HLP-10, and that the second aromatic residue may act synergistically with W23 in developing and enhancing the activity of this cationic peptide.

Monitoring gramicidin conformations in membranes: a fluorescence approach

We have monitored the membrane-bound channel and nonchannel conformations of gramicidin utilizing red-edge excitation shift (REES), and related fluorescence parameters. In particular, we have used fluorescence lifetime, polarization, quenching, chemical modification, and membrane penetration depth analysis in addition to REES measurements to distinguish these two conformations. Our results show that REES of gramicidin tryptophans can be effectively used to distinguish conformations of membrane-bound gramicidin. The interfacially localized tryptophans in the channel conformation display REES of 7 nm whereas the tryptophans in the nonchannel conformation exhibit REES of 2 nm which highlights the difference in their average environments in terms of localization in the membrane. This is supported by tryptophan penetration depth measurements using the parallax method and fluorescence lifetime and polarization measurements. Further differences in the average tryptophan microenvironments in the two conformations are brought out by fluorescence quenching experiments using acrylamide and chemical modification of the tryptophans by N-bromosuccinimide. In summary, we report novel fluorescence-based approaches to monitor conformations of this important ion channel peptide. Our results offer vital information on the organization and dynamics of the functionally important tryptophan residues in gramicidin.

A new type of antimicrobial protein with multiple histidines from the hard tick, Amblyomma hebraeum

A novel 11 kDa antimicrobial protein, named as hebraein, and having a unique amino acid sequence, was purified from the hemolymph of fed female Amblyomma hebraeum ticks. A full-length cDNA clone encoding hebraein was isolated from a cDNA library made from tick synganglia. Hebraein consists of 102 amino acids, including 6 cysteine residues; has 9 histidines in its C-terminal domain that are mainly present as HX repeats; and has no significant similarity to any known protein. The secondary structure prediction is very clearly all alpha-helical (4-6 helices) except for a very short extension at the C terminus. Such high alpha-helical content is quite different from known antimicrobial proteins. Recombinant hebraein and a mutant lacking the histidine residues in the C-terminal domain were constructed and expressed. Assayed at the slightly acidic pH equivalent of fed female tick hemolymph, the wild-type and the histidine-rich recombinant hebraein had stronger antimicrobial activities than the histidine-deficient mutant. The pH-dependent properties of histidine-rich antimicrobial proteins may allow the design of agents that would function selectively in specific pH environments. The results from protein profiling of hemolymph, analyzed by surface-enhanced laser desorption/ionization time-of-flight (SELDI-TOF) mass spectrometry combined with ProteinChip technology and RT-PCR analysis suggested that this antimicrobial protein was up-regulated by blood feeding. Our findings describe a new type of antimicrobial protein with multiple cysteine and histidine residues, and with unique secondary structure.

Chemical and Photochemical Modification of Colicin E1 and Gramicidin A in Bilayer Lipid Membranes

Chemical modification and photodynamic treatment of the colicin E1 channel-forming domain (P178) in vesicular and planar bilayer lipid membranes (BLMs) was used to elucidate the role of tryptophan residues in colicin E1 channel activity. Modification of colicin tryptophan residues by N-bromosuccinimide (NBS), as judged by the loss of tryptophan fluorescence, resulted in complete suppression of wild-type P178 channel activity in BLMs formed from fully saturated (diphytanoyl) phospholipids, both at the macroscopic-current and single-channel levels. The similar effect on both the tryptophan fluorescence and the electric current across BLM was observed also after NBS treatment of gramicidin channels. Of the single-tryptophan P178 mutants studied, W460 showed the highest sensitivity to NBS treatment, pointing to the importance of the water-exposed Trp460 in colicin channel activity. In line with previous work, the photodynamic treatment (illumination with visible light in the presence of a photosensitizer) led to suppression of P178 channel activity in diphytanoyl-phospholipid membranes concomitant with the damage to tryptophan residues detected here by a decrease in tryptophan fluorescence. The present work revealed novel effects: activation of P178 channels as a result of both NBS and photodynamic treatments was observed with BLMs formed from unsaturated (dioleoyl) phospholipids. These phenomena are ascribed to the effect of oxidative modification of double-bond-containing lipids on P178 channel formation. The pronounced stimulation of the colicin-mediated ionic current observed after both pretreatment with NBS and sensitized photomodification of the BLMs support the idea that distortion of membrane structure can facilitate channel formation.

Ion channel-like activity of the antimicrobial peptide tritrpticin in planar lipid bilayers

The cationic peptide tritrpticin (VRRFPWWWPFLRR, Trp3) has a broad action spectrum, acting against Gram-positive and Gram-negative bacteria, as well as some fungi, while also displaying hemolytic activity. We have studied the behavior of Trp3 in planar lipid bilayers (or black lipid membrane - BLM) and were able to demonstrate its ion channel-like activity. Channel-like activity was observed in negatively charged azolectin BLM as a sudden appearance of discrete current fluctuations upon application of a constant voltage across the membrane. Trp3 formed large conductance channels (500-2000 pS) both at positive and negative potentials. In azolectin bilayers, the predominant ion-channel activity was characterized by very regular and discrete current steps (corresponding to openings) of uniform amplitude, which exhibited relatively long residence times (of the order of seconds). Occasionally, multiple conductance steps were observed, indicating the simultaneous presence of more than one open pore. In bilayers of zwitterionic diphytanoylphosphatidyl choline (DPhPC) Trp3 also showed ion-channel activity, but in a much less frequent and less prominent way. Studies of ion selectivity indicated that Trp3 forms a cation-selective channel. These results should contribute to the understanding of the molecular interactions and mechanism of action of Trp3 in lipid bilayers and biological membranes.

Conformation of a bactericidal domain of puroindoline a: structure and mechanism of action of a 13-residue antimicrobial peptide

Puroindoline a, a wheat endosperm-specific protein containing a tryptophan-rich domain, was reported to have antimicrobial activities. We found that a 13-residue fragment of puroindoline a (FPVTWRWWKWWKG-NH(2)) (puroA) exhibits activity against both gram-positive and gram-negative bacteria. This suggests that puroA may be a bactericidal domain of puroindoline a. PuroA interacted strongly with negatively charged phospholipid vesicles and induced efficient dye release from these vesicles, suggesting that the microbicidal effect of puroA may be due to interactions with bacterial membranes. A variety of biophysical and biochemical methods, including fluorescence spectroscopy and microcalorimetry, were used to examine the mode of action of puroA. These studies showed that puroA is located at the membrane interface, probably due to its high content of Trp residues that have a high propensity to partition into the membrane interface. The penetration of these Trp residues in negatively charged phospholipid vesicles resembling bacterial membranes was more extensive than the penetration in neutral vesicles mimicking eukaryotic membranes. Peptide binding had a significant influence on the phase behavior of the former vesicles. The three-dimensional structure of micelle-bound puroA determined by two-dimensional nuclear magnetic resonance spectroscopy indicated that all the positively charged residues are oriented close to the face of Trp indole rings, forming energetically favorable cation-pi interactions. This characteristic, along with its well-defined amphipathic structure upon binding to membrane mimetic systems, allows puroA to insert more deeply into bacterial membranes and disrupt the regular membrane bilayer structure.