Synthesis of acylated gramicidins and the influence of acylation on the interfacial properties and conformational behavior of gramicidin A

Idealizing ion channel recordings by a jump segmentation multiresolution filter

Based on a combination of jump segmentation and statistical multiresolution analysis for dependent data, a new approach called J-SMURF to idealize ion channel recordings has been developed. It is model-free in the sense that no a-priori assumptions about the channel’s characteristics have to be made; it thus complements existing methods which assume a model for the channel's dynamics, like hidden Markov models. The method accounts for the effect of an analog filter being applied before the data analysis, which results in colored noise, by adapting existing muliresolution statistics to this situation. J-SMURF’s ability to denoise the signal without missing events even when the signal-to-noise ratio is low is demonstrated on simulations as well as on ion current traces obtained from gramicidin A channels reconstituted into solvent-free planar membranes. When analyzing a newly synthesized acylated system of a fatty acid modified gramicidin channel, we are able to give statistical evidence for unknown gating characteristics such as subgating.

Design and characterization of gramicidin channels

This article summarizes methods for the chemical synthesis and biophysical characterization of gramicidins with varying sequences and labels. The family of gramicidin channels has developed into a powerful model system for understanding fundamental properties, interactions, and dynamics of proteins and lipids generally, and ion channels specifically, in biological membranes.

The monolayer technique: a potent tool for studying the interfacial properties of antimicrobial and membrane-lytic peptides and their interactions with lipid membranes

Erudites of the antiquity already knew the calming effect of oil films on the sea waves. But one had to wait until 1774 to read the first scientific report on oil films from B. Franklin and again 1878 to learn the thermodynamic analysis on adsorption developed by J. Gibbs. Then, in 1891, Agnes Pockels described a technique to manipulate oil films by using barriers. Finally, in 1917, I. Langmuir introduced the experimental and theoretical modern concepts on insoluble monolayers. Since that time, and because it has been found to provide invaluable information at the molecular scale, the monolayer technique has been more and more extensively used, and, during the past decade, an explosive increase in the number of publications has occurred. Over the same period, considerable and ever-increasing interest in the antimicrobial peptides of various plants, bacteria, insects, amphibians and mammals has grown. Because many of these antimicrobial peptides act at the cell membrane level, the monolayer technique is entirely suitable for studying their physicochemical and biological properties. This review describes monolayer experiments performed with some of these antimicrobial peptides, especially gramicidin A, melittin, cardiotoxins and defensin A. After giving a few basic notions of surface chemistry, the surface-active properties of these peptides and their behavior when they are arranged in monomolecular films are reported and discussed in relation to their tridimensional structure and their amphipathic character. The penetration of these antimicrobial peptides into phospholipid monolayer model membranes, as well as their interactions with lipids in mixed films, are also emphasized.

Effect of tryptophan-N-formylated gramicidin on growth of Plasmodium berghei in mice

The effect of tryptophan-N-formylated gramicidin (NFG) on the growth of Plasmodium berghei in mice was tested in three different experiments. NFG was shown to be capable of inhibiting the growth of the parasite in a dose-dependent way, although its action did not result in elimination of the parasite and was only temporary, preventing mice from early death, presumably due to cerebral malaria, but not from fatal generalized malaria. Intriguingly, a similar observation was made with two other drugs, (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine, an inhibitor of viral and eukaryotic DNA polymerases, and the presumed topoisomerase II inhibitor, a bisquaternary quinolinium salt. A rise in the level of parasitemia after 8 days, despite continued treatment, was not due to parasite-induced reticulocytosis, as demonstrated in experiments in which this condition was induced artificially. NFG was added in the form of lipid vesicles in which the peptide had been incorporated. The inhibitory action of NFG was not modulated by the lipid composition of the vesicles. Control experiments did not demonstrate any toxicity of NFG when it was administered in lipid vesicles. The main observation is that NFG is able to inhibit the growth of a malaria parasite in vivo at concentrations that are well tolerated by the host.

Effects of gramicidin and tryptophan-N-formylated gramicidin on the sodium and potassium content of human erythrocytes

In order to get a better understanding in the mechanism by which tryptophan-N-formylated gramicidin (NFG) and gramicidin kill the malaria parasite Plasmodium falciparum in vitro, we studied the capacity of these peptides to change the potassium, as well as the sodium, composition of normal human erythrocytes, and their ability to cause cell lysis. It is shown that both peptides are able to induce potassium leakage from, and sodium flux into, erythrocytes in such a manner that it is most likely that they are able to form cation channels in the membrane of these cells. For both peptides, potassium efflux proceeds at a faster rate than sodium influx, but this difference is greater for NFG than for gramicidin. This explains the observation that gramicidin is more lytic than NFG is, even when comparing concentrations that show the same antimalarial activity. The finding that gramicidin is approximately 10 times more active than NFG in causing potassium efflux from normal erythrocytes, as well as in killing the malaria parasite, supports the hypothesis that peptide-induced parasite death is related to their capacity to induce potassium leakage from infected erythrocytes. Finally, the observation that erythrocytes are able to restore their normal ion contents after losing more than 50% of their potassium content by incubation with NFG or gramicidin, suggests that, in vivo, and upon treatment with drug concentrations that cause full inhibition of parasite growth, these cells would not be irreversibly damaged by action of the drugs.

Palmitoylation-induced conformational changes of specific side chains in the gramicidin transmembrane channel

To gain insight into the structural consequences of acylation for membrane proteins, we have covalently attached palmitic acid to the ethanolamine end of gramicidin A (gA), which functions as a well-characterized cation-selective membrane channel. Next, we investigated by NMR methods the effect of acylation on the side chains of Trp9, Leu10, and Trp11, which are expected to be close to the acyl chain, and of Val7, which is expected to be far from the acyl chain. Two-dimensional NMR spectroscopy in a sodium dodecyl sulfate (SDS) environment suggests that one of the beta-hydrogens of Leu10 of gA is severely shielded by a nearby aromatic ring. This shielding disappears upon acylation. Deuterium NMR spectra for labeled samples in hydrated dimyristoylphosphatidylcholine (DMPC) bilayers show that, for the major gA conformation, the (deuterated) side chains of Trp9 and Leu10 are markedly influenced by acylation, whereas the side chains of Val7 and Trp11 are essentially unaffected. The NMR results in both environments suggest that the indole ring of Trp9 is situated near the side chain of Leu10 and moves away upon acylation. We propose that acylation provides a subtle mechanism to modulate protein and lipid interactions and to regulate the stability and function of proteins within membranes.

Stage-dependent effects of analogs of gramicidin A on the growth of Plasmodium falciparum in vitro

Tryptophan-N-formylated gramicidin A, a nonhemolytic derivative of the toxic peptide antibiotic gramicidin A, has previously been shown to induce potassium leakage from Plasmodium falciparum-infected erythrocytes in vitro and to inhibit the growth of the parasite. In the present study the antimalarial activities of two other nonhemolytic derivatives of gramicidin A, viz., acylated gramicidin A and desformylated gramicidin A, were tested and compared with those of gramicidin A and tryptophan-N-formylated gramicidin A. The 50% growth-inhibitory concentrations (IC50 values) of the four compounds varied from 0.3 to 18.3 nM, and complete growth inhibition was detected within one parasitic growth cycle. Using highly synchronized cultures of P. falciparum, it was furthermore shown that the gramicidin analogs are inhibitory to all developmental stages of the parasite, although their efficiency in accomplishing growth inhibition was found, as expected, to be clearly stage-dependent and to increase with the age of the parasite.

The influence of acylation on the lipid structure modulating properties of the transmembrane polypeptide gramicidin

In order to get insight into the effect of acylation of a transmembrane polypeptide on the interaction of the polypeptide with the membrane lipids we used 31P-NMR to investigate the influence of acylated gramicidins on the polymorphic phase behavior of hydrated dispersions of 1-palmitoyllysophosphatidylcholine (lyso-PC), 1,2-dioleoylphosphatidylcholine (DOPC) and 1,2-dielaidoylphosphatidylethanolamine (DEPE). Palmitoylgramicidin induces a micelle to extended bilayer organization in lyso-PC with a slightly lower efficiency than the parent gramicidin molecule. In DOPC and DEPE acylgramicidins induce the formation of HII phase at the expense of a bilayer organization with a similar high efficiency as gramicidin. The ability of acylgramicidin to induce lipid mixing between vesicles prepared of DOPC was decreased relative to gramicidin. The results are discussed in the light of the proposed models for gramicidin-induced HII phase formation and emphasize that gramicidin itself has a very strong lipid structure modulating activity.