Kinetically different populations of O-pyromellityl-gramicidin channels induced by poly-L-lysines in lipid bilayers

Electrostatically induced recruitment of membrane peptides into clusters requires ligand binding at both interfaces

Protein recruitment to specific membrane locations may be governed or facilitated by electrostatic attraction, which originates from a multivalent ligand. Here we explored the energetics of a model system in which this simple electrostatic recruitment mechanism failed. That is, basic poly-L-lysine binding to one leaflet of a planar lipid bilayer did not recruit the triply-charged peptide (O-Pyromellitylgramicidin). Clustering was only observed in cases where PLL was bound to both channel ends. Clustering was indicated (i) by the decreased diffusional PLL mobility D_PLL and (ii) by an increased lifetime τ_PLL of the clustered channels. In contrast, if PLL was bound to only one leaflet, neither D_PLL nor τ_P changed. Simple calculations suggest that electrostatic repulsion of the unbound ends prevented neighboring OPg dimers from approaching each other. We believe that a similar mechanism may also operate in cell signaling and that it may e.g. contribute to the controversial results obtained for the ligand driven dimerization of G protein-coupled receptors.

Molecular Motion at Soft and Hard Interfaces: From Phospholipid Bilayers to Polymers and Lubricants

Spatially resolved and time-resolved understanding of complex fluid situations compose a new frontier in physical chemistry. Here we draw attention to the significance of spatially resolving systems whose ensemble average differs fundamentally from the spatially resolved individual elements. We take examples from the field of fluid phospholipid bilayers, to which macromolecules adsorb; the field of polymer physics, when flexible chains adsorb to the solid-liquid interface; and from the field of lubrication, when two solids are squeezed close together with confined fluid retained between them.

Sensitized photoinactivation of minigramicidin channels in bilayer lipid membranes

The method of sensitized photoinactivation based on the photosensitized damage of gramicidin A (gA) molecules was applied here to study ionic channels formed by minigramicidin (the 11-residue analogue of gramicidin A) in a planar bilayer lipid membrane (BLM) of different thickness. Irradiation of BLM with a single flash of visible light in the presence of a photosensitizer (aluminum phthalocyanine or Rose Bengal) generating singlet oxygen provoked a decrease in the minigramicidin-induced electric current across BLM, the kinetics of which had the characteristic time of several seconds, as observed with gA. For gA, there is good correlation between the characteristic time of photoinactivation and the single-channel lifetime. In contrast to the covalent dimer of gA characterized by extremely long single-channel lifetime and the absence of current relaxation upon flash excitation, the covalent head-to-head dimer of minigramicidin displayed the flash-induced current decrease with the kinetics being strongly dependent on the membrane thickness. The current decrease became slower both upon increasing the concentration of the minigramicidin covalent dimer and upon including cholesterol in the membrane composition. These data in combination with the quadratic dependence of the current on the peptide concentration can be rationalized by hypothesizing that the macroscopic current across BLM measured at high concentrations of the peptide is provided by dimers of minigramicidin covalent dimers in the double beta(5.7)-helical conformation having the lifetime of about 0.4 s, while single channels with the lifetime of 0.01 s, observed at a very low peptide concentration, correspond to the single-stranded beta(6.3)-helical conformation. Alternatively the results can be explained by clustering of channels at high concentrations of the minigramicidin covalent dimer.

Grafting of polylysine with polyethylenoxide prevents demixing of O-pyromellitylgramicidin in lipid membranes

Both natural and synthetic polycations can induce demixing of negatively charged components in artificial and possibly in natural membranes. This process can result in formation of clusters (binding of several components to a polycation chain) and/or domains (aggregation of clusters and formation of a separate phase enriched in some particular component). In order to distinguish between these two phenomena, a model lipid membrane system containing ion channels, formed by a negatively charged peptide, O-pyromellitylgramicidin, and polycations of different structures was used. Microelectrophoresis of liposomes, changes in boundary potential of planar bilayers, the shape of compression curves and potentials of lipid and lipid/peptide monolayers were used to monitor the electrostatic factors in polymer adsorption to the membrane and peptide-polymer interactions. The synthesized PEO-grafted polylysine, PLL-PEO20000, did not induce peptide demixing monitored by stabilization of the gramicidin channels, in contrast to parent polylysine (PLL). Both polymers were shown to bind effectively to negatively charged liposomes and lipid monolayers, suggesting that the ineffectiveness of PLL-PEO20000 was not due to reduction of its binding. It was hypothesized that PLL-PEO20000 could not induce domain formation due to steric hindrance of long PEO chains preventing lateral fusion of clusters. Another copolymer, PLL-PEO4000, having four PEO chains of 4000 Da, exhibited intermediate effect between PLL and PLL-PEO20000, which shows the importance of the copolymer architecture for the effect on the lateral distribution of OPg channels. The model system can be relevant to regulation of lateral organization of ion channels and other components in natural membrane systems.

Gramicidin-based channel systems for the detection of protein-ligand interaction

To detect protein-ligand interaction a gramicidin-based sensor was developed. Biotin was tagged to the C-terminus of gramicidin (Gram-bio 1). The biotin-moiety, which faces the electrolyte, gave little effect on single-channel conductance. Streptavidin added to the electrolyte was detected by Gram-bio 1 through the monitoring channel current using the planar bilayer system. The suppression of macroscopic currents and the acceleration of their decaying time course were observed in a concentration dependent manner. In the single-channel level, however, no significant effect on the single-channel conductance and the open dwell time was observed upon addition of streptavidin. Therefore, streptavidin neither blocked the open channel nor changed the stability of the conducting dimer. Insertion of a linker between gramicidin and biotin did not change the streptavidin-sensitivity of the current reduction. We conclude that the binding of streptavidin to the Gram-bio 1 shifted the distribution of the complex from the membrane to the electrolyte and, thus, reduced the formation of conducting dimer of Gram-bio 1 in the membrane. Interaction of biotin with an anti-biotin antibody was also observed using this system, indicating that this system is applicable for the detection of protein-ligand interaction having a binding constant of approximately 10(8-9) M(-1) or more. Both the adamantane-tagged gramicidin for detection of beta-cyclodextrin and the Strep Tag-II-tagged gramicidin for detection of streptavidin (binding constant: approximately 10(5) M(-1) or less) failed to respond. Thus, high-affinity ligands upon tagging to gramicidin render the gramicidin-based sensor able to execute as a real-time monitoring system for protein-ligand interaction.

Cytochrome c decelerates channel kinetics of negatively charged gramicidin due to electrostatic interaction

The effect of cytochrome c on the kinetic properties of ion channels formed by O-pyromellitylgramicidin (OPg), the negatively charged analogue of gramicidin A (gA), in bilayer lipid membranes was studied by the method of sensitized photoinactivation. The addition of cytochrome c to both sides of the membrane caused substantial deceleration of the photoinactivation kinetics of OPg channels which expose three negative charges to the aqueous phase at both sides of the membrane. By contrast, the gA photoinactivation kinetics was unaltered by the addition of cytochrome c. Based on the sensitivity of the observed effect to the ionic strength of the bathing solution, the cytochrome c-induced deceleration of the OPg photoinactivation kinetics reflecting the increase in the OPg channel lifetime was ascribed to electrostatic interaction of positive charges of cytochrome c with negative charges of OPg that resulted in channel clustering. Formation of clusters of OPg channels was previously inferred to explain the polylysine effect on the OPg channel kinetics. The decelerating effect of cytochrome c on OPg channels was observed only at a high number of OPg channels in the membrane, thus suggesting that the interaction between cytochrome c and the charged transmembrane protein requires sufficiently high negative charge density on the surface of the membrane.