It is thought that magainin 2, an antimicrobial peptide, acts by binding to lipid membranes. Recent studies using a suspension of large unilamellar vesicles (LUVs) indicate that magainin 2 causes gradual leakage from LUVs containing negatively charged lipids. However, the details of the characteristics of the membrane permeability and the mechanism of pore formation remain unclear. In this report, we investigated the interaction of magainin 2 with single giant unilamellar vesicles (GUVs) composed of a dioleoylphosphatidylcholine and dioleoylphosphatidylglycerol mixture (50% DOPG/50% DOPC GUVs) containing the fluorescent dye, calcein, by phase contrast, fluorescence microscopy using the single GUV method. Low concentrations (3-10 microM) of magainin 2 caused the rapid leakage of calcein from single GUVs but did not disrupt the liposomes or change the membrane structure, showing directly that magainin 2 forms membrane pores through which calcein leaked. The rapid leakage of calcein from a GUV started stochastically, and once it began, the complete leakage occurred rapidly (6-60 s). The fraction of completely leaked GUV, P(L), increased with time and also with an increase in magainin 2 concentration. Shape changes in these GUVs occurred prior to the pore formation and also at lower concentrations of magainin 2, which could not induce the pore formation. Their analysis indicates that binding of magainin 2 to the external monolayer of the GUV increases its membrane area, thereby raising its surface pressure. The addition of lysophosphatidylcholine into the external monolayer of GUVs increased P(L). On the basis of these results, we propose the two-state transition model for the pore formation.

Antimicrobial peptide magainin 2 forms pores in lipid membranes and induces membrane permeation of the cellular contents. Although this permeation is likely the main cause of its bactericidal activity, the mechanism of pore formation remains poorly understood. We therefore investigated in detail the interaction of magainin 2 with lipid membranes using single giant unilamellar vesicles (GUVs). The binding of magainin 2 to the lipid membrane of GUVs increased the fractional change in the area of the membrane, δ, which was proportional to the surface concentration of magainin 2, X. This indicates that the rate constant of the magainin 2-induced two-state transition from the intact state to the pore state greatly increased with an increase in δ. The tension of a lipid membrane following aspiration of a GUV also activated magainin 2-induced pore formation. To reveal the location of magainin 2, the interaction of carboxyfluorescein (CF)-labeled magainin 2 (CF-magainin 2) with single GUVs containing a water-soluble fluorescent probe, AF647, was investigated using confocal microscopy. In the absence of tension due to aspiration, after the interaction of magainin 2 the fluorescence intensity of the GUV rim due to CF-magainin 2 increased rapidly to a steady value, which remained constant for a long time, and at 4-32 s before the start of leakage of AF647 the rim intensity began to increase rapidly to another steady value. In contrast, in the presence of the tension, no increase in rim intensity just before the start of leakage was observed. These results indicate that magainin 2 cannot translocate from the outer to the inner monolayer until just before pore formation. Based on these results, we conclude that a magainin 2-induced pore is a stretch-activated pore and the stretch of the inner monolayer is a main driving force of the pore formation.

The cell-penetrating peptide, transportan 10 (TP10), can translocate across the plasma membrane of living cells and thus can be used for the intracellular delivery of biological cargo such as proteins. However, the mechanisms underlying its translocation and the delivery of large cargo remain unclear. In this report we investigated the entry of TP10 into a single giant unilamellar vesicle (GUV) and the TP10-induced leakage of fluorescent probes using the single GUV method. GUVs of 20% dioleoylphosphatidylglycerol (DOPG)/80% dioleoylphosphatidylcholine (DOPC) were prepared, and they contained a water-soluble fluorescent dye, Alexa Fluor 647 hydrazide (AF647), and smaller vesicles composed of 20% DOPG/80% DOPC. The interaction of carboxyfluorescein (CF)-labeled TP10 (CF-TP10) with these loaded GUVs was investigated using confocal microscopy. The fluorescence intensity of the GUV membrane increased with time to a saturated value, then the fluorescence intensity due to the membranes of the smaller vesicles inside the GUV increased prior to leakage of AF647. This result indicates that CF-TP10 entered the GUV from the outside by translocating across the lipid membrane before CF-TP10-induced pore formation. The rate constant of TP10-induced pore formation in lipid membranes increased with an increase in TP10 concentration. Large molecules such as Texas Red Dextran 40,000, and vesicles with a diameter of 1-2 μm, permeated through the TP10-induced pores or local rupture in the lipid membrane. These results provide the first direct experimental evidence that TP10 can deliver large cargo through lipid membranes, without the need for special transport mechanisms such as those found in cells.

Antimicrobial peptide magainin 2 forms pores in lipid membranes to induce leakage of internal contents of cells, which is a main cause of its bactericidal activity. However, the conditions and the mechanism of its pore formation remain unclear. In this report, to reveal the effect of the surface charge density of membranes on magainin 2-induced pore formation, we investigated the interaction of magainin 2 with giant unilamellar vesicles (GUVs) composed of a mixture of electrically neutral dioleoylphosphatidylcholine (DOPC) and negatively charged dioleoylphosphatidylglycerol (DOPG) in various ratios, using the single GUV method. We found that magainin 2 induced pores in the membranes of all kinds of single GUVs. For GUVs with the same charge density, the rate of the pore formation increased with magainin 2 concentration. The magainin 2 concentrations in a buffer required to induce the same rate of the pore formation greatly increased with a decrease in the surface charge density; e.g., the magainin 2 concentrations required for the pore formation in 30% DOPG/70% DOPC-GUVs were 50 times higher than those in 60% DOPG/40% DOPC-GUVs. However, after we converted the magainin 2 concentration in the buffer into that in the membrane interface, Xbmag, we found that Xbmag mainly determines the rate of the pore formation in various GUVs. These data support our model of two-state transition from the binding state to the pore state of the GUV for magainin 2-induced pore formation.

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