The effects of gramicidin on electroporation of lipid bilayers

The effects of the channel-forming peptide gramicidin D (gD) on the conductance and electroporation thresholds of planar bilayer lipid membranes, made of the synthetic lipid 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC), was studied. High-amplitude (~200-900 mV) rectangular voltage pulses of 15 ms duration were used to perturb the bilayers and monitor the transmembrane conductance. Electroporation voltage thresholds were found, and conductance was recorded before and after electroporation Gramicidin was added to the system in peptide/lipid ratios of 1:10,000, 1:500, and 1:15. The addition of gD in a ratio of 1:10,000 had no effect on electroporation, but ratios of 1:500 and 1:15 significantly increased the thresholds by 16% (p < 0.0001) and 40% (p < 0.0001), respectively. Membrane conductance before electroporation was measurable only after the addition of gD and increased monotonically as the peptide/lipid ratio increased. The effect of gD on the membrane area expansivity modulus (K) was tested using giant unilamellar vesicles (GUVs). When gD was incorporated into the vesicles in a 1:15 ratio, K increased by 110%, consistent with the increase in thresholds predicted by an electromechanical model. These findings suggest that the presence of membrane proteins may affect the electroporation of lipid bilayers by changing their mechanical properties.