Estimation of negative membrane tension in lipid bilayers and its effect on antimicrobial peptide magainin 2-induced pore formation

Positive membrane tension in the stretched plasma membrane of cells and in the stretched lipid bilayer of vesicles has been well analyzed quantitatively, whereas there is limited quantitative information on negative membrane tension in compressed plasma membranes and lipid bilayers. Here, we examined negative membrane tension quantitatively. First, we developed a theory to describe negative membrane tension by analyzing the free energy of lipid bilayers to obtain a theoretical equation for negative membrane tension. This allowed us to obtain an equation describing the negative membrane tension (sigma(osm)) for giant unilamellar vesicles (GUVs) in hypertonic solutions due to negative osmotic pressure (Pi). Then, we experimentally estimated the negative membrane tension for GUVs in hypertonic solutions by measuring the rate constant (k(r)) of rupture of the GUVs induced by the constant tension (sigma(ex)) due to an external force as a function of sigma(ex). We found that larger sigma(ex) values were required to induce the rupture of GUVs under negative Pi compared with GUVs in isotonic solution and quantitatively determined the negative membrane tension induced by Pi (sigma(osm)) by the difference between these sigma(ex) values. At small negative Pi, the experimental values of negative sigma(osm) agree with their theoretical values within experimental error, but as negative Pi increases, the deviation increases. Negative tension increased the stability of GUVs because higher tensions were required for GUV rupture, and the rate constant of antimicrobial peptide magainin 2-induced pore formation decreased.