Adsorption of Bovine Serum Albumin to Lipid Membranes Increases the Number and Stability of Ion Channels of Gramicidin A

Abstract

Local mechanical properties of lipid membranes participate in the regulation of biological processes. Adsorption of peripheral proteins may induce lateral inhomogeneity in membranes. We studied the interaction of bovine serum albumin (BSA) with lipid membranes and its effect on the characteristics of ion channels formed by gramicidin A (gA). We proposed a qualitative mechanism for the induction of local membrane deformations by adsorbed BSA molecules. In the presence of gA in the membrane, BSA adsorption changes the characteristics of the transmembrane ion current, presumably due to gA-BSA lateral interactions mediated by membrane deformations induced by gA and BSA. With the one-sided addition of gA to the membrane, subsequent one-sided or two-sided addition of BSA significantly increased the gA-mediated ion current. We assumed that this is due to an increase in the rate of equalization of the gA concentration in two membrane monolayers by its transition through the membrane (flip-flop). In addition, it was experimentally observed that the addition of BSA to a membrane with symmetrically inserted gA increased the ion current many-fold, suggesting an increase in the average number of conducting gA dimers. An increase in the average lifetime of the gA channels was also shown. We developed a theoretical model that accounts for membrane deformations in three states of gA in the membrane: two monomers, coaxial pair, and conducting dimer. The calculated energy of membrane deformations in these states depended strongly on the presence of BSA, leading to the shift of the monomer–dimer equilibrium of gA toward dimers. The model qualitatively describes the experimentally observed effects of BSA on the characteristics of the gA channel.