Mechanical Properties of Membranes under Asymmetric Buffer Conditions

Abstract

Biological membranes consist of molecular bilayers which are intrinsically asymmetric in nature. This asymmetry can be induced not only by leaflet composition and specific adsorption but also by differences in the cytosolic and periplasmic solutions containing macromolecules and ions. Membranes are surrounded by aqueous buffers inside and outside the cell exhibiting strong concentration asymmetry of e.g. sodium, potassium and chlorine ions. There has been a long quest to understand the effect of these ions on the physical and morphological properties of membranes. Ion-lipid interactions and, in particular, the effect of ion trans membrane asymmetry are crucial not only for the membrane phase state [Kubsch et al. Biophys. J. 110:2581-2584, 2016] but also influence the mechanical properties of membranes. Here, we set to explore the changes in the mechanical properties of membranes exposed to asymmetric buffer conditions. As a model membrane, we employed giant unilamellar vesicles (GUVs) and first improved existing protocols for generating GUVs in physiologically relevant salt concentrations. To assess the membrane mechanical properties, we aspirate a GUV into a micropipette and by means of an attached bead manipulated via optical tweezers, we pull an outward tube to measure the spontaneous curvature and the bending rigidity of the bilayer. With increasing the aspiration pressure, the bead is displaced from the equilibrium position in the optical trap, which in return gives us the bending rigidity and spontaneous curvature of GUVs [Lipowsky, Faraday Discuss. 161:305-331, 2013]. We explore the effect of asymmetric distribution of salt and sugars across the membrane. This work is part of the MaxSynBio consortium which is jointly funded by the Federal Ministry of Education and Research of Germany and the Max Planck Society.