The Effect of Proteins and Lipids on Membrane Stiffness

Abstract

The dynamical and elastic properties of membranes are important for a wide range of biological functions, such as endo- and exocytosis, autophagy and tabulation. The key elastic parameter of a membrane, as defined by Helfrich-Canham theory, is the bending modulus [1]. Proteins that can either sense regions with a low bending modulus or can locally reduce the bending modulus, leading to increased membrane curvature, such as BAR domains, have been extensively studied. Little is known, however, about what effect homeostatic integral membrane proteins, such as ion channels and pores, have on the elastic properties of membranes. For example the bending modulus is reduced when SERCA1a, a calcium ATPase, is inserted into GUVs [2]. Measuring the bending modulus experimentally is difficult. We therefore have simulated the dynamics of a range of large (50,000+ lipids) unary, binary and ternary membranes using a coarse-grained description, MARTINI. Our computer simulations suggest that adding integral membrane proteins (such as Kir2.2, Aqp0 or BtuB) at physiological concentrations reduces the bending rigidity of the membrane. Furthermore, increasing the proportion of cholesterol in the ternary lipid mixture increases the bending rigidity. Finally, we demonstrate that MARTINI describes the dynamics of the membranes by a careful comparison to a long atomistic simulation of 1,500 lipids.1. Brown FLH (2008) Elastic modeling of biomembranes and lipid bilayers. Ann Rev Phys Chem 59:6852. Girard P, Prost J, Bassereau P (2005) Passive or active fluctuations in membranes containing proteins Phys Rev Lett 94:2