Formation and Stability of Membrane Necks from Molecular Simulation

Abstract

In their fluid state, lipid membranes are able to execute a variety of shape transformations. One intriguing example is provided by vesicle budding, i.e., by the expulsion of a small, spherical bud connected to the mother vesicle by a narrow membrane neck. The formation of such necks provides an essential step of many biological processes (cellular uptake, secretion as well as cell division or cytokinesis). In the framework of curvature elasticity, formation and stability of such a narrow neck are determined by local stability conditions, which depend both on the local mean curvature and on the spontaneous curvature of the membrane. However, these conditions ignore the molecular structure of the membrane, which may affect the shape of the neck. Furthermore, because of their locality, the stability conditions cannot describe the overall response of a neck to mechanical perturbations. In order to address these questions, we use molecular simulations to study the neck formation. Budding and neck formation are induced by small solutes such as monosaccharides that adsorb onto the bilayer membranes. If the two leaflets of a membrane are exposed to different solute concentrations, the membrane becomes asymmetric and acquires a certain spontaneous curvature. We start from a spherical vesicle that is exposed to a certain solute concentration in the exterior solution. When we reduce the volume of this vesicle, we observe formation of narrow membrane necks provided the concentration exceeds a certain threshold value. We induce mechanical perturbation using a geometry that resembles micropipette aspiration on the micrometer scale to observe neck opening for which we calculate the associated free energy landscape.