Budding of membranes induced by intramembrane domains

Abstract

Abstract. — Membranes are often composed of a mixture of amphiphilic molecules whichaggregate into clusters or domains. A simple theoretical model is introduced which predictsthat flat or weakly curved domains become unstable at a certain limiting size and then undergoa budding or invagination process. This shape transformation is primarily driven by the linetension of the domain edge. It is also predicted that the budding domain can rupture themembrane and then pinch off from the matrix. The size of the bud and the time scales involvedin the budding dynamics are estimated for model membranes composed of lipid mixtures. Thisinstability mechanism should also be effective for the budding of biomembranes. 1. Introduction. Membranes such as lipid bilayers are highly flexible and thus can easily change their shape.Recently, a variety of shape transformations have been observed by phase contrast microscopyof giant lipid vesicles [1-7]. It was found that these vesicles can exhibit budding or invaginationprocesses in which small vesicles bud off from a larger membrane surface. These processescan be simply induced by a change in temperature. Such temperature-induced budding canbe understood theoretically if one assumes that the lipid bilayer of the vesicle is laterallyhomogeneous. Its shape is then determined, to a large extent, by the area to volume ratio.The thermal expansivity of the bilayer membrane is large compared to that of the water.Therefore, as the temperature is increased, the bilayer membrane expands more rapidly thanthe enclosed water, and the area to volume ratio of the vesicles is increased.In biological cells, budding is a rather frequent event since it represents the first step inthe production of transport vesicles which shuttle between different compartments of the cell[8-11]. Two budding processes can be distinguished: (i) Endocytosis of the plasma membrane,i.e., budding of the plasma membrane towards the