Energetics of Membrane Necking: Role of Forces and Protein Induced Curvature

Abstract

The formation and constriction of membrane necks is a fundamental step that immediately precedes fission in membrane remodeling processes like endocytosis, viral egress, and cytokinesis. Extensive experimental studies have identified key molecular players in these processes, such as the GTPase dynamin in the case of several endocytic pathways. Structural and biophysical studies have identified that constriction of the neck by dynamin helices triggers membrane fission. However, a detailed physical understanding of the force and energetic requirement for neck constriction is difficult to ascertain using experimental techniques due to the small length scales nearing self contact. In this work, we apply a modified Helfrich theory of lipid membranes to investigate membrane necking in cylindrical tubes. Specifically, we seek to understand how applied forces and protein-induced spontaneous curvature contribute to induce necking and energy distribution on the membrane. Application of a radial constricting force, mimicking the action of dynamin, leads to a snapthrough instability in the shape of the membrane. We observe a large energy barrier that can be overcome by force to achieve a narrow neck. This instability is eliminated by applying a region of spontaneous curvature. Additionally, we achieve a closed, tear drop shaped bud when we apply a squeezing tangential force to the membrane. Finally, an out of plane twisting force is also considered in a cylindrical co-ordinate system. These results will help to elucidate the mechanical requirements for dynamin-catalyzed membrane necking and other techniques of constriction and scission.