Protein-Induced Membrane Shape Instability: Dynamics and Membrane Tension Dependence

Abstract

Cellular membranes undergo constant shape remodeling involving the formation of highly curved structures. However, the physical and molecular mechanisms governing the transition of membrane shapes are still not clear. Here, we seek to demystify this problem by quantifying the dynamics of membrane tubulation induced by the binding of peripheral proteins. A recently developed single giant unilamellar vesicle (GUV) transfer method is utilized to record the association of fluorescence labeled-proteins onto single GUVs as well as the simultaneous changes in GUV shape. Endophilin, a BAR domain-containing protein, is chosen due to its well-know role in promoting the formation of transport vesicles during clathrin-mediated endocytosis.During endophilin-membrane association, we observe membrane tubulation when protein density on the relatively flat GUV reaches a critical level, as marked by a sudden decrease of GUV membrane area. The critical protein density is found to increase with membrane tension, which is accurately controlled through micro-pipette aspiration. Furthermore, the roles of lipid composition are investigated. PE lipids which provide more lipid packing defects in the bilayer are found to reduce the protein density required for initiating membrane tubulation. The negatively-charged PS lipids, while playing essential roles in determining the membrane binding rates and affinity of endophilin, do not alter the membrane instability criterion.The observed membrane instability criterion as a function of both membrane tension and critical protein density, agrees well with a theoretical curvature instability model. In this model, membrane-bound diffusive proteins, which can be treated as two-dimensional van der Waals gas, couple with the local out-of-plane fluctuations of the bilayer to drive the planar membrane unstable. The results give new insights into the dynamics of biological membranes and support a mechanism in which membrane tension is used to regulate dynamic transport and signaling processes in cells.