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Curvature and Composition Coupling In Lipid Bilayer Membrane Tubes

Aiwei Tian, Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104-6391 and Tobias Baumgart, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104.

Membrane curvature and composition are expected to couple in many fundamental cell-biological processes, such as lipid and protein sorting, and membrane trafficking. These processes are modulated by protein coats in numerous intracellular trafficking pathways. The biophysical mechanisms of these phenomena are not well understood. It is therefore important to quantitatively investigate curvature/composition coupling and test existing biochemical and mechanical models. In this study, we focus on curvature sorting of membrane components, including lipids, peptides and membrane proteins.

In order to vary the degree of membrane curvature, we pull membrane tubes (tethers) from lipid model membranes (giant vesicles) via a combination of micropipette aspiration and a membrane-attached bead. Tether radii were measured using a stepwise pulling and releasing method. Fluorescence intensities of tether were measured by confocal fluorescence imaging in a plane orthogonal to the membrane tether. By correlating tether radii with tether intensities, we found no curvature sorting for double leaflets or outer leaflet labeled DiI derivatives down to the radii about 10nm, Cholera Toxin subunit B (CTB) prefers binding to low curvature membrane. We furthermore examined the diffusion of DiO and CTB in membrane tubes by Fluorescence Recovery after Photobleaching.