Formation of Membrane Tubular Protrusions upon Localized Application of Calcium Ions to the Surface of Giant Lipid Vesicles

We present a study on the formation of lipid nanotubular protrusions as a result of localized application of calcium ions to the membrane of giant lipid vesicles. Our findings demonstrate that a point source of calcium ions acting upon a negatively charged membrane generates spontaneous curvature sufficient to induce membrane bending and the formation of nanotubular protrusions. The observed membrane behaviour can be explained as the response to the trans-bilayer mismatch of surface charge density upon calcium ion binding from the exposed side of the bilayer. By using molecular dynamics simulations, we validate this experimental observation, finding that calcium ion binding to a lipid membrane is sufficient to generate spontaneous curvature, bending the membrane away from the ion source. The observed calcium-ion guided tubulation in lipid membranes can be a significant step towards gaining insights into cell membrane re-shaping in response to chemical stimulation, and can further our understanding of the fundamental mechanisms by which lipids redistribute within cell membranes.