Proteins Shaping Membranes : Quantitative Measurements

Membrane transport between intracellular compartments, entry or exit out of the cell, imply similar sequential events: membrane deformation and lipid/protein sorting during the formation of the transport intermediate (vesicle or tube), fission from the donor compartment, transport and eventually fusion with the acceptor membrane. The mechanisms behind these biological processes of membrane transformation are actively studied both in the cell biology and the biophysics contexts. Membrane nanotubes with a controlled diameter (15-500 nm) pulled out of Giant Unilamellar Vesicles (GUV) are very convenient tools to address the role of curvature in trafficking events and to measure mechanical effects due to protein binding using optical tweezers.As an example of this type of approach combining in vitro experiments and theoretical modeling, I will present our results on two proteins implied in clathrin-mediated endocytosis. Amphiphysin 1 contains a N-BAR membrane-binding domain. We have shown that at low protein density on the GUV, the distribution of proteins and the mechanical effects induced are well described by a model based on spontaneous curvature induction. At high densities, the radius and force are independent of tension and vesicle protein density, resulting from the formation of a scaffold around the tube. For the entire density range, protein was found to be enriched on the tube as compared to the GUV, showing a concomitant curvature-sensing ability. I will compare this behavior with that of another protein, the dynamin, induced the scission of the clathrin-coated vesicle in cells. In contrast, there is a threshold for the tube radius above which no binding occurs, but below which dynamin forms a scaffold constricting the tube.