Abstract
Curved membranes are an essential feature of dynamic cellular structures, including endocytic pits, filopodia protrusions and most organelles1,2. It has been proposed that specialized proteins induce curvature by binding to membranes through two primary mechanisms: membrane scaffolding by curved proteins or complexes3,4; and insertion of wedge-like amphipathic helices into the membrane5,6. Recent computational studies have raised questions about the efficiency of the helix-insertion mechanism, predicting that proteins must cover nearly 100% of the membrane surface to generate high curvature7,8,9, an improbable physiological situation. Thus, at present, we lack a sufficient physical explanation of how protein attachment bends membranes efficiently. On the basis of studies of epsin1 and AP180, proteins involved in clathrin-mediated endocytosis, we propose a third general mechanism for bending fluid cellular membranes: protein–protein crowding. By correlating membrane tubulation with measurements of protein densities on membrane surfaces, we demonstrate that lateral pressure generated by collisions between bound proteins drives bending. Whether proteins attach by inserting a helix or by binding lipid heads with an engineered tag, protein coverage above ~20% is sufficient to bend membranes. Consistent with this crowding mechanism, we find that even proteins unrelated to membrane curvature, such as green fluorescent protein (GFP), can bend membranes when sufficiently concentrated. These findings demonstrate a highly efficient mechanism by which the crowded protein environment on the surface of cellular membranes can contribute to membrane shape change.
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Acknowledgements
We acknowledge H. McMahon (LMB, Cambridge, UK) and M. Ford (UC Davis, USA) for discussions on this work and contribution of reagents. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering (membrane studies and development, J.C.S., D.Y.S., Sandia), DEAC02-05CH11231 (membrane modelling, P.L.G., C.J.R.) and Division of Chemical Sciences, Geosciences, and Biosciences (fluorescence imaging and analysis, C.C.H., Sandia); as well as the Laboratory Directed Research and Development program at Sandia National Laboratories (engineered vesicle design, J.C.S., D.Y.S., C.C.H., D.A.F.); the NIH NIGMS and Nanomedicine Development Centers (protein membrane interactions, D.A.F., E.M.S., H.S.A.); a Sealy and Smith Foundation grant to the Sealy Center for Structural Biology and Molecular Biophysics (cryo-electron microscopy facility, M.B.S.); and the Miller Institute for Basic Research in Science (E.M.S.). Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
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J.C.S., E.M.S., M.B.S. and C.C.H. performed experiments. E.M.S., H.S.A. and D.Y.S. created unique materials. C.J.R., P.L.G. and C.C.H. performed simulations and modelling. All authors designed experiments. J.C.S., E.M.S., C.J.R., D.A.F. and C.C.H. wrote the manuscript. All authors discussed the results and commented on the manuscript.
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Stachowiak, J., Schmid, E., Ryan, C. et al. Membrane bending by protein–protein crowding. Nat Cell Biol 14, 944–949 (2012). https://doi.org/10.1038/ncb2561
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DOI: https://doi.org/10.1038/ncb2561
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