Friction Mediates Scission of Tubular Membranes Scaffolded by BAR Proteins

International audienceMembrane scission is essential for intracellular trafficking. While BAR domain proteins such as endophilin have been reported in dynamin-independent scission of tubular membrane necks, the cutting mechanism has yet to be deciphered. Here, we combine a theoretical model, in vitro, and in vivo experiments revealing how protein scaffolds may cut tubular membranes. We demonstrate that the protein scaffold bound to the underlying tube creates a frictional barrier for lipid diffusion; tube elongation thus builds local membrane tension until the membrane undergoes scission through lysis. We call this mechanism friction-driven scission (FDS). In cells, motors pull tubes, particularly during endocytosis. Through reconstitution, we show that motors not only can pull out and extend protein-scaffolded tubes but also can cut them by FDS. FDS is generic, operating even in the absence of amphipathic helices in the BAR domain, and could in principle apply to any high-friction protein and membrane assembly

Contrôle de la courbure et de la mécanique des membranes durant l’endocytose par les protéines à domaines BAR

Many biological phenomena are accompanied by the change in shape of the cell membrane. This process is often mediated by curvature-generating proteins, most notably by those containing one of many BAR domains. At the same time, membrane curvature controls the way proteins interact with one another and so it acts as a vital signaling mechanism in the cell. In our work, presented in two theses, we combine theoretical modeling, high-resolution imaging, and quantitative microscopy techniques to study the assembly of BAR proteins on the membrane and its influence on membrane shape and mechanics. Our simulations elucidate the molecular mechanism underlying the self-assembly of BAR proteins on the membrane and the way their collective behavior affects the large-scale membrane reshaping. Experimental biophysical methods demonstrate a novel mechanism of membrane fission mediated by BAR proteins and molecular motors. It also quantifies how the formation of protein scaffolds alters the mechanical behavior of the membrane. These results are essential for understanding a newly discovered pathway of endocytosis, mediated by a BAR protein endophilin. Our combined theoretical and experimental approach gives vital clues on how the mechanical properties of the membrane may regulate protein dynamics in living cells.De nombreux phénomènes biologiques s'accompagnent de déformations de la membrane cellulaire. Ce processus est souvent induit par des protéines,tout particulièrement par des protéines possédant un des différents domaines BAR. Dans le même temps, la courbure membranaire contrôle l’interactionentre protéines, donc elle représente un mécanisme essentiel de signalisation dans la cellule. Dans mon travail, je combine la modélisation théorique,l’imagerie à haute résolution, et la microscopie quantitative pour étudier l'assemblage des protéines à domaine BAR sur les membranes et leur influencesur la forme et la mécanique membranaire. Mes simulations expliquent le mécanisme moléculaire sous-jacent de l'auto-assemblage des protéinesBAR et la façon dont leur comportement collectif affecte le remodelage de la membrane à grande échelle. Grâce à des expériences de biophysique,j'ai pu mettre en évidence un nouveau mécanisme de fission des tubules membranaires induit par des protéines BAR et des moteurs moléculaires. J'aiégalement étudié la formation de structures en "scaffold" par ces protéines et comment elles modifient le comportement mécanique de la membrane. Ces résultats sont essentiels pour comprendre la voie d'endocytose, découverte récemment, qui est contrôlées par une protéine BAR, l'endophiline. Mon travail qui combine théorie et expériences propose des explications sur la manière dont les propriétés mécaniques de la membrane peuvent réguler la dynamique des protéines dans la cellule

Curving Cells Inside and Out: Roles of BAR Domain Proteins in Membrane Shaping and Its Cellular Implications

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Long-Range Organization of Membrane-Curving Proteins

Biological membranes have a central role in mediating the organization of membrane-curving proteins, a dynamic process that has proven to be challenging to probe experimentally. Using atomic force microscopy, we capture the hierarchically organized assemblies of Bin/amphiphysin/Rvs (BAR) proteins on supported lipid membranes. Their structure reveals distinct long linear aggregates of proteins, regularly spaced by up to 300 nm. Employing accurate free-energy calculations from large-scale coarse-grained computer simulations, we found that the membrane mediates the interaction among protein filaments as a combination of short- and long-ranged interactions. The long-ranged component acts at strikingly long distances, giving rise to a variety of micron-sized ordered patterns. This mechanism may contribute to the long-ranged spatiotemporal control of membrane remodeling by proteins in the cell

How curvature-generating proteins build scaffolds on membrane nanotubes

International audienceBin/Amphiphysin/Rvs (BAR) domain proteins control the curvature of lipid membranes in endocytosis, trafficking, cell motility, the formation of complex subcellular structures, and many other cellular phenomena. They form 3D assemblies that act as molecular scaffolds to reshape the membrane and alter its mechanical properties. It is unknown, however, how a protein scaffold forms and how BAR domains interact in these assemblies at protein densities relevant for a cell. In this work, we use various experimental, theoretical, and simulation approaches to explore how BAR proteins organize to form a scaffold on a membrane nanotube. By combining quantitative microscopy with analytical modeling, we demonstrate that a highly curving BAR protein endophilin nucleates its scaffolds at the ends of a membrane tube, contrary to a weaker curving protein centaurin, which binds evenly along the tube's length. Our work implies that the nature of local protein-membrane interactions can affect the specific localization of proteins on membrane-remodeling sites. Furthermore, we show that amphipathic helices are dispensable in forming protein scaffolds. Finally, we explore a possible molecular structure of a BAR-domain scaffold using coarse-grained molecular dynamics simulations. Together with fluorescence microscopy, the simulations show that proteins need only to cover 30-40% of a tube's surface to form a rigid assembly. Our work provides mechanical and structural insights into the way BAR proteins may sculpt the membrane as a high-order cooperative assembly in important biological processes