During endocytosis, energy is invested to narrow the necks of cargo-containing plasma membrane invaginations to radii at which the opposing segments spontaneously coalesce, thereby leading to the detachment by scission of endocytic uptake carriers1. In the clathrin pathway, dynamin uses mechanical energy from GTP hydrolysis to this effect2–4, assisted by the BIN/amphiphysin/Rvs (BAR) domain-containing protein endophilin5,6. Clathrin-independent endocytic events are often less reliant on dynamin7, and whether in these cases BAR domain proteins such as endophilin contribute to scission has remained unexplored. Here we found that endophilin-A2 (endoA2) specifically and functionally associates with very early uptake structures that are induced by the bacterial Shiga and cholera toxins, which both are clathrin-independent endocytic cargoes8. In controlled in vitro systems, endoA2 reshapes membranes prior to scission. Furthermore, we demonstrate that endoA2, dynamin, and actin contribute in parallel to the scission of Shiga toxin-induced tubules. Our results establish a novel function of endoA2 in clathrin-independent endocytosis. They document that distinct scission factors operate in an additive manner, and predict that specificity within a given uptake process arises from defined combinations of universal modules. Our findings finally highlight a previously unnoticed link between membrane scaffolding by endoA2 and pulling force-driven dynamic scission.

Cell membranes become highly curved during membrane trafficking, cytokinesis, infection, immune response or cell motion. Bin/amphiphysin/Rvs (BAR) domain proteins with their intrinsically curved and anisotropic shape are involved in many of these processes, but with a large spectrum of modes of action. In vitro experiments and multiscale computer simulations have contributed in identifying a minimal set of physical parameters, namely protein density on the membrane, membrane tension, and membrane shape, that control how bound BAR domain proteins behave on the membrane. In this review, we summarize the multifaceted coupling of BAR proteins to membrane mechanics and propose a simple phase diagram that recapitulates the effects of these parameters.

SummaryMembrane 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.

Adhesion and insertion of curvature-mediating proteins can induce dramatic structural changes in cell membranes, allowing them to participate in several key cellular tasks. The way proteins interact to generate curvature remains largely unclear, especially at early stages of membrane remodeling. Using a coarse-grained model of Bin/ amphiphysin/Rvs domain with an N-terminal helix (N-BAR) interacting with flat membranes and vesicles, we demonstrate that at low protein surface densities, binding of N-BAR domain proteins to the membrane is followed by a linear aggregation and the formation of meshes on the surface. In this process, the proteins assemble at the base of emerging membrane buds. Our work shows that beyond a more straightforward scaffolding mechanism at high bound densities, the interplay of anisotropic interactions and the local stress imposed by the N-BAR proteins results in deep invaginations and endocytic vesicular bud-like deformations, an order of magnitude larger than the size of the individual protein. Our results imply that by virtue of this mechanism, cell membranes may achieve rapid local increases in protein concentration.coarse-grained simulation | membrane stress | self-assembly | membrane curvature | Gaussian curvature L ipid membranes protect cells and their organelles and serve as mechanical support for proteins involved in signal transduction and cellular trafficking (1). Owing to the way lipids are assembled into bilayers of mesoscopic length and molecular width, membranes behave as elastic and highly dynamic molecular sheets. Their innate multiscale nature is further evident in the local interplay between proteins and lipids, which results in large-scale membrane remodeling characterized by an impressive array of morphologies (2). These properties allow biological membranes to take part in several important cellular processes, as their restructuring is key to enabling communication between cells, formation of organelles, trafficking, division, and cell migration (1).The interactions between individual proteins and lipid molecules alone are insufficient to remodel flat membrane sheets to an appreciable extent, implying that the experimentally observed membrane remodeling is a result of the concerted action of multiple proteins (3) in ways mechanistically differing from the mode of action of a single protein. Considering the multiple timescales required to capture protein dynamics as the membrane deformations are induced, as well as the difficulty in separating individual events of the remodeling process, this mechanism remains largely unclear. It previously was demonstrated that the binding of curvature-inducing nanoparticles may form tubular and vesicular structures, in which particles interact with one another solely via curvature (3-6). Furthermore, analytical theory predicts that anisotropic inclusions may induce budding on the membrane surface (7). However, the way proteins assemble on the membrane and how their oligomerization couples to both local and long-wavelength transformat...

Proteins containing a Bin/Amphiphysin/Rvs (BAR) domain regulate membrane curvature in the cell. Recent simulations have revealed that BAR proteins assemble into linear aggregates, strongly affecting membrane curvature and its in-plane stress profile. Here, we explore the opposite question: do mechanical properties of the membrane impact protein association? By using coarse-grained molecular dynamics simulations, we show that increased surface tension significantly impacts the dynamics of protein assembly. While tensionless membranes promote a rapid formation of long-living linear aggregates of N-BAR proteins, increase in tension alters the geometry of protein association. At high tension, protein interactions are strongly inhibited. Increasing surface density of proteins leads to a wider range of protein association geometries, promoting the formation of meshes, which can be broken apart with membrane tension. Our work indicates that surface tension may play a key role in recruiting proteins to membrane-remodelling sites in the cell.

Fate allocation in the gastrulating embryo is spatially organized as cells differentiate to specialized cell types depending on their positions with respect to the body axes. There is a need for in vitro protocols that allow the study of spatial organization associated with this developmental transition. While embryoid bodies and organoids can exhibit some spatial organization of differentiated cells, these methods do not yield consistent and fully reproducible results. Here, we describe a micropatterning approach where human embryonic stem cells are confined to disk-shaped, sub-millimeter colonies. After 42 hours of BMP4 stimulation, cells form self-organized differentiation patterns in concentric radial domains, which express specific markers associated with the embryonic germ layers, reminiscent of gastrulating embryos. Our protocol takes 3 days; it uses commercial microfabricated slides (CYTOO), human laminin-521 (LN-521) as extra-cellular matrix coating, and either conditioned or chemically-defined medium (mTeSR). Differentiation patterns within individual colonies can be determined by immunofluorescence and analyzed with cellular resolution. Both the size of the micropattern and the type of medium affect the patterning outcome. The protocol is appropriate for personnel with basic stem cell culture training. This protocol describes a robust platform for quantitative analysis of the mechanisms associated with pattern formation at the onset of gastrulation.

Cells have an intrinsic ability to self-assemble and self-organize into complex and functional tissues and organs. By taking advantage of this ability, embryoids, organoids and gastruloids have recently been generated in vitro, providing a unique opportunity to explore complex embryological events in a detailed and highly quantitative manner. Here, we examine how such approaches are being used to answer fundamental questions in embryology, such as how cells selforganize and assemble, how the embryo breaks symmetry, and what controls timing and size in development. We also highlight how further improvements to these exciting technologies, based on the development of quantitative platforms to precisely follow and measure subcellular and molecular events, are paving the way for a more complete understanding of the complex events that help build the human embryo.

Bin/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.protein scaffold | BAR proteins | membrane curvature | self-assembly | endocytosis