How curvature-generating proteins build scaffolds on membrane nanotubes

Abstract

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

The Matrix protein M1 from influenza C virus induces tubular membrane invaginations in an in vitro cell membrane model

“…In the context of this work, membrane rigidity can be excluded as a likely cause for the difference in tubulation behavior observed for the two membrane compositions. Indeed, the bending rigidity for the DOPS-GUV was found to be approximately four times lower than the bending rigidity of the TBE-GUVs (12 ± 4K B T59 vs 46 ± 4.5K B T67) indicating that from a membrane-mechanics perspective, membrane deformation should be more favorable for the DOPS-GUVs (see for instance ref. 68).…”

Membrane structure formation induced by two types of banana-shaped proteins

“…Some of the BAR superfamily proteins, such as N-BAR proteins, also have hydrophobic insertions. Experimentally, the membrane tubulation and curvaturesensing by various types of BAR superfamily proteins have been observed [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”

The Unsolved Problem of How Cells Sense Micron-Scale Curvature

“…Membrane curvature sensing is best understood for nanometer-sized molecules binding nanometer curvatures such as ArfGAP1 [10–12], α-synuclein [13], and BAR proteins [14, 15]. When compared to typical globular proteins, curvatures at the micron scale would have a thousand-fold larger area (Figure 1A).…”

“…To address this question, we will discuss mechanisms employed by nanometer-scale curvature sensors and how those mechanisms may relate to micron-scale curvature sensors. Nanometer-scale curvature sensors utilize amphipathic helices [11], tune protein-membrane affinity through electrostatic interactions [42], and formation of polymeric scaffolds [15] to sense curvature (a subset of nanometer sensors summarized in Table 2). Interestingly, some evidence suggests that both SpoVM and septins might also use amphipathic helices and electrostatic interactions.…”