A large collaboration work led by Patricia Bassereau Group describes the general mechanism of how a protein scaffold can cut tubular membranes. This discovery was published in the prestigious Cell revue.
Membrane scission is essential for intracellular trafficking. Dynamin-mediated scission occurring during clathrin-mediated endocytosis is a relatively well-understood process. However, recent work, both from the Johannes lab at the Institut Curie (Renard et al, Nature 2015) and the McMahon lab in Cambridge (Boucrot el al, Nature 2015) have shown that curved BAR-domain proteins such as endophilin A2 are involved in scission of tubular membrane structures that mediate clathrin-independent endocytosis. The scission mechanism, however, had yet to be deciphered.
In the Membranes and Cellular Functions group most recent paper, appearing on the cover of the June 29th 2017 edition of Cell, it combined in vitro and in vivo experiments as well as theoretical modeling that reveal a general mechanism of how a protein scaffold — a self-assembled rigid sleeve made up of BAR proteins — can cut tubular membranes. In vitro, researchers reconstituted the scission process using purified BAR-domain proteins interacting with membrane nanotubes. These nanotubes, that mimic the in vivo geometry of endocytosis, were mechanically pulled from Giant Unilamellar Vesicles (GUVs) using micromanipulation and optical tweezers. They demonstrated that the protein scaffold bound to the underlying tube creates a frictional barrier for lipid diffusion or advection. Upon tube elongation, which is produced in vivo by molecular motors that pull on the tubular membrane when moving along the cytoskeleton, local membrane tension builds up until the membrane undergoes scission through lysis. Time-lapse imaging, using confocal fluorescence microscopy in Figure 1A, shows an example of scission of a thin tubular membrane. Here, a 20-nm wide tubular membrane, seen as a green line, is scaffolded by a BAR protein endophilin and is connected to the GUV on one side (top) and the optical trap on the other side (bottom). Upon extending the tube, it breaks. To understand what is happening at the microscopic level, the group developed a physical model for scission, which relies on a stochastic pore nucleation event at the scaffold/bare tube boundary that causes scission. Biophysicists called this new scission mechanism “friction-driven scission” (FDS) Through reconstitution using kinesins as molecular motors, they showed that motors can not only pull out and extend protein-scaffolded tubes, but also cut them by FDS. Their combined experimental and theoretical work allowed to understand how a membrane can rupture in a cell without being cut though protein conformational change, as in dynamin; rather, it relies on relatively simple, yet well orchestrated, physics.
Although very different at the molecular level, one can macroscopically model this mechanism by representing the tubular membrane using cooked spaghetti, which has elastic properties, and pulling it out of honey, a highly viscous medium representing the BAR protein scaffold. Due to the friction between spaghetti and honey, spaghetti noodle gets stretched up to its rupture point (see Figure 1B).
This work results from a large collaboration between experimentalist biophysicists of Institut Curie (P. Bassereau’s group, and J. B. Manneville), theoretical biophysicists from University Paris Diderot (A. Callan-Jones, MSC lab.) and Institut Curie (J. Prost), together with cell biologists from Institut Curie (L. Johannes’s group), Cambridge University (H. McMahon’s group, MRC Lab of Molecular Biology) and Vanderbilt University in Nashville (A. Kenworthy’s group), as well as with a computational biophysicist from the University of Chicago (G. Voth).
Friction Mediates Scission of Tubular Membranes Scaffolded by BAR Proteins
Mijo Simunovic, Jean-Baptiste Manneville, Henri-François Renard, Emma Evergren, Krishnan Raghunathan, Dhiraj Bhatia, Anne K. Kenworthy, Gregory A. Voth, Jacques Prost, Harvey T. McMahon, Ludger Johannes, Patricia Bassereau
Cell / DOI: http://dx.doi.org/10.1016/j.cell.2017.05.047 / June 22, 2017