BAR scaffolds drive membrane fission by crowding disordered domains

Wilton T. Snead; Wade F. Zeno; Grace Kago; Ryan W. Perkins; J Blair Richter; Chi Zhao; Eileen M. Lafer; Jeanne C. Stachowiak

We next asked how the membrane remodeling ability of crowded disordered domains compares with that of BAR scaffolds. To make this comparison, we created a chimeric protein that places the two mechanisms in direct competition within the same molecule. Specifically, we fused the inverted BAR (I-BAR) domain of human IRSp53 to the bulky, C-terminal disordered domain of rat AP180 (569 disordered amino acids) to form I-BAR-AP180 CTD (Fig. 5A ). While the I-BAR domain is known to generate inverted membrane curvature (Mattila et al., 2007; Saarikangas et al., 2009) , the attached disordered domains should generate steric pressure that will tend to bend the membrane in the opposite direction. Exposing GUVs to the I-BAR domain alone drove inverted membrane tubulation, as expected (Fig. 5B , left and Movie S6). In contrast, the I-BAR-AP180 CTD chimera drove neither inward nor outward tubulation. Instead, the protein caused the GUV membrane to fluctuate dynamically through non-spherical shapes in which regions of gentle membrane curvature extending both inward and outward were apparent, (Fig. 5B , right and Movie S7). These "frustrated" fluctuations demonstrate that the disordered domain effectively neutralized the ability of the I-BAR domain to drive inward membrane bending. This result suggests that crowding among disordered domains and scaffolding by BAR domains make comparable contributions to membrane remodeling.