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

How does crowding among disordered domains overcome the ability of BAR scaffolds to stabilize lipid tubules? One explanation is that steric pressure among the bulky disordered domains of Amph-FL inhibits the assembly of a long-range N-BAR scaffold, which, if allowed to form, would inhibit fission. In support of this hypothesis, when Amph-FL reached over 70% surface coverage as described above (Fig. S2F) , the underlying N-BAR domain covered only about 16% of the membrane, based on membrane footprints for Amph-FL and N-BAR of 79 and 16.5 nm 2 per monomer, respectively (see methods). This coverage is significantly lower than expected for a fully-assembled N-BAR scaffold, which approaches complete coverage (Adam et al., 2015; . Furthermore, the volume available per amphiphysin disordered domain above the N-BAR scaffold is only about 50% of the volume that each domain would be expected to occupy in solution, based on its radius of gyration (Fig. 3I , see calculation in methods). Therefore, the disordered domains would be required to compress substantially to fit around a fullyassembled N-BAR scaffold. Previous work has shown that substantial compression of disordered domains is energetically costly, likely exceeding the cost of membrane deformation . Collectively, these arguments suggest that the presence of amphiphysin's bulky disordered domains inhibits assembly of long-range N-BAR scaffolds.