Septin-Mediated Uniform Bracing of Phospholipid Membranes

SummaryCell shape is determined by the interplay between the lipid bilayer and the underlying network of protein polymers [1]. We explored unknown determinants involved in cell morphogenesis as factors that transform phospholipid-based liposomes (diameter 5–20 μm). Unlabeled giant liposomes, observed through dark-field optics, were metastable in an aqueous suspension. In contrast, liposomes robustly protruded uniform tubules immediately after the addition of a brain extract to the suspension. The tubulation reaction was greatly facilitated when the liposomes contained PIP or PIP2. Biochemical analysis of the brain extract revealed that heteromeric complexes of septins, a family of polymerizing GTP/GDP-binding proteins, are responsible for the membrane transformation. Ultrastructural analysis established that each membrane tubule (diameter 0.43 ± 0.079 μm) is braced by a circumferential array of septin filaments. Although submembranous septin assemblies are associated with diverse cortical morphogenesis from yeast to mammals 2, 3, 4 and 5, the biophysical basis for the septin-membrane interplay remains largely unknown. Further, there is a biochemical discrepancy between the fast septin remodeling in cells and their slow self-assembly in vitro 6 and 7. This membrane-facilitated fast septin assembly demonstrated for the first time by our unique experimental system should provide important clues to characterize these processes.