Three- dimensional architecture of ESCRT- III flat spirals on the membrane

The endosomal sorting complexes required for transport (ESCRTs) are responsible for membrane remodeling in many cellular processes, such as multivesicular body biogenesis, viral budding, and cytokinetic abscission. ESCRT- III, the most abundant ESCRT subunit, assembles into flat spirals as the primed state, essential to initiate membrane invagination. However, the three- dimensional architecture of ESCRT- III flat spirals remained vague for decades due to highly curved filaments with a small diameter and a single preferred orientation on the membrane. Here, we unveiled that yeast Snf7, a component of ESCRT- III, forms flat spirals on the lipid monolayers using cryogenic electron microscopy. We developed a geometry- constrained Euler angle-assigned reconstruction strategy and obtained moderate- resolution structures of Snf7 flat spirals with varying curvatures. Our analyses showed that Snf7 subunits recline on the membrane with N- terminal motifs alpha 0 as anchors, adopt an open state with fused alpha 2/3 helices, and bend alpha 2/3 gradually from the outer to inner parts of flat spirals. In all, we provide the orientation and conformations of ESCRT- III flat spirals on the membrane and unveil the underlying assembly mechanism, which will serve as the initial step in understanding how ESCRTs drive membrane abscission.