ESCRTing Receptor Down‐Regulation: Assembly and Function of the ESCRT‐III Complex

Abstract

The Endosomal Sorting Complexes Required for Transport (ESCRTs) comprise a highly conserved set of five protein complexes that function as a ubiquitin‐dependent protein sorting machine which recognizes and directs the down‐regulation of membrane proteins including signaling receptors. Recent studies have identified the ESCRT‐III complex as a vesicle budding machine that can sort cargo into intraluminal vesicles (ILV) which bud into an endosomal multi‐vesicular body (MVB). The mechanisms by which soluble ESCRT‐III subunits (Vps20, Snf7, Vps24 and Vps2) mediate ILV budding are not yet known. Here, we present data indicating that conformational changes within the ESCRT‐III subunit Snf7 (displacement of helix‐5) activate Snf7 for membrane binding. Using electron microscopy techniques, we directly visualize ESCRT‐III assembly on lipid monolayers. Activated Snf7 generates long ~9nm‐wide protofilaments composed of two ~4nm sub‐filaments. These protofilaments form 2‐D spiral arrays on the monolayers. Strikingly, the addition of Vps24 and Vps2 transforms the flat spirals into 3‐D membranesculpting helices. Finally, addition of ESCRT‐II stabilizes the formation of ESCRT‐III rings that are ~65 nm in diameter. We will discuss the role of domains in ESCRT‐III subunits that function in cargo capture, membrane binding/deformation and scission during intraluminal vesicle formation.