Plasma membrane deformation by circular arrays of ESCRT-III protein filaments

TLDR

ESCRT‑III proteins are recruited from the cytoplasm to membranes, where they assemble into large complexes that mediate multivesicular body biogenesis and viral budding. We employed deep‑etch electron microscopy to visualize polymers formed by the ESCRT‑III proteins hSnf7‑1 (CHMP4A) and hSnf7‑2 (CHMP4B). Overexpressed hSnf7‑1 and hSnf7‑2 form ~5‑nm filaments that curve into circular arrays, and when recruited to the plasma membrane—especially with an ATPase‑deficient VPS4B mutant or a C‑terminal‑deleted hSnf7‑1—these filaments bend the membrane outward to create buds and tubules, demonstrating that ESCRT‑III polymers can generate negative curvature and help form multivesicular body vesicles.

Abstract

Endosomal sorting complex required for transport III (ESCRT-III) proteins function in multivesicular body biogenesis and viral budding. They are recruited from the cytoplasm to the membrane, where they assemble into large complexes. We used “deep-etch” electron microscopy to examine polymers formed by the ESCRT-III proteins hSnf7-1 (CHMP4A) and hSnf7-2 (CHMP4B). When overexpressed, these proteins target to endosomes and the plasma membrane. Both hSnf7 proteins assemble into regular approximately 5-nm filaments that curve and self-associate to create circular arrays. Binding to a coexpressed adenosine triphosphate hydrolysis–deficient mutant of VPS4B draws these filaments together into tight circular scaffolds that bend the membrane away from the cytoplasm to form buds and tubules protruding from the cell surface. Similar buds develop in the absence of mutant VPS4B when hSnf7-1 is expressed without its regulatory C-terminal domain. We demonstrate that hSnf7 proteins form novel membrane-attached filaments that can promote or stabilize negative curvature and outward budding. We suggest that ESCRT-III polymers delineate and help generate the luminal vesicles of multivesicular bodies.