Molecule of the Month: ESCRT-III

ESCRT-III forms helical assemblies that remodel cellular membranes

Helical filament of ESCRT-III proteins CHMP1B (blue) and IST1 (green). The membrane is shown schematically in gray.
Helical filament of ESCRT-III proteins CHMP1B (blue) and IST1 (green). The membrane is shown schematically in gray.
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Membranes are the walls that structure our cells, forming a protective barrier around each cell and forming compartments such as the nucleus inside. However, membranes are not fixed like the walls in our houses. Cells constantly modify their membranes to respond to their minute-by-minute needs, shaping compartments correctly and controlling the traffic between different compartments. ESCRT proteins help with this process, remodeling membranes and pinching off pieces to form vesicles. This role is reflected in their name: “Endosomal Sorting Complexes Required for Transport.” ESCRT-III plays the central role, providing the force that’s necessary to remodel cell membranes.

Pinching Proteins

Our cells build several different types of ESCRT-III proteins for different functions. The one shown here (PDB ID 6tz4) assembles into helices or spirals that form a collar around a tubular section of membrane. This large assembly is a combination of two proteins: CHMP1B and IST1. Together they form a narrow constriction at the center that thins the membrane and ultimately pinches it off. This process is used, for example, to bud small vesicles from a larger endosome compartment.

Inside Out

Amazingly, many types of ESCRT-III work in the opposite way, pulling membranes around the outside of the helix like a sleeve. In this way, they constrict membranes from the inside, as seen in the picture in the section below. This inside-out mechanism plays many essential roles in our cells. For example, when a cell divides, ESCRT-III helps narrow the final connection, allowing the two daughter cells to separate. Similarly, ESCRT-III seals up holes in membranes and helps repair subcellular wounds. It’s also important in managing cell-specific membrane shapes such as the long extensions of cilia and the branched structures of nerve cells.

Frame from an animation of HIV-1 budding from the surface of an infected cell. ESCRT-III is depicted in turquoise.
Frame from an animation of HIV-1 budding from the surface of an infected cell. ESCRT-III is depicted in turquoise.

Hijacked Helpers

HIV-1 hijacks the ESCRT system and forces it to assist with the final steps of the viral life cycle. As the virus buds from the surface of the infected cell, ESCRT-III narrows the neck connecting the virus to the cell, ultimately allowing the virus to pinch off and escape. As seen in this illustration by Janet Iwasa, ESCRT-III is thought to form a spiral that progressively constricts the membrane. To see the whole process, take a look at the full animation of the life cycle of HIV-1.

VIPP1 in two helical assemblies, with 18-fold symmetry at the top and 14-fold symmetry at the bottom.
VIPP1 in two helical assemblies, with 18-fold symmetry at the top and 14-fold symmetry at the bottom.
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Shaping Photosynthesis

Proteins similar to our ESCRT-III are used in many types of cells to shape their membranes. The one shown here, VIPP1, helps manage the complex photosynthetic membranes in a cyanobacterium. Related proteins are thought to play the same role in green algae and plants. Two structures of the protein are shown here (PDB ID 7o3w and 7o3z) revealing how it forms helical assemblies of different diameters as it constricts around a membrane tubule.

Exploring the Structure

ESCRT-III Conformations

ESCRT-III proteins are highly dynamic, adopting different conformations as they bind to membranes to form progressively smaller helices. For example, in the cytoplasm the ESCRT-III protein CHMP3 adopts a closed, inactive form, as seen in PDB ID 3frt. With the help of other ESCRT proteins, CHMP3 opens up and binds side-by-side with CHMP2A to form the helical assembly that constricts membranes, as seen in PDB ID 7zcg. To compare these two structures, click on the JSmol tab for an interactive view.

Topics for Further Discussion

  1. Many proteins assist ESCRT-III in the remodeling of membranes. For example, look at PDB ID 6ap1 to see how the protein Vps4 disassembles ESCRT-III filaments during the process of constriction.

References

  1. 7zcg: Azad, K., Guilligay, D., Boscheron, C., Maity, S., De Franceschi, N., Sulbaran, G., Effantin, G., Wang, H., Kleman, J.P., Bassereau, P., Schoehn, G., Roos, W.H., Desfosses, A., Weissenhorn, W. (2023) Structural basis of CHMP2A-CHMP3 ESCRT-III polymer assembly and membrane cleavage. Nat Struct Mol Biol 30: 81-90
  2. Schlosser, L., Sachse, C., Low, H.H., Schneier, D. (2023) Conserved structures of ESCRT-III superfamily members across domains of life. Trends Biochem Sci 48: 993-1004
  3. 7o3w, 7o3z: Gupta, T.K., Klumpe, S., Gries, K., Heinz, S., Wietrzynski, W., Ohnishi, N., Niemeyer, J., Spaniol, B., Schaffer, M., Rast, A., Ostermeier, M., Strauss, M., Plitzko, J.M., Baumeister, W., Rudack, T., Sakamoto, W., Nickelsen, J., Schuller, J.M., Schroda, M., Engel, B.D. (2021) Structural basis for VIPP1 oligomerization and maintenance of thylakoid membrane integrity. Cell 184: 3643-3659.e23
  4. Liu, J., Tassinari, M., Souza, D.P., Naskar, S., Noel, J.K., Bohuszewicz, O., Buck, M., Williams, T.A., Baum, B., Low, H.H. (2021) Bacterial Vipp1 and PspA are members of the ancient ESCRT-III membrane-remodeling superfamily. Cell 184: 3660-3673.e18
  5. 6tz4: Nguyen, H.C., Talledge, N., McCullough, J., Sharma, A., Moss 3rd, F.R., Iwasa, J.H., Vershinin, M.D., Sundquist, W.I., Frost, A. (2020) Membrane constriction and thinning by sequential ESCRT-III polymerization. Nat Struct Mol Biol 27: 392-399
  6. McCullough, J., Frost, A., Sundquist, W.I. (2018) Structures, functions, and dynamics of ESCRT-III/Vps4 membrane remodeling and fission complexes. Ann Rev Cell Develop Biol 34: 85-109
  7. Han, H., Monroe, N., Sundquist, W.I., Shen, P.S., Hill, C.P. (2017) The AAA ATPase Vps4 binds ESCRT-III substrates through a repeating array of dipeptide-binding pockets. Elife 6: e31324
  8. McCullough, J., Clippinger, A.K., Talledge, N., Skowyra, M.L., Saunders, M.G., Naismith, T.V., Colf, L.A., Afonine, P., Arthur, C., Sundquist, W.I, Hanson, P.I, Frost, A. (2015) Structure and membrane remodeling activity of ESCRT-III helical polymers. Science 350, 1548-1551
  9. 3frt: Bajorek, M., Schubert, H.L., McCullough, J., Langelier, C., Eckert, D.M., Stubblefield, W.M., Uter, N.T., Myszka, D.G., Hill, C.P., Sundquist, W.I. (2009) Structural basis for ESCRT-III protein autoinhibition. Nat Struct Mol Biol 16: 754-762

August 2024, David Goodsell

http://doi.org/10.2210/rcsb_pdb/mom_2024_8
About Molecule of the Month
The RCSB PDB Molecule of the Month by David S. Goodsell (The Scripps Research Institute and the RCSB PDB) presents short accounts on selected molecules from the Protein Data Bank. Each installment includes an introduction to the structure and function of the molecule, a discussion of the relevance of the molecule to human health and welfare, and suggestions for how visitors might view these structures and access further details.More