The endosomal system internalizes membrane and protein from the plasma membrane, and then in a topologically distinct process internalizes membrane into itself to form multivesicular bodies (MVBs). Internalization of membrane and associated cargo into the MVB is essential for digestion and degradation in the lysosome, and is also used by specialized cells to generate exosomes. A set of eighteen """"""""class E"""""""" proteins conserved from yeast to mammals is thought to regulate and probably drive the formation of intralumenal vesicles. Some or possibly all of these proteins are also used by nonlytic viruses such as HIV to bud from the plasma membrane of infected cells in a process that is topologically equivalent to budding into the lumen of the endosome. Elegant studies in yeast and more recently mammalian systems have lent insight into how these proteins associate with each other, cargo molecules, and the endosomal membrane, but essentially nothing is known about how these or other proteins drive membrane invagination and vesicle release. Because the required membrane curvature is opposite to that of traditional coated vesicle-driven endocytosis, it is likely that novel mechanisms are involved. We found using quick-freeze deep-etch electron microscopy that ESCRT-III proteins related to Snf7 (hSnf7/CHMP4 in mammalian systems) assemble into filaments on the plasma membrane that can be induced to form tight circular arrays and bend the membrane away from the cytoplasm, creating buds and eventually tubules on the cell surface. We hypothesize that similar ESCRT-III containing polymers normally create the neck of nascent endosomal vesicles, both confining the vesicle's contents and inducing the requisite deformation in the membrane. Our plan is to define the structure and dynamics of ESCRT-III polymers in vitro, on model membranes, and in the context of normal MVB biogenesis in order to determine how assembly and disassembly of ESCRT-III polymers participate in MVB biogenesis.
Aim 1 will explore the structure of ESCRT-III homo- and hetero- polymers.
Aim 2 will define mechanisms that regulate ESCRT-III polymer assembly.
Aim 3 will study disassembly of ESCRT-III polymers by the AAA+ ATPase VPS4. Finally, Aim 4 will study endogenous ESCRT-III proteins in cells during the process of receptor downregulation before and after silencing expression of VPS4 and hSnf7 proteins.

Public Health Relevance

These studies will illuminate cellular mechanisms involved in receptor downregulation, lipid homeostasis, and the release of many enveloped viruses including HIV from the cell. Mutations in two of the proteins to be studied are directly responsible for familial forms of frontotemporal dementia and early-onset cataracts. Insight into the pathophysiology of these processes requires the mechanistic understanding at which this work is aimed.

Agency
National Institute of Health (NIH)
Institute
National Institute of General Medical Sciences (NIGMS)
Type
Research Project (R01)
Project #
5R01GM076686-04
Application #
8134458
Study Section
Cell Structure and Function (CSF)
Program Officer
Ainsztein, Alexandra M
Project Start
2008-09-29
Project End
2013-08-31
Budget Start
2011-09-01
Budget End
2013-08-31
Support Year
4
Fiscal Year
2011
Total Cost
$312,848
Indirect Cost
Name
Washington University
Department
Anatomy/Cell Biology
Type
Schools of Medicine
DUNS #
068552207
City
Saint Louis
State
MO
Country
United States
Zip Code
63130
Jackson, Charles E; Scruggs, Benjamin S; Schaffer, Jean E et al. (2017) Effects of Inhibiting VPS4 Support a General Role for ESCRTs in Extracellular Vesicle Biogenesis. Biophys J 113:1342-1352
McCullough, John; Clippinger, Amy K; Talledge, Nathaniel et al. (2015) Structure and membrane remodeling activity of ESCRT-III helical polymers. Science 350:1548-51
Cashikar, Anil G; Shim, Soomin; Roth, Robyn et al. (2014) Structure of cellular ESCRT-III spirals and their relationship to HIV budding. Elife 3:
Hanson, Phyllis I; Cashikar, Anil (2012) Multivesicular body morphogenesis. Annu Rev Cell Dev Biol 28:337-62
Thomas, Stefani N; Wan, Yunhu; Liao, Zhongping et al. (2011) Stable isotope labeling with amino acids in cell culture based mass spectrometry approach to detect transient protein interactions using substrate trapping. Anal Chem 83:5511-8
Merrill, Samuel A; Hanson, Phyllis I (2010) Activation of human VPS4A by ESCRT-III proteins reveals ability of substrates to relieve enzyme autoinhibition. J Biol Chem 285:35428-38
Hanson, Phyllis I; Shim, Soomin; Merrill, Samuel A (2009) Cell biology of the ESCRT machinery. Curr Opin Cell Biol 21:568-74