Mechanics of ESCRT-III mediated membrane scission (MEM_FIZZ)

Framework: FP7 — Ideas
Programme: European Research Council
Theme: European research council
Destination: Starting grants
Action type: Support for frontier research (ERC)
Panel: Molecular and Structural Biology and Biochemistry
Countries involved: United Kingdom
Call:  (ERC-2010-StG_20091118, 2010)
Start date: 2010-10-01
End date: 2016-01-31
Duration (months): 64.0
Total project cost (EUR): 1499655.0
EU funding (EUR): 1499655.0
Participants: 1
Project status: CLOSED

Cellular processes such as cytokinesis, the budding of enveloped retrovirus (e.g. HIV-1), and multivesicular biogenesis have direct links to several human diseases including carcinogenesis and neuro-degeration etc. While seemingly unrelated, these processes all involve membrane abscission for generating two newly formed membrane bound structures - a process aided by the cytosolic proteins collectively termed ESCRT-III. Understanding these processes for therapeutic intervention has so far focused on identification of the factors involved, their structures, and the interactions between them. However, given that membrane-abcission is the key event in all these processes, the mechanics of membrane scission cannot be neglected. Due to fast and highly localised transformations, protein mediated membrane remodelling in general has proven difficult for quantitative mechanistic scrutiny (perhaps with the single exception of dynamin which, unlike the ESCRT-III, acts from the outside of a membrane neck). In humans ESCRT-III members are called CHMPs. Major advances have been recently made in (i) determination of polymeric structures formed by human (yeast) CHMP4 (Snf7), CHMP3 (vps24) and CHMP2A; (ii) membrane splitting activity has been attributed to the sequential binding of the yeast proteins vps20 (CHMP6), Snf7 and vps24, (iii) vps2 (CHMP2), which binds vps24, recruits a AAA ATPAse vps4 which then recycles the membrane bound ESCRT-III. Several models have since been proposed where protein polymers constricting the membrane neck for fission is the common theme. However, there is considerable debate over the essential molecular mechanism of the process. Therefore, I will address: 1. How do CHMP2, 3, 4 and 6 assemblies form on membranes and dissociate in a VPS4 dependent manner? 2. What are the structures, composition and direction of growth of ESCRT-III assemblies as they mature on lipid membranes? 3. Since ESCRT-III polymer must form through the central pore of a membrane tubule, thereby posing a steric hindrance for fusion, how does pore closure followed by scission take place? 4. As CHMPs are predominantly cytosolic, how do their binding partners such as VPS4, AMSH (deubiquitin isopeptidase), and Alix (adaptor molecule) get selectively targeted to the membrane-bound fraction of CHMPs to exert their membrane proximal function? Answering the posed questions will not only advance our understanding of HIV egress from cells, it may also help open new therapeutic intervention points for several ESCRT-III related dysfunction. These studies will further form the basis for in vivo investigation of the mechanism by which ESCRT-III functions.

Keywords: escrts,membrane remodelling,membrane scission,peripheral membrane proteins,advanced fluorescence detection,electrophysiology,model membrane,membrane nanotubes