Cargo Sorting and Intralumenal Vesicle Budding by the ESCRT Complexes? ? ? Membrane budding and fission is a fundamental process of eukaryotic cell biology. Endocytosis, the formation of intracellular transport and secretory vesicles, and mitochondrial fission are examples of inward budding. In the classical example of clathrin-mediated endocytosis, the cytosolic protein dynamin forms arrays on the outside of the membrane neck, and membrane fission is driven thermodynamically by the hydrolysis of GTP. The formation of multivesicular bodies (MVBs) is the prototypical example of outward budding. MVBs are formed during the maturation of endosomes destined to fuse with lysosomes, and mediate the sorting of ubiquitinated membrane proteins to the lysosome. Portions of the limiting membrane of the endosome are internalized to form intralumenal vesicles (ILVs). When the MVB fuses with the lysosome, ILV contents are degraded by lysosomal hydrolases. When ILVs are released through fusion with the plasma membrane, they are referred to as exosomes. The budding of enveloped viruses from the plasma membrane and cell division (cytokinesis) are other examples of outward budding events. Outward budding events in MVB formation, viral budding, and cytokinesis are directed from the cytosol. Since cytosol is in contact with the inside, not the outside of the neck of the nascent bud, the mechanics of membrane fission differ fundamentally from inward budding, and utilize a completely distinct protein machinery. A major breakthrough in understanding outward budding came from the identification in yeast of the ESCRT machinery responsible for MVB formation. The ESCRT machinery is conserved throughout eukaryotes, and many enveloped viruses of mammals use the ESCRT pathway to bud, including HIV-1. The closure of the membrane neck in cytokinesis also uses the ESCRT pathway.? The assembly of ESCRT complexes on endosomes is triggered by the presence of phosphatidylinositol 3-phosphate (PI(3)P) and ubiquitinated cargo proteins. ESCRT-I and II directly bind to cargo, and in turn recruit ESCRT-III. There are four ESCRT-III subunits in yeast, Vps2, Vps20, Vps24, and Snf7, together with two associated ESCRT-III-like proteins, Did2 and Vps60. ESCRT-III subunits exist in the cytosol as monomers, and assemble with each other on membranes in large multimeric arrays. ESCRT-II is a Y-shaped complex that contains two copies of the Vps25 subunit, which recruits ESCRT-III by directly binding to Vps20. Vps20 binds to Snf7, comprising a subcomplex of ESCRT-III. Snf7, in turn, directly binds to the Bro1 domain of the ESCRT-associated protein Alix (known as Bro1 in yeast). The Vps20:Snf7 complex recruits the Vps2:Vps24 subcomplex to form the complete ESCRT-III complex. A subset of ESCRT-III proteins directly bind to the N-terminal MIT domain of the AAA ATPase Vps4. Vps4 is a central player in the MVB pathway that is required for the disassembly of the ESCRT-III complex. ESCRT function can be conceptually separated into two phases: cargo recruitment and concentration, followed by membrane invagination and budding. The long term objectives of this project are to: 1) determine the structures of ESCRT complexes by x-ray crystallography, abetted where necessary by electron microscopy, hydrodynamics, molecular simulations, and small angle x-ray scattering; 2) to determine how ESCRTs assemble on membranes containing PI(3)P and cargo using binding and spectroscopic techniques; and 3) to study the mechanism of ILV formation by a microscopic, spectroscopic, and structure/function approaches.? ? ? Progress in FY2008 included the structure determination of the human ESCRT-II complex and analysis of its membrane targeting mechanism, as described in more detail under the ubiquitin binding domains project. Other projects involved analysis of MIT domain interactions and the assembly of reagents and assays for in vitro studies of ESCRT function.
| Wollert, Thomas; Yang, Dong; Ren, Xuefeng et al. (2009) The ESCRT machinery at a glance. J Cell Sci 122:2163-6 |
| Hurley, James H; Im, Young Jun; Lee, Hyung Ho et al. (2009) Piecing together the ESCRTs. Biochem Soc Trans 37:161-6 |
| Wollert, Thomas; Wunder, Christian; Lippincott-Schwartz, Jennifer et al. (2009) Membrane scission by the ESCRT-III complex. Nature 458:172-7 |
| Yang, Dong; Rismanchi, Neggy; Renvoise, Benoit et al. (2008) Structural basis for midbody targeting of spastin by the ESCRT-III protein CHMP1B. Nat Struct Mol Biol 15:1278-86 |
| Hurley, James H (2008) ESCRT complexes and the biogenesis of multivesicular bodies. Curr Opin Cell Biol 20:4-11 |
| Im, Young Jun; Hurley, James H (2008) Integrated structural model and membrane targeting mechanism of the human ESCRT-II complex. Dev Cell 14:902-13 |
| Kostelansky, Michael S; Schluter, Cayetana; Tam, Yuen Yi C et al. (2007) Molecular architecture and functional model of the complete yeast ESCRT-I heterotetramer. Cell 129:485-98 |
| Hurley, James H; Emr, Scott D (2006) The ESCRT complexes: structure and mechanism of a membrane-trafficking network. Annu Rev Biophys Biomol Struct 35:277-98 |
| Kostelansky, Michael S; Sun, Ji; Lee, Sangho et al. (2006) Structural and functional organization of the ESCRT-I trafficking complex. Cell 125:113-26 |
