ESCRT complexes and the biogenesis of multivesicular bodies

Abstract

Multivesicular bodies (MVBs) are crucial intermediates in the trafficking of ubiquitinated receptors and other cargo destined for lysosomes. The formation of MVBs by invagination of the endosomal limiting membrane is catalyzed by the endosomal sorting complex required for transport (ESCRT) complexes, a process that has recently been visualized in three-dimensional detail by electron tomography. Structural and biochemical analysis of the upstream components, Vps27-Hse1, ESCRT-I, and ESCRT-II, shows how these complexes assemble and cluster cargo. Rapid progress has been made in understanding the assembly and disassembly of the ESCRT-III complex and the interactions of its subunits with MIT domain and other proteins. A key role for deubiquitination in the regulation of the system has been demonstrated. One central question remains largely unanswered, which is how the ESCRTs actually promote the invagination of the endosomal membrane.

Figure 1

A model for the organization of the ESCRT system. In this scheme, Vps27-Hse1, ESCRT-I, and ESCRT-II are targeted to membranes by lipids (green), where they cluster cargo (red). ESCRT-I and II interact at a 1:1 stoichiometry. The stoichiometry of ESCRT-I binding to Vps27-Hse1 is undefined and for simplicity is shown as 1:1. ESCRT-II recruits ESCRT-III via its Vps20 subunit. Vps20 might nucleate the polymerization of Snf7, which forms filamentous arrays as demonstrated by Hanson, Heuser, and colleagues (personal communication). Other ESCRT-III subunits probably polymerize in a similar manner, and it is not clear whether homo- or heteromeric polymers predominate. Incorporation of Vps2 and Did2 into ESCRT-III initiates the depolymerization of ESCRT-III by Vps4:Vta1. ILVs are thought to be formed at some point during the polymerization/depolymerization cycle. This model is similar to the recently proposed “concentric circle” model [17].

Figure 2

Multivesicular bodies and the class E compartment. Genotypes are identified at left. Panels A, D, and G show tomographic sections, and the remaining panels show three-dimensional reconstructions. Panels A, B, C, show normal yeast multivesicular bodies; D, E, F show the class E compartment; and G, H, I show multivesicular bodies under conditions of defective cargo deubiquitination. Reproduced from the Journal of Cell Biology, 2006, 175: 715−720 [18]. Copyright 2006, Rockefeller University Press.

Figure 3

A structural model for the assembly and cargo clustering by A) Vps27-Hse1, B) ESCRT-I, and C) ESCRT-II. The structures shown are described in more detail in references [16,26,35].

Figure 4

MIT domains and MIM motifs coordinate the assembly and disassembly of ESCRT-III. The MIM of DID2B (red) bound to the MIT domain of VPS4A (orange) [48] illustrates the recognition of the C-terminal MIM regions of ESCRT-III subunits by Vps4. The structure of the C-terminally truncated dimer of VPS24 [49] is shown to illustrate the putative membrane binding “tiles” of the ESCRT-III array. The structure is colored according to electrostatic potential, with blue electropositive, and red electronegative.