A concentric circle model of multivesicular body cargo sorting

Abstract

Targeting of ubiquitylated transmembrane proteins into luminal vesicles of endosomal multivesicular bodies (MVBs) depends on their recognition by endosomal sorting complexes required for transport (ESCRTs), which are also required for MVB vesicle formation. The model originally proposed for how ESCRTs function succinctly summarizes much of the protein-protein interaction and genetic data but oversimplifies the coordination of cargo recognition and cannot explain why ESCRTs are required for the budding of MVB vesicles. Recent structural and functional studies of ESCRT complexes suggest an alternative model that might direct the next series of breakthroughs in understanding protein sorting through the MVB pathway.

Figure 1

The multivesicular body pathway. Endocytic and biosynthetic transmembrane cargoes are ubiquitylated to facilitate their entry into luminal vesicles of MVBs. Monoubiquitylation is the most common form of modification of MVB cargoes, although polyubiquitylation and multiple monoubiquitylation are also known to occur. When the MVB fuses with the lysosome/vacuole, its luminal contents are degraded. MVB, multivesicular body; Ub, ubiquitin.

Figure 2

The conveyor belt model of ESCRT function (reproduced from Hurley & Emr, 2006). According to this model, ESCRT complexes are recruited sequentially to the endosome and recognize ubiquitylated transmembrane proteins, passing cargo from one complex to the next to facilitate sorting to MVB vesicles. Deubiquitylation of cargoes by the Ub hydrolase Doa4 and dissassembly of ESCRTs by the ATPase Vps4 precede invagination. See main text and Hurley & Emr (2006) for further description. Bro, BCK1-like resistance to osmotic shock; CPS, carboxypeptidase S; Doa, degradation of alpha; ESCRT, endosomal sorting complex required for transport; FYVE, ‘Fab1, YOTB, Vac1, EEA1'; Hse, has symptoms of class E mutants; MVB, multivesicular body; NZF, Npl4 zinc finger; PI(3)P, phosphatidylinositol 3-phosphate; PTVP, proline threonine valine proline; SH3, Src-homology 3; Snf, sucrose non-fermenting; STAM, signal transducing adaptor molecule; Ub, ubiquitin; UEV, Ub E2 variant; UIM, Ub-interacting motif; VHS, ‘Vps27, Hrs, STAM'; Vps, vacuolar protein sorting.

Figure 3

Cross-section of concentric circle model of yeast ESCRT function in MVB cargo sorting. (A) Crucial domains of ESCRT-0, -I and -II mediate cargo recognition, lipid binding and complex assembly, resulting in (B) the formation of an ESCRT-0/I/II supercomplex on the endosomal membrane with MVB cargo proteins concentrated beneath. Subunits of ESCRT-III (C) assemble to form a perimeter that recruits both Bro1 and Doa4 and (D) promote assembly of Vps4 dimers into active decamers, although a link between nucleotide exchange in Vps4 and interaction with ESCRT-III is unknown. (E) Dissociation of the ESCRT-0/I/II core precedes vesicle formation, making sequestered MVB cargoes available for Doa4-mediated deubiquitylation before vesicle scission. Vps4 hydrolyses ATP to dissociate ESCRT-III from the membrane after vesicle release. Note that ESCRT-0 is depicted as a barrel having radial symmetry oriented perpendicular to the membrane, although direct evidence for this configuration has yet to be obtained. Bro, BCK1-like resistance to osmotic shock; Doa, degradation of alpha; ESCRT, endosomal sorting complex required for transport; FYVE, ‘Fab1, YOTB, Vac1, EEA1'; GLUE, GRAM-like Ub binding in Eap45; MVB, multivesicular body; NZF, Npl4 zinc finger; PI(3)P, phosphatidylinositol 3-phosphate; PSDP, proline serine aspartate proline; Snf, sucrose non-fermenting; Ub, ubiquitin; UEV, Ub E2 variant; UIM, Ub-interacting motif; Vps, vacuolar protein sorting.

Figure 4

Top-down view of concentric circle assembly of ESCRT in yeast. With an ESCRT-0 hexamer at the centre, the 48 nm diameter membrane domain accommodates one ring each of ESCRT-I and ESCRT-II, including bound ubiquitylated cargoes. Although the interaction between ESCRT-I and ESCRT-II occurs in vitro at 1:1 stoichiometry (Gill et al, 2007), an excess of ESCRT-II is depicted here to account for the potential of ESCRT-II to oligomerize (Teo et al, 2006). ESCRT-III defines the perimeter of the membrane invagination area, oligomerizing outside the ESCRT-0/I/II core. The ESCRT-II/III boundary is depicted as ‘fuzzy' owing to the unknown assembly pattern of ESCRT-III and its potential to interact with ESCRT-I. ESCRT, endosomal sorting complex required for transport.

Daniel P. Nickerson

Matthew R.G. Russell

Greg Odorizzi