Many plasma membrane proteins are in a constant flux throughout the internal trafficking pathways of the cell. Some receptors are continuously internalized into recycling endosomes and returned to the cell surface. Others are sorted into intralumenal vesicles of morphologically distinctive endosomes that are known as multivesicular bodies (MVBs). These MVBs fuse with lysosomes, resulting in degradation of their cargo by lysosomal acidic hydrolases.
Endosomes can be operationally defined as being either early or late, referring to the relative time it takes for endocytosed material to reach either stage. Ultrastructural studies indicate that early endosomes are predominantly tubulovesicular structures, which constitute a major sorting platform in the cell, whereas late endosomes show the characteristics of typical MVBs and are capable of fusing with lysosomes.
A well characterized signal for shunting membrane proteins into the degradative MVB pathway is the ubiquitylation of these cargoes. At the center of a vast protein:protein and protein:lipid interaction network that underpins ubiquitin mediated sorting to the lysosome are the endosomal sorting complexes required for transport (ESCRTs), which are conserved throughout all major eukaryotic taxa.
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Disassembly Phase
The AAA-ATPase, Vps4 disassembles ESCRT-III and catalyzes the final step of the MVB pathway. The microtubule interacting and trafficking (MIT) domain of Vps4 interacts directly with the C-terminal region of Vps2 (MIM1) and Vps20 (MIM2). The association of Vta1, which contains two MIT domains, greatly enhances the activity of Vps4. Please note that the recomended names of the Vacuolar protein sorting-associated proteins (Vps) are Charged multivesicular body proteins or CHMPs.
ESCRT-0 recruits ESCRT-I and thereby initiates the MVB pathway. Cargo Sorting ESCRT-I is a heterotetramer consisting of Vps23, Vps28, Vps37 (VPS37A, VPS37B, VPS37C or VPS37D), and Mvb12/Ubap1 (MVB12A, MVB12B or UBAP1). The UEV domain of Vps23 binds ubiquitinated membrane proteins. Vps28 interacts with the GLUE domain of Vps36 in ESCRT-II. Cargo Sorting ESCRT-II is a heterotetramer formed of Vps36, Vps22, and two Vps25 molecules. The GLUE domain of Vps36 binds PI3P, Vps28, and ubiquitinated membrane proteins. Vps25 interacts with Vps20 of ESCRT-III.
ESCRT-III assembles into a highly ordered filament-like hetero-oligomer. Vps20 nucleates the homo-oligomerization of Snf7 that is capped by Vps24. Vps24 recruits Vps2 and initiates Vps4-dependent ESCRT-III disassembly. ESCRT-III is required for the last steps of MVB sorting, cargo sequestration, and MVB vesicle formation.
Initiation/Cargo Recognition is mediated by ESCRT-0, a heterodimer consisting of Vps27 and Hse1. ESCRT-0 binds Phosphatidyl inositol 3- phosphate (PI3P) on endosomes via a FVYE domain and ubiquitinated cargo via two UIM domains.
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Endosmal Membrane
Bound CargoAnnotated Interactions
The AAA-ATPase, Vps4 disassembles ESCRT-III and catalyzes the final step of the MVB pathway. The microtubule interacting and trafficking (MIT) domain of Vps4 interacts directly with the C-terminal region of Vps2 (MIM1) and Vps20 (MIM2). The association of Vta1, which contains two MIT domains, greatly enhances the activity of Vps4. Please note that the recomended names of the Vacuolar protein sorting-associated proteins (Vps) are Charged multivesicular body proteins or CHMPs.
Endosmal Membrane
Bound Cargo