An endosome is a membrane-enclosed sorting compartment in the endocytosis pathway. Cargo may return to the cell membrane or proceed toward lysosomes; entry therefore does not by itself establish a digestive fate.[ref]
A multivesicular body is an endosome containing multiple small vesicles within its lumen. These internal vesicles form when regions of the boundary cell membrane bud inward and undergo separation. This reverses the familiar direction of a transport vesicle budding outward into the cytosol.
Membrane topology explains a useful consequence. A cytosol-facing portion of a membrane protein becomes enclosed inside an internal vesicle; subsequent delivery to a lysosome can expose the whole cargo to degradation. Sorting onto an internal vesicle can therefore remove a signalling receptor from the cytosolic machinery that it previously contacted.
A reconstitution experiment added purified ESCRT-III components to large artificial vesicles and observed formation and detachment of inward buds. This demonstrated a direct membrane-separation activity in a simplified system, rather than merely finding the proteins near cellular multivesicular bodies.[ref]
Membrane scission by the ESCRT-III complex by Thomas Wollert and others. 2009, abstract and giant-vesicle reconstitution
Progressive changes in endosomal membrane identity and lumenal processing, accompanied by sorting and retrieval rather than uniform conversion of all endocytosed cargo into a degradative route.
A maturing degradative compartment distinguished from early sorting endosomes by changes in proteins, lipids, lumenal conditions and fusion competence.
An early endosome is an early sorting compartment for internalized material. Delivery to it, recycling from it and onward trafficking are experimentally distinct from uptake across the plasma membrane.
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