ESCRTII

ESCRT-II (Endosomal Sorting Complex Required for Transport II) is a conserved multi-subunit protein complex that forms part of the ESCRT machinery involved in endosomal sorting and multivesicular body biogenesis in eukaryotic cells. It functions downstream of ESCRT-0 and ESCRT-I to promote sorting of ubiquitinated transmembrane proteins into intraluminal vesicles within late endosomes, a process essential for downregulation of surface receptors and for MVB formation.

Composition: In yeast and mammals, ESCRT-II comprises three core subunits: Vps22, Vps25, and Vps36. These subunits assemble into a curved, three-subunit complex that acts as a bridge linking ESCRT-I to ESCRT-III and organizes the recruitment of ESCRT-III components. In higher eukaryotes these subunits are homologous to the mammalian VPS22, VPS25, and VPS36 proteins.

Mechanism: The complex is recruited to endosomal membranes by ESCRT-I and contributes to cargo selection and concentration, facilitating the subsequent assembly of ESCRT-III. By coordinating the timing and localization of ESCRT-III polymerization, ESCRT-II promotes membrane scission that forms intraluminal vesicles inside the endosome.

Other roles: In addition to endosomal sorting, ESCRT-II participates in other cellular remodeling events such as cytokinesis and has been implicated in exosome biogenesis and viral budding, where the ESCRT machinery is co-opted to mediate membrane scission.

Conservation and significance: ESCRT-II is evolutionarily conserved from yeast to humans and is essential for proper endosomal trafficking. Disruption of ESCRT-II function impairs MVB formation and receptor downregulation.