The Endosomal Sorting Complex Required for Transport (ESCRT) complexes form the machinery driving protein sorting from endosomes to lysosomes. ESCRT complexes are central to receptor down-regulation, lysosome biogenesis, and budding of HIV. Yeast ESCRT-I consists of three protein subunits, Vps23, Vps28, and Vps37. In humans, ESCRT-I comprises TSG101, VPS28, and one of four potential human VPS37 homologues. The main role of ESCRT-I is to recognise ubiquitinated cargo via the UEV domain of the VPS23/TSG101 subunit. The assembly of the ESCRT-I complex is directed by the C-terminal steadiness box (SB) of VPS23, the N-terminal half of VPS28, and the C-terminal half of VPS37. The structure is primarily composed of three long, parallel helical hairpins, each corresponding to a different subunit. The additional domains and motifs extending beyond the core serve as gripping tools for ESCRT-I critical functions [, ].
A key aspect of eukaryotic intracellular trafficking is the sorting of cell-surface proteins into multi-vesicular endosomes or bodies (MVBs), which eventually fuse with the lysosome, where they are degraded by lipases and peptidases. This is the primary mechanism for down-regulation of signaling via transmembrane receptors and removal of misfolded or defective membrane proteins. This process is also utilised by several viruses (e.g. HIV-1) to facilitate budding of their virions from the cell-membrane. Studies in animals and fungi have shown that it depends on an intricate series of interactions, which is initiated via ubiquitination (typically one or more mono-ubiquitinations) of the cytoplasmic tails of membrane proteins by specific E3 ligases. Ubiquitinated membrane proteins are then captured into endosomes by the ESCRT system and prevented from being recycled back to the plasma membrane via the retrograde trafficking system. The ESCRT system also folds the endosomal membranes into invaginations that are concentrated in these ubiquitinated targets and catalyzes their abscission into intra-luminal-vesicles inside the endosome. This largely seals the fate of these membrane proteins as targets for lysosomal degradation. The ESCRT system is comprised of 4 major protein complexes, ESCRT-0 to ESCRT-III, which are successively involved in the above-described steps [].ESCRT-I contains three subunits that are conserved between yeast and animals, namely the inactive E2-ligase protein TSG101/VPS23, VPS28 and VPS37. Additionally, both yeast and metazoan ESCRT-I contain a fourth subunit termed MVB12 (multivesicular body sorting factor of 12 kD); however, the MVB12 subunits from the two lineages do not show significant sequence similarity. The metazoan MVB12 proteins contain two distinct conserved domains that occur independently in various proteins. The C-terminal region of MVB12, which is shared with ubiquitin associated protein-1 (UBAP1), forms the UBAP1-MVB12 associated (UMA) domain. Human UBAP1 is implicated in nasopharyngeal carcinoma risk and fronto-temporal lobar degeneration. The UMA domain is also found in several other poorly characterised proteins, including at leat one orthologous group of proteins conserved in vertebrates prototyped be the human protein LOC390595 and another group conserved across Metazoa typified by human tcag7.903. The UMA domain found in MVB12 and UBAP1 defines a novel adaptor that might recruit diverse targets to ESCRT-I. The different UMA proteins might function as alternative as MVB12-like subunit that recruit different targets via their specific intercation modules (such as MABP) or UBA or the specific extensions) to the ESCRT-I complex [].This entry represents the UMA domain, which contains a conserved proline followed by a hydrophobic residue in the N terminus and a nearly absolutely conserved glutamate at the C terminus. It is predicted to adopt an alpha+beta fold [].
A key aspect of eukaryotic intracellular trafficking is the sorting of cell-surface proteins into multi-vesicular endosomes or bodies (MVBs), which eventually fuse with the lysosome, where they are degraded by lipases and peptidases. This is the primary mechanism for down-regulation of signaling via transmembrane receptors and removal of misfolded or defective membrane proteins. This process is also utilised by several viruses (e.g. HIV-1) to facilitate budding of their virions from the cell-membrane. Studies in animals and fungi have shown that it depends on an intricate series of interactions, which is initiated via ubiquitination (typically one or more mono-ubiquitinations) of the cytoplasmic tails of membrane proteins by specific E3 ligases. Ubiquitinated membrane proteins are then captured into endosomes by the ESCRT system and prevented from being recycled back to the plasma membrane via the retrograde trafficking system. The ESCRT system also folds the endosomal membranes into invaginations that are concentrated in these ubiquitinated targets and catalyzes their abscission into intra-luminal-vesicles inside the endosome. This largely seals the fate of these membrane proteins as targets for lysosomal degradation. The ESCRT system is comprised of 4 major protein complexes, ESCRT-0 to ESCRT-III, which are successively involved in the above-described steps [].ESCRT-I contains three subunits that are conserved between yeast and animals, namely the inactive E2-ligase protein TSG101/VPS23, VPS28 and VPS37. Additionally, both yeast and metazoan ESCRT-I contain a fourth subunit termed MVB12 (multivesicular body sorting factor of 12 kD); however, the MVB12 subunits from the two lineages do not show significant sequence similarity. The metazoan MVB12 proteins contain two distinct conserved domains that occur independently in various proteins. The N-terminal region of MVB12 forms the MVB12-associated β-prism (MABP), which is also found in DENND4A/B/C from vertebrates, the membrane trafficking regulator Crag from Drosophila, bacterial proteins typified by the MAC/perforin (MACPF)-like protein plu1415 from Photorhabdus luminescens and uncharacterised proteins from choanoflagellates and stamenopiles. It has been suggested that the MABP domain has a membrane-associated function, perhaps even specific interactions with membrane components. It is plausible that the eukaryotic MABP domains are adaptators that help linking other associated domains found in the same polypeptide to vesicular membranes [].The MABP domain has an internal repeat structure of three homologous segments. Consitent with this, the structurally characterised representative Photorhabdus plu1415, showed that this region precisely corresponds to a type-I β-prism domain with an internal three fold symetry. Each of the three sub-domains of the β-prism structure is a distinctive three-stranded β-sheet. The MABP domain shares a triradial symmetry with β-sheets parallel to the prism axis. The β-prism fold is associated with membrane interaction. The majority of the eukaryotic MABP domain versions contain a conserved cysteine in the first and third subdomain of the β-prism [, ].
