The mammalian protein epsin is required for endocytosis. In this study, two homologous yeast proteins, Ent1p and Ent2p, both similar to mammalian epsin, have been characterized. An essential function for the highly conserved N-terminal epsin N-terminal homology (ENTH) domain was revealed using deletions and randomly generated temperature-sensitive ent1 alleles. Changes in conserved ENTH domain residues in ent1(ts) cells revealed defects in endocytosis and actin cytoskeleton structure. The Ent1 protein was localized to peripheral and internal punctate structures, and biochemical fractionation studies found the protein associated with a large, Triton X-100-insoluble pellet. Finally, an Ent1p clathrin-binding domain was mapped to the final eight amino acids (RGYTLIDL*) in the Ent1 protein sequence. Based on these and other data, it is proposed that the yeast epsin-like proteins are essential components of an endocytic complex that may act at multiple stages in the endocytic pathway (Wendland, 1999).
The yeast actin-regulating kinases Ark1p and Prk1p are signaling proteins localized to cortical actin patches, which may be sites of endocytosis. Interactions between the endocytic proteins Pan1p and End3p may be regulated by Prk1p-dependent threonine phosphorylation of Pan1p within the consensus sequence [L/I]xxQxTG. Two Prk1p phosphorylation sites have been identified within the Pan1p-binding protein Ent1p, a yeast epsin homologue, and Prk1p-dependent phosphorylation of both threonines has been demonstrated. Converting both threonines to either glutamate or alanine mimics constitutively phosphorylated or dephosphorylated Ent1p, respectively. Synthetic growth defects were observed in a pan1-20 ENT1(EE) double mutant, suggesting that Ent1p phosphorylation negatively regulates the formation/activity of a Pan1p-Ent1p complex. Interestingly, pan1-20 ent2 Delta but not pan1-20 ent1 Delta double mutants had improved growth and endocytosis over the pan1-20 mutant. Actin-regulating Ser/Thr kinase (ARK) mutants exhibit endocytic defects, and overexpressing either wild-type or alanine-substituted Ent1p partially suppresses phenotypes associated with loss of ARK kinases, including growth, endocytosis, and actin localization defects. Consistent with synthetic growth defects of pan1-20 ENT1(EE) cells, overexpressing glutamate-substituted Ent1p is deleterious to ARK mutants. Surprisingly, overexpressing the related Ent2p protein could not suppress ARK kinase mutant phenotypes. These results suggest that Ent1p and Ent2p are not completely redundant and may perform opposing functions in endocytosis. These data support the model that, as for clathrin-dependent recycling of synaptic vesicles, yeast endocytic protein phosphorylation inhibits endocytic functions (Watson, 2001).
In addition to its well known role in targeting proteins for proteasomal degradation, ubiquitin (Ub) is also involved in promoting internalization of cell surface proteins into the endocytic pathway. Moreover, putative Ub interaction motifs (UIMs) as well as Ub-associated (UBA) domains have been identified in key yeast endocytic proteins (the epsins Ent1 and Ent2, and the Eps15 homolog Ede1). The interaction of Ub with the Ede1 UBA domain and with the UIMs of Ent1 has been characterized. The data suggest that the UIMs and the UBA are involved in binding these proteins to biological membranes. The Ent1 ENTH domain binds to phosphoinositides in vitro and Ent1 NPF motifs interact with the EH domain-containing proteins Ede1 and Pan1. These findings indicate that the ENTH domain interaction with membrane lipids cooperates with the binding of membrane-associated Ub moieties. These events may in turn favor the occurrence of other interactions, for instance EH-NPF recognition, thus stabilizing networks of low affinity binding partners at endocytic sites (Aquilar, 2003).
Clathrin-coated vesicles (CCVs) are a central component of endocytosis and traffic between the trans-Golgi network (TGN) and endosomes. Although endocytic CCV formation is well characterized, much less is known about CCV formation at internal membranes. Two epsin amino-terminal homology (ENTH) domain-containing proteins are described, Ent3p and Ent5p, that are intimately involved in clathrin function at the Golgi. Both proteins associate with the clathrin adaptor Gga2p in vivo; Ent5p also interacts with the clathrin adaptor complex AP-1 and clathrin. A novel, conserved motif that mediates the interaction of Ent3p and Ent5p with gamma-ear domains of Gga2p and AP-1 is defined. Ent3p and Ent5p colocalize with clathrin, and cells lacking both Ent proteins exhibit defects in clathrin localization and traffic between the Golgi and endosomes. The findings suggest that Ent3p and Ent5p constitute a functionally related pair that co-operate with Gga proteins and AP-1 to recruit clathrin and promote formation of clathrin coats at the Golgi/endosomes. On the basis of these results and the established roles of epsin and epsin-related proteins in endocytosis, it is proposed that ENTH-domain-containing proteins are a universal component of CCV formation (Duncan, 2003).
PtdIns(3,5)P(2) is required for cargo-selective sorting to the vacuolar lumen via the multivesicular body (MVB). Ent3p, a yeast epsin N-terminal homology (ENTH) domain-containing protein, is a specific PtdIns(3,5)P(2) effector localized to endosomes. The ENTH domain of Ent3p is essential for its PtdIns(3,5)P(2) binding activity and for its membrane interaction in vitro and in vivo. Ent3p is required for protein sorting into the MVB but not for the internalization step of endocytosis. Ent3p is associated with clathrin and is necessary for normal actin cytoskeleton organization. These results show that Ent3p is required for protein sorting into intralumenal vesicles of the MVB through PtdIns(3,5)P(2) binding via its ENTH domain (Friant, 2003).
At the late endosomes, cargoes destined for the interior of the vacuole are sorted into invaginating vesicles of the multivesicular body. Both PtdIns(3,5)P(2) and ubiquitin are necessary for proper sorting of some of these cargoes. Ent5p, a yeast protein of the epsin family homologous to Ent3p, localizes to endosomes and specifically binds to PtdIns(3,5)P(2) via its ENTH domain. In cells lacking Ent3p and Ent5p, ubiquitin-dependent sorting of biosynthetic and endocytic cargo into the multivesicular body is disrupted, whereas other trafficking routes to the vacuole are not affected. Ent3p and Ent5p are associated with Vps27p, a FYVE domain containing protein that interacts with ubiquitinated cargoes and is required for protein sorting into the multivesicular body. Therefore, Ent3p and Ent5p are the first proteins shown to be connectors between PtdIns(3,5)P(2)- and the Vps27p-ubiquitin-driven sorting machinery at the multivesicular body (Eugster, 2004).
Epsin and AP180/CALM are important endocytic accessory proteins that are believed to be involved in the formation of clathrin coats. Both proteins associate with phosphorylated membrane inositol lipids through their epsin N-terminal homology domains and with other components of the endocytic machinery through short peptide motifs in their carboxyl-terminal segments. Using hydrodynamic and spectroscopic methods, it was demonstrates that the parts of epsin 1 and AP180 that are involved in protein-protein interactions behave as poorly structured flexible polypeptide chains with little or no conventional secondary structure. The predominant cytosolic forms of both proteins are monomers. Recombinant epsin 1, like AP180, drives in vitro assembly of clathrin cages. It is concluded that the epsin N-terminal homology domain-containing proteins AP180/CALM and epsin 1 have a very similar molecular architecture that is designed for the rapid and efficient recruitment of the principal coat components clathrin and AP-2 at the sites of coated pit assembly (Kalthoff, 2002).
Epsin is a protein that binds to the Eps15 homology (EH) domains, and is involved in clathrin-mediated endocytosis. The epsin N-terminal homology (ENTH) domain (about 140 amino acid residues) is well conserved in eukaryotes and is considered to be important for actin cytoskeleton organization in endocytosis. The solution structure of the ENTH domain (residues 1-144) of human epsin has been determined by multidimensional nuclear magnetic resonance spectroscopy. In the ENTH-domain structure, seven alpha-helices form a superhelical fold, consisting of two antiparallel two-helix HEAT motifs and one three-helix ARM motif, with a continuous hydrophobic core in the center. It is concluded that the seven-helix superhelical fold defines the ENTH domain, and that the previously-reported eight-helix fold of a longer fragment of rat epsin 1 is divided into the authentic ENTH domain and a C-terminal flanking alpha-helix (Koshida, 2002).
The covalent attachment of ubiquitin to proteins is an evolutionarily conserved signal for rapid protein degradation. However, additional cellular functions for ubiquitination are now emerging, including regulation of protein trafficking and endocytosis. For example, recent genetic studies have suggested a role for ubiquitination in regulating epsin, a modular endocytic adaptor protein that functions in the assembly of clathrin-coated vesicles; however, biochemical evidence for this notion has been lacking. Epsin consists of an epsin NH(2)-terminal homology (ENTH) domain that promotes the interaction with phospholipids, several AP2 binding sites, two clathrin binding sequences, and several Eps15 homology (EH) domain binding motifs. Interestingly, epsin also possesses several recently described ubiquitin-interacting motifs (UIMs) that have been postulated to bind ubiquitin. Epsin is demonstrated to be predominantly monoubiquitinated and resistant to proteasomal degradation. The UIMs are necessary for epsin ubiquitination but are not the site of ubiquitination. The isolated UIMs from both epsin and an unrelated monoubiquitinated protein, Eps15, are sufficient to promote ubiquitination of a chimeric glutathione-S-transferase (GST)-UIM fusion protein. Thus, these data suggest that UIMs may serve as a general signal for ubiquitination (Oldham, 2002).
Ubiquitination is a post-translation modification in which ubiquitin chains or single ubiquitin molecules are appended to target proteins, giving rise to poly- or mono-ubiquitination, respectively. Polyubiquitination targets proteins for destruction by the proteasome. The role of monoubiquitination is less understood, although a function in membrane trafficking is emerging, at least in yeast. A short amino-acid stretch at the carboxy-termini of the monoubiquitinated endocytic proteins Eps15 and eps15R is indispensable for their monoubiquitination. A similar sequence, also required for this modification, is found in other cytosolic endocytic proteins, such as epsins and Hrs. These sequences comprise a protein motif, UIM, which has been proposed to bind to ubiquitin. This has been confirmed for the UIMs of eps15, eps15R, epsins and Hrs. Thus, the same motif in several endocytic proteins is responsible for ubiquitin recognition and monoubiquitination. These results predict the existence of a UIM:ubiquitin-based intracellular network. Eps15/eps15R, epsins and Hrs may function as adaptors between ubiquitinated membrane cargo and either the clathrin coat or other endocytic scaffolds. In addition, through their own ubiquitination, they may further contribute to the amplification of this network in the endocytic pathway (Polo, 2002).
Ubiquitin functions as a signal for sorting cargo at multiple steps of the endocytic pathway and controls the activity of trans-acting components of the endocytic machinery. By contrast to proteasome degradation, which generally requires a polyubiquitin chain that is at least four subunits long, internalization and sorting of endocytic cargo at the late endosome are mediated by mono-ubiquitination. Ubiquitin-interacting motifs (UIMs) found in epsins and Vps27p from Saccharomyces cerevisiae are required for ubiquitin binding and protein transport. Epsin UIMs are important for the internalization of receptors into vesicles at the plasma membrane. Vps27p UIMs are necessary to sort biosynthetic and endocytic cargo into vesicles that bud into the lumen of a late endosomal compartment, the multivesicular body. It is proposed that mono-ubiquitin regulates internalization and endosomal sorting by interacting with modular ubiquitin-binding domains in core components of the protein transport machinery. UIM domains are found in a broad spectrum of proteins, consistent with the idea that mono-ubiquitin can function as a regulatory signal to control diverse biological activities (Shih, 2002).
During endocytosis, clathrin and the clathrin adaptor protein AP-2, assisted by a variety of accessory factors, help to generate an invaginated bud at the cell membrane. One of these factors is Eps15, a clathrin-coat-associated protein that binds the alpha-adaptin subunit of AP-2. The function of Eps15 was investigated by characterizing an important binding partner for its region containing EH domains; this protein, epsin, is closely related to the Xenopus mitotic phosphoprotein MP90 and has a ubiquitous tissue distribution. It is concentrated together with Eps15 in presynaptic nerve terminals, which are sites specialized for the clathrin-mediated endocytosis of synaptic vesicles. The central region of epsin binds AP-2 and its carboxy-terminal region binds Eps15. Epsin is associated with clathrin coats in situ, can be co-precipitated with AP-2 and Eps15 from brain extracts, but does not co-purify with clathrin coat components in a clathrin-coated vesicle fraction. When epsin function is disrupted, clathrin-mediated endocytosis is blocked. It is proposed that epsin may participate, together with Eps15, in the molecular rearrangement of the clathrin coats that are required for coated-pit invagination and vesicle fission (Chen, 1998).
Clathrin-mediated endocytosis was shown to be arrested in mitosis due to a block in the invagination of clathrin-coated pits. A Xenopus mitotic phosphoprotein, MP90, is very similar to an abundant mammalian nerve terminal protein, epsin, which binds the Eps15 homology (EH) domain of Eps15 and the alpha-adaptin subunit of the clathrin adaptor AP-2. Both rat epsin and Eps15 are mitotic phosphoproteins, and their mitotic phosphorylation inhibits binding to the appendage domain of alpha-adaptin. Both epsin and Eps15, like other cytosolic components of the synaptic vesicle endocytic machinery, undergo constitutive phosphorylation and depolarization-dependent dephosphorylation in nerve terminals. Furthermore, their binding to AP-2 in brain extracts is enhanced by dephosphorylation. Epsin together with Eps15 have been proposed to assist the clathrin coat in its dynamic rearrangements during the invagination/fission reactions. Their mitotic phosphorylation may be one of the mechanisms by which the invagination of clathrin-coated pits is blocked in mitosis and their stimulation-dependent dephosphorylation at synapses may contribute to the compensatory burst of endocytosis after a secretory stimulus (Chen, 1999).
Epsin (epsin 1) is an interacting partner for the EH domain-containing region of Eps15 and has been implicated in conjunction with Eps15 in clathrin-mediated endocytosis. A similar protein, epsin 2, has been cloned from human and rat brain libraries. Epsin 1 and 2 are most similar in their NH(2)-terminal region, which represents a module [epsin NH(2) terminal homology domain, ENTH domain] found in a variety of other proteins of the data base. The multiple DPW motifs, typical of the central region of epsin 1, are only partially conserved in epsin 2. Both proteins, however, interact through this central region with the clathrin adaptor AP-2. In addition, both epsin 1 and 2 interact with clathrin. The three NPF motifs of the COOH-terminal region of epsin 1 are conserved in the corresponding region of epsin 2, consistent with the binding of both proteins to Eps15. Epsin 2, like epsin 1, is enriched in brain, is present in a brain-derived clathrin-coated vesicle fraction, is concentrated in the peri-Golgi region and at the cell periphery of transfected cells, and partially colocalizes with clathrin. High overexpression of green fluorescent protein-epsin 2 mislocalizes components of the clathrin coat and inhibits clathrin-mediated endocytosis. The epsins define a new protein family implicated in membrane dynamics at the cell surface (Rosenthal, 1999).
Epsin is a recently identified protein that appears to play an important role in clathrin-mediated endocytosis. The central region of epsin 1, the so-called DPW domain, binds to the heterotetrameric AP-2 adaptor complex by associating directly with the globular appendage of the alpha subunit. This central portion of epsin 1 also associates with clathrin. The interaction with clathrin is direct and not mediated by epsin-bound AP-2. Alanine scanning mutagenesis shows that clathrin binding depends on the sequence (257)LMDLADV located within the epsin 1 DPW domain. This sequence, related to the known clathrin-binding sequences in the adaptor beta subunits, amphiphysin, and beta-arrestin, facilitates the association of epsin 1 with the terminal domain of the clathrin heavy chain. Unexpectedly, inhibiting the binding of AP-2 to the GST-epsin DPW fusion protein by progressively deleting DPW triplets but leaving the LMDLADV sequence intact, diminishes the association of clathrin in parallel with AP-2. Because the beta subunit of the AP-2 complex also contains a clathrin-binding site, optimal association with soluble clathrin appears to depend on the presence of at least two distinct clathrin-binding sites, and a second clathrin-binding sequence (480)LVDLD, located within the carboxyl-terminal segment of epsin 1, also interacts with clathrin directly. The LMDLADV and LVDLD sequences act cooperatively in clathrin recruitment assays, suggesting that they bind to different sites on the clathrin-terminal domain. The evolutionary conservation of similar clathrin-binding sequences in several metazoan epsin-like molecules suggests that the ability to establish multiple protein-protein contacts within a developing clathrin-coated bud is an important aspect of epsin function (Drake, 2000).
The epsin N-terminal homology (ENTH) domain is a protein module of approximately 150 amino acids found at the N terminus of a variety of proteins identified in yeast, plants, nematode, frog, and mammals. ENTH domains comprise multiple alpha-helices folded upon each other to form a compact globular structure that has been implicated in interactions with lipids and proteins. In characterizing this evolutionarily conserved domain, tubulin was isolated and identified as an ENTH domain-binding partner. The interaction, which is direct and has a dissociation constant of approximately 1 microm, was observed with ENTH domains of proteins present in various species. Tubulin is co-immunoprecipitated from rat brain extracts with the ENTH domain-containing proteins, epsins 1 and 2, and punctate epsin staining is observed along the microtubule cytoskeleton of dissociated cortical neurons. Consistent with a role in microtubule processes, the over-expression of epsin ENTH domain in PC12 cells stimulates neurite outgrowth. These data demonstrate an evolutionarily conserved property of ENTH domains to interact with tubulin and microtubules (Hussain, 2003).
SNARE proteins on transport vesicles and target membranes have important roles in vesicle targeting and fusion. Therefore, localization and activity of SNAREs have to be tightly controlled. Regulatory proteins bind to N-terminal domains of some SNAREs. vti1b is a mammalian SNARE that functions in late endosomal fusion. To investigate the role of the N terminus of vti1b, a yeast two-hybrid screen was performed. The N terminus of vti1b interacts specifically with the epsin N-terminal homology (ENTH) domain of enthoprotin/CLINT/epsinR. The interaction was confirmed using in vitro binding assays. This complex formation between a SNARE and an ENTH domain was conserved between mammals and yeast. Yeast Vti1p interacted with the ENTH domain of Ent3p. ENTH proteins are involved in the formation of clathrin-coated vesicles. Both epsinR and Ent3p bind adaptor proteins at the trans-Golgi network. Vti1p is required for multiple transport steps in the endosomal system. Genetic interactions between VTI1 and ENT3 were investigated. Synthetic defects suggested that Vti1p and Ent3p cooperate in transport from the trans-Golgi network to the prevacuolar endosome. These experiments identified the first cytoplasmic protein binding to specific ENTH domains. These results point toward a novel function of the ENTH domain and a connection between proteins that function either in vesicle formation or in vesicle fusion (Chidambaram, 2004).
Plasma membrane receptors can be endocytosed through clathrin-dependent and clathrin-independent pathways. The epidermal growth factor receptor (EGFR), when stimulated with low doses of EGF, is internalized almost exclusively through the clathrin pathway, and it is not ubiquitinated. At higher concentrations of ligand, however, a substantial fraction of the receptor is endocytosed through a clathrin-independent, lipid raft-dependent route, as the receptor becomes ubiquitinated. A ubiquitination-impaired EGFR mutant is internalizes through the clathrin pathway, whereas an EGFR/ubiquitin chimera, that can signal solely through its ubiquitin (Ub) moiety, is internalizes exclusively by the non-clathrin pathway. Non-clathrin internalization of ubiquitinated EGFR depends on its interaction with proteins harboring the Ub-interacting motif, as shown through the ablation of three Ub-interacting motif-containing proteins: eps15, eps15R, and epsin. Thus, eps15s and epsin perform an important function in coupling ubiquitinated cargo to clathrin-independent internalization (Sigismund, 2005).
Monoubiquitination of plasma membrane proteins is a mechanism to control their endocytic trafficking by promoting their interaction with cytosolic adaptor proteins that contain ubiquitin (Ub)-binding domains. Epsin, which contains Ub interaction motifs (UIMs), as well as binding sites for the clathrin coat and clathrin accessory factors, is thought to function as one of such adaptors. The importance of clathrin in the internalization of ubiquitinated cargo, however, has been questioned. This study shows that a GFP-Ub chimera directly targeted to the plasma membrane via a lipid-based interaction is efficiently taken up by endocytosis and delivered to the same endosomes that accumulate internalized EGF. Internalization of the chimera requires integrity of the UIM binding interface of Ub, but does not require clathrin. Surprisingly, WT epsin shows little colocalization with this chimera, whereas UIM-containing epsin constructs that lack the clathrin and AP2 binding region, strikingly colocalizes with this chimera on endocytic vacuoles. In addition, extensive colocalization of WT epsin with the chimera on endocytic structures can be observed in cells where clathrin levels are drastically reduced by RNA interference. These results reveal an important regulatory mechanism in epsin function. The mutually exclusive colocalization of epsin with membrane-bound Ub or clathrin may play a role in controlling the endocytic route taken by ubiquitinated cargo (Chen, 2005).
Clathrin-mediated endocytosis involves cargo selection and membrane budding into vesicles with the aid of a protein coat. Formation of invaginated pits on the plasma membrane and subsequent budding of vesicles is an energetically demanding process that involves the cooperation of clathrin with many different proteins. The role of the brain-enriched protein epsin 1 in this process has been investigated. Epsin is targeted to areas of endocytosis by binding the membrane lipid phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)]. Epsin 1 directly modifies membrane curvature on binding to PtdIns(4,5)P(2) in conjunction with clathrin polymerization. Formation of an amphipathic alpha-helix in epsin is coupled to PtdIns(4,5)P(2) binding. Mutation of residues on the hydrophobic region of this helix abolishes the ability to curve membranes. It is proposed that this helix is inserted into one leaflet of the lipid bilayer, inducing curvature. On lipid monolayers epsin alone is sufficient to facilitate the formation of clathrin-coated invaginations (Ford, 2002).
Epsin and AP180/CALM are endocytotic accessory proteins that have been implicated in the formation of clathrin-coated pits. Both proteins have phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]-binding domains in their N termini, but these domains are structurally and functionally different. To understand the basis of their distinct properties, the PtdIns(4,5)P2-dependent membrane binding of the epsin N-terminal homology (ENTH) domain and the AP180 N-terminal homology (ANTH) domain were measured by means of surface plasmon resonance and monolayer penetration techniques and also the effect of PtdIns(4,5)P2 on the electrostatic potential of these domains was calculated. PtdIns(4,5)P2 enhances the electrostatic membrane association of both domains; however, PtdIns(4,5)P2 binding exerts distinct effects on their membrane dissociation. Specifically, PtdIns(4,5)P2 induces the membrane penetration of the N-terminal alpha-helix of the ENTH domain, which slows the membrane dissociation of the domain and triggers the membrane deformation. These results provide the biophysical explanation for the membrane bending activity of epsin and its ENTH domain (Stahelin, 2003).
The Ral signaling pathway is critically involved in Ras-dependent oncogenesis. One of its key actors, RLIP/RalBP1, which participates in receptor endocytosis during interphase, is also involved in mitotic processes when endocytosis is switched off. During mitosis, RLIP76 is located on the duplicated centrosomes and is required for their proper separation and movement to the poles. Factors have been sought that associate with RLIP during mitosis. RLIP/RalBP1 interacts with an active p34cdc2.cyclinB1 (cdk1) enzyme and this interaction is crucial for the mitotic phosphorylation of Epsin that, once phosphorylated, is no longer competent for endocytosis. This latter phosphorylation is dependent on Ral signaling. It is proposed that RLIP/RalBP1 is used as a platform by the mitotic cdk1 to facilitate the phosphorylation of Epsin, which makes Epsin incompetent for endocytosis during mitosis, when endocytosis is switched off (Rosse, 2003).
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