The association of epsin with ubiquitinated cargo along the endocytic pathway is negatively regulated by its interaction with clathrin

Molecular basis of oligoubiquitin-dependent internalization of membrane proteins in Mammalian cells

Ubiquitination induced down-regulation of cell surface proteins by internalization and lysosomal targeting plays a fundamental role in cell physiology and pathogenesis of diseases. The molecular basis of a single ubiquitin (Ub) as an autonomous endocytic signal, the widely accepted mechanism, however, remains elusive in higher eukaryotes. Using Ub containing reporter proteins without signalling abilities, we present evidence that only multiple Ub moieties, linked either covalently or assembled as oligomers with an intact interface for recognition by Ub-interacting motifs (UIMs), are recognized by the endocytic machinery in vivo and associate with a subset of Ub-binding clathrin adaptors in vitro. Genetic and pharmacological approaches show that internalization of plasma membrane proteins harbouring multiple Ub moieties is clathrin-dependent, but caveolin-independent. Functional assays demonstrate the cargo-dependent involvement of eps15/15R and epsin, UIM containing clathrin adaptors, in the endocytosis of model proteins, CD4 and the activated beta(2)-adrenergic receptor complex, containing polymeric or oligomeric Ub. These results provide a paradigm for the clathrin-mediated uptake of ubiquitinated membrane proteins in mammalian cells, requiring the assembly of multiple UIM-Ub interactions to overcome the low affinity binding of mono-Ub to UIM.

Ubiquitin binding in endocytosis--how tight should it be and where does it happen?

Ubiquitin is an important tag in membrane transport. From studies in yeast, monoubiquitin has been considered sufficient to elicit uptake of cell surface transporters and receptors into endosomes. Two articles in the current issue of Traffic (Hawryluk et al. and Barriere et al.) indicate that stronger binding is required to retain and concentrate cargo in endocytic microdomains of the plasma membrane. High avidity interactions can be obtained by tandemly arrayed ubiquitin interaction motifs (UIM), in proteins such as the endocytic adaptors epsin and Eps15, interacting with polyubiquitin or by UIM-containing proteins binding several ubiquitins brought together through oligomerization of receptors. A controversial issue has been where such interactions take place. One view is that the association of epsin with ubiquitinated cargo is negatively regulated by its interaction with clathrin (Chen H and De Camilli P. Proc Natl Acad Sci USA 2005;102:2766-2771). This contention is now challenged by the articles of Hawryluk et al. and Barriere et al. Hawryluk et al. demonstrate that epsin and Eps15 consistently co-localize with clathrin but never with caveolin.

Riding the DUBway: regulation of protein trafficking by deubiquitylating enzymes

Ubiquitylation is a key regulator of protein trafficking, and much about the functions of ubiquitin ligases, which add ubiquitin to substrates in this regulation, has recently come to light. However, a clear understanding of ubiquitin-dependent protein localization cannot be achieved without knowledge of the role of deubiquitylating enzymes (DUBs). DUBs, by definition, function downstream in ubiquitin pathways and, as such, have the potential to be the final editors of protein ubiquitylation status, thus determining substrate fate. This paper assimilates the current evidence concerning the substrates and activities of DUBs that regulate protein trafficking.

Ligands for clathrin-mediated endocytosis are differentially sorted into distinct populations of early endosomes

Cells rely on the correct sorting of endocytic ligands and receptors for proper function. Early endosomes have been considered as the initial sorting station where cargos for degradation separate from those for recycling. Using live-cell imaging to monitor individual endosomes and ligand particles in real time, we have discovered a sorting mechanism that takes place prior to early endosome entry. We show that early endosomes are in fact comprised of two distinct populations: a dynamic population that is highly mobile on microtubules and matures rapidly toward late endosomes and a static population that matures much more slowly. Several cargos destined for degradation are preferentially targeted to the dynamic endosomes, whereas the recycling ligand transferrin is nonselectively delivered to all early endosomes and effectively enriched in the larger, static population. This pre-early endosome sorting process begins at clathrin-coated vesicles, depends on microtubule-dependent motility, and appears to involve endocytic adaptors.

Endocytosis conducts the cell signaling orchestra

Endocytosis is used by eukaryotic cells to regulate nutrient internalization, signal transduction, and the composition of the plasma membrane. However, a more complex picture is emerging, in which endocytic pathways integrate diverse signals, thereby contributing to a higher level of cellular and organismal organization. In this way, endocytosis and cell signaling are intertwined in many biological processes, such as cell motility and cell fate determination.

Role of Ubiquitylation in Cellular Membrane Transport

Ubiquitylation of membrane proteins has gained considerable interest in recent years. It has been recognized as a signal that negatively regulates the cell surface expression of many plasma membrane proteins both in yeast and in mammalian cells. Moreover, it is also involved in endoplasmic reticulum-associated degradation of membrane proteins, and it acts as a sorting signal both in the secretory pathway and in endosomes, where it targets proteins into multivesicular bodies in the lumen of vacuoles/lysosomes. In this review we discuss the progress in understanding these processes, achieved during the past several years.

Sorting of major cargo glycoproteins into clathrin-coated vesicles

The AP-1 and AP-2 complexes are the most abundant adaptors in clathrin-coated vesicles (CCVs), but clathrin-mediated trafficking can still occur in the absence of any detectable AP-1 or AP-2. To find out whether adaptor abundance reflects cargo abundance, we used lectin pulldowns to identify the major membrane glycoproteins in CCVs from human placenta and rat liver. Both preparations contained three prominent high molecular-weight proteins: the cation-independent mannose 6-phosphate receptor (CIMPR), carboxypeptidase D (CPD) and low-density lipoprotein receptor-related protein 1 (LRP1). To investigate how these proteins are sorted, we constructed and stably transfected CD8 chimeras into HeLa cells. CD8-CIMPR localized mainly to early/tubular endosomes, CD8-CPD to the trans Golgi network and CD8-LRP1 to late/multivesicular endosomes. All three constructs redistributed to the plasma membrane when clathrin was depleted by siRNA. CD8-CIMPR was also strongly affected by AP-2 depletion. CD8-CPD was moderately affected by AP-2 depletion but strongly affected by depleting AP-1 and AP-2 together. CD8-LRP1 was only slightly affected by AP-2 depletion; however, mutating an NPXY motif in the LRP1 tail caused it to become AP-2 dependent. These results indicate that all three proteins have AP-dependent sorting signals, which may help to explain the relative abundance of AP complexes in CCVs. However, the relatively low abundance of cargo proteins in CCV preparations suggests either that some of the APs may be empty or that the preparations may be dominated by empty coats.

Clathrin-independent endocytosis: new insights into caveolae and non-caveolar lipid raft carriers

A number of recent studies have provided new insights into the complexity of the endocytic pathways originating at the plasma membrane of mammalian cells. Many of the molecules involved in clathrin coated pit internalization are now well understood but other pathways are less well defined. Caveolae appear to represent a low capacity but highly regulated pathway in a restricted set of tissues in vivo. A third pathway, which is both clathrin- and caveolae-independent, may constitute a specialized high capacity endocytic pathway for lipids and fluid. The relationship of this pathway, if any, to macropinocytosis or to the endocytic pathways of lower eukaryotes remains an interesting open question. Our understanding of the regulatory mechanisms and molecular components involved in this pathway are at a relatively primitive stage. In this review, we will consider some of the characteristics of different endocytic pathways in high and lower eukaryotes and consider some of the common themes in endocytosis. One theme which becomes apparent from comparison of these pathways is that apparently different pathways can share common molecular machinery and that pathways considered to be distinct actually represent similar basic pathways to which additional levels of regulatory complexity have been added.

Ubiquitylation of leptin receptor OB-Ra regulates its clathrin-mediated endocytosis

Leptin receptors are constitutively endocytosed in a ligand-independent manner. To study their endocytosis, leptin receptors OB-Ra and OB-Rb were expressed in HeLa cells. Both receptor isoforms were ubiquitylated, internalized by clathrin-mediated endocytosis and transported to Hrs-positive endosomes after their internalization. Proteasome inhibitors inhibited OB-Ra but not OB-Rb internalization from the cell surface. OB-Ra ubiquitylation occurred on lysine residues K877 and K889 in the cytoplasmic tail, the mutation of which abolished OB-Ra internalization. Fusion of an ubiquitin molecule at the C-terminus of an OB-Ra construct defective both in ubiquitylation and endocytosis restored clathrin-dependent endocytosis of the receptor. The internalization of this constitutively mono-ubiquitylated construct was no longer sensitive to proteasome inhibitors, which inhibited OB-Ra endocytosis by blocking its ubiquitylation. Fusion of an ubiquitin molecule to a transferrin receptor deleted from its own endocytosis motif restored clathrin-mediated endocytosis. We propose that mono-ubiquitin conjugates act as internalization motifs for clathrin-dependent endocytosis of leptin receptor OB-Ra.

Clathrin-independent endocytosis: new insights into caveolae and non-caveolar lipid raft carriers

A number of recent studies have provided new insights into the complexity of the endocytic pathways originating at the plasma membrane of mammalian cells. Many of the molecules involved in clathrin coated pit internalization are now well understood but other pathways are less well defined. Caveolae appear to represent a low capacity but highly regulated pathway in a restricted set of tissues in vivo. A third pathway, which is both clathrin- and caveolae-independent, may constitute a specialized high capacity endocytic pathway for lipids and fluid. The relationship of this pathway, if any, to macropinocytosis or to the endocytic pathways of lower eukaryotes remains an interesting open question. Our understanding of the regulatory mechanisms and molecular components involved in this pathway are at a relatively primitive stage. In this review, we will consider some of the characteristics of different endocytic pathways in high and lower eukaryotes and consider some of the common themes in endocytosis. One theme which becomes apparent from comparison of these pathways is that apparently different pathways can share common molecular machinery and that pathways considered to be distinct actually represent similar basic pathways to which additional levels of regulatory complexity have been added.

Partially overlapping distribution of epsin1 and HIP1 at the synapse: analysis by immunoelectron microscopy

Synapses of neurons use clathrin-mediated endocytic pathways for recycling of synaptic vesicles and trafficking of neurotransmitter receptors. Epsin 1 and huntingtin-interacting protein 1 (HIP1) are endocytic accessory proteins. Both proteins interact with clathrin and the AP2 adaptor complex and also bind to the phosphoinositide-containing plasma membrane via an epsin/AP180 N-terminal homology (ENTH/ANTH) domain. Epsin1 and HIP1 are found in neurons; however, their precise roles in synapses remain largely unknown. Using immunogold electron microscopy, we examine and compare the synaptic distribution of epsin1 and HIP1 in rat CA1 hippocampal synapse. We find that epsin1 is located across both sides of the synapse, whereas HIP1 displays a preference for the postsynaptic compartment. Within the synaptic compartments, espin1 is distributed similarly throughout, whereas postsynaptic HIP1 is concentrated near the plasma membrane. Our results suggest a dual role for epsin1 and HIP1 in the synapse: as broadly required factors for promoting clathrin assembly and as adaptors for specific endocytic pathways.

Regulation of ubiquitin-binding proteins by monoubiquitination

Proteins containing ubiquitin-binding domains (UBDs) interact with ubiquitinated targets and regulate diverse biological processes, including endocytosis, signal transduction, transcription and DNA repair. Many of the UBD-containing proteins are also themselves monoubiquitinated, but the functional role and the mechanisms that underlie this modification are less well understood. Here, we demonstrate that monoubiquitination of the endocytic proteins Sts1, Sts2, Eps15 and Hrs results in intramolecular interactions between ubiquitin and their UBDs, thereby preventing them from binding in trans to ubiquitinated targets. Permanent monoubiquitination of these proteins, mimicked by the fusion of ubiquitin to their carboxyl termini, impairs their ability to regulate trafficking of ubiquitinated receptors. Moreover, we mapped the in vivo monoubiquitination site in Sts2 and demonstrated that its mutation enhances the Sts2-mediated effects of epidermal-growth-factor-receptor downregulation. We propose that monoubiquitination of ubiquitin-binding proteins inhibits their capacity to bind to and control the functions of ubiquitinated targets in vivo.

Role of lipids in the MAPK signaling pathway

The mitogen-activated protein kinase (MAPK) signaling pathway is activated in response to a variety of extracellular stimuli such as growth factor stimulation. The best-characterized MAPK pathway involves the sequential activation of Raf, MEK and ERK proteins, capable of regulating the gene expression required for cell proliferation. Binding to specific lipids can regulate both the subcellular localization of these MAPK signaling proteins as well as their kinase activities. More recently it has become increasingly clear that the majority of MAPK signaling takes place intracellularly on endosomes and that the perturbation of endocytic pathways has dramatic effects on the MAPK pathway. This review highlights the direct effects of lipids on the localization and regulation of MAPK pathway proteins. In addition, the indirect effects lipids have on MAPK signaling via their regulation of endocytosis and the biophysical properties of different membrane lipids as a result of growth factor stimulation are discussed. The ability of a protein to bind to both lipids and proteins at the same time may act like a "ZIP code" to target that protein to a highly specific microlocation and could also allow a protein to be "handed off" to maintain tight control over its binding partners and location.

Regulation of Epithelial Na+ Channel Activity by Conserved Serine/Threonine Switches within Sorting Signals

The PY and YXXphi motifs are canonical sorting signals involved in trafficking. Nedd4-2 and the mu(2)-subunit of the AP-2 complex target these motifs to facilitate internalization. Epithelial Na(+) channel (ENaC) subunits contain both motifs in their cytosolic COOH termini where they overlap ((S/T)PPPXYX(S/T)phi). Just preceding the PY and embedded within the YXXphi motifs are conserved serine/threonine. We test here whether these conserved Ser/Thr modulate ENaC activity by influencing the function of the internalization domains. We find that co-expression of dominant-negative dynamin (K44A) with ENaC increases channel activity. Conversely, co-expression of Nedd4-2 and epsin with ENaC decrease activity. Alanine substitution of the conserved Thr(628) preceding the PY motif in gamma-mENaC had no effect on basal activity. Channels with this mutation, however, responded to K44A and epsin but not Nedd4-2. Similarly, mutation of the proline repeat in the PY motif of gamma-mENaC disrupted only Nedd4-2 regulation having no effect on regulation by K44A and epsin. Alanine substitution of the conserved Thr within the YXX motif of gamma-mENaC (T635A) increased basal activity. Channels containing this mutation responded to Nedd4-2 but not K44A and epsin. Channels containing the T635(D/E) substitution in gamma-mENaC did not have increased basal activity and responded to Nedd4-2 but not K44A. The double mutant T628A,T635A did not respond to Nedd4-2 or K44A. Mutation of Thr(628) and Thr(635) also disrupted ENaC precipitation with the mu(2)-subunit of the AP-2 complex. Moreover, the YXXphi motif, independent of the PY motif, was sufficient to target degradation with T635A disrupting this effect. These results demonstrate that the overlapping PY and YXXphi motifs in ENaC are, in some instances, capable of independent function and that the Ser/Thr just preceding and within these domains impact this function.

Evolving endosomes: how many varieties and why?

The cell biologist's insight into endosomal diversity, in terms of both form and function, has increased dramatically in the past few years. This understanding has been promoted by the availability of powerful new techniques that allow imaging of both cargo and machinery in the endocytic process in real time, and by our ability to inhibit components of this machinery by RNA interference. The emerging picture from these studies is of a highly complex, dynamic and adaptable endosomal system that interacts at various points with the secretory system of the cell.

E3 ubiquitin ligases as regulators of membrane protein trafficking and degradation

Ubiquitination is a regulated post-translational modification that conjugates ubiquitin (Ub) to lysine residues of target proteins and determines their intracellular fate. The canonical role of ubiquitination is to mediate degradation by the proteasome of short-lived cytoplasmic proteins that carry a single, polymeric chain of Ub on a specific lysine residue. However, protein modification by Ub has much broader and diverse functions involved in a myriad of cellular processes. Monoubiquitination, at one or multiple lysine residues of transmembrane proteins, influences their stability, protein-protein recognition, activity and intracellular localization. In these processes, Ub functions as an internalization signal that sends the modified substrate to the endocytic/sorting compartments, followed by recycling to the plasma membrane or degradation in the lysosome. E3 ligases play a pivotal role in ubiquitination, because they recognize the acceptor protein and hence dictate the high specificity of the reaction. The multitude of E3s present in nature suggests their nonredundant mode of action and the need for their controlled regulation. Here we give a short account of E3 ligases that specifically modify and regulate membrane proteins. We emphasize the intricate network of interacting proteins that contribute to the substrate-E3 recognition and determine the substrate's cellular fate.

Distinct roles for Mind bomb, Neuralized and Epsin in mediating DSL endocytosis and signaling in Drosophila

Ligands of the Delta/Serrate/Lag2 (DSL) family must normally be endocytosed in signal-sending cells to activate Notch in signal-receiving cells. DSL internalization and signaling are promoted in zebrafish and Drosophila, respectively, by the ubiquitin ligases Mind bomb (Mib) and Neuralized (Neur). DSL signaling activity also depends on Epsin, a conserved endocytic adaptor thought to target mono-ubiquitinated membrane proteins for internalization. Here, we present evidence that the Drosophila ortholog of Mib (Dmib) is required for ubiquitination and signaling activity of DSL ligands in cells that normally do not express Neur, and can be functionally replaced by ectopically expressed Neur. Furthermore, we show that both Dmib and Epsin are required in these cells for some of the endocytic events that internalize DSL ligands, and that the two Drosophila DSL ligands Delta and Serrate differ in their utilization of these Dmib- and Epsin-dependent pathways: most Serrate is endocytosed via the actions of Dmib and Epsin, whereas most Delta enters by other pathways. Nevertheless, only those Serrate and Delta proteins that are internalized via the action of Dmib and Epsin can signal. These results support and extend our previous proposal that mono-ubiquitination of DSL ligands allows them to gain access to a select, Epsin-dependent, endocytic pathway that they must normally enter to activate Notch.

Caveolae: Stable Membrane Domains with a Potential for Internalization

The role of caveolae in endocytosis is hotly debated. Here, we argue that most caveolae are stable microdomains at the cell surface. Only a small fraction of caveolae is constitutively internalized, leading to a quantitatively minor uptake of ligands and receptors. In addition, we suggest that a more pronounced downregulation of caveolae from the plasma membrane can occur, presumably stimulated by receptor cross-linking and clustering in caveolae. Finally, we propose that future studies dealing with internalization of caveolae should actually document such internalization and include kinetic data.

The roles of receptor and ligand endocytosis in regulating Notch signaling

Cell-cell signaling is a central process in the formation of multicellular organisms. Notch (N) is the receptor of a conserved signaling pathway that regulates numerous developmental decisions, and the misregulation of N has been linked to various physiological and developmental disorders. The endocytosis of N and its ligands is a key mechanism by which N-mediated cell-cell signaling is developmentally regulated. We review here the recent findings that have highlighted the importance and complexity of this regulation.

Herpes simplex virus 1 infected cell protein 0 forms a complex with CIN85 and Cbl and mediates the degradation of EGF receptor from cell surfaces

Infected cell protein 0 (ICP0) is a 775-residue multifunctional herpes simplex virus protein associated with numerous functions related to transactivation of gene expression and repression of host defenses to infection. We report that an uncharted domain of ICP0 located between residues 245 and 510 contains multiple SH3 domain binding motifs similar to those required for binding to CIN85, the M(r) 85,000 protein that interacts with Cbl. CIN85 and Cbl are involved in endocytosis and negative regulation of numerous receptor tyrosine kinases. We report that ICP0 binds CIN85 in a reciprocal manner and that the complexes pulled down by ICP0 also contain Cbl. We tested the role of ICP0 in the down-regulation of receptor tyrosine kinases by using epidermal growth factor receptor (EGFR) as a prototypic receptor. In transfection assays, ICP0, in the absence of other viral genes, down-regulated EGF-dependent expression of a reporter gene (luciferase). ICP0 also down-regulated both total and cell surface levels of EGFR in EGF-independent manner. In wild-type virus-infected cells, the surface levels of EGFR were also decreased in the absence of EGF stimulation. Stimulation by EGF enhanced the decrease in surface EGFR. We conclude that ICP0 encodes SH3 domain binding sites that function to down-regulate signaling pathways associated with receptor tyrosine kinases. The results suggest that ICP0 precludes signaling to the infected cells through the receptor tyrosine kinases.