The endosomal sorting complexes required for transport (ESCRT) drive multivesicular body (MVB) biogenesis and cytokinetic abscission. Originally identified through genetics and cell biology, more recent work has begun to elucidate the molecular mechanisms of ESCRT-mediated membrane remodeling, with special focus on the ESCRT-III complex. In particular, several light and electron microscopic studies provide high-resolution imaging of ESCRT-III rings and spirals that purportedly drive MVB morphogenesis and abscission. These studies highlight unifying principles to ESCRT-III function, in particular: (1) the ordered assembly of the ESCRT-III monomers into a heteropolymer, (2) ESCRT-III as a dynamic complex, and (3) the role of the AAA ATPase Vps4 as a contributing factor in membrane scission. Mechanistic comparisons of ESCRT-III function in MVB morphogenesis and cytokinesis suggest common mechanisms in membrane remodeling.
The ESCRT pathway mediates certain types of membrane remodeling (e.g., cellular abscission). Recent light and electron microscopy studies focusing on one of the ESCRT complexes, ESCRT-III, are revealing mechanistic details.
Abscission is the final step of cytokinesis that involves the cleavage of the intercellular bridge connecting the two daughter cells. Recent studies have given novel insight into the spatiotemporal regulation and molecular mechanisms controlling abscission in cultured yeast and human cells. The mechanisms of abscission in living metazoan tissues are however not well understood. Here we show that ALIX and the ESCRT-III component Shrub are required for completion of abscission during Drosophila female germline stem cell (fGSC) division. Loss of ALIX or Shrub function in fGSCs leads to delayed abscission and the consequent formation of stem cysts in which chains of daughter cells remain interconnected to the fGSC via midbody rings and fusome. We demonstrate that ALIX and Shrub interact and that they co-localize at midbody rings and midbodies during cytokinetic abscission in fGSCs. Mechanistically, we show that the direct interaction between ALIX and Shrub is required to ensure cytokinesis completion with normal kinetics in fGSCs. We conclude that ALIX and ESCRT-III coordinately control abscission in Drosophila fGSCs and that their complex formation is required for accurate abscission timing in GSCs in vivo.
Cytokinesis, the final step of cell division, concludes with a process termed abscission, during which the two daughter cells physically separate. In spite of their importance, the molecular machineries controlling abscission are poorly characterized especially in the context of living metazoan tissues. Here we provide molecular insight into the mechanism of abscission using the fruit fly Drosophila melanogaster as a model organism. We show that the scaffold protein ALIX and the ESCRT-III component Shrub are required for completion of abscission in Drosophila female germline stem cells (fGSCs). ESCRT-III has been implicated in topologically similar membrane scission events as abscission, namely intraluminal vesicle formation at endosomes and virus budding. Here we demonstrate that ALIX and Shrub co-localize and interact to promote abscission with correct timing in Drosophila fGSCs. We thus show that ALIX and ESCRT-III coordinately control abscission in Drosophila fGSCs cells and report an evolutionarily conserved function of the ALIX/ESCRT-III pathway during cytokinesis in a multi-cellular organism.
The occurrence of cancer is often associated with a dysfunction in one of the three central membrane-involution processes—autophagy, endocytosis or cytokinesis. Interestingly, all three pathways are controlled by the same central signaling module: the class III phosphatidylinositol 3-kinase (PI3K-III) complex and its catalytic product, the phosphorylated lipid phosphatidylinositol 3-phosphate (PtdIns3P). The activity of the catalytic subunit of the PI3K-III complex, the lipid-kinase VPS34, requires the presence of the membrane-targeting factor VPS15 as well as the adaptor protein Beclin 1. Furthermore, a growing list of regulatory proteins associates with VPS34 via Beclin 1. These accessory factors define distinct subunit compositions and thereby guide the PI3K-III complex to its different cellular and physiological roles. Here we discuss the regulation of the PI3K-III complex components by ubiquitination and SUMOylation. Especially Beclin 1 has emerged as a highly regulated protein, which can be modified with Lys11-, Lys48- or Lys63-linked polyubiquitin chains catalyzed by distinct E3 ligases from the RING-, HECT-, RBR- or Cullin-type. We also point out other cross-links of these ligases with autophagy in order to discuss how these data might be merged into a general concept.
VPS34; Beclin 1; Ambra 1; ubiquitin; SUMO; autophagy; tumor suppressor
The C-terminal ends of small GTPases contain hypervariable sequences which may be posttranslationally modified by defined lipid moieties. The diverse structural motifs generated direct proteins towards specific cellular membranes or organelles. However, knowledge on the factors that determine these selective associations is limited. Here we show, using advanced microscopy, that the isoprenylation and palmitoylation motif of human RhoB (–CINCCKVL) targets chimeric proteins to intraluminal vesicles of endolysosomes in human cells, displaying preferential co-localization with components of the late endocytic pathway. Moreover, this distribution is conserved in distant species, including cells from amphibians, insects and fungi. Blocking lipidic modifications results in accumulation of CINCCKVL chimeras in the cytosol, from where they can reach endolysosomes upon release of this block. Remarkably, CINCCKVL constructs are sorted to intraluminal vesicles in a cholesterol-dependent process. In the lower species, neither the C-terminal sequence of RhoB, nor the endosomal distribution of its homologs are conserved; in spite of this, CINCCKVL constructs also reach endolysosomes in Xenopus laevis and insect cells. Strikingly, this behavior is prominent in the filamentous ascomycete fungus Aspergillus nidulans, in which GFP-CINCCKVL is sorted into endosomes and vacuoles in a lipidation-dependent manner and allows monitoring endosomal movement in live fungi. In summary, the isoprenylated and palmitoylated CINCCKVL sequence constitutes a specific structure which delineates an endolysosomal sorting strategy operative in phylogenetically diverse organisms.
Autophagy is a mechanism of cellular self-degradation that is very important for cellular homeostasis and differentiation.
Components of the endosomal sorting complex required for transport (ESCRT) machinery are required for endosomal sorting and also for autophagy
and the completion of cytokinesis. Here we show that the ESCRT-III subunit CHMP4B not only localizes to normal cytokinetic bridges but also to chromosome
bridges and micronuclei, the latter surrounded by lysosomes and autophagosomes. Moreover, CHMP4B can be co-immunoprecipitated with chromatin. Interestingly,
a CHMP4B mutation associated with autosomal dominant posterior polar cataract abolishes the ability of CHMP4B to localize to micronuclei. We propose that CHMP4B,
through its association with chromatin, may participate in the autophagolysosomal degradation of micronuclei and other extranuclear chromatin. This may have implications
for DNA degradation during lens cell differentiation, thus potentially protecting lens cells from cataract development.
SNX18 promotes autophagosome formation by remodeling membranes and providing membrane to forming autophagosomes.
The membrane remodeling events required for autophagosome biogenesis are still poorly understood. Because PX domain proteins mediate membrane remodeling and trafficking, we conducted an imaging-based siRNA screen for autophagosome formation targeting human PX proteins. The PX-BAR protein SNX18 was identified as a positive regulator of autophagosome formation, and its Drosophila melanogaster homologue SH3PX1 was found to be required for efficient autophagosome formation in the larval fat body. We show that SNX18 is required for recruitment of Atg16L1-positive recycling endosomes to a perinuclear area and for delivery of Atg16L1- and LC3-positive membranes to autophagosome precursors. We identify a direct interaction of SNX18 with LC3 and show that the pro-autophagic activity of SNX18 depends on its membrane binding and tubulation capacity. We also show that the function of SNX18 in membrane tubulation and autophagy is negatively regulated by phosphorylation of S233. We conclude that SNX18 promotes autophagosome formation by virtue of its ability to remodel membranes and provide membrane to forming autophagosomes.
Recently, we demonstrated that integrin adhesion to the extracellular matrix at the cleavage furrow is essential for cytokinesis of adherent cells. Here, we report that tight junction protein ZO-1 (Zonula Occludens-1) is required for successful cytokinesis in NCI-H460 cells plated on fibronectin. This function of ZO-1 involves interaction with the cytoplasmic domain of α5-integrin to facilitate recruitment of active fibronectin-binding integrins to the base of the cleavage furrow. In the absence of ZO-1, or a functional ZO-1/α5β1-integrin complex, proper actin-dependent constriction between daughter cells is impaired and cells fail cytokinesis. Super-resolution microscopy reveals that in ZO-1 depleted cells the furrow becomes delocalized from the matrix. We also show that PKCε-dependent phosphorylation at Serine168 is required for ZO-1 localization to the furrow and successful cell division. Altogether, our results identify a novel regulatory pathway involving the interplay between ZO-1, α5-integrin and PKCε in the late stages of mammalian cell division.
The yeast gene fab1 and its mammalian orthologue Pip5k3 encode the phosphatidylinositol 3-phosphate [PtdIns(3)P] 5-kinases Fab1p and PIKfyve, respectively, enzymes that generates phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P2]. A shared feature of fab1Δ yeast cells and mammalian cells overexpressing a kinase-dead PIKfyve mutant is the formation of a swollen vacuolar phenotype: a phenotype that is suggestive of a conserved function for these enzymes and their product, PtdIns(3,5)P2, in the regulation of endomembrane homeostasis. In the current study, fixed and live cell imaging has established that, when overexpressed at low levels in HeLa cells, PIKfyve is predominantly associated with dynamic tubular and vesicular elements of the early endosomal compartment. Moreover, through the use of small interfering RNA, it has been shown that suppression of PIKfyve induces the formation of swollen endosomal structures that maintain their early and late endosomal identity. Although internalisation, recycling and degradative sorting of receptors for epidermal growth factor and transferrin was unperturbed in PIKfyve suppressed cells, a clear defect in endosome to trans-Golgi-network (TGN) retrograde traffic was observed. These data argue that PIKfyve is predominantly associated with the early endosome, from where it regulates retrograde membrane trafficking to the TGN. It follows that the swollen endosomal phenotype observed in PIKfyve-suppressed cells results primarily from a reduction in retrograde membrane fission rather than a defect in multivesicular body biogenesis.
PIKfyve; Fab1p; Early endosome; Phosphatidylinositol (3,5)-bisphosphate; Endosomal sorting
Eukaryotic cells are distinguished by their compartmentalization into membrane-enclosed organelles that exchange membranes and content in a highly ordered manner. Central in defining membrane identity are the Rabs, a large family of small GTPases that localize to distinct membranes and recruit specific regulators of membrane traffic. Two recent papers, including one by Klöpper et al. in BMC Biology, present phylogenomic evidence that the Rab repertoire was established very early in eukaryotic evolution, and correlates with interspecies variations in organelles.
See research article http://www.biomedcentral.com/1741-7007/10/71
Over the past fourteen years, ubiquitination has emerged as a centrally important mechanism governing the subcellular trafficking of proteins. Ubiquitination, interaction with sorting factors that contain ubiquitin binding domains, and finally deubiquitination govern the itineraries of cargo proteins that include yeast carboxypeptidase S, the epithelial sodium channel ENaC, and epidermal growth factor receptor. The molecular structures and mechanisms of the paradigmatic HECT and RING domain ubiquitin ligases, JAMM and USP domain deubiquitinating enzymes, and numerous ubiquitin binding domains involved in these pathways, have been worked out in recent years and are described.
Lysosome; vacuole; yeast genetics; EGF; EGF receptor; growth factor receptor; epithelial sodium channel; ENaC; yeast genetics; carboxypeptidase S; protein structure; crystal structure; ubiquitin; RING domain; HECT domain; JAMM domain; isopeptidase; ubiquitin ligase; deubiquitinating enzyme; ubiquitin binding domain; ESCRT complex
Beclin 1, a subunit of the class III phosphatidylinositol 3-kinase complex, is a tumour suppressor with a central role in endocytic trafficking, cytokinesis and the cross-regulation between autophagy and apoptosis. Interestingly, not only reduced expression but also overexpression of Beclin 1 is correlated with cancer development and metastasis. Thus it seems necessary for the cell to balance the protein levels of Beclin 1. In the present study we describe a regulatory link between Beclin 1 and the ubiquitin ligase Nedd4 (neural-precursor-cell-expressed developmentally down-regulated 4). We establish Nedd4 as a novel binding partner of Beclin 1 and demonstrate that Nedd4 polyubiquitinates Beclin 1 with Lys11- and Lys63-linked chains. Importantly, Nedd4 expression controls the stability of Beclin 1, and depletion of the Beclin 1-interacting protein VPS34 causes Nedd4-mediated proteasomal degradation of Beclin 1 via Lys11-linked polyubiquitin chains. Beclin 1 is thus the first tumour suppressor reported to be controlled by Lys11-linked polyubiquitination.
cancer; homologous with E6-associated protein C-terminus (HECT); neural-precursor-cell-expressed developmentally down-regulated 4 (Nedd4); phosphoinositide; ubiquitination; VPS34; AIP4, atrophin-interacting protein 4; BH3, Bcl-2 homology 3; CCD, coiled-coil domain; CHX, cycloheximide; DMEM, Dulbecco's modified Eagle's medium; DTT, dithiothreitol; ECD, evolutionarily conserved domain; GST, glutathione transferase; HA, haemagglutinin; HECT, homologous with E6-associated protein C-terminus; HRP, horseradish peroxidase; Nedd4, neural-precursor-cell-expressed developmentally down-regulated 4; PI3K-III, class III PI 3-kinase; siRNA, small interfering RNA; TRAF6, tumour-necrosis-factor-receptor-associated factor 6; Ub, ubiquitin
Autophagy has been reported to contribute to cell death, but the underlying mechanisms remain largely unknown and controversial. We have been studying oogenesis in Drosophila melanogaster as a model system to understand the interplay between autophagy and cell death. Using a novel autophagy reporter we found that autophagy occurs during developmental cell death of nurse cells in late oogenesis. Genetic inhibition of autophagy-related genes atg1, atg13 and vps34 results in late-stage egg chambers containing persisting nurse cell nuclei without fragmented DNA and attenuation of caspase-3 cleavage. We found that Drosophila inhibitor of apoptosis dBruce is degraded by autophagy and this degradation promotes DNA fragmentation and subsequent nurse cell death. These studies demonstrate that autophagic degradation of an inhibitor of apoptosis is a novel mechanism of triggering cell death.
apoptosis; autophagy; Drosophila; IAPs; nurse cells; oogenesis; programmed cell death
There is growing evidence that macroautophagic cargo is not limited to bulk cytosol in response to starvation, and can occur selectively for substrates including aggregated proteins. It remains unclear, however, if starvation-induced and selective macroautophagy share identical adapter molecules to capture their cargo. Here we report that Alfy, a phosphatidylinositol 3-phosphate binding protein, is central to the selective elimination of aggregated proteins. We report that the loss of Alfy inhibits the clearance of inclusions, with little to no effect on the starvation response. Alfy is recruited to intracellular inclusions and scaffolds a complex between p62(SQSTM1)-positive proteins and the autophagic effectors Atg5, Atg12, Atg16L and LC3. Alfy overexpression leads to elimination of aggregates in an Atg5-dependent manner, and likewise, to protection in a neuronal and Drosophila model of polyglutamine toxicity. We propose that Alfy plays a key role in selective macroautophagy, by bridging cargo to the molecular machinery that builds autophagosomes.
The tumor suppressor activity of Beclin 1 (BECN1), a subunit of class III phosphatidylinositol 3-kinase complex, has been attributed to its regulation of apoptosis and autophagy. Here, we identify FYVE-CENT (ZFYVE26), a phosphatidylinositol 3-phosphate binding protein important for cytokinesis, as a novel interacting protein of Beclin 1. A mutation in FYVE-CENT (R1945Q) associated with breast cancer abolished the interaction between FYVE-CENT and Beclin 1, and reduced the localization of these proteins at the intercellular bridge during cytokinesis. Breast cancer cells containing the FYVE-CENT R1945Q mutation displayed a significant increase in cytokinetic profiles and bi - multinuclear phenotype. Both Beclin 1 and FYVE-CENT were found to be downregulated in advanced breast cancers. These findings suggest a positive feedback loop for recruitment of FYVE-CENT and Beclin 1 to the intercellular bridge during cytokinesis, and reveal a novel potential tumor suppressor mechanism for Beclin 1.
Blocking autophagy protects the apoptosis inhibitor dBruce from destruction and promotes nurse cell survival in developing egg chambers.
Autophagy is an evolutionarily conserved pathway responsible for degradation of cytoplasmic material via the lysosome. Although autophagy has been reported to contribute to cell death, the underlying mechanisms remain largely unknown. In this study, we show that autophagy controls DNA fragmentation during late oogenesis in Drosophila melanogaster. Inhibition of autophagy by genetically removing the function of the autophagy genes atg1, atg13, and vps34 resulted in late stage egg chambers that contained persisting nurse cell nuclei without fragmented DNA and attenuation of caspase-3 cleavage. The Drosophila inhibitor of apoptosis (IAP) dBruce was found to colocalize with the autophagic marker GFP-Atg8a and accumulated in autophagy mutants. Nurse cells lacking Atg1 or Vps34 in addition to dBruce contained persisting nurse cell nuclei with fragmented DNA. This indicates that autophagic degradation of dBruce controls DNA fragmentation in nurse cells. Our results reveal autophagic degradation of an IAP as a novel mechanism of triggering cell death and thereby provide a mechanistic link between autophagy and cell death.
Cytokinesis, the final step of cell division, normally proceeds to completion in living organisms, so that daughter cells physically separate by abscission. In certain tissues and developmental stages, on the other hand, the cytokinesis process is incomplete, giving rise to cells interconnected in syncytia by stable intercellular bridges. This evolutionarily conserved physiological process occurs in the female and male germline in species ranging from insects to humans, and has also been observed in some somatic tissues in invertebrates. Stable intercellular bridges have fascinated cell biologists ever since they were first described more than 50 years ago, and even though substantial progress has been made concerning their ultrastructure and molecular composition, much remains to be understood about their biological functions. Another major question is by which mechanisms complete versus incomplete cytokinesis is determined. In this mini-review we will try to give an overview of the current knowledge about the structure, composition and functions of stable intercellular bridges, and discuss recent insights into the molecular control of the incomplete cytokinesis process.
incomplete cytokinesis; stable intercellular bridge; cytokinesis; intercellular communication; ring canal; oogenesis; spermatogenesis; anillin; MKLP1; Pav-Klp; cindr
Recent findings have shown that ubiquitination is involved in regulating several proteins required for cell adhesion and migration. We showed that α5 integrin is ubiquitinated at its cytoplasmic lysines in response to fibronectin binding, and that this is required for its sorting to lysosomes together with fibronectin. Here we speculate whether other α integrin tails may also be ubiquitinated, and discuss the significance of ubiquitin linkages in the regulation of cell adhesion and migration.
integrin; ubiquitin; cell migration; endocytosis; lysosome
Research in autophagy continues to accelerate,1 and as a result many new scientists are entering the field. Accordingly, it is important to establish a standard set of criteria for monitoring macroautophagy in different organisms. Recent reviews have described the range of assays that have been used for this purpose.2,3 There are many useful and convenient methods that can be used to monitor macroautophagy in yeast, but relatively few in other model systems, and there is much confusion regarding acceptable methods to measure macroautophagy in higher eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers of autophagosomes versus those that measure flux through the autophagy pathway; thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from fully functional autophagy that includes delivery to, and degradation within, lysosomes (in most higher eukaryotes) or the vacuole (in plants and fungi). Here, we present a set of guidelines for the selection and interpretation of the methods that can be used by investigators who are attempting to examine macroautophagy and related processes, as well as by reviewers who need to provide realistic and reasonable critiques of papers that investigate these processes. This set of guidelines is not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to verify an autophagic response.
autolysosome; autophagosome; flux; lysosome; phagophore; stress; vacuole
Since the first morphological description of autophagosomes in the early 1960s, two critical questions have been a matter of intense investigation and debate: what is the origin of the autophagosomal membrane and how is it formed? A study by Axe et al. (E.L. Axe, S.A. Walker, M. Manifava, P. Chandra, H.L. Roderick, A. Habermann, G. Griffiths, and N.T. Ktistakis. 2008. J. Cell Biol. 182:685–701) provides evidence that cup-shaped protrusions from the endoplasmic reticulum, named omegasomes, serve as platforms for autophagosome biogenesis in mammalian cells.
Several regulators of endocytic trafficking have recently been identified as tumour suppressors in Drosophila. These include components of the endosomal sorting complex required for transport (ESCRT) machinery. Disruption of subunits of ESCRT-I and –II leads to cell-autonomous endosomal accumulation of ubiquitinated receptors, loss of apicobasal polarity and epithelial integrity, and increased cell death. Here we report that disruption of the ATPase dVps4, the most downstream component of the ESCRT machinery, causes the same array of cellular phenotypes. We find that loss of epithelial integrity and increased apoptosis, but not loss of cell polarity, require the activation of JNK signalling. Abrogation of JNK signalling prevents apoptosis in dVps4 deficient cells. Indeed double deficiency in dVps4 and JNK signalling leads to the formation of neoplastic tumours. We conclude that dvps4 is a tumour suppressor in Drosophila and that JNK is central to the cell-autonomous phenotypes of ESCRT-deficient cells.
The small GTPase Rab5 has emerged as an important regulator of animal development, and it is essential for endocytic trafficking. However, the mechanisms that link Rab5 activation to cargo entry into early endosomes remain unclear. We show here that Drosophila Rabenosyn (Rbsn) is a Rab5 effector that bridges an interaction between Rab5 and the Sec1/Munc18-family protein Vps45, and we further identify the syntaxin Avalanche (Avl) as a target for Vps45 activity. Rbsn and Vps45, like Avl and Rab5, are specifically localized to early endosomes and are required for endocytosis. Ultrastructural analysis of rbsn, Vps45, avl, and Rab5 null mutant cells, which show identical defects, demonstrates that all four proteins are required for vesicle fusion to form early endosomes. These defects lead to loss of epithelial polarity in mutant tissues, which overproliferate to form neoplastic tumors. This work represents the first characterization of a Rab5 effector as a tumor suppressor, and it provides in vivo evidence for a Rbsn–Vps45 complex on early endosomes that links Rab5 to the SNARE fusion machinery.
c-Cbl is the E3 ubiquitin ligase that ubiquitinates the epidermal growth factor (EGF) receptor (EGFR). On the basis of localization, knockdown, and in vitro activity analyses, we have identified the E2 ubiquitin-conjugating enzyme that cooperates with c-Cbl as Ubc4/5. Upon EGF stimulation, both Ubc4/5 and c-Cbl were relocated to the plasma membrane and then to Hrs-positive endosomes, strongly suggesting that EGFR continues to be ubiquitinated after internalization. Our time-course experiment showed that EGFR undergoes polyubiquitination, which seemed to be facilitated during the transport to Hrs-positive endosomes. Use of a conjugation-defective ubiquitin mutant suggested that receptor polyubiquitination is required for efficient interaction with Hrs and subsequent sorting to lysosomes. Abrupt inhibition of the EGFR kinase activity resulted in dissociation of c-Cbl from EGFR. Concomitantly, EGFR was rapidly deubiquitinated and its degradation was delayed. We propose that sustained tyrosine phosphorylation of EGFR facilitates its polyubiquitination in endosomes and counteracts rapid deubiquitination, thereby ensuring Hrs-dependent lysosomal sorting.
p62 has been proposed to mark ubiquitinated protein bodies for autophagic degradation. We report that the Drosophila melanogaster p62 orthologue, Ref(2)P, is a regulator of protein aggregation in the adult brain. We demonstrate that Ref(2)P localizes to age-induced protein aggregates as well as to aggregates caused by reduced autophagic or proteasomal activity. A similar localization to protein aggregates is also observed in D. melanogaster models of human neurodegenerative diseases. Although atg8a autophagy mutant flies show accumulation of ubiquitin- and Ref(2)P-positive protein aggregates, this is abrogated in atg8a/ref(2)P double mutants. Both the multimerization and ubiquitin binding domains of Ref(2)P are required for aggregate formation in vivo. Our findings reveal a major role for Ref(2)P in the formation of ubiquitin-positive protein aggregates both under physiological conditions and when normal protein turnover is inhibited.
Down-regulation of activated and ubiquitinated growth factor (GF) receptors by endocytosis and subsequent lysosomal degradation ensures attenuation of GF signaling. The ubiquitin-binding adaptor protein Eps15 (epidermal growth factor receptor [EGFR] pathway substrate 15) functions in endocytosis of such receptors. Here, we identify an Eps15 isoform, Eps15b, and demonstrate its expression in human cells and conservation across vertebrate species. Although both Eps15 and Eps15b interact with the endosomal sorting protein Hrs (hepatocyte growth factor–regulated tyrosine kinase substrate) in vitro, we find that Hrs specifically binds Eps15b in vivo (whereas adaptor protein 2 preferentially interacts with Eps15). Although Eps15 mainly localizes to clathrin-coated pits at the plasma membrane, Eps15b localizes to Hrs-positive microdomains on endosomes. Eps15b overexpression, similarly to Hrs overexpression, inhibits ligand-mediated degradation of EGFR, whereas Eps15 is without effect. Similarly, depletion of Eps15b but not Eps15 delays degradation and promotes recycling of EGFR. These results indicate that Eps15b is an endosomally localized isoform of Eps15 that is present in the Hrs complex via direct Hrs interaction and important for the sorting function of this complex.
A recent EMBO-FEBS workshop entitled Endocytic Systems: Mechanism and Function, organized by Howard Riezman in Villars-sur-Ollon (Switzerland), showcased the multifaceted approaches and model systems used to study endocytosis. The meeting revealed how endocytosis controls multiple aspects of biology, ranging from development to immunity and neurotransmission.