Alix/AIP1 antagonizes epidermal growth factor receptor downregulation by the Cbl-SETA/CIN85 complex

Galectin-3 negatively regulates TCR-mediated CD4+ T-cell activation at the immunological synapse

We have investigated the function of endogenous galectin-3 in T cells. Galectin-3-deficient (gal3(-/-)) CD4(+) T cells secreted more IFN-gamma and IL-4 than gal3(+/+)CD4(+) T cells after T-cell receptor (TCR) engagement. Galectin-3 was recruited to the cytoplasmic side of the immunological synapse (IS) in activated T cells. In T cells stimulated on supported lipid bilayers, galectin-3 was primarily located at the peripheral supramolecular activation cluster (pSMAC). Gal3(+/+) T cells formed central SMAC on lipid bilayers less effectively and adhered to antigen-presenting cells less firmly than gal3(-/-) T cells, suggesting that galectin-3 destabilizes the IS. Galectin-3 expression was associated with lower levels of early signaling events and phosphotyrosine signals at the pSMAC. Additional data suggest that galectin-3 potentiates down-regulation of TCR in T cells. By yeast two-hybrid screening, we identified as a galectin-3-binding partner, Alix, which is known to be involved in protein transport and regulation of cell surface expression of certain receptors. Co-immunoprecipitation confirmed galectin-3-Alix association and immunofluorescence analysis demonstrated the translocation of Alix to the IS in activated T cells. We conclude that galectin-3 is an inhibitory regulator of T-cell activation and functions intracellularly by promoting TCR down-regulation, possibly through modulating Alix's function at the IS.

Enterococcus faecalis strains differentially regulate Alix/AIP1 protein expression and ERK 1/2 activation in intestinal epithelial cells in the context of chronic experimental colitis

Monoassociation of germfree Interleukin 10 gene deficient (IL-10-/-) 129SvEv but not wild-type mice with Enterococcus faecalis induces severe chronic colitis. Bacterial strain-specific effects on the development of chronic intestinal inflammation are not understood. We investigated the molecular mechanisms of E. faecalis OG1RF (human clinical isolate, colitogenic) and E. faecalis ms2 (endogenous isolate from an IL-10-/- mouse) in initiating chronic experimental colitis using IL-10-/- mice. Monoassociation of IL-10-/- mice for 14 weeks revealed significant differences in colonic inflammation (3.6 +/- 0.2 and 2.4 +/- 0.6 for OG1RF and ms2, respectively) (n = 5 mice in each group) (histological scoring (0-4)). Consistent with the tissue pathology, gene expression of the pro-inflammatory chemokine interferon-gamma inducible protein-10 (IP-10) was significantly higher in intestinal epithelial cells (IEC) derived from E. faecalis OG1RF monoassociated IL-10-/- mice. We further compared the differentially E. faecalis induced colitis on the epithelial level by 2D-SDS PAGE coupled with MALDI-TOF MS. Proteome analysis identified 13 proteins which were differentially regulated during disease progression in the epithelium of E. faecalis-monoassociated IL-10-/- mice. Regulation of Alix/AIP1 protein expression and ERK1/2 phosphorylation was validated in primary IEC and epithelial cell lines, suggesting a protective role for Alix/AIP1 in the process of disease progression. Alix/AIP1 protein expression was further characterized in epithelial cell lines using siRNA-mediated knock-down. Our study demonstrates E. faecalis strain-specific induction of colitis in IL-10-/- mice after 14 weeks of monoassociation. Our study suggests that Alix/AIP1 protein expression and ERK1/2 activation are decreased in severe colitis.

Switching of vascular phenotypes within a murine breast cancer model induced by angiopoietin-2

Sustained growth of solid tumours can rely on both the formation of new and the co-option of existing blood vessels. Current models suggest that binding of angiopoietin-2 (Ang-2) to its endothelial Tie2 receptor prevents receptor phosphorylation, destabilizes blood vessels, and promotes vascular permeability. In contrast, binding of angiopoietin-1 (Ang-1) induces Tie2 receptor activation and supports the formation of mature blood vessels covered by pericytes. Despite the intense research to decipher the role of angiopoietins during physiological neovascularization and tumour angiogenesis, a mechanistic understanding of angiopoietin function on vascular integrity and remodelling is still incomplete. We therefore assessed the vascular morphology of two mouse mammary carcinoma xenotransplants (M6378 and M6363) which differ in their natural angiopoietin expression. M6378 displayed Ang-1 in tumour cells but no Ang-2 in tumour endothelial cells in vivo. In contrast, M6363 tumours expressed Ang-2 in the tumour vasculature, whereas no Ang-1 expression was present in tumour cells. We stably transfected M6378 mouse mammary carcinoma cells with human Ang-1 or Ang-2 and investigated the consequences on the host vasculature, including ultrastructural morphology. Interestingly, M6378/Ang-2 and M6363 tumours displayed a similar vascular morphology, with intratumoural haemorrhage and non-functional and abnormal blood vessels. Pericyte loss was prominent in these tumours and was accompanied by increased endothelial cell apoptosis. Thus, overexpression of Ang-2 converted the vascular phenotype of M6378 tumours into a phenotype similar to M6363 tumours. Our results support the hypothesis that Ang-1/Tie2 signalling is essential for vessel stabilization and endothelial cell/pericyte interaction, and suggest that Ang-2 is able to induce a switch of vascular phenotypes within tumours.

Endocytosis and intracellular trafficking of ErbBs

This review article describes the pathways and mechanisms of endocytosis and post-endocytic sorting of the EGF receptor (EGFR/ErbB1) and other members of the ErbB family. Growth factor binding to EGFR accelerates its internalization through clathrin-coated pits which is followed by the efficient lysosomal targeting of internalized receptors and results in receptor down-regulation. The role of EGFR interaction with the Grb2 adaptor protein and Cbl ubiquitin ligase, and receptor ubiquitination in the clathrin-dependent internalization and sorting of EGFR in multivesicular endosomes is discussed. Activation and phosphorylation of ErbB2, ErbB3 and ErbB4 also results in their ubiquitination. However, these ErbBs are internalized and targeted to lysosomes less efficiently than EGFR. When overexpressed endocytosis-impaired ErbBs may inhibit the internalization and degradation of EGFR.

Structure and function of ESCRT-III

ESCRT-III (endosomal sorting complex required for transport III) is required for the formation and abscission of intraluminal endosomal vesicles, which gives rise to multivesicular bodies, budding of some enveloped viruses and cytokinesis. ESCRT-III is composed of 11 members in humans, which, except for one, correspond to the six ESCRT-III-like proteins in yeast. At least CHMP (charged multivesicular body protein) 2A and CHMP3 assemble into helical tubular structures that provide a platform for membrane interaction and VPS (vacuolar protein sorting) 4-catalysed effects leading to disassembly of ESCRT-III CHMP2A-CHMP3 polymers in vitro. Progress towards the understanding of the structures and function of ESCRT-III, its activation, its regulation by accessory factors and its role in abscission of membrane enveloped structures in concert with VPS4 are discussed.

Molecular assemblies and membrane domains in multivesicular endosome dynamics

Along the degradation pathway, endosomes exhibit a characteristic multivesicular organization, resulting from the budding of vesicles into the endosomal lumen. After endocytosis and transport to early endosomes, activated signaling receptors are incorporated into these intralumenal vesicles through the action of the ESCRT machinery, a process that contributes to terminate signaling. Then, the vesicles and their protein cargo are further transported towards lysosomes for degradation. Evidence also shows that intralumenal vesicles can undergo "back-fusion" with the late endosome limiting membrane, a route exploited by some pathogens and presumably followed by proteins and lipids that need to be recycled from within the endosomal lumen. This process depends on the late endosomal lipid lysobisphosphatidic acid and its putative effector Alix/AIP1, and is presumably coupled to the invagination of the endosomal limiting membrane at the molecular level via ESCRT proteins. In this review, we discuss the intra-endosomal transport routes in mammalian cells, and in particular the different mechanisms involved in membrane invagination, vesicle formation and fusion in a space inaccessible to proteins known to control intracellular membrane traffic.

Systems biology of growth factor-induced receptor endocytosis

Clathrin-mediated endocytosis sorts for degradation of more than 50 different growth factor receptors capable of relaying growth and differentiation signals by means of their cytoplasm-facing, intrinsic tyrosine kinase activity. The kinetics and alternative routings of receptor endocytosis critically regulate growth factor signaling, which underscores the importance of understanding mechanisms underlying fail-safe operation (robustness) and fidelity of the pathway. Like other robust systems, a layered hub-centric network controls receptor endocytosis. Characteristically, the modular hubs (e.g., AP2-Eps15 and Hrs) contain a membrane-anchoring lipid-binding domain, an ubiquitin-binding module, which recruits ubiquitinylated cargo, and a machinery enabling homo-assembly. Scheduled hub transitions, as well as cascades of Rab family guanosine triphosphatases and membrane bending machineries, define points of commitment to vesicle budding, thereby securing unidirectional trafficking. System's bistability permits stimulation by a growth factor, which oscillates a series of switches based on posttranslational protein modifications (i.e., phosphorylation, ubiquitinylation and neddylation), as well as transient low-affinity/high-avidity protein assemblies. Cbl family ubiquitin ligases, along with a set of phosphotyrosine-binding adaptors (e.g., Grb2), integrate receptor endocytosis into the densely wired networks of signal transduction pathways, which are involved in health and disease.

Alix and ALG-2 are involved in tumor necrosis factor receptor 1-induced cell death

Alix/AIP1 regulates cell death in a way involving interactions with the calcium-binding protein ALG-2 and with proteins of ESCRT (endosomal sorting complex required for transport). Using mass spectrometry we identified caspase-8 among proteins co-immunoprecipitating with Alix in dying neurons. We next demonstrated that Alix and ALG-2 interact with pro-caspase-8 and that Alix forms a complex with the TNFalpha receptor-1 (TNF-R1), depending on its capacity to bind ESCRT proteins. Thus, Alix and ALG-2 may allow the recruitment of pro-caspase-8 onto endosomes containing TNF-R1, a step thought to be necessary for activation of the apical caspase. In line with this, expression of Alix deleted of its ALG-2-binding site (AlixDeltaALG-2) significantly reduced TNF-R1-induced cell death, without affecting endocytosis of the receptor. In a more physiological setting, we found that programmed cell death of motoneurons, which can be inhibited by AlixDeltaALG-2, is regulated by TNF-R1. Taken together, these results highlight Alix and ALG-2 as new actors of the TNF-R1 pathway.

The ESCRT-I subunit TSG101 controls endosome-to-cytosol release of viral RNA

Like other enveloped viruses, vesicular stomatitis virus infects cells through endosomes. There, the viral envelope undergoes fusion with endosomal membranes, thereby releasing the nucleocapsid into the cytoplasm and allowing infection to proceed. Previously, we reported that the viral envelope fuses preferentially with the membrane of vesicles present within multivesicular endosomes. Then, these intra-endosomal vesicles (containing nucleocapsids) are transported to late endosomes, where back-fusion with the endosome limiting membrane delivers the nucleocapsid into the cytoplasm. In this study, we show that the tumor susceptibility gene 101 (Tsg101) subunit of the endosomal sorting complexes required for transport (ESCRT)-I complex, which mediates receptor sorting into multivesicular endosomes, is dispensable for viral envelope fusion with endosomal membranes and viral RNA transport to late endosomes but is necessary for infection. Our data indicate that Tsg101, in contrast to the ESCRT-0 component Hrs, plays a direct role in nucleocapsid release from within multivesicular endosomes to the cytoplasm, presumably by controlling the back-fusion process. We conclude that Tsg101, through selective interactions with its partners including Hrs and Alix, may link receptor sorting and lysosome targeting to the back-fusion process involved in viral capsid release.

In vitro budding of intralumenal vesicles into late endosomes is regulated by Alix and Tsg101

Endosomes along the degradation pathway leading to lysosomes accumulate membranes in their lumen and thus exhibit a characteristic multivesicular appearance. These lumenal membranes typically incorporate down-regulated EGF receptor destined for degradation, but the mechanisms that control their formation remain poorly characterized. Here, we describe a novel quantitative biochemical assay that reconstitutes the formation of lumenal vesicles within late endosomes in vitro. Vesicle budding into the endosome lumen was time-, temperature-, pH-, and energy-dependent and required cytosolic factors and endosome membrane components. Our light and electron microscopy analysis showed that the compartment supporting the budding process was accessible to endocytosed bulk tracers and EGF receptor. We also found that the EGF receptor became protected against trypsin in our assay, indicating that it was sorted into the intraendosomal vesicles that were formed in vitro. Our data show that the formation of intralumenal vesicles is ESCRT-dependent, because the process was inhibited by the K173Q dominant negative mutant of hVps4. Moreover, we find that the ESCRT-I subunit Tsg101 and its partner Alix control intralumenal vesicle formation, by acting as positive and negative regulators, respectively. We conclude that budding of the limiting membrane toward the late endosome lumen, which leads to the formation of intraendosomal vesicles, is controlled by the positive and negative functions of Tsg101 and Alix, respectively.

Endocytosis and intracellular trafficking of ErbBs

This review article describes the pathways and mechanisms of endocytosis and post-endocytic sorting of the EGF receptor (EGFR/ErbB1) and other members of the ErbB family. Growth factor binding to EGFR accelerates its internalization through clathrin-coated pits which is followed by the efficient lysosomal targeting of internalized receptors and results in receptor down-regulation. The role of EGFR interaction with the Grb2 adaptor protein and Cbl ubiquitin ligase, and receptor ubiquitination in the clathrin-dependent internalization and sorting of EGFR in multivesicular endosomes is discussed. Activation and phosphorylation of ErbB2, ErbB3 and ErbB4 also results in their ubiquitination. However, these ErbBs are internalized and targeted to lysosomes less efficiently than EGFR. When overexpressed endocytosis-impaired ErbBs may inhibit the internalization and degradation of EGFR.

Alix differs from ESCRT proteins in the control of autophagy

Alix/AIP1 is a cytosolic protein that regulates cell death through mechanisms that remain unclear. Alix binds to two protein members of the so-called Endosomal Sorting Complex Required for Transport (ESCRT), which facilitates membrane fission events during multivesicular endosome formation, enveloped virus budding and cytokinesis. Alix itself has been suggested to participate in these cellular events and is thus often considered to function in the ESCRT pathway. ESCRT proteins were recently implicated in autophagy, a process involved in bulk degradation of cytoplasmic constituents in lysosomes, which can also participate in cell death. In this study, we shown that, unlike ESCRT proteins, Alix is not involved in autophagy. These results strongly suggest that the capacity of several mutants of Alix to block both caspase-dependent and independent cell death does not relate to their capacity to modulate autophagy. Furthermore, they reinforce the conclusion of other studies demonstrating that the role of Alix is different from that of classical ESCRT proteins.

Lysobisphosphatidic acid controls endosomal cholesterol levels

Most cell types acquire cholesterol by endocytosis of circulating low density lipoprotein, but little is known about the mechanisms of intra-endosomal cholesterol transport and about the primary cause of its aberrant accumulation in the cholesterol storage disorder Niemann-Pick type C (NPC). Here we report that lysobisphosphatidic acid (LBPA), an unconventional phospholipid that is only detected in late endosomes, regulates endosomal cholesterol levels under the control of Alix/AlP1, which is an LBPA-interacting protein involved in sorting into multivesicular endosomes. We find that Alix down-expression decreases both LBPA levels and the lumenal vesicle content of late endosomes. Cellular cholesterol levels are also decreased, presumably because the storage capacity of endosomes is affected and thus cholesterol clearance accelerated. Both lumenal membranes and cholesterol can be restored in Alix knockdown cells by exogenously added LBPA. Conversely, we also find that LBPA becomes limiting upon pathological cholesterol accumulation in NPC cells, because the addition of exogenous LBPA, but not of LBPA isoforms or analogues, partially reverts the NPC phenotype. We conclude that LBPA controls the cholesterol capacity of endosomes.

Extracellular Alix regulates integrin-mediated cell adhesions and extracellular matrix assembly

Alix (ALG-2-interacting protein X), a cytoplasmic adaptor protein involved in endosomal sorting and actin cytoskeleton assembly, is required for the maintenance of fibroblast morphology. As Alix has sequence similarity to adhesin in Entamoeba histolytica, and we observed that Alix is secreted, we determined whether extracellular Alix affects fibroblast morphology. Here, we demonstrate that secreted Alix is deposited on the substratum of non-immortalized WI38 fibroblasts. Antibody binding to extracellular Alix retards WI38 cell adhesion and spreading on fibronectin and vitronectin. Alix knockdown in WI38 cells reduces spreading and fibronectin assembly, and the effect is partially complemented by coating recombinant Alix on the cell substratum. Immortalized NIH/3T3 fibroblasts deposit less Alix on the substratum and have defects in alpha5beta1-integrin functions. Coating recombinant Alix on the culture substratum for NIH/3T3 cells promotes alpha5beta1-integrin-mediated cell adhesions and fibronectin assembly, and these effects require the aa 605-709 region of Alix. These findings demonstrate that a sub-population of Alix localizes extracellularly and regulates integrin-mediated cell adhesions and fibronectin matrix assembly.

ALIX-CHMP4 interactions in the human ESCRT pathway

The ESCRT pathway facilitates membrane fission events during enveloped virus budding, multivesicular body formation, and cytokinesis. To promote HIV budding and cytokinesis, the ALIX protein must bind and recruit CHMP4 subunits of the ESCRT-III complex, which in turn participate in essential membrane remodeling functions. Here, we report that the Bro1 domain of ALIX binds specifically to C-terminal residues of the human CHMP4 proteins (CHMP4A-C). Crystal structures of the complexes reveal that the CHMP4 C-terminal peptides form amphipathic helices that bind across the conserved concave surface of ALIX(Bro1). ALIX-dependent HIV-1 budding is blocked by mutations in exposed ALIX(Bro1) residues that help contribute to the binding sites for three essential hydrophobic residues that are displayed on one side of the CHMP4 recognition helix (M/L/IxxLxxW). The homologous CHMP1-3 classes of ESCRT-III proteins also have C-terminal amphipathic helices, but, in those cases, the three hydrophobic residues are arrayed with L/I/MxxxLxxL spacing. Thus, the distinct patterns of hydrophobic residues provide a "code" that allows the different ESCRT-III subunits to bind different ESCRT pathway partners, with CHMP1-3 proteins binding MIT domain-containing proteins, such as VPS4 and Vta1/LIP5, and CHMP4 proteins binding Bro1 domain-containing proteins, such as ALIX.

The Bro1-related protein HD-PTP/PTPN23 is required for endosomal cargo sorting and multivesicular body morphogenesis

The Saccharomyces cerevisiae protein Bro1p is required for sorting endocytic cargo to the lumen of multivesicular bodies (MVBs). The mammalian ortholog of Bro1p is not known; although Alix, a structurally related protein, supports the topologically similar process of virus budding, functional studies have so far failed to identify a role for Alix in MVB formation. To establish whether Alix or similar protein(s) participate in endosomal sorting, we attached a retroviral peptide that binds Alix to a reporter receptor. This chimera was sorted efficiently away from the early endosome to the lumen of late endocytic compartments. Surprisingly, sorting was not prevented by depleting Alix but instead required the Alix-related protein His domain phosphotyrosine phosphatase (HD-PTP)/His-Domain/Type N23 protein tyrosine phosphatase (PTPN23). Depletion of HD-PTP also reduced transfer of fluid-phase markers and EGF receptor to lysosomes, caused the accumulation of ubiquitinated proteins on endosomal compartments and disrupted the morphogenesis of MVBs. Rescue experiments using an RNAi-resistant version of HD-PTP and HD-PTP mutants demonstrated an essential role for the HD-PTP Bro1 domain, with ESCRT-III binding correlating with full biological activity.

Myopic acts in the endocytic pathway to enhance signaling by the Drosophila EGF receptor

Endocytosis of activated receptors can control signaling levels by exposing the receptors to novel downstream molecules or by instigating their degradation. Epidermal growth factor receptor (EGFR) signaling has crucial roles in development and is misregulated in many cancers. We report here that Myopic, the Drosophila homolog of the Bro1-domain tyrosine phosphatase HD-PTP, promotes EGFR signaling in vivo and in cultured cells. myopic is not required in the presence of activated Ras or in the absence of the ubiquitin ligase Cbl, indicating that it acts on internalized EGFR, and its overexpression enhances the activity of an activated form of EGFR. Myopic is localized to intracellular vesicles adjacent to Rab5-containing early endosomes, and its absence results in the enlargement of endosomal compartments. Loss of Myopic prevents cleavage of the EGFR cytoplasmic domain, a process controlled by the endocytic regulators Cbl and Sprouty. We suggest that Myopic promotes EGFR signaling by mediating its progression through the endocytic pathway.

Transcription-dependent epidermal growth factor receptor activation by hepatocyte growth factor

The mechanisms and biological implications of coordinated receptor tyrosine kinase coactivation remain poorly appreciated. Epidermal growth factor receptor (EGFR) and c-Met are frequently coexpressed in cancers, including those associated with hepatocyte growth factor (HGF) overexpression, such as malignant astrocytoma. In a previous analysis of the HGF-induced transcriptome, we found that two EGFR agonists, transforming growth factor-alpha and heparin-binding epidermal growth factor-like growth factor (HB-EGF), are prominently up-regulated by HGF in human glioma cells. We now report that stimulating human glioblastoma cells with recombinant HGF induces biologically relevant EGFR activation. EGFR phosphorylation at Tyr(845) and Tyr(1068) increased 6 to 24 h after cell stimulation with HGF and temporally coincided with the induction of transforming growth factor-alpha (~5-fold) and HB-EGF (~23-fold) expression. Tyr(845) and Tyr(1068) phosphorylation, in response to HGF, was inhibited by cycloheximide and actinomycin D, consistent with a requirement for DNA transcription and RNA translation. Specifically, blocking HB-EGF binding to EGFR with the antagonist CRM197 inhibited HGF-induced EGFR phosphorylation by 60% to 80% and inhibited HGF-induced S-G(2)-M transition. CRM197 also inhibited HGF-induced anchorage-dependent cell proliferation but had no effect on HGF-mediated cytoprotection. These findings establish that EGFR can be activated with functional consequences by HGF as a result of EGFR ligand expression. This transcription-dependent cross-talk between the HGF receptor c-Met and EGFR expands our understanding of receptor tyrosine kinase signaling networks and may have considerable consequences for oncogenic mechanisms and cancer therapeutics.

A novel requirement for C. elegans Alix/ALX-1 in RME-1-mediated membrane transport

BACKGROUND

Alix/Bro1p family proteins have recently been identified as important components of multivesicular endosomes (MVEs) and are involved in the sorting of endocytosed integral membrane proteins, interacting with components of the ESCRT complex, the unconventional phospholipid LBPA, and other known endocytosis regulators. During infection, Alix can be co-opted by enveloped retroviruses, including HIV, providing an important function during virus budding from the plasma membrane. In addition, Alix is associated with the actin cytoskeleton and might regulate cytoskeletal dynamics.

RESULTS

Here we demonstrate a novel physical interaction between the only apparent Alix/Bro1p family protein in C. elegans, ALX-1, and a key regulator of receptor recycling from endosomes to the plasma membrane, called RME-1. The analysis of alx-1 mutants indicates that ALX-1 is required for the endocytic recycling of specific basolateral cargo in the C. elegans intestine, a pathway previously defined by the analysis of rme-1 mutants. The expression of truncated human Alix in HeLa cells disrupts the recycling of major histocompatibility complex class I, a known Ehd1/RME-1-dependent transport step, suggesting the phylogenetic conservation of this function. We show that the interaction of ALX-1 with RME-1 in C. elegans, mediated by RME-1/YPSL and ALX-1/NPF motifs, is required for this recycling process. In the C. elegans intestine, ALX-1 localizes to both recycling endosomes and MVEs, but the ALX-1/RME-1 interaction appears to be dispensable for ALX-1 function in MVEs and/or late endosomes.

CONCLUSIONS

This work provides the first demonstration of a requirement for an Alix/Bro1p family member in the endocytic recycling pathway in association with the recycling regulator RME-1.

The Cbl interactome and its functions

Cbl proteins are ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-associated proteins can assemble together at a given time or space inside the cell, and such an interactome can form signal competent networks that control many physiological processes. Dysregulation of spatial or temporal constraints in the Cbl interactome results in the development of human pathologies such as immune diseases, diabetes and cancer.

Proteome changes induced by knock-down of the deubiquitylating enzyme HAUSP/USP7

Modification of proteins by ubiquitin plays a major role in a broad array of biological processes. Reversal of this process through deubiquitylation likely represents an important regulatory step in the maintenance of cellular homeostasis. However, the biological functions of deubiquitylating enzymes still remain poorly characterized. To investigate the biological role of the herpes virus-associated ubiquitin-specific protease HAUSP/USP7, we have generated stably transfected cells carrying inducible shRNA expression plasmids. USP7 mRNA and protein were strongly down-regulated 48-72 h after shRNA induction. We used a selected clone to compare whole-cell proteomes by 2D-SDS-PAGE before and after knockdown of USP7. Alterations in 36 proteins were detected and their identities were revealed by mass spectrometry analysis. Components of the replication machinery, DNA/RNA binding proteins, enzymes involved in apoptosis and metabolism were found to be down-regulated upon USP7 removal, representing proteins that are either more rapidly turned over or synthesized less efficiently in the absence of USP7-mediated deubiquitylation. Alix/HP95, a protein implicated in endosomal organization and virus budding, was confirmed by immunoblotting to become down-regulated when USP7 levels were reduced. Our results extend the current list of USP7-dependent biological processes and suggest a role for this enzyme not only in transcriptional regulation but also in DNA replication, apoptosis, and possibly endosomal organization.

Trafficking and developmental signaling: Alix at the crossroads

Alix is a phylogenetically conserved protein that participates in mammals in programmed cell death in association with ALG-2, a penta-EF-hand calciprotein. It contains an N-terminal Bro1 domain, a coiled-coil region and a C-terminal proline-rich domain containing several SH3- and WW-binding sites that contribute to its scaffolding properties. Recent data showed that by virtue of its Bro1 domain, Alix is functionally associated to the ESCRT complexes involved in the biogenesis of the multivesicular body and sorting of transmembrane proteins within this specific endosomal compartment. In Dictyostelium, an alx null strain shows a markedly perturbed starvation-induced morphogenetic program while ALG-2 disruptants remain unaffected. This review summarizes Dictyostelium data on Alix and ALG-2 homologues and evaluates whether known functions of Alix in other organisms can account for the developmental arrest of the alx null mutant and how Dictyostelium studies can substantiate the current understanding of the function(s) of this versatile and conserved signaling molecule.

Malfunctions within the Cbl interactome uncouple receptor tyrosine kinases from destructive transport

Proteins of the Cbl family are adaptor molecules and ubiquitin ligases with major functions in the regulation, intracellular transport and degradation of receptor tyrosine kinases (RTKs). Due to this central role, mutations that cause malfunctions of Cbl or their associated proteins - termed the Cbl interactome - easily lead to the transformation of affected cells and eventually the development of cancer. This review intends to give an overview on the mechanisms of Cbl-mediated cell transformation in light of the dysregulated intracellular trafficking of RTKs.

Angiopoietin-2 impairs revascularization after limb ischemia

Angiopoietins play important roles in the formation of neovessels and complex vascular networks. Angiopoietin (Ang)-1 and Ang-2 belong to a family of growth factors that display opposing effects on the activation of Tie2 (tyrosine kinase with immunoglobulin and epidermal growth factor homology domain 2). Endothelial Ang-2 expression is associated with vessel destabilization and regulates a balance between vascular regression and growth. To elucidate, in particular, the role of Ang-2 after arterial artery occlusion in the mouse limb, we applied a transgenic animal model with targeted Ang-2 expression in endothelial cells. We show here that restoration of blood flow in Ang-2:Tie1 transgenic mice is dramatically impaired when Ang-2 expression is induced in the vasculature. The defective restoration of perfusion in Ang-2 transgenic mice is evidenced by reduced collateral artery growth, which typically occurs to compensate for flow deficits after occlusion of the large conductance artery. Furthermore, reduced movement capacities and higher incidents of necrosis are consequently observed in the transgenic limbs as compared with controls. Mechanistically, the observed effects are attributed to defective smooth muscle cell recruitment in Ang-2 transgenic mice. Moreover, distinct Ang-2 levels in the genetically modified animals clearly correlated with the magnitude of reduced perfusion. In conclusion, our studies define Ang-2 as an important molecule for the progression of collateral artery growth and angiogenesis during ischemia and suggest precise Ang-2 dosage activities to accomplish blood vessel growth.

The multiple personalities of Alix

Alix is a cytosolic protein in mammalian cells that was originally identified on the basis of its association with pro-apoptotic signaling. More recent evidence has established that Alix has a hand in regulating other cellular mechanisms, including endocytic membrane trafficking and cell adhesion. Although Alix appears to participate directly in these various activities, the role it plays in each process has largely been inferred from the functions of proteins with which it interacts. For example, recruitment of Alix to endosomes is mediated by its N-terminal Bro1 domain, the structure of which was recently solved for its yeast orthologue, Bro1. The diversity of Alix functions is due to its proline-rich C-terminus, which provides multiple protein-binding sites. With this blueprint in hand, we can now ask whether Alix acts simply as an adaptor that links different proteins into networks or, instead, contributes a specific function to distinct molecular machineries.

Structural and biochemical studies of ALIX/AIP1 and its role in retrovirus budding

ALIX/AIP1 functions in enveloped virus budding, endosomal protein sorting, and many other cellular processes. Retroviruses, including HIV-1, SIV, and EIAV, bind and recruit ALIX through YPX(n)L late-domain motifs (X = any residue; n = 1-3). Crystal structures reveal that human ALIX is composed of an N-terminal Bro1 domain and a central domain that is composed of two extended three-helix bundles that form elongated arms that fold back into a "V." The structures also reveal conformational flexibility in the arms that suggests that the V domain may act as a flexible hinge in response to ligand binding. YPX(n)L late domains bind in a conserved hydrophobic pocket on the second arm near the apex of the V, whereas CHMP4/ESCRT-III proteins bind a conserved hydrophobic patch on the Bro1 domain, and both interactions are required for virus budding. ALIX therefore serves as a flexible, extended scaffold that connects retroviral Gag proteins to ESCRT-III and other cellular-budding machinery.

Potent rescue of human immunodeficiency virus type 1 late domain mutants by ALIX/AIP1 depends on its CHMP4 binding site

The release of human immunodeficiency virus type 1 (HIV-1) and of other retroviruses from certain cells requires the presence of distinct regions in Gag that have been termed late assembly (L) domains. HIV-1 harbors a PTAP-type L domain in the p6 region of Gag that engages an endosomal budding machinery through Tsg101. In addition, an auxiliary L domain near the C terminus of p6 binds to ALIX/AIP1, which functions in the same endosomal sorting pathway as Tsg101. In the present study, we show that the profound release defect of HIV-1 L domain mutants can be completely rescued by increasing the cellular expression levels of ALIX and that this rescue depends on an intact ALIX binding site in p6. Furthermore, the ability of ALIX to rescue viral budding in this system depended on two putative surface-exposed hydrophobic patches on its N-terminal Bro1 domain. One of these patches mediates the interaction between ALIX and the ESCRT-III component CHMP4B, and mutations which disrupt the interaction also abolish the activity of ALIX in viral budding. The ability of ALIX to rescue a PTAP mutant also depends on its C-terminal proline-rich domain (PRD), but not on the binding sites for Tsg101, endophilin, CIN85, or for the newly identified binding partner, CMS, within the PRD. Our data establish that ALIX can have a dramatic effect on HIV-1 release and suggest that the ability to use ALIX may allow HIV-1 to replicate in cells that express only low levels of Tsg101.

Phosphorylation of the proline-rich domain of Xp95 modulates Xp95 interaction with partner proteins

The mammalian adaptor protein Alix [ALG-2 (apoptosis-linked-gene-2 product)-interacting protein X] belongs to a conserved family of proteins that have in common an N-terminal Bro1 domain and a C-terminal PRD (proline-rich domain), both of which mediate partner protein interactions. Following our previous finding that Xp95, the Xenopus orthologue of Alix, undergoes a phosphorylation-dependent gel mobility shift during progesteroneinduced oocyte meiotic maturation, we explored potential regulation of Xp95/Alix by protein phosphorylation in hormone-induced cell cycle re-entry or M-phase induction. By MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS analyses and gel mobility-shift assays, Xp95 is phosphorylated at multiple sites within the N-terminal half of the PRD during Xenopus oocyte maturation, and a similar region in Alix is phosphorylated in mitotically arrested but not serum-stimulated mammalian cells. By tandem MS, Thr745 within this region, which localizes in a conserved binding site to the adaptor protein SETA [SH3 (Src homology 3) domain-containing, expressed in tumorigenic astrocytes] CIN85 (a-cyano-4-hydroxycinnamate)/SH3KBP1 (SH3-domain kinase-binding protein 1), is one of the phosphorylation sites in Xp95. Results from GST (glutathione S-transferase)-pull down and peptide binding/competition assays further demonstrate that the Thr745 phosphorylation inhibits Xp95 interaction with the second SH3 domain of SETA. However, immunoprecipitates of Xp95 from extracts of M-phase-arrested mature oocytes contained additional partner proteins as compared with immunoprecipitates from extracts of G2-arrested immature oocytes. The deubiquitinase AMSH (associated molecule with the SH3 domain of signal transducing adaptor molecule) specifically interacts with phosphorylated Xp95 in M-phase cell lysates. These findings establish that Xp95/Alix is phosphorylated within the PRD during M-phase induction, and indicate that the phosphorylation may both positively and negatively modulate their interaction with partner proteins.

An Alix fragment potently inhibits HIV-1 budding: characterization of binding to retroviral YPXL late domains

The retroviral structural protein, Gag, contains small peptide motifs known as late domains that promote efficient virus release from the infected cell. In addition to the well characterized PTAP late domain, the p6 region of HIV-1 Gag contains a binding site for the host cell protein Alix. To better understand the functional role of the Gag/Alix interaction, we overexpressed an Alix fragment composed of residues 364-716 (Alix 364-716) and examined the effect on release of wild type (WT) and Alix binding site mutant HIV-1. We observed that Alix 364-716 expression significantly inhibited WT virus release and Gag processing and that mutation of the Alix binding site largely relieved this inhibition. Furthermore, Alix 364-716 expression induced a severe defect on WT but not mutant particle morphology. Intriguingly, the impact of Alix 364-716 expression on HIV-1 release and Gag processing was markedly different from that induced by mutation of the Alix binding site in p6. The association of Alix 364-716 with HIV-1 and equine infectious anemia virus late domains was quantitatively evaluated by isothermal titration calorimetry and surface plasmon resonance techniques, and the effects of mutations in these viral sequences on Alix 364-716 binding was determined. This study identifies a novel Alix-derived dominant negative inhibitor of HIV-1 release and Gag processing and provides quantitative information on the interaction between Alix and viral late domains.

HD-PTP and Alix share some membrane-traffic related proteins that interact with their Bro1 domains or proline-rich regions

Mammalian Alix is a multifunctional adaptor protein involved in cell death, receptor endocytosis, endosomal protein sorting and cell adhesion by associating with various proteins such as ALG-2, CIN85/Rukl/SETA, endophilins, CHMP4s and TSG101. HD-PTP is a paralog of Alix and a putative protein tyrosine phosphatase (PTP) that contains a Bro1 domain, coiled-coils, a proline-rich region (PRR) in addition to a PTP domain. We investigated interactions between HD-PTP and Alix-binding proteins. In the yeast two-hybrid assay, HD-PTP showed positive interactions with CHMP4b/Shax1, TSG101, endophilin A1 and ALG-2 but not with either RabGAPLP or CIN85. We confirmed the interactions in a mammalian system by Strep-pulldown assays in which pulldown products from the lysates of HEK293T cells expressing either Strep-tagged HD-PTP alone or co-expressing with epitope-tagged proteins were analyzed by Western blotting using specific antibodies. While Alix associated with both ALG-2 and TSG101 in a Ca2+-dependent manner, HD-PTP interacted with ALG-2 Ca2+-dependently but with TSG101 Ca2+-independently.

Alix facilitates the interaction between c-Cbl and platelet-derived growth factor beta-receptor and thereby modulates receptor down-regulation

Alix (ALG-2-interacting protein X) is an adaptor protein involved in down-regulation and sorting of cell surface receptors through the endosomal compartments toward the lysosome. In this study, we show that Alix interacts with the C-terminal region of the platelet-derived growth factor (PDGF) beta-receptor (PDGFRbeta) and becomes transiently tyrosine-phosphorylated in response to PDGF-BB stimulation. Increased expression levels of Alix resulted in a reduced rate of PDGFRbeta removal from the cell surface following receptor activation, and this was associated with decreased receptor degradation. Furthermore, Alix was found to co-immunoprecipitate with the ubiquitin ligase c-Cbl, and elevated Alix levels increased the interaction between c-Cbl and PDGFRbeta. Interestingly, Alix interacted constitutively with both c-Cbl and PDGFRbeta. Moreover, c-Cbl was found to be hyperphosphorylated in cells engineered to overexpress Alix compared with control cells. The increased c-Cbl phosphorylation correlated with enhanced proteasomal degradation of c-Cbl, which in turn correlated with a decreased ubiquitination of PDGFRbeta. Our data suggest that Alix inhibits down-regulation of PDGFRbeta by modulating the interaction between c-Cbl and the receptor, thereby affecting the ubiquitination of the receptor.

Intra-endosomal membrane traffic

Following endocytosis, ubiquitinated signaling receptors are incorporated within intraluminal vesicles of forming multivesicular endosomes. These vesicles then follow the pathway from early to late endosomes, remaining within the endosomal lumen, and are eventually delivered to lysosomes, where they are degraded together with their protein cargo. However, intraluminal vesicles do not always end up in lysosomes for degradation; they can also fuse back with the limiting membrane of late endosomes. This route, which might be regulated by lyso-bisphosphatidic acid and its putative effector Alix, can be hijacked by the anthrax toxin and vesicular stomatitis virus and is presumably exploited by proteins and lipids that transit through intraluminal vesicles. Alternatively, these vesicles can be released extracellularly, like HIV in macrophages, upon fusion of endosomes or lysosomes with the plasma membrane.

Vinexin β regulates the phosphorylation of epidermal growth factor receptor on the cell surface

Epidermal growth factor (EGF) regulates various cellular events, including proliferation, differentiation, migration and oncogenesis. In this study, we found that exogenous expression of vinexin beta enhanced the phosphorylation of 180-kDa proteins in an EGF-dependent manner in Cos-7 cells. Western blot analysis using phospho-specific antibodies against EGFR identified EGFR as a phosphorylated 180-kDa protein. Vinexin beta did not stimulate the phosphorylation of EGFR but suppressed the dephosphorylation, resulting in a sustained phosphorylation. Mutational analyses revealed that both the first and third SH3 domains were required for a sustained phosphorylation of EGFR. Small interfering RNA-mediated knockdown of vinexin beta reduced the phosphorylation of EGFR on the cell surface in HeLa cells. The sustained phosphorylation of EGFR induced by vinexin beta was completely abolished by adding the EGFR-specific inhibitor AG1478 even after EGF stimulation, suggesting that the kinase activity of EGFR is required for the sustained phosphorylation induced by vinexin beta. We also found that E3 ubiquitin ligase c-Cbl is a binding partner of vinexin beta through the third SH3 domain. Expression of wild-type vinexin beta but not a mutant containing a mutation in the third SH3 domain decreased the cytosolic pool of c-Cbl and increased the amount of membrane-associated c-Cbl. Furthermore, over-expression of c-Cbl suppressed the sustained phosphorylation of EGFR induced by vinexin beta. These results suggest that vinexin beta plays a role in maintaining the phosphorylation of EGFR on the plasma membrane through the regulation of c-Cbl.

The ESCRT complexes: structure and mechanism of a membrane-trafficking network

The ESCRT complexes and associated proteins comprise a major pathway for the lysosomal degradation of transmembrane proteins and are critical for receptor downregulation, budding of the HIV virus, and other normal and pathological cell processes. The ESCRT system is conserved from yeast to humans. The ESCRT complexes form a network that recruits monoubiquitinated proteins and drives their internalization into lumenal vesicles within a type of endosome known as a multivesicular body. The structures and interactions of many of the components have been determined over the past three years, revealing mechanisms for membrane and cargo recruitment and for complex assembly.

Phosphoproteomic analysis of Her2/neu signaling and inhibition

Her2/neu (Her2) is a tyrosine kinase belonging to the EGF receptor (EGFR)/ErbB family and is overexpressed in 20-30% of human breast cancers. We sought to characterize Her2 signal transduction pathways further by using MS-based quantitative proteomics. Stably transfected cell lines overexpressing Her2 or empty vector were generated, and the effect of an EGFR and Her2 selective tyrosine kinase inhibitor, PD168393, on these cells was characterized. Quantitative measurements were obtained on 462 proteins by using the SILAC (stable isotope labeling with amino acids in cell culture) method to monitor three conditions simultaneously. Of these proteins, 198 showed a significant increase in tyrosine phosphorylation in Her2-overexpressing cells, and 81 showed a significant decrease in phosphorylation. Treatment of Her2-overexpressing cells with PD168393 showed rapid reversibility of the majority of the Her2-triggered phosphorylation events. Phosphoproteins that were identified included many known Her2 signaling molecules as well as known EGFR signaling proteins that had not been previously linked to Her2, such as Stat1, Dok1, and delta-catenin. Importantly, several previously uncharacterized Her2 signaling proteins were identified, including Axl tyrosine kinase, the adaptor protein Fyb, and the calcium-binding protein Pdcd-6/Alg-2. We also identified a phosphorylation site in Her2, Y877, which is located in the activation loop of the kinase domain, is distinct from the known C-terminal tail autophosphorylation sites, and may have important implications for regulation of Her2 signaling. Network modeling, which combined phosphoproteomic results with literature-curated protein-protein interaction data, was used to suggest roles for some of the previously unidentified Her2 signaling proteins.

Do Alix and ALG-2 really control endosomes for better or for worse?

Alix/AIP1 (ALG-2-interacting protein X/apoptosis-linked-gene-2-interacting protein 1) is an adaptor protein that was first described for its capacity to bind to the calcium-binding protein ALG-2 (apoptosis-linked gene 2), the expression of which seemed necessary for cell death. Over-expression of truncated forms of Alix blocks caspase-dependent and -independent mechanisms of cell death. Numerous observations in yeast and in mammalian cells suggest that Alix controls the making of and trafficking through endosomes called MVBs (multivesicular bodies), which are crucial intermediates within the endolysosomal system. In particular, deletion of Bro1, one of the yeast homologues of Alix, leads to an impairment in the function of MVBs, leading to mis-sorting of proteins normally destined to the vacuole. Mammalian Alix may have a similar function and has been shown to bind to lyso(bis)phosphatidic acid, ESCRT (endosomal sorting complex required for transport) proteins, endophilins and CIN85 (Cbl-interacting protein of 85 kDa), which are all main regulators of the endosomal system. EIAV (equine infectious anaemia virus) and HIV late domains use Alix to recruit the ESCRT machinery in order to bud from the cell surface, underscoring the crucial role of the protein in orchestrating membrane deformation. In this review I develop the hypothesis that the normal function of Alix in the endolysosomal system may be deviated by ALG-2 towards a destructive role during active cell death.

Endosomal and non-endosomal functions of ESCRT proteins

The three endosomal sorting complexes required for transport (ESCRTs) are integral to the degradation of endocytosed membrane proteins and multivesicular body (MVB) biogenesis. Here, we review evidence that ESCRTs have evolved as a specialized machinery for the degradative sorting of ubiquitinated membrane proteins and we highlight recent studies that have shed light on the mechanisms by which these complexes mediate protein sorting, MVB biogenesis, tumour suppression and viral budding. We also discuss evidence that some ESCRT subunits have evolved additional functions that are unrelated to membrane trafficking.

HIV-1 Vpu promotes release and prevents endocytosis of nascent retrovirus particles from the plasma membrane

The human immunodeficiency virus (HIV) type-1 viral protein U (Vpu) protein enhances the release of diverse retroviruses from human, but not monkey, cells and is thought to do so by ablating a dominant restriction to particle release. Here, we determined how Vpu expression affects the subcellular distribution of HIV-1 and murine leukemia virus (MLV) Gag proteins in human cells where Vpu is, or is not, required for efficient particle release. In HeLa cells, where Vpu enhances HIV-1 and MLV release approximately 10-fold, concentrations of HIV-1 Gag and MLV Gag fused to cyan fluorescent protein (CFP) were initially detected at the plasma membrane, but then accumulated over time in early and late endosomes. Endosomal accumulation of Gag-CFP was prevented by Vpu expression and, importantly, inhibition of plasma membrane to early endosome transport by dominant negative mutants of Rab5a, dynamin, and EPS-15. Additionally, accumulation of both HIV and MLV Gag in endosomes required a functional late-budding domain. In human HOS cells, where HIV-1 and MLV release was efficient even in the absence of Vpu, Gag proteins were localized predominantly at the plasma membrane, irrespective of Vpu expression or manipulation of endocytic transport. While these data indicated that Vpu inhibits nascent virion endocytosis, Vpu did not affect transferrin endocytosis. Moreover, inhibition of endocytosis did not restore Vpu-defective HIV-1 release in HeLa cells, but instead resulted in accumulation of mature virions that could be released from the cell surface by protease treatment. Thus, these findings suggest that a specific activity that is present in HeLa cells, but not in HOS cells, and is counteracted by Vpu, traps assembled retrovirus particles at the cell surface. This entrapment leads to subsequent endocytosis by a Rab5a- and clathrin-dependent mechanism and intracellular sequestration of virions in endosomes.

Vesicular Trafficking of Tyrosine Kinase Receptors and Associated Proteins in the Regulation of Signaling and Vascular Function

Receptor tyrosine kinases (RTKs) play a pivotal role in the development and function of the cardiovascular system. Ligand-activated RTKs promote numerous downstream signal transduction pathways that lead to vascular permeability, as well as proliferation, migration, and differentiation of vascular endothelia and smooth muscle cells. Ligand binding also promotes internalization of the activated receptors either to downregulate the signaling via degradation of the ligand/receptor complex or to signal from endosomes. However, the outcomes of receptor internalization via clathrin-dependent or caveolar pathways and trafficking mechanisms are incompletely clarified in vascular systems. Activity modulation through endocytosis and vesicular trafficking significantly impacts downstream targets of RTKs such as endothelial nitric oxide synthase (eNOS) and VE-cadherin. RTKs and their associated targets are also transported to the nucleus, where they may directly impact nuclear signaling. Although the nuclear transport pathways are just beginning to be unraveled, it appears that endocytosis and vesicular trafficking are involved. In this review, we discuss the mechanisms by which activated RTKs and the downstream targets eNOS and VE-cadherin may be internalized and transported to various intracellular compartments. How localization and interacting proteins impact protein function and influence signaling is an important theme, as is the potential for modulating signaling through therapeutic targeting of activated receptors and components of the endocytic machinery.

A novel paraptosis pathway involving LEI/L-DNaseII for EGF-induced cell death in somato-lactotrope pituitary cells

We have recently reported that EGF triggers an original form of cell death in pituitary cell line (GH4C1) with a phenotype sharing some characteristics of both apoptosis (internucleosomal DNA fragmentation) and paraptosis (caspase-independence and cytoplasmic vacuolization). However, the endonuclease involved in EGF-induced DNA fragmentation has not been assessed so far. In the present work we therefore further explored the putative paraptosis involvement in EGF-induced cell death and asked whether L-DNaseII might be involved. Indeed, this endonuclease is known to mediate internucleosomal DNA fragmentation in caspase independent manner. Our Western blot, immunocytochemistry and enzymatic measurement assays show that EGF triggers a cleavage of Leukocyte Elastase Inhibitor (LEI) precursor into L-DNaseII, its subsequent enzymatic activation and nuclear translocation thus pointing to the involvement of this endonuclease pathway in caspase-independent DNA fragmentation. In addition, EGF-induced cell death can be blocked by paraptosis inhibitor AIP-1/Alix, but not with its anti-apoptotic C-terminal fragment (Alix-CT). Altogether these data suggest that EGF-induced cell death defines a novel, L-DNaseII-mediated form of paraptosis.

Degradation of endocytosed epidermal growth factor and virally ubiquitinated major histocompatibility complex class I is independent of mammalian ESCRTII

Models for protein sorting at multivesicular bodies in the endocytic pathway of mammalian cells have relied largely on data obtained from yeast. These data suggest the essential role of four ESCRT complexes in multivesicular body protein sorting. However, the putative mammalian ESCRTII complex (hVps25p, hVps22p, and hVps36p) has no proven functional role in endosomal transport. We have characterized the human ESCRTII complex and investigated its function in endosomal trafficking. The human ESCRTII proteins interact with one another, with hVps20p (a component of ESCRTIII), and with their yeast homologues. Our interaction data from yeast two-hybrid studies along with experiments with purified proteins suggest an essential role for the N-terminal domain of hVps22p in the formation of a heterotetrameric ESCRTII complex. Although human ESCRTII is found in the cytoplasm and in the nucleus, it can be recruited to endosomes upon overexpression of dominant-negative hVps4Bp. Interestingly, we find that small interference RNA depletion of mammalian ESCRTII does not affect degradation of epidermal growth factor, a known cargo of the multivesicular body protein sorting pathway. We also show that depletion of the deubiquitinating enzymes AMSH (associated molecule with the SH3 domain of STAM (signal transducing adaptor molecule)) and UBPY (ubiquitin isopeptidase Y) have opposite effects on epidermal growth factor degradation, with UBPY depletion causing dramatic swelling of endosomes. Down-regulation of another cargo, the major histocompatibility complex class I in cells expressing the Kaposi sarcoma-associated herpesvirus protein K3, is unaffected in ESCRTII-depleted cells. Our data suggest that mammalian ESCRTII may be redundant, cargo-specific, or not required for protein sorting at the multivesicular body.

The penta-EF-hand protein ALG-2 interacts directly with the ESCRT-I component TSG101, and Ca2+-dependently co-localizes to aberrant endosomes with dominant-negative AAA ATPase SKD1/Vps4B

ALG-2 (apoptosis-linked gene 2) is a Ca2+-binding protein that belongs to the PEF (penta-EF-hand) protein family. Alix (ALG-2-interacting protein X)/AIP1 (ALG-2-interacting protein 1), one of its binding partners, interacts with TSG101 and CHMP4 (charged multivesicular body protein 4), which are components of ESCRT-I (endosomal sorting complex required for transport I) and ESCRT-III respectively. In the present study, we investigated the association between ALG-2 and ESCRT-I. By a GST (glutathione S-transferase) pull-down assay using HEK-293T (human embryonic kidney 293T) cell lysates, endogenous TSG101 and two other exogenously expressed ESCRT-I components [hVps28 (human vacuolar protein sorting 28) and hVps37A] were shown to associate with GST-ALG-2 in the presence of Ca2+. By the yeast two-hybrid assay, however, a positive interaction was observed with only TSG101 among the three ESCRT-I components, suggesting that ALG-2 associates with hVps28 and hVps37A indirectly through TSG101. Using various deletion mutants of TSG101, the central PRR (proline-rich region) was found to be sufficient for interaction with ALG-2 by the GST-pull-down assay. Direct binding of ALG-2 to the TSG101 PRR was demonstrated by an overlay assay using biotin-labelled ALG-2 as a probe. In immunofluorescence microscopic analysis of HeLa cells that overexpressed a GFP (green fluorescent protein)-fused ATPase-defective dominant-negative form of SKD1/Vps4B (GFP-SKD1(E235Q)), ALG-2 exhibited a punctate distribution at the perinuclear area and co-localized with GFP-SKD1(E235Q) to aberrant endosomes. This punctate distribution of ALG-2 was markedly diminished by treatment of HeLa cells with a membrane-permeant Ca2+ chelator. Moreover, a Ca2+-binding-defective mutant of ALG-2 did not co-localize with GFP-SKD1(E235Q). Our findings suggest that ALG-2 may function as a Ca2+-dependent accessory protein of the endosomal sorting machinery by interacting directly with TSG101 as well as with Alix.

Regulating the regulator: negative regulation of Cbl ubiquitin ligases

Cbl proteins are regulators of signal transduction through many pathways and, consequently, regulate cell function and development. They are ubiquitin ligases that ubiquitinate and target many signaling molecules for degradation. The Cbl proteins themselves are regulated by an increasingly complex network of interactions that fine-tune the effects that Cbl proteins have on signaling. The negative regulation of Cbl protein function can occur via cis-acting structural elements that prevent inappropriate ubiquitin ligase activity, degradation of the Cbl proteins, inhibition without degradation owing to interaction with other signaling proteins, deubiquitination of Cbl substrates, and regulation of assembly of the endosomal ESCRT-I complex. Defects in the regulatory mechanisms that control Cbl function are implicated in the development of immunological and malignant diseases.

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.

Identification of Rab GTPase-activating protein-like protein (RabGAPLP) as a novel Alix/AIP1-interacting protein

Alix/AIP1 is a multifunctional adaptor protein involved in endocytosis, cell adhesion, and cell death. By yeast two-hybrid screening we identified a novel Alix/AIP1-interacting protein named Rab GTPase-activating protein-like protein (RabGAPLP). Interaction between Alix and RabGAPLP was confirmed by pull-down assays using fusion proteins of either glutathione-S-transferase (GST) or chitin-binding domain (CBD) and lysates of cultured mammalian cells expressing the respective proteins. Partial colocalization of FLAG-tagged RabGAPLP and green fluorescent protein (GFP)-fused Alix was observed at cell edges and filopodia-like structures by fluorescence confocal laser scanning microscopic analysis. The identity of RabGAPLP to merlin-associated protein (MAP), one of the interacting partners of neurofibromatosis type 2 (NF2) tumor suppressor gene product (merlin), implies cross-talk of membrane traffic and cell adhesion.

Structural basis for endosomal targeting by the Bro1 domain

Proteins delivered to the lysosome or the yeast vacuole via late endosomes are sorted by the ESCRT complexes and by associated proteins, including Alix and its yeast homolog Bro1. Alix, Bro1, and several other late endosomal proteins share a conserved 160 residue Bro1 domain whose boundaries, structure, and function have not been characterized. The crystal structure of the Bro1 domain of Bro1 reveals a folded core of 367 residues. The extended Bro1 domain is necessary and sufficient for binding to the ESCRT-III subunit Snf7 and for the recruitment of Bro1 to late endosomes. The structure resembles a boomerang with its concave face filled in and contains a triple tetratricopeptide repeat domain as a substructure. Snf7 binds to a conserved hydrophobic patch on Bro1 that is required for protein complex formation and for the protein-sorting function of Bro1. These results define a conserved mechanism whereby Bro1 domain-containing proteins are targeted to endosomes by Snf7 and its orthologs.

Alix regulates cortical actin and the spatial distribution of endosomes

Alix/AIP1 is a proline-rich protein that has been implicated in apoptosis, endocytic membrane trafficking and viral budding. To further elucidate the functions of Alix, we used RNA interference to specifically suppress its expression. Depletion of Alix caused a striking redistribution of early endosomes from a peripheral to a perinuclear location. The redistribution of endosomes did not affect transferrin recycling or degradation of endocytosed epidermal growth factor receptors, although the uptake of transferrin was mildly reduced when Alix was downregulated. Quantitative immunoelectron microscopy showed that multivesicular endosomes of Alix-depleted cells contained normal amounts of CD63, whereas their levels of lysobisphosphatidic acid were reduced. Alix depletion also caused an accumulation of unusual actin structures that contained clathrin and cortactin, a protein that couples membrane dynamics to the cortical actin cytoskeleton. Our results suggest that Alix functions in the actin-dependent intracellular positioning of endosomes, but that it is not essential for endocytic recycling or for trafficking of membrane proteins between early and late endosomes in non-polarised cells.

Compartmentalization of growth factor receptor signalling

Spatial and temporal separation of signal transduction pathways often determines the specificity in cellular responses. Recent advances have improved our understanding of how growth factor signalling is influenced by the formation of molecular complexes (signalosomes) in distinct cellular compartments. There has also been new insight into the mechanisms that determine the signalling competence of these complexes and their role in receptor endocytosis, retrograde trafficking in neurons and restricted protein biosynthesis, and many examples have been found where signalosome deregulation leads to disease.

Studying protein isoforms of the adaptor SETA/CIN85/Ruk with monoclonal antibodies

SETA/CIN85/Ruk is a multifunctional adaptor protein involved in signal transduction and attenuation downstream of receptor tyrosine kinases. It has a modular structure, and various isoforms that combine different protein-protein interaction domains have been proposed based on cDNA analysis. As a first step towards understanding SETA/CIN85/Ruk isoforms at the protein level, we have characterized 5 monoclonal antibodies against this protein. Three of these were used to study lysates fractionated on a pH gradient, leading to the identification of various SETA/CIN85/Ruk proteins on the basis of pI and apparent molecular weight. While good correspondence with proteins predicted from cDNA analysis was found for two isoforms, in most cases it was not possible to make an unequivocal assignment. We conclude that additional splice variants remain to be described, and that a deeper understanding of SETA/CIN85/Ruk post-translational processing and modification is necessary to gain further understanding of this complex gene product.