Phosphorylation of Hrs downstream of the epidermal growth factor receptor

STAM and Hrs interact sequentially with IFN-α Receptor to control spatiotemporal JAK-STAT endosomal activation

Activation of the JAK-STAT pathway by type I interferons (IFNs) requires clathrin-dependent endocytosis of the IFN-α and -β receptor (IFNAR), indicating a role for endosomal sorting in this process. The molecular machinery that brings the selective activation of IFN-α/β-induced JAK-STAT signalling on endosomes remains unknown. Here we show that the constitutive association of STAM with IFNAR1 and TYK2 kinase at the plasma membrane prevents TYK2 activation by type I IFNs. IFN-α-stimulated IFNAR endocytosis delivers the STAM-IFNAR complex to early endosomes where it interacts with Hrs, thereby relieving TYK2 inhibition by STAM and triggering signalling of IFNAR at the endosome. In contrast, when stimulated by IFN-β, IFNAR signalling occurs independently of Hrs as IFNAR is sorted to a distinct endosomal subdomain. Our results identify the molecular machinery that controls the spatiotemporal activation of IFNAR by IFN-α and establish the central role of endosomal sorting in the differential regulation of JAK-STAT signalling by IFN-α and IFN-β.

Connecting the dots: combined control of endocytic recycling and degradation

Endocytosis is an essential process where proteins and lipids are internalised from the plasma membrane in membrane-bound carriers, such as clathrin-coated vesicles. Once internalised into the cell these vesicles fuse with the endocytic network where their contents are sorted towards degradation in the lysosome or recycling to their origin. Initially, it was thought that cargo recycling is a passive process, but in recent years the identification and characterisation of specialised recycling complexes has established a hitherto unthought-of level of complexity that actively opposes degradation. This review will summarise recent developments regarding the composition and regulation of the recycling machineries and their relationship with the degradative pathways of the endosome.

Involvement of CASP9 (caspase 9) in IGF2R/CI-MPR endosomal transport

Recently, we reported that increased expression of CASP9 pro-domain, at the endosomal membrane in response to HSP90 inhibition, mediates a cell-protective effect that does not involve CASP9 apoptotic activity. We report here that a non-apoptotic activity of endosomal membrane CASP9 facilitates the retrograde transport of IGF2R/CI-MPR from the endosomes to the trans-Golgi network, indicating the involvement of CASP9 in endosomal sorting and lysosomal biogenesis. CASP9-deficient cells demonstrate the missorting of CTSD (cathepsin D) and other acid hydrolases, accumulation of late endosomes, and reduced degradation of bafilomycin A₁-sensitive proteins. In the absence of CASP9, IGF2R undergoes significant degradation, and its rescue is achieved by the re-expression of a non-catalytic CASP9 mutant. This endosomal activity of CASP9 is potentially mediated by herein newly identified interactions of CASP9 with the components of the endosomal membrane transport complexes. These endosomal complexes include the retromer VPS35 and the SNX dimers, SNX1-SNX5 and SNX2-SNX6, which are involved in the IGF2R retrieval mechanism. Additionally, CASP9 interacts with HGS/HRS/ESCRT-0 and the CLTC (clathrin heavy chain) that participate in the initiation of the endosomal ESCRT degradation pathway. We propose that endosomal CASP9 inhibits the endosomal membrane degradative subdomain(s) from initiating the ESCRT-mediated degradation of IGF2R, allowing its retrieval to transport-designated endosomal membrane subdomain(s). These findings are the first to identify a cell survival, non-apoptotic function for CASP9 at the endosomal membrane, a site distinctly removed from the cytoplasmic apoptosome. Via its non-apoptotic endosomal function, CASP9 impacts the retrograde transport of IGF2R and, consequently, lysosomal biogenesis.Abbreviations: ACTB: actin beta; ATG7: autophagy related 7; BafA1: bafilomycin A₁; CASP: caspase; CLTC/CHC: clathrin, heavy chain; CTSD: cathepsin D; ESCRT: endosomal sorting complexes required for transport; HEXB: hexosaminidase subunit beta; HGS/HRS/ESCRT-0: hepatocyte growth factor-regulated tyrosine kinase substrate; IGF2R/CI-MPR: insulin like growth factor 2 receptor; ILV: intraluminal vesicles; KD: knockdown; KO: knockout; M6PR/CD-MPR: mannose-6-phosphate receptor, cation dependent; MEF: murine embryonic fibroblasts; MWU: Mann-Whitney U test; PepA: pepstatin A; RAB7A: RAB7, member RAS oncogene family; SNX-BAR: sorting nexin dimers with a Bin/Amphiphysin/Rvs (BAR) domain each; TGN: trans-Golgi network; TUBB: tubulin beta; VPS26: VPS26 retromer complex component; VPS29: VPS29 retromer complex component; VPS35: VPS35 retromer complex component.

Beclin 1 Promotes Endosome Recruitment of Hepatocyte Growth Factor Tyrosine Kinase Substrate to Suppress Tumor Proliferation

Beclin 1 has nonautophagic functions that include its ability to regulate endocytic receptor trafficking. However, the contribution of this function to tumor suppression is poorly understood. Here, we provide in vivo evidence that Beclin 1 suppresses tumor proliferation by regulating the endocytic trafficking and degradation of the EGFR and transferrin (TFR1) receptors. Beclin 1 promoted endosomal recruitment of hepatocyte growth factor tyrosine kinase substrate (HRS), which was necessary for sorting surface receptors to intraluminal vesicles for signal silencing and lysosomal degradation. In tumors with low Beclin 1 expression, endosomal HRS recruitment was diminished and receptor function was sustained. Collectively, our results demonstrate a novel role for Beclin 1 in impeding tumor growth by coordinating the regulation of key growth factor and nutrient receptors. These data provide an explanation for how low levels of Beclin 1 facilitate tumor proliferation and contribute to poor cancer outcomes. SIGNIFICANCE: Beclin 1 controls the trafficking fate of growth regulatory receptors to suppress tumor proliferation.

Growth factor stimulation promotes multivesicular endosome biogenesis by prolonging recruitment of the late-acting ESCRT machinery

The formation of multivesicular endosomes (MVEs) mediates the turnover of numerous integral membrane proteins and has been implicated in the down-regulation of growth factor signaling, thereby exhibiting properties of a tumor suppressor. The endosomal sorting complex required for transport (ESCRT) machinery plays a key role in MVE biogenesis, enabling cargo selection and intralumenal vesicle (ILV) budding. However, the spatiotemporal pattern of endogenous ESCRT complex assembly and disassembly in mammalian cells remains poorly defined. By combining CRISPR/Cas9-mediated genome editing and live cell imaging using lattice light sheet microscopy (LLSM), we determined the native dynamics of both early- and late-acting ESCRT components at MVEs under multiple growth conditions. Specifically, our data indicate that ESCRT-0 accumulates quickly on endosomes, typically in less than 30 seconds, and its levels oscillate in a manner dependent on the downstream recruitment of ESCRT-I. Similarly, levels of the ESCRT-I complex also fluctuate on endosomes, but its average residency time is more than fivefold shorter compared with ESCRT-0. Vps4 accumulation is the most transient, however, suggesting that the completion of ILV formation occurs rapidly. Upon addition of epidermal growth factor (EGF), both ESCRT-I and Vps4 are retained at endosomes for dramatically extended periods of time, while ESCRT-0 dynamics are only modestly affected. Our findings are consistent with a model in which growth factor stimulation stabilizes late-acting components of the ESCRT machinery at endosomes to accelerate the rate of ILV biogenesis and attenuate signal transduction initiated by receptor activation.

Endosomal "sort" of signaling control: The role of ESCRT machinery in regulation of receptor-mediated signaling pathways

The endosomal sorting complexes required for transport (ESCRTs) machinery consists of four protein assemblies (ESCRT-0 to -III subcomplexes) which mediate various processes of membrane remodeling in the cell. In the endocytic pathway, ESCRTs sort cargo destined for degradation into intraluminal vesicles (ILVs) of endosomes. Cargos targeted by ESCRTs include various signaling molecules, mainly internalized cell-surface receptors but also some cytosolic proteins. It is therefore expected that aberrant trafficking caused by ESCRT dysfunction affects different signaling pathways. Here we review how perturbation of ESCRT activity alters intracellular transport of membrane receptors, causing their accumulation on endocytic compartments, decreased degradation and/or altered recycling to the plasma membrane. We further describe how perturbed trafficking of receptors impacts the activity of their downstream signaling pathways, with or without changes in transcriptional responses. Finally, we present evidence that ESCRT components can also control activity and intracellular distribution of cytosolic signaling proteins (kinases, other effectors and soluble receptors). The underlying mechanisms involve sequestration of such proteins in ILVs, their sorting for degradation or towards non-lysosomal destinations, and regulating their availability in various cellular compartments. All these ESCRT-mediated processes can modulate final outputs of multiple signaling pathways.

Endocytosis and the Src family of non-receptor tyrosine kinases

The regulated intracellular transport of nutrient, adhesion, and growth factor receptors is crucial for maintaining cell and tissue homeostasis. Endocytosis, or endocytic membrane trafficking, involves the steps of intracellular transport that include, but are not limited to, internalization from the plasma membrane, sorting in early endosomes, transport to late endosomes/lysosomes followed by degradation, and/or recycling back to the plasma membrane through tubular recycling endosomes. In addition to regulating the localization of transmembrane receptor proteins, the endocytic pathway also controls the localization of non-receptor molecules. The non-receptor tyrosine kinase c-Src (Src) and its closely related family members Yes and Fyn represent three proteins whose localization and signaling activities are tightly regulated by endocytic trafficking. Here, we provide a brief overview of endocytosis, Src function and its biochemical regulation. We will then concentrate on recent advances in understanding how Src intracellular localization is regulated and how its subcellular localization ultimately dictates downstream functioning. As Src kinases are hyperactive in many cancers, it is essential to decipher the spatiotemporal regulation of this important family of tyrosine kinases.

Regulation of EGFR signal transduction by analogue-to-digital conversion in endosomes

An outstanding question is how receptor tyrosine kinases (RTKs) determine different cell-fate decisions despite sharing the same signalling cascades. Here, we uncovered an unexpected mechanism of RTK trafficking in this process. By quantitative high-resolution FRET microscopy, we found that phosphorylated epidermal growth factor receptor (p-EGFR) is not randomly distributed but packaged at constant mean amounts in endosomes. Cells respond to higher EGF concentrations by increasing the number of endosomes but keeping the mean p-EGFR content per endosome almost constant. By mathematical modelling, we found that this mechanism confers both robustness and regulation to signalling output. Different growth factors caused specific changes in endosome number and size in various cell systems and changing the distribution of p-EGFR between endosomes was sufficient to reprogram cell-fate decision upon EGF stimulation. We propose that the packaging of p-RTKs in endosomes is a general mechanism to ensure the fidelity and specificity of the signalling response.

Recycling of EGFR and ErbB2 is associated with impaired Hrs tyrosine phosphorylation and decreased deubiquitination by AMSH

ErbB receptors play an important role in normal cellular growth, differentiation and development, but overexpression or poor downregulation can result in enhanced signaling and cancerous growth. ErbB signaling is terminated by clathrin-dependent receptor-mediated endocytosis, followed by incorporation in multi-vesicular bodies and subsequent degradation in lysosomes. In contrast to EGFR, ErbB2 displays poor ligand-induced downregulation and enhanced recycling, but the molecular mechanisms underlying this difference are poorly understood. Given our previous observation that both EGFR and an EGFR-ErbB2 chimera undergo Cbl-mediated K63-polyubiquitination, we investigated in the present study whether activation of the EGFR and the EGFR-ErbB2 chimera is associated with tyrosine phosphorylation of the ESCRT-0 complex subunit Hrs and AMSH-mediated deubiquitination. EGF stimulation of the EGFR resulted in efficient Hrs tyrosine phosphorylation and deubiquitination by the K63-polyubiquitin chain-specific deubiquitinating enzyme AMSH. In contrast, EGF activation of EGFR-ErbB2 showed significantly decreased Hrs tyrosine phosphorylation and deubiquitination by AMSH. To test whether this phenotype is the result of endosomal recycling, we induced recycling of the EGFR by stimulation with TGFα. Indeed, even though TGFα-stimulation of EGFR is associated with efficient ligand-stimulated K63-polyubiquitination, we observed that Hrs tyrosine phosphorylation as well as AMSH-mediated deubiquitination is significantly reduced under these conditions. Using various EGFR-ErbB2 chimeras, we demonstrate that enhanced recycling, decreased Hrs tyrosine phosphorylation and decreased AMSH mediated deubiquitination of EGFR-ErbB2 chimeras is primarily due to the presence of ErbB2 sequences or the absence of EGFR sequences C-terminal to the Cbl binding site. We conclude that endosomal recycling of the EGFR and ErbB2 receptors is associated with significantly impaired tyrosine phosphorylation of the ESCRT-0 subunit Hrs as well as decreased deubiquitination by AMSH, which is consistent with the finding that recycling receptors are not efficiently incorporated in the MVB pathway.

The late stage of autophagy: cellular events and molecular regulation

Autophagy is an intracellular degradation system that delivers cytoplasmic contents to the lysosome for degradation. It is a "self-eating" process and plays a "house-cleaner" role in cells. The complex process consists of several sequential steps-induction, autophagosome formation, fusion of lysosome and autophagosome, degradation, efflux transportation of degradation products, and autophagic lysosome reformation. In this review, the cellular and molecular regulations of late stage of autophagy, including cellular events after fusion step, are summarized.

Cbl controls EGFR fate by regulating early endosome fusion

Amino acid residues 1 to 434 of the E3 ubiquitin ligase Cbl control signaling of the epidermal growth factor receptor (EGFR) by enhancing its ubiquitination, down-regulation, and lysosomal degradation. This region of Cbl comprises a tyrosine kinase-binding domain, a linker region, a really interesting new gene finger (RF), and a subset of the residues of the RF tail. In experiments with full-length alanine substitution mutants, we demonstrated that the RF tail of Cbl regulated biochemically distinct checkpoints in the endocytosis of EGFR. The Cbl- and ubiquitin-dependent degradation of the regulator of internalization hSprouty2 was compromised by the Val(431)--> Ala mutation, whereas the Cbl- and EGFR-dependent dephosphorylation or degradation of the endosomal trafficking regulator Hrs was compromised by the Phe(434)--> Ala mutation. Deregulated phosphorylation of Hrs correlated with inhibition of the fusion of early endosomes and of the degradation of EGFR. This study provides the first evidence that Cbl regulates receptor fate by controlling the fusion of sorting endosomes. We postulate that it does so by modulating the abundance of tyrosine-phosphorylated Hrs.

Src promotes delta opioid receptor (DOR) desensitization by interfering with receptor recycling

Abstract An important limitation in the clinical use of opiates is progressive loss of analgesic efficacy over time. Development of analgesic tolerance is tightly linked to receptor desensitization. In the case of delta opioid receptors (DOR), desensitization is especially swift because receptors are rapidly internalized and are poorly recycled to the membrane. In the present study, we investigated whether Src activity contributed to this sorting pattern and to functional desensitization of DORs. A first series of experiments demonstrated that agonist binding activates Src and destabilizes a constitutive complex formed by the spontaneous association of DORs with the kinase. Src contribution to DOR desensitization was then established by showing that pre-treatment with Src inhibitor PP2 (20 microM; 1 hr) or transfection of a dominant negative Src mutant preserved DOR signalling following sustained exposure to an agonist. This protection was afforded without interfering with endocytosis, but suboptimal internalization interfered with PP2 ability to preserve DOR signalling, suggesting a post-endocytic site of action for the kinase. This assumption was confirmed by demonstrating that Src inhibition by PP2 or its silencing by siRNA increased membrane recovery of internalized DORs and was further corroborated by showing that inhibition of recycling by monensin or dominant negative Rab11 (Rab11S25N) abolished the ability of Src blockers to prevent desensitization. Finally, Src inhibitors accelerated recovery of DOR-Galphal3 coupling after desensitization. Taken together, these results indicate that Src dynamically regulates DOR recycling and by doing so contributes to desensitization of these receptors.

Ubc4/5 and c-Cbl continue to ubiquitinate EGF receptor after internalization to facilitate polyubiquitination and degradation

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.

Ubc4/5 and c-Cbl continue to ubiquitinate EGF receptor after internalization to facilitate polyubiquitination and degradation

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.

Stable isotopic labeling by amino acids in cultured primary neurons: application to brain-derived neurotrophic factor-dependent phosphotyrosine-associated signaling

Cultured primary neurons are a well established model for the study of neuronal function in vitro. Here we demonstrated that stable isotope labeling by amino acids in cell culture (SILAC) can be applied to a differentiated, non-dividing cell type such as primary neurons, and we applied this technique to assess changes in the neuronal phosphotyrosine proteome in response to stimulation by brain-derived neurotrophic factor (BDNF), an important molecule for the development and regulation of neuronal connections. We found that 13 proteins had SILAC ratios above 1.50 or below 0.67 in phosphotyrosine immunoprecipitations comparing BDNF-treated and control samples, and an additional 18 proteins had ratios above 1.25 or below 0.80. These proteins include TrkB, the receptor tyrosine kinase for BDNF, and others such as hepatocyte growth factor-regulated tyrosine kinase substrate and signal-transducing adaptor molecule, which are proteins known to regulate intracellular trafficking of receptor tyrosine kinases. These results demonstrate that the combination of primary neuronal cell culture and SILAC can be a powerful tool for the study of the proteomes of neuronal molecular and cellular dynamics.

Regulation of GPCRs by endocytic membrane trafficking and its potential implications

The endocytic pathway tightly controls the activity of G protein-coupled receptors (GPCRs). Ligand-induced endocytosis can drive receptors into divergent lysosomal and recycling pathways, producing essentially opposite effects on the strength and duration of cellular signaling via heterotrimeric G proteins, and may also promote distinct signaling events from intracellular membranes. This chapter reviews recent developments toward understanding the molecular machinery and functional implications of GPCR sorting in the endocytic pathway, focusing on mammalian GPCRs whose ligand-induced endocytosis is mediated primarily by clathrin-coated pits. Lysosomal sorting of a number of GPCRs occurs via a highly conserved mechanism requiring covalent tagging of receptors with ubiquitin. There is increasing evidence that additional, noncovalent mechanisms control the sorting of endocytosed GPCRs to lysosomes in mammalian cells. Recycling of several GPCRs to the plasma membrane is also specifically sorted, via a mechanism requiring both receptor-specific and shared sorting proteins. The current data reveal an unprecedented degree of specificity and plasticity in the cellular regulation of mammalian GPCRs by endocytic membrane trafficking. These developments have fundamental implications for GPCR pharmacology, and suggest new mechanisms that could be exploited in GPCR-directed pharmacotherapy.

Role of Hrs in maturation of autophagosomes in mammalian cells

Autophagy is an evolutionarily conserved system responsible for the degradation of cellular components and contributes to the increasing of amino acid pool, organelle turnover, and elimination of intracellular bacteria. The molecular process of autophagy is still unclear. Here we demonstrate that Hrs, a master regulator in endosomal protein sorting, plays critical roles for the autophagic degradation of non-specific proteins and Streptococcus pyogenes. We found that Hrs containing FYVE domain is localized to autophagosomes. Hrs depletion resulted in a significant decrease in the number of mature autophagosomes (autophagolysosomes) detected by the co-localization of autophagosome marker LC3 and lysosome marker LAMP-1. In contrast, formation of the primary autophagosome, detected by LC3 immunoblotting and lysosomal degradation of non-specific proteins, were not significantly altered by Hrs depletion. Based on these results, we propose a novel function of Hrs, as a crucial player in the maturation of autophagosomes.

Hrs is a positive regulator of VEGF and insulin signaling

Both VEGF and insulin are implicated in the pathogenesis of diabetic retinopathy. While it has been established for many years that the number of cell surface receptors impacts upon VEGF and insulin action, little is known about the precise machinery and proteins driving VEGF-R2 and IR degradation. Here, we investigate the role of Hepatocyte growth factor-Regulated tyrosine kinase Substrate (Hrs), a regulator of RTK trafficking, in VEGF and insulin signaling. We report that ectopic expression of Hrs increases VEGF-R2 and IR number and tyrosine phosphorylation, leading to amplification of their downstream signaling. The UIM (Ubiquitin Interacting Motif) domain of Hrs is required for Hrs-induced increases in VEGF-R2, but not in IR. Furthermore, Hrs is tyrosine-phosphorylated in response to VEGF and insulin. We show that the UIM domain is required for Hrs phosphorylation in response to VEGF, but not to insulin. Importantly, Hrs co-localizes with both VEGF-R2 and IR and co-immunoprecipitates with both in a manner independent of the Hrs-UIM domain. Finally, we demonstrate that Hrs inhibits Nedd4-mediated VEGF-R2 degradation and acts additively with Grb10. We conclude that Hrs is a positive regulator of VEGF-R2 and IR signaling and that ectopic expression of Hrs protects both VEGF-R2 and IR from degradation.

Epidermal growth factor receptor fate is controlled by Hrs tyrosine phosphorylation sites that regulate Hrs degradation

Hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) is an endosomal protein essential for the efficient sorting of activated growth factor receptors into the lysosomal degradation pathway. Hrs undergoes ligand-induced tyrosine phosphorylation on residues Y329 and Y334 downstream of epidermal growth factor receptor (EGFR) activation. It has been difficult to investigate the functional roles of phosphoHrs, as only a small proportion of the cellular Hrs pool is detectably phosphorylated. Using an HEK 293 model system, we found that ectopic expression of the protein Cbl enhances Hrs ubiquitination and increases Hrs phosphorylation following cell stimulation with EGF. We exploited Cbl's expansion of the phosphoHrs pool to determine whether Hrs tyrosine phosphorylation controls EGFR fate. In structure-function studies of Cbl and EGFR mutants, the level of Hrs phosphorylation and rapidity of apparent Hrs dephosphorylation correlated directly with EGFR degradation. Differential expression of wild-type versus Y329,334F mutant Hrs in Hrs-depleted cells revealed that one or both tyrosines regulate ligand-dependent Hrs degradation, as well as EGFR degradation. By modulating Hrs ubiquitination, phosphorylation, and protein levels, Cbl may control the composition of the endosomal sorting machinery and its ability to target EGFR for lysosomal degradation.

Endocytic pathways regulate Toll-like receptor 4 signaling and link innate and adaptive immunity

Immune responses are initiated when molecules of microbial origin are sensed by the Toll-like receptors (TLRs). We now report the identification of essential molecular components for the trafficking of the lipopolysaccharide (LPS) receptor complex. LPS was endocytosed by a receptor-mediated mechanism dependent on dynamin and clathrin and colocalized with TLR4 on early/sorting endosomes. TLR4 was ubiquitinated and associated with the ubiquitin-binding endosomal sorting protein hepatocyte growth factor-regulated tyrosine kinase substrate, Hrs. Inhibition of endocytosis and endosomal sorting increased LPS signaling. Finally, the LPS receptor complex was sorted to late endosomes/lysosomes for degradation and loading of associated antigens onto HLA class II molecules for presentation to CD4+ T cells. Our results show that endosomal trafficking of the LPS receptor complex is essential for signal termination and LPS-associated antigen presentation, thus controlling both innate and adaptive immunity through TLR4.

Growth factors induce differential phosphorylation profiles of the Hrs-STAM complex: a common node in signalling networks with signal-specific properties

Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate) and STAM (signal-transducing adaptor molecule) form a heterodimeric complex that associates with endosomal membranes and is tyrosine-phosphorylated in response to a variety of growth factors including EGF (epidermal growth factor), HGF (hepatocyte growth factor) and PDGF (platelet-derived growth factor). Phosphorylation of the Hrs-STAM complex requires receptor endocytosis. We show that an intact UIM (ubiquitin interaction motif) within Hrs is a conserved requirement for Hrs phosphorylation downstream of both EGF and HGF stimulations. Consistent with this, expression of a dominant-negative form of the E3 ubiquitin ligase, c-Cbl, inhibits EGF- and HGF-dependent Hrs phosphorylation. Despite this conservation, kinase inhibitor profiles using PP1 (4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) and SU6656 indicate that distinct non-receptor tyrosine kinases couple EGF, HGF and PDGF stimulation with the tyrosine phosphorylation of the Hrs-STAM complex. Crucially, analysis with phospho-specific antibodies indicates that these kinases generate a signal-specific, combinatorial phosphorylation profile of the Hrs-STAM complex, with the potential of diversifying tyrosine kinase receptor signalling through a common element.

Epidermal growth factor-dependent phosphorylation of the GGA3 adaptor protein regulates its recruitment to membranes

The Golgi-localized, Gamma-ear-containing, Arf-binding (GGA) proteins are monomeric clathrin adaptors that mediate the sorting of transmembrane cargo at the trans-Golgi network and endosomes. Here we report that one of these proteins, GGA3, becomes transiently phosphorylated upon activation of the epidermal growth factor (EGF) receptor. This phosphorylation takes place on a previously unrecognized site in the "hinge" segment of the protein, S368, and is strictly dependent on the constitutive phosphorylation of another site, S372. The EGF-induced phosphorylation of S368 does not require internalization of the EGF receptor or association of GGA3 with membranes. This phosphorylation can be blocked by inhibitors of both the mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways that function downstream of the activated EGF receptor. Phosphorylation of GGA3 on S368 causes an increase in the hydrodynamic radius of the protein, indicating a transition to a more asymmetric shape. Mutation of S368 and S372 to a phosphomimic aspartate residue decreases the association of GGA3 with membranes. These observations indicate that EGF signaling elicits phosphorylation events that regulate the association of GGA3 with organellar membranes.

Clathrin- and non-clathrin-mediated endocytic regulation of cell signalling

The internalization of various cargo proteins and lipids from the mammalian cell surface occurs through the clathrin and lipid-raft endocytic pathways. Protein-lipid and protein-protein interactions control the targeting of signalling molecules and their partners to various specialized membrane compartments in these pathways. This functions to control the activity of signalling cascades and the termination of signalling events, and therefore has a key role in defining how a cell responds to its environment.

Immunoaffinity profiling of tyrosine phosphorylation in cancer cells

Tyrosine kinases play a prominent role in human cancer, yet the oncogenic signaling pathways driving cell proliferation and survival have been difficult to identify, in part because of the complexity of the pathways and in part because of low cellular levels of tyrosine phosphorylation. In general, global phosphoproteomic approaches reveal small numbers of peptides containing phosphotyrosine. We have developed a strategy that emphasizes the phosphotyrosine component of the phosphoproteome and identifies large numbers of tyrosine phosphorylation sites. Peptides containing phosphotyrosine are isolated directly from protease-digested cellular protein extracts with a phosphotyrosine-specific antibody and are identified by tandem mass spectrometry. Applying this approach to several cell systems, including cancer cell lines, shows it can be used to identify activated protein kinases and their phosphorylated substrates without prior knowledge of the signaling networks that are activated, a first step in profiling normal and oncogenic signaling networks.

Interactions of TOM1L1 with the multivesicular body sorting machinery

Tom1L1 (Tom1-like1) and related proteins Tom1 (Target of Myb1) and Tom1L2 (Tom1-like2) constitute a new protein family characterized by the presence of a VHS (Vps27p/Hrs/Stam) domain in the N-terminal portion followed by a GAT (GGA and Tom) domain. Recently it was demonstrated that the GAT domain of both Tom1 and Tom1L1 binds ubiquitin, suggesting that these proteins might participate in the sorting of ubiquitinated proteins into multivesicular bodies (MVBs). Here we report a novel interaction between Tom1L1 and members of the MVB sorting machinery. Specifically, we found that the VHS domain of Tom1L1 interacts with Hrs (Hepatocyte growth factor-regulated tyrosine kinase substrate), whereas a PTAP motif, located between the VHS and GAT domain of Tom1L1, is responsible for binding to TSG101 (tumor susceptibility gene 101). Myc epitope-tagged Tom1L1 showed a cytosolic distribution but was recruited to endosomes following Hrs expression. In addition, Tom1L1 possesses several tyrosine motifs at the C-terminal region that mediate interactions with members of the Src family kinases and other signaling proteins such as Grb2 and p85. We showed that a fraction of Fyn kinase localizes at endosomes and that this distribution becomes more evident after epidermal growth factor internalization. Moreover, expression of a constitutive active form of Fyn also promoted the recruitment of Tom1L1 to enlarged endosomes. Taken together, we propose that Tom1L1 could act as an intermediary between signaling and degradative pathways.

The UIM domain of Hrs couples receptor sorting to vesicle formation

Hepatocyte growth factor regulated tyrosine kinase substrate (Hrs), a main component of the 'bilayered' clathrin coat on sorting endosomes, was originally identified as a substrate of activated tyrosine kinase receptors. We have analysed Hrs phosphorylation in response to epidermal growth factor (EGF) stimulation and show that the evolutionary conserved tyrosines Y329 and Y334 provide the principal phosphorylation sites. Hrs is proposed to concentrate ubiquitinated receptors within clathrin-coated regions via direct interaction with its UIM (ubiquitin interaction motif) domain. We show that the same UIM domain is necessary for EGF-stimulated tyrosine phosphorylation of Hrs. Over-expression of wild-type Hrs or a double mutant, Y329/334F, defective in EGF-dependent phosphorylation, both substantially retard EGF receptor (EGFR) degradation by inhibiting internal vesicle formation and thereby preventing EGFR incorporation into lumenal vesicles of the multivesicular bodies. In contrast, mutation or deletion of the Hrs-UIM domain strongly suppresses this effect. In addition the UIM-deletion and point mutants are also observed on internal membranes, indicating a failure to dissociate from the endosomal membrane prior to incorporation of the receptor complex into lumenal vesicles. Our data suggest a role for the UIM-domain of Hrs in actively retaining EGFR at the limiting membrane of endosomes as a prelude to lumenal vesicle formation.

HIV Gag mimics the Tsg101-recruiting activity of the human Hrs protein

The HIV-1 Gag protein recruits the cellular factor Tsg101 to facilitate the final stages of virus budding. A conserved P(S/T)AP tetrapeptide motif within Gag (the "late domain") binds directly to the NH2-terminal ubiquitin E2 variant (UEV) domain of Tsg101. In the cell, Tsg101 is required for biogenesis of vesicles that bud into the lumen of late endosomal compartments called multivesicular bodies (MVBs). However, the mechanism by which Tsg101 is recruited from the cytoplasm onto the endosomal membrane has not been known. Now, we report that Tsg101 binds the COOH-terminal region of the endosomal protein hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs; residues 222-777). This interaction is mediated, in part, by binding of the Tsg101 UEV domain to the Hrs 348PSAP351 motif. Importantly, Hrs222-777 can recruit Tsg101 and rescue the budding of virus-like Gag particles that are missing native late domains. These observations indicate that Hrs normally functions to recruit Tsg101 to the endosomal membrane. HIV-1 Gag apparently mimics this Hrs activity, and thereby usurps Tsg101 and other components of the MVB vesicle fission machinery to facilitate viral budding.

STAM and Hrs are subunits of a multivalent ubiquitin-binding complex on early endosomes

STAM1 and STAM2, which have been identified as regulators of receptor signaling and trafficking, interact directly with Hrs, which mediates the endocytic sorting of ubiquitinated membrane proteins. The STAM proteins interact with the same coiled-coil domain that is involved in the targeting of Hrs to endosomes. In this work, we show that STAM1 and STAM2, as well as an endocytic regulator protein, Eps15, can be co-immunoprecipitated with Hrs both from membrane and cytosolic fractions and that recombinant Hrs, STAM1/STAM2, and Eps15 form a ternary complex. We find that overexpression of Hrs causes a strong recruitment of STAM2 to endosome membranes. Moreover, STAM2, like Hrs and Eps15, binds ubiquitin, and Hrs, STAM2, and Eps15 colocalize with ubiquitinated proteins in clathrin-containing endosomal microdomains. The localization of Hrs, STAM2, Eps15, and clathrin to endosome membranes is controlled by the AAA ATPase mVps4, which has been implicated in multivesicular body formation. Depletion of cellular Hrs by small interfering RNA results in a strongly reduced recruitment of STAM2 to endosome membranes and an impaired degradation of endocytosed epidermal growth factor receptors. We propose that Hrs, Eps15, and STAM proteins function in a multivalent complex that sorts ubiquitinated proteins into the multivesicular body pathway.

Hrs and endocytic sorting of ubiquitinated membrane proteins

Endocytosed receptors are either recycled to the plasma membrane or trapped within intralumenal vesicles of multi-vesicular bodies for subsequent degradation in lysosomes. How the cell is able to sort receptors in endosomes has so far been largely unknown. The hepatocyte growth factor regulated tyrosine kinase substrate, Hrs, is an essential protein that has been implicated in cell signalling and intracellular membrane trafficking. Very recently, several reports have demonstrated a role for Hrs in endocytic sorting of ubiquitinated membrane proteins. Here, we review current knowledge about how Hrs recognises ubiquitinated cargo that is destined for lysosomal degradation, and how Hrs may act as a key regulator of the molecular machinery involved in receptor sorting and multivesicular body formation.