Ubc13 and COOH terminus of Hsp70-interacting protein (CHIP) are required for growth hormone receptor endocytosis

Chaperone-assisted E3 ligase CHIP: A double agent in cancer

The carboxy-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin ligase and co-chaperone belonging to Ubox family that plays a crucial role in the maintenance of cellular homeostasis by switching the equilibrium of the folding-refolding mechanism towards the proteasomal or lysosomal degradation pathway. It links molecular chaperones viz. HSC70, HSP70 and HSP90 with ubiquitin proteasome system (UPS), acting as a quality control system. CHIP contains charged domain in between N-terminal tetratricopeptide repeat (TPR) and C-terminal Ubox domain. TPR domain interacts with the aberrant client proteins via chaperones while Ubox domain facilitates the ubiquitin transfer to the client proteins for ubiquitination. Thus, CHIP is a classic molecule that executes ubiquitination for degradation of client proteins. Further, CHIP has been found to be indulged in cellular differentiation, proliferation, metastasis and tumorigenesis. Additionally, CHIP can play its dual role as a tumor suppressor as well as an oncogene in numerous malignancies, thus acting as a double agent. Here, in this review, we have reported almost all substrates of CHIP established till date and classified them according to the hallmarks of cancer. In addition, we discussed about its architectural alignment, tissue specific expression, sub-cellular localization, folding-refolding mechanisms of client proteins, E4 ligase activity, normal physiological roles, as well as involvement in various diseases and tumor biology. Further, we aim to discuss its importance in HSP90 inhibitors mediated cancer therapy. Thus, this report concludes that CHIP may be a promising and worthy drug target towards pharmaceutical industry for drug development.

The Heat Shock Protein 40-Type Chaperone MASH Supports the Endoplasmic Reticulum-Associated Degradation E3 Ubiquitin Ligase MAKIBISHI1 in Medicago truncatula

Jasmonates (JA) are oxylipin-derived phytohormones that trigger the production of specialized metabolites that often serve in defense against biotic stresses. In Medicago truncatula, a JA-induced endoplasmic reticulum-associated degradation (ERAD)-type machinery manages the production of bioactive triterpenes and thereby secures correct plant metabolism, growth, and development. This machinery involves the conserved RING membrane-anchor (RMA)-type E3 ubiquitin ligase MAKIBISHI1 (MKB1). Here, we discovered two additional members of this protein control apparatus via a yeast-based protein-protein interaction screen and characterized their function. First, a cognate E2 ubiquitin-conjugating enzyme was identified that interacts with MKB1 to deliver activated ubiquitin and to mediate its ubiquitination activity. Second, we identified a heat shock protein 40 (HSP40) that interacts with MKB1 to support its activity and was therefore designated MKB1-supporting HSP40 (MASH). MASH expression was found to be co-regulated with that of MKB1. The presence of MASH is critical for MKB1 and ERAD functioning because the dramatic morphological, transcriptional, and metabolic phenotype of MKB1 knock-down M. truncatula hairy roots was phenocopied by silencing of MASH. Interaction was also observed between the Arabidopsis thaliana (Arabidopsis) homologs of MASH and MKB1, suggesting that MASH represents an essential and plant-specific component of this vital and conserved eukaryotic protein quality control machinery.

GHR signalling: Receptor activation and degradation mechanisms

Growth hormone (GH) actions via initiating cell signalling through the GH receptor (GHR) are important for many physiological processes, in addition to its well-known role in regulating growth. The activation of JAK-STAT signalling by GH is well characterized, however knowledge on GH activation of SRC family kinases (SFKs) is still limited. In this review we summarise the collective knowledge on the activation, regulation, and downstream signalling of GHR. We highlight studies on GH activation of SFKs and the important outcome of this signalling pathway with a focus on the different degradation mechanisms that can regulate GHR availability since this is an area that warrants further study considering its role in tumour progression.

Growth Hormone Receptor Regulation in Cancer and Chronic Diseases

The GHR signaling pathway plays important roles in growth, metabolism, cell cycle control, immunity, homeostatic processes, and chemoresistance via both the JAK/STAT and the SRC pathways. Dysregulation of GHR signaling is associated with various diseases and chronic conditions such as acromegaly, cancer, aging, metabolic disease, fibroses, inflammation and autoimmunity. Numerous studies entailing the GHR signaling pathway have been conducted for various cancers. Diverse factors mediate the up- or down-regulation of GHR signaling through post-translational modifications. Of the numerous modifications, ubiquitination and deubiquitination are prominent events. Ubiquitination by E3 ligase attaches ubiquitins to target proteins and induces proteasomal degradation or starts the sequence of events that leads to endocytosis and lysosomal degradation. In this review, we discuss the role of first line effectors that act directly on the GHR at the cell surface including ADAM17, JAK2, SRC family member Lyn, Ubc13/CHIP, proteasome, βTrCP, CK2, STAT5b, and SOCS2. Activity of all, except JAK2, Lyn and STAT5b, counteract GHR signaling. Loss of their function increases the GH-induced signaling in favor of aging and certain chronic diseases, exemplified by increased lung cancer risk in case of a mutation in the SOCS2-GHR interaction site. Insight in their roles in GHR signaling can be applied for cancer and other therapeutic strategies.

Ubiquitylation Pathways In Insulin Signaling and Organismal Homeostasis

The insulin/insulin-like growth factor-1 (IGF-1) signaling (IIS) pathway is a pivotal genetic program regulating cell growth, tissue development, metabolic physiology, and longevity of multicellular organisms. IIS integrates a fine-tuned cascade of signaling events induced by insulin/IGF-1, which is precisely controlled by post-translational modifications. The ubiquitin/proteasome-system (UPS) influences the functionality of IIS through inducible ubiquitylation pathways that regulate internalization of the insulin/IGF-1 receptor, the stability of downstream insulin/IGF-1 signaling targets, and activity of nuclear receptors for control of gene expression. An age-related decline in UPS activity is often associated with an impairment of IIS, contributing to pathologies such as cancer, diabetes, cardiovascular, and neurodegenerative disorders. Recent findings identified a key role of diverse ubiquitin modifications in insulin signaling decisions, which governs dynamic adaption upon environmental and physiological changes. In this review, we discuss the mutual crosstalk between ubiquitin and insulin signaling pathways in the context of cellular and organismal homeostasis.

Drosophila bendless catalyzes K63-linked polyubiquitination and is involved in the response to DNA damage

In this study, we report the identification and functional characterization of the Drosophila ben/ubc13 gene, encoding a unique ubiquitin-conjugating enzyme (Ubc or E2), in DNA-damage response. Ben forms a heterodimer with DmUev1a, the only Ubc/E2 variant (Uev) in Drosophila. Ben and DmUev1a act together to catalyze K63-linked polyubiquitination in vitro. ben can functionally rescue the yeast ubc13 null mutant from killing by DNA-damaging agents. We also find that Ben^P97S, which was previously described to affect the connectivity between the giant fiber and the tergotrochanter motor neuron, fails to interact with the RING protein Chfr but retains interaction with DmUev1a as well as Uevs from other species. The corresponding yeast Ubc13^P97S interacts with Mms2 but fails to bind Rad5. Consequently, neither ben^P97S nor ubc13^P97S is able to complement the yeast ubc13 mutant defective in error-free DNA-damage tolerance. More importantly, the ben^P97S mutant flies are more sensitive to a DNA-damaging agent, suggesting that Ben functions in a manner similar to its yeast and mammalian counterparts. Collectively, our observations imply that Ben-DmUev1a-promoted K63-linked polyubiquitination and involvement in DNA-damage response are highly conserved in eukaryotes including flies.

Ubc13: the Lys63 ubiquitin chain building machine

Ubc13 is an ubiquitin E2 conjugating enzyme that participates with many different E3 ligases to form lysine 63-linked (Lys63) ubiquitin chains that are critical to signaling in inflammatory and DNA damage response pathways. Recent studies have suggested Ubc13 as a potential therapeutic target for intervention in various human diseases including several different cancers, alleviation of anti-cancer drug resistance, chronic inflammation, and viral infections. Understanding a potential therapeutic target from different angles is important to assess its usefulness and potential pitfalls. Here we present a global review of Ubc13 from its structure, function, and cellular activities, to its natural and chemical inhibition. The aim of this article is to review the literature that directly implicates Ubc13 in a biological function, and to integrate structural and mechanistic insights into the larger role of this critical E2 enzyme. We discuss observations of multiple Ubc13 structures that suggest a novel mechanism for activation of Ubc13 that involves conformational change of the active site loop.

CHIP: A new modulator of human malignant disorders

Carboxyl terminus of Hsc70-interacting protein (CHIP) is known as a chaperone-associated E3 for a variety of protein substrates. It acts as a link between molecular chaperones and ubiquitin-proteasome system. Involved in the process of protein clearance, CHIP plays a critical role in maintaining protein homeostasis in diverse conditions. Here, we provide a comprehensive review of our current understanding of CHIP and summarize recent advances in CHIP biology, with a focus on CHIP in the setting of malignancies.

Ubiquitination of the PI3-kinase VPS-34 promotes VPS-34 stability and phagosome maturation

The class III phosphatidylinositol 3-kinase VPS-34 promotes phagosome closure and maturation. Liu et al. identify UBC-13 as an E2-conjugating enzyme working with the E3 ligase CHN-1 to catalyze K63-linked poly-ubiquitination on VPS-34, which promotes VPS-34 stability and phagosome maturation.

Apoptotic cells generated by programmed cell death are engulfed by phagocytes and enclosed within membrane-bound phagosomes. Maturation of apoptotic cell–containing phagosomes leads to formation of phagolysosomes where cell corpses are degraded. The class III phosphatidylinositol 3-kinase (PI3-kinase) VPS-34 coordinates with PIKI-1, a class II PI3-kinase, to produce PtdIns3P on phagosomes, thus promoting phagosome closure and maturation. Here, we identified UBC-13, an E2 ubiquitin–conjugating enzyme that functions in the same pathway with VPS-34 but in parallel to PIKI-1 to regulate PtdIns3P generation on phagosomes. Loss of ubc-13 affects early steps of phagosome maturation, causing accumulation of cell corpses. We found that UBC-13 functions with UEV-1, a noncatalytic E2 variant, and CHN-1, a U-box–containing E3 ubiquitin ligase, to catalyze K63-linked poly-ubiquitination on VPS-34 both in vitro and in Caenorhabditis elegans. Loss of ubc-13, uev-1, or chn-1 disrupts ubiquitin modification of VPS-34 and causes significantly reduced VPS-34 protein levels. Our data suggest that K63-linked ubiquitin modification serves as a general mechanism to modulate VPS-34 stability in multiple processes.

Uev1A-Ubc13 catalyzes K63-linked ubiquitination of RHBDF2 to promote TACE maturation

The TNFα-induced NF-κB signaling pathway plays critical roles in multiple biological processes. Extensive studies have explored the mechanisms regulating this signaling cascade, and identified an E2 complex, Uev1A-Ubc13, that mediates K63-linked poly-Ub chain formation and thus recruits NEMO to activate the signaling transduction. In this study, we demonstrate that the Uev1A-Ubc13 complex simultaneously serves as a repressor of the NF-κB pathway. It was found that cells overexpressing UEV1A silence the signal cascade earlier than control cells. Importantly, UEV1A overexpression enhances TACE maturation to shed the TNFα receptor. The Uev1A-Ubc13 complex interacts with RHBDF2, a key factor promoting TACE maturation, and inhibition of the Uev1A-Ubc13 activity interferes with RHBDF2-promoted TACE maturation. Furthermore, upon TNFα stimulation, the Uev1A-Ubc13 complex cooperates with CHIP to promote K63-linked ubiquitination of RHBDF2, enhancing its activity toward TACE maturation and subsequently blocking TNFα-induced NF-κB signaling.

Chaperone-directed ubiquitylation maintains proteostasis at the expense of longevity

The integrity of the cellular proteome is supported by quality control networks, which govern protein synthesis, folding, and degradation. It is generally accepted that an age-related decline in protein homeostasis (proteostasis) contributes to protein aggregation diseases. However, the mechanistic principles underlying proteostasis imbalance and the impact on life expectancy are not well understood. We recently demonstrated that this interrelation is affected by chaperone-directed ubiquitylation, shifting the amount of the conserved DAF-2/insulin receptor both in Caenorhabditis elegans and Drosophila melanogaster. The ubiquitin ligase CHIP either targets the membrane bound insulin receptor or misfolded proteins for degradation, which depends on the cellular proteostasis status. Increased proteotoxicity triggers chaperone-assisted redirection of CHIP toward protein aggregates, limiting its capacity to degrade the insulin receptor and prevent premature aging. In light of these findings, we discuss a new concept for understanding the impact of proteome imbalance on longevity risk.

The Ubiquitin Ligase CHIP Integrates Proteostasis and Aging by Regulation of Insulin Receptor Turnover

Aging is attended by a progressive decline in protein homeostasis (proteostasis), aggravating the risk for protein aggregation diseases. To understand the coordination between proteome imbalance and longevity, we addressed the mechanistic role of the quality-control ubiquitin ligase CHIP, which is a key regulator of proteostasis. We observed that CHIP deficiency leads to increased levels of the insulin receptor (INSR) and reduced lifespan of worms and flies. The membrane-bound INSR regulates the insulin and IGF1 signaling (IIS) pathway and thereby defines metabolism and aging. INSR is a direct target of CHIP, which triggers receptor monoubiquitylation and endocytic-lysosomal turnover to promote longevity. However, upon proteotoxic stress conditions and during aging, CHIP is recruited toward disposal of misfolded proteins, reducing its capacity to degrade the INSR. Our study indicates a competitive relationship between proteostasis and longevity regulation through CHIP-assisted proteolysis, providing a mechanistic concept for understanding the impact of proteome imbalance on aging.

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The ubiquitin ligase CHIP triggers insulin receptor turnover

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Insulin receptor level is linked to insulin and IGF1 signaling and longevity

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Engagement of CHIP in protein quality control limits insulin receptor degradation

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Proteotoxic stress aggravates insulin receptor stability, drives aging, and shortens lifespan

Cytokine Receptor Endocytosis: New Kinase Activity-Dependent and -Independent Roles of PI3K

Type I and II cytokine receptors are cell surface sensors that bind cytokines in the extracellular environment and initiate intracellular signaling to control processes such as hematopoiesis, immune function, and cellular growth and development. One key mechanism that regulates signaling from cytokine receptors is through receptor endocytosis. In this mini-review, we describe recent advances in endocytic regulations of cytokine receptors, focusing on new paradigms by which PI3K controls receptor endocytosis through both kinase activity-dependent and -independent mechanisms. These advances underscore the notion that the p85 regulatory subunit of PI3K has functions beyond regulating PI3K kinase activity, and that PI3K plays both positive and negative roles in receptor signaling. On the one hand, the PI3K/Akt pathway controls various aspects downstream of cytokine receptors. On the other hand, it stimulates receptor endocytosis and downregulation, thus contributing to signaling attenuation.

A Decade of Boon or Burden: What Has the CHIP Ever Done for Cellular Protein Quality Control Mechanism Implicated in Neurodegeneration and Aging?

Cells regularly synthesize new proteins to replace old and abnormal proteins for normal cellular functions. Two significant protein quality control pathways inside the cellular milieu are ubiquitin proteasome system (UPS) and autophagy. Autophagy is known for bulk clearance of cytoplasmic aggregated proteins, whereas the specificity of protein degradation by UPS comes from E3 ubiquitin ligases. Few E3 ubiquitin ligases, like C-terminus of Hsc70-interacting protein (CHIP) not only take part in protein quality control pathways, but also plays a key regulatory role in other cellular processes like signaling, development, DNA damage repair, immunity and aging. CHIP targets misfolded proteins for their degradation through proteasome, as well as autophagy; simultaneously, with the help of chaperones, it also regulates folding attempts for misfolded proteins. The broad range of CHIP substrates and their associations with multiple pathologies make it a key molecule to work upon and focus for future therapeutic interventions. E3 ubiquitin ligase CHIP interacts and degrades many protein inclusions formed in neurodegenerative diseases. The presence of CHIP at various nodes of cellular protein-protein interaction network presents this molecule as a potential candidate for further research. In this review, we have explored a wide range of functionality of CHIP inside cells by a detailed presentation of its co-chaperone, E3 and E4 enzyme like functions, with central focus on its protein quality control roles in neurodegenerative diseases. We have also raised many unexplored but expected fundamental questions regarding CHIP functions, which generate hopes for its future applications in research, as well as drug discovery.

Fos-Zippered GH Receptor Cytosolic Tails Act as Jak2 Substrates and Signal Transducers

Members of the Janus kinase (Jak) family initiate the majority of downstream signaling events of the cytokine receptor family. The prevailing principle is that the receptors act in dimers: 2 Jak2 molecules bind to the cytosolic tails of a cytokine receptor family member and initiate Jak-signal transducer and activator of transcription signaling upon a conformational change in the receptor complex, induced by the cognate cytokine. Due to the complexity of signaling complexes, there is a strong need for in vitro model systems. To investigate the molecular details of the Jak2 interaction with the GH receptor (GHR), we used cytosolic tails provided with leucine zippers derived from c-Fos to mimic the dimerized state of GHR. Expressed together with Jak2, fos-zippered tails, but not unzippered tails, were stabilized. In addition, the Jak-signal transducer and activator of transcription signaling pathway was activated by the fos-zippered tails. The stabilization depended also on α-helix rotation of the zippers. Fos-zippered GHR tails and Jak2, both purified from baculovirus-infected insect cells, interacted via box1 with a binding affinity of approximately 40nM. As expected, the Jak kinase inhibitor Ruxolitinib inhibited the stabilization but did not affect the c-Fos-zippered GHR tail-Jak2 interaction. Analysis by blue-native gel electrophoresis revealed high molecular-weight complexes containing both Jak2 and nonphosphorylated GHR tails, whereas Jak2-dissociated tails were highly phosphorylated and monomeric, implying that Jak2 detaches from its substrate upon phosphorylation.

Versatile roles of k63-linked ubiquitin chains in trafficking

Modification by Lys63-linked ubiquitin (UbK63) chains is the second most abundant form of ubiquitylation. In addition to their role in DNA repair or kinase activation, UbK63 chains interfere with multiple steps of intracellular trafficking. UbK63 chains decorate many plasma membrane proteins, providing a signal that is often, but not always, required for their internalization. In yeast, plants, worms and mammals, this same modification appears to be critical for efficient sorting to multivesicular bodies and subsequent lysosomal degradation. UbK63 chains are also one of the modifications involved in various forms of autophagy (mitophagy, xenophagy, or aggrephagy). Here, in the context of trafficking, we report recent structural studies investigating UbK63 chains assembly by various E2/E3 pairs, disassembly by deubiquitylases, and specifically recognition as sorting signals by receptors carrying Ub-binding domains, often acting in tandem. In addition, we address emerging and unanticipated roles of UbK63 chains in various recycling pathways that function by activating nucleators required for actin polymerization, as well as in the transient recruitment of signaling molecules at the plasma or ER membrane. In this review, we describe recent advances that converge to elucidate the mechanisms underlying the wealth of trafficking functions of UbK63 chains.

A CHIPotle in physiology and disease

The carboxy-terminus of Hsc70 interacting protein (CHIP) is known to function as a chaperone associated E3 ligase for several proteins and regulates a variety of physiological processes. Being a connecting link between molecular chaperones and 26S proteasomes, it is widely regarded as the central player in the cellular protein quality control system. Recent analyses have provided new insights on the biochemical and functional dynamics of CHIP. In this review article, we give a comprehensive account of our current knowledge on the biology of CHIP, which apart from shedding light on fundamental biological questions promises to provide a potential target for therapeutic intervention.

Eliminative signaling by Janus kinases: role in the downregulation of associated receptors

Activation of cytokine receptor-associated Janus kinases (JAKs) mediates most, if not all, of the cellular responses to peptide hormones and cytokines. Consequently, JAKs play a paramount role in homeostasis and immunity. Members of this family of tyrosine kinases control the cytokine/hormone-induced alterations in cell gene expression program. This function is largely mediated through an ability to signal toward activation of the signal transducer and activator of transcription proteins (STAT), as well as toward some other pathways. Importantly, JAKs are also instrumental in tightly controlling the expression of associated cytokine and hormone receptors, and, accordingly, in regulating the cell sensitivity to these cytokines and hormones. This review highlights the enzymatic and non-enzymatic mechanisms of this regulation and discusses the importance of the ambidextrous nature of JAK as a key signaling node that integrates the combining functions of forward signaling and eliminative signaling. Attention to the latter aspect of JAK function may contribute to emancipating our approaches to the pharmacological modulation of JAKs.

Garbage on, garbage off: new insights into plasma membrane protein quality control

Maintenance of cellular protein quality - by restoring misfolded proteins to their native state and by targeting terminally misfolded or damaged proteins for degradation - is a critical function of all cells. To ensure protein quality, cells have evolved various organelle-specific quality control mechanisms responsible for recognizing and responding to misfolded proteins at different subcellular locations of the cell. Recently, several publications have begun to elucidate mechanisms of quality control that operate at the plasma membrane (PM), recognizing misfolded PM proteins and targeting their endocytic trafficking and lysosomal degradation. Here, I discuss these recent developments in our understanding of PM quality control mechanisms and how they relate to global protein quality control strategies in the cell.

The E3 ligase CHIP: insights into its structure and regulation

The carboxy-terminus of Hsc70 interacting protein (CHIP) is a cochaperone E3 ligase containing three tandem repeats of tetratricopeptide (TPR) motifs and a C-terminal U-box domain separated by a charged coiled-coil region. CHIP is known to function as a central quality control E3 ligase and regulates several proteins involved in a myriad of physiological and pathological processes. Recent studies have highlighted varied regulatory mechanisms operating on the activity of CHIP which is crucial for cellular homeostasis. In this review article, we give a concise account of our current knowledge on the biochemistry and regulation of CHIP.

Regulation of IL-4 receptor signaling by STUB1 in lung inflammation

RATIONALE

IL-4Rα, the common receptor component for IL-4 and IL-13, plays a critical role in IL-4- and IL-13-mediated signaling pathways that regulate airway inflammation and remodeling. However, the regulatory mechanisms underlying IL-4Rα turnover and its signal termination remain elusive.

OBJECTIVES

To evaluate the role of STUB1 (STIP1 homology and U-Box containing protein 1) in regulating IL-4R signaling in airway inflammation.

METHODS

The roles of STUB1 in IL-4Rα degradation and its signaling were investigated by immunoblot, immunoprecipitation, and flow cytometry. The involvement of STUB1 in airway inflammation was determined in vivo by measuring lung inflammatory cells infiltration, mucus production, serum lgE levels, and alveolar macrophage M2 activation in STUB1(-/-) mice. STUB1 expression was evaluated in airway epithelium of patients with asthma and lung tissues of subjects with chronic obstructive pulmonary disease.

MEASUREMENTS AND MAIN RESULTS

STUB1 interacted with IL-4Rα and targeted it for ubiquitination-mediated proteasomal degradation, terminating IL-4 or IL-13 signaling. STUB1 knockout cells showed increased levels of IL-4Rα and sustained STAT6 activation, whereas STUB1 overexpression reduced IL-4Rα levels. Mice deficient in STUB1 had spontaneous airway inflammation, alternative M2 activation of alveolar macrophage, and increased serum IgE. STUB1 levels were increased in airways of subjects with asthma or chronic obstructive pulmonary disease, suggesting that up-regulation of STUB1 might be an important feedback mechanism to dampen IL-4R signaling in airway inflammation.

CONCLUSIONS

Our study identified a previously uncharacterized role for STUB1 in regulating IL-4R signaling, which might provide a new strategy for attenuating airway inflammation.

Multimeric growth hormone receptor complexes serve as signaling platforms

Growth hormone (GH) signaling is required for promoting longitudinal body growth, stem cell activation, differentiation, and survival and for regulation of metabolism. Failure to adequately regulate GH signaling leads to disease: excessive GH signaling has been connected to cancer, and GH insensitivity has been reported in cachexia patients. Since its discovery in 1989, the receptor has served a pivotal role as the prototype cytokine receptor both structurally and functionally. Phosphorylation and ubiquitylation regulate the GH receptor (GHR) at the cell surface: two ubiquitin ligases (SCF(βTrCP2) and CHIP) determine the GH responsiveness of cells by controlling its endocytosis, whereas JAK2 initiates the JAK/STAT pathway. We used blue native electrophoresis to identify phosphorylated and ubiquitylated receptor intermediates. We show that GHRs occur as ∼500-kDa complexes that dimerize into active ∼900-kDa complexes upon GH binding. The dimerized complexes act as platforms for transient interaction with JAK2 and ubiquitin ligases. If GH and receptors are made in the same cell (autocrine mode), only limited numbers of ∼900-kDa complexes are formed. The experiments reveal the dynamic changes in post-translational modifications during GH-induced signaling events and show that relatively simple cytokine receptors like GHRs are able to form higher order protein complexes. Insight in the complex formation of cytokine receptors is crucially important for engineering cytokines that control ligand-induced cell responses and for generating a new class of therapeutic agents for a wide range of diseases.

βTrCP interacts with the ubiquitin-dependent endocytosis motif of the GH receptor in an unconventional manner

GH (growth hormone) binding to the GHR (GH receptor) triggers essential signalling pathways that promote growth and metabolic regulation. The sensitivity of the cells to GH is mainly controlled by the endocytosis of the receptor via βTrCP (β-transducin repeat-containing protein). In the present study, we show that βTrCP interacts directly via its WD40 domain with the UbE (ubiquitin-dependent endocytosis) motif in GHR, promoting GHR ubiquitination in vitro. NMR experiments demonstrated that the UbE motif is essentially unstructured, and, together with functional mapping of the UbE and βTrCP WD40 residues necessary for binding, led to a unique interaction model of βTrCP with GHR-UbE. This interaction is different from the conventional βTrCP-substrate interactions described to date. This interaction therefore represents a promising specific target to develop drugs that inhibit GHR endocytosis and increase GH sensitivity in cachexia patients.

The tomato Fni3 lysine-63-specific ubiquitin-conjugating enzyme and suv ubiquitin E2 variant positively regulate plant immunity

The activation of an immune response in tomato (Solanum lycopersicum) against Pseudomonas syringae relies on the recognition of E3 ligase-deficient forms of AvrPtoB by the host protein kinase, Fen. To investigate the mechanisms by which Fen-mediated immunity is regulated, we characterize in this study a Fen-interacting protein, Fni3, and its cofactor, S. lycoperiscum Uev (Suv). Fni3 encodes a homolog of the Ubc13-type ubiquitin-conjugating enzyme that catalyzes exclusively Lys-63-linked ubiquitination, whereas Suv is a ubiquitin-conjugating enzyme variant. The C-terminal region of Fen was necessary for interaction with Fni3, and this interaction was required for cell death triggered by overexpression of Fen in Nicotiana benthamiana leaves. Fni3 was shown to be an active E2 enzyme, but Suv displayed no ubiquitin-conjugating activity; Fni3 and Suv together directed Lys-63-linked ubiquitination. Decreased expression of Fni3, another tomato Ubc13 homolog, Sl-Ubc13-2, or Suv in N. benthamiana leaves diminished cell death associated with Fen-mediated immunity and cell death elicited by several other resistance (R) proteins and their cognate effectors. We also discovered that coexpression of Fen and other R proteins/effectors with a Fni3 mutant that is compromised for ubiquitin-conjugating activity diminished the cell death. These results suggest that Fni3/Sl-Ubc13-2 and Suv regulate the immune response mediated by Fen and other R proteins through Lys-63-linked ubiquitination.

Complex structure of OspI and Ubc13: the molecular basis of Ubc13 deamidation and convergence of bacterial and host E2 recognition

Ubc13 is an important ubiquitin-conjugating (E2) enzyme in the NF-κB signaling pathway. The Shigella effector OspI targets Ubc13 and deamidates Gln100 of Ubc13 to a glutamic acid residue, leading to the inhibition of host inflammatory responses. Here we report the crystal structure of the OspI-Ubc13 complex at 2.3 Å resolution. The structure reveals that OspI uses two differently charged regions to extensively interact with the α1 helix, L1 loop and L2 loop of Ubc13. The Gln100 residue is bound within the hydrophilic catalytic pocket of OspI. A comparison between Ubc13-bound and wild-type free OspI structures revealed that Ubc13 binding induces notable structural reassembly of the catalytic pocket, suggesting that substrate binding might be involved in the catalysis of OspI. The OspI-binding sites in Ubc13 largely overlap with the binding residues for host ubiquitin E3 ligases and a deubiquitinating enzyme, which suggests that the bacterial effector and host proteins exploit the same surface on Ubc13 for specific recognition. Biochemical results indicate that both of the differently charged regions in OspI are important for the interaction with Ubc13, and the specificity determinants in Ubc13 for OspI recognition reside in the distinct residues in the α1 helix and L2 region. Our study reveals the molecular basis of Ubc13 deamidation by OspI, as well as a convergence of E2 recognition by bacterial and host proteins.

The Gram-negative pathogenic bacterium Shigella infects human intestinal epithelium cells and causes severe inflammatory colitis (bacillary dysentery). Shigella harbors an approximately 220-kb virulence plasmid that encodes a type III secretion system (T3SS) protein secretion apparatus and many effector proteins. Using the T3SS, Shigella delivers the effector proteins into the host cells, targeting key signal molecules and manipulating the host physiological processes and thereby promoting infection and multiplication. OspI, a newly identified Shigella effector, targets the host Ubc13 protein, a critical ubiquitin-conjugating enzyme in the NF-κB signaling pathway. OspI deamidates Gln100 of Ubc13 to a glutamic acid residue, thereby disrupting TRAF6-catalyzed polyubiquitination and dampening host inflammatory responses. However, the structural mechanism of this specific deamidation is unclear. Through crystallography, we have determined the structure of the OspI-Ubc13 complex. The structure illustrates how OspI interacts with Ubc13 and how Ubc13 induces conformational changes in OspI. Combining structural analysis and biochemical assays, we revealed how OspI distinguishes Ubc13 from other ubiquitin conjugating enzymes and found that OspI binds to the same surface region on Ubc13 as host TRAF6, CHIP and OTUB1. Our study sheds light on the molecular mechanism of Ubc13 deamidation by OspI and provides new insights into E2 recognition by bacterial and host proteins.

Reciprocal cross-regulation between RNF41 and USP8 controls cytokine receptor sorting and processing

The mechanisms controlling the steady-state cytokine receptor cell surface levels, and consequently the cellular response to cytokines, remain poorly understood. The number of surface-exposed receptors is a dynamic balance of de novo synthesis, transport to the plasma membrane, internalization, recycling, degradation and ectodomain shedding. We previously reported that the E3 ubiquitin ligase Ring Finger Protein 41 (RNF41) inhibits basal lysosomal degradation and enhance ectodomain shedding of JAK2-associated cytokine receptors. Ubiquitin-specific protease 8 (USP8), an RNF41 interacting deubiquitinating enzyme (DUB) stabilizes RNF41 and is involved in trafficking of various transmembrane proteins. The present study identifies USP8 as a substrate of RNF41 and reveals that loss of USP8 explains the aforementioned RNF41 effects. RNF41 redistributes and ubiquitinates USP8, and reduces USP8 levels. In addition, USP8 knockdown functionally matches the effects of RNF41 ectopic expression on the model leptin and leukemia inhibitory factor (LIF) receptors. Moreover, RNF41 indirectly destabilizes the ESCRT-0 complex via USP8 suppression. Collectively, our findings demonstrate that RNF41 controls JAK2-associated cytokine receptor trafficking by acting as a key regulator of USP8 and ESCRT-0 stability. Balanced reciprocal cross-regulation between RNF41 and USP8 thus decides if receptors are sorted for lysosomal degradation or recycling, this way regulating basal cytokine receptor levels.