Over-expression of Rififylin, a new RING finger and FYVE-like domain-containing protein, inhibits recycling from the endocytic recycling compartment

E3 ubiquitin ligase rififylin has yin and yang effects on rabbit cardiac transient outward potassium currents (Ito) and corresponding channel proteins

Genome-wide association studies have reported a correlation between a SNP of the RING finger E3 ubiquitin protein ligase rififylin (RFFL) and QT interval variability in humans (Newton-Cheh et al., 2009). Previously, we have shown that RFFL downregulates expression and function of the human-like ether-a-go-go-related gene potassium channel and corresponding rapidly activating delayed rectifier potassium current (IKr) in adult rabbit ventricular cardiomyocytes. Here, we report that RFFL also affects the transient outward current (Ito), but in a peculiar way. RFFL overexpression in adult rabbit ventricular cardiomyocytes significantly decreases the contribution of its fast component (Ito,f) from 35% to 21% and increases the contribution of its slow component (Ito,s) from 65% to 79%. Since Ito,f in rabbits is mainly conducted by Kv4.3, we investigated the effect of RFFL on Kv4.3 expressed in HEK293A cells. We found that RFFL overexpression reduced Kv4.3 expression and corresponding Ito,f in a RING domain-dependent manner in the presence or absence of its accessory subunit Kv channel-interacting protein 2. On the other hand, RFFL overexpression in Kv1.4-expressing HEK cells leads to an increase in both Kv1.4 expression level and Ito,s, similarly in a RING domain-dependent manner. Our physiologically detailed rabbit ventricular myocyte computational model shows that these yin and yang effects of RFFL overexpression on Ito,f, and Ito,s affect phase 1 of the action potential waveform and slightly decrease its duration in addition to suppressing IKr. Thus, RFFL modifies cardiac repolarization reserve via ubiquitination of multiple proteins that differently affect various potassium channels and cardiac action potential duration.

Identification of α-Tocopherol succinate as an RFFL-substrate interaction inhibitor inducing peripheral CFTR stabilization and apoptosis

The E3 ubiquitin ligase RFFL is an apoptotic inhibitor highly expressed in cancers and its knockdown suppresses cancer cell growth and sensitizes to chemotherapy. RFFL also participates in peripheral protein quality control which removes the functional cell surface ΔF508-CFTR channel and reduces the efficacy of pharmaceutical therapy for cystic fibrosis (CF). Although RFFL inhibitors have therapeutic potential for both cancer and CF, they remain undiscovered. Here, a chemical array screening has identified α-tocopherol succinate (αTOS) as an RFFL ligand. NMR analysis revealed that αTOS directly binds to RFFL's substrate-binding region without affecting the E3 enzymatic activity. Consequently, αTOS inhibits the RFFL-substrate interaction, ΔF508-CFTR ubiquitination and elimination from the plasma membrane of epithelial cells, resulting in the increased functional CFTR channel. Among the α-tocopherol (αTOL) analogs we tested, only αTOS inhibited the RFFL-substrate interaction and increased the cell surface ΔF508-CFTR, depending on RFFL expression. Similarly, the unique proapoptotic effect of αTOS was dependent on RFFL expression. Thus, unlike other αTOL analogs, αTOS acts as an RFFL protein-protein interaction inhibitor which may explain its unique biological properties among αTOL analogs. Moreover, αTOS may act as a CFTR stabilizer, a novel class of drugs that extend cell surface ΔF508-CFTR lifetime.

Endosomal-associated RFFL facilitates mitochondrial clearance by enhancing PRKN/parkin recruitment to mitochondria

Mutations in the ubiquitin ligase PRKN (parkin RBR E3 ubiquitin protein ligase) are associated with Parkinson disease and defective mitophagy. Conceptually, PRKN-dependent mitophagy is classified into two phases: 1. PRKN recruits to and ubiquitinates mitochondrial proteins; 2. formation of phagophore membrane, sequestering mitochondria for degradation. Recently, endosomal machineries are reported to contribute to the later stage for membrane assembly. We reported a role for endosomes in the events upstream of phase 1. We demonstrate that the endosomal ubiquitin ligase RFFL (ring finger and FYVE like domain containing E3 ubiquitin protein ligase) associated with damaged mitochondria, and this association preceded that of PRKN. RFFL interacted with PRKN, and stable recruitment of PRKN to damaged mitochondria was substantially reduced in RFFL KO cells. Our study unraveled a novel role of endosomes in modulating upstream pathways of PRKN-dependent mitophagy initiation.Abbreviations CCCP: carbonyl cyanide 3-chlorophenylhydrazone; DMSO: dimethyl sulfoxide; EGFP: enhanced green fluorescence protein; KO: knockout; PRKN: parkin RBR E3 ubiquitin protein ligase; RFFL: ring finger and FYVE like domain containing E3 ubiquitin protein ligase; UQCRC1: ubiquinol-cytochrome c reductase core protein 1; WT: wild-type.

Membrane compartmentalisation of the ubiquitin system

We now have a comprehensive inventory of ubiquitin system components. Understanding of any system also needs an appreciation of how components are organised together. Quantitative proteomics has provided us with a census of their relative populations in several model cell types. Here, by examining large scale unbiased data sets, we seek to identify and map those components, which principally reside on the major organelles of the endomembrane system. We present the consensus distribution of > 50 ubiquitin modifying enzymes, E2s, E3s and DUBs, that possess transmembrane domains. This analysis reveals that the ER and endosomal compartments have a diverse cast of resident E3s, whilst the Golgi and mitochondria operate with a more restricted palette. We describe key functions of ubiquitylation that are specific to each compartment and relate this to their signature complement of ubiquitin modifying components.

Sulfatase 2 Is Associated with Steroid Resistance in Childhood Nephrotic Syndrome

Glucocorticoid (GC) resistance complicates the treatment of ~10-20% of children with nephrotic syndrome (NS), yet the molecular basis for resistance remains unclear. We used RNAseq analysis and in silico algorithm-based approaches on peripheral blood leukocytes from 12 children both at initial NS presentation and after ~7 weeks of GC therapy to identify a 12-gene panel able to differentiate steroid resistant NS (SRNS) from steroid-sensitive NS (SSNS). Among this panel, subsequent validation and analyses of one biologically relevant candidate, sulfatase 2 (SULF2), in up to a total of 66 children, revealed that both SULF2 leukocyte expression and plasma arylsulfatase activity Post/Pre therapy ratios were greater in SSNS vs. SRNS. However, neither plasma SULF2 endosulfatase activity (measured by VEGF binding activity) nor plasma VEGF levels, distinguished SSNS from SRNS, despite VEGF's reported role as a downstream mediator of SULF2's effects in glomeruli. Experimental studies of NS-related injury in both rat glomeruli and cultured podocytes also revealed decreased SULF2 expression, which were partially reversible by GC treatment of podocytes. These findings together suggest that SULF2 levels and activity are associated with GC resistance in NS, and that SULF2 may play a protective role in NS via the modulation of downstream mediators distinct from VEGF.

Identification of phostensin in association with Eps 15 homology domain-containing protein 1 (EHD1) and EHD4

Phostensin (PTS) encoded by KIAA1949 is a protein phosphatase 1 (PP1)-binding protein. In order to explore the cellular functions of PTS, we have searched PTS-binding proteins by using co-immunoprecipitation in combination with shotgun proteomics. Here, we report two novel PTS-binding proteins, Eps 15 homology domain-containing protein 1 (EHD1) and EHD4. PTS associated with EHD proteins was also observed in GST pull-down assays. Immunofluorescence microscopy demonstrated that the complex was co-localized at the endocytic vesicles. EHD proteins have been known to play a critical role in regulation of endocytic transport. Overexpression of PTS-β can attenuate the endocytic trafficking of transferrin.

Cytomegalovirus Generates Assembly Compartment in the Early Phase of Infection by Perturbation of Host-Cell Factors Recruitment at the Early Endosome/Endosomal Recycling Compartment/Trans-Golgi Interface

Beta-herpesviruses develop a unique structure within the infected cell known as an assembly compartment (AC). This structure, as large as the nucleus, is composed of host-cell-derived membranous elements. The biogenesis of the AC and its contribution to the final stages of beta-herpesvirus assembly are still unclear. In this study, we performed a spatial and temporal analysis of the AC in cells infected with murine CMV (MCMV), a member of the beta-herpesvirus family, using a panel of markers that characterize membranous organelle system. Out of 64 markers that were analyzed, 52 were cytosolic proteins that are recruited to membranes as components of membrane-shaping regulatory cascades. The analysis demonstrates that MCMV infection extensively reorganizes interface between early endosomes (EE), endosomal recycling compartment (ERC), and the trans-Golgi network (TGN), resulting in expansion of various EE-ERC-TGN intermediates that fill the broad area of the inner AC. These intermediates are displayed as over-recruitment of host-cell factors that control membrane flow at the EE-ERC-TGN interface. Most of the reorganization is accomplished in the early (E) phase of infection, indicating that the AC biogenesis is controlled by MCMV early genes. Although it is known that CMV infection affects the expression of a large number of host-cell factors that control membranous system, analysis of the host-cell transcriptome and protein expression in the E phase of infection demonstrated no sufficiently significant alteration in expression levels of analyzed markers. Thus, our study demonstrates that MCMV-encoded early phase function targets recruitment cascades of host cell-factors that control membranous flow at the EE-ERC-TGN interface in order to initiate the development of the AC.

Ubc13-Mms2 cooperates with a family of RING E3 proteins in budding yeast membrane protein sorting

Polyubiquitin chains linked via lysine (K) 63 play an important role in endocytosis and membrane trafficking. Their primary source is the ubiquitin protein ligase (E3) Rsp5/NEDD4, which acts as a key regulator of membrane protein sorting. The heterodimeric ubiquitin-conjugating enzyme (E2), Ubc13-Mms2, catalyses K63-specific polyubiquitylation in genome maintenance and inflammatory signalling. In budding yeast, the only E3 proteins known to cooperate with Ubc13-Mms2 so far is a nuclear RING finger protein, Rad5, involved in the replication of damaged DNA. Here, we report a contribution of Ubc13-Mms2 to the sorting of membrane proteins to the yeast vacuole via the multivesicular body (MVB) pathway. In this context, Ubc13-Mms2 cooperates with Pib1, a FYVE-RING finger protein associated with internal membranes. Moreover, we identified a family of membrane-associated FYVE-(type)-RING finger proteins as cognate E3 proteins for Ubc13-Mms2 in several species, and genetic analysis indicates that the contribution of Ubc13-Mms2 to membrane trafficking in budding yeast goes beyond its cooperation with Pib1. Thus, our results widely implicate Ubc13-Mms2 as an Rsp5-independent source of K63-linked polyubiquitin chains in the regulation of membrane protein sorting.This article has an associated First Person interview with the first author of the paper.

The integral function of the endocytic recycling compartment is regulated by RFFL-mediated ubiquitylation of Rab11 effectors

Endocytic trafficking is regulated by ubiquitylation (also known as ubiquitination) of cargoes and endocytic machineries. The role of ubiquitylation in lysosomal delivery has been well documented, but its role in the recycling pathway is largely unknown. Here, we report that the ubiquitin (Ub) ligase RFFL regulates ubiquitylation of endocytic recycling regulators. An RFFL dominant-negative (DN) mutant induced clustering of endocytic recycling compartments (ERCs) and delayed endocytic cargo recycling without affecting lysosomal traffic. A BioID RFFL interactome analysis revealed that RFFL interacts with the Rab11 effectors EHD1, MICALL1 and class I Rab11-FIPs. The RFFL DN mutant strongly captured these Rab11 effectors and inhibited their ubiquitylation. The prolonged interaction of RFFL with Rab11 effectors was sufficient to induce the clustered ERC phenotype and to delay cargo recycling. RFFL directly ubiquitylates these Rab11 effectors in vitro, but RFFL knockout (KO) only reduced the ubiquitylation of Rab11-FIP1. RFFL KO had a minimal effect on the ubiquitylation of EHD1, MICALL1, and Rab11-FIP2, and failed to delay transferrin recycling. These results suggest that multiple Ub ligases including RFFL regulate the ubiquitylation of Rab11 effectors, determining the integral function of the ERC.

Trafficking of the human ether-a-go-go-related gene (hERG) potassium channel is regulated by the ubiquitin ligase rififylin (RFFL)

The QT interval is an important diagnostic feature on surface electrocardiograms because it reflects the duration of the ventricular action potential. A previous genome-wide association study has reported a significant linkage between a single-nucleotide polymorphism ∼11.7 kb downstream of the gene encoding the RING finger ubiquitin ligase rififylin (RFFL) and variability in the QT interval. This, along with results in animal studies, suggests that RFFL may have effects on cardiac repolarization. Here, we sought to determine the role of RFFL in cardiac electrophysiology. Adult rabbit cardiomyocytes with adenovirus-expressed RFFL exhibited reduced rapid delayed rectifier current (I _Kr). Neonatal rabbit cardiomyocytes transduced with RFFL-expressing adenovirus exhibited reduced total expression of the potassium channel ether-a-go-go-related gene (rbERG). Using transfections of 293A cells and Western blotting experiments, we observed that RFFL and the core-glycosylated form of the human ether-a-go-go-related gene (hERG) potassium channel interact. Furthermore, RFFL overexpression led to increased polyubiquitination and proteasomal degradation of hERG protein and to an almost complete disappearance of I _Kr, which depended on the intact RING domain of RFFL. Blocking the ER-associated degradation (ERAD) pathway with a dominant-negative form of the ERAD core component, valosin-containing protein (VCP), in 293A cells partially abolished RFFL-mediated hERG degradation. We further substantiated the link between RFFL and ERAD by showing an interaction between RFFL and VCP in vitro We conclude that RFFL is an important regulator of voltage-gated hERG potassium channel activity and therefore cardiac repolarization and that this ubiquitination-mediated regulation requires parts of the ERAD pathway.

Chaperone-Independent Peripheral Quality Control of CFTR by RFFL E3 Ligase

The peripheral protein quality control (QC) system removes non-native membrane proteins, including ΔF508-CFTR, the most common CFTR mutant in cystic fibrosis (CF), from the plasma membrane (PM) for lysosomal degradation by ubiquitination. It remains unclear how unfolded membrane proteins are recognized and targeted for ubiquitination and how they are removed from the apical PM. Using comprehensive siRNA screens, we identified RFFL, an E3 ubiquitin (Ub) ligase that directly and selectively recognizes unfolded ΔF508-CFTR through its disordered regions. RFFL retrieves the unfolded CFTR from the PM for lysosomal degradation by chaperone-independent K63-linked poly-ubiquitination. RFFL ablation enhanced the functional expression of cell-surface ΔF508-CFTR in the presence of folding corrector molecules, and this effect was further improved by inhibiting the Hsc70-dependent ubiquitination machinery. We propose that multiple peripheral QC mechanisms evolved to dispose of non-native PM proteins and to preserve cellular proteostasis, even at the cost of eliminating partially functional polypeptides.

A FYVE zinc finger domain protein specifically links mRNA transport to endosome trafficking

An emerging theme in cellular logistics is the close connection between mRNA and membrane trafficking. A prominent example is the microtubule-dependent transport of mRNAs and associated ribosomes on endosomes. This coordinated process is crucial for correct septin filamentation and efficient growth of polarised cells, such as fungal hyphae. Despite detailed knowledge on the key RNA-binding protein and the molecular motors involved, it is unclear how mRNAs are connected to membranes during transport. Here, we identify a novel factor containing a FYVE zinc finger domain for interaction with endosomal lipids and a new PAM2-like domain required for interaction with the MLLE domain of the key RNA-binding protein. Consistently, loss of this FYVE domain protein leads to specific defects in mRNA, ribosome, and septin transport without affecting general functions of endosomes or their movement. Hence, this is the first endosomal component specific for mRNP trafficking uncovering a new mechanism to couple mRNPs to endosomes.

DOI:http://dx.doi.org/10.7554/eLife.06041.001

DNA contains the instructions to build proteins. These instructions are first copied to make a molecule of messenger RNA (or mRNA for short). A large machine called the ribosome then reads the mRNA molecule and translates it to build a protein.

Many proteins must get to particular locations in a cell to carry out their roles. For some proteins, this is achieved by transporting the mRNAs to the right location before they get translated, via a process called ‘mRNA trafficking’. However, mRNAs do not move by themselves; instead they bind to a host of mRNA-binding proteins, and the ribosomes that are required for translation to take place. Cells also move proteins between different locations using small bubble-like structures called vesicles. These vesicles are surrounded by a membrane, and so this process is known as ‘membrane trafficking’. Previous work has shown that these two processes are often linked, as vesicles can also carry mRNA molecules. But it is not fully understood how mRNA molecules are connected to vesicles.

Now, Pohlmann et al. have used a fungus called Ustilago maydis as a model system to investigate how mRNAs and vesicles can move together in cells that grow to form filament-like structures called hyphae. This fungus uses these filaments to penetrate into plant tissues and causes a disease called corn smut. The experiments revealed a vesicle protein called Upa1 that contains a new type of binding site that allows Upa1 to bring an important RNA-binding protein to the surface of vesicles. Since the RNA-binding protein binds mRNA and the translating ribosomes, this can explain how mRNAs can associate with membranes to move together along hyphae.

When Pohlmann et al. engineered fungi that lacked the gene for Upa1, these mutants had problems transporting their mRNAs and associated ribosomes. These findings reveal a direct connection between mRNA trafficking and membrane trafficking. Future studies could now investigate whether similar processes take place in other cells that grow as long filaments, such as plant pollen tubes or nerve cells. These studies might provide new insights into plant reproduction or brain activity.

DOI:http://dx.doi.org/10.7554/eLife.06041.002

DNA damage-induced activation of ATM promotes β-TRCP-mediated Mdm2 ubiquitination and destruction

The Mdm2 oncoprotein promotes p53 ubiquitination and destruction. Yet, exact molecular mechanisms of Mdm2 destruction itself, under DNA damaging conditions, remain unclear. Recently, we identified SCF^β-TRCP as a novel E3 ligase that targets Mdm2 for ubiquitination and destruction in a Casein Kinase Iδ (CKIδ)-dependent manner. However, it remains elusive how the β-TRCP/CKIδ/Mdm2 signaling axis is regulated by DNA damage signals to govern p53 activity. Consistent with previous studies, we found that inactivation of the Ataxia Telangiectasia Mutated (ATM) kinase, in turn, impaired DNA damage-induced Mdm2 destruction. Although phosphorylation of Mdm2 at Ser395 (an ATM phosphorylation site) facilitated Mdm2 interaction with β-TRCP, Ser395A-Mdm2 was degraded non-distinguishably from WT-Mdm2 by SCF^β-TRCP upon DNA damaging treatments. This indicates that in addition to phosphorylating Mdm2 at Ser395, ATM may govern Mdm2 stability through other unknown mechanisms. We further demonstrated that DNA damage-induced activation of ATM directly phosphorylated CKIδ at two well-conserved S/TQ sites, which promotes CKIδ nuclear localization to increase CKIδ-mediated phosphorylation of Mdm2, thereby facilitating subsequent Mdm2 ubiquitination by SCF^β-TRCP. Our studies provide a molecular mechanism of how ATM could govern DNA damage-induced destruction of Mdm2 in part by phosphorylating both Mdm2 and CKIδ to modulate SCF^β-TRCP–mediated Mdm2 ubiquitination. Given the pivotal role of Mdm2 in the negative regulation of p53, this work will also provide a rationale for developing CKIδ or ATM agonists as anti-cancer agents.

Shigella effector IpaB-induced cholesterol relocation disrupts the Golgi complex and recycling network to inhibit host cell secretion

Shigella infection causes destruction of the human colonic epithelial barrier. The Golgi network and recycling endosomes are essential for maintaining epithelial barrier function. Here we show that Shigella epithelial invasion induces fragmentation of the Golgi complex with consequent inhibition of both secretion and retrograde transport in the infected host cell. Shigella induces tubulation of the Rab11-positive compartment, thereby affecting cell surface receptor recycling. The molecular process underlying the observed damage to the Golgi complex and receptor recycling is a massive redistribution of plasma membrane cholesterol to the sites of Shigella entry. IpaB, a virulence factor of Shigella that is known to bind cholesterol, is necessary and sufficient to induce Golgi fragmentation and reorganization of the recycling compartment. Shigella infection-induced Golgi disorganization was also observed in vivo, suggesting that this mechanism affecting the sorting of cell surface molecules likely contributes to host epithelial barrier disruption associated with Shigella pathogenesis.

Increased Expression of Rififylin in A < 330 Kb Congenic Strain is Linked to Impaired Endosomal Recycling in Proximal Tubules

Cell surface proteins are internalized into the cell through endocytosis and either degraded within lysosomes or recycled back to the plasma membrane. While perturbations in endosomal internalization are known to modulate renal function, it is not known whether similar alterations in recycling affect renal function. Rififylin is a known regulator of endocytic recycling with E3 ubiquitin protein ligase activity. In this study, using two genetically similar strains, the Dahl Salt-sensitive rat and an S.LEW congenic strain, which had allelic variants within a < 330 kb segment containing rififylin, we tested the hypothesis that alterations in endosomal recycling affect renal function. The congenic strain had 1.59-fold higher renal expression of rififylin. Transcriptome analysis indicated that components of both endocytosis and recycling were upregulated in the congenic strain. Transcription of Atp1a1 and cell surface content of the protein product of Atp1a1, the alpha subunit of Na⁺K⁺ATPase were increased in the proximal tubules from the congenic strain. Because rififylin does not directly regulate endocytosis and it is also a differentially expressed gene within the congenic segment, we reasoned that the observed alterations in the transcriptome of the congenic strain constitute a feedback response to the primary functional alteration of recycling caused by rififylin. To test this, recycling of transferrin was studied in isolated proximal tubules. Recycling was significantly delayed within isolated proximal tubules of the congenic strain, which also had a higher level of polyubiquitinated proteins and proteinuria compared with S. These data provide evidence to suggest that delayed endosomal recycling caused by excess of rififylin indirectly affects endocytosis, enhances intracellular protein polyubiquitination and contributes to proteinuria.

PRR5L degradation promotes mTORC2-mediated PKC-δ phosphorylation and cell migration downstream of Gα12

Mammalian target of rapamycin complex (MTORC) 2 phosphorylates AGC protein kinases including PKC and regulates cellular functions including cell migration. However, its regulation remains poorly understood. Here we show that LPA induces two phases of PKCδ hydrophobic motif (HM) phosphorylation. The late phase is mediated by Gα12, which specifically activates ARAF, leading to upregulation of the expression of an E3 ubiquitin ligase RFFL and subsequent ubiquitination and degradation of PRR5L. Destabilization of PRR5L, a suppressor of mTORC2-mediated HM phosphorylation of PKCδ, but not AKT, results in PKCδ HM phosphorylation and activation. This Gα12-mediated pathway is critically important for fibroblast migration and pulmonary fibrosis development. Thus, our study unravels a signaling pathway for mTORC2 regulation and fibroblast migration.

Augmented rififylin is a risk factor linked to aberrant cardiomyocyte function, short-QT interval and hypertension

Using congenic strains of the Dahl salt-sensitive (S) rat introgressed with genomic segments from the normotensive Lewis rat, a blood pressure quantitative trait locus was previously mapped within 104 kb on chromosome 10. The goal of the current study was to conduct extensive phenotypic studies and to further fine-map this locus. At 14 weeks of age, the blood pressure of the congenic rats fed a low-salt diet was significantly higher by 47 mm Hg (P<0.001) compared with that of the S rat. A time-course study showed that the blood pressure effect was significant from very young ages of 50 to 52 days (13 mm Hg; P<0.01). The congenic strain implanted with electrocardiography transmitters demonstrated shorter-QT intervals and increased heart rate compared with S rats (P<0.01). The average survival of the congenic strain was shorter (134 days) compared with the S rat (175 days; P<0.0007). The critical region was narrowed to <42.5 kb containing 171 variants and a single gene, rififylin. Both the mRNA and protein levels of rififylin were significantly higher in the hearts of the congenic strain. Overexpression of rififylin is known to delay endocytic recycling. Endocytic recycling of fluorescently labeled holotransferrin from cardiomyocytes of the congenic strain was slower than that of S rats (P<0.01). Frequency of cardiomyocyte beats in the congenic strain (62±9 bpm) was significantly higher than that of the S rat (24±6 bpm; P<0.001). Taken together, our study provides evidence to suggest that early perturbations in endocytic recycling caused by the overexpression of Rffl is a novel physiological mechanism potentially underlying the development of hypertension.

Ubiquitination and TNFR1 signaling

Death receptors are a subset of the tumor necrosis factor receptor (TNFR) family of proteins and share a characteristic cytoplasmic motif called the "death domain". In addition to mediating cell death, these receptors regulate cell proliferation, inflammatory responses, and tumor progression. Receptor occupancy triggers the assembly of several cytoplasmic molecules into distinct complexes, each initiating separate signaling events leading to different biological responses. Post-translational modifications involving ubiquitin, a peptide of 76 amino acids, regulate events at nearly all stages of signaling. All ubiquitin chains function as docking platforms for molecules with specific recognition motifs that either propagate the signal or target the protein for proteasomal degradation. Moreover, enzymes with ubiquitin thioesterase activity (deubiquitinating enzymes, or DUBs) reverse modifications by removing the ubiquitin chains, allowing ubiquitin editing at the molecular level. Ubiquitin protein ligases (E3s), DUBs, and signaling molecules with ubiquitin recognition motifs control TNFR1 mediated cell death and activation of NF-kappaB and JNK. Here, we discuss the current understanding of how these proteins regulate TNFR1 signaling.

CARP-2 is an endosome-associated ubiquitin ligase for RIP and regulates TNF-induced NF-kappaB activation

BACKGROUND

The proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) elicits cellular responses by signaling through a receptor complex that includes the essential adaptor molecule RIP. One important consequence of signaling is activation of the transcription factor NF-kappaB, and failure to downregulate TNF-induced NF-kappaB transcriptional activity results in chronic inflammation and death. Internalization of the receptor complex plays an important regulatory role in TNF signaling.

RESULTS

We report that CARP-2, a RING domain-containing ubiquitin protein ligase (E3), is a negative regulator of TNF-induced NF-kappaB activation. By virtue of its phospholipid-binding FYVE domain, CARP-2 localized to endocytic vesicles, where it interacted with internalized TNF-receptor complex, resulting in RIP ubiquitination and degradation. Knockdown of CARP-2 stabilized TNFR1-associated polyubiquitinated RIP levels after TNF simulation and enhanced activation of NF-kappaB.

CONCLUSIONS

CARP-2 acts at the level of endocytic vesicles to limit the intensity of TNF-induced NF-kappaB activation by the regulated elimination of a necessary signaling component within the receptor complex.

Three-dimensional structure of the human cytomegalovirus cytoplasmic virion assembly complex includes a reoriented secretory apparatus

Human cytomegalovirus (HCMV) induces profound changes in infected cell morphology, including a large cytoplasmic inclusion that corresponds to the virion assembly complex (AC). In electron micrographs, the AC is a highly vacuolated part of the cytoplasm. Markers of cellular secretory organelles have been visualized at the outer edge of the AC, and we recently showed that a marker for early endosomes (i.e., early endosome antigen 1) localizes to the center of the AC. Here, we examined the relationship between the AC and components of the secretory apparatus, studied temporal aspects of the dramatic infection-induced cytoplasmic remodeling, examined the three-dimensional structure of the AC, and considered the implications of our observations for models of HCMV virion maturation and egress. We made three major observations. First, in addition to being relocated, the expression levels of some organelle markers change markedly during the period while the AC is developing. Second, based on three-dimensional reconstructions from z-series confocal microscopic images, the observed concentric rings of vesicles derived from the several compartments (Golgi bodies, the trans-Golgi network [TGN], and early endosomes) are arranged as nested cylinders of organelle-specific vesicles. Third, the membrane protein biosynthetic and exocytic pathways from the endoplasmic reticulum to the Golgi bodies, TGN, and early endosomes are in an unusual arrangement that nonetheless allows for a conventional order of biosynthesis and transport. Our model of AC structure suggests a mechanism by which the virus can regulate the order of tegument assembly.

Shared as well as distinct roles of EHD proteins revealed by biochemical and functional comparisons in mammalian cells and C. elegans

Background

The four highly homologous human EHD proteins (EHD1-4) form a distinct subfamily of the Eps15 homology domain-containing protein family and are thought to regulate endocytic recycling. Certain members of this family have been studied in different cellular contexts; however, a lack of concurrent analyses of all four proteins has impeded an appreciation of their redundant versus distinct functions.

Results

Here, we analyzed the four EHD proteins both in mammalian cells and in a cross-species complementation assay using a C. elegans mutant lacking the EHD ortholog RME-1. We show that all human EHD proteins rescue the vacuolated intestinal phenotype of C. elegans rme-1 mutant, are simultaneously expressed in a panel of mammalian cell lines and tissues tested, and variably homo- and hetero-oligomerize and colocalize with each other and Rab11, a recycling endosome marker. Small interfering RNA (siRNA) knock-down of EHD1, 2 and 4, and expression of dominant-negative EH domain deletion mutants showed that loss of EHD1 and 3 (and to a lesser extent EHD4) but not EHD2 function retarded transferrin exit from the endocytic recycling compartment. EH domain deletion mutants of EHD1 and 3 but not 2 or 4, induced a striking perinuclear clustering of co-transfected Rab11. Knock-down analyses indicated that EHD1 and 2 regulate the exit of cargo from the recycling endosome while EHD4, similar to that reported for EHD3 (Naslavsky et al. (2006) Mol. Biol. Cell 17, 163), regulates transport from the early endosome to the recycling endosome.

Conclusion

Altogether, our studies suggest that concurrently expressed human EHD proteins perform shared as well as discrete functions in the endocytic recycling pathway and lay a foundation for future studies to identify and characterize the molecular pathways involved.

Endocytic recycling compartments altered in cisplatin-resistant cancer cells

The clinical utility of cisplatin to treat human malignancies is often limited by the development of drug resistance. We have previously shown that cisplatin-resistant human KB adenocarcinoma cells that are cross-resistant to methotrexate and heavy metals have altered endocytic recycling. In this work, we tracked lipids in the endocytic recycling compartment (ERC) and found that the distribution of the ERC is altered in KB-CP.5 cells compared with parental KB-3-1 cells. A tightly clustered ERC is located near the nucleus in parental KB-3-1 cells but it appears loosely arranged and widely dispersed throughout the cytoplasm in KB-CP.5 cells. The altered distribution of the ERC in KB-CP.5 cells is related to the amount and distribution of stable detyrosinated microtubules (Glu-alpha-tubulin), as previously shown in Chinese hamster ovary B104-5 cells that carry a temperature-sensitive Glu-alpha-tubulin allele. In addition, B104-5 cells with a dispersed ERC under nonpermissive conditions were more resistant to cisplatin compared with B104-5 cells with a clustered ERC under permissive conditions. We conclude that resistance to cisplatin might be due, in part, to reduced uptake of cisplatin resulting from an endocytic defect reflecting defective formation of the ERC, possibly related to a shift in the relative amounts and distributions of stable microtubules.

A Cell-Surface Receptor for Lipocalin 24p3 Selectively Mediates Apoptosis and Iron Uptake

The lipocalin mouse 24p3 has been implicated in diverse physiological processes, including apoptosis due to interleukin-3 (IL-3) deprivation and iron transport. Here we report cloning of the 24p3 cell-surface receptor (24p3R). Ectopic 24p3R expression confers on cells the ability to undergo either iron uptake or apoptosis, dependent upon the iron content of the ligand: Iron-loaded 24p3 increases intracellular iron concentration without promoting apoptosis; iron-lacking 24p3 decreases intracellular iron levels, which induces expression of the proapoptotic protein Bim, resulting in apoptosis. Intracellular iron delivery blocks Bim induction and suppresses apoptosis due to 24p3 addition or IL-3 deprivation. We find, unexpectedly, that the BCR-ABL oncoprotein activates expression of 24p3 and represses 24p3R expression, rendering BCR-ABL(+) cells refractory to secreted 24p3. By inhibiting BCR-ABL, imatinib induces 24p3R expression and, consequently, apoptosis. Our results reveal an unanticipated role for intracellular iron regulation in an apoptotic pathway relevant to BCR-ABL-induced myeloproliferative disease and its treatment.