Guanylate-binding protein 1 acts as a pro-viral factor for the life cycle of hepatitis C virus.

Viral infections trigger the expression of interferons (IFNs) and interferon stimulated genes (ISGs), which are crucial to modulate an antiviral response. The human guanylate binding protein 1 (GBP1) is an ISG and exhibits antiviral activity against several viruses. In a previous study, GBP1 was described to impair replication of the hepatitis C virus (HCV). However, the impact of GBP1 on the HCV life cycle is still enigmatic. To monitor the expression and subcellular distribution of GBP1 and HCV we performed qPCR, Western blot, CLSM and STED microscopy, virus titration and reporter gene assays. In contrast to previous reports, we observed that HCV induces the expression of GBP1. Further, to induce GBP1 expression, the cells were stimulated with IFNγ. GBP1 modulation was achieved either by overexpression of GBP1-Wt or by siRNA-mediated knockdown. Silencing of GBP1 impaired the release of viral particles and resulted in intracellular HCV core accumulation, while overexpression of GBP1 favored viral replication and release. CLSM and STED analyses revealed a vesicular distribution of GBP1 in the perinuclear region. Here, it colocalizes with HCV core around lipid droplets, where it acts as assembly platform and thereby favors HCV morphogenesis and release. Collectively, our results identify an unprecedented function of GBP1 as a pro-viral factor. As such, it is essential for viral assembly and release acting through tethering factors involved in HCV morphogenesis onto the surface of lipid droplets.

Identification of the interferon-inducible GTPase GBP1 as major restriction factor for the Hepatitis E virus.

This study aims to gain deeper insight into HEV-induced innate immunity by characterizing the crosstalk between the virus and the host factor guanylate-binding protein 1 (GBP1). We observe that the amount of GBP1 is elevated upon infection, although number of transcripts is decreased, which is explained by a prolonged protein half-life. Modulation of GBP1 levels via overexpression significantly inhibits the viral life cycle. Use of various GBP-1 mutants revealed that the antiviral effect of GBP-1 on HEV is independent from the GTPase-activity, but depends on the capacity of GBP-1 to form GBP1 homodimers. This connects GBP-1 to the autophagosomal pathway. Indeed, dimerization competent GBP1 targets the viral capsid protein to the lysosomal compartment leading to inactivation of the viral particle. Most importantly, silencing of GBP1 abolishes the antiviral effect of IFNγ on HEV. In IFNγ treated cells the virus is targeted to lysosomal structures and destroyed therein. This process depends in part on GBP1. These observations about the relevance of GBP1 for type II interferon-mediated innate immunity against HEV could be a base for tailoring novel antivirals and improvement of disease management.IMPORTANCE Although HEV represents a worldwide public health problem with 20 million infections and 44.000 death cases per year, there are still no specific antivirals available and many aspects of the viral life cycle are not well understood. Here we identify the guanylate binding protein 1 (GBP1) as a restriction factor affecting life cycle of HEV. Surprisingly, the antiviral effect of GBP1 does not depend on its GTPase function, but on its capacity to homodimerize. We revealed that GBP1 exerts its antiviral activity by targeting HEV to the lysosomal compartment where the virus is inactivated. Most importantly, we observed that the antiviral effect of interferon-γ on HEV strongly depends on GBP1. Our observation that GBP1 impairs HEV and is crucial for the antiviral effect of interferons on HEV extends understanding of host defense-mechanisms. As the interferon-system represents a universal defense-mechanism, our study could help to design novel antivirals targeting.

Arkadia/RNF111 is a SUMO-targeted ubiquitin ligase with preference for substrates marked with SUMO1-capped SUMO2/3 chain.

Modification with SUMO regulates many eukaryotic proteins. Down-regulation of sumoylated forms of proteins involves either their desumoylation, and hence recycling of the unmodified form, or their proteolytic targeting by ubiquitin ligases that recognize their SUMO modification (termed STUbL or ULS). STUbL enzymes such as Uls1 and Slx5-Slx8 in budding yeast or RNF4 and Arkadia/RNF111 in humans bear multiple SUMO interaction motifs to recognize substrates carrying poly-SUMO chains. Using yeast as experimental system and isothermal titration calorimetry, we here show that Arkadia specifically selects substrates carrying SUMO1-capped SUMO2/3 hybrid conjugates and targets them for proteasomal degradation. Our data suggest that a SUMO1-specific binding site in Arkadia with sequence similarity to a SUMO1-binding site in DPP9 is required for targeting endogenous hybrid SUMO conjugates and PML nuclear bodies in human cells. We thus characterize Arkadia as a STUbL with a preference for substrate proteins marked with distinct hybrid SUMO chains.

Mechanism and chain specificity of RNF216/TRIAD3, the ubiquitin ligase mutated in Gordon Holmes syndrome.

Gordon Holmes syndrome (GDHS) is an adult-onset neurodegenerative disorder characterized by ataxia and hypogonadotropic hypogonadism. GDHS is caused by mutations in the gene encoding the RING-between-RING (RBR)-type ubiquitin ligase RNF216, also known as TRIAD3. The molecular pathology of GDHS is not understood, although RNF216 has been reported to modify several substrates with K48-linked ubiquitin chains, thereby targeting them for proteasomal degradation. We identified RNF216 in a bioinformatical screen for putative SUMO-targeted ubiquitin ligases and confirmed that a cluster of predicted SUMO-interaction motifs (SIMs) indeed recognizes SUMO2 chains without targeting them for ubiquitination. Surprisingly, purified RNF216 turned out to be a highly active ubiquitin ligase that exclusively forms K63-linked ubiquitin chains, suggesting that the previously reported increase of K48-linked chains after RNF216 overexpression is an indirect effect. The linkage-determining region of RNF216 was mapped to a narrow window encompassing the last two Zn-fingers of the RBR triad, including a short C-terminal extension. Neither the SIMs nor a newly discovered ubiquitin-binding domain in the central portion of RNF216 contributes to chain specificity. Both missense mutations reported in GDHS patients completely abrogate the ubiquitin ligase activity. For the R660C mutation, ligase activity could be restored by using a chemical ubiquitin loading protocol that circumvents the requirement for ubiquitin-conjugating (E2) enzymes. This result suggests Arg-660 to be required for the ubiquitin transfer from the E2 to the catalytic cysteine. Our findings necessitate a re-evaluation of the previously assumed degradative role of RNF216 and rather argue for a non-degradative K63 ubiquitination, potentially acting on SUMOylated substrates.

Regulation of innate immune functions by guanylate-binding proteins.

Guanylate-binding proteins (GBP) are a family of dynamin-related large GTPases which are expressed in response to interferons and other pro-inflammatory cytokines. GBPs mediate a broad spectrum of innate immune functions against intracellular pathogens ranging from viruses to bacteria and protozoa. Several binding partners for individual GBPs have been identified and several different mechanisms of action have been proposed depending on the organisms, the cell type and the pathogen used. Many of these anti-pathogenic functions of GBPs involve the recruitment to and the subsequent destruction of pathogen containing vacuolar compartments, the assembly of large oligomeric innate immune complexes such as the inflammasome, or the induction of autophagy. Furthermore, GBPs often cooperate with immunity-related GTPases (IRGs), another family of dynamin-related GTPases, to exert their anti-pathogenic function, but since most IRGs have been lost in the evolution of higher primates, the anti-pathogenic function of human GBPs seems to be IRG-independent. GBPs and IRGs share biochemical and structural properties with the other members of the dynamin superfamily such as low nucleotide affinity and a high intrinsic GTPase activity which can be further enhanced by oligomerisation. Furthermore, GBPs and IRGs can interact with lipid membranes. In the case of three human and murine GBP isoforms this interaction is mediated by C-terminal isoprenylation. Based on cell biological studies, and in analogy to the function of other dynamins in membrane scission events, it has been postulated that both GBPs and IRGs might actively disrupt the outer membrane of pathogen-containing vacuole leading to the detection and destruction of the pathogen by the cytosolic innate immune system of the host. Recent evidence, however, indicates that GBPs might rather function by mediating membrane tethering events similar to the dynamin-related atlastin and mitofusin proteins, which mediate fusion of the ER and mitochondria, respectively. The aim of this review is to highlight the current knowledge on the function of GBPs in innate immunity and to combine it with the recent progress in the biochemical characterisation of this protein family.

Disruption of the plant-specific CFS1 gene impairs autophagosome turnover and triggers EDS1-dependent cell death.

Cell death, autophagy and endosomal sorting contribute to many physiological, developmental and immunological processes in plants. They are mechanistically interconnected and interdependent, but the molecular basis of their mutual regulation has only begun to emerge in plants. Here, we describe the identification and molecular characterization of CELL DEATH RELATED ENDOSOMAL FYVE/SYLF PROTEIN 1 (CFS1). The CFS1 protein interacts with the ENDOSOMAL SORTING COMPLEX REQUIRED FOR TRANSPORT I (ESCRT-I) component ELCH (ELC) and is localized at ESCRT-I-positive late endosomes likely through its PI3P and actin binding SH3YL1 Ysc84/Lsb4p Lsb3p plant FYVE (SYLF) domain. Mutant alleles of cfs1 exhibit auto-immune phenotypes including spontaneous lesions that show characteristics of hypersensitive response (HR). Autoimmunity in cfs1 is dependent on ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1)-mediated effector-triggered immunity (ETI) but independent from salicylic acid. Additionally, cfs1 mutants accumulate the autophagy markers ATG8 and NBR1 independently from EDS1. We hypothesize that CFS1 acts at the intersection of autophagosomes and endosomes and contributes to cellular homeostasis by mediating autophagosome turnover.

Nucleotide-dependent farnesyl switch orchestrates polymerization and membrane binding of human guanylate-binding protein 1.

Dynamin-like proteins (DLPs) mediate various membrane fusion and fission processes within the cell, which often require the polymerization of DLPs. An IFN-inducible family of DLPs, the guanylate-binding proteins (GBPs), is involved in antimicrobial and antiviral responses within the cell. Human guanylate-binding protein 1 (hGBP1), the founding member of GBPs, is also engaged in the regulation of cell adhesion and migration. Here, we show how the GTPase cycle of farnesylated hGBP1 (hGBP1F) regulates its self-assembly and membrane interaction. Using vesicles of various sizes as a lipid bilayer model, we show GTP-dependent membrane binding of hGBP1F In addition, we demonstrate nucleotide-dependent tethering ability of hGBP1F Furthermore, we report nucleotide-dependent polymerization of hGBP1F, which competes with membrane binding of the protein. Our results show that hGBP1F acts as a nucleotide-controlled molecular switch by modulating the accessibility of its farnesyl moiety, which does not require any supportive proteins.

Invited review: Mechanisms of GTP hydrolysis and conformational transitions in the dynamin superfamily.

Dynamin superfamily proteins are multidomain mechano-chemical GTPases which are implicated in nucleotide-dependent membrane remodeling events. A prominent feature of these proteins is their assembly- stimulated mechanism of GTP hydrolysis. The molecular basis for this reaction has been initially clarified for the dynamin-related guanylate binding protein 1 (GBP1) and involves the transient dimerization of the GTPase domains in a parallel head-to-head fashion. A catalytic arginine finger from the phosphate binding (P-) loop is repositioned toward the nucleotide of the same molecule to stabilize the transition state of GTP hydrolysis. Dynamin uses a related dimerization-dependent mechanism, but instead of the catalytic arginine, a monovalent cation is involved in catalysis. Still another variation of the GTP hydrolysis mechanism has been revealed for the dynamin-like Irga6 which bears a glycine at the corresponding position in the P-loop. Here, we highlight conserved and divergent features of GTP hydrolysis in dynamin superfamily proteins and show how nucleotide binding and hydrolysis are converted into mechano-chemical movements. We also describe models how the energy of GTP hydrolysis can be harnessed for diverse membrane remodeling events, such as membrane fission or fusion. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 580-593, 2016.

Structural and Mechanistic Insights into the Regulation of the Fundamental Rho Regulator RhoGDIα by Lysine Acetylation.

Rho proteins are small GTP/GDP-binding proteins primarily involved in cytoskeleton regulation. Their GTP/GDP cycle is often tightly connected to a membrane/cytosol cycle regulated by the Rho guanine nucleotide dissociation inhibitor α (RhoGDIα). RhoGDIα has been regarded as a housekeeping regulator essential to control homeostasis of Rho proteins. Recent proteomic screens showed that RhoGDIα is extensively lysine-acetylated. Here, we present the first comprehensive structural and mechanistic study to show how RhoGDIα function is regulated by lysine acetylation. We discover that lysine acetylation impairs Rho protein binding and increases guanine nucleotide exchange factor-catalyzed nucleotide exchange on RhoA, these two functions being prerequisites to constitute a bona fide GDI displacement factor. RhoGDIα acetylation interferes with Rho signaling, resulting in alteration of cellular filamentous actin. Finally, we discover that RhoGDIα is endogenously acetylated in mammalian cells, and we identify CBP, p300, and pCAF as RhoGDIα-acetyltransferases and Sirt2 and HDAC6 as specific deacetylases, showing the biological significance of this post-translational modification.

Gamma interferon-induced guanylate binding protein 1 is a novel actin cytoskeleton remodeling factor.

Gamma interferon (IFN-γ) regulates immune defenses against viruses, intracellular pathogens, and tumors by modulating cell proliferation, migration, invasion, and vesicle trafficking processes. The large GTPase guanylate binding protein 1 (GBP-1) is among the cellular proteins that is the most abundantly induced by IFN-γ and mediates its cell biologic effects. As yet, the molecular mechanisms of action of GBP-1 remain unknown. Applying an interaction proteomics approach, we identified actin as a strong and specific binding partner of GBP-1. Furthermore, GBP-1 colocalized with actin at the subcellular level and was both necessary and sufficient for the extensive remodeling of the fibrous actin structure observed in IFN-γ-exposed cells. These effects were dependent on the oligomerization and the GTPase activity of GBP-1. Purified GBP-1 and actin bound to each other, and this interaction was sufficient to impair the formation of actin filaments in vitro, as demonstrated by atomic force microscopy, dynamic light scattering, and fluorescence-monitored polymerization. Cosedimentation and band shift analyses demonstrated that GBP-1 binds robustly to globular actin and slightly to filamentous actin. This indicated that GBP-1 may induce actin remodeling via globular actin sequestering and/or filament capping. These results establish GBP-1 as a novel member within the family of actin-remodeling proteins specifically mediating IFN-γ-dependent defense strategies.

Multivalent interactions of the SUMO-interaction motifs in RING finger protein 4 determine the specificity for chains of the SUMO.

RNF4 (RING finger protein 4) is a STUbL [SUMO (small ubiquitin-related modifier)-targeted ubiquitin ligase] controlling PML (promyelocytic leukaemia) nuclear bodies, DNA double strand break repair and other nuclear functions. In the present paper, we describe that the sequence and spacing of the SIMs (SUMO-interaction motifs) in RNF4 regulate the avidity-driven recognition of substrate proteins carrying SUMO chains of variable length.

The GTPase activity of murine guanylate-binding protein 2 (mGBP2) controls the intracellular localization and recruitment to the parasitophorous vacuole of Toxoplasma gondii.

One of the most abundantly IFN-γ-induced protein families in different cell types is the 65-kDa guanylate-binding protein family that is recruited to the parasitophorous vacuole of the intracellular parasite Toxoplasma gondii. Here, we elucidate the relationship between biochemistry and cellular host defense functions of mGBP2 in response to Toxoplasma gondii. The wild type protein exhibits low affinities to guanine nucleotides, self-assembles upon GTP binding, forming tetramers in the activated state, and stimulates the GTPase activity in a cooperative manner. The products of the two consecutive hydrolysis reactions are both GDP and GMP. The biochemical characterization of point mutants in the GTP-binding motifs of mGBP2 revealed amino acid residues that decrease the GTPase activity by orders of magnitude and strongly impair nucleotide binding and multimerization ability. Live cell imaging employing multiparameter fluorescence image spectroscopy (MFIS) using a Homo-FRET assay shows that the inducible multimerization of mGBP2 is dependent on a functional GTPase domain. The consistent results indicate that GTP binding, self-assembly, and stimulated hydrolysis activity are required for physiological localization of the protein in infected and uninfected cells. Ultimately, we show that the GTPase domain regulates efficient recruitment to T. gondii in response to IFN-γ.

Covalent protein modification with ISG15 via a conserved cysteine in the hinge region.

The ubiquitin-like protein ISG15 (interferon-stimulated gene of 15 kDa) is strongly induced by type I interferons and displays antiviral activity. As other ubiquitin-like proteins (Ubls), ISG15 is post-translationally conjugated to substrate proteins by an isopeptide bond between the C-terminal glycine of ISG15 and the side chains of lysine residues in the substrates (ISGylation). ISG15 consists of two ubiquitin-like domains that are separated by a hinge region. In many orthologs, this region contains a single highly reactive cysteine residue. Several hundred potential substrates for ISGylation have been identified but only a few of them have been rigorously verified. In order to investigate the modification of several ISG15 substrates, we have purified ISG15 conjugates from cell extracts by metal-chelate affinity purification and immunoprecipitations. We found that the levels of proteins modified by human ISG15 can be decreased by the addition of reducing agents. With the help of thiol blocking reagents, a mutational analysis and miRNA mediated knock-down of ISG15 expression, we revealed that this modification occurs in living cells via a disulphide bridge between the substrates and Cys78 in the hinge region of ISG15. While the ISG15 activating enzyme UBE1L is conjugated by ISG15 in the classical way, we show that the ubiquitin conjugating enzyme Ubc13 can either be classically conjugated by ISG15 or can form a disulphide bridge with ISG15 at the active site cysteine 87. The latter modification would interfere with its function as ubiquitin conjugating enzyme. However, we found no evidence for an ISG15 modification of the dynamin-like GTPases MxA and hGBP1. These findings indicate that the analysis of potential substrates for ISG15 conjugation must be performed with great care to distinguish between the two types of modification since many assays such as immunoprecipitation or metal-chelate affinity purification are performed with little or no reducing agent present.

Analysis of cellular SUMO and SUMO-ubiquitin hybrid conjugates.

Posttranslational modification of proteins with the small ubiquitin-related modifier (SUMO) has been implicated in many important physiological functions, including the regulation of transcription and DNA repair. In most cases, only a small fraction of the total cellular amounts of a given protein is sumoylated at a certain point in time. Sensitive detection of sumoylated forms of proteins by western blotting is, therefore, an important step in the identification and/or characterization of a protein control by sumoylation. Polysumoylated proteins are recognized and targeted to the proteasome by specific ubiquitin ligases bearing SUMO interaction motifs. Sumoylation itself is reversible by the action of desumoylating enzymes. Their activities cause a rapid loss of SUMO conjugates in most standard cell extracts. To preserve SUMO-protein conjugates, therefore, a preparation of extracts under denaturing conditions is recommended. Here, we describe the application of an alkaline lysis procedure and a western blot protocol for the analysis of SUMO conjugates in yeast and human cells. In addition, we describe the application of another extraction procedure combined with immobilized metal affinity chromatography for the analysis of ubiquitin-SUMO hybrid conjugates from yeast and human cells.

Reconstitution of SUMO-dependent ubiquitylation in vitro.

In eukaryotic cells, most soluble proteins are degraded via the ubiquitin proteasome system. The recognition signal for the proteasome consists of a lysine 48-linked ubiquitin chain which is posttranslationally conjugated to lysine residues in target proteins. This conjugation reaction is mediated by an enzymatic cascade consisting of specific E1, E2, and E3 enzymes. The small ubiquitin-related modifier (SUMO) is conjugated to target proteins via a similar cascade of SUMO-specific enzymes. Contrary to the long-standing assumption that SUMO does not participate in proteolytic targeting, proteasomal inhibition stabilizes both ubiquitin and SUMO conjugates (SCs). This led to the discovery of ubiquitin ligases for SUMO conjugates (ULS proteins or SUMO-targeted ubiquitin ligases) that target SUMOylated proteins for proteasomal degradation. The so far identified ULS proteins each contains a really interesting new gene domain with ubiquitin-E3 ligase activity and several SUMO interaction motifs that noncovalently bind SUMO. In order to identify ULS proteins and characterize their substrates, it is important to reconstitute this reaction in vitro. In this chapter, we describe step-by-step protocols for the production and purification of recombinant SUMOylated substrates as well as their in vitro ubiquitylation by ULS proteins.

SUMO playing tag with ubiquitin.

In addition to being structurally related, the protein modifiers ubiquitin and SUMO (small ubiquitin-related modifier), share a multitude of functional interrelations. These include the targeting of the same attachment sites in certain substrates, and SUMO-dependent ubiquitylation in others. Notably, several cellular processes, including the targeting of repair machinery to DNA damage sites, require the sequential sumoylation and ubiquitylation of distinct substrates. Some proteins promote both modifications. By contrast, the activity of some enzymes that control either sumoylation or ubiquitylation is regulated by the respective other modification. In this review, we summarize recent findings regarding intersections between SUMO and ubiquitin that influence genome stability and cell growth and which are relevant in pathogen resistance and cancer treatment.

Ugo1 and Mdm30 act sequentially during Fzo1-mediated mitochondrial outer membrane fusion.

Dynamin-related GTPase proteins (DRPs) are main players in membrane remodelling. Conserved DRPs called mitofusins (Mfn1/Mfn2/Fzo1) mediate the fusion of mitochondrial outer membranes (OM). OM fusion depends on self-assembly and GTPase activity of mitofusins as well as on two other proteins, Ugo1 and Mdm30. Here, we define distinct steps of the OM fusion cycle using in vitro and in vivo approaches. We demonstrate that yeast Fzo1 assembles into homo-dimers, depending on Ugo1 and on GTP binding to Fzo1. Fzo1 homo-dimers further associate upon formation of mitochondrial contacts, allowing membrane tethering. Subsequent GTP hydrolysis is required for Fzo1 ubiquitylation by the F-box protein Mdm30. Finally, Mdm30-dependent degradation of Fzo1 completes Fzo1 function in OM fusion. Our results thus unravel functions of Ugo1 and Mdm30 at distinct steps during OM fusion and suggest that protein clearance confers a non-cycling mechanism to mitofusins, which is distinct from other cellular membrane fusion events.

Purification of the CaaX-modified, dynamin-related large GTPase hGBP1 by coexpression with farnesyltransferase.

Over a hundred proteins in eukaryotic cells carry a C-terminal CaaX box sequence, which targets them for posttranslational isoprenylation of the cysteine residue. This modification, catalyzed by either farnesyl or geranylgeranyl transferase, converts them into peripheral membrane proteins. Isoprenylation is usually followed by proteolytic cleavage of the aaX tripeptide and methylation of the carboxyl group of the newly exposed isoprenylcysteine. The C-terminal modification regulates the cellular localization and biological activity of isoprenylated proteins. We have established a strategy to produce and purify recombinant farnesylated guanylate-binding protein 1 (hGBP1), a dynamin-related large GTPase. Our system is based on the coexpression of hGBP1 with the two subunits of human farnesyltransferase in Escherichia coli and a chromatographic separation of farnesylated and unmodified protein. Farnesylated hGBP1 displays altered GTPase activity and is able to interact with liposomes in the activated state.