The exonuclease ISG20 mainly localizes in the nucleolus and the Cajal (Coiled) bodies and is associated with nuclear SMN protein‐containing complexes

Proteomics analyses of human nucleoli provided molecular bases for an understanding of the multiple functions fulfilled by these nuclear domains. However, the biological roles of about 100 of the identified proteins are unpredictable. The present study describes the functional characterization of one of these proteins, ISG20L2. We demonstrate that ISG20L2 is a 3 to 5 exoribonuclease involved in ribosome biogenesis at the level of 5.8 S rRNA maturation, more specifically in the processing of the 12 S precursor rRNA. The use of truncated forms of ISG20L2 demonstrated that its N-terminal half promotes the nucleolar localization and suggested that its C-terminal half bears the exoribonuclease activity. Nuclei of eukaryotic cells are highly organized organelles in which more than 30 different kinds of transient structures, called nuclear domains or nuclear bodies, support the nuclear functions (1). Since the 1960s, it has been known that nucleoli, the most prominent of the nuclear domains, are the sites of ribosome biogenesis. This function gives rise to their characteristic ultrastructural organization in three main compartments, the fibrillar center and the dense fibrillar and granular components (2).Ribosome biogenesis is a very complex process that involves the synthesis of ribosomal RNAs (rRNAs) 1 coupled with their extensive processing, the assembly of these rRNAs with ribosomal proteins, and the export of the resulting preribosomal subunits from nuclei to cytoplasm where the final maturation occurs (3). rRNA processing mainly consists of base modifications and cleavages (4). Precursor rRNAs (pre-rRNAs) contain external and internal sequences that are not present within mature rRNAs. Removal of these sequences is achieved through sequential endonucleolytic and exonucleolytic cleavages, producing mature 18 S, 5.8 S, and 28 S rRNAs. Studies in yeast have identified several factors responsible for such cleavages: the ribonucleoprotein complex RNase MRP and Ngl2p catalyzing endoribonucleolytic processing, Xrn1p and Rat1p that are 5Ј to 3Ј exonucleases, Rex1p and Rex2p that are 3Ј to 5Ј exonucleases, and a protein complex named exosome composed of 3Ј to 5Ј exoribonucleases and putative RNA-binding proteins (5-9). However, the list of the catalytic factors required for the complete processing of pre-rRNAs into mature species is still incomplete.Correct rRNA maturation that provides finely folded and packaged mature rRNAs is crucial because it is highly probable that eukaryotic ribosomes are, as it has clearly been demonstrated for prokaryote ribosomes, ribozymes (10, 11). In yeast, incorrectly maturated rRNAs lead to translational infidelity of ribosomes (12). In mammals, defects in pre-rRNA processing are therefore likely to entail the development of severe pathologies. Indeed results on DKC1, the gene altered in dyskeratosis congenita, indicated that altered rRNA processing plays a direct role in tumorigenesis (13). Recently Gleizes and co-workers (14) showed that ribosome biogenesis and notably pre-rRNA processing...

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“…2A), notably Cajal bodies (Fig. 2B), contrary to ISG20 (40). These observations confirm that ISG20L2 is a nucleolar protein.…”

Section: In Silico Prediction Of Potential Functional Domains Of Isg2supporting

confidence: 72%

ISG20L2, a Novel Vertebrate Nucleolar Exoribonuclease Involved in Ribosome Biogenesis

Couté

Kindbeiter²,

Belin

et al. 2008

Molecular & Cellular Proteomics

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“…It has been shown that this exonuclease is a component of the nuclear PML body (21,27). Previous studies demonstrated that ISG20 expression confers resistance to the infection of various RNA viruses, including VSV and human immunodeficiency virus, but not DNA viruses, such as adenoviruses (19,20,68).…”

Section: Discussionmentioning

confidence: 99%

“…ISG20 is a 3Ј-to-5Ј exonuclease with a strong preference for single-stranded RNA and minor activity toward single-stranded DNA (51). According to the bioinformatic analysis and recently published crystal structural data, ISG20 belongs to the DEDD superfamily of 3Ј-to-5Ј exonucleases that is defined by four conserved acidic residues, three aspartates (D) and one glutamate (E), distributed among three separate sequence motifs (Exo I to III) (19,21,34,74). Previous studies demonstrated that replacement of Asp96 in Exo II with a glycine inactivates the enzymatic activity of ISG20 (19).…”

Section: Pkrmentioning

confidence: 99%

Identification of Three Interferon-Inducible Cellular Enzymes That Inhibit the Replication of Hepatitis C Virus

Jiang

Guo

et al. 2008

J Virol

256

299

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Hepatitis C virus (HCV) infection is a common cause of chronic hepatitis and is currently treated with alpha interferon (IFN-␣)-based therapies.However, the underlying mechanism of IFN-␣ therapy remains to be elucidated. To identify the cellular proteins that mediate the antiviral effects of IFN-␣, we created a HEK293-based cell culture system to inducibly express individual interferon-stimulated genes (ISGs) and determined their antiviral effects against HCV. By screening 29 ISGs that are induced in Huh7 cells by IFN-␣ and/or up-regulated in HCV-infected livers, we discovered that viperin, ISG20, and double-stranded RNA-dependent protein kinase (PKR) noncytolytically inhibited the replication of HCV replicons. Mechanistically, inhibition of HCV replication by ISG20 and PKR depends on their 3-5 exonuclease and protein kinase activities, respectively. Moreover, our work, for the first time, provides strong evidence suggesting that viperin is a putative radical S-adenosyl-L-methionine (SAM) enzyme. In addition to demonstrating that the antiviral activity of viperin depends on its radical SAM domain, which contains conserved motifs to coordinate [4Fe-4S] cluster and cofactor SAM and is essential for its enzymatic activity, mutagenesis studies also revealed that viperin requires an aromatic amino acid residue at its C terminus for proper antiviral function. Furthermore, although the N-terminal 70 amino acid residues of viperin are not absolutely required, deletion of this region significantly compromises its antiviral activity against HCV. Our findings suggest that viperin represents a novel antiviral pathway that works together with other antiviral proteins, such as ISG20 and PKR, to mediate the IFN response against HCV infection. Hepatitis C virus (HCV) is the sole member of the genusHepacivirus in the family Flaviviridae (43). It establishes persistent infections in the vast majority of infected individuals and is the only known positive-stranded RNA virus that causes persistent life-long infections in humans. Currently, HCV chronically infects more than 170 million people worldwide. Although the initial infection is largely asymptomatic, prolonged infection carries a high risk of chronic hepatitis, cirrhosis, and primary hepatocellular carcinoma (2).Although it has been elegantly demonstrated that HCV can evade the host cellular innate defense response through proteolytic cleavage of RIG-I/MDA5 adaptor protein MAVS and Tolllike receptor 3 adaptor protein TRIF (7,22,25,42,44,48,69), microarray studies performed with liver samples obtained from HCV transiently infected chimpanzees and chronically infected humans revealed that the induction of interferon (IFN)-stimulated genes (ISGs) in HCV-infected livers is a hallmark of the virus infection (5,6,33,39,58,61). These discoveries suggest that the HCV-infected liver is a constant battlefield between the virus and host innate immunity defense systems, and thus IFN-mediated innate responses induced by HCV may play an important role in shaping the pathogenesis and clini...

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“…However, ISG20 can also be expressed at high levels in peripheral blood leukocytes, lymphoid tissues (such as spleen or thymus), the colon, and the lungs without exogenous IFN treatment (2). When the location of ISG20 protein was assessed immunohistochemically in human HeLa and lymphoblastoid Daudi cells, diffuse cytoplasmic and nucleoplasmic localization was observed, but ISG20 also appeared in the nucleus, including the nucleolus and Cajal bodies (CBs) (14). Further, electron microcopy analysis revealed that ISG20 was principally concentrated in the dense fibrillar component of the nucleolus, the major site for rRNA processing (14).…”

Section: Expressionmentioning

confidence: 99%

“…When the location of ISG20 protein was assessed immunohistochemically in human HeLa and lymphoblastoid Daudi cells, diffuse cytoplasmic and nucleoplasmic localization was observed, but ISG20 also appeared in the nucleus, including the nucleolus and Cajal bodies (CBs) (14). Further, electron microcopy analysis revealed that ISG20 was principally concentrated in the dense fibrillar component of the nucleolus, the major site for rRNA processing (14). Similarly, laser confocal microscopy detected porcine ISG20 primarily in the nucleus, with only a small amount in the cytoplasm (15).…”

Section: Expressionmentioning

confidence: 99%

Interferon-stimulated gene 20-kDa protein (ISG20) in infection and disease: Review and outlook

Zheng

Wang

Pan

2017

IRDR

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The exonuclease ISG20 mainly localizes in the nucleolus and the Cajal (Coiled) bodies and is associated with nuclear SMN protein‐containing complexes

Cited by 30 publications

References 89 publications

ISG20L2, a Novel Vertebrate Nucleolar Exoribonuclease Involved in Ribosome Biogenesis

ISG20L2, a Novel Vertebrate Nucleolar Exoribonuclease Involved in Ribosome Biogenesis

Identification of Three Interferon-Inducible Cellular Enzymes That Inhibit the Replication of Hepatitis C Virus

Interferon-stimulated gene 20-kDa protein (ISG20) in infection and disease: Review and outlook

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