Natural occurrence of 2-5A in interferon-treated EMC virus-infected L cells

TRIM7 inhibits encephalomyocarditis virus replication by activating interferon-β signaling pathway

Tripartite motif-containing protein 7 (TRIM7), the member of tripartite motif (TRIM) family, plays an important role in innate immune responses against viral infection. Among them, the function of TRIM7 in Encephalomyocarditis virus (EMCV) infection has not been reported. Here, we found that TRIM7 inhibited the replication of EMCV through the type I interferon (IFN) signaling pathway. Interestingly, TRIM7 was down-regulated after EMCV infection in HEK293T cells. Further, overexpression of TRIM7 suppressed the replication of EMCV in HEK293T cells and enhanced the activity of IFN-β promoter. On the other hand, knockdown of the endogenous TRIM7 promoted EMCV infection and impaired the activity of IFN-β promoter. TRIM7 could regulate retinoic acid-inducible gene I (RIG-I)/ melanoma differentiation-associated gene 5 (MDA5)/ mitochondrial antiviral-signaling protein (MAVS) mediated IFN-β signaling pathway. Moreover, TRIM7 interacted with MAVS and they were co-located in HEK293T cells. We demonstrate that TRIM7 plays a positive role in IFN-β signaling pathway during EMCV infection and suppresses EMCV replication. Taken together, the presented results suggest that TRIM7 has a pivotal function in anti-EMCV infection, thereby providing a potential target for further development of anti-EMCV inhibitors.

Mitochondrial C11orf83 is a potent Antiviral Protein Independent of interferon production

Mitochondria have a central position in innate immune response via the adaptor protein MAVS in mitochondrial outer membrane to limit viral replication by inducing interferon production. Here, we reported that C11orf83, a component of complex III of electronic transfer chain in mitochondrial inner membrane, was a potent antiviral protein independent of interferon production. C11orf83 expression significantly increased in response to viral infection, and endows cells with stronger capability of inhibiting viral replication. Deletion of C11orf83 permits viral replication easier and cells were more vulnerable to viral killing. These effects mainly were mediated by triggering OAS3-RNase L system. C11orf83 overexpression induced higher transcription of OAS3, and knockdown either OAS3 or RNase L impaired the antiviral capability of C11orf83. Interestingly, the signaling from C11orf83 to OAS3-RNase L was independent of interferon production. Thus, our findings suggested a new antiviral mechanism by bridging cell metabolic machinery component with antiviral effectors.

The Roles of RNase-L in Antimicrobial Immunity and the Cytoskeleton-Associated Innate Response

The interferon (IFN)-regulated endoribonuclease RNase-L is involved in multiple aspects of the antimicrobial innate immune response. It is the terminal component of an RNA cleavage pathway in which dsRNA induces the production of RNase-L-activating 2-5A by the 2′-5′-oligoadenylate synthetase. The active nuclease then cleaves ssRNAs, both cellular and viral, leading to downregulation of their expression and the generation of small RNAs capable of activating retinoic acid-inducible gene-I (RIG-I)-like receptors or the nucleotide-binding oligomerization domain-like receptor 3 (NLRP3) inflammasome. This leads to IFNβ expression and IL-1β activation respectively, in addition to broader effects on immune cell function. RNase-L is also one of a growing number of innate immune components that interact with the cell cytoskeleton. It can bind to several cytoskeletal proteins, including filamin A, an actin-binding protein that collaborates with RNase-L to maintain the cellular barrier to viral entry. This antiviral activity is independent of catalytic function, a unique mechanism for RNase-L. We also describe here the interaction of RNase-L with the E3 ubiquitin ligase and scaffolding protein, ligand of nump protein X (LNX), a regulator of tight junction proteins. In order to better understand the significance and context of these novel binding partners in the antimicrobial response, other innate immune protein interactions with the cytoskeleton are also discussed.

Structural and functional analysis reveals that human OASL binds dsRNA to enhance RIG-I signaling

The oligoadenylate synthetase (OAS) enzymes are cytoplasmic dsRNA sensors belonging to the antiviral innate immune system. Upon binding to viral dsRNA, the OAS enzymes synthesize 2'-5' linked oligoadenylates (2-5As) that initiate an RNA decay pathway to impair viral replication. The human OAS-like (OASL) protein, however, does not harbor the catalytic activity required for synthesizing 2-5As and differs from the other human OAS family members by having two C-terminal ubiquitin-like domains. In spite of its lack of enzymatic activity, human OASL possesses antiviral activity. It was recently demonstrated that the ubiquitin-like domains of OASL could substitute for K63-linked poly-ubiquitin and interact with the CARDs of RIG-I and thereby enhance RIG-I signaling. However, the role of the OAS-like domain of OASL remains unclear. Here we present the crystal structure of the OAS-like domain, which shows a striking similarity with activated OAS1. Furthermore, the structure of the OAS-like domain shows that OASL has a dsRNA binding groove. We demonstrate that the OAS-like domain can bind dsRNA and that mutating key residues in the dsRNA binding site is detrimental to the RIG-I signaling enhancement. Hence, binding to dsRNA is an important feature of OASL that is required for enhancing RIG-I signaling.

The 2'-5'-oligoadenylate synthetase 3 enzyme potently synthesizes the 2'-5'-oligoadenylates required for RNase L activation

The members of the oligoadenylate synthetase (OAS) family of proteins are antiviral restriction factors that target a wide range of RNA and DNA viruses. They function as intracellular double-stranded RNA (dsRNA) sensors that, upon binding to dsRNA, undergo a conformational change and are activated to synthesize 2'-5'-linked oligoadenylates (2-5As). 2-5As of sufficient length act as second messengers to activate RNase L and thereby restrict viral replication. We expressed human OAS3 using the baculovirus system and purified it to homogeneity. We show that recombinant OAS3 is activated at a substantially lower concentration of dsRNA than OAS1, making it a potent in vivo sensor of dsRNA. Moreover, we find that OAS3 synthesizes considerably longer 2-5As than previously reported, and that OAS3 can activate RNase L intracellularly. The combined high affinity for dsRNA and the capability to produce 2-5As of sufficient length to activate RNase L suggests that OAS3 is a potent activator of RNase L. In addition, we provide experimental evidence to support one active site of OAS3 located in the C-terminal OAS domain and generate a low-resolution structure of OAS3 using SAXS.

IMPORTANCE

We are the first to purify the OAS3 enzyme to homogeneity, which allowed us to characterize the mechanism utilized by OAS3 and identify the active site. We provide compelling evidence that OAS3 can produce 2'-5'-oligoadenylates of sufficient length to activate RNase L. This is contrary to what is described in the current literature but agrees with recent in vivo data showing that OAS3 harbors an antiviral activity requiring RNase L. Thus, our work redefines our understanding of the biological role of OAS3. Furthermore, we used a combination of mutagenesis and small-angle X-ray scattering to describe the active site and low-resolution structure of OAS3.

Role of ribonuclease L in viral pathogen-associated molecular pattern/influenza virus and cigarette smoke-induced inflammation and remodeling

Interactions between cigarette smoke (CS) exposure and viral infection play an important role(s) in the pathogenesis of chronic obstructive pulmonary disease and a variety of other disorders. A variety of lines of evidence suggest that this interaction induces exaggerated inflammatory, cytokine, and tissue remodeling responses. We hypothesized that the 2'-5' oligoadenylate synthetase (OAS)/RNase L system, an innate immune antiviral pathway, plays an important role in the pathogenesis of these exaggerated responses. To test this hypothesis, we characterize the activation of 2'-5' OAS in lungs from mice exposed to CS and viral pathogen-associated molecular patterns (PAMPs)/live virus, alone and in combination. We also evaluated the inflammatory and remodeling responses induced by CS and virus/viral PAMPs in lungs from RNase L null and wild-type mice. These studies demonstrate that CS and viral PAMPs/live virus interact in a synergistic manner to stimulate the production of select OAS moieties. They also demonstrate that RNase L plays a critical role in the pathogenesis of the exaggerated inflammatory, fibrotic, emphysematous, apoptotic, TGF-β1, and type I IFN responses induced by CS plus virus/viral PAMP in combination. These studies demonstrate that CS is an important regulator of antiviral innate immunity, highlight novel roles of RNase L in CS plus virus induced inflammation, tissue remodeling, apoptosis, and cytokine elaboration and highlight pathways that may be operative in chronic obstructive pulmonary disease and mechanistically related disorders.

Pathologic effects of RNase-L dysregulation in immunity and proliferative control

The endoribonuclease RNase-L is the terminal component of an RNA cleavage pathway that mediates antiviral, antiproliferative and immunomodulatory activities. Inactivation or dysregulation of RNase-L is associated with a compromised immune response and increased risk of cancer, accordingly its activity is tightly controlled and requires an allosteric activator, 2',5'-linked oligoadenylates, for enzymatic activity. The biological activities of RNase-L are a result of direct and indirect effects of RNA cleavage and microarray analyses have revealed that RNase-L impacts the gene expression program at multiple levels. The identification of RNase-L-regulated RNAs has provided insights into potential mechanisms by which it exerts antiproliferative, proapoptotic, senescence-inducing and innate immune activities. RNase-L protein interactors have been identified that serve regulatory functions and are implicated as alternate mechanisms of its biologic functions. Thus, while the molecular details are understood for only a subset of RNase-L activities, its regulation by small molecules and critical roles in host defense and as a candidate tumor suppressor make it a promising therapeutic target.

Activation of human RNase L by 2'- and 5'-O-methylphosphonate-modified oligoadenylates

To determine the influence of internucleotide linkage and sugar ring conformation, and the role of 5'-terminal phosphate, on the activation of human RNase L, a series of 2'- and 5'-O-methylphosphonate-modified tetramers were synthesized from appropriate monomeric units and evaluated for their ability to activate human RNase L. Tetramers pAAAp(c)X modified by ribo, arabino or xylo 5'-phosphonate unit p(c)X activated RNase L with efficiency comparable to that of natural activator. Moreover, incorporation of phosphonate linkages ensured the stability against cleavage by nucleases. The substitution of 5'-terminal phosphate for 5'-terminal phosphonate in tetramer p(c)XAAA afforded tetramers with excellent activation efficiency and with complete stability against cleavage by phosphomonoesterases.

Extracellular 2'-5' oligoadenylate synthetase stimulates RNase L-independent antiviral activity: a novel mechanism of virus-induced innate immunity

The 2'-5' oligoadenylate synthetase (OAS) proteins are traditionally considered intracellular antiviral proteins. However, several studies demonstrate a correlation between the concentration of freely circulating OAS protein in sera from hepatitis C patients and their clinical prognosis. Here we demonstrate that extracellular OAS1 enters into cells and possesses a strong antiviral activity, both in vitro and in vivo, which is independent of RNase L. The OAS protein directly inhibits viral proliferation and does not require the activation of known antiviral signaling pathways. We propose that OAS produced by cells infected with viruses is released to the extracellular space, where it acts as a paracrine antiviral agent. Thus, the OAS protein represents the first direct antiviral compound released by virus-infected cells.

Ribosomal protein mRNAs are primary targets of regulation in RNase-L-induced senescence

The endoribonuclease RNase-L requires 2',5'-linked oligoadenylates for activation, and mediates antiviral and antiproliferative activities. We previously determined that RNase-L activation induces senescence; to determine potential mechanisms underlying this activity, we used microarrays to identify RNase-L-regulated mRNAs. RNase-L activation affected affected a finite number of transcripts, and thus does not lead to a global change in mRNA turnover. The largest classes of downregulated transcripts, that represent candidate RNase-L substrates, function in protein biosynthesis, metabolism and proliferation. Among these, mRNAs encoding ribosomal proteins (RPs) were particularly enriched. The reduced levels of four RP mRNAs corresponded with a decrease in their half lives and a physical association with an RNase-L-ribonucleoprotein (RNP) complex in cells, suggesting that they represent authentic RNase-L substrates. Sequence and structural analysis of the downregulated mRNAs identified a putative RNase-L target motif that was used for the in silico identification of a novel RNase-L-RNP-interacting transcript. The downregulation of RP mRNAs corresponded with a marked reduction in protein translation, consistent with the roles of RP proteins in ribosome function. Our data support a model in which the RNase-L-mediated degradation of RP mRNAs inhibits translation, and may contribute to its antiproliferative, senescence inducing and tumor suppressor activities.

A scientific journey through the 2-5A/RNase L system

The antiviral and antitumor actions of interferons are caused, in part, by a remarkable regulated RNA cleavage pathway known as the 2-5A/RNase L system. 2'-5' linked oligoadenylates (2-5A) are produced from ATP by interferon-inducible synthetases. 2-5A activates pre-existing RNase L, resulting in the cleavage of RNAs within single-stranded regions. Activation of RNase L by 2-5A leads to an antiviral response, although precisely how this happens is a subject of ongoing investigations. Recently, RNase L was identified as the hereditary prostate cancer 1 gene. That finding has led to the discovery of a novel human retrovirus, XMRV. My scientific journey through the 2-5A system recounts some of the highlights of these efforts. Knowledge gained from studies on the 2-5A system could have an impact on development of therapies for important viral pathogens and cancer.

Diverse functions of RNase L and implications in pathology

The endoribonuclease L (RNase L) is the effector of the 2-5A system, a major enzymatic pathway involved in the molecular mechanism of interferons (IFNs). RNase L is a very unusual nuclease with a complex mechanism of regulation. It is a latent enzyme, expressed in nearly every mammalian cell type. Its activation requires its binding to a small oligonucleotide, 2-5A. 2-5A is a series of unique 5'-triphosphorylated oligoadenylates with 2'-5' phosphodiester bonds. By regulating viral and cellular RNA expression, RNase L plays an important role in the antiviral and antiproliferative activities of IFN and contributes to innate immunity and cell metabolism. The 2-5A/RNase L pathway is implicated in mediating apoptosis in response to viral infections and to several types of external stimuli. Several recent studies have suggested that RNase L could have a role in cancer biology and evidence of a tumor suppressor function of RNase L has emerged from studies on the genetics of hereditary prostate cancer.

The human 2'-5'oligoadenylate synthetase family: unique interferon-inducible enzymes catalyzing 2'-5' instead of 3'-5' phosphodiester bond formation

The demonstration by Kerr and colleagues that double-stranded (ds) RNA inhibits drastically protein synthesis in cell-free systems prepared from interferon-treated cells, suggested the existence of an interferon-induced enzyme, which is dependent on dsRNA. Consequently, two distinct dsRNA-dependent enzymes were discovered: a serine/threonine protein kinase that nowadays is referred to as PKR and a 2'-5'oligoadenylate synthetase (2'-5'OAS) that polymerizes ATP to 2'-5'-linked oligomers of adenosine with the general formula pppA(2'p5'A)(n), n>or=1. The product is pppG2'p5'G when GTP is used as a substrate. Three distinct forms of 2'-5'OAS exist in human cells, small, medium, and large, which contain one, two, and three OAS units, respectively, and are encoded by distinct genes clustered on the 2'-5'OAS locus on human chromosome 12. OASL is an OAS like IFN-induced protein encoded by a gene located about 8 Mb telomeric from the 2'-5'OAS locus. OASL is composed of one OAS unit fused at its C-terminus with two ubiquitin-like repeats. The human OASL is devoid of the typical 2'-5'OAS catalytic activity. In addition to these structural differences between the various OAS proteins, the three forms of 2'-5'OAS are characterized by different subcellular locations and enzymatic parameters. These findings illustrate the apparent structural and functional complexity of the human 2'-5'OAS family, and suggest that these proteins may have distinct roles in the cell.

RNase L plays a role in the antiviral response to West Nile virus

Alleles at the Flv locus determine disease outcome after a flavivirus infection in mice. Although comparable numbers of congenic resistant and susceptible mouse embryo fibroblasts (MEFs) are infected by the flavivirus West Nile virus (WNV), resistant MEFs produce approximately 100- to 150-fold lower titers than susceptible ones and flavivirus titers in the brains of resistant and susceptible animals can differ by >10,000-fold. The Flv locus was previously identified as the 2'-5' oligoadenylate synthetase 1b (Oas1b) gene. Oas gene expression is up-regulated by interferon (IFN), and after activation by double-stranded RNA, some mouse synthetases produce 2-5A, which activates latent RNase L to degrade viral and cellular RNAs. To determine whether the lower levels of intracellular flavivirus genomic RNA from resistant mice detected in cells at all times after infection were mediated by RNase L, RNase L activity levels in congenic resistant and susceptible cells were compared. Similar moderate levels of RNase L activation by transfected 2-5A were observed in both types of uninfected cells. After WNV infection, the mRNAs of IFN-beta and three Oas genes were up-regulated to similar levels in both types of cells. However, significant levels of RNase L activity were not detected until 72 h after WNV infection and the patterns of viral RNA cleavage products generated were similar in both types of cells. When RNase L activity was down-regulated in resistant cells via stable expression of a dominant negative RNase L mutant, approximately 5- to 10-times-higher yields of WNV were produced. Similarly, about approximately 5- to 10-times-higher virus yields were produced by susceptible C57BL/6 RNase L-/- cells compared to RNase L+/+ cells that were either left untreated or pretreated with IFN and/or poly(I) . poly(C). The data indicate that WNV genomic RNA is susceptible to RNase L cleavage and that RNase L plays a role in the cellular antiviral response to flaviviruses. The results suggest that RNase L activation is not a major component of the Oas1b-mediated flavivirus resistance phenotype.

Anti-mitogenic function of interferon-induced (2'-5')oligo(adenylate) and growth-related variations in enzymes that synthesize and degrade this oligonucleotide

Addition of (2'5')ApApA to concanavalin-A-stimulated mouse spleen lymphocytes strongly inhibits the large increase in RNA and protein synthesis which takes place 24-48 h after stimulation. The inhibitory effect on protein synthesis precedes the effect on RNA synthesis and takes at least 6 h to be detected. Histone synthesis is preferentially inhibited at 48 h. No effect on protein synthesis was detected in unstimulated resting lymphocytes, or in stimulated lymphocytes during the first 24 h after concanavalin A treatment. The anti-mitogenic effect of the (2'-5')oligo(adenylate) seems to result, therefore, from inhibition of protein synthesis taking place before initiation of DNA replication. The mitogenic stimulus produced by the lectin enhances, in lymphocytes, the level of the 2'-phosphodiesterase which degrades (2'-5')oligo(adenylate). Enhancement of the 2'-phosphodiesterase was also observed after serum stimulation of confluent monkey kidney cells. Furthermore, the ratio of (2'-5')oligo(adenylate) synthetase to 2'-phosphodiesterase is ten-times lower in fast-growing kidney cells than in quiescent serum-starved cells. A model for the role of (2'-5')oligo(adenylate) synthesis and degradation in the regulation of cell proliferation by interferon and by mitogens is presented.

2',5'-Oligoadenylate size is critical to protect RNase L against proteolytic cleavage in chronic fatigue syndrome

A dysregulation in the 2',5'-oligoadenylate (2-5A)-dependent RNase L antiviral pathway has been detected in peripheral blood mononuclear cells (PBMC) of chronic fatigue syndrome (CFS) patients, which is characterized by upregulated 2-5A synthetase and RNase L activities, as well as by the presence of a low molecular weight (LMW) 2-5A-binding protein of 37-kDa related to RNase L. This truncated protein has been shown to originate from proteolytic cleavage of the native 83-kDa RNase L by m-calpain and human leukocyte elastase (HLE). We investigated the possible role of 2-5A oligomers in the proteolytic action toward the endonuclease and show that incubation of CFS PBMC extracts with 2-5A trimer and tetramer, but not with the dimer, results in a significant protection of the native 83-kDa RNase L against cleavage by endogenous and purified proteases. Similar results are obtained with a purified recombinant RNase L. An analysis of the size of 2-5A oligomers produced by the catalytic activity of the 2-5A synthetase present in PBMC extracts further shows that samples containing the 37-kDa RNase L preferentially produce 2-5A dimers instead of higher oligomers. Taken together, our results indicate that homodimerization of RNase L by 2-5A oligomers higher than the dimer prevents its cleavage by proteolytic enzymes. The presence of the truncated 37-kDa RNase L in PBMC extracts is therefore likely to result, not only from the abnormal activation of inflammatory proteases, but also from a dysregulation in 2-5A synthetase induction or activation towards the preferential production of 2-5A dimers.

Blockade of interferon induction and action by the E3L double-stranded RNA binding proteins of vaccinia virus

The vaccinia virus E3L gene encodes two double-stranded RNA binding proteins that promote viral growth and pathogenesis through suppression of innate immunity. To explore how E3L enables vaccinia virus to evade the interferon system, cells and mice deficient in the principal interferon-regulated antiviral enzymes, PKR and RNase L, were infected with wild-type vaccinia virus and strains of vaccinia virus from which E3L had been deleted (E3L-deleted strains). While wild-type virus was unaffected by RNase L and PKR, virus lacking E3L replicated only in the deficient cells. Nevertheless, E3L-deleted virus failed to replicate to high titers or to cause significant morbidity or mortality in triply deficient mice lacking RNase L, PKR, and Mx1. To investigate the underlying cause, we determined the effect of E3L on interferon regulatory factor 3 (IRF3), a transcription factor required for viral induction of subtypes of type I interferons. Results showed that IRF3 activation and interferon-beta induction occurred after infections with E3L-deleted virus but not with wild-type virus. These findings demonstrate that E3L plays an essential role in the pathogenesis of vaccinia virus by blocking the interferon system at multiple levels. Furthermore, our results indicate the existence of an interferon-mediated antipoxvirus pathway that operates independently of PKR, Mx1, or the 2-5A/RNase L system.

Upregulation of type I interferon receptor by IFN-gamma

Type I interferon (IFN) receptor has a multichain structure composed of at least two distinct subunits, IFNAR-1 and IFNAR-2. In the present study, we demonstrated that IFN-gamma induced the expression of mRNA for IFNAR-1 and IFNAR-2 in a human hepatoma cell line, HepG2 cells. The induction was dose and time dependent. Because of this result, we examined the effect of combined treatment with type I IFN and IFN-gamma. The intracellular 2-5A-synthetase activity induced by combined treatment was significantly higher than that by type I IFN alone. This study suggests that combined treatment with type I IFN and IFN-gamma may be more effective than that of type I IFN alone and that the upregulation of type I IFN receptor may be one of the reasons. Our findings may have some relevance to the clinical use of IFN.

2'-adenylated derivatives of Ap3A activate RNase L

The exact physiological function of Ap3A (A5'ppp5"A, 5'5" diadenosine triphosphate) remains unclear. Previously we have demonstrated that the human p46 2-5A synthetase (OAS1) efficiently utilises Ap3A as an acceptor substrate for oligoadenylate synthesis. Here we show that Ap3A(2'p5'A)n oligonucleotides can activate the 2-5A-dependent RNase (RNase L), when the number of 2',5'-linked adenyl residues is two or more. Under the experimental conditions applied the half-maximal activation (AC50) of RNase L for 2'-adenylated Ap3A derivatives was determined to be in nanomolar range while the AC50 for 2-5A3 was 0.4 nM. The Ap3A(2'p5'A)n oligonucleotides are thus less effective in activating RNase L than 2-5A. We also investigated the occurrence of 2'-adenylated Ap3A in interferon and poly(I).poly(C)-treated HeLa cells. In purified trichloroacetic acid-soluble extracts about 40% of RNase L-activating material is resistant to phosphatase treatment, whereas the removal of 5'-terminal phosphates greatly reduces the activating properties of 2-5A. We assume that this activity at least partly may be associated with the presence of 2'-adenylated ApnA derivatives with blocked 5'-terminal phosphates.

The expression of both domains of the 69/71 kDa 2',5' oligoadenylate synthetase generates a catalytically active enzyme and mediates an anti-viral response

The 2',5' oligoadenylate synthetase (OAS) represents a family of interferon-induced proteins which, when activated by double-stranded (ds) RNA, polymerizes ATP into 2',5'-linked oligomers with the general formula pppA(2'p5'A)n, where n >/= 1. The 69-kDa form of human OAS has two isoforms (p69 and p71) that are identical for their first 683 amino acids and consist of two homologous and adjacent domains, each homologous to the small 40-kDa OAS. Here, we demonstrate that mRNA species specific for the isoforms p69 and p71 are enhanced in interferon-treated cells, with the p69 mRNA being more abundant than that of p71. In transfected cells, both isoforms could be expressed independently to generate enzymes with similar catalytic activity, typical of the natural 69-kDa OAS from interferon-treated cells. On the other hand, deletion mutants expressing either the N- or C-terminal domain common in p69 and p71 were greatly unstable and were found to be devoid of catalytic activity, in spite of the capacity of the C-terminal domain to bind dsRNA. Finally, we show that murine cell lines stably expressing either p69 or p71 isoforms partially resist infection by the encephalomyocarditis virus. These results indicate that both isoforms of the 69-kDa form of 2',5' OAS are expressed in interferon-treated cells, and that each isoform could be implicated in the mechanism of the anti-viral action of interferon.

The human 2',5'-oligoadenylate synthetase family: interferon-induced proteins with unique enzymatic properties

2',5'-Oligoadenylate synthetase (2',5'-OAS) was discovered and characterized as an interferon (IFN)-induced enzyme that in the presence of double-stranded (ds) RNA converts ATP into 2',5'-linked oligomers of adenosine with the general formula pppA(2'p'A)n, n > or = 1. The product is pppG2'p5'G when GTP is used as a substrate. Now, 20 years later, this activity is attributed to several well-characterized, homologous, and IFN-induced proteins in human cells. Three distinct forms of 2',5'-OAS exist, small, medium, and large, which contain 1, 2, and 3 OAS units, respectively, and are encoded by distinct genes clustered on the 2',5'-OAS locus on human chromosome 12. Recently, other IFN-induced proteins homologous to the OAS unit but devoid of the typical 2',5'-OAS catalytic activity have been described. These OAS-related proteins are encoded by a gene located at the proximity of the 2',5'-OAS locus. These findings illustrate the apparent structural and functional complexity of the human 2',5'-OAS family.

The 100-kDa 2',5'-oligoadenylate synthetase catalyzing preferentially the synthesis of dimeric pppA2'p5'A molecules is composed of three homologous domains

The 2-5A synthetases represent a family of proteins implicated in the mechanism of the antiviral action of interferon. When activated by double-stranded RNA, these proteins polymerize ATP into 2'-5'-linked oligomers with the general formula pppA(2'p5'A)n, n >/= 1. Three forms of human 2-5A synthetases have been described corresponding to proteins of 40/46 (p40/p46), 69/71 (p69/p71), and 100 kDa (p100). Here we describe the molecular cloning and characterization of p100. By screening a cDNA expression library with a specific p100 polyclonal antibody, we first isolated a 590-nucleotide cDNA fragment which was subsequently used to isolate the full-length 6365-nucleotide cDNA. This cDNA recognizes a distinct interferon-induced messenger RNA of 7 kilobases. It has an open reading frame encoding a protein of 1087 amino acids including the sequence of seven peptides obtained by microsequencing of the natural p100 protein, which was purified from interferon-treated human cells. p100 is composed of three adjacent domains, each homologous to the previously defined catalytic unit of 350 amino acids, which is present as one unit in p40/p46 and as two units in p69/p71. The recombinant p100 synthesized preferentially dimeric 2', 5'-oligoadenylate molecules and displayed parameters for maximum enzyme activity similar to the natural p100. These results confirm that the enzymatic activity of p100 is distinct compared with that of p40/p46 and p69/p71.

RNase L inhibitor is induced during human immunodeficiency virus type 1 infection and down regulates the 2-5A/RNase L pathway in human T cells

The interferon-regulated 2-5A/RNase L pathway plays a major role in the antiviral and antiproliferative activities of these cytokines. Several viruses, however, have evolved strategies to escape the antiviral activity of the 2-5A/RNase L pathway. In this context, we have cloned a cDNA coding for the RNase L inhibitor (RLI), a protein that specifically inhibits RNase L and whose regulated expression in picornavirus-infected cells down regulates the activity of the 2-5A/RNase L pathway. We show here that RLI increases during the course of human immunodeficiency virus type 1 (HIV-1) infection, which may be related to the downregulation of RNase L activity that has been described to occur in HIV-infected cells. In order to establish a possible causal relationship between these observations, we have stably transfected H9 cells with RLI sense or antisense cDNA-expressing vectors. The overexpression of RLI causes a decrease in RNase L activity and a twofold enhancement of HIV production. This increase in HIV replication correlates with an increase in HIV RNA and proteins. In contrast, reduction of RLI levels in RLI antisense cDNA-expressing clones reverses the inhibition of RNase L activity associated with HIV multiplication and leads to a threefold decrease in the viral load. This anti-HIV activity correlated with a decrease in HIV RNA and proteins. These findings demonstrate that the level of RLI, via its modulation of RNase L activity, can severely impair HIV replication and suggest the involvement of RLI in the inhibition of the 2-5A/RNase L system observed during HIV infection.

The 2-5A system in viral infection and apoptosis

The 2-5A system is an established endogenous antiviral pathway. Interferon treatment of cells leads to an increase in basal, but latent, levels of 2-5A-dependent RNase (RNase L) and the family of 2'-5' oligoadenylate synthetases (OAS). Double-stranded RNA, thought to be derived from viral replication intermediates, activates OAS. Activated OAS converts ATP into unusual short 2'-5' linked oligoadenylates called 2-5A [ppp5'(A2'p5')2A]. The 2-5A binds to and activates RNase L which cleaves single stranded RNA with moderate specificity for sites 3' of UpUp and UpAp sequences, and thus leads to degradation of cellular rRNA. During apoptosis, generalized cellular RNA degradation, distinct from the differential expression of mRNA species that may regulate specific gene expression during apoptosis, has been observed. The mechanism of RNA breakdown during apoptosis has been commonly considered a non-specific event that reflects the generalized shut down of translation and homeostatic regulation during cell death. Due to the similar RNA degradation that occurs during both apoptosis and viral infection we investigated the potential role of RNase L in apoptosis. To investigate whether RNase L activity could lead to apoptosis, NIH3T3 cells were transfected with a lac-inducible vector containing the human RNase L gene. Treatment of these cells with isopropylthiogalactoside (IPTG) caused loss of cell viability that was confirmed as an apoptotic cell death by morphological and biochemical criteria. Similarly, specific allosteric activation of endogenous RNase L by introduction of 2-5A directly into L929 cells also induced apoptosis. In L929 cells poly(I).poly(C) treatment in combination with interferon caused an increase in apoptosis whereas neither interferon or double stranded RNA alone altered cell viability. Therefore, increased expression or activation of RNase L causes apoptosis. Inhibition of RNase L, specifically with a dominant negative mutant, suppressed poly(I)Ypoly(C)-induced apoptosis in interferon-primed fibroblasts. Poliovirus, a picornovirus with a single-stranded RNA genome, causes apoptosis of HeLa cells. Expression of the dominant negative inhibitor of RNase L in HeLa prevented virus-induced apoptosis and maintained cell viability. Thus, reduction or inhibition of RNase L activity prevents apoptosis. Both apoptosis and the 2-5A system can provide defense against viral infection in multicellular organisms by preventing production and therefore spread of progeny virus. RNase L appears to function in both mechanisms, therefore, initiation of apoptosis may be one mechanism for the antiviral activity of the 2-5A system.

Impact of RNase L overexpression on viral and cellular growth and death

The biologic actions of interferons (IFNs) are complex and involve multiple biochemical mechanisms, including the 2-5A system, a regulated RNA decay pathway. The 2-5A system is implicated in the antipicornavirus activity of IFN and in the control of apoptosis. To further investigate involvement of the 2-5A system in the control of viral and cellular growth and death, human RNase L cDNA was stably expressed in murine 3T3 cells from a constitutive cytomegalovirus (CMV) promoter. A clonal cell line, 3T3/pLZ, was isolated that overexpressed RNase L by >100-fold compared with levels of the endogenous murine RNase L. Interestingly, human RNase L levels in 3T3/pLZ cells decreased 3-fold as cells entered a confluent, growth arrest state, suggesting autoregulation. Overexpression of human RNase L greatly enhanced both the cell growth inhibitory activity of IFN and the proapoptotic activity of staurosporine. Furthermore, high levels of RNase L suppressed the replication of diverse viruses: encephalomyocarditis virus, vesicular stomatitis virus, human parainfluenza virus-3, and vaccinia virus. Additional reductions in viral growth were obtained by treating 3T3/pLZ cells with IFN (a + beta) before infections. These results directly demonstrate the anticellular and antiviral potential of the 2-5A system.

The 2-5A system: modulation of viral and cellular processes through acceleration of RNA degradation

The 2-5A system is an RNA degradation pathway that can be induced by the interferons (IFNs). Treatment of cells with IFN activates genes encoding several double-stranded RNA (dsRNA)-dependent synthetases. These enzymes generate 5'-triphosphorylated, 2',5'-phosphodiester-linked oligoadenylates (2-5A) from ATP. The effects of 2-5A in cells are transient since 2-5A is unstable in cells due to the activities of phosphodiesterase and phosphatase. 2-5A activates the endoribonuclease 2-5A-dependent RNase L, causing degradation of single-stranded RNA with moderate specificity. The human 2-5A-dependent RNase is an 83.5 kDa polypeptide that has little, if any, RNase activity, unless 2-5A is present. 2-5A binding to RNase L switches the enzyme from its off-state to its on-state. At least three 2',5'-linked oligoadenylates and a single 5'-phosphoryl group are required for maximal activation of the RNase. Even though the constitutive presence of 2-5A-dependent RNase is observed in nearly all mammalian cell types, cellular amounts of 2-5A-dependent mRNA and activity can increase after IFN treatment. One well-established role of the 2-5A system is as a host defense against some types of viruses. Since virus infection of cells results in the production and secretion of IFNs, and since dsRNA is both a frequent product of virus infection and an activator of 2-5A synthesis, the replication of encephalomyocarditis virus, which produces dsRNA during its life cycle, is greatly suppressed in IFN-treated cells as a direct result of RNA decay by the activated 2-5A-dependent RNase. This review covers the organic chemistry, enzymology, and molecular biology of 2-5A and its associated enzymes. Additional possible biological roles of the 2-5A system, such as in cell growth and differentiation, human immunodeficiency virus replication, heat shock, atherosclerotic plaque, pathogenesis of Type I diabetes, and apoptosis, are presented.

RNase L mediates the antiviral effect of interferon through a selective reduction in viral RNA during encephalomyocarditis virus infection

The 2',5'-oligoadenylate (2-5A) system is an RNA degradation pathway which plays an important role in the antipicornavirus effects of interferon (IFN). RNase L, the terminal component of the 2-5A system, is thought to mediate this antiviral activity through the degradation of viral RNA; however, the capacity of RNase L to selectively target viral RNA has not been carefully examined in intact cells. Therefore, the mechanism of RNase L-mediated antiviral activity was investigated following encephalomyocarditis virus (EMCV) infection of cell lines in which expression of transfected RNase L was induced or endogenous RNase L activity was inhibited. RNase L induction markedly enhanced the anti-EMCV activity of IFN via a reduction in EMCV RNA. Inhibition of endogenous RNase L activity inhibited this reduction in viral RNA. RNase L had no effect on IFN-mediated protection from vesicular stomatitis virus. RNase L induction reduced the rate of EMCV RNA synthesis, suggesting that RNase L may target viral RNAs involved in replication early in the virus life cycle. The RNase L-mediated reduction in viral RNA occurred in the absence of detectable effects on specific cellular mRNAs and without any global alteration in the cellular RNA profile. Extensive rRNA cleavage, indicative of high levels of 2-5A, was not observed in RNase L-induced, EMCV-infected cells; however, transfection of 2-5A into cells resulted in widespread degradation of cellular RNAs. These findings provide the first demonstration of the selective capacity of RNase L in intact cells and link this selective activity to cellular levels of 2-5A.

Ultrastructural localization of interferon-inducible double-stranded RNA-activated enzymes in human cells

The protein kinase PKR and the 2',5'-oligoadenylate (2-5A) synthetase are two interferon-induced and double-stranded RNA-activated enzymes which are implicated in the mechanism of action of interferon. Their distribution was undertaken here at the ultrastructural level by the immunogold procedure, following the use of specific monoclonal antibodies directed against PKR and 69- and 100-kDa forms of the 2-5A synthetase. These enzymes were detected as a pool of nonaggregated proteins scattered throughout the cell and as aggregates often associated with electron-dense doughnut-like structures showing a similar aspect whatever their subcellular localization: the cytoplasm, the nuclear envelope, and the nucleus. In general, the 2-5A synthetases were present in much more lower amounts than the PKR, probably due to the difficulty of detecting traces of proteins by electron microscopy. To circumvent this, we used a human lymphoblastoid cell line overexpressing the 69-kDa form of the 2-5A synthetase. In such cells, the synthetase was then clearly observed in both the cytoplasm and the nucleus; isolated or small clusters of gold particles were numerous in the cell mainly over the RNP fibrils of the interchromatin space, nucleolus, and ribosomes. Interestingly, gold particles were also found to be associated with the membranes of nuclear envelope and rough endoplasmic reticulum probably due to the myristilated motif of this form of 2-5A synthetase. Finally, intensely labeled electron-opaque dots sometimes associated with the nuclear pore complexes were present in the nucleus and in the cytoplasm of cells which might suggest their transport from the nucleus to the cytoplasm or reciprocally through the nuclear pore complexes. These observations indicate the wider distribution of the dsRNA-activated enzymes in the cell, thus pointing out their potential implication in as yet undetermined physiological function(s) necessary for various cellular metabolic reactions.

Interferon action and apoptosis are defective in mice devoid of 2',5'-oligoadenylate-dependent RNase L

2',5'-Oligoadenylate-dependent RNase L functions in the interferon-inducible, RNA decay pathway known as the 2-5A system. To determine the physiological roles of the 2-5A system, mice were generated with a targeted disruption of the RNase L gene. The antiviral effect of interferon alpha was impaired in RNase L-/- mice providing the first evidence that the 2-5A system functions as an antiviral pathway in animals. In addition, remarkably enlarged thymuses in the RNase L-/- mice resulted from a suppression of apoptosis. There was a 2-fold decrease in apoptosis in vivo in the thymuses and spleens of RNase L-/- mice. Furthermore, apoptosis was substantially suppressed in RNase L-/- thymocytes and fibroblasts treated with different apoptotic agents. These results suggest that both interferon action and apoptosis can be controlled at the level of RNA stability by RNase L. Another implication is that the 2-5A system is likely to contribute to the antiviral activity of interferon by inducing apoptosis of infected cells.

69-kDa and 100-kDa isoforms of interferon-induced (2'-5')oligoadenylate synthetase exhibit differential catalytic parameters

The (2'-5')oligoadenylate synthetase represents a family of interferon-induced proteins which when activated by double-stranded (ds)RNA polymerizes ATP into 2'-5'-linked oligomers with the general formula pppA(2'p5'A)n, where n > 1, which for convenience are referred to as 2-5A. We studied here the influence of pH, dsRNA concentration and time on oligomeric composition of 2-5A synthesized by purified 69-kDa and 100-kDa isoforms of (2'-5')oligo(adenylate) synthetase. In optimal conditions for activity, the 69-kDa form synthesized higher oligomers of 2-5A molecules whereas the 100 kDa form synthesized preferentially dimeric molecules, which are known not to be functional for the activation of RNase L. This difference does not reflect a differential affinity of the enzymes for the preformed 2-5A dimer, which is found to be a very poor substrate for both enzymes. This latter strongly suggests that the mechanism of elongation is more likely processive. Moreover, we show that both isoforms have efficient nucleotidyl-transferase activity and provide evidence that, in optimized conditions, GTP can be used alone as substrate by these enzymes to generate pppG2'p5'G. Our results clearly demonstrate that the 69-kDa and 100-kDa forms of (2'-5')oligoadenylate synthetase manifest various differential catalytic activities, and favor the hypothesis that these enzymes might have other functions in the cell besides those in the 2-5A system.

A bipartite model of 2-5A-dependent RNase L

The 2-5A-dependent RNase (RNase L) is a tightly regulated endoribonuclease of higher vertebrates that is catalytically active only after engaging unusual effector molecules consisting of the 2',5'-linked oligoadenylates, p1-3A(2'p5'A)>/=2 (2-5A). Progressive truncations from either terminus have provided insight into the structure, function, and regulation of RNase L. We determined that deletion of the N-terminal 335 amino acids of RNase L, about 45% of the enzyme, produced a constitutively active endoribonuclease, thus effectively eliminating the requirement for 2-5A. The truncated nuclease had 6-fold lower catalytic activity against an oligo(rU) substrate than wild type RNase L. However, the two enzymes showed identical RNA cleavage site preferences with an mRNA as substrate. The repressor function required only the last three of a series of nine ankyrin-like repeats present in the N-terminal part of RNase L. In contrast, the entire ankyrin repeat region was necessary and sufficient for 2-5A binding activity. Deletion of a 10-amino acid sequence near the C terminus of RNase L, between residues 710 and 720, eliminated both the catalytic and RNA substrate binding functions of the enzyme. The ability to bind native RNase L in response to 2-5A required amino acid sequences near both termini of the protein. A bipartite model for the structure of RNase L emerged in which the regulatory functions of the molecule are located in the N-terminal half, while the catalytic domain is present in the C-terminal half.

Targeting RNA decay with 2',5' oligoadenylate-antisense in respiratory syncytial virus-infected cells

Treatment of human cells with 2',5' oligoadenylate covalently linked to antisense (2-5A-antisense) results in the selective cleavage of targeted RNA species by 2-5A-dependent RNase L. Here we show that 2-5A-antisense containing stabilizing modifications at both termini are effective in suppressing the replication of respiratory syncytial virus (RSV) in human tracheal epithelial cells. The affinity of 2-5A-antisense for different regions in the RSV M2 and L mRNAs was predicted from a computer-generated model of the RNA secondary structure. The most potent 2-5A-antisense molecule caused a highly effective, dose-dependent suppression of RSV yields when added to previously infected cells. In contrast, control oligonucleotides, including an inactive dimeric form of 2-5A linked to antisense, 2-5A linked to a randomized sequence of nucleotides, and antisense molecules lacking 2-5A, had minimal effects on virus replication. The specificity of this approach was shown by reverse transcriptase-coupled PCR analysis of RSV M2, P, and N mRNA and of cellular glyceraldehyde-3-phosphate dehydrogenase mRNA. The RSV M2 mRNA amounts were depleted after treating RSV-infected cells with 2-5A-antisense targeted to this mRNA, whereas the amounts of the other RNA species were unchanged. These studies demonstrate that 2',5' oligoadenylate covalently linked to antisense (2-5A-antisense) can effectively suppress RSV replication by directing the cellular RNase L to selectively degrade an essential viral mRNA.

Brefeldin A inhibits the antiviral action of interferon against encephalomyocarditis virus

Brefeldin A (BFA), a unique fungal metabolite of a 13-membered lactone ring, exhibits various biological actions, including antitumor, antifungal and antiviral activities. In the present study, mouse LB cells were treated with various concentrations of interferon (IFN) and/or BFA overnight and infected with encephalomyocarditis virus (EMCV) after removal of IFN and BFA. Doses of BFA which neither inhibit the metabolism of the cell nor the infectivity of EMCV, decreased the IFN-induced antiviral activity against EMCV as demonstrated by virus titer from supernatants. Since 2-5A synthetase and double-stranded RNA (dsRNA)-dependent protein kinase (PKR) have been suggested to be involved in the antiviral action of IFN against EMCV, their activities were investigated in LB cells after BFA treatment. Northern blot analysis and in situ hybridization showed a decrease (2-3-fold) in the mRNA of 2'-5' oligoadenylate (2-5A) synthetase after BFA treatment. BFA also inhibited the activity of 2-5A synthetase, 2-5A dependent RNase and phosphorylation of PKR in cellular extracts, indicating that BFA may be exerting its inhibitory effect both at the transcriptional and post-transcriptional levels. This study reports a new biological action of BFA, demonstrating that BFA antagonized the antiviral action of IFN by inhibiting IFN-induced enzymatic pathways. These studies also suggest that both 2-5A and PKR are important in the antiviral activity of IFN against EMCV.

Mechanisms of viral inhibition by interferons

Interferons (IFNs) are a family of related proteins grouped in four species (alpha, beta, gamma and omega) according to their cellular origin, inducing agents and antigenic and functional properties. Their binding to specific receptors leads to the activation of signal transduction pathways that stimulate a defined set of genes, whose products are eventually responsible for the IFN antiviral effects. Their action against viruses is a complex phenomenon. It has been reported that IFNs restrict virus growth at the levels of penetration, uncoating, synthesis of mRNA, protein synthesis and assembly. This review will attempt to evaluate evidence of the involvement of the IFN-inducible proteins in the expression of the antiviral state against RNA or DNA viruses.

2',5'-oligoadenylate synthetase in interferon-alpha- and acyclovir-treated herpes simplex virus-infected cells

The 2',5'-oligoadenylate (2-5A) synthetase pathway, induced by interferon-alpha (IFN-alpha), has been shown to be responsible for the antiviral action of IFN-alpha against some viruses. Studies were done to determine the role of this pathway in the anti-herpes simplex virus (HSV) action of IFN-alpha alone or in combination with acyclovir (ACV), a combination that leads to synergistic anti-HSV activity. Treatment of human corneal cells or Vero cells with 100 IU/ml of IFN-alpha induced expression of 2-5A synthetase mRNA and a 10-fold increase in 2-5A synthetase production compared with untreated cells. HSV infection alone did not induce 2-5A synthetase production, but when IFN-alpha-treated cells were infected with HSV, enzyme level was significantly increased (p < 0.05) compared with that in IFN-alpha-treated, uninfected cells. HSV infection actually decreased the level of 2-5A synthetase mRNA in IFN-alpha-treated cells. Although IFN-alpha treatment induced high levels of 2-5A synthetase with or without HSV infection, no activation of the latent endonuclease was detected by specific cleavage of ribosomal RNA. Treatment of infected cells with 5 microM ACV alone or combined with IFN-alpha did not increase 2-5A synthetase or endonuclease activities above those detected in cells not treated with ACV. The data indicate that the 2-5A synthetase pathway was inducible in corneal cells and Vero cells but did not appear to contribute to the anti-HSV activity of IFN-alpha alone or the synergistic activity of IFN-alpha combined with ACV.

Defective transport of herpes simplex virus glycoprotein in interferon-treated cells: role of intracellular pH

We have investigated the mechanism(s) of interferon (IFN)-induced inhibition of assembly steps of herpes simplex virus (HSV-1) in mouse LB cells. Data showed that physiological doses of mouse IFN-beta (10-100 IU/ml) significantly inhibited the infectivity (5- to 100-fold) of HSV-1; however, viral protein synthesis was marginally inhibited (2- to 5-fold). Immunofluorescence studies showed that most of the HSV-1gD glycoprotein accumulated intracellularly in IFN-treated LB and LMtk- cells transfected with gD cDNA, as compared to untreated controls, where most of the gD was localized on the plasma membrane. Double-immunofluorescence studies demonstrated that rhodamine-labeled wheat germ agglutinin (WGA) was co-localized with gD protein, suggesting the block was in the transport from the trans-Golgi to the plasma membrane. IFN treatment of LB and LMtk- cells raised the intracellular pH as measured by DAMP distribution and SNARF-1 using laser spectroscopy; this could play an important role in the inhibition of transport of HSV-1gD.

A sensitive assay for the IFN-regulated 2-5A synthetase enzyme

2-5A synthetase is the central enzyme of the 2-5A system, an important mediator of interferon action. An assay capable of detecting low, yet biologically important levels of 2-5A synthetase enzyme activity is described. The purification of enzyme reaction products on SepPak C-18 cartridges resulted in a significant reduction in background, when a high specific activity substrate was used to label the 2-5A. Quantitation of labeled 2-5A by chromatography and scintillation counting provided a means of detecting femptomolar amounts of 2-5A. The combination of these procedures accounts for a 3-4 log increase in sensitivity over existing assays. This degree of sensitivity should permit a more accurate determination of the 2-5A synthetase activity in vivo leading to a better understanding of the role of the 2-5A system in virus infection and other cellular processes.

Preparation of cyclic 2',3'-monophosphates of oligoadenylates (A2'p)nA > p and A3'p(A2'p)n-1A > p

The action of the guanylyl-preferring RNase from Bacillus intermedius (binase) on a mixture of oligoadenylates with randomly distributed 2'-5' and 3'-5' internucleotide bonds [(A2'/3'p)n] under conditions sufficient for complete hydrolysis of poly(A) results in a mixture containing a single circular oligoadenylate and two series of linear oligoadenylates ending in cyclic 2',3'-phosphate. Individual compounds formed upon digestion of (A2'/3'p)n with binase have been isolated. Their structure was determined on the basis of their chemical and enzymatic conversions and confirmed by 1H-, 13C- and 31P-NMR spectra. According to these data, the circular triadenylate contains one 2'-5' and two 3'-5' internucleotide bonds, linear oligoadenylates of one series contain exclusively 2'-5' internucleotide bonds [(A2'p)nA > p], while each compound of the other series contains a single 3'-5' internucleotide bond connecting the 5'-ultimate nucleotide residue with the penultimate one [A3'p(A2'p)n-1A > p]. The incubation of compounds of the former series A3' p(A2'p)n > p at pH 1.0 and the subsequent action of phosphatase results in successive formation of compounds of two other new series: A3'p(A2'p)nA2'(3')p and A3'p(A2'p)nA.

Homologies between different forms of 2-5A synthetases

Sequence analyses of 2-5A synthetases of class I (M(r) 40,000-46,000) revealed high homology among them. The cDNA coding for the M(r) 69,000 2-5A synthetase of class II displayed in the second half a likewise high homology to the complete sequences of class I enzymes. This high degree of conservation of the 2-5A synthetases supports the assumption that these enzymes play important roles during virus infection (Williams et al. 1979; Coccia et al. 1990) and in the control of growth and differentiation of mammalian cells (Williams and Silverman 1985).

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.

A dominant negative mutant of 2-5A-dependent RNase suppresses antiproliferative and antiviral effects of interferon

2-5A-dependent RNase is the terminal factor in the interferon-regulated 2-5A system thought to function in both the molecular mechanism of interferon action and in the general control of RNA stability. However, direct evidence for specific functions of 2-5A-dependent RNase has been generally lacking. Therefore, we developed a strategy to block the 2-5A system using a truncated form of 2-5A-dependent RNase which retains 2-5A binding activity while lacking RNase activity. When the truncated RNase was stably expressed to high levels in murine cells, it prevented specific rRNA cleavage in response to 2-5A transfection and the cells were unresponsive to the antiviral activity of interferon alpha/beta for encephalomyocarditis virus. Remarkably, cells expressing the truncated RNase were also resistant to the antiproliferative activity of interferon. The truncated RNase is a dominant negative mutant that binds 2-5A and that may interfere with normal protein-protein interactions through nine ankyrin-like repeats.

Tissue-related and species-specific differences in the 2-5A oligomer size requirement for activation of 2-5A-dependent RNase

2',5'-Oligoadenylate (2-5A)-dependent RNase (L or F) is the final enzyme in the 2-5A pathway and a key component in the molecular mechanism of interferon (IFN) action. Here we demonstrate differences in the 2-5A oligomer size requirement between rabbit 2-5A-dependent RNase from reticulocytes and from cultured kidney cells. The rabbit reticulocyte enzyme was activated by tetramer 2-5A, whereas the ribonuclease from rabbit kidney cells required only trimer 2-5A. Interestingly, in contrast to the 2-5A-dependent RNase from rabbit reticulocytes, that from murine reticulocytes could be activated by trimer 2-5A. Partial proteolysis of affinity-labeled, 80-kD 2-5A-dependent RNase from rabbit reticulocytes and rabbit kidney cells resulted in the same pattern of labeled peptides. However, the affinity labeling reaction with a 32P-labeled 2-5A analog did produce some different labeled polypeptides in rabbit kidney cell extract and rabbit reticulocyte lysate. These results could indicate specialized functions for the 2-5A system in different organ systems.