Identification of the substrate-binding sites of 2'-5'-oligoadenylate synthetase

Distinct domain organization and diversity of 2'-5'-oligoadenylate synthetases

The 2'-5'-oligoadenylate synthetases (OAS) are important components of the innate immune system that recognize viral double-stranded RNA (dsRNA). Upon dsRNA binding, OAS generate 2'-5'-linked oligoadenylates (2-5A) that activate ribonuclease L (RNase L), halting viral replication. The OAS/RNase L pathway is thus an important antiviral pathway and viruses have devised strategies to circumvent OAS activation. OAS enzymes are divided into four classes according to size: small (OAS1), medium (OAS2), and large (OAS3) that consist of one, two, and three OAS domains, respectively, and the OAS-like protein (OASL) that consists of one OAS domain and tandem domains similar to ubiquitin. Early investigation of the OAS enzymes hinted at the recognition of dsRNA by OAS, but due to size differences amongst OAS family members combined with the lack of structural information on full-length OAS2 and OAS3, the regulation of OAS catalytic activity by dsRNA was not well understood. However, the recent biophysical studies of OAS have highlighted overall structure and domain organization. In this review, we present a detailed examination of the OAS literature and summarized the investigation on 2'-5'-oligoadenylate synthetases.

A potential robust antiviral defense state in the common vampire bat: Expression, induction and molecular characterization of the three interferon-stimulated genes -OAS1, ADAR1 and PKR

Bats are known to harbor many zoonotic viruses, some of which are pathogenic to other mammals while they seem to be harmless in bats. As the interferon (IFN) response represents the first line of defense against viral infections in mammals, it is hypothesized that activation of the IFN system is one of the mechanisms enabling bats to co-exist with viruses. We have previously reported induction of type I IFN in a cell line from the common vampire bat, Desmodus rotundus, upon polyinosinic:polycytidylic acid (poly(I:C)) stimulation. To deepen our knowledge on D. rotundus' IFN-I antiviral response, we molecularly characterized three interferon-stimulated genes (ISGs), OAS1, PKR and ADAR1, closely implicated in the IFN-I antiviral response, and tested their functionality in our cellular model. We first found that D. rotundus encoded two OAS1 paralogs, OAS1a and OAS1b, and that the functional domains of the four ISGs characterized were highly conserved with those of other mammals. Despite their significant transcription level in the absence of stimulation, the transcription of the four ISGs characterized was enhanced by poly(I:C). In addition, the transcription of OAS1a and OAS1b appears to be differentially regulated. These findings demonstrate an active ISG antiviral response in D. rotundus in which OAS1b may play an important role.

Bat Mx1 and Oas1, but not Pkr are highly induced by bat interferon and viral infection

Bats harbour many emerging and re-emerging viruses, several of which are highly pathogenic in other mammals but cause no diseases in bats. As the interferon (IFN) response represents a first line of defence against viral infection, the ability of bats to control viral replication may be linked to the activation of the IFN system. The three most studied antiviral IFN-stimulated genes (ISGs) in other mammals; Pkr, Mx1 and Oas1 were examined in our model bat species, Pteropus alecto. Our results demonstrate that the three ISGs from P. alecto are highly conserved in their functional domains and promoter elements compared to corresponding genes from other mammals. However, P. alecto Oas1 contains two IFN-stimulated response elements (ISRE) in its promoter region compared with the single ISRE present in human OAS1 which may lead to higher IFN inducibility of the bat gene. Both Oas1 and Mx1 were induced in a highly IFN-dependent manner following stimulation with IFN or synthetic double-strand RNA (dsRNA) whereas Pkr showed evidence of being induced in an IFN-independent manner. Furthermore, bat Oas1 appeared to be the most inducible of the three ISGs following either IFN stimulation or viral infection, providing evidence that Oas1 may play a more important role in antiviral activity in bats compared with Mx1 or Pkr. Our results have important implications for the different roles of ISGs in bats and provide the first step in understanding the role of these molecules in the ability of bats to coexist with viruses.

Sponge OAS has a distinct genomic structure within the 2-5A synthetase family

2',5'-Oligoadenylate synthetases (2-5A synthetases, OAS) are enzymes that play an important role in the interferon-induced antiviral defense mechanisms in mammals. Sponges, the evolutionarily lowest multicellular animals, also possess OAS; however, their function is presently unclear. Low homology between primary structures of 2-5A synthetases from vertebrates and sponges renders their evolutionary relationship obscure. The genomic structure of vertebrate OASs has been thoroughly examined, making it possible to elucidate molecular evolution and expansion of this gene family. Until now, no OAS gene structure was available from sponges to compare it with the corresponding genes from higher organisms. In the present work, we determined the exon/intron structure of the OAS gene from the marine sponge Geodia cydonium and found it to be completely different from the strictly conserved exon/intron pattern of the OAS genes from vertebrates. This finding was corroborated by the analysis of OAS genes from another sponge, Amphimedon queenslandica, whose genome was recently sequenced. Our data suggest that vertebrate and sponge OAS genes have no direct common intron-containing ancestor and two (sub)types of OAS may be discriminated. This study opens new perspectives for understanding the phylogenesis and evolution of 2-5A synthetases as well as functional aspects of this multigene family.

Viral stress-inducible genes

Virus-infection of mammalian cells causes transcriptional induction of many cellular genes, collectively called as "viral stress-inducible genes." The proteins encoded by these genes are essential to maintain cell-virus homeostasis, which is required for both virus replication and host survival. Many viral products, including RNA, DNA, and proteins, can induce these genes by using distinct, but partially overlapping, signaling pathways. Type I interferons, direct products of virus infection, can also induce many of these genes, thus providing a positive feedback loop. Double-stranded RNA, a common by-product of virus replication, can induce them by multiple signaling pathways initiated by Toll-like receptor 3 or RIG-I/Mda-5. Several viral stress-inducible proteins inhibit protein synthesis. Proteins of the P56 family bind to the translation initiation factor, eIF-3, and block translation initiation. PKR, a protein kinase, phosphorylates a different initiation factor, eIF-2, and inhibits translation initiation. However, unlike P56, PKR needs to be first activated by dsRNA or PACT, another cellular protein. Another family of enzymes, the 2'-5' oligoadenylate synthetases, synthesizes 2'-5' linked oligoadenylates [2-5(A)] in the presence of dsRNA; 2-5(A) activates the latent ribonuclease, RNase L, which degrades mRNA. Many viruses have evolved mechanisms to evade these genes by blocking their induction or actions; often more than one strategy is used by the same virus to achieve this goal. Thus, in an infected cell, equilibrium is reached between the virus and the cell with regards to the viral stress-inducible genes.

Characterization of the equine 2'-5' oligoadenylate synthetase 1 (OAS1) and ribonuclease L (RNASEL) innate immunity genes

Background

The mammalian OAS/RNASEL pathway plays an important role in antiviral host defense. A premature stop-codon within the murine Oas1b gene results in the increased susceptibility of mice to a number of flaviviruses, including West Nile virus (WNV). Mutations in either the OAS1 or RNASEL genes may also modulate the outcome of WNV-induced disease or other viral infections in horses. Polymorphisms in the human OAS gene cluster have been previously utilized for case-control analysis of virus-induced disease in humans. No polymorphisms have yet been identified in either the equine OAS1 or RNASEL genes for use in similar case-control studies.

Results

Genomic sequence for equine OAS1 was obtained from a contig assembly generated from a shotgun subclone library of CHORI-241 BAC 100I10. Specific amplification of regions of the OAS1 gene from 13 horses of various breeds identified 33 single nucleotide polymorphisms (SNP) and two microsatellites. RNASEL cDNA sequences were determined for 8 mammals and utilized in a phylogenetic analysis. The chromosomal location of the RNASEL gene was assigned by FISH to ECA5p17-p16 using two selected CHORI-241 BAC clones. The horse genomic RNASEL sequence was assembled. Specific amplification of regions of the RNASEL gene from 13 horses identified 31 SNPs.

Conclusion

In this report, two dinucleotide microsatellites and 64 single nucleotide polymorphisms within the equine OAS1 and RNASEL genes were identified. These polymorphisms are the first to be reported for these genes and will facilitate future case-control studies of horse susceptibility to infectious diseases.

On the discovery of interferon-inducible, double-stranded RNA activated enzymes: the 2'-5'oligoadenylate synthetases and the protein kinase PKR

The demonstration that double-stranded (ds) RNA inhibits protein synthesis in cell-free systems prepared from interferon-treated cells, lead to the discovery of the two interferon-induced, dsRNA-dependent enzymes: the serine/threonine protein kinase that is referred to as PKR and the 2',5'-oligoadenylate synthetase (2',5'-OAS), which converts ATP to 2',5'-linked oligoadenylates with the unusual 2'-5' instead of 3'-5' phosphodiesterase bond. We raised monoclonal and polyclonal antibodies against human PKR and the two larger forms of the 2',5'-OAS. Such specific antibodies proved to be indispensable for the detailed characterization of these enzyme and the cloning of cDNAs corresponding to the human PKR and the 69-71 and 100 kDa forms of the 2',5'-OAS. When activated by dsRNA, PKR becomes autophosphorylated and catalyzes phosphorylation of the protein synthesis initiation factor eIF2, whereas the 2'-5'OAS forms 2',5'-oligoadenylates that activate the latent endoribonuclease, the RNAse L. By inhibiting initiation of protein synthesis or by degrading RNA, these enzymes play key roles in two independent pathways that regulate overall protein synthesis and the mechanism of the antiviral action of interferon. In addition, these enzymes are now shown to regulate other cellular events, such as gene induction, normal control of cell growth, differentiation and apoptosis.

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.

Structural and functional genomics and evolutionary relationships in the cluster of genes encoding murine 2′,5′-oligoadenylate synthetases

2',5'-Oligoadenylate synthetases (2',5'-OASs) are interferon-inducible enzymes. Some of these proteins play an important role in cellular physiology, in particular, in the innate defense mechanisms against RNA virus infections. In the present publication we report the complete genomic structure of the cluster of genes encoding mouse 2',5'-OAS, with all its transcription units, their predicted functions, and their evolutionary relationships. We found that mouse Oas2/Oas3 genes have a genomic structure similar to that of human OAS2/OAS3, while the mouse equivalent of human OAS1 is composed of eight (Oas1a to Oas1h) tandemly arranged transcription units. For all these eight genes a specific inducible promoter controls transcription. The possible functions of this family of proteins are discussed.

Crisscross enzymatic reaction between the two molecules in the active dimeric P69 form of the 2'-5' oligodenylate synthetase

2'-5' oligoadenylate (2-5 (A)) synthetases are major components of the antiviral pathways induced by interferons. In the presence of double-stranded RNA, they polymerize ATP to form 2-5 (A) oligomers that, in turn, activate the latent ribonuclease RNase L, causing mRNA degradation. These enzymes, unlike other nucleotidyl transferases, catalyze 2'-5', not 3'-5', phosphodiester bond formation between substrates bound to the acceptor and donor sites. Moreover, unlike other members of this extended family, the P69 isozyme of 2-5 (A) synthetase functions as a homodimer. Here, we report that the need for P69 dimerization is because of a crisscross enzyme reaction joining two substrate molecules bound to two opposite subunits. Consequently, although homodimers of mutants in the previously identified acceptor site, the donor site, or the catalytic site were inactive, selective heterodimers of the mutants were active because of subunit complementation. The catalytic site had to be present in the same subunit that contained the acceptor site, whereas the donor site had to be provided by the other subunit. These results allowed us to design a mutant protein that acted as a dominant-negative inhibitor of wt P69 but not of another isozyme of 2-5 (A) synthetase.