Identification of a novel member of 2H phosphoesterases, 2',5'-oligoadenylate degrading ribonuclease from the oyster Crassostrea gigas.

Several genes of IFN-mediated pathways in vertebrates, among them the genes that participate in the 2',5'-oligoadenylate synthetase (OAS)/RNase L pathway, have been identified in C. gigas. In the present study, we identified genes, which encode proteins having 2',5'-oligoadenylate degrading activity in C. gigas. These proteins belong to the 2H phosphoesterase superfamily and have sequence similarity to the mammalian A kinase anchoring protein 7 (AKAP7) central domain, which is responsible for the 2',5'-phosphodiesterase (2',5'-PDE) activity. Comparison of the genomic structures of C. gigas proteins with that of AKAP7 suggests that these enzymes originate from a direct common ancestor. However, the identified nucleases are not typical 2',5'-PDEs. The found enzymes catalyse the degradation of 2',5'-linked oligoadenylates in a metal-ion-independent way, yielding products with 2',3' -cyclic phosphate and 5'-OH termini similarly to the 3'-5' bond cleavage in RNA, catalyzed by metal-independent ribonucleases. 3',5'-linked oligoadenylates are not substrates for them. The preferred substrates for the C. gigas enzymes are 5'-triphosphorylated 2',5'-oligoadenylates, whose major cleavage reaction results in the removal of the 5'-triphosphorylated 2',3'-cyclic phosphate derivative, leaving behind the respective unphosphorylated 2',5'-oligoadenylate. Such a cleavage reaction results in the direct inactivation of the biologically active 2-5A molecule. The 2',5'-ribonucleases (2',5'-RNases) from C. gigas could be members of the ancient group of ribonucleases, specific to 2'-5' phosphodiester bond, together with the enzyme that was characterized previously from the marine sponge Tethya aurantium. The novel 2',5'-RNases may play a role in the control of cellular 2-5A levels, thereby limiting damage to host cells after viral infection.

Enzymatically active 2',5'-oligoadenylate synthetases are widely distributed among Metazoa, including protostome lineage.

2',5'-Oligoadenylate synthetases (OASs) belong to the nucleotidyl transferase family together with poly(A) polymerases, CCA-adding enzymes and the recently discovered cyclic-GMP-AMP synthase (cGAS). Mammalian OASs have been thoroughly characterized as components of the interferon-induced antiviral system. The OAS activity and the respective genes were also discovered in marine sponges where the interferon system is absent. In this study the recombinant OASs from several multicellular animals and their closest unicellular relative, a choanoflagellate, were expressed in a bacterial expression system and their enzymatic activities were examined. We demonstrated 2-5A synthesizing activities of OASs from the marine sponge Tedania ignis, a representative of the phylogenetically oldest metazoan phylum (Porifera), from an invertebrate of the protostome lineage, the mollusk Mytilus californianus (Mollusca), and from a vertebrate species, a cartilaginous fish Leucoraja erinacea (Chordata). However, the expressed proteins from an amphibian, the salamander Ambystoma mexicanum (Chordata), and from a protozoan, the marine choanoflagellate Monosiga brevicollis (Choanozoa), did not show 2-5A synthesizing activity. Differently from other studied OASs, OAS from the marine sponge T. ignis was able to catalyze the formation of oligomers having both 2',5'- and 3',5'-phosphodiester linkages. Our data suggest that OASs from sponges and evolutionarily higher animals have similar activation mechanisms which still include different affinities and possibly different structural requirements for the activating RNAs. Considering their 2'- and 3'-specificities, sponge OASs could represent a link between evolutionarily earlier nucleotidyl transferases and 2'-specific OASs from higher animals.

A novel endoribonuclease from the marine sponge Tethya aurantium specific to 2',5'-phosphodiester bonds.

In the marine sponge Tethya aurantium a novel endoribonuclease was found which specifically catalyzed the degradation of 2',5'-phosphodiester linkages and was therefore named endo-2',5'-ribonuclease. This enzymatic reaction yielded 2',3'-cyclic phosphate and 5'-OH products similarly to the 3'-5' bond cleavage in RNA, catalyzed by metal-independent ribonucleases. The partially purified enzyme preparation was used for its biochemical characterization. The enzyme did not require the presence of metal ions for its activity. The novel nuclease exhibited a preference for 5'-phosphorylated 2',5'-oligoadenylates, but 2'-5' linkage in 5'-triphosphorylated hetero-oligomers or homo-dimers comprising guanylate or uridylate residues instead of adenylate was cleaved as well. The enzyme was also able to catalyze the degradation of 5'-unphosphorylated 2',5'-oligoadenylates, except for 2',5'-diadenylate, which were weaker substrates for the enzyme than the respective 5'-triphosphorylated forms. The observed substrate specificity may refer to the specific role of the enzyme in the degradation of natural 2',5'-oligoadenylates (2-5A) that function in the interferon-induced mammalian 2-5A system as allosteric regulators of ribonuclease L. They are produced by 2-5A synthetases (OAS) that are also present in sponges, the most ancient phylum of Metazoa. We suggest that the newly discovered endoribonuclease found in the marine sponge T. aurantium could be a representative of the group of 2',5'-specific ribonucleases that primarily control the cellular levels of 2',5'-oligoadenylates.

Expressed 2-5A synthetase genes and pseudogenes in the marine sponge Geodia barretti.

The 2',5'-oligoadenylate synthetases (2-5A synthetases, OAS) form a family of proteins presented in many branches of Metazoa. The phylum Porifera (sponges) contains OAS proteins which are different from those in vertebrates and form a distinct OAS subfamily. In turn, OAS proteins from different genera of Demospongia show rather low similarities in their primary structures. To ascertain divergence of the OAS genes within a particular sponge genus, we identified the OAS genes from the marine sponge Geodia barretti and compared them with those from another member of the genus Geodia, Geodia cydonium. The identity and similarity of the OAS sequences found in G. barretti with those from G. cydonium were considerably higher than identities and similarities compared with those from other sponges, 75% and 85% versus 27-30% and 42-47%, respectively. We also established the presence of a transcriptionally active polymorphic OAS pseudogene in the genome of G. barretti. The transcripts of the OAS pseudogene(s) lack several internal exons encoding necessary motifs for OAS enzymatic activity. The maintenance and further diversification of OAS gene(s) and pseudogene(s) suggest the prevalence of gene duplication events over the loss of gene duplicates in Geodia genomes during the evolution.

Natural occurrence of 2',5'-linked heteronucleotides in marine sponges.

2',5'-oligoadenylate synthetases (OAS) as a component of mammalian interferon-induced antiviral enzymatic system catalyze the oligomerization of cellular ATP into 2',5'-linked oligoadenylates (2-5A). Though vertebrate OASs have been characterized as 2'-nucleotidyl transferases under in vitro conditions, the natural occurrence of 2',5'-oligonucleotides other than 2-5A has never been demonstrated. Here we have demonstrated that OASs from the marine sponges Thenea muricata and Chondrilla nucula are able to catalyze in vivo synthesis of 2-5A as well as the synthesis of a series 2',5'-linked heteronucleotides which accompanied high levels of 2',5'-diadenylates. In dephosphorylated perchloric acid extracts of the sponges, these heteronucleotides were identified as A2'p5'G, A2' p5'U, A2'p5'C, G2'p5'A and G2' p5'U. The natural occurrence of 2'-adenylated NAD(+) was also detected. In vitro assays demonstrated that besides ATP, GTP was a good substrate for the sponge OAS, especially for OAS from C. nucula. Pyrimidine nucleotides UTP and CTP were also used as substrates for oligomerization, giving 2',5'-linked homo-oligomers. These data refer to the substrate specificity of sponge OASs that is remarkably different from that of vertebrate OASs. Further studies of OASs from sponges may help to elucidate evolutionary and functional aspects of OASs as proteins of the nucleotidyltransferase family.

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.

Expression and characterization of recombinant 2',5'-oligoadenylate synthetase from the marine sponge Geodia cydonium.

2',5'-oligoadenylate (2-5A) synthetases are known as components of the interferon-induced cellular defence mechanism in mammals. The existence of 2-5A synthetases in the evolutionarily lowest multicellular animals, the marine sponges, has been demonstrated and the respective candidate genes from Geodia cydonium and Suberites domuncula have been identified. In the present study, the putative 2-5A synthetase cDNA from G. cydonium was expressed in an Escherichia coli expression system to characterize the enzymatic activity of the recombinant polypeptide. Our studies reveal that, unlike the porcine recombinant 2-5A synthetase, the sponge recombinant protein associates strongly with RNA from E. coli, forming a heterogeneous set of complexes. No complete dissociation of the complex occurs during purification of the recombinant protein and the RNA constituent is partially protected from RNase degradation. We demonstrate that the sponge recombinant 2-5A synthetase in complex with E. coli RNA catalyzes the synthesis of 2',5'-phosphodiester-linked 5'-triphosphorylated oligoadenylates from ATP, although with a low specific activity. Poly(I).poly(C), an efficient artificial activator of the mammalian 2-5A synthetases, has only a minimal effect (an approximate two-fold increase) on the sponge recombinant 2-5A synthetase/bacterial RNA complex activity.

ATP N-glycosidase - a novel ATP-converting activity from a marine sponge Axinella polypoides.

A novel nucleosidase enzymatic activity was discovered in the marine sponge Axinella polypoides. This enzyme, designated as ATP N-glycosidase, converts adenosine-5'-triphosphate into adenine and ribose-5-triphosphate. The crude extract of A. polypoides was capable of hydrolysing 25 micro mol ATP.min-1 per g wet weight of sponge. The catalytic activity of a sponge crude extract per mg total protein is comparable with specific activities of purified plant adenosine and bacterial AMP nucleosidases. The preferred substrate for the novel enzyme is ATP but any compound containing adenosine-5'-diphosphoryl fragment is also cleaved. The biochemical properties (Km, Kip, environmental requirements) of ATP N-glycosidase show similarities with previously described adenine-specific nucleosidases; however, the pattern of its biochemical characteristics does not match with that of any of those enzymes.

Qualitative and quantitative aspects of 2-5A synthesizing capacity of different marine sponges.

2-5A synthetase is an important component of the mammalian antiviral 2-5A system. At present, the existence of 2-5A synthetase in the lowest animals, the marine sponges, has been demonstrated, although this enzyme has not been found in bacteria, yeast or plants. Here, we studied the 2-5A synthesizing capacity and the product profile of a variety of marine sponges belonging to Demospongia subclasses Tetractinomorpha and Ceractinomorpha. The 2-5A synthetase activity varied largely, in the range of four orders of magnitude, depending on the sponge species. Compared with the enzymes of the mammalian 2-5A synthetase family, the most active sponge species exhibited a surprisingly high 2-5A synthetase specific activity. Unlike the mammalian 2-5A synthetases that produce 2-5A oligomers in the presence of a double-stranded RNA activator, the 2-5A synthetase(s) from sponges were active without the addition of dsRNA. The sponge species differed in their product profiles. A novel product pool formed by Chondrosia reniformis was identified as a series of long 2-5A oligomers (up to 17-mers) with the prevalence of heptamers and octamers. The large variability of qualitative and quantitative characteristics of sponge 2-5A synthetases may refer to the occurrence of a variety of 2-5A synthetase isozymes in sponges.

Sponge (2',5')oligoadenylate synthetase activity in the whole sponge organism and in a primary cell culture.

A high (2',5')oligoadenylate (2-5A) synthetase activity was found in the marine sponge Geodia cydonium. Here we demonstrate that the 2-5A synthetase activity is present also in other sponge species although the level of the 2-5A synthetase activity varies in several magnitudes in different sponges. The 2-5A synthesizing activity was maintained in the primary culture produced from a sponge.

2',5'-oligoadenylate synthetase from a lower invertebrate, the marine sponge Geodia cydonium, does not need dsRNA for its enzymatic activity.

Recently, the presence of 2',5'-linked oligoadenylates and a high 2',5'-oligoadenylate synthetase activity were discovered in a lower invertebrate, the marine sponge Geodia cydonium. It has been demonstrated that mammalian 2-5A synthetase isozymes require a dsRNA cofactor for their enzymatic activity. Our results show that, unlike mammalian 2-5A synthetases, the 2-5A synthetase from the sponge acts in a dsRNA-independent manner in vitro. A prolonged incubation of the G. cydonium extract with a high concentration of a micrococcal nuclease had no effect on the activity of the 2-5A synthetase. At the same time, the micrococcal nuclease was effective within 30 min in degrading dsRNA needed for the enzymatic activity in IFN-induced PC12 cells. These results indicate that the 2-5A synthetase from G. cydonium may be active per se or is activated by some other mechanism. The sponge enzyme is capable of synthesizing a series of 2-5A oligomers ranging from dimers to octamers. The accumulation of a dimer in the predominant proportion during the first stage of the reaction was observed, followed by a gradual increase in longer oligoadenylates. By its product profile and kinetics of formation, the sponge 2-5A synthetase behaves like a specific isoform of enzymes of the 2-5A synthetase family.

Induction of (2'-5')oligoadenylate synthetase in the marine sponges Suberites domuncula and Geodia cydonium by the bacterial endotoxin lipopolysaccharide.

Recent studies have shown that the Porifera, with the examples of the demosponges Suberites domuncula and Geodia cydonium, comprise a series of pathways found also in the immune system of Deuterostomia, such as vertebrates, but are absent in Protostomia, with insects or nematodes as examples. One pathway is the (2'-5')oligoadenylate synthetase [(2-5)A synthetase] system. In the present study we show that crude extracts from tissue of S. domuncula collected from the sea display a considerable amount of (2-5)A synthetase activity; 16% of the ATP substrate is converted to the (2-5)A product, while tissue from specimens which were kept for 6 months in an aquarium shows only 1% of conversion. As aquarium animals show a lower bacterial load, those specimens were treated for the experiments with the bacterial endotoxin lipopolysaccharide (LPS); they responded to LPS with a stimulation of the (2-5)A synthetase activity. To monitor if this effect can be obtained also on the in vitro level, primmorphs which comprise proliferating and differentiating cells, were incubated with LPS. Extracts obtained from LPS-treated primmorphs also convert ATP to the (2-5)A products mediated by the synthetase. In parallel to this effect on protein level, LPS causes after an incubation period of 12 h also an increase in the steady-state level of the transcripts encoding the putative (2-5)A synthetase. It is postulated that in sponges the (2-5)A synthetase is involved in antimicrobial defense of the animals.