From the Department of Molecular and Structural
Biology, University of Aarhus, C. F. Møllers Allé, Building
130, DK-8000 Aarhus C, Denmark, the § Division of
Biochemistry, Department of Cellular and Molecular Sciences, St.
George's Hospital Medical School, Cranmer Terrace, London SW 17 ORE,
United Kingdom, and the ¶ Department of Medical Biochemistry,
University of Aarhus, Ole Worms Allé, Building 170, DK-8000 Aarhus C, Denmark
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ABSTRACT |
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A number of small RNA molecules that are high
affinity ligands for the 46-kDa form of human 2-5
oligoadenylate
synthetase have been identified by the SELEX method. Surface plasmon
resonance analysis indicates that these RNAs bind to the enzyme with
dissociation constants in the nanomolar range. Competition experiments
indicate that the binding site for the small RNAs on the 2
-5
oligoadenylate synthetase molecule at least partially overlaps that for
the synthetic double-stranded RNA, poly(I)·poly(C). Several of the
RNAs function as potent activators of 2
-5
oligoadenylate synthetase
in vitro, although there is no correlation between binding
affinity and ability to activate. The RNA aptamers having the strongest
activation potential appear to have few base-paired regions. This
suggests that 2
-5
oligoadenylate synthetase, which has previously
been believed to be activated only by double-stranded RNA, can also be
activated by RNA ligands with little secondary structure. Since 2
-5
oligoadenylate synthetase possesses no homology to other known
RNA-binding proteins, the development of small specific ligands by
SELEX should facilitate studies of RNA-protein interactions and may
reveal novel features of the structure-function relationships involving
this enzyme.
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INTRODUCTION |
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Double-stranded RNA
(dsRNA)1 binds to and
activates a number of interferon-induced proteins, namely the family of
2-5
oligoadenylate synthetases (2-5A synthetase) and the protein
kinase PKR (reviewed in Refs. 1-3). The synthetic dsRNA
poly(I)·poly(C) has been commonly used to stimulate these enzymes
in vitro. In previous work, the interaction between 2-5A
synthetase and the stimulatory dsRNA has been studied with a particular
focus on poly(I)·poly(C) (and modifications thereof), showing that a
10-base pair ideal helix in a 60-nucleotide-long dsRNA was necessary
for the stimulation of 2-5A synthetase activity (4, 5). However, the
RNA species used in those pioneering studies were all synthetic
polymers not having a well defined length or structure.
Several small viral RNAs such as VAI encoded by adenovirus and the TAR RNA of HIV-1 have been found to activate 2-5A synthetase (6, 7), but inhibit PKR (8, 9), whereas the Rex response element from HTLV-1 activates both 2-5A synthetase and PKR (10). It was shown recently that mutating the VAI RNA in a way that induced extra base pairing in the stem structure of this RNA increased the activation of 2-5A synthetase by VAI (6).
Many dsRNA-binding proteins, including PKR, possess dsRNA binding domains (dsRBMs) with related structures, and consensus sequences for these structures have been proposed (Refs. 11 and 12; reviewed in Ref. 13). However, despite the fact that the members of the 2-5A synthetase family are in general believed to be activated by similar types of RNA to those that activate PKR, the structural basis of the RNA-protein interactions in these enzymes might be rather different. Notably, the 2-5A synthetase family does not contain characteristic dsRBMs (14). For example, the binding of TAR to PKR involves interaction of the minor groove of the double-stranded helix with a dsRNA binding motif (dsRBM) (15). However, this motif is not present in any of the known 2-5A synthetase sequences, and no other structural similarity has been found between PKR and 2-5A synthetase. Several other RNA binding motifs have been identified over the last few years in both cellular and viral proteins (16). No obvious homology is found between these proteins and 2-5A synthetase, but deletion mapping has suggested that the RNA binding domain of 2-5A synthetase is situated in the N-terminal region of the protein (17).
To investigate the nature of the interaction between 2-5A synthetase and its activating RNAs, we have produced the 46-kDa form of human 2-5A synthetase in a baculovirus expression system and purified it from the infected insect cells. Subsequently, we tested the stimulation of enzymatic activity using a number of artificial aptamers selected through a SELEX procedure from a population of RNA molecules of 103 nucleotides in length containing internal sequences of 34 random nucleotides. We report the identification of small well characterized RNA molecules, which activate 2-5A synthetase at lower concentrations than any previously reported RNAs. Despite the absence of extensive secondary structure in these RNA aptamers, they function as potent activators of 2-5A synthetase in vitro. Annealing of complementary RNAs to the single-stranded aptamers increased the ability of these molecules to activate the enzyme in a sequence-specific manner.
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MATERIALS AND METHODS |
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Baculovirus Expression of Human 2-5
Oligoadenylate
Synthetase
Complementary DNAs for the 1.6- and 1.8-kilobase forms of 2-5A synthetase (18, 19) were cloned into the EcoRI site of the baculovirus transfer vector pVL1393. Co-transfection with CsCl-purified recombinant transfer vector and linearized baculovirus (Autographa californica multiple nuclear polyhedrosis virus) DNA was performed as described by the suppliers (BaculoGold, PharMingen). Spodoptora frugiperda (Sf9) cells (from ATCC CRL1711) were grown in Ex-Cell 401 medium (Sera Lab), supplemented with 5% fetal calf serum and antibiotics (100 international units/ml penicillin and 50 µg/ml streptomycin) (Life Technologies, Inc.), and adapted to growth in suspension in roller flasks. Pure recombinant viruses were isolated using either plaque purification or a virus dilution method. Insect cells in 96-well microtiter plates were infected with serial dilutions of transfection solution, and cellular DNA was then examined for the presence of the 2-5A synthetase gene by dot-blot analysis (20). Recombinant baculoviruses were tested for their ability to produce 2-5A synthetase by an enzymatic activity test (21).
Purification of Enzyme from High FiveTM Cells
High FiveTM (Trichoplucia ni from InVitrogen) cells were grown to 1.5 × 106 cells/ml and infected with recombinant virus at a multiplicity of infection of 10. The infected cultures were harvested after 72 h by centrifugation at 13,000 rpm for 20 min. Large scale protein production was optimized in 400-ml roller flasks (Nunc, Roskilde, Denmark).
Cells were lysed on ice for 10 min in 1% Nonidet P-40 in buffer E (20 mM Tris-HCl, pH 7.5, 10 mM KOAc, 10 mM Mg(OAc)2, and 1 mM EDTA). Cell extracts were cleared by centrifugation, and ammonium sulfate was added to a final concentration of 1.1 M. After incubation for 30 min on ice and centrifugation for 10 min at 20,000 × g, ammonium sulfate was added to the supernatant to a final concentration of 1.7 M and the mixture was loaded on a Phenyl-Sepharose 6FF column (Pharmacia Biotech Inc.). The column was washed with a decreasing ammonium sulfate gradient (1.7 M-0 M). The proteins were eluted with a gradient of ethylene glycol (0-50%) in buffer E. Peak fractions were identified by enzyme activity tests using the rapid pyrophosphate assay (see below), and protein concentrations were measured using the BCA method (Pierce).
SELEX Selection of RNA Ligands
Generation of Library-- An oligonucleotide library containing a random insert of 34 nucleotides was constructed as summarized in Fig. 1. Oligonucleotides 3, 4, and 5 were ligated together using oligonucleotides 1 and 2 as bridging molecules (22, 23). Ligation was performed using the temperature cycling method described in Ref. 24 using a Perkin-Elmer 480 thermal cycler. Oligonucleotides were denatured by heating to 94 °C for 5 min, followed by cooling to 10 °C over 30 min. T4 DNA ligase was added and a temperature cycling program started, cycling between 10 °C and 30 °C over a period of 45 s for 20 cycles. The temperature was then raised to 94 °C and reduced to 10 °C again. Fresh enzyme was added and another 20 cycles of ligation performed. In total, three such rounds were performed. The 130-nucleotide ligation products were purified on a 12% sequencing polyacrylamide gel (Sequagel, National Diagnostic).
Template for the first transcription was prepared by annealing 400 pmol (18 µg) of the ligation products to the same molar amount of oligonucleotide 1 (forward primer). This mixture was added to 900 µl of transcription mix (40 mM Tris-HCl, pH 7.5, 50 mM NaCl, 8 mM MgCl2, 2 mM spermidine, 30 mM DTT, 0.5 mM rNTPs, 0.8 unit/µl RNasin (Promega) and 2000 units of T7 RNA polymerase) and incubated at 37 °C for 4.5 h. After digestion with 0.4 units/µl DNase I (Stratagene, RNase-free) for 15 min at 37 °C, the products were precipitated using isopropanol and ammonium acetate. The RNA products were redissolved in renaturation buffer (150 mM KCl, 10 mM MgCl2, 10 mM Hepes-KOH, pH 7.2, and 0.25 mg/ml BSA). A total of 4.3 nmol (158 µg) of RNA was recovered. This RNA was split in two pools, one for the selection of species binding to 2-5A synthetase and one for the Rev control experiment (see below). The same procedure was used to transcribe RNA in later rounds, except that the templates were the products of RT-PCR reactions on selected RNAs.Selection of RNAs-- RNA (2.2 nmol, 10 µl) was denatured for 3 min at 70 °C and renatured at 4 °C for 5 min. It was then mixed with 12 pmol of the 46-kDa form of human 2-5A synthetase and 2 µg of poly(I)·poly(C) in buffer E in a final volume of 100 µl on ice for 10 min, and then incubated for 10 min at 37 °C.
The RNA-protein complex was retained on a nitrocellulose filter (BA85, Schleicher & Schuell) by filtering the mixture through a manifold under vacuum, and was washed twice for 5 min with 2-3 ml of buffer E. The retained RNA was eluted by incubating the filter in elution buffer (300 mM NaCl, 1% SDS, 10 mM Tris-HCl, pH 8, and 1 mM EDTA) for 10 min. The recovered RNA was phenol-extracted and precipitated. The isolated RNA species were reverse transcribed in 50 µl using avian myeloblastosis virus reverse transcriptase (U. S. Biochemical Corp.). This first strand synthesized DNA was used for amplification in a 500-µl PCR reaction using oligonucleotides 1 and 5 as primers (see Fig. 1). PCR was performed as follows: 94 °C for 1 min, 60 °C for 1 min, 72 °C for 1 min. In the first round of selection, 25 cycles were used, and this was gradually decreased to 15 cycles in the final round of selection. The PCR product was purified on a 2% NuSieve (FMC) low melting point agarose gel. After the first round of selection, the yield of pure PCR product was about 5.5 µg of DNA. A control experiment using the HIV-1 Rev protein instead of 2-5A synthetase was performed using the same method, except that the binding reaction was performed in Rev binding buffer (50 mM Tris, pH 7.5, 200 mM potassium acetate, 2 mM MgCl2, 10 mM DTT). Rev protein was a gift from Dr J. Kjems (IMSB, University of Aarhus) and was prepared as described in Ref. 25.Cloning and Sequencing-- After either 8 or 11 rounds of selection, a portion of the RT-PCR product was cut with BamHI and HindIII and cloned into pBluescript-KS+ cloning vector (Stratagene). DNA was prepared from a number of colonies and sequenced using standard primers (M13-21 and M13 reverse). Sequencing was done automatically on an Applied Biosystems 373A sequencer, using the ABI Prism dye terminator cycle sequencing kit (Perkin-Elmer).
Preparation of Template for Antisense RNA Synthesis
The template was prepared by PCR, using a primer containing a
T7 promoter at its 5 end and 21 bases complementary to the 3
end of the aptamer (antisense T7 primer) and a primer
identical to the 5
end of the aptamer (antisense 5
primer). The
antisense RNA product was extended with an additional three G residues
since the T7 RNA polymerase initiates most efficiently if
the first three bases are GGG. The primer sequences were as follows:
antisense T7 primer,
5
-TAATACGACTCACTATAGGGCCGGATCCGTTTCAATAGAG-3
; antisense 5
-primer,
5
-GGGAGCCAACACCACAAT-3
.
Preparation and Purification of RNA
RNA was transcribed using the T7-MEGAshortscriptTM kit (Ambion). Templates were generated by PCR, using the same primer as in the SELEX experiment. The transcription product was phenol-extracted, precipitated with ammonium acetate and isopropanol, washed with 80% ethanol, and redissolved in formamide loading buffer. After electrophoresis on a 6% sequencing gel, the RNA was visualized by UV shadowing, cut out, and eluted overnight in 0.25 M sodium acetate, 1 mM EDTA, then phenol-extracted and isopropanol-precipitated. RNA used for assaying PKR activity in the rabbit reticulocyte lysates system was additionally purified on a 10% native polyacrylamide gel, to avoid low levels of contaminating dsRNA known to be associated with T7 transcript (26). RNA was redissolved in 50 mM Hepes, pH 7.0, 150 mM potassium acetate, and 10 mM magnesium acetate and renatured by heating to 85 °C for 5 min, followed by slow cooling to 37 °C.
RPA Assay for 2-5A Synthetase Activity
The rapid pyrophosphate (RPA) assay for 2-5A synthetase activity was performed essentially as described in Ref. 27, except that buffer EM containing the coupling enzymes was prepared as two separate buffers. Buffer EMa contained 1.5 ml of 100 mM NADP+, 1 ml of 100 mM UDP-Glc, 150 µl of 10 mM glucose 1,6-bisphosphate, 15 ml of TM (70 mM triethanolamine base, 20 mM Mg(OAc)2, 20 mM KOAc, 2 mM EDTA, and 2 mM DTT) buffer and 32.5 ml of H2O. Buffer EMb contained 50 units of UDP-galactose pyrophosphorylase, 34 units of phosphoglucomutase, and 27 units of glucose-6-phosphate dehydrogenase (all from Boehringer Mannheim) in 700 µl of buffer TM with 50% glycerol. This change was made to prolong the lifetime of the enzymes in buffer EM. A master mix containing 2.4 µg (3 µl) of pure 2-5A synthetase (46-kDa form), 35 µl of buffer EMa, 1 µl of buffer EMb, and 11 µl of buffer AM (25 mM ATP, 75 mM triethanolamine base, 50 mM Mg(OAc)2 and 2 mM EDTA) per assay was made prior to each experiment. In the kinetic experiments, the mix contained 0.5 µg of 2-5A synthetase, 35 µl of EMa, 4 µl of EMb, and 11 µl of AM, to avoid saturation of the coupling enzyme system. RNA was diluted in buffer TM and added to 96-well microtiter plates in a total volume of 50 µl on ice. The reaction was started by adding the master mix with a multi-pipette (Ripette®) and raising the temperature to 37 °C. Optical density at 340 nm was measured in an enzyme-linked immunosorbent assay reader (Bio-TekTM, Bio Kinetics EL 340) connected to a computer.
Collection and Manipulation of Data
Data obtained from the enzyme-linked immunosorbent assay reader were used to calculate 2-5A synthetase-catalyzed reaction velocities by linear regression within the linear range of the time courses (at least four points). The apparent dissociation constant (Kapp) and the maximal reaction velocity (Vmax) were estimated using the following relations: V = (Vmax/(1 + Kapp/[RNA])), where [RNA] is the concentration of the RNA agonist. Calculations were performed by making a non-linear curve fit of V to [RNA] using Sigmaplot® software.
Thin Layer Chromatography of 2-5A Synthetase Products
2-5A synthetases were synthesized according to Ref. 21 using
[-33P]ATP and separated on polyethyleneimine-cellulose
thin layer plates, developed with 2 M Tris-HCl, pH 8.63. Only intact triphosphorylated oligo-adenylates were detected, as well
as the produced PPi. No Pi due to contaminating
phosphatase activity was detected.
Filter Binding Assays
The filter binding assay was performed using
[-32P]UTP-labeled RNA. Approximately 50,000 cpm was
incubated with the indicated concentration of 2-5A synthetase in
binding buffer (10 mM Tris-HCl, pH 7.6, 50 mM
KCl, 8 mM MgCl2, and 0.2 mg/ml RNase-free BSA
(Boehringer Mannheim)) in a total volume of 25 µl for 15 min at
30 °C. 20 µl of the binding reaction was passed through a
nitrocellulose membrane under vacuum. The membranes were washed three
times with 300 µl of wash buffer (10 mM Tris HCl, pH 7.6, 50 mM KCl, 8 mM MgCl2). The
stability of the RNA-protein complex was measured by diluting the
binding reaction into 2 ml of wash buffer and incubating for various
times prior to loading onto the nitrocellulose membrane. Complexes were
stable for at least 10 min under these conditions.
Biosensor Measurements
All measurements were performed on a BIAcore 2000 instrument (Biosensor, Uppsala, Sweden) equipped with SA sensor chip. The SA sensor chip has a streptavidin-coated surface in all four flow channels ready to bind biotinylated ligands. A continuous flow of Mg2+-HBS buffer (10 mM HEPES, pH 7.4, 1 mM EDTA, 2 mM MgCl2, 50 mM NaCl, 0.005% surfactant P20) passing over the sensor surface was maintained at 5 µl/min.
Biotinylated RNA was produced using the biotin RNA labeling mix from Boehringer Mannheim. The reaction mixture was precipitated using ammonium acetate and isopropanol and subsequently passed through a G50 Sephadex spin column to remove unincorporated biotin-UTP. The biotinylated RNA samples were diluted in 10 mM sodium acetate, pH 5.0, to a final concentration of 10 µg/ml and injected over the immobilized streptavidin surface. Typically, 500-1000 response units (RU) of each RNA species was bound to the immobilized streptavidin corresponding to 0.016-0.032 pmol/mm2.
Analyses of the 2-5A synthetase protein were performed by injecting 40 µl of protein solution into the derivatized sensor chip flow channels using a blank flow channel as a control. The protein was dissolved in Mg2+-HBS buffer and injected in concentrations of 40-600 nM. Regeneration of the sensor chip after each analysis cycle was performed by injecting 2 × 5 µl of 0.05% SDS. Kinetic parameters were determined by using the BIAevaluation 2.1 software.
Protein Synthesis in Rabbit Reticulocyte Lysates
Assays were performed as described in Ref. 28. The reaction volume was 10 µl. At the end of the incubation, 7 µl was taken for precipitation of acid-insoluble radioactivity, which was counted in a liquid scintillation counter.
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RESULTS |
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Selection of RNA Ligands for 2-5A Synthetase-- The 46-kDa isoform of 2-5A synthetase was used as the protein target in a SELEX experiment, using an RNA population of 103-mers containing randomized sequences within a 34-nucleotide stretch near the center of the molecule (Fig. 1B). RNA ligands for 2-5A synthetase were selected by either 8 or 11 rounds of selection using binding of the RNA-protein complexes on nitrocellulose filters. After elution of the RNA and amplification by RT-PCR, the products were cloned and sequenced. Out of a total of 20 aptamers obtained, two were identical (sx112 and sx118). Two others (sx106 and sx108) differ only at one nucleotide position. The sequences of the aptamers are shown in Fig. 2.
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Activation of 2-5A Synthetase by ssRNA or dsRNA Aptamers-- Following the selection of aptamers by binding to 2-5A synthetase, RNA was prepared from the series of 20 aptamer clones and the ability of each RNA aptamer to activate 2-5A synthetase (46-kDa isoform) was tested in an in vitro assay (Fig. 3). The activity observed in the presence of the different aptamers varied between 0.5- and 3.8-fold of the activity obtained with the random (preselection) RNA mixture. It should be noted that the RNA used in this initial screen was not gel-purified. Subsequently, we found that the activity stimulated by the random RNA decreased substantially if the RNA was gel-purified, as in the case of all subsequent experiment (see Table I). The activity stimulated by aptamers was not influenced significantly by the degree of purification.
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2-5A Synthetase Binding Assay-- Aptamers from different groups (sx93, sx97, sx106, and sx112) were individually analyzed for binding to 2-5A synthetase by filter binding assays. All these RNAs bound to nitrocellulose filters in the presence, but not in the absence, of the enzyme. Fig. 7 shows the result obtained with the sx112 aptamer. This aptamer also bound to 2-5A synthetase in a concentration-dependent manner (data not shown), and binding was partially competed by poly(I)·poly(C).
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Consensus Motifs in Activators of 2-5A Synthetase-- The three groups of aptamers (according to stimulatory activity) were compared at the sequence level (Fig. 2). In general, the selected aptamers have a high content of cytosine, and the presence of oligo(C) stretches is striking. This suggests an involvement of such sequences in the high activation of 2-5A synthetase by those RNAs. The strong agonists have a low content of G residues, with an average of only 3 G nucleotides in the 34 central randomized nucleotides. They are also characterized by two motifs, APyAPy(N)nCC and UU(N)nACCC in different parts of the molecule. These motifs are also found in some of the medium and poor activators, but they are only partial (Fig. 2). We used the RNA DRAW computer program to examine possible secondary structures of the selected aptamers. A surprisingly low extent of base pairing was obtained for the aptamers, which were the best 2-5A synthetase agonists. The sx137 RNA had only a 3-base pair putative stem structure within the random region and the sx112 RNA had only a 6-base pair stem structure, which could be formed between the random region and part of the linker sequences (Fig. 8).
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Effects on Protein Synthesis--
In addition to the 2-5A
synthetase another IFN-inducible enzyme, the protein kinase PKR, is
also activated by binding structured RNA species. PKR inhibits protein
synthesis by virtue of its ability to phosphorylate the subunit of
polypeptide chain initiation factor eIF2 (30). We have examined the
ability of the most efficient 2-5A synthetase aptamer to activate PKR
by monitoring protein synthesis in the rabbit reticulocyte lysate
system, which contains endogenous latent PKR. Incubations were carried
out in the presence and absence of 2-aminopurine. The latter inhibits
PKR but has no effect on the activation of 2-5A synthetase by dsRNA.
Thus, 2-aminopurine can be used to distinguish between inhibition of translation caused by PKR and that caused by the 2-5A/RNase L system
(31, 32). The abilities of various concentrations of the sx112 aptamer
to inhibit protein synthesis are shown in Fig. 9. Poly(I)·poly(C) was used as a
positive control, and the unselected random RNA was used as a negative
control. Whereas poly(I)·poly(C) at 1 µg/ml was a potent inhibitor,
and most of its effect was blocked in the presence of 2-aminopurine,
much higher concentrations of the sx112 aptamer were required to
inhibit translation. Moreover, this inhibition was not reversed by
2-aminopurine. This suggests that sx112 is not a good activator of PKR
and that its main effect on protein synthesis (observed at 5 µg/ml
and above) is mediated by the activation of 2-5A synthetase. Addition
of the random RNA resulted in 2-aminopurine-sensitive inhibition of
protein synthesis, even at concentrations as low as 1 µg/ml,
suggesting that there was significant PKR activation (presumably caused
by double-stranded regions present in the total RNA population).
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Effects of Single-stranded and Double-stranded Forms of the Aptamers on 2-5A Synthetase-- Since the sx112 aptamer was predicted by computer analysis to have a largely single-stranded structure, we went on to investigate the activation of 2-5A synthetase by the dsRNA formed by annealing sx112 sense RNA to sx112 antisense RNA. A construct was made from which RNA antisense to the aptamer could be transcribed using T7 RNA polymerase. Sense and antisense RNA were annealed by the same method as used for normal renaturation of RNA. Annealing of the complementary RNA led to a mobility shift in native polyacrylamide gels (data not shown). Similar hybrids were also made from three additional aptamers, sx137 representing another strong agonist and sx95 and sx139 representing the weak agonists. A concentration of 3 µg/ml total RNA was used in each assay of enzyme activity (corresponding to 82 nM for the ssRNA and 41 nM for the dsRNA hybrids) and the results are shown in Table II.
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DISCUSSION |
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We have obtained a series of RNA aptamers using the SELEX method to select for binding to the 46-kDa form of human 2-5A synthetase. We examined 20 of these aptamers for in vitro activation of 2-5A synthetase using a simple assay previously developed in our laboratory (27) and have thereby identified a group of highly potent small RNA agonists of this enzyme. These agonists were also shown to be active in a direct assay of 2-5A synthetase activity. The RNAs selected for their high affinity for 2-5A synthetase are the strongest activators of this enzyme yet identified. Subsequently, we demonstrated that one of the most potent aptamers, sx112, is unlikely to be a strong activator of the dsRNA-dependent protein kinase PKR. This suggests a difference in the nature of the RNA structures necessary to activate 2-5A synthetase and PKR.
For 2-5A synthetase, no particular RNA species has been identified as a natural activator. However, certain small viral RNAs, such as VAI RNA of adenovirus, Rex RNA of HTLV-1, and TAR RNA of HIV-1, function as activators of the enzyme in vitro and have been suggested to act in this capacity upon viral infection (6, 7, 10). A similar effect has been observed in the case of the small RNA of Epstein-Barr Virus, EBER-1.2 The synthetic compound poly(I)·poly(C) is a widely used in vitro activator of 2-5A synthetase, but it is clearly not a physiological agonist for the enzyme in vivo. 2-5A synthetase is strongly stimulated by several other synthetic dsRNAs, including poly(I)n·poly(C12U)n (Ampligen) and, to a lesser extent, poly(A)·poly(U) (5). In all cases, single-stranded homopolymers are inactive, and poly(G) is actually inhibitory.3 Double-stranded polymers containing a poly(C) stretch are generally the best activators, which is in agreement with our finding that all the selected aptamers were rich in C residues, whereas they contained few G nucleotides.
The first step in the evaluation of a SELEX experiment is normally to
search for a consensus motif in the selected aptamers. The consensus
found in the experiments reported here is rather weak. However, we
confirmed the validity of the selection protocol by selecting for RNAs
with high affinity for the HIV-1 protein Rev, from which we obtained
consensus sequences similar to the one previously published (29) (data
not shown). Moreover, if one takes into consideration the nature of the
protein under investigation, the absence of a long linear consensus
sequence necessary for binding to 2-5A synthetase is not entirely
unexpected. This enzyme does not show any primary sequence specificity
for its activation, but rather is activated by a variety of structured
RNA molecules. In this way, the enzyme meets the same challenges as
other parts of the immune system in that it must recognize a variety of
foreign molecules (in this case double-stranded RNAs), but must not be stimulated by endogenous RNA structures. Most other protein-RNA interactions previously analyzed in SELEX experiments have been based
on specific interactions in vivo between a protein and an already well defined RNA element, such as phenylalanyl-tRNA synthetase and its cognate tRNA (33) or the HIV-1 Rev protein and the
Rev-responsive element (29, 34). These considerations notwithstanding,
two small consensus motifs were identified among the strong 2-5A
synthetase agonists. There is no doubt that sequences within the unique
regions of the aptamers are responsible for the strong and specific
activation seen with this group, since other RNAs with the same common
5 and 3
sequences were poor activators.
The computer program RNA DRAW was used to predict the secondary structures of the aptamers belonging to the group of strong agonists. The longest potential double helix was found in sx131; this was 9 base pairs long but contained several internal bulges (data not shown). Surprisingly, the most active aptamer, sx112, had only a 3-base pair putative dsRNA stem within the central region (on which the SELEX selection is based). The accuracy of computer programs for predicting RNA secondary structure is uncertain, especially when a number of possible helices exist. However, the RNA DRAW program uses the minimal energy algorithm devised by Zuker (35), which searches for all possible base pairings and is more likely to overestimate than underestimate the actual number of base pairs within a given RNA molecule. We therefore conclude that the 2-5A synthetase activators described in this paper, despite being the strongest yet identified, have a predominantly single-stranded rather than the expected double-stranded structure. At first sight, this is a surprising conclusion, but it is supported by the fact that one of the strongest agonists, the sx112 aptamer, was a poor activator of the dsRNA-dependent enzyme PKR. In contrast, the random RNA pool used in the SELEX procedure was apparently able to activate PKR, suggesting the presence of dsRNA molecules in the pre-selection RNA population.
PKR and 2-5A synthetase have no significant similarities in their amino acid sequences, and the consensus dsRBM motif that is present in many dsRNA-binding proteins (11, 12) is notably absent from the latter enzyme (14). Furthermore, 2-5A synthetase possesses no known homology to other RNA-binding proteins. It is therefore likely that the region of the 2-5A synthetase protein interacting with RNA is quite distinct from that of PKR and recognizes different RNA structural motifs. On the basis of the data obtained in this paper and the lack of sequence similarity, we therefore suggest that PKR and 2-5A synthetase interact with RNA molecules through different structural mechanisms.
Annealing complementary RNA to different aptamers led to an increase in the stimulation of 2-5A synthetase. The ratio of stimulation between the good and bad aptamers was roughly the same whether ssRNA or dsRNA versions were used. Since the sx112 ssRNA had a stimulatory ability comparable with perfectly base-paired dsRNA forms of the otherwise weakly activating aptamers, the strong activation seen with sx112 appears to arise from interactions between the enzyme and a motif within the sx112 sense strand alone. Annealing complementary RNAs to aptamer RNAs, converting them into normal Watson-Crick base-paired helices, probably eliminates all previous secondary structures that were present. Nevertheless, it promotes activation of the synthetase in a sequence-specific manner, leading us to favor a model where the protein interacts directly with bases in the sense strand.
We estimated the Kapp and the Vmax for single- and double-stranded versions of sx112 RNA, and for poly(I)·poly(C), as activators of 2-5A synthetase. The Kapp determined for sx112 ssRNA was about 4 times lower (9.3 µg/ml) than for poly(I)·poly(C) (42 µg/ml), but the Vmax found with single-stranded sx112 RNA was lower than that with poly(I)·poly(C) or double-stranded sx112 RNA.
We propose an interpretation of the kinetic data according to the "two-state model" of Del Castillo and Katz, reviewed in Ref. 36. This model has widespread applicability in describing the activation of receptors by agonists, but can equally well be applied to allosteric activation of enzymes. In short the model characterizes an agonist by two dissociation constants rather than one, KA* being the dissociation constant for the active state (E*) of the enzyme and KA being the dissociation constant for the inactive state of the enzyme (E). The efficacy of an agonist then depends on the ratio of KA to KA* (i.e. if KA:KA* is above 1, then the molecule behaves as a agonist, the higher the ratio is the more effective the agonist is). In our case, the sx112 ssRNA has an lower efficiency for activation of 2-5A synthetase than the corresponding dsRNA. However, it is reasonable that a ssRNA aptamer might bind well to both active and inactive forms of the enzyme (resulting in a relatively low efficiency of activation), since it is selected solely upon its ability to bind 2-5A synthetase.
In a given system, the maximal fraction of an enzyme trapped in the active state by an agonist at saturating concentration is then determined solely upon the ratio of KA:KA*. According to this model, an agonist can have a high affinity for the enzyme, inducing activity at low concentrations, but still leading to a lower Vmax, as seen for the single-stranded sx112. It is not surprising that an aptamer selected by its ability to bind to the protein under conditions of competition for a limited number of binding sites has a high affinity for the inactive state as well as the active state. The dsRNA agonists have a higher efficacy than single-stranded sx112, as indicated by the higher Vmax achieved, implying that dsRNA structures preferentially interact with the active state of 2-5A synthetase.
We found a large difference between the Kd measured on the Biacore machine and the RNA concentration necessary to give half-maximal activity. A similar difference in binding and activation was found in Ref. 10 using UV cross-linking as the method for measuring Kd. This implies that the mechanism of activation of 2-5A synthetase is more complex than the simple binding of a suitable RNA ligand.
Although dsRNA has been demonstrated to be the preferred activator of 2-5A synthetase, the ssRNA molecules described here activated the enzyme as strongly as any previously described dsRNA agonist. However, other dsRNA-dependent pathways, including the activation of PKR, may not be stimulated by these ssRNA molecules. Future experiments will be required to show whether the aptamers induce the biosynthesis of interferon as efficiently as does poly(I)·poly(C). Our well defined aptamers could be expressed in cells in a number of ways, thus giving valuable information on the in vivo activation and function of 2-5A synthetase in the absence of other dsRNA-sensitive events.
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ACKNOWLEDGEMENTS |
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We thank Joan Hansen, Inger Bjørndal, and Anne Marie Bundsgaard for excellent technical assistance; Nanni Din and Nick J. Hoogenraad for help with establishing the baculovirus system; and Niels Ole Kjeldgaard, Anette Bejder, Max T. M. Søgaard, Janne L. Simonsen, and Jan Egebjerg Jensen for critical reading of the manuscript and helpful discussions.
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FOOTNOTES |
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* This work was supported by the Danish Cancer Society, the Karen Elise Jensen Foundation, the Danish Natural Science Research Council, the Wellcome Trust, the Cancer Research Campaign and the Leukaemia Research Fund.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed. Tel.: 45-89422682;
Fax: 45-89422637; E-mail: jj{at}mbio.aau.dk.
1
The abbreviations used are: dsRNA,
double-stranded RNA; ssNA, single-stranded RNA; 2-5A synthetase, 2-5
oligoadenylate synthetase; PKR, protein kinase RNA regulated; dsRBM,
double-stranded RNA binding domains; TAR, transactivating response
element; HIV, human immunodeficiency virus; HTLV, human T cell
lymphotropic virus; DTT, dithiothreitol; BSA, bovine serum albumin;
PCR, polymerase chain reaction; RT-PCR, reverse
transcription-polymerase chain reaction; RPA, rapid pyrophosphate
assay.
2 T. V. Sharp and M. J. Clemens, unpublished data.
3 R. Hartmann and J. Justesen, unpublished results.
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REFERENCES |
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