2- and 8-azido photoaffinity probes. 2. Studies on the binding process of 2-5A synthetase by photosensitive ATP analogues

Synthesis, characterization, and biological properties of 8-azido- and 8-amino-substituted 2',5'-oligoadenylates

A series of 8-azido- and 8-amino-substituted 2',5'-oligoadenylatyes was prepared by a uranyl-ion catalyzed polymerization of the corresponding 8-substituted adenosine phosphorimidazolide. Subsequent 5'-dephosphorylation of the resulting 5'-phosphoryl 2',5'-linked oligomers with alkaline phosphatase gave the corresponding core oligomers. The CD spectra indicated that the 8-aminoadenosine analogue of the 2',5'-linked trimer has an anti-orientation as in naturally occurring 2',5'-oligoadenylates, while 8-azido-substituted 2',5'-oligoadenylates have a syn-orientation. The 8-substituted oligomers showed enhanced resistance against digestion by snake venom phosphodiesterase. The 2',5'-linked 8-azidoadenylate trimer and tetramer displayed strong RNase L binding and activating ability, although the corresponding dimer is devoid of such activities. In contrast, very low or no RNase L binding and activating ability were observed in the 8-aminoadenosine analogue of 2',5'-oligoadenylates. Results indicate that the bulkiness and ionic character of the 8-substituting group have significant effects on the ability of these analogues to bind and activate RNase L. Furthermore, the orientation of the glycosidic base in the 2-5A analogues may change from syn to anti during binding to RNase L. The 8-azidoadenosine analogues of 2-5A will be useful tools in the photoaffinity labeling of RNase L, due to their strong RNase L binding ability. In addition, these 8-azidoadenosine compounds may be considered as candidates for experimental therapeutic agents because they have enhanced stability to enzyme degradation while retaining the ability to activate RNase L.

Ap3A and Ap4A are primers for oligoadenylate synthesis catalyzed by interferon-inducible 2-5A synthetase

The biological role of Ap3A synthesized in cells by tryptophanyl-tRNA synthetase (WRS) is unknown. Previously we have demonstrated that the cellular level of Ap3A significantly increases after interferon treatment. Here we show that the human 46 kDa 2-5A synthetase efficiently utilizes Ap3A as a primer for oligoadenylate synthesis. The Km for Ap3A is several-fold lower than for Ap4A and 100-fold lower than for ATP. This implies that Ap3A might be a natural primer for the 2'-adenylation reaction catalysed by 2-5A synthetase. Since WRS and 2-5A synthetase are both interferon-inducible proteins, a new link between two interferon-dependent enzymes is established.

Identification of the ATP binding domain of recombinant human 40-kDa 2',5'-oligoadenylate synthetase by photoaffinity labeling with 8-azido-[alpha-32P]ATP

Three isoforms of the interferon-inducible 2',5'-oligoadenylate (2-5A) synthetase that require double-stranded RNA have been isolated and cloned. However, identification of the amino acid(s) of 2-5A synthetase directly interacting with ATP is crucial to the elucidation of the mechanism of the enzymatic conversion of ATP to 2',5'-oligoadenylates by 2-5A synthetase. Recombinant human 40-kDa 2-5A synthetase has been expressed as a glutathione S-transferase fusion protein in E. coli and purified to near homogeneity in milligram quantities. The azido photoprobe, 8-azido-[alpha-32P]ATP, has been used to identify the ATP binding domain of the recombinant human 40-kDa 2-5A synthetase. Specific covalent photoincorporation of 8-azido-[alpha-32P]ATP into the 2-5A synthetase, tryptic digestion of the covalently 32P-labeled enzyme, isolation of the photolabeled phosphopeptide by metal (Al3+) chelate chromatography, and high pressure liquid chromatography identified a 32P-pentapeptide, which has been assigned to the ATP binding domain of 2-5A synthetase. The radioactive pentapeptide has the sequence D196FLKQ200 in which the photoprobe, 8-azido-[alpha-32P]ATP, chemically modified the amino acid lysine 199. The catalytic importance of Lys199 was further established by mutation of lysine 199 to arginine 199 and histidine 199 using site-directed mutagenesis. The K199R and K199H recombinant human 40-kDa 2-5A synthetase mutants bind 8-azido-ATP and the allosteric activator, poly(I) poly(C) but are enzymatically inactive. These photoaffinity labeling and mutation data strongly suggest that lysine 199 is essential for the formation of a productive 2-5A synthetase-ATP-double-stranded RNA complex for the enzymatic conversion of ATP to 2-5A.

Study of the rat liver S-adenosylmethionine synthetase active site with 8-azido ATP

The active site of rat liver S-adenosylmethionine synthetase was studied using 8-azido ATP, a photolabile analogue of ATP. Both forms of the enzyme, tetramer and dimer, could be labelled by using concentrations of the analogue similar to the KmATP values for each form, 350 microM and 1 mM respectively. Labelling of both S-adenosylmethionine synthetase forms with 8-azido [alpha-32P]ATP, followed by tryptic digestion and purification by HPLC, afforded one specifically labelled peptide in each case. Identification of the labelled peptide by amino acid analysis and peptide sequencing, and comparison with the enzyme sequence, indicated that the same peptide (267-286) was modified in both enzyme forms. The results are discussed on the basis of the high degree of similarity that this peptide shows in all the known S-adenosylmethionine synthetase sequences.

Inhibition of HIV-1 replication and activation of RNase L by phosphorothioate/phosphodiester 2',5'-oligoadenylate derivatives

2',5'-Oligoadenylate (2-5A) derivatives have been designed to act distal to the human immunodeficiency virus-1 (HIV-1)-induced blockade in the 2-5A synthetase/RNase L antiviral pathway. Stereochemical modification of individual internucleotide linkages of the 2-5A molecule was accomplished by phosphoramidite and phosphotriester chemical syntheses. Phosphorothioate/phosphodiester trimer and tetramer 2-5A derivatives revealed differences in the stereodynamics of activation of RNase L and inhibition of HIV-1 replication. The first and second internucleotide linkages are critical for activation of recombinant, human RNase L; A(Rp)ApA, A(Sp)ApA and ApA(Rp)A are agonists (IC50 = 2 x 10(-7), 2 x 10(-6) and 8 x 10(-6) M); ApA(Sp)A is an antagonist. The second and third internucleotide linkages are crucial for activation of murine RNase L; ApA(Rp)A, ApA(Rp)ApA, and ApApA(Rp)A are agonists (IC50 = 5 x 10(-7) M); ApA(Sp)A, ApA(Sp)ApA, and ApApA(Sp)A are antagonists. Inhibition of HIV-1-induced syncytia formation by the phosphorothioate/phosphodiester derivatives is specific for derivatives with substitution at the 2',3'-terminus. ApA(Rp)A, ApA(Sp)A, ApApA(Rp)A, and ApApA(Sp)A are potent inhibitors of HIV-1-induced syncytia formation (80-, 10-, 40-, and 15-fold more inhibitory, respectively, than solvent control). HIV-1 infection results in enhanced uptake and accumulation of ApA(Rp)A and ApA(Sp)A (7- and 10-fold, respectively). These stereochemically modified 2-5A derivatives are taken up preferentially by HIV-1-infected cells and show promise in anti-HIV-1 chemotherapy.

2',5' A synthetase: allosteric activation by fructose 1,6-bisphosphate

Fructose 1,6-bisphosphate (fru-1,6-P2), but not other glycolytic intermediates, activates highly purified 2',5' A synthetases from rabbit reticulocyte lysates and from 2',5'-ADP-agarose purified extracts of interferon-treated HeLa cells without the addition of dsRNA. The 2',5' A was structurally and biologically identical to authentic 2',5' A. Micrococcal nuclease inhibited the activation of 2',5' A synthetase by poly(I)-poly(C), but did not affect activation by fru-1,6-P2. Addition of fru-1,6-P2 aldolase prevented the activation of 2',5' A synthetase by fru-1,6-P2.

Photoaffinity labeling of ATP and NAD+ binding sites on recombinant human interleukin 2

Interleukin 2 (IL-2) is a T-cell-derived lymphokine critical in the activation and proliferation of T cells, B cells, and lymphokine-activated killer cells. It is a glycoprotein of approximately 15,500 daltons that is synthesized and secreted after activation by antigen or mitogen. By using the analogs 8-azidoadenosine 5'-[gamma-32P]triphosphate [( gamma-32P]8N3ATP) and nicotinamide 2-azidoadenine [adenylate-32P]dinucleotide [( alpha-32P]2N3NAD+) as photoaffinity probes, we have detected specific, metal ion-requiring nucleotide binding sites on recombinant human IL-2 (rhIL-2). The specificity of these nucleotide interactions with rhIL-2 was demonstrated by saturation effects and by competition by the parent nucleotides at physiologically relevant concentrations. Saturation of photoinsertion into rhIL-2 occurred at 50 microM [gamma-32P]8N3ATP; a half-maximal decrease of its photoinsertion at 10 microM was obtained with 22 microM ATP. Saturation of photoinsertion with [alpha-32P]2N3NAD+ was observed at 180 microM; a half-maximal decrease of its photoinsertion at 10 microM was effected by 10 microM NAD+ and by 5 microM 3-aminobenzamide. The extent of photoinsertion of both photoprobes into rhIL-2 varied with the presence of different divalent metal ions. rhIL-2 photolabeling with [gamma-32P]8N3ATP appeared to be dependent on the presence of metal ion. It was effectively labeled in the presence of Mg2+ and photoinsertion was increased with the addition of Zn2+ at micromolar concentrations. Also, rhIL-2 underwent slow autophosphorylation by an intramolecular mechanism using [gamma-32P]8N3ATP as well as nonphotoactive nuceotide. The biological significance of these interactions is unknown, but their specificity suggests that nucleotide binding may be involved in the bioactivity of IL-2.