2',3'-Dialdehyde ATP analog labels the Ca(2+)-ATPase of sarcoplasmic reticulum via the catalytic adenosine-nucleotide-binding site

Oxidized ATP protection against anthrax lethal toxin

Bacillus anthracis lethal toxin (LT) induces rapid lysis (<90 min) of murine macrophages from certain inbred strains. The mechanism for LT-induced cytolysis is currently unknown. We hypothesized that the ATP-activated macrophage P2X7 receptors implicated in nucleotide-mediated macrophage lysis could play a role in LT-mediated cytolysis and discovered that a potent P2X7 antagonist, oxidized ATP (o-ATP), protects macrophages against LT. Other P2X7 receptor antagonists, however, had no effect on LT function, while oxidized nucleotides, o-ADP, o-GTP, and o-ITP, which did not act as receptor ligands, provided protection. Cleavage of the LT substrates, the mitogen-activated protein kinases, was inhibited by o-ATP in RAW274.6 macrophages and CHO cells. We investigated the various steps in the intoxication pathway and found that binding of the protective-antigen (PA) component of LT to cells and the enzymatic proteolytic ability of the lethal factor (LF) component of LT were unaffected by o-ATP. Instead, the drug inhibited formation of the sodium dodecyl sulfate-resistant PA oligomer, which occurs in acidified endosomes, but did not prevent cell surface PA oligomerization, as evidenced by binding and translocation of LF to a protease-resistant intracellular location. We found that o-ATP also protected cells from anthrax edema toxin and diphtheria toxin, which also require an acidic environment for escape from endosomes. Confocal microscopy using pH-sensitive fluorescent dyes showed that o-ATP increased endosomal pH. Finally, BALB/cJ mice injected with o-ATP and LT were completely protected against lethality.

Metal-dependent nucleotide binding to the Escherichia coli rotamase SlyD

Upon expression and purification of the first catalytic domain of mammalian adenylate cyclase type 1 (IC1), a 27 kDa contaminant was observed, which was labelled by three radioactive ATP analogues (8-azido-ATP, 3'-O-(4-benzoyl)benzoyl-ATP and 2',3'-dialdehyde-ATP); the protein was purified separately and identified as Escherichia coli SlyD by N-terminal amino acid sequence determination. SlyD is the host protein required for lysis of E. coli upon infection with bacteriophage PhiX174 and has recently been shown to display rotamase (peptidylproline cis-trans-isomerase) activity. The covalent incorporation of ATP analogues into SlyD was promoted by bivalent transition metal ions (Zn(2+)>/=Ni(2+)>Co(2+)>Cu(2+)) but not by Mg(2+) or Ca(2+); this is consistent with the known metal ion specificity of SlyD. ATP, ADP, GTP and UTP suppressed labelling of SlyD with comparable potencies. Similarly, SlyD bound 2',3'-O-(-2,4, 6-trinitrophenyl)-ATP with an affinity in the range of 10 microM, as determined by fluorescence enhancement. This interaction was further augmented in the presence of Zn(2+) (K(d)= approximately 2 microM at saturating Zn(2+)) but not of Mg(2+). Irrespective of the assay conditions, hydrolysis of nucleotides by SlyD was not detected. Upon gel filtration on a Superose HR12 column, SlyD (predicted molecular mass=21 kDa) migrated with an apparent molecular mass of 44 kDa, indicating that the protein was a dimer. However, the migration of SlyD was not affected by the presence of Zn(2+) or of Zn(2+) and ATP. Thus we concluded that SlyD binds nucleotides in the presence of metal ions. These findings suggest that SlyD serves a physiological role that goes beyond that accounted for by its intrinsic rotamase activity, which is observed in the absence of metal ions.

The C2 catalytic domain of adenylyl cyclase contains the second metal ion (Mn2+) binding site

Membrane-bound mammalian adenylyl cyclase isoforms contain two internally homologous cytoplasmic domains (C1 and C2). When expressed separately, C1 and C2 are catalytically inactive, but conversion of ATP to cAMP is observed if C1 and C2 are combined. By analogy with DNA polymerases, adenylyl cyclases are thought to require two divalent metal ions for nucleotide binding and phosphodiester formation; however, only one Mg2+ ion (liganded to C1) has been visualized in the recently solved crystal structure of a C1-C2 complex [Tesmer, J. J. G., Sunahara, R. K., Gilman, A. G., and Sprang, S. R. (1997) Science 278, 1907-1916]. Here, we have studied the binding of ATP to IIC2 (from type II adenylyl cyclase) using ATP analogues [2',3'-dialdehyde ATP (oATP), a quasi-irreversible inhibitor that is covalently incorporated via reduction of a Schiff base, the photoaffinity ligand 8-azido-ATP (8N3-ATP), and trinitrophenyl-ATP (TNP-ATP), a fluorescent analogue] and fluorescein isothiocyanate (FITC). [alpha-32P]oATP and 8N-[alpha-32P]ATP are specifically incorporated into IIC2. Labeling of IIC2 by [alpha-32P]oATP and by FITC is greatly enhanced by Mn2+ and to a much lesser extent by Mg2+. Similarly, TNP-ATP binds to IIC2 as determined by fluorescence enhancement, and this binding is promoted by Mn2+. Thus, a second metal ion binding site (preferring Mn2+) is contained within the C2 domain, and this finding highlights the analogy in the reaction catalyzed by DNA polymerases and adenylyl cyclases.

Drugs interacting with G protein alpha subunits: selectivity and perspectives

Extracellular signal molecules as diverse as hormones, neurotransmitters and photons use a signal transduction pathway involving a receptor, a G protein and effectors. Compounds that interact directly with G proteins can mimic the receptor-G protein interaction or can block the activation of G proteins by receptors. Several binding sites exist on the G alpha protein that may be exploited for the design of synthetic stimulatory or inhibitory ligands. The effector binding site is regulated by endogenous proteins and appears to be a target for selective exogenous ligands. The GTP binding site presents a large homology within the G protein families and therefore the nucleotide analogs might not be considered as a tool to discriminate between the G protein subclasses. In contrast, different experimental strategies have substantiated the specificity in the interaction between a receptor and a G protein, the receptor binding site of G proteins should be considered as potential drug targets. Drugs interfering with this site such as mastoparan and related peptides, GPAnt-2 and suramin, are lead compounds in the design of selective G protein antagonists. Benzalkonium chloride and methoctramine have agonist or antagonist properties, depending on G protein subtypes. Such compounds would be very useful to delineate the functions of G proteins and G protein-coupled receptors, to understand some side effects of drugs used in therapy and to develop new therapeutic agents.

Periodate-oxidized ATP stimulates the permeability transition of rat liver mitochondria

Periodate-oxidized ADP (oADP)2 and periodate-oxidized ATP (oATP) stimulate the permeability transition in energized rat liver mitochondria measured as the Ca2+-efflux induced by Ca2+ and Pi. In the presence of Mg2+ and Pi, mitochondria lose intramitochondrial adenine nucleotides at a slow rate. oATP induces a strong decrease of the matrix adenine nucleotides which is inhibited by carboxyatractyloside. Under these conditions, Mg2+ prevents the opening of the permeability transition pore. EGTA prevents the Pi-induced slow efflux of adenine nucleotides, but is without effect on the oATP-induced strong decrease of adenine nucleotides. This oATP-induced strong adenine nucleotide efflux is inhibited by ADP. oATP reduces the increase of matrix adenine nucleotides occurring when the mitochondria are incubated with Mg2+ and ATP. This effect of oATP is also prevented by carboxyatractyloside. oATP is not taken up by the mitochondria. It is suggested that oATP induces a strong efflux of matrix adenine nucleotides by the interaction with the ADP/ATP carrier from the cytosolic side. The induction of the mitochondrial permeability transition by oADP and oATP is attributed to two mechanisms-a strong decrease in the intramitochondrial adenine nucleotide content, especially that of ADP, and a stabilization of the c-conformation of the ADP/ATP carrier.

Inactivation of Escherichia coli phosphoribosylpyrophosphate synthetase by the 2',3'-dialdehyde derivative of ATP. Identification of active site lysines

The enzyme 5-phosphoribosyl-alpha-1-pyrophosphate (PRPP) synthetase from Escherichia coli was irreversibly inactivated on exposure to the affinity analog 2',3'-dialdehyde ATP (oATP). The reaction displayed complex saturation kinetics with respect to oATP with an apparent KD of approximately 0.8 mM. Reaction with radioactive oATP demonstrated that complete inactivation of the enzyme corresponded to reaction at two or more sites with limiting stoichiometries of approximately 0.7 and 1.3 mol of oATP incorporated/mol of PRPP synthetase subunit. oATP served as a substrate in the presence of ribose-5-phosphate, and the enzyme could be protected against inactivation by ADP or ATP. Isolation of radioactive peptides from the enzyme modified with radioactive oATP, followed by automated Edman sequencing allowed identification of Lys181, Lys193, and Lys230 as probable sites of reaction with the analog. Cysteine 229 may also be labeled by oATP. Of these four residues, Lys193 is completely conserved within the family of PRPP synthetases, and Lys181 is found at a position in the sequence where the cognate amino acid (Asp181) in human isozyme I PRPP synthetase has been previously implicated in the regulation of enzymatic activity. These results imply a functional role for at least two of the identified amino acid residues.

Activation and labelling of the purified skeletal muscle ryanodine receptor by an oxidized ATP analogue

We have tested the periodate-oxidized ATP analogue 2',3'-dialdehyde adenosine triphosphate (oATP) as a ligand for the skeletal muscle ryanodine receptor/Ca(2+)-release channel. Ca2+ efflux from passively loaded heavy sarcoplasmic reticulum vesicles of skeletal muscle is biphasic. oATP stimulates the initial phase of Ca2+ release in a concentration-dependent manner (EC50 160 microM), and the efflux proceeds with a half-time in the range 100-200 ms. This oATP-modulated initial rapid Ca2+ release was specifically inhibited by millimolar concentrations of Mg2+ and micromolar concentrations of Ruthenium Red, indicating that the effect of oATP was mediated via the ryanodine receptor. The purified Ca(2+)-release channel was incorporated into planar lipid bilayers, and single-channel recordings were carried out to verify a direct interaction of oATP with the ryanodine receptor. Addition of oATP to the cytoplasmic side activated the channel with an EC50 of 76 microM, which is roughly 30-fold higher than the apparent affinity of ATP. The oATP-induced increase in the open probability of the ryanodine receptor displays a steep concentration-response curve with a Hill coefficient of approximately 2, which suggests a co-operativity of the ATP binding sites in the tetrameric protein. oATP binds to the ryanodine receptor in a quasi-irreversible manner via Schiff base formation between the aldehyde groups of oATP and amino groups in the nucleotide binding pocket. This allows for the covalent specific incorporation of [alpha-32P]oATP by borhydride reduction. A typical adenine nucleotide binding site cannot be identified in the primary sequence of the ryanodine receptor. Our results demonstrate that oATP can be used to probe the structure and function of the nucleotide binding pocket of the ryanodine receptor and presumably of other ATP-regulated ion channels.