Nucleoside diphosphate kinase: role in bacterial growth, virulence, cell signalling and polysaccharide synthesis

Succinyl coenzyme A synthetase of Pseudomonas aeruginosa with a broad specificity for nucleoside triphosphate (NTP) synthesis modulates specificity for NTP synthesis by the 12-kilodalton form of nucleoside diphosphate kinase

Pseudomonas aeruginosa secretes copious amounts of an exopolysaccharide called alginate during infection in the lungs of cystic fibrosis patients. A mutation in the algR2 gene of mucoid P. aeruginosa is known to exhibit a nonmucoid (nonalginate-producing) phenotype and showed reduced activities of succinyl-coenzyme A (CoA) synthetase (Scs) and nucleoside diphosphate kinase (Ndk), implying coregulation of Ndk and Scs in alginate synthesis. We have cloned and characterized the sucCD operon encoding the alpha and beta subunits of Scs from P. aeruginosa and have studied the role of Scs in generating GTP, an important precursor in alginate synthesis. We demonstrate that, in the presence of GDP, Scs synthesizes GTP using ATP as the phosphodonor and, in the presence of ADP, Scs synthesizes ATP using GTP as a phosphodonor. In the presence of inorganic orthophosphate, succinyl-CoA, and an equimolar amount of ADP and GDP, Scs synthesizes essentially an equimolar amount of ATP and GTP. Such a mechanism of GTP synthesis can be an alternate source for the synthesis of alginate as well as for the synthesis of other macromolecules requiring GTP such as RNA and protein. Scs from P. aeruginosa is also shown to exhibit a broad NDP kinase activity. In the presence of inorganic orthophosphate (P(i)), succinyl-CoA, and either GDP, ADP, UDP or CDP, it synthesizes GTP, ATP, UTP, or CTP. Scs was previously shown to copurify with Ndk, presumably as a complex. In mucoid cells of P. aeruginosa, Ndk is also known to exist in two forms, a 16-kDa cytoplasmic form predominant in the log phase and a 12-kDa membrane-associated form predominant in the stationary phase. We have observed that the 16-kDa Ndk-Scs complex present in nonmucoid cells, synthesizes all three of the nucleoside triphosphates from a mixture of GDP, UDP, and CDP, whereas the 12-kDa Ndk-Scs complex specifically present in mucoid cell predominantly synthesizes GTP and UTP but not CTP. Such regulation may promote GTP synthesis in the stationary phase when the bulk of alginate is synthesized by mucoid P. aeruginosa.

The multiple activities of polyphosphate kinase of Escherichia coli and their subunit structure determined by radiation target analysis

Polyphosphate kinase (PPK), the principal enzyme required for the synthesis of inorganic polyphosphate (polyP) from ATP, also exhibits other enzymatic activities, which differ significantly in their biochemical optima and responses to chemical agents. These several activities include: polyP synthesis (forward reaction), nATP --> polyP(n) + nADP (Equation 1); ATP synthesis from polyP (reverse reaction), ADP + polyP(n) --> ATP + polyP(n - 1) (Equation 2); general nucleoside-diphosphate kinase, GDP + polyP(n) --> GTP + polyP(n - 1) (Equation 3); linear guanosine 5'-tetraphosphate (ppppG) synthesis, GDP + polyP(n) --> ppppG + polyP(n - 2) (Equation 4); and autophosphorylation, PPK + ATP --> PPK-P + ADP (Equation 5). The Mg(2+) optima are 5, 2, 1, and 0.2 mM, respectively, for the activities in Equations 1, 2, 3, and 4. Inorganic pyrophosphate inhibits the activities in Equations 1 and 3 but stimulates that in Equation 4. The kinetics of the activities in Equations 1, 2, and 3 are highly processive, whereas the transfer of a pyrophosphoryl group from polyP to GDP (Equation 4) is distributive and demonstrates a rapid equilibrium, random Bi-Bi catalytic mechanism. Radiation target analysis revealed that the principal functional unit of the homotetrameric PPK is a dimer. Exceptions are a trimer for the synthesis of ppppG (Equation 4) and a tetrameric state for the autophosphorylation of PPK (Equation 5) at low ATP concentrations. Thus, the diverse functions of this enzyme involve different subunit organizations and conformations. The highly conserved homology of PPK among 18 microorganisms was used to determine important residues and conserved regions by alanine substitution, by site-directed mutagenesis, and by deletion mutagenesis. Of 46 single-site mutants, seven exhibit none of the five enzymatic activities; in one mutant, ATP synthesis from polyP is reduced relative to GTP synthesis. Among deletion mutants, some lost all five PPK activities, but others retained partial activity for some reactions but not for others.

P2Z-Independent and P2Z receptor-mediated macrophage killing by Pseudomonas aeruginosa isolated from cystic fibrosis patients

We demonstrate that a mucoid, alginate-producing strain of Pseudomonas aeruginosa isolated from the lungs of a cystic fibrosis (CF) patient secretes multiple enzymes with nucleoside diphosphate kinase (Ndk), ATPase, adenylate kinase, 5'-nucleotidase, and ATP-modifying enzymatic activities. The secretion is triggered at high cell density and in complex media but is greatly reduced when the mucoid cells are grown in mineral salts media or in presence of 5.0 mM Ca2+ or Mg2+. Interestingly, the secretion is triggered primarily in the mucoid CF isolate of strain 8821M (or in strain FRD1) but not in a nonmucoid laboratory strain, PAO1. The purified secreted Ndk shows 100% match in its N-terminal amino acid sequence with that of purified intracellular Ndk and demonstrates similar enzymatic properties. The N-terminal sequence of the purified ATPase isolated from an ndk knockout mutant shows its identity with that of the heat shock chaperonin Hsp60. During fractionation, the flowthrough fraction from a Mono Q column demonstrates the presence of 5'-nucleotidase, adenylate kinase, and a putative ATP reductase activity. These fractions demonstrate high cytotoxic activities for murine peritoneal primary macrophages which can be further stimulated in the presence of ATP or inhibited by pretreatment of macrophages with oxidized ATP (oATP). The cytotoxicity associated with ATP-induced stimulation is believed to be due to activation of macrophage surface-associated P2Z (P2X7) receptors, which are one of the purinergic receptors responsible for pore formation on macrophage membrane. Blocking of these receptors by pretreatment with oATP blocks ATP-induced macrophage cell death. Thus mucoid P. aeruginosa cells elaborate enzymes that modulate the external ATP levels of macrophages, thereby modulating macrophage cell death through P2Z receptor activation. Evidence for the presence of secreted cytotoxic agents that act independently of P2Z receptor activation is also presented.

Secretion of ATP-utilizing enzymes, nucleoside diphosphate kinase and ATPase, by Mycobacterium bovis BCG: sequestration of ATP from macrophage P2Z receptors?

Mycobacterium bovis BCG secretes two ATP-scavenging enzymes, nucleoside diphosphate kinase (Ndk) and ATPase, during growth in Middlebrook 7H9 medium. In synthetic Sauton medium without any protein supplements, there is less secretion of these two enzymes unless proteins such as bovine serum albumin (BSA), ovalbumin or extracts of macrophages are added to the medium. There is a gradient of activity among various proteins in triggering the induction of secretion of these two enzymes. Other mycobacteria, such as M. smegmatis, primarily secrete Ndk, while M. chelonae does not appear to secrete either of these two enzymes. Purification of the enzymes from the culture filtrate of 7H9-grown M. bovis BCG cells and determination of the N-terminal amino-acid sequence have demonstrated a high level of sequence identity of one of the ATPases with DnaK, a heat shock chaperone, of M. tuberculosis and M. leprae, while that of Ndk shows significant identity with the Ndk of Myxococcus xanthus. As both Ndk and ATPase use ATP as a substrate, the physiological significance of the secretion of these two ATP-utilizing enzymes was explored. External ATP is important in the activation of macrophage surface-associated P2Z receptors, whose activation has been postulated to allow phagosome-lysosome fusion and macrophage cell death. We demonstrate that the presence of the filtrate containing these enzymes prevents ATP-induced macrophage cell death, as measured by the release of an intracellular enzyme, lactate dehydrogenase. In vitro complexation studies with purified Ndk/ATPase and hyperproduced P2Z receptor protein will demonstrate whether these enzymes may be used by mycobacteria to sequester ATP from the macrophage P2Z receptors, thereby preventing phagosome-lysosome fusion or macrophage apoptotic death.

Cellular function of elastase in Pseudomonas aeruginosa: role in the cleavage of nucleoside diphosphate kinase and in alginate synthesis

Elastase is a major virulence factor in Pseudomonas aeruginosa that is believed to cause extensive tissue damage during infection in the human host. Elastase is secreted in non-mucoid P. aeruginosa. It is known that secretion of most virulence factors such as elastase, lipase, exotoxin A, etc., in P. aeruginosa is greatly reduced in alginate-secreting mucoid cells isolated from the lungs of cystic fibrosis (CF) patients. We have previously reported that in mucoid P. aeruginosa, an intracellular protease cleaves the 16 kDa form of nucleoside diphosphate kinase (Ndk) to a truncated 12 kDa form. This smaller form is membrane associated and has been observed to form complexes with specific proteins to predominantly generate GTP, an important molecule in alginate synthesis. The main aim of this study was to purify and characterize this protease. The protease was purified by hydrophobic interaction chromatography of the crude extract of mucoid P. aeruginosa 8821, a CF isolate. Further analysis using a gelatin containing SDS-polyacrylamide gel detected the presence of a 103 kDa protease, which when boiled, migrated as a 33 kDa protein on a SDS-polyacrylamide gel. The first 10 amino acids from the N-terminus of the 33 kDa protease showed 100% identity to the mature form of elastase. An elastase-negative lasB::Cm knock-out mutant in the mucoid 8821 background was constructed, and it showed a non-mucoid phenotype. This mutant showed the presence of only the 16 kDa form of Ndk both in the cytoplasm and membrane fractions. We present evidence for the retention of active elastase in the periplasm of mucoid P. aeruginosa and its role in the generation of the 12 kDa form of Ndk. Finally, we demonstrate that elastase, when overproduced in both mucoid and non-mucoid cells, stimulates alginate synthesis. This suggests that the genetic rearrangements that trigger mucoidy in P. aeruginosa also allow retention of elastase in the periplasm in an active oligomeric form that facilitates cleavage of 16 kDa Ndk to its 12 kDa form for the generation of GTP, required for alginate synthesis.