Regulatory GTP-binding proteins (ADP-ribosylation factor, Gt, and RAS) are not activated directly by nucleoside diphosphate kinase

Modulation of small GTPase activity by NME proteins

NME proteins are reported to influence signal transduction activity of small GTPases from the Ras superfamily by diverse mechanisms in addition to their generic NDP kinase activity, which replenishes the cytoplasmic pool of GTP. Comprehensive evidence shows that NME proteins modulate the activity of Ras GTPases, in particular members of the Rho family, via binding to their major activators GEFs. Direct interaction between several NMEs and Ras GTPases were also indicated in vitro and in vivo. These modes of regulation are mainly independent of the NME's kinase activity. NMEs also modulate the Ras-mediated signal transduction by interfering with the formation of a Ras signaling complex at the plasma membrane. In several examples, NMEs were proposed to perform the role of GAP proteins by promoting hydrolysis of the bound GTP, but this activity still requires additional verification. Early suggestions that NMEs can activate small GTPases by direct phosphorylation of the bound GDP, or by high-rate loading of GTP onto a closely apposed GTPase, were largely dismissed. In this review article, we survey and put into perspective published examples of identified and hypothetical mechanisms of Ras signaling modulation by NME proteins. We also point out involvement of NMEs in the transcriptional regulation of components of Ras GTPases-mediated signal transduction pathways, and reciprocal regulation of NME function by small GTPases, particularly related to NME's binding to membranes.

A Computational Approach for Understanding the Interactions between Graphene Oxide and Nucleoside Diphosphate Kinase with Implications for Heart Failure

During a heart failure, an increased content and activity of nucleoside diphosphate kinase (NDPK) in the sarcolemmal membrane is responsible for suppressing the formation of the second messenger cyclic adenosine monophosphate (cAMP)-a key component required for calcium ion homeostasis for the proper systolic and diastolic functions. Typically, this increased NDPK content lets the surplus NDPK react with a mutated G protein in the beta-adrenergic signal transduction pathway, thereby inhibiting cAMP synthesis. Thus, it is thus that inhibition of NDPK may cause a substantial increase in adenylate cyclase activity, which in turn may be a potential therapy for end-stage heart failure patients. However, there is little information available about the molecular events at the interface of NDPK and any prospective molecule that may potentially influence its reactive site (His118). Here we report a novel computational approach for understanding the interactions between graphene oxide (GO) and NDPK. Using molecular dynamics, it is found that GO interacts favorably with the His118 residue of NDPK to potentially prevent its binding with adenosine triphosphate (ATP), which otherwise would trigger the phosphorylation of the mutated G protein. Therefore, this will result in an increase in cAMP levels during heart failure.

RGS19 inhibits Ras signaling through Nm23H1/2-mediated phosphorylation of the kinase suppressor of Ras

Besides serving as signal terminators for G protein pathways, several regulators of G protein signaling (RGS) can also modulate cell proliferation. RGS19 has previously been shown to enhance Akt signaling despite impaired Ras signaling. The present study examines the mechanism by which RGS19 inhibits Ras signaling. In HEK293 cells stably expressing RGS19, serum-induced Ras activation and phosphorylations of Raf/MEK/ERK were significantly inhibited, while cells expressing RGS2, 4, 7, 8, 10, or 20 did not exhibit this inhibitory phenotype. Conversely, siRNA-mediated knockdown of RGS19 enabled partial recovery of serum-induced ERK phosphorylation. Interestingly, two isoforms of the tumor metastasis suppressor Nm23 (H1 and H2) were upregulated in 293/RGS19 cells. As a nucleoside diphosphate kinase, Nm23H1 can phosphorylate the kinase suppressor of Ras (KSR). Elevated levels of phosphorylated KSR were indeed detected in the nuclear fractions of 293/RGS19 cells. Co-immunoprecipitation assays revealed that Nm23H1/2 can form complexes with RGS19, Ras, or KSR. siRNA-mediated knockdown of Nm23H1/2 allowed 293/RGS19 cells to partially recover their ERK responses to serum treatment, while overexpression of Nm23H1/2 in HEK293 cells suppressed the serum-induced ERK response. This study demonstrates that expression of RGS19 can suppress Ras-mediated signaling via upregulation of Nm23.

Singular features of trypanosomatids' phosphotransferases involved in cell energy management

Trypanosomatids are responsible for economically important veterinary affections and severe human diseases. In Africa, Trypanosoma brucei causes sleeping sickness or African trypanosomiasis, while in America, Trypanosoma cruzi is the etiological agent of Chagas disease. These parasites have complex life cycles which involve a wide variety of environments with very different compositions, physicochemical properties, and availability of metabolites. As the environment changes there is a need to maintain the nucleoside homeostasis, requiring a quick and regulated response. Most of the enzymes required for energy management are phosphotransferases. These enzymes present a nitrogenous group or a phosphate as acceptors, and the most clear examples are arginine kinase, nucleoside diphosphate kinase, and adenylate kinase. Trypanosoma and Leishmania have the largest number of phosphotransferase isoforms ever found in a single cell; some of them are absent in mammals, suggesting that these enzymes are required in many cellular compartments associated to different biological processes. The presence of such number of phosphotransferases support the hypothesis of the existence of an intracellular enzymatic phosphotransfer network that communicates the spatially separated intracellular ATP consumption and production processes. All these unique features make phosphotransferases a promising start point for rational drug design for the treatment of human trypanosomiasis.

Nucleotide-dependent self-assembly of Nucleoside Diphosphate Kinase (NDPK) in vitro

In addition to their role in nucleotide homeostasis, members of the Nucleoside Diphosphate Kinase (NDPK) family have been implicated in tumor metastasis, cell migration and vesicle trafficking. Although its role in most cases depends on nucleotide catalysis, a precise understanding of how the catalytic activity of NDPK supports its function in diverse processes is lacking. Here we report that wild type, but not catalytically inactive (H118C) NDPKB, undergoes dynamic self-assembly into ordered 20-25 nm diameter filaments in vitro. Self-assembly is nucleoside triphosphate dependent, GTP being most effective at promoting polymer formation. In addition, polymerization appears to depend on formation of the phosphoryl-Histidine intermediate of the enzyme, suggesting a previously unappreciated conformational change in NDPK during its catalytic cycle. We hypothesize that the observed nucleotide-dependent self-assembly property of NDPKB may reflect a key feature of NDPK enzymes that enables their function in diverse processes.

Trypanosoma cruzi: multiple nucleoside diphosphate kinase isoforms in a single cell

Nucleoside diphosphate kinases (NDPKs) are multifunctional enzymes involved mainly in the conservation of nucleotides and deoxynucleotides at intracellular levels. Here we report the characterization of two NDPKs from the protozoan parasite Trypanosoma cruzi, the etiological agent of Chagas disease. TcNDPK1 and TcNDPK2 were biochemically characterized presenting different kinetic parameters and regulation mechanisms. NDPK activity was mainly detected in soluble fractions according to the digitonin extraction technique; however 20% of the activity remains insoluble at digitonin concentrations up to 5 mg ml(-1). TcNDPK1 is a short enzyme isoform, whereas TcNDPK2 is a long one containing a DM10 motif. In addition, two other putative NDPK genes (TcNPDK3 and TcNDPK4) were detected by data mining at the T. cruzi genome database. The large number and diversity of NDPK isoforms are in agreement with those previously observed for other T. cruzi phosphotransferases, such as adenylate kinases.

A cell-physiological description of GE, a GTP-binding protein that mediates exocytosis

Introduction of GTP gamma S or other non-metabolic analogues of GTP into permeabilized myeloid granulocytes (mast cells, eosinophils, neutrophils) constitutes a sufficient stimulus to induce exocytosis. We concentrate on mast cells. Exocytosis from cells permeabilized in isotonic glutamate solution proceeds in the absence of ATP and at exceedingly low levels (< 10(-9) M) of Ca2+. Mg2+ strongly promotes GTP gamma S-induced exocytosis but this requirement can be spared and then obliterated by lifting Ca2+ through 10(-7) to 10(-6) M. GTP provides only a modest support to exocytosis but becomes almost equipotent with GTP gamma S when Mg2+ is excluded. Ca2+ alone is unable to induce exocytosis. We envisage that the terminal stage of exocytosis (membrane fusion) requires activation of GE, a putative GTPase so far undefined as a molecular entity. Ca2+, presumed to act through a Ca(2+)-binding protein (CE, also undefined) supports exocytosis by promoting the exchange of guanine nucleotides on GE. In the absence of Mg2+ the onset of exocytosis is characterized by delays that have concentration-dependent (binding) and independent components. The latter are sensitive to the identity of the stimulating nucleotide (GTP < GTP gamma S < Gpp [NH]p) and may reflect activation of GE. The activation by Ca2+ and Mg2+ and the delays preceding onset of GTP gamma S-triggered exocytosis are reminiscent of the action of glucagon and Mg2+ in the activation of adenylate cyclase in hepatocyte membranes. The cell-physiological description predicts GE to be an alpha beta gamma heterotrimeric GTP-binding protein with functional similarity to GS.

High energy phosphate transfer by NDPK B/Gbetagammacomplexes--an alternative signaling pathway involved in the regulation of basal cAMP production

The activation of heterotrimeric G proteins induced by G protein coupled receptors (GPCR) is generally believed to occur by a GDP/GTP exchange at the G protein alpha -subunit. Nevertheless, nucleoside diphosphate kinase (NDPK) and the beta-subunit of G proteins (Gbeta) participate in G protein activation by phosphate transfer reactions leading to the formation of GTP from GDP. Recent work elucidated the role of these reactions. Apparently, the NDPK isoform B (NDPK B) forms a complex with Gbetagamma dimers in which NDPK B acts as a histidine kinase phosphorylating Gbeta at His266. Out of this high energetic phosphoamidate bond the phosphate can be transferred specifically onto GDP. The formed GTP binds to the G protein alpha-subunit and thus activates the respective G protein. Evidence is presented, that this process occurs independent of the classical GPCR-induced GTP/GTP exchange und thus contributes, e.g. to the regulation of basal cAMP synthesis in cells.

Interaction of nucleoside diphosphate kinase B with heterotrimeric G protein betagamma dimers: consequences on G protein activation and stability

It is generally accepted that G protein coupled receptors (GPCR) activate heterotrimeric G proteins by inducing a GDP/GTP exchange at the G protein alpha subunit. In addition, the transfer of high energetic phosphate by nucleoside diphosphate kinase (NDPK) and/or the beta subunit of G proteins (Gbeta) can induce G protein activation. Recent evidence suggests that the NDPK isoform B (NDPK B) forms a complex with Gbetagamma dimers. In this complex, NDPK B acts as a protein histidine kinase phosphorylating Gbeta at histidine residue 266 (His266). The high energetic phosphoamidate bond on His266 allows for a phosphate transfer specifically onto GDP and thus local formation of GTP, which binds to and thereby activates the respective G protein alpha subunit. Apparently, this process occurs independent of the classical GPCR-induced GDP/GTP exchange at least for members of the G(s) and G(i) subfamilies of heterotrimeric G proteins. By using a mutant of Gbeta(1) in which His266 was replaced by Leu, it was recently demonstrated that NDPK B/Gbetagamma-mediated G(s) activation contributes by about 50% to basal cAMP formation and contractility in rat cardiac myocytes. Besides its apparent role in G protein activation, the complex formation of NDPK B with Gbetagamma dimers might be essential for G protein stability. Depletion of either the NDPK B orthologue or Gbeta(1) isoforms in zebrafish embryos led to a similar phenotype displaying contractile dysfunction in the heart accompanied by a complete loss of heterotrimeric G protein expression. In conclusion, the interaction of NDKP B with Gbetagamma dimers might play an important role in signal transduction, and alterations in this novel pathway might be of pathophysiological importance.

Activation of the phagocyte NADPH oxidase by Rac Guanine nucleotide exchange factors in conjunction with ATP and nucleoside diphosphate kinase

Activation of the phagocyte NADPH oxidase is the consequence of the assembly of membranal cytochrome b559 with the cytosolic components p47phox, p67phox, and the GTPase Rac and is mimicked by a cell-free system comprising these components and an activator. We designed a variant of this system, consisting of membranes, p67phox) prenylated Rac1-GDP, and the Rac-specific guanine nucleotide exchange factor (GEF) Trio, in which oxidase activation is induced in the absence of an activator and p47phox. We now show that: 1) Trio and another Rac GEF (Tiam1) act by inducing GDP to GTP exchange on prenylated Rac1-GDP and that our earlier assertion that activation is GTP-independent is explained by contamination of p67phox preparations with GTP and/or ATP. 2) Oxidase activation by Rac GEFs is supported not only by GTP but also by ATP. 3) Non-hydrolysable GTP analogs are active, whereas ATP analogs, incapable of gamma-phosphoryl transfer, are inactive. 4) The ability of ATP to support GEF-induced oxidase activation is explained by ATP serving as a gamma-phosphoryl donor for a membrane-localized nucleoside diphosphate kinase (NDPK), converting GDP to GTP. 5) The existence of a NDPK in macrophage membranes is proven by functional, enzymatic, and immunologic criteria. 6) NDPK acts on free GDP, and the newly formed GTP is bound again to Rac. 7) Free GDP is derived exclusively by dissociation from prenylated Rac1-GDP, mediated by GEF. NDPK and GEF appear to be functionally linked in the sense that the availability of GDP, serving as substrate for NDPK, is dependent on the level of activity of GEF.

Nucleoside diphosphate kinase of Mycobacterium tuberculosis acts as GTPase-activating protein for Rho-GTPases

Several bacterial pathogens secrete proteins into the host cells that act as GTPase-activating proteins (GAPs) for Rho-GTPases and convert GTP-bound active form to GDP-bound inactive form. However, no such effector molecule has been identified in Mycobacterium tuberculosis. In this study, we show that culture supernatant of M. tuberculosis H(37)Rv harbors a protein that stimulates the conversion of GTP-bound Rho-GTPases to the GDP-bound form. Nucleoside diphosphate kinase (Ndk) was identified as this culture supernatant protein that stimulated in vitro GTP hydrolysis by members of Rho-GTPases. The histidine-117 mutant of Ndk, which is impaired for autophosphorylation and nucleotide-binding activities, shows GAP activity. These results suggest that Ndk of M. tuberculosis functions as a Rho-GAP to downregulate Rho-GTPases, and this activity may aid in pathogenesis of the bacteria.

Point mutations affecting the oligomeric structure of Nm23-H1 abrogates its inhibitory activity on colonization and invasion of prostate cancer cells

In order to identify Nm23-H1's structural motifs influencing its metastasis-inhibitory activity, we transfected DU 145 human prostate carcinoma cells with the expression vector encoding the Nm23-H1 protein with mutations at the following amino acids: serine-44, a phosphorylation site; proline-96, a site corresponding to the k-pn mutation that causes developmental defects in Drosophila; and serine-120, a site of mutation in human neuroblastoma and phosphorylation. Significant decrease in colonization in soft agar and invasiveness of DU 145 cells was observed in the wild type nm23-H1 transfectants, and also in the serine-44 and serine-120 to alanine mutant nm23-H1-transfected cell lines. However, the k-pn type proline-96 to serine (P96S) and neuroblastoma type serine-120 to glycine (S120G) mutations of Nm23-H1 abrogated its inhibitory activity on colonization and invasion. Meanwhile, all of the recombinant mutant Nm23-H1 proteins produced in Escherichia coli exhibited NDP kinase activity levels at the wild type protein, although the P96S and S120G mutant proteins exhibited decreased histidine protein kinase activity and autophosphorylation level, respectively. Interestingly, only two of the mutant recombinant Nm23-H1 proteins examined, P96S and S120G, exhibited reduced hexameric and increased dimeric oligomerization relative to the wild type. These correlative data suggest that the metastasis-suppressing activity of Nm23-H1 may depend on its oligomeric structure, but not on its NDP kinase activity.

Metastasis suppressor genes: basic biology and potential clinical use

Metastatic disease remains a significant contributor to morbidity and mortality in patients with breast cancer. An improved molecular and biochemical understanding of the metastatic process is expected to fuel the development of new therapeutic approaches. The suppression of tumor metastasis, despite tumor cell expression of oncogenes and metastasis-promoting events, has become a diverse and fruitful field of investigation. Although many genetic events promote metastasis, several genes show relatively reduced expression levels in metastatic tumor cells in mouse model systems and in aggressive human tumors. Re-expression of a metastasis-suppressor gene in a metastatic tumor cell line results in a significant reduction in metastatic behavior in vivo with no effect on tumorigenicity. The known metastasis-suppressor gene products nm23, KAI1, mitogen-activated protein kinase kinase 4, breast cancer metastasis suppressor-1, KiSS1, RHOGDI2, CRSP3, and vitamin D3-upregulated protein/thioredoxin interacting protein exhibit unexpected biochemical functions that have shed new light on signaling events that are important in metastasis. Most metastasis suppressors function at the translationally important stage of outgrowth of micrometastatic tumor cells at a distant site. We hypothesize that elevation of metastasis suppressor gene expression in micrometastatic tumor cells in the adjuvant high-risk population of patients with breast cancer will halt metastatic colonization and have a clinical benefit. DNA methylation inhibitors have shown limited promise in increasing metastasis-suppressor gene expression, and ligands of the nuclear hormone receptor family are currently under investigation in vitro and in vivo. Clinical testing of agents that increase metastasis-suppressor gene expression is expected to require tailored trial designs.

Mammalian protein histidine kinases

The existence of protein kinases, known as histidine kinases, which phosphorylate their substrates on histidine residues has been well documented in bacteria and also in lower eukaryotes such as yeast and plants. Their biological roles in cellular signalling pathways within these organisms have also been well characterised. The evidence for the existence of such enzymes in mammalian cells is much less well established and little has been determined about their cellular functions. The aim of the current review is to present a summary of what is known about mammalian histidine kinases. In addition, by consideration of the chemistry of phosphohistidine, what is currently known of some mammalian histidine kinases and the way in which they act in bacteria and other eukaryotes, a general role for mammalian histidine kinases is proposed. A histidine kinase phosphorylates a substrate protein, by virtue of the relatively high free energy of hydrolysis of phosphohistidine the phosphate group is easily transferred to either a small molecule or another protein with which the phosphorylated substrate protein specifically interacts. This allows a signalling process to occur, which may be downregulated by the action of phosphatases. Given the known importance of protein phosphorylation to the regulation of almost all aspects of cellular function, the investigation of the largely unexplored area of histidine phosphorylation in mammalian cells is likely to provide a greater understanding of cellular action and possibly provide a new set of therapeutic drug targets.

Activation of heterotrimeric G proteins by a high energy phosphate transfer via nucleoside diphosphate kinase (NDPK) B and Gbeta subunits. Specific activation of Gsalpha by an NDPK B.Gbetagamma complex in H10 cells

Formation of GTP by nucleoside diphosphate kinase (NDPK) can contribute to G protein activation in vitro. To study the effect of NDPK on G protein activity in living cells, the NDPK isoforms A and B were stably expressed in H10 cells, a cell line derived from neonatal rat cardiomyocytes. Overexpression of either NDPK isoform had no effect on cellular GTP and ATP levels, basal cAMP levels, basal adenylyl cyclase activity, and the expression of G(s)alpha and G(i)alpha proteins. However, co-expression of G(s)alpha led to an increase in cAMP synthesis that was largely enhanced by the expression of NDPK B, but not NDPK A, and that was confirmed by direct measurement of adenylyl cyclase activity. Cells expressing an inactive NDPK B mutant (H118N) exhibited a decreased cAMP formation in response to G(s)alpha. Co-immunoprecipitation studies demonstrated a complex formation of the NDPK with Gbetagamma dimers. The overexpression of NDPK B, but not its inactive mutant or NDPK A, increased the phosphorylation of Gbeta subunits. In summary, our data demonstrate a specific NDPK B-mediated activation of a G protein in intact cells, which is apparently caused by formation of NDPK B.Gbetagamma complexes and which appears to contribute to the receptor-independent activation of heterotrimeric G proteins.

Histidine kinases and histidine phosphorylated proteins in mammalian cell biology, signal transduction and cancer

Intensive investigation of protein tyrosine, serine and threonine phosphorylation has lead to advances in signal transduction research and cancer treatment. This feature summarizes research on mammalian proteins exhibiting histidine phosphorylation. Histidine kinases are well known in prokaryotic and lower eukaryotic systems where they form the 'two-component' signal transduction system. The relative invisibility of histidine phosphorylation in mammalian cells may result from technical obstacles such as its acid lability, which precludes detection in electrophoretic systems, amino acid sequencing, etc. Emerging data have identified mammalian histidine kinases for the kinase suppressor of ras, a scaffold molecule for the Map kinase pathway, as well as histone H4, aldolase C and the beta-subunit of heterotrimeric G proteins. Additional mammalian proteins of interest to signal transduction and cancer research exhibit histidine phosphorylation, including P-selectin, annexin I and the 20S proteasome. Other candidate histidine phosphorylated proteins are identified. These data suggest the existence of another series of phosphorylation patterns in signal transduction.

Nm23-H1 metastasis suppressor phosphorylation of kinase suppressor of Ras via a histidine protein kinase pathway

The metastasis-suppressive activity of Nm23-H1 was previously correlated with its in vitro histidine protein kinase activity, but physiological substrates have not been identified. We hypothesized that proteins that interact with histidine kinases throughout evolution may represent partners for Nm23-H1 and focused on the interaction of Arabidopsis "two-component" histidine kinase ERS with CTR1. A mammalian homolog of CTR1 was previously reported to be c-Raf; we now report that CTR1 also exhibits homology to the kinase suppressor of Ras (KSR), a scaffold protein for the mitogen-activated protein kinase (MAPK) cascade. Nm23-H1 co-immunoprecipitated KSR from lysates of transiently transfected 293T cells and at endogenous protein expression levels in MDA-MB-435 breast carcinoma cells. Autophosphorylated recombinant Nm23-H1 phosphorylated KSR in vitro. Phosphoamino acid analysis identified serine as the major target, and two peaks of Nm23-H1 phosphorylation were identified upon high performance liquid chromatography analysis of KSR tryptic peptides. Using site-directed mutagenesis, we found that Nm23-H1 phosphorylated KSR serine 392, a 14-3-3-binding site, as well as serine 434 when serine 392 was mutated. Phosphorylated MAPK but not total MAPK levels were reduced in an nm23-H1 transfectant of MDA-MB-435 cells. The data identify a complex in vitro histidine-to-serine protein kinase pathway, which may contribute to signal transduction and metastasis.

Tumor metastasis suppressor nm23H1 regulates Rac1 GTPase by interaction with Tiam1

The putative tumor metastasis suppressor nm23H1 was originally identified in murine melanomas by subtraction cloning. It displays nucleoside diphosphate kinase activity and regulates cellular events, including growth and development. Recently nm23H1 has been reported to also act as a GTPase-activating protein of the Ras-related GTPase Rad. We attempted to determine whether nm23H1 also regulates Rho-family GTPases. Although we were unable to detect a direct association between nm23H1 and Rho-family GTPases, nm23H1 was shown to be associated with a Rac1-specific nucleotide exchange factor, Tiam1, by interaction with its amino-terminal region in extracts from the cells expressing exogenous Tiam1 and from native tissue. Overexpression of nm23H1 inhibited the Tiam1-induced production of GTP-bound Rac1 and activation of c-Jun kinase. On the other hand, forced overexpression of the wild type, but not the kinase-inactivated mutant of nm23H1, converted the GDP-bound forms of Rac1, Cdc42, and RhoA to their GTP-bound forms in vitro by its nucleoside diphosphate kinase activity, but nm23H1 alone apparently did not produce the GTP-bound form of these GTPases in vivo. These results suggest that nm23H1 negatively regulates Tiam1 and inhibits Rac1 activation in vivo. Moreover, adhesion-stimulated membrane ruffles of Rat1 fibroblasts were reduced by overexpression of nm23H1. Based on these observations, we concluded that we had identified a function of nm23H1 as a regulator of Rac1 and that it may be related to the effect of nm23H1 as a tumor metastasis suppressor.

Interaction of the Ras-related protein associated with diabetes rad and the putative tumor metastasis suppressor NM23 provides a novel mechanism of GTPase regulation

Rad is the prototypic member of a new class of Ras-related GTPases. Purification of the GTPase-activating protein (GAP) for Rad revealed nm23, a putative tumor metastasis suppressor and a development gene in Drosophila. Antibodies against nm23 depleted Rad-GAP activity from human skeletal muscle cytosol, and bacterially expressed nm23 reconstituted the activity. The GAP activity of nm23 was specific for Rad, was absent with the S105N putative dominant negative mutant of Rad, and was reduced with mutations of nm23. In the presence of ATP, GDP.Rad was also reconverted to GTP.Rad by the nucleoside diphosphate (NDP) kinase activity of nm23. Simultaneously, Rad regulated nm23 by enhancing its NDP kinase activity and decreasing its autophosphorylation. Melanoma cells transfected with wild-type Rad, but not the S105N-Rad, showed enhanced DNA synthesis in response to serum; this effect was lost with coexpression of nm23. Thus, the interaction of nm23 and Rad provides a potential novel mechanism for bidirectional, bimolecular regulation in which nm23 stimulates both GTP hydrolysis and GTP loading of Rad whereas Rad regulates activity of nm23. This interaction may play important roles in the effects of Rad on glucose metabolism and the effects of nm23 on tumor metastasis and developmental regulation.

Na+ and K+ regulate the phosphorylation state of nucleoside diphosphate kinase in human airway epithelium

We describe how cations, in the presence of ATP, regulate the phosphorylated form of 19- and 21-kDa nucleoside diphosphate kinase (NDPK; EC 2.7.4.6), a kinase controlling K+ channels, G proteins, cell secretion, cellular energy production, and UTP synthesis. In apically enriched human nasal epithelial membranes, 10 mM Na+ inhibits phosphorylation of NDPK relative to other cations. Dose response showed that, whereas K+ induces a fourfold greater phosphate incorporation (EC50 10 mM), Na+ is inhibitory (EC50 10 mM) compared with respective buffer controls. Cation discrimination is nucleotide selective (not seen with [gamma-32P]GTP) and NDPK specific (not seen with p37h, a previously characterized Cl--sensitive phosphoprotein). Na+ does not exert an inhibitory effect on NDPK phosphorylation directly but is likely to act via an okadaic acid-insensitive phosphatase. We speculate that the ability of NDPK to discriminate between physiologically relevant cation concentrations provides a novel example of cross talk within the apical membrane.

ATP-dependent activation of the atrial acetylcholine-induced K+ channel does not require nucleoside diphosphate kinase activity

Prior reports by others have shown that cytoplasmically applied ATP can activate the acetylcholine-induced K+ channel in inside-out atrial membrane patches when no guanine nucleotides are present in the solution bathing the cytosolic face of the membrane. A nucleoside diphosphate kinase mechanism was proposed to explain the activation by ATP. We show in the present study that cytoplasmic adenylylimidodiphosphate mimics the activation by ATP. Unlike ATP, the activation by adenylylimidodiphosphate does not subside on washout. Although commercially available adenylylimidodiphosphate is contaminated by guanylylimidodiphosphate, the activation by adenylylimidodiphosphate still occurs after HPLC purification to remove guanine nucleotide contamination. Adenylylimidodiphosphate does not support phosphotransferase activity by nucleoside diphosphate kinase. Therefore, nucleoside diphosphate kinase activity cannot explain the activation of atrial acetylcholine-induced K+ current by ATP and adenylylimidodiphosphate. We hypothesize that the activation by millimolar concentrations of ATP is due to binding of adenine nucleotide to the guanine nucleotide binding site of the G protein(s) responsible for stimulating the acetylcholine-induced K+ current.

Opioid inhibition of adenylyl cyclase in membranes from pertussis toxin-treated NG108-15 cells

Gi/Go proteins are uncoupled from receptors by ADP-ribosylation with pertussis toxin (PTX). However, PTX treatment of delta opioid receptor-containing NG108-15 cells reduces, but does not eliminate, opioid inhibition of adenylyl cyclase. The present study explored potential mechanisms of this residual inhibition. Overnight treatment of NG108-15 cells with 100 ng/ml PTX eliminated both PTX-catalyzed [adenylyl-32P]NAD+-labeling of G proteins and agonist stimulation of low Km GTPase in membranes. Although PTX-treatment decreased the maximal opioid inhibition of adenylyl cyclase by 50-65%, the inhibition that remained was concentration-dependent and antagonist-reversible. This inhibition persisted in the absence of GTP (even though opioid inhibition of adenylyl cyclase in untreated membranes was GTP-dependent), but was eliminated by hydrolysis-resistant guanine nucleotide analogs, indicating that G-proteins were still involved in the coupling mechanism. However, assays of agonist-stimulated [35S]GTPgammaS binding in the presence of excess GDP indicated that PTX pretreatment eliminated stimulation of guanine nucleotide exchange by opioid agonists. These results suggest that in membranes from PTX-treated NG108-15 cells, a subpopulation of G proteins may transduce an inhibitory signal from agonist-bound opioid receptors without involvement of guanine nucleotide exchange.

Site-directed mutation of Nm23-H1. Mutations lacking motility suppressive capacity upon transfection are deficient in histidine-dependent protein phosphotransferase pathways in vitro

We previously compared the structure and motility suppressive capacity of nm23-H1 by transfection of wild type and site-directed mutant forms into breast carcinoma cells. Wild type nm23-H1 and an nm23-H1(S44A) (serine 44 to alanine) mutant suppressed motility, whereas the nm23-H1(P96S), nm23-H1(S120G), and to a lesser extent, nm23-H1(S120A) mutant forms failed to do so. In the present study wild type and mutant recombinant Nm23-H1 proteins have been produced, purified, and assayed for phosphorylation and phosphotransfer activities. We report the first association of Nm23-H1 mutations lacking motility suppressive capacity with decreased in vitro activity in histidine-dependent protein phosphotransferase assays. Nm23-H1(P96S), a Drosophila developmental mutation homolog, exhibited normal autophosphorylation and nucleoside-diphosphate kinase (NDPK) characteristics but deficient phosphotransfer activity in three histidine protein kinase assays, using succinic thiokinase, Nm23-H2, and GST-Nm23-H1 as substrates. Nm23-H1(S120G), found in advanced human neuroblastomas, exhibited deficient activity in several histidine-dependent protein phosphotransfer reactions, including histidine autophosphorylation, downstream phosphorylation on serines, and slightly decreased histidine protein kinase activity; significant NDPK activity was observed. The Nm23-H1(S120A) mutant was deficient in only histidine-dependent serine autophosphorylation. Nm23-H1 and Nm23-H1(S44A) exhibited normal activity in all assays conducted. Based on this correlation, we hypothesize that a histidine-dependent protein phosphotransfer activity of Nm23-H1 may be responsible for its biological suppressive effects.

Mutational analysis of nucleoside diphosphate kinase from Pseudomonas aeruginosa: characterization of critical amino acid residues involved in exopolysaccharide alginate synthesis

We report the utilization of site-directed and random mutagenesis procedures in the gene encoding nucleoside diphosphate kinase (ndk) from Pseudomonas aeruginosa in order to examine the role of Ndk in the production of alginate by this organism. Cellular levels of the 16-kDa form of the Ndk enzyme are greatly reduced in P. aeruginosa 8830 with a knockout mutation in the algR2 gene (8830R2::Cm); this strain is also defective in the production of the exopolysaccharide alginate. In this study, we isolated four mutations in ndk (Ala-14-->Pro [Ala14Pro], Gly21Val, His117Gln, and Ala125Arg) which resulted in the loss of Ndk biochemical activity; hyperexpression of any of these four mutant genes did not restore alginate production to 8830R2::Cm. We identified six additional amino acid residues (Ser-43, Ala-56, Ser-69, Glu-80, Gly-91, and Asp-135) whose alteration resulted in the inability of Ndk to complement alginate production. After hyperproduction in 8830R2::Cm, it was determined that each of these six mutant Ndks was biochemically active. However, in four cases, the in vivo levels of Ndk were reduced, which consequently affected the growth of 8830R2::Cm in the presence of Tween 20. Two mutant Ndk proteins which could not complement the alginate synthesis defect in 8830R2::Cm were not affected in any characteristic examined in the present study. All of the mutant Ndks characterized which were still biochemically active formed membrane complexes with Pk, resulting in GTP synthesis. Two of the four Ndk activity mutants (His117Gln and Ala125Arg) identified were capable of being truncated to 12 kDa and formed a membrane complex with Pk; however, the complexes formed were inactive for GTP synthesis. The other two Ndk activity mutants could be truncated to 12 kDa but were not detected in membrane fractions. These results further our understanding of the role of Ndk in alginate synthesis and identify amino acid residues in Ndk which have not previously been studied as critical to this process.

Nucleoside diphosphate kinase from Pseudomonas aeruginosa: characterization of the gene and its role in cellular growth and exopolysaccharide alginate synthesis

We report the cloning and determination of the nucleotide sequence of the gene encoding nucleoside diphosphate kinase (Ndk) from Pseudomonas aeruginosa. The amino acid sequence of Ndk was highly homologous with other known bacterial and eukaryotic Ndks (39.9 to 58.3% amino acid identity). We have previously reported that P. aeruginosa strains with mutations in the genes algR2 and algR2 algH produce extremely low levels of Ndk and, as a consequence, are defective in their ability to grow in the presence of Tween 20, a detergent that inhibits a kinase which can substitute for Ndk. Hyperexpression of ndk from the clone pGWS95 in trans in the P. aeruginosa algR2 and algR2 algH double mutant restored Ndk production to levels which equalled or exceeded wild-type levels and enabled these strains to grow in the presence of Tween 20. Hyperexpression of ndk from pGWS95 in the P. aeruginosa algR2 mutant also restored alginate production to levels that were approximately 60% of wild type. Nucleoside diphosphate kinase activity was present in both the cytosolic and membrane-associated fractions of P. aeruginosa. The cytosolic Ndk was non-specific in its transfer activity of the terminal phosphate from ATP to other nucleoside diphosphates. However, the membrane form of Ndk was more active in the transfer of the terminal phosphate from ATP to GDP resulting in the predominant formation of GTP. We report in this work that pyruvate kinase and Ndk form a complex which alters the specificity of Ndk substantially to GTP. The significance of GTP in signal transduction events within the cell and in the production of GDP-mannose, an essential alginate precursor, clearly indicates the importance of Ndk in cellular processes as well as in alginate synthesis.

Nucleotides regulate the binding affinity of the recombinant type A cholecystokinin receptor in CHO K1 cells

Cholecystokinin (CCK) receptors on rat pancreatic acinar cells display two binding affinity states in the presence of adeninine and guanine triphosphates with the effect of ATP mediated by the enzyme nucleoside diphosphate kinase. To determine whether this behavior was intrinsic to a single receptor protein we studied the binding affinity of CHO cells stably transfected with a cloned rat CCKA receptor. 125I-CCK binding to intact cells at 37 degrees C revealed two affinity states for CCK of Kd values 20 pM and 2.4 nM. Membranes prepared from these cells displayed a single affinity state for CCK but two affinity states could be restored in the presence of GTP[gamma S], ATP and ATP[gamma S] but not AMP-PCP. ATP and ATP[gamma S] but not AMP-PCP were substrates for nucleoside diphosphate kinase present in CHO cell membranes and transferred their terminal phosphate to GDP. These findings indicate that the interconvertible affinity states of the CCK receptor are inherent in a single receptor protein and that nucleoside diphosphate kinase mediates the effect of ATP to regulate these two affinity states.

Activation of GTP formation and high-affinity GTP hydrolysis by mastoparan in various cell membranes. G-protein activation via nucleoside diphosphate kinase, a possible general mechanism of mastoparan action

The wasp venom, mastoparan (MP), is a direct activator of reconstituted pertussis toxin-sensitive G-proteins and of purified nucleoside diphosphate kinase (NDPK) [E.C. 2.6.4.6.]. In HL-60 membranes, MP activates high-affinity GTPase [E.C. 3.6.1.-] and NDPK-catalyzed GTP formation, but not photolabeling of G-protein alpha-subunits with GTP azidoanilide; this suggests that the venom activates G-proteins in this system indirectly via stimulation of NDPK. Moreover, the MP analogue, mastoparan 7 (MP 7), is a much more effective activator of reconstituted G-proteins than MP, whereas with regard to NDPK and GTPase in HL-60 membranes, the two peptides are similarly effective. In our present study, we investigated NDPK- and G-protein activation by MP in membranes of the human neuroblastoma cell line, SH-SY5Y, the human erythroleukemia cell line, HEL, the rat basophilic leukemia cell line, RBL 2H3, and the hamster ductus deferens smooth muscle cell line, DDT1MF-2. All these membranes exhibited high NDPK activities that were increased by MP. Compared to basal GTP formation rates, basal rates of high-affinity GTP hydrolysis in cell membranes were low. MP activated high-affinity GTP hydrolysis in cell membranes but did not enhance incorporation of GTP azidoanilide into G-protein alpha-subunits. As with HL-60 membranes, MP and MP 7 were similarly effective activators of NDPK and GTPase in SH-SY5Y membranes. Pertussis toxin inhibited MP-stimulated GTP hydrolyses in SH-SY5Y- and HEL membranes, whereas NDPK activations by MP were pertussis toxin-insensitive. Our data suggest that indirect G-protein activation via NDPK is not restricted to HL-60 membranes but is a more general mechanism of MP action in cell membranes. Pertussis toxin-catalyzed ADP-ribosylation of alpha-subunits may inhibit the transfer of GTP from NDPK to G-proteins. NDPK may play a much more important role in transmembrane signal transduction than was previously appreciated and, moreover, the GTPase of G-protein alpha-subunits may serve as GDP-synthase for NDPK.

NDP kinase can modulate contraction of Dictyostelium cytoskeletons

Extraction of Dictyostelium amoebae with Triton X-100 produces robust cytoskeletons composed mainly of actin and myosin II. These cytoskeletons rapidly contract when mixed with Mg-ATP in simple buffers. The Triton-soluble fraction was found to contain a GTP-dependent activity that prevented contraction by Mg-ATP. This activity was purified, and identified, as nucleoside diphosphate kinase (NDP kinase). The apparent inhibition resulted from pre-contraction of the cytoskeletons. Tightly bound cytoskeletal ADP was presumably phosphorylated, and the resulting ATP powered contraction. NDP kinase appeared to be unique in this capacity, since other regenerating systems did not cause pre-contraction. Reconstitution experiments demonstrated that the kinase must be in physical contact with the cytoskeleton. These results suggest that Dictyostelium NDP kinase is able to channel ATP to the myosin molecule, and this could play a role in directly regulating cytoskeletal contraction or in facilitating contraction under conditions where intracellular ATP concentrations are low. This ability to modulate cytoskeletal contraction could help to explain observations in other systems whereby defects in NDP kinase result in abnormal development or changes in the metastatic potential of cancer cells.

Direct and indirect receptor-independent G-protein activation by cationic-amphiphilic substances. Studies with mast cells, HL-60 human leukemic cells and purified G-proteins

Studies from several laboratories have revealed that structurally diverse substances including the wasp venom, mastoparan (MP), activate purified regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins) in a receptor-independent manner, presumably by mimicking the effects of heptahelical receptors. Mast cells and differentiated HL-60 human leukemic cells are useful model systems for the analysis of receptor-independent G-protein activation. We compared the effects of 2-phenylhistamines which are cationic-amphiphilic, too, and of MP on G-protein activation in dibutyryl cAMP-differentiated HL-60 cells and in the rat basophilic leukemia cell line, RBL 2H3. In HL-60 cells, 2-phenylhistamines show stimulatory effects which resemble those of formyl peptide receptor agonists but which cannot be attributed to agonism at classical receptors. 2-phenylhistamines do not, however, activate RBL 2H3 cells and various other myeloid cell types, pointing to cell type-specificity of receptor-independent G-protein activation. In HL-60 cells, MP shows effects on G-protein activation which differ substantially from those of formyl peptides. In RBL 2H3 membranes, MP shows similar effects on G-protein activation as in HL-60 membranes. We develop a model according to which receptor-independent G-protein activation can be subdivided into direct and indirect receptor-independent G-protein activation. In case of the former mechanism, substances like 2-phenylhistamines interact with G-protein alpha-subunits and in case of the latter mechanism, substances like MP interact with nucleoside diphosphate kinase which catalyzes the formation of GTP. This newly formed GTP is then transferred to, and cleaved by, G-protein alpha-subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by cromoglycate and some flavonoids of nucleoside diphosphate kinase and of exocytosis from permeabilized mast cells

1. The anti-allergic compound, cromoglycate, is reported to possess affinity for, and to suppress the autophosphorylation of a 72kDa protein having the sequence of nucleoside diphosphate kinase (NDPK). 2. We have tested the ability of cromoglycate, and a panel of ten structurally related flavonoids of plant origin, to inhibit the NDPK reaction and the exocytotic process of permeabilized mast cells. The conditions of permeabilization (use of an isotonic medium based on sodium glutamate) were selected to ensure that NDPK activity would be an essential component in the induction of Ca(2+)-induced exocytosis in which ATP is required for generation of GTP. For comparison, we also measured the inhibition of exocytosis induced by GTP-gamma-S; this proceeds in the absence of ATP and bypasses the need for NDPK activity. 3. We found that cromoglycate does not discriminate between Ca2+ and GTP-gamma-S-induced exocytosis and is a poor inhibitor of NDPK activity. Concentrations in the millimolar range are required for inhibition of all these functions. By comparison, many of the flavonoids are effective at concentrations in the micromolar range. 4. While we were unable to discern any systematic relationships between their ability to inhibit the three functions, two compounds, quercetin and genistein, inhibit Ca(2+)-induced, but not GTP-gamma-S-induced exocytosis. Inhibition of the late stages of the stimulus-response pathway in mast cells by these compounds is therefore likely to be due to inhibition of NDPK and the consequent failure to generate GTP.

Increased expression of the NME1 gene is associated with metastasis in epithelial ovarian cancer

The genetic events involved in the development of metastases of epithelial ovarian cancer are largely unknown. One gene postulated to play a role in tumour metastasis suppression is NME1 (nm23-H1), and an inverse relationship between NME1 expression and metastatic potential has been observed for some solid tumours. In this study we have investigated the levels of mRNA expression of the 2 isoforms of the NME gene, NME1 and NME2. A maximum of 45 tumours samples from 33 patients were available for Northern blot analysis. We observed variable levels expression of NME1 and NME2 mRNA. The average level of NME1, but not NME2, mRNA expression was statistically higher in metastatic biopsies when compared with primary tumour biopsies. To examine the possible tumour suppressor gene role of NME1 in ovarian tumours, 76 patients were investigated by Southern blot analysis to determine the rate of allelic deletion. Allele loss at 5 other chromosome 17 loci (D17S5, TP53, NF1, D17S74, D17S4) was also evaluated for many of these 76 patients. Allele loss was observed in 22/30 (73%) informative patients at the NME1 locus. We also observed high rates of allele loss at the other loci evaluated. No correlations with clinical stage, histological subtype or patient survival were observed in either mRNA or DNA analyses. We have established that tumour progression in ovarian cancer is accompanied by over-expression of the NME1 gene; however, despite high rates of allele loss at the NME1 locus, the concept that NME1 may be a candidate tumour suppressor gene in ovarian cancer cannot be confirmed by this study.

The myristoylated amino terminus of ADP-ribosylation factor 1 is a phospholipid- and GTP-sensitive switch

ADP-ribosylation factor 1 (Arf1) is an essential N-myristoylated 21-kDa GTP-binding protein with activities that include the regulation of membrane traffic and phospholipase D activity. Both the N terminus of the protein and the N-myristate bound to glycine 2 have previously been shown to be essential to the function of Arf in cells. We show that the bound nucleotide affects the conformation of either the N terminus or residues of Arf1 that are in direct contact with the N terminus. This was demonstrated by examining the effects of mutations in this N-terminal domain on guanosine 5'-O-(3-thio)triphosphate (GTP gamma S) and GDP binding and dissociation kinetics. Arf1 mutants, lacking 13 or 17 residues from the N terminus or mutated at residues 3-7, had a greater affinity for GTP gamma S and a lower affinity for GDP than did the wild-type protein. As the N terminus is required for interactions with target proteins, we conclude that the N terminus of Arf1 is a GTP-sensitive effector domain. When Arf1 was acylated, the GTP-dependent conformational changes were codependent on added phospholipids. In the absence of phospholipids, myristoylated Arf1 has a lower affinity for GTP gamma S than for GDP, and in the presence of phospholipids, the myristoylated protein has a greater affinity for GTP gamma S than for GDP. Thus, N-myristoylation is a critical component in the construction of this phospholipid- and GTP-dependent switch.

Effect of retinoic acid on Nm/23 nucleoside diphosphate kinase and components of cyclic adenosine monophosphate-dependent signalling in human neuroblastoma cell lines

The effects of retinoic acid on components of the cAMP-dependent signalling system were examined in two related human neuroblastoma cell lines SK-N-SH-F (SHF) and SK-N-SH-N (SHN). Retinoid treatment for a week significantly increased the concentration of intracellular cAMP and the levels of activity of protein kinase A and adenylate cyclase in both cell lines. Retinoic acid treatment also caused a very marked translocation of nucleoside diphosphate kinase from the cytosol to the membrane fraction. The increases in cyclic nucleotide and protein kinase A activity were observed to occur as early as within 1 and 2 days respectively and preceded or were concurrent with the onset of observable morphological differentiation. Results also indicated that agents which elevated intracellular cAMP caused neuronal differentiation and blunted retinoic acid-induced melanocytic differentiation in SHF cells. However, increases in cAMP brought about by treatment of SHF cells with retinoic acid alone were several-fold smaller and thus insufficient to induce neuritogenesis in these cells. The results as a whole indicate that one overall effect of retinoic acid treatment is to upgrade the activity of components of the cAMP-dependent signalling system in both neuroblastoma cell lines. However, retinoic acid causes the SH-F and SH-N cell lines to differentiate along different routes which means that the upgrading responses may be related to more general aspects of differentiation rather than to specific phenotype expression.

Expression of functional proteins of cDNA encoding rice nucleoside diphosphate kinase (NDK) in Escherichia coli and organ-related alteration of NDK activities during rice seed germination (Oryza sativa L.)

The GST (glutathione S-transferase)-NDK (nucleoside diphosphate kinase) fusion protein was expressed in Escherichia coli. The GST-NDK protein was capable of transferring gamma-phosphate from ATP to nucleoside diphosphates such as GDP, CDP, TDP and UDP. Western blot analysis using anti-NDK antibody indicated that NDK in endosperm gradually decreased during 36 h of imbibition. On the contrary, NDK in embryo increased during the same period. NDK activities in both tissues were in accord with these observations. Whereas the NDK protein in roots of rice seedlings during 7 days of imbibition remained constant, in shoots it declined after 5 days of imbibition. Thus, NDK may play a significant role in the cellular event modulated by adenylate energy charge level.

Purification and characterization of a soluble nucleoside diphosphate kinase in Trypanosoma cruzi

A soluble nucleoside diphosphate kinase (NDP kinase) was purified and characterized in epimastigote forms of Trypanosoma cruzi. The enzyme was purified by affinity chromatography on Blue-agarose and Q-Sepharose columns and by FPLC on a Superose 12 column. A membrane-associated NDP kinase was identified which accounts for 30% of total enzymatic activity. Western blot analysis of the soluble NDP kinase revealed a 16.5-kDa monomer recognized by polyclonal antibodies to NDP kinase from Dictyostelium discoideum, Candida albicans or human. Most of the T. cruzi NDP kinase is found in the cell as a hexamer composed of 16.5-kDa monomers. The Km values of the enzyme for ATP, GDP and dTDP were 0.2 +/- 0.008 mM, 0.125 +/- 0.012 mM and 0.4 +/- 0.009 mM, respectively. The parasite enzyme was stable, remained active at 65 degrees C and was found to tolerate up to 2.5 M urea. The 16.5-kDa subunit was phosphorylated with [gamma-32P]ATP or thiophosphorylated with [35S]GTP gamma S. The incubation of the 32P-labelled phosphoenzyme with unlabelled nucleoside 5'-diphosphates resulted in the formation of 32P-labelled nucleoside 5'-triphosphates without strict base specificity, indicating that the reaction mechanism of the T. cruzi enzyme is the same as reported for other NDP kinases. When the phosphoenzyme was incubated with a mixture of nucleoside 5'-diphosphates, GTP was preferentially formed.

Isolation of the myc transcription factor nucleoside diphosphate kinase and the multifunctional enzyme glyceraldehyde-3-phosphate dehydrogenase by cAMP affinity chromatography

Cyclic AMP affinity chromatography applied to various mammalian tissue extracts yielded two proteins in addition to the regulatory subunits of protein kinase. This paper characterizes these proteins and provides a simple procedure for their preparation. The polypeptides (36 kDa and a 19 kDa/21 kDa doublet) were isolated from the cAMP matrix by sequential elution with cAMP solutions of increasing concentrations. Microsequencing was accomplished following chemical or enzymic degradation of isolated polypeptides. Partial amino acid sequences of the 36 kDa protein and analyses of its enzymic activity indicated identity with glyceraldehyde-3-phosphate dehydrogenase whilst the lower MW protein proved to be identical with mammalian nucleoside diphosphate kinase subunits. In both cases, binding to cAMP appeared to occur at the nucleotide (NAD and ATP, respectively) sites. In conclusion, we present a one step-procedure, applicable to tissue and cell extracts, which allows the simultaneous isolation of both glyceraldehyde-3-phosphate dehydrogenase and nucleoside diphosphate kinase. This procedure may help to elucidate the multiple functions of these two important enzymes.

Nm23/nucleoside diphosphate kinase: toward a structural and biochemical understanding of its biological functions

The nm23 gene, a putative metastasis suppressor gene, was originally identified by its reduced expression in highly metastatic K-1735 murine melanoma cell lines, as compared to related, low metastatic melanoma cell lines. Transfection of nm23 cDNA has been reported to suppress malignant progression in Drosophila and mammalian cells. Highly conserved homologues of nm23 have been found in organisms ranging from the prokaryote Myxococcus xanthus to Drosophila, where the gene is involved in normal development and differentiation. The product of the nm23 gene exhibits a nucleoside diphosphate kinase activity, yet the nucleoside diphosphate kinase activity of Nm23 does not correlate with its apparent biological functions. We review recent cellular, genetic, biochemical and X-ray crystallographic data to formulate and evaluate hypotheses concerning the molecular mechanism of nm23 action.

Measurement of nucleoside diphosphate kinase-Nm23 activity by anion-exchange high-performance liquid chromatography

A first-order assay to detect the activity of nucleoside diphosphate kinase (NDP-kinase; EC 2.7.4.6) was developed. In this assay, the activity of NDP-kinase is measured using various deoxy- and ribonucleotide triphosphates as phosphate donors and dADP as phosphate acceptor. The enzyme activity is determined by quantifying, after anion-exchange HPLC, the amount of newly synthesized dATP. Contrary to the most common coupled enzymic assays or isotopic assays the use of different donor-acceptor pairs is not restricted. The resolution of the procedure described is limited only by the chromatographic separation of substrate and product pairs participating in the reaction.

A plant nucleoside diphosphate kinase homologous to the human Nm23 gene product: purification and characterization

Nucleoside diphosphate kinases (NDPKs) catalyze the transfer of high-energy phosphates from nucleoside triphosphates to nucleoside diphosphates and may be involved in the regulation of growth, development, and signal transduction processes. We report here the purification and characterization of NDPK from detergent-solubilized extracts of dark-grown oat (Avena) tissue. The purification was achieved primarily through adsorption to GTP-agarose, followed by elution with ATP. SDS-polyacrylamide gel electrophoresis and gel filtration chromatography indicated that the purified protein is composed of six 18 kDa subunits. Substrate specificity experiments indicated that the purified kinase is capable of using all tested nucleosides as substrates. N-terminal sequencing of the Avena protein revealed that 87% of the 23 amino acids sequenced were identical to the human Nm23 protein, a nucleoside diphosphate kinase identified as a possible tumor metastasis suppressor and transcriptional activator of the myc oncogene.

Molecular cloning, sequence determination and heterologous expression of nucleoside diphosphate kinase from Pisum sativum

Protein sequence data derived from the N-terminal region of a 17 kDa polypeptide associated with the microsomal membrane fraction from Pisum sativum was used to design degenerate oligonucleotides which were used to amplify P. sativum cDNA via the polymerase chain reaction (PCR). Amplified cDNA was used as a probe to screen a P. sativum cDNA library and a cDNA clone, NDK-P1 was isolated and sequenced. The protein encoded by NDK-P1 had a calculated molecular mass of 16,485 Da and possessed substantial homology with nucleoside diphosphate kinases (NDKs) isolated and cloned from other sources. High levels of expression of NDK-P1 protein were achieved in Escherichia coli using a T7-driven expression system. Recombinant NDK-P1 protein was shown to possess NDK activity and had similar biochemical characteristics to NDKs isolated from other sources. The Michaelis constants for a variety of nucleoside diphosphate (NDP) substrates were found to be broadly similar to those reported for other NDKs, with thymidine nucleotides being the substrates of greatest affinity.

Human c-myc transcription factor PuF identified as nm23-H2 nucleoside diphosphate kinase, a candidate suppressor of tumor metastasis

A human gene encoding the c-myc purine-binding transcription factor PuF was identified by screening of a cervical carcinoma cell complementary DNA library with a DNA fragment containing PuF binding sites. The 17-kilodalton bacterially produced PuF was shown to have biological activity and properties similar to that of human PuF. DNA sequence analysis of recombinant PuF revealed perfect identity with the human nm23-H2 nucleoside diphosphate kinase gene, a potential negative regulator of cancer metastasis. These results provide a link between nm23 and the c-myc oncogene and suggest that the nm23 protein can function in vitro in the transcriptional regulation of c-myc expression.

Nm23 and breast cancer metastasis

The majority of breast cancer patients succumb to metastatic disease. We summarize published and recent research concerning the nm23 gene in breast cancer metastasis. In a murine developmental study, nm23 expression increased with the functional differentiation of the mammary gland in nulliparous and pregnant animals. In human breast cancer, five studies have now demonstrated a significant association between reduced nm23 expression, at the RNA or protein levels, and aggressive tumor behavior. Nm23-negative tumor cells have been observed in comedo ductal carcinoma in situ lesions in two independent studies, indicating that decreases in nm23 expression begin prior to actual histologically identifiable invasion. Transfection studies, in which human nm23-H1 cDNA was expressed in the metastatic human MDA-MB-435 breast carcinoma cell line, indicate that nm23-H1 suppresses in vivo metastatic potential by 50-90%. Finally, our data in melanoma and breast carcinoma transfection systems suggest that the biochemical mechanism of nm23 suppressive activity is likely not due to its nucleoside diphosphate kinase activity, association with GAP proteins, or secretion from cells.