A point mutation of human nucleoside diphosphate kinase A found in aggressive neuroblastoma affects protein folding

Evidence for interaction between human PRUNE and nm23-H1 NDPKinase

We have isolated a human and murine homologue of the Drosophila prune gene through dbEST searches. The gene is ubiquitously expressed in human adult tissues, while in mouse developing embryos a high level of expression is confined to the nervous system particularly in the dorsal root ganglia, cranial nerves, and neural retina. The gene is composed of eight exons and is located in the 1q21.3 chromosomal region. A pseudogene has been sequenced and mapped to chromosomal region 13q12. PRUNE protein retains the four characteristic domains of DHH phosphoesterases. The synergism between prune and awdK-pn in Drosophila has led various authors to propose an interaction between these genes. However, such an interaction has never been supported by biochemical data. By using interaction-mating and in vitro co-immunoprecipitation experiments, we show for the first time the ability of human PRUNE to interact with the human homologue of awd protein (nm23-H1). In contrast, PRUNE is impaired in its interaction with nm-23-H1-S120G mutant, a gain-of-function mutation associated with advanced neuroblastoma stages. Consistently, PRUNE and nm23-H1 proteins partially colocalize in the cytoplasm. The data presented are consistent with the view that PRUNE acts as a negative regulator of the nm23-H1 protein. We discuss how PRUNE regulates nm23-H1 protein and postulate possible implications of PRUNE in neuroblastoma progression.

Human nucleoside diphosphate kinase B (Nm23-H2) from melanoma cells shows altered phosphoryl transfer activity due to the S122P mutation

The Ser122 --> Pro mutation in human nucleoside diphosphate kinase (NDK)-B/Nm23-H2 was recently found in melanoma cells. In comparison to the wild-type enzyme, steady state activity of NDKS122P with ATP and TDP as substrates was slowed down 5-fold. We have utilized transient kinetic techniques to analyze phosphoryl transfer between the mutant enzyme and various pairs of nucleoside triphosphates and nucleoside diphosphates. The two half-reactions of phosphorylation and dephosphorylation of the active site histidine residue (His118) were studied separately by making use of the intrinsic fluorescence changes which occur during these reactions. All apparent second order rate constants are drastically reduced, falling 5-fold for phosphorylation and 40-200-fold for dephosphorylation. Also, the reactivity of the mutant with pyrimidine nucleotides and deoxy nucleotides is more than 100-fold reduced compared with the wild-type. Thus, the rate-limiting step of the NDK-BS122P-catalyzed reaction is phosphoryl transfer from the phospho-enzyme intermediate to the nucleoside diphosphate and not phosphoryl transfer from the nucleoside triphosphate to the enzyme as was found for the wild-type protein. This results in a pronounced shift of the equilibrium between unphosphorylated and phosphorylated enzyme. Moreover, like the Killer-of-prune mutation in Drosophila NDK and the neuroblastoma Ser120 --> Gly mutation in human NDK-A/Nm23-H1, the Ser122 --> Pro substitution in NDK-B affects the stability of the protein toward heat and urea. These significantly altered properties may be relevant to the role of the mutant enzyme in various intracellular processes.

The structure and dynamics of partially folded actin

Steady-state and time-resolved intrinsic fluorescence, fluorescence quenching by acrylamide, and surface testing by hydrophobic label ANS were used to study the structure of inactivated alpha-actin. The results are discussed together with that of earlier experiments on sedimentation, anisotropy of fluorescence, and CD spectrum in the near- and far-UV regions. A dramatic increase in ANS binding to inactivated actin in comparison with native and unfolded protein indicates that the inactivated actin has solvent-exposed hydrophobic clusters on the surface. It results in specific association of actin macromolecules (sedimentation constants for native and inactivated actin are 3 and 20 S, respectively) and, consequently, in irreversibility of native-inactivated actin transition. It was found that, though the fluorescence spectrum of inactivated actin is red-shifted, the efficiency of the acrylamide collision quenching is even lower than that of the intact protein. It suggests that tryptophan residues of inactivated actin are located in the inner region of protein formed by polar groups, which are highly packed. It correlates with the pronounced near-UV CD spectrum of inactivated actin. The experimentally found tryptophan fluorescence lifetimes allowed evaluation rotational correlation times on the basis of Perrin plots. It is found that oscillations of tryptophan residues in inactivated actin are restricted in comparison with native one. The inactivated actin properties were invariant with experimental conditions (ionic strength, the presence of reducing agents), the way of inactivation (Ca2+ and/or ATP removal, heating, 3-5 M urea or 1.5 M GdmCl treatment), and protein concentration (within the limits 0.005-1.0 mg/mL). The same state of actin appears on the refolding from the completely unfolded state. Thermodynamic stability, pronounced secondary structure, and the existing hydrophobic clusters, tested by ANS fluorescence and reversibility of transition inactivated-unfolded forms, allowed us to suggest that inactivated actin can be intermediate in the folding-unfolding pathway.

Wild-type NM23-H1, but not its S120 mutants, suppresses desensitization of muscarinic potassium current

NM23 (NDP kinase) modulates the gating of muscarinic K+ channels by agonists through a mechanism distinct from GTP regeneration. To better define the function of NM23 in this pathway and to identify sites in NM23 that are important for its role in muscarinic K+ channel function, we utilized MDA-MB-435 human breast carcinoma cells that express low levels of NM23-H1. M2 muscarinic receptors and GIRK1/GIRK4 channel subunits were co-expressed in cells stably transfected with vector only (control), wild-type NM23-H1, or several NM23-H1 mutants. Lysates from all cell lines tested exhibit comparable nucleoside diphosphate (NDP) kinase activity. Whole cell patch clamp recordings revealed a substantial reduction of the acute desensitization of muscarinic K+ currents in cells overexpressing NM23-H1. The mutants NM23-H1P96S and NM23-H1S44A resembled wild-type NM23-H1 in their ability to reduce desensitization. In contrast, mutants NM23-H1S120G and NM23-H1S120A completely abolished the effect of NM23-H1 on desensitization of muscarinic K+ currents. Furthermore, NM23-H1S120G potentiated acute desensitization, indicating that this mutant retains the ability to interact with the muscarinic pathway, but has properties antithetical to those of the wild-type protein. We conclude that NM23 acts as a suppressor of the processes leading to the desensitization of muscarinic K+ currents, and that Ser-120 is essential for its actions.

Cloning of a second nm23-M1 cDNA: expression in the central nervous system of adult mouse and comparison with nm23-M2 mRNA distribution

Nm23 has been identified as a gene family encoding different isoforms of the nucleoside diphosphate kinase. This protein is a key enzyme in the control of cellular concentrations of nucleoside triphosphates. Moreover, it has been shown to play important roles in various cellular functions such as differentiation and metastasis. In the present study, a second cDNA for nucleoside diphosphate kinase A (Nm23-M1) was isolated from a cDNA library of mouse embryonic stem cells. This clone encodes the same putative 152 aminoacids long protein as an already published cDNA but is longer in both its 5' and 3' untranslated regions. Tissue and cellular distribution of nm23-M1 mRNA was investigated by using Northern blot analysis and in situ hybridization. Nm23-M1 transcripts were found to be widely distributed throughout the mouse central nervous system with prominent expression in several restricted areas. No differences were noticed between the distribution of long and short transcripts. Furthermore, a similar pattern of expression was described in the central nervous system for nm23-M2 mRNA, encoding a second isoform of the nucleoside diphosphate kinase. However, the transcript of this isoform displayed a wider distribution and was expressed in all organs analysed by northern blotting. The possible involvement of nm23-M1 in differentiation of mouse nervous system is further discussed.

Coupling between catalysis and oligomeric structure in nucleoside diphosphate kinase

A dimeric Dictyostelium nucleoside diphosphate kinase has been stabilized by the double mutation P100S-N150stop which targets residues involved in the trimer interface (Karlsson, A., Mesnildrey, S., Xu, Y., Moréra, S., Janin, J., and Veron, M. (1996) J. Biol. Chem. 271, 19928-19934). The reassociation of this dimeric form into a hexamer similar to the wild-type enzyme is induced by the presence of a nucleotide substrate. Equilibrium sedimentation and gel filtration experiments, as well as enzymatic activity measurements, show that reactivation of the enzyme closely parallels its reassociation. A phosphorylatable intermediate with low activity participates in the association pathway while the dimeric form is shown totally devoid of enzymatic activity. Our results support the hypothesis that different oligomeric species of nucleoside diphosphate kinase are involved in different cellular processes where the enzymatic activity is not required.