Ras and the awd couple

NM23/nucleoside diphosphate kinase and signal transduction

NM23s (or NDP kinases) regulate a fascinating variety of cellular activities, including proliferation, development, and differentiation. All these processes are modulated by external stimuli, leading to the idea that this family of proteins modulates transmembrane signaling pathways. This review summarizes the evidence indicating that NM23/NDP kinases participate in transmembrane signaling in eukaryotic cells and discusses the molecular mechanisms proposed to account for these actions.

Isolation of cDNA clones encoding two isoforms of nucleoside diphosphate kinase from bovine retina

The cDNA encoding bovine retinal isoforms of nucleoside diphosphate kinase (NDP-kinase, EC 2.7.4.6) has been cloned and sequenced. Based on the partial amino acid sequence of the enzyme determined after trypsin digestion of purified NDP-kinase, primers were synthesized and used to isolate two different cDNA clones encoding the full length of two NDP-kinase isoforms. The nucleotide sequences of these clones contained open reading frames encoding 152-residue polypeptides with calculated molecular masses of 17.262 and 17.299 kDa, similar to that determined for the subunits of purified enzyme (17.5 and 18.5 kDa). The deduced NDP-kinase sequences showed high similarity with the known NDP-kinase sequences from other sources.

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.

Point mutations in awdKpn which revert the prune/Killer of prune lethal interaction affect conserved residues that are involved in nucleoside diphosphate kinase substrate binding and catalysis

The awd gene of Drosophila melanogaster encodes a nucleoside diphosphate kinase. Killer of prune (Kpn) is a mutation in the awd gene which substitutes Ser for Pro at position 97 and causes dominant lethality in individuals that do not have a functional prune gene. This lethality is not due to an inadequate amount of nucleoside diphosphate (NDP) kinase activity. In order to understand why the prune/Killer of prune combination is lethal, even in the presence of an adequate NDP kinase specific activity level, and to understand the biochemical basis for the conditional lethality of the awdKpn mutation, we generated second site mutations which revert this lethal interaction. All of the 12 revertants we recovered are second site mutations of the awdKpn gene. Three revertants have deletions of the awdKpn protein coding region. Two revertants have substitutions of the initiator methionine and do not accumulate KPN protein. Seven revertants have amino acid substitutions of conserved residues that are likely to affect the active site: five of these have no enzymatic activity and two have a very low level of specific activity. These data suggest that an altered NDP kinase activity is involved in the mechanism underlying the conditional lethality of the awdKpn mutation.

Recombinant rat nucleoside diphosphate kinase isoforms (alpha and beta): purification, properties and application to immunological detection of native isoforms in rat tissues

We previously demonstrated that at least two isoforms of nucleoside diphosphate (NDP) kinase, the products of two different tandemly arrayed genes, are present in rat. To understand the physiological role of each isoform, some biochemical properties of recombinant rat NDP kinase alpha- and beta-isoforms, produced in large amount, were studied. cDNAs of the two isoforms were inserted in an expression vector pET3b and recombinant enzymes were overproduced in Escherichia coli. Their primary structures were different from the native enzymes in that the latter suffer from modification of the NH2-terminal end. The two recombinant isoforms were purified from the cell lysate to apparent homogeneity by ammonium sulfate fractionation followed by three successive column chromatographies. Despite their extreme similarity in the amino-acid sequences, the two showed somewhat different enzymic properties in terms of di- and triphosphate nucleotide substrate specificity. They showed similar mobilities on SDS-PAGE as expected from their calculated molecular weight (alpha-isoform, 17,283 versus beta-isoform, 17,192) but differed in isoelectric point (alpha-isoform, pI 6.7; beta-isoform, pI 7.8) and heat stability. Polyclonal antibody which reacted with both isoforms and alpha-isoform-specific monoclonal antibodies differentially recognized native enzymes from rat tissues after the tissue extracts were separated by isoelectric focusing gel electrophoresis under a denaturation condition. The results showed that the alpha-isoform, though its amount varied from one tissue to another, was the major form in rat tissues examined compared with the beta-isoform which was detectable in brain and testis. There was no preference in their subcellular localization when examined with myelin, synaptosomal supernatant and total homogenate fractions from the rat cerebrum and cerebellum.

The postsynaptic density

The postsynaptic density is a specialization of the nerve cell's submembrane cytoskeleton that is hypothesized to participate in the regulation of synaptic adhesion, transmitter receptor clustering, and modulation of receptor sensitivity. Until recently, many of the major proteins in the highly insoluble postsynaptic density fraction remained uncharacterized. Modern immunological and microsequencing methods now make it possible to define more precisely the molecular composition and function of this intriguing organelle.

Tumour suppressors and the regulation of GTP-binding protein activity

GTP-binding proteins regulate a wide variety of intracellular signalling pathways in eukaryotic cells. The Ras GTP-binding proteins have received a great deal of attention since they were found to be modified by amino acid substitutions in a large number of cancers. It is now clear that Ras plays an essential role in regulating normal cell growth and differentiation, although how this is achieved biochemically is not known. The cellular concentration of Ras bound to GTP appears to be the limiting factor for signalling, and, not surprisingly, it is tightly controlled by both positive and negative regulators. There is now convincing evidence that the loss of one of these negative regulators of Ras, neurofibromin, can contribute to the development of malignancy; thus, neurofibromin behaves as a tumour suppressor gene product.

A Drosophila homolog of bovine smg p25a GDP dissociation inhibitor undergoes a shift in isoelectric point in the developmental mutant quartet

The Drosophila developmental mutation quartet causes late larval lethality and small imaginal discs and, when expressed in the adult female, has a lethal effect on early embryogenesis. These developmental defects are associated with mitotic defects, which include a low mitotic index in larval brains and incomplete separation of chromosomes in mitosis in the early embryo. quartet mutations also have a biochemical effect, i.e., a basic shift in isoelectric point in three proteins. We have purified one of these proteins, raised an antibody to it, and isolated and sequenced its cDNA. At the amino acid level, the sequence shows 68% identity and 81% similarity to bovine smg p25a GDP dissociation inhibitor (GDI), a regulator of ras-like small GTPases of the rab/SEC4/YPT1 subfamily. The correlation between a basic shift in isoelectric point in Drosophila GDI in quartet mutant tissue and the quartet developmental phenotype raises the possibility that a posttranslational modification of GDI is necessary for its function and that GDI function is essential for development.

A Drosophila homolog of bovine smg p25a GDP dissociation inhibitor undergoes a shift in isoelectric point in the developmental mutant quartet

The Drosophila developmental mutation quartet causes late larval lethality and small imaginal discs and, when expressed in the adult female, has a lethal effect on early embryogenesis. These developmental defects are associated with mitotic defects, which include a low mitotic index in larval brains and incomplete separation of chromosomes in mitosis in the early embryo. quartet mutations also have a biochemical effect, i.e., a basic shift in isoelectric point in three proteins. We have purified one of these proteins, raised an antibody to it, and isolated and sequenced its cDNA. At the amino acid level, the sequence shows 68% identity and 81% similarity to bovine smg p25a GDP dissociation inhibitor (GDI), a regulator of ras-like small GTPases of the rab/SEC4/YPT1 subfamily. The correlation between a basic shift in isoelectric point in Drosophila GDI in quartet mutant tissue and the quartet developmental phenotype raises the possibility that a posttranslational modification of GDI is necessary for its function and that GDI function is essential for development.

The rat brain postsynaptic density fraction contains a homolog of the Drosophila discs-large tumor suppressor protein

In CNS synapses, the synaptic junctional complex with associated postsynaptic density is presumed to contain proteins responsible for adhesion between pre- and postsynaptic membranes and for postsynaptic signal transduction. We have found that a prominent, brain-specific protein (PSD-95) enriched in the postsynaptic density fraction from rat brain is highly similar to the Drosophila lethal(1)discs-large-1 (dlg) tumor suppressor protein. The dlg protein is associated with septate junctions in developing flies and contains a guanylate kinase domain that is required for normal control of cell division. The sequence similarity between dlg and PSD-95 suggests that molecular mechanisms critical for growth control in developing organisms may also regulate synapse formation, stabilization, or function in the adult brain.

Ras is essential for nerve growth factor- and phorbol ester-induced tyrosine phosphorylation of MAP kinases

Treatment of PC12 cells with nerve growth factor (NGF) induces a rapid increase in tyrosine phosphorylation of multiple cellular proteins. Expression of a dominant inhibitory Ras mutant specifically blocked NGF- and TPA-induced tyrosine phosphorylation of two proteins of approximately 42 and 44 kd. Conversely, expression of an oncogenic variant of Ras induced tyrosine phosphorylation of the same 42 and 44 kd proteins. The 44 kd protein was immunoprecipitated with an antibody directed against extracellular signal-regulated kinase 1/mitogen-activated protein kinase (MAPK) and the 42 kd protein comigrated with a 42 kd MAPK, indicating that at least one and probably both Ras-regulated phosphoproteins are MAPKs. In addition, MAPK activation, as measured by in vitro phosphorylation of myelin basic protein, was also regulated by Ras. Ras was not required for NGF-induced activation of Trk or tyrosine phosphorylation of PLC-gamma 1. Thus, NGF-induced tyrosine phosphorylation occurs both prior to and following Ras action, and Ras plays a critical role in the NGF- and TPA-induced tyrosine phosphorylation of MAPKs.