Search

Two complementary DNAs (cDNAs) previously isolated, one by functional screening and the other by immunological screening of a Dictyostelium discoideum expression library, encode two proteins, Gip17 and Guk7.2, sharing 71% homology. In the present study, we found that the expression of their messenger RNAs (mRNAs) is developmentally regulated, with a sharp decrease during the first hours of differentiation. The Gip17 protein was purified to homogeneity from D. discoideum amoebas and from recombinant Escherichia coli and was conclusively identified as a nucleoside diphosphate (NDP) kinase. NDP kinases play a major role in synthesis of nucleoside triphosphates and, in many systems, are found associated with guanosine triphosphate (GTP)-binding proteins. We found the Gip17 protein to be 77% homologous to the human Nm23 protein and 75% homologous to the Drosophila melanogaster Awd protein. The levels of murine and human nm23 mRNA and Nm23 protein are significantly reduced in tumor cells of high metastatic potential, suggesting that Nm23 is involved in suppression of mammalian tumor metastasis, and mutants of the awd gene exhibit widespread development abnormalities, suggesting that Awd is involved in D. melanogaster development. The high percentage of homology of the Gip17 and Guk7.2 proteins with the Nm23 and Awd proteins indicates that Nm23 and Awd also have nucleoside diphosphate kinase activity. Possible modulations in the activity of this metabolic enzyme could be related to the altered metabolism of tumor cells and the control of metastatic potential. Our results point to an unexpected role of NDP kinase in development, growth control, and oncogenic transformation.

A lambda gt11 cDNA library from Dictyostelium discoideum was screened by direct labeling of filter replicas with [35S]guanosine 5'-O-(thiotriphosphate) (GTP gamma S). A positive clone was obtained and used as probe to isolate additional clones from which a complete cDNA sequence was determined. The cDNA hybridizes to a single copy gene that is expressed as a 0.6-kilobase mRNA in vegetatively growing amoeba. The open reading frame encodes a protein of 155 amino acids (calculated Mr 16,775), devoid of cysteine residues. The protein contains most of the short consensus motifs characteristic of the catalytic domain of protein kinases although the overall homology with this class of enzymes is not greater than 20%. Its size and amino acid composition indicated that it could be the monomer of a nucleoside diphosphate (NDP) kinase, an enzyme which catalyzes the phosphate transfer from triphospho- to diphosphonucleotides. Indeed, specific NDP kinase activity was found in extracts of bacteria transformed with a plasmid expressing the protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the extracts incubated in the presence of [35S]GTP gamma S revealed a single 35S-labeled band of size corresponding to the protein, which likely represents the stable thiophosphorylated reaction intermediate characteristic for the ping-pong reaction mechanism of NDP kinases. The formation of this labeled intermediate probably allowed the detection of the enzyme on the filters during the screening procedure. Although NDP kinases from a great variety of sources have been characterized, the primary structure of the D. discoideum NDP kinase is the first reported for an enzyme of eukaryotic origin.