The gene upstream of DmRP128 codes for a novel GTP-binding protein of Drosophila melanogaster

Transfection of Sponge Cells and Intracellular Localization of Cancer-Related MYC, RRAS2, and DRG1 Proteins

The determination of the protein's intracellular localization is essential for understanding its biological function. Protein localization studies are mainly performed on primary and secondary vertebrate cell lines for which most protocols have been optimized. In spite of experimental difficulties, studies on invertebrate cells, including basal Metazoa, have greatly advanced. In recent years, the interest in studying human diseases from an evolutionary perspective has significantly increased. Sponges, placed at the base of the animal tree, are simple animals without true tissues and organs but with a complex genome containing many genes whose human homologs have been implicated in human diseases, including cancer. Therefore, sponges are an innovative model for elucidating the fundamental role of the proteins involved in cancer. In this study, we overexpressed human cancer-related proteins and their sponge homologs in human cancer cells, human fibroblasts, and sponge cells. We demonstrated that human and sponge MYC proteins localize in the nucleus, the RRAS2 in the plasma membrane, the membranes of the endolysosomal vesicles, and the DRG1 in the cell's cytosol. Despite the very low transfection efficiency of sponge cells, we observed an identical localization of human proteins and their sponge homologs, indicating their similar cellular functions.

Structure and function of cancer-related developmentally regulated GTP-binding protein 1 (DRG1) is conserved between sponges and humans

Cancer is a disease caused by errors within the multicellular system and it represents a major health issue in multicellular organisms. Although cancer research has advanced substantially, new approaches focusing on fundamental aspects of cancer origin and mechanisms of spreading are necessary. Comparative genomic studies have shown that most genes linked to human cancer emerged during the early evolution of Metazoa. Thus, basal animals without true tissues and organs, such as sponges (Porifera), might be an innovative model system for understanding the molecular mechanisms of proteins involved in cancer biology. One of these proteins is developmentally regulated GTP-binding protein 1 (DRG1), a GTPase stabilized by interaction with DRG family regulatory protein 1 (DFRP1). This study reveals a high evolutionary conservation of DRG1 gene/protein in metazoans. Our biochemical analysis and structural predictions show that both recombinant sponge and human DRG1 are predominantly monomers that form complexes with DFRP1 and bind non-specifically to RNA and DNA. We demonstrate the conservation of sponge and human DRG1 biological features, including intracellular localization and DRG1:DFRP1 binding, function of DRG1 in α-tubulin dynamics, and its role in cancer biology demonstrated by increased proliferation, migration and colonization in human cancer cells. These results suggest that the ancestor of all Metazoa already possessed DRG1 that is structurally and functionally similar to the human DRG1, even before the development of real tissues or tumors, indicating an important function of DRG1 in fundamental cellular pathways.

DRG1 is a potential oncogene in lung adenocarcinoma and promotes tumor progression via spindle checkpoint signaling regulation

Developmentally regulated GTP binding protein 1 (DRG1), a member of the DRG family, plays important roles in regulating cell growth. However, the molecular basis of DRG1 in cell proliferation regulation and the relationship between DRG1 and tumor progression remain poorly understood. Here, we demonstrate that DRG1 is elevated in lung adenocarcinomas while weakly expressed in adjacent lung tissues. DRG1 knockdown causes growth inhibition of tumor cells by significantly increasing the proportion of cells in M phase. Overexpression of DRG1 leads to chromosome missegregation which is an important index for tumorigenesis. Interestingly, ectopic of DRG1 reduces taxol induced apoptosis of lung adenocarcinoma cells. Mechanistic analyses confirm that DRG1 localizes at mitotic spindles in dividing cells and binds to spindle checkpoint signaling proteins in vivo. These studies highlight the expanding role of DRG1 in tumorigenesis and reveal a mechanism of DRG1 in taxol resistance.

Developmentally Regulated GTP binding protein 1 (DRG1) controls microtubule dynamics

The mitotic spindle, essential for segregating the sister chromatids into the two evolving daughter cells, is composed of highly dynamic cytoskeletal filaments, the microtubules. The dynamics of microtubules are regulated by numerous microtubule associated proteins. We identify here Developmentally regulated GTP binding protein 1 (DRG1) as a microtubule binding protein with diverse microtubule-associated functions. In vitro, DRG1 can diffuse on microtubules, promote their polymerization, drive microtubule formation into bundles, and stabilize microtubules. HeLa cells with reduced DRG1 levels show delayed progression from prophase to anaphase because spindle formation is slowed down. To perform its microtubule-associated functions, DRG1, although being a GTPase, does not require GTP hydrolysis. However, all domains are required as truncated versions show none of the mentioned activities besides microtubule binding.

Rbg1-Tma46 dimer structure reveals new functional domains and their role in polysome recruitment

Developmentally Regulated GTP-binding (DRG) proteins are highly conserved GTPases that associate with DRG Family Regulatory Proteins (DFRP). The resulting complexes have recently been shown to participate in eukaryotic translation. The structure of the Rbg1 GTPase, a yeast DRG protein, in complex with the C-terminal region of its DFRP partner, Tma46, was solved by X-ray diffraction. These data reveal that DRG proteins are multimodular factors with three additional domains, helix–turn–helix (HTH), S5D2L and TGS, packing against the GTPase platform. Surprisingly, the S5D2L domain is inserted in the middle of the GTPase sequence. In contrast, the region of Tma46 interacting with Rbg1 adopts an extended conformation typical of intrinsically unstructured proteins and contacts the GTPase and TGS domains. Functional analyses demonstrate that the various domains of Rbg1, as well as Tma46, modulate the GTPase activity of Rbg1 and contribute to the function of these proteins in vivo. Dissecting the role of the different domains revealed that the Rbg1 TGS domain is essential for the recruitment of this factor in polysomes, supporting further the implication of these conserved factors in translation.

The highly conserved eukaryotic DRG factors are required for efficient translation in a manner redundant with the putative RNA helicase Slh1

Eukaryotic and archaeal DRG factors are highly conserved proteins with characteristic GTPase motifs. This suggests their implication in a central biological process, which has so far escaped detection. We show here that the two Saccharomyces cerevisiae DRGs form distinct complexes, RBG1 and RBG2, and that the former co-fractionate with translating ribosomes. A genetic screen for triple synthetic interaction demonstrates that yeast DRGs have redundant function with Slh1, a putative RNA helicase also associating with translating ribosomes. Translation and cell growth are severely impaired in a triple mutant lacking both yeast DRGs and Slh1, but not in double mutants. This new genetic assay allowed us to characterize the roles of conserved motifs present in these proteins for efficient translation and/or association with ribosomes. Altogether, our results demonstrate for the first time a direct role of the highly conserved DRG factors in translation and indicate that this function is redundantly shared by three factors. Furthermore, our data suggest that important cellular processes are highly buffered against external perturbation and, consequently, that redundantly acting factors may escape detection in current high-throughput binary genetic interaction screens.

Molecular Cloning and Expression of Two Closely Related GTP-binding Proteins from Zebrafish

Developmentally regulated GTP-binding proteins (DRGs) are a subclass of GTP-binding proteins that have been discovered recently. Here we report two zebrafish DRG cDNA clones closely related to human and mouse DRG genes. The two DRG sequences showed a high degree of similarity (55% identity, 72% similarity) at the amino acids level. Whole mount in situ hybridization revealed expression of zebrafish DRGs maternally, following the onset of zygotic transcription at the mid-blastula transition (MBT) and throughout embryonic. The expression of these two genes in different tissues follows a similar pattern, suggesting that they may serve a similar function.

Biochemical characterization of Arabidopsis developmentally regulated G-proteins (DRGs)

Developmentally regulated G-proteins (DRGs) are a highly conserved family of GTP-binding proteins found in archaea, plants, fungi and animals, indicating important roles in fundamental pathways. Their function is poorly understood, but they have been implicated in cell division, proliferation, and growth, as well as several medical conditions. Individual subfamilies within the G-protein superfamily possess unique nucleotide binding and hydrolysis rates that are intrinsic to their cellular function, and so characterization of these rates for a particular G-protein may provide insight into its cellular activity. We have produced recombinant active DRG protein using a bacterial expression system and refolding, and performed biochemical characterization of their GTP binding and hydrolysis. We show that recombinant Arabidopsis thaliana atDRG1 and atDRG2a are able to bind GDP and GTP. We also show that DRGs can hydrolyze GTP in vitro without the assistance of GTPase-activating proteins and guanine exchange factors. The atDRG proteins hydrolyze GTP at a relatively slow rate (0.94x10(-3)min(-1) for DRG1 and 1.36x10(-3)min(-1) for DRG2) that is consistent with their nearest characterized relatives, the Obg subfamily. The ability of DRGs to bind nucleotide substrates without assistance, their slow rate of GTP hydrolysis, heat stress activation and domain conservation suggest a possible role as a chaperone in ribosome assembly in response to stress as it has been suggested for the Obg proteins, a different but related G-protein subfamily.

A conserved G protein (Drg1p) plays a role in regulation of invasive filamentation in Candida albicans

During infection, the opportunistic fungal pathogenCandida albicansgrows invasively into the tissues of its host, forming filaments that penetrate the host tissue. To search for genes that are important for invasive filamentation, a screen for mutants that were defective in invasion of agar medium was conducted. A mutant carrying an insertion mutation in the locus of a gene, termed hereDRG1, was identified.DRG1encodes a highly conserved cytoplasmic G protein, with orthologues in the genomes of organisms from humans to yeast and archaea.C. albicansstrains lacking Drg1p were defective in producing filaments that penetrated agar media, but produced filaments normally under other conditions, such as during liquid growth. When inoculated intravenously into mice, thedrg1null mutant caused delayed lethality accompanied by delayed invasive growth in the kidneys of the host, in comparison with those of the wild-type strain. These results implicate Drg1p in the control of invasive filamentation in the laboratory, and in the progression of invasive disease in the host.

Cloning and characterization of Xenopus laevis drg2, a member of the developmentally regulated GTP-binding protein subfamily

The developmentally regulated GTP-binding protein (DRG) subfamily is an uncharacterized member of the Obg family, an evolutional branch of GTPase superfamily proteins. GTPases act as molecular switches regulating diverse cellular processes. DRG2 and DRG1 comprise the DRG subfamily in eucaryotes. Although drg1 was first identified as a gene predominantly expressed during early development of the mouse central nervous system, comparative analysis of drg2 and drg1 expression during embryogenesis has never been reported, and the biochemical properties of the DRG family proteins remain to be elucidated. Thus, we first cloned Xenopus drg2 (Xdrg2) and examined the temporal and spatial expression patterns of Xdrg2 mRNA in comparison to those of Xdrg1. Both Xdrg2 and Xdrg1 are induced at late gastrula and subsequently increased during later stages of embryos (stage 13-41). Whole-mount in situ hybridization showed that Xdrg2 and Xdrg1 expression patterns are almost identical except that only Xdrg2 expression is detected in the stage 22 pronephric anlage. Strong transcripts of both genes are also observed at this stage in neural crest cells, blood islands, and developing eyes, and in brain, eyes, otic vesicle, branchial arches, pronephroses, spinal cord, notochord, head mesenchyme, and somites at stages 27 and 32. Northern blot analysis of adult tissues revealed that both genes are expressed highly in ovary and testis and rather moderately in other organs, except that Xdrg1 transcripts are scarcely detected in heart, lung, and liver. Accordingly, transcription or stability of Xdrg2 and Xdrg1 mRNAs may be regulated by different mechanisms. In addition, by generating recombinant XDRG2 and XDRG1 proteins, we found the RNA binding activity of these proteins in vitro. Our results suggest that the DRG proteins may play their physiological roles via RNA binding.

Six GTP-binding proteins of the Era/Obg family are essential for cell growth in Bacillus subtilis

GTP-binding proteins are found in all domains of life and are involved in various essential cellular processes. With the recent explosion of available genome sequence data, a widely distributed bacterial subfamily of GTP-binding proteins was discovered, represented by the Escherichia coli Era and the Bacillus subtilis Obg proteins. Although only a limited number of the GTP-binding proteins belonging to the subfamily have been experimentally characterized, and their function remains unknown, the available data suggests that many of them are essential to bacterial growth. When the complete genomic sequence of B. subtilis was surveyed for genes encoding GTP-binding proteins of the Era/Obg family, nine such genes were identified. As a first step in elucidating the functional networks of those nine GTP-binding proteins, data presented here indicates that six of them are essential for B. subtilis viability. Additionally, it is shown that the six essential proteins are able to specifically bind GTP and GDP in vitro. Experimental depletion of the essential GTP-binding proteins was examined in the context of cell morphology and chromosome replication, and it was found that two proteins, Bex and YqeH, appeared to participate in the regulation of initiation of chromosome replication. Collectively, these results suggest that members of the GTP-binding Era/Obg family are important proteins with precise, yet still not fully understood, roles in bacterial growth and viability.

DRG represents a family of two closely related GTP-binding proteins

In a previous publication we identified a novel human GTP-binding protein that was related to DRG, a developmentally regulated GTP-binding protein from the central nervous system of mouse. Here we demonstrate that both the human and the mouse genome possess two closely related drg genes, termed drg1 and drg2. The two genes share 62% sequence identity at the nucleotide and 58% identity at the protein level. The corresponding proteins appear to constitute a separate family within the superfamily of the GTP-binding proteins. The DRG1 and the DRG2 mRNA are widely expressed in human and mouse tissues and show a very similar distribution pattern. The human drg1 gene is located on chromosome 22q12, the human drg2 gene on chromosome 17p12. Distantly related species including Caenorhabditis elegans, Schizosaccharomyces pombe and Saccharomyces cerevisiae also possess two drg genes. In contrast, the genomes of archaebacteria (Halobium, Methanococcus, Thermoplasma) harbor only one drg gene, while eubacteria do not seem to contain any. The high conservation of the polypeptide sequences between distantly related organisms indicates an important role for DRG1 and DRG2 in a fundamental pathway.

The Caulobacter crescentus CgtA protein displays unusual guanine nucleotide binding and exchange properties

The Caulobacter crescentus CgtA protein is a member of the Obg-GTP1 subfamily of monomeric GTP-binding proteins. In vitro, CgtA specifically bound GTP and GDP but not GMP or ATP. CgtA bound GTP and GDP with moderate affinity at 30 degrees C and displayed equilibrium binding constants of 1.2 and 0.5 microM, respectively, in the presence of Mg(2+). In the absence of Mg(2+), the affinity of CgtA for GTP and GDP was reduced 59- and 6-fold, respectively. N-Methyl-3'-O-anthranoyl (mant)-guanine nucleotide analogs were used to quantify GDP and GTP exchange. Spontaneous dissociation of both GDP and GTP in the presence of 5 to 12 mM Mg(2+) was extremely rapid (k(d) = 1.4 and 1.5 s(-1), respectively), 10(3)- to 10(5)-fold faster than that of the well-characterized eukaryotic Ras-like GTP-binding proteins. The dissociation rate constant of GDP increased sevenfold in the absence of Mg(2+). Finally, there was a low inherent GTPase activity with a single-turnover rate constant of 5.0 x 10(-4) s(-1) corresponding to a half-life of hydrolysis of 23 min. These data clearly demonstrate that the guanine nucleotide binding and exchange properties of CgtA are different from those of the well-characterized Ras-like GTP-binding proteins. Furthermore, these data are consistent with a model whereby the nucleotide occupancy of CgtA is controlled by the intracellular levels of guanine nucleotides.

An essential GTP-binding protein functions as a regulator for differentiation in Streptomyces coelicolor

The Streptomyces coelicolor obg gene, which encodes a putative GTP-binding protein of the Obg/Gtp1 family, was characterized. The obg gene was essential for viability. Introduction of multiple copies of obg into wild-type S. coelicolor suppressed aerial mycelium formation. A single amino acid substitution at any of six positions was introduced into the GTP binding site of Obg, and the mutated proteins were expressed in wild-type cells. Obg(P168-->V) exerted a more accentuated suppressive effect on aerial mycelium formation than did the wild-type Obg protein. In contrast, Obg(G171-->A) accelerated the development of aerial mycelium. These results show that Obg protein functions as a pivotal regulator for the onset of cell differentiation through its ability to bind GTP. Western analysis revealed that expression of obg is regulated in a growth phase-dependent manner, indicating a sharp decrease just after onset of aerial mycelium development or at the end of vegetative growth. Obg was a membrane-bound protein as determined by immunoelectron microscopy.

Molecular cloning of a novel GTP-binding protein induced in fish cells by rhabdovirus infection

We have cloned and sequenced a cDNA encoding GTP-binding protein from a fish cell, CHSE-214. The clone was 1493 bp long and contained an open reading frame encoding 364 amino acids. It has the five sequence motifs G1-G5 that are conserved in all GTP-binding proteins. Its amino acid sequences are strikingly different from those of the well-characterized G-proteins. However, sequences closely related to this protein are found in various kinds of species including human, Arabidopsis, Drosophila and archaebacteria, suggesting a novel subfamily within the superfamily of the GTP-binding proteins. Northern analysis indicates that this gene is constitutively expressed at a low level in normal cells but is induced by fish rhabdovirus infection at about 24 h post infection and disappears thereafter. Based on these observations, we propose that this protein represents an evolutionarily conserved novel subfamily of GTP-binding proteins which may play an important role in fish rhabdovirus infection.

Transcription of DmRP140, the gene coding for the second-largest subunit of RNA polymerase II

To analyze transcriptional control regions of Drosophila melanogaster housekeeping genes, we have characterized the promoter of the gene coding for the second-largest subunit of RNA polymerase II (DmRP140). Upstream of DmRP140 the genomic region harbors a gene which is transcribed in the opposite direction (DmRP140up). By determination of the transcription start sites of both genes we found a short non-transcribed intergenic region of 220 bp. Functional analysis of various promoter reportergene constructs by transient transfection of cultured cells revealed that sequences important for transcription of DmRP140 are located in the untranslated leader of the upstream gene. The onset of DmRP140 transcription during embryonic development was studied in transgenic flies using beta-galactosidase as reportergene. To distinguish between the maternally provided DmRP140 transcripts and the embryonically transcribed RNA the offspring of nontransformed females and male transformants was examined. The development of a sensitive detection assay based on a chemiluminescent substrate for beta-galactosidase allowed us to determine the onset of DmRP140 transcription to between 8-10 h after oviposition. Thus, DmRP140 transcription does not start following the transcriptional transition period between 2-3 h of development but occurs much later in embryogenesis coinciding with decreasing DNA synthesis and cell division rates.