Regulation of a ras-related protein during development of Dictyostelium discoideum

A novel Ras-interacting protein required for chemotaxis and cyclic adenosine monophosphate signal relay in Dictyostelium

We have identified a novel Ras-interacting protein from Dictyostelium, RIP3, whose function is required for both chemotaxis and the synthesis and relay of the cyclic AMP (cAMP) chemoattractant signal. rip3 null cells are unable to aggregate and lack receptor activation of adenylyl cyclase but are able, in response to cAMP, to induce aggregation-stage, postaggregative, and cell-type-specific gene expression in suspension culture. In addition, rip3 null cells are unable to properly polarize in a cAMP gradient and chemotaxis is highly impaired. We demonstrate that cAMP stimulation of guanylyl cyclase, which is required for chemotaxis, is reduced approximately 60% in rip3 null cells. This reduced activation of guanylyl cyclase may account, in part, for the defect in chemotaxis. When cells are pulsed with cAMP for 5 h to mimic the endogenous cAMP oscillations that occur in wild-type strains, the cells will form aggregates, most of which, however, arrest at the mound stage. Unlike the response seen in wild-type strains, the rip3 null cell aggregates that form under these experimental conditions are very small, which is probably due to the rip3 null cell chemotaxis defect. Many of the phenotypes of the rip3 null cell, including the inability to activate adenylyl cyclase in response to cAMP and defects in chemotaxis, are very similar to those of strains carrying a disruption of the gene encoding the putative Ras exchange factor AleA. We demonstrate that aleA null cells also exhibit a defect in cAMP-mediated activation of guanylyl cyclase similar to that of rip3 null cells. A double-knockout mutant (rip3/aleA null cells) exhibits a further reduction in receptor activation of guanylyl cyclase, and these cells display almost no cell polarization or movement in cAMP gradients. As RIP3 preferentially interacts with an activated form of the Dictyostelium Ras protein RasG, which itself is important for cell movement, we propose that RIP3 and AleA are components of a Ras-regulated pathway involved in integrating chemotaxis and signal relay pathways that are essential for aggregation.

Rapid changes of nucleotide excision repair gene expression following UV-irradiation and cisplatin treatment of Dictyostelium discoideum

Organisms use different mechanisms to detect and repair different types of DNA damage, and different species vary in their sensitivity to DNA damaging agents. The cellular slime mold Dictyostelium discoideum has long been recognized for its unusual resistance to UV and ionizing radiation. We have recently cloned three nucleotide excision repair (NER) genes from Dictyostelium , the rep B, D and E genes (the homologs of the human xeroderma pigmentosum group B, D and E genes, respectively). Each of these genes has a unique pattern of expression during the multicellular development of this organism. We have now examined the response of these genes to DNA damage. The rep B and D DNA helicase genes are rapidly and transiently induced in a dose dependent manner following exposure to both UV-light and the widely used chemotherapeutic agent cisplatin. Interestingly, the rep E mRNA level is repressed by UV but not by cisplatin, implying unique signal transduction pathways for recognizing and repairing different types of damage. Cells from all stages of growth and development display the same pattern of NER gene expression following exposure to UV-light. These results suggest that the response to UV is independent of DNA replication, and that all the factors necessary for rapid transcription of these NER genes are either stable throughout development, or are continuously synthesized. It is significant that the up-regulation of the rep B and D genes in response to UV and chemical damage has not been observed to occur in cells from other species. We suggest that this rapid expression of NER genes is at least in part responsible for the unusual resistance of Dictyostelium to DNA damage.

A Ras GAP is essential for cytokinesis and spatial patterning in Dictyostelium

Using the yeast two-hybrid system, we have identified developmentally regulated Dictyostelium genes whose encoded proteins interact with Ras-GTP but not Ras-GDP. By sequence homology and biochemical function, one of these genes encodes a Ras GAP (DdRasGAP1). Cells carrying a DdRasGAP1 gene disruption (ddrasgap1 null cells) have multiple, very distinct growth and developmental defects as elucidated by examining the phenotypes of ddrasgap1 null strains. First, vegetative ddrasgap1 null cells are very large and highly multinucleate cells when grown in suspension, indicating a severe defect in cytokinesis. When suspension-grown cells are plated in growth medium on plastic where they attach and can move, the cells rapidly become mono- and dinucleate by traction-mediated cell fission and continue to grow vegetatively with a number of nuclei (1–2) per cell, similar to wild-type cells. The multinucleate phenotype, combined with results indicating that constitutive expression of activated Ras does not yield highly multinucleate cells and data on Ras null mutants, suggest that Ras may need to cycle between GTP- and GDP-bound states for proper cytokinesis. After starvation, the large null cells undergo rapid fission when they start to move at the onset of aggregation, producing mononucleate cells that form a normal aggregate. Second, ddrasgap1 null cells also have multiple developmental phenotypes that indicate an essential role of DdRasGAP1 in controlling cell patterning. Multicellular development is normal through the mid-slug stage, after which morphological differentiation is very abnormal and no culminant is formed: no stalk cells and very few spores are detected. lacZ reporter studies show that by the mid-finger stage, much of the normal cell-type patterning is lost, indicating that proper DdRasGAP1 function and possibly normal Ras activity are necessary to maintain spatial organization and for induction of prestalk to stalk and prespore to spore cell differentiation. The inability of ddrasgap1 null cells to initiate terminal differentiation and form stalk cells is consistent with a model in which Ras functions as a mediator of inhibitory signals in cell-type differentiation at this stage. Third, DdRasGAP1 and cAMP dependent protein kinase (PKA) interact to control spatial organization within the organism. Overexpression of the PKA catalytic subunit in ddrasgap1 cells yields terminal structures that are multiply branched but lack spores. This suggests that RasGAP and PKA may mediate common pathways that regulate apical tip differentiation and organizer function, which in turn control spatial organization during multicellular development. It also suggests that DdRasGAP1 either lies downstream from PKA in the prespore to spore pathway or in a parallel pathway that is also essential for spore differentiation. Our results indicate that DdRasGAP1 plays an essential role in controlling multiple, potentially novel pathways regulating growth and differentiation in Dictyostelium and suggest a role for Ras in these processes.

The Dictyostelium mitogen-activated protein kinase ERK2 is regulated by Ras and cAMP-dependent protein kinase (PKA) and mediates PKA function

The chemoattractant cAMP, acting through serpentine cAMP receptors, results in a rapid and transient stimulation of the Dictyostelium mitogen-activated protein kinase ERK2 activity (). In this study we show that other pathways required for aggregation, including Ras and cAMP-dependent protein kinase (PKA), are important regulators of ERK2 activation and adaptation. By examining both the level and kinetics of activation and adaptation of ERK2, we show that Ras is a negative regulator of ERK2. Activated Ras or disruption of a Ras GAP gene results in reduced ERK2 activation whereas disruption of putative Ras GEF or expression of dominant negative Ras proteins have a more rapid, higher, and extended activation. CRAC, a PH domain-containing protein required for adenylyl cyclase activation, is also required for proper ERK2 adaptation. PKA overexpression results in a more rapid, higher level of activation, whereas pka null cells show a lower level but more extended ERK2 activation. Furthermore, we show that constitutive expression of PKA catalytic subunit bypasses the requirement of ERK2 for aggregation and later development, indicating that PKA lies downstream from ERK2 and that ERK2 may regulate one or more components of the signaling pathway required for mediating PKA function, possibly by directly regulating PKA R or a protein controlling the intracellular level of cAMP.

Ras-related genes in Dictyostelium discoideum

Dictyostelium discoideum, like other eukaryotes, has been shown to express several ras-related genes. Two gene products, Ddras and DdrasG, are highly conserved relative to the human ras proteins. Ddras is expressed at the pseudoplasmodial stage of development, whereas DdrasG is expressed in vegetative cells and during early development. In addition, Dictyostelium possesses three ras-related genes, SAS1, SAS2 and Ddrap1, whose gene products are only partially conserved relative to those of the ras genes. The expression of these three genes is also developmentally regulated.

Regulated expression and phosphorylation of the 23-26-kDa ras protein in the sponge Geodia cydonium

We have cloned, sequenced and examined the sponge Geodia cydonium cDNA encoding a protein homologous to ras proteins. The sponge ras protein has a more conserved N-terminal region and a less conserved C-terminal region, especially in comparison to Dictyostelium discoideum; the similarity to human c-Ha-ras-1 and to Saccharomyces cerevisiae is less pronounced. The sponge ras cDNA comprises five TAG triplets; at the translational level these UAG termination codons are suppressed by a Gln-tRNA. The sponge ras protein was isolated and partially purified (23-26 kDa) and found to undergo phosphorylation at a threonine moiety, when dissociated cells were incubated in the presence of a homologous aggregation factor and insulin. Insulin-mediated phosphorylation of the ras protein resulted in a decrease in its Kd with GTP from 2 microM to 80 nM. The activated ras protein displayed high GTPase activity if the partially purified protein was incubated with homologous lectin and lectin receptor molecules. These results suggest that in the sponge, ras is activated by the insulin/insulin(insulin-like)-receptor system. This transition enables the ras protein to interact with the lectin-receptor/lectin complex, a process which may ultimately lead to an initiation of an intracellular signal-transduction chain.

Oncogenic proteins new targets for chemotherapeutic agents against cancer

Over the past 10 years, more than 40 potentially oncogenic genes, termed protooncogenes, have been identified in the human genome. Little is known of the physiological role of the proteins encoded by these genes, but they seem to be involved in the reception and transmission of hormonal and other environmental information from the cell membrane to the nucleus. These proteins may acquire transforming properties when over-expressed or if structurally altered following partial deletions or point mutations. Cytogenetic analysis shows loss of genetic material from specific chromosomal loci in many human tumors, suggesting that the absence of a functional gene at these loci may permit tumor development. The genes involved have been termed "anti-oncogenes". Understanding the control mechanisms of cell proliferation is essential in order to understand how cancer cells escape from this control. To this end, numerous oncogenes have been cloned, permitting the production of modified forms of oncogenic proteins and identification of the regions essential for their biological activity. Availability of large amounts of protein also allows the production of specific antibody which can be used to verify whether blockage of a given protein results in reversion of the transformed phenotype. If it can be shown that the expression of an oncogenic protein is essential for transformation, it should be possible to search for molecules that inhibit its action or which mimic the effects of an anti-oncogene. This type of research is already well advanced for the oncogenic ras proteins, and models have been established that permit both screening for potential inhibitors and design of specific antagonists.

Developmental regulation of cell-type-enriched mRNAs in Dictyostelium discoideum

We describe sixteen new families of cDNA clones representing mRNAs that are expressed preferentially in either prespore or prestalk cells during development of Dictyostelium discoideum and two new mRNAs that are expressed in a non-cell-type-specific manner. None of the prespore-enriched mRNAs are detectable in Dictyostelium cells until 13-15 h of development but then they increase dramatically and peak at 18-22 h. Upon dissociation of developing aggregates, all these mRNAs rapidly decay to low levels. In marked contrast to data presented for prespore genes by other workers, cyclic AMP either has no effect on the mRNA levels in dissociated cells or is only weakly effective in restoring normal expression. A prestalk-enriched mRNA examined, 5G mRNA, is similarly expressed late in development but is also expressed in vegetative cells. The level of 5G mRNA is only moderately affected by cell disaggregation.

A ras-encoded protein in Dictyostelium discoideum is acylated and membrane-associated

Dictyostelium discoideum synthesizes a 23,000 Mr protein, p23dd-ras, closely related to the mammalian oncogene-encoded protein p21ras. To investigate the subcellular localization of p23dd-ras, conditions were optimized to reduce protein degradation following cell breakage. Subcellular fractionation of D. discoideum showed that p23dd-ras was associated predominantly with the membrane fraction during both vegetative growth and differentiation. In the absence of suitable protease inhibitors considerable amounts of a truncated form of p23dd-ras were recovered in the cytosol fraction, suggesting that intact p23dd-ras is attached to the membrane by a short terminal peptide sequence. Radio-isotope labelling of D. discoideum with myristic acid or palmitic acid in the presence of excess unlabelled acetate resulted in radio-isotope incorporation into a select group of proteins including p23dd-ras. No acyl label appeared in the truncated cytoplasmic form of p23dd-ras when cell breakage was performed in the absence of suitable protease inhibitors, indicating that the acyl group is associated with the short terminal peptide that is cleaved. These data suggest that p23dd-ras, like its mammalian counterpart, is acylated and associated with the plasma membrane. There was no evidence during a 30-minute pulse of methionine for a cytoplasmic precursor to the membrane-bound p23dd-ras, suggesting that the turnover of the presumptive precursor must be much more rapid in D. discoideum than for pro-p21ras in mammalian cells.

A ras-like protein is required for a post-Golgi event in yeast secretion

Secretion is blocked at the post-Golgi stage within 5 min of a shift of sec4-8 cells from 25 degrees C to 37 degrees C. Analysis of SEC4 predicts a protein product of 23.5 kd molecular weight that shares 32% homology with ras proteins and is essential for growth. The regions of best homology are those involved in the binding and hydrolysis of GTP. Duplication of SEC4 suppresses post-Golgi-blocked mutations in three sec genes. These mutations are lethal when combined with sec4-8 at 25 degrees C. Mutations that block elsewhere on the pathway are not suppressed by the SEC4 duplication and are not lethal when combined with sec4-8. We propose that the SEC4 product is a GTP-binding protein that plays an essential role in controlling a late stage of the secretory pathway.

The synthesis and degradation of ras-related gene products during growth and differentiation in Dictyostelium discoideum

A Dictyostelium discoideum protein with an Mr of 23,000 (p23dd-ras) is structurally related to the mammalian proto-oncogene ras-gene product, p21ras, and is specifically precipitated from cell-free extracts of D. discoideum by the Y13-259 monoclonal antibody against p21ras. p23dd-ras was degraded at rates that were very similar to those observed for total protein during both growth and differentiation, suggesting that the previously reported decline in p23dd-ras levels during differentiation is due to a change in the rate of synthesis rather than a change in the rate of degradation. p23dd-ras synthesis did not decrease immediately after the initiation of differentiation, but rather its rate of synthesis increased for the first 1-2 h, suggesting that p23dd-ras is not rapidly down-regulated in response to nutrient deprivation. There were differences in the extent of p23dd-ras turnover during the differentiation of the three tested strains, A-3, NC4, and V12-M2. The relative level of p23dd-ras dropped most rapidly in V12-M2, which may reflect the slightly faster differentiation process exhibited by this strain. In all three strains, very little p23dd-ras was present by the end of the differentiation process. A second protein with an Mr of 24,000 (p24dd-ras) was also immunoprecipitated using the Y13-259 antibody. The amount of p24dd-ras was small or undetectable in vegetative cells, but relatively larger amounts of p24dd-ras were synthesized in pseudoplasmodial cells. We found no evidence to suggest that p24dd-ras is a precursor of p23dd-ras.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of growth factor-induced differentiation of PC12 cells by microinjection of antibody to ras p21

The protein products (p21) of the ras cellular proto-oncogenes are thought to transduce membrane signals necessary for the induction of cell division. However, there is uncertainty as to the precise role of ras p21 in mediating ligand-membrane receptor signals leading to cell differentiation. Treatment of rat phaeochromocytoma cells (PC12) with nerve growth factor (NGF) results in the induction of a number of phenotypic characteristics of sympathetic neurones, including cessation of cell division and outgrowth of neuronal processes (neurites). Here we report that microinjection of antibody to ras p21 into PC12 cells inhibited neurite formation and resulted in temporary regression of partially extended neurites, an effect which was observed up to 36 h after initiation of NGF treatment. Neurite formation induced by cyclic AMP was unaffected by injection of anti-p21 antibody. These results indicate that p21 is involved in the initiation phase of NGF-induced neurite formation in PC12 cells and has a role in hormone-mediated cellular responses distinct from cell proliferation.

Expression of viral p21ras during acquisition of a transformed phenotype by rat adrenal cortex cells infected with Kirsten murine sarcoma virus

Rat adrenal cortex cells infected with Kirsten murine sarcoma virus acquire a transformed phenotype in a progressive fashion. The expression of the viral p21ras does not appear to correlate with the degree of transformation of the adrenocortical cells but rather is produced at similar levels as the culture becomes transformed. This indicates that the expression of an oncogenic form of p21ras is not of itself sufficient to completely transform rat adrenal cortex cells.