Growing and developing Dictyostelium cells express different ras genes

Functional and structural insights into RAS effector proteins

RAS proteins are conserved guanosine triphosphate (GTP) hydrolases (GTPases) that act as molecular binary switches and play vital roles in numerous cellular processes. Upon GTP binding, RAS GTPases adopt an active conformation and interact with specific proteins termed RAS effectors that contain a conserved ubiquitin-like domain, thereby facilitating downstream signaling. Over 50 effector proteins have been identified in the human proteome, and many have been studied as potential mediators of RAS-dependent signaling pathways. Biochemical and structural analyses have provided mechanistic insights into these effectors, and studies using model organisms have complemented our understanding of their role in physiology and disease. Yet, many critical aspects regarding the dynamics and biological function of RAS-effector complexes remain to be elucidated. In this review, we discuss the mechanisms and functions of known RAS effector proteins, provide structural perspectives on RAS-effector interactions, evaluate their significance in RAS-mediated signaling, and explore their potential as therapeutic targets.

IQGAP-related protein IqgC suppresses Ras signaling during large-scale endocytosis

Macropinocytosis and phagocytosis are evolutionarily conserved forms of bulk endocytosis used by cells to ingest large volumes of fluid and solid particles, respectively. Both processes are regulated by Ras signaling, which is precisely controlled by mechanisms involving Ras GTPase activating proteins (RasGAPs) responsible for terminating Ras activity on early endosomes. While regulation of Ras signaling during large-scale endocytosis in WT Dictyostelium has been, for the most part, attributed to the Dictyostelium ortholog of human RasGAP NF1, in commonly used axenic laboratory strains, this gene is mutated and inactive. Moreover, none of the RasGAPs characterized so far have been implicated in the regulation of Ras signaling in large-scale endocytosis in axenic strains. In this study, we establish, using biochemical approaches and complementation assays in live cells, that Dictyostelium IQGAP-related protein IqgC interacts with active RasG and exhibits RasGAP activity toward this GTPase. Analyses of iqgC ^- and IqgC-overexpressing cells further revealed participation of this GAP in the regulation of both types of large-scale endocytosis and in cytokinesis. Moreover, given the localization of IqgC to phagosomes and, most prominently, to macropinosomes, we propose IqgC acting as a RasG-specific GAP in large-scale endocytosis. The data presented here functionally distinguish IqgC from other members of the Dictyostelium IQGAP family and call for repositioning of this genuine RasGAP outside of the IQGAP group.

A Diaphanous-related formin links Ras signaling directly to actin assembly in macropinocytosis and phagocytosis

Phagocytosis and macropinocytosis are Ras-regulated and actin-driven processes that depend on the dynamic rearrangements of the plasma membrane that protrudes and internalizes extracellular material by cup-shaped structures. However, the regulatory mechanisms underlying actin assembly in large-scale endocytosis remain elusive. Here, we show that the Diaphanous-related formin G (ForG) from the professional phagocyte Dictyostelium discoideum localizes to endocytic cups. Biochemical analyses revealed that ForG is a rather weak nucleator but efficiently elongates actin filaments in the presence of profilin. Notably, genetic inactivation of ForG is associated with a strongly impaired endocytosis and a markedly diminished F-actin content at the base of the cups. By contrast, ablation of the Arp2/3 (actin-related protein-2/3) complex activator SCAR (suppressor of cAMP receptor) diminishes F-actin mainly at the cup rim, being consistent with its known localization. These data therefore suggest that ForG acts as an actin polymerase of Arp2/3-nucleated filaments to allow for efficient membrane expansion and engulfment of extracellular material. Finally, we show that ForG is directly regulated in large-scale endocytosis by RasB and RasG, which are highly related to the human proto-oncogene KRas.

Daydreamer, a Ras effector and GSK-3 substrate, is important for directional sensing and cell motility

Daydreamer (DydA), a new Mig10/RIAM/lamellipodin family adaptor protein, is a Ras effector required for cell polarization and directional movement during chemotaxis. DydA is phosphorylated by glycogen synthase kinase-3, which is required for some, but not all, of DydA's functions. gskA⁻ cells exhibit very strong chemotactic phenotypes, a subset of which are exhibited by dydA⁻ cells.

How independent signaling pathways are integrated to holistically control a biological process is not well understood. We have identified Daydreamer (DydA), a new member of the Mig10/RIAM/lamellipodin (MRL) family of adaptor proteins that localizes to the leading edge of the cell. DydA is a putative Ras effector that is required for cell polarization and directional movement during chemotaxis. dydA⁻ cells exhibit elevated F-actin and assembled myosin II (MyoII), increased and extended phosphoinositide-3-kinase (PI3K) activity, and extended phosphorylation of the activation loop of PKB and PKBR1, suggesting that DydA is involved in the negative regulation of these pathways. DydA is phosphorylated by glycogen synthase kinase-3 (GSK-3), which is required for some, but not all, of DydA's functions, including the proper regulation of PKB and PKBR1 and MyoII assembly. gskA⁻ cells exhibit very strong chemotactic phenotypes, as previously described, but exhibit an increased rate of random motility. gskA⁻ cells have a reduced MyoII response and a reduced level of phosphatidylinositol (3,4,5)-triphosphate production, but a highly extended recruitment of PI3K to the plasma membrane and highly extended kinetics of PKB and PKBR1 activation. Our results demonstrate that GSK-3 function is essential for chemotaxis, regulating multiple substrates, and that one of these effectors, DydA, plays a key function in the dynamic regulation of chemotaxis.

Determinants of RasC specificity during Dictyostelium aggregation

RasC is required for optimum activation of adenylyl cyclase A and for aggregate stream formation during the early differentiation of Dictyostelium discoideum. RasG is unable to substitute for this requirement despite its sequence similarity to RasC. A critical question is which amino acids in RasC are required for its specific function. Each of the amino acids within the switch 1 and 2 domains in the N-terminal portion of RasG was changed to the corresponding amino acid from RasC, and the ability of the mutated RasG protein to reverse the phenotype of rasC(-) cells was determined. Only the change from aspartate at position 30 of RasG to alanine (the equivalent position 31 in RasC) resulted in a significant increase in adenylyl cyclase A activation and a partial reversal of the aggregation-deficient phenotype of rasC(-) cells. All other single amino acid changes were without effect. Expression of a chimeric protein, RasG(1-77)-RasC(79-189), also resulted in a partial reversal of the rasC(-) cell phenotype, indicating the importance of the C-terminal portion of RasC. Furthermore, expression of the chimeric protein, with alanine changed to aspartate (RasG(1-77(D30A))-RasC(79-189)), resulted in a full rescue the rasC(-) aggregation-deficient phenotype. Finally, the expression of either a mutated RasC, with the aspartate 31 replaced by alanine, or the chimeric protein, RasC(1-78)-RasG(78-189), only generated a partial rescue. These results emphasize the importance of both the single amino acid at position 31 and the C-terminal sequence for the specific function of RasC during Dictyostelium aggregation.

Ras proteins have multiple functions in vegetative cells of Dictyostelium

During the aggregation of Dictyostelium cells, signaling through RasG is more important in regulating cyclic AMP (cAMP) chemotaxis, whereas signaling through RasC is more important in regulating the cAMP relay. However, RasC is capable of substituting for RasG for chemotaxis, since rasG⁻ cells are only partially deficient in chemotaxis, whereas rasC⁻/rasG⁻ cells are totally incapable of chemotaxis. In this study we have examined the possible functional overlap between RasG and RasC in vegetative cells by comparing the vegetative cell properties of rasG⁻, rasC⁻, and rasC⁻/rasG⁻ cells. In addition, since RasD, a protein not normally found in vegetative cells, is expressed in vegetative rasG⁻ and rasC⁻/rasG⁻ cells and appears to partially compensate for the absence of RasG, we have also examined the possible functional overlap between RasG and RasD by comparing the properties of rasG⁻ and rasC⁻/rasG⁻ cells with those of the mutant cells expressing higher levels of RasD. The results of these two lines of investigation show that RasD is capable of totally substituting for RasG for cytokinesis and growth in suspension, whereas RasC is without effect. In contrast, for chemotaxis to folate, RasC is capable of partially substituting for RasG, but RasD is totally without effect. Finally, neither RasC nor RasD is able to substitute for the role that RasG plays in regulating actin distribution and random motility. These specificity studies therefore delineate three distinct and none-overlapping functions for RasG in vegetative cells.

Rap1 activation in response to cAMP occurs downstream of ras activation during Dictyostelium aggregation

We have used a doubly disrupted rasC(-)/rasG(-) strain of Dictyostelium discoideum, which ectopically expresses the carA gene, to explore the relationship between the activation of RasC and RasG, the two proteins that are necessary for optimum cAMP signaling, and the activation of Rap1, a Ras subfamily protein, that is also activated by cAMP. The ectopic expression of carA restored early developmental gene expression to the rasC(-)/rasG(-) strain, rendering it suitable for an analysis of cAMP signal transduction. Because there was negligible signaling through both the cAMP chemotactic pathway and the adenylyl cyclase activation pathway in the rasC(-)/rasG(-)/[act15]:carA strain, it is clear that RasG and RasC are the only two Ras subfamily proteins that directly control these pathways. The position of Rap1 in the signal transduction cascade was clarified by the finding that Rap1 activation was totally abolished in rasC(-)/rasG(-)/[act15]:carA and rasG(-) cells but only slightly reduced in rasC(-) cells. Rap1 activation, therefore, occurs downstream of the Ras proteins and predominantly, if not exclusively, downstream of RasG. The finding that in vitro guanylyl cyclase activation is also abolished in the rasC(-)/rasG(-)/[act15]:carA strain identifies RasG/RasC as the presumptive monomeric GTPases required for this activation.

Delineation of the roles played by RasG and RasC in cAMP-dependent signal transduction during the early development of Dictyostelium discoideum

On starvation, the cellular slime mold Dictyostelium discoideum initiates a program of development leading to formation of multicellular structures. The initial cell aggregation requires chemotaxis to cyclic AMP (cAMP) and relay of the cAMP signal by the activation of adenylyl cyclase (ACA), and it has been shown previously that the Ras protein RasC is involved in both processes. Insertional inactivation of the rasG gene resulted in delayed aggregation and a partial inhibition of early gene expression, suggesting that RasG also has a role in early development. Both chemotaxis and ACA activation were reduced in the rasG- cells, but the effect on chemotaxis was more pronounced. When the responses of rasG- cells to cAMP were compared with the responses of rasC- and rasC- rasG- strains, generated in otherwise isogenic backgrounds, these studies revealed that signal transduction through RasG is more important in chemotaxis and early gene expression, but that signal transduction through RasC is more important in ACA activation. Because the loss of either of the two Ras proteins alone did not result in a total loss of signal output down either of the branches of the cAMP signal-response pathway, there appears to be some overlap of function.

Cytoskeletal regulation by Dictyostelium Ras subfamily proteins

The Ras subfamily proteins are monomeric GTPases that function as molecular switches in cellular signal transduction. The roles of six of these proteins in regulating actin cytoskeletal functions in Dictyostelium discoideum are discussed in this review.

Roles played by Ras subfamily proteins in the cell and developmental biology of microorganisms

The Ras subfamily proteins are monomeric GTPases that function as molecular switches in cellular signal transduction pathways. This review describes our current knowledge of the roles that these proteins play in the growth and differentiation of single celled microorganisms.

Lysophosphatidic acid- and Gbeta-dependent activation of Dictyostelium MAP kinase ERK2

Exogenous lysophosphatidic acid (LPA) has been shown to evoke a chemotactic response in aggregative cells of the social amoeba Dictyostelium discoideum. In this paper, we demonstrate that extracellular LPA is also able to induce activation of mitogen-activated protein (MAP) kinase DdERK2 (extracellular signal regulated kinase 2) in these cells. This activation is independent of cyclic AMP receptors, yet fully dependent on the single Gbeta subunit, hinting to the presence of functional heptahelical LPA receptors in a primitive eukaryote. We did not observe LPA-dependent cyclic GMP accumulation, which suggests that the pathways for LPA-induced and "classical" chemotaxis of D. discoideum cells are substantially different.

Expression of activated Ras during Dictyostelium development alters cell localization and changes cell fate

There is now a body of evidence to indicate that Ras proteins play important roles in development. Dictyostelium expresses several ras genes and each appears to perform a distinct function. Previous data had indicated that the overexpression of an activated form of the major developmentally regulated gene, rasD, caused a major aberration in morphogenesis and cell type determination. We now show that the developmental expression of an activated rasG gene under the control of the rasD promoter causes a similar defect. Our results indicate that the expression of activated rasG in prespore cells results in their transdifferentiation into prestalk cells, whereas activated rasG expression in prestalk causes gross mislocalization of the prestalk cell populations.

Dictyostelium: an ideal organism for genetic dissection of Ras signalling networks

Signalling pathways based on the small GTPase Ras regulate a multitude of cellular events in eukaryotic cells. Dictyostelium expresses a large and varied family of Ras proteins. It also uses a range of known Ras regulators, in particular RasGEFs, and effectors. The genetic tractability of Dictyostelium, together with the wide range of Ras proteins and regulators, make it an ideal model for the genetic dissection of Ras pathways. This review highlights the recent advances in our understanding of Ras function in Dictyostelium, and considers the implications of these findings for our understanding of eukaryotic signal transduction.

A novel Dictyostelium RasGEF is required for normal endocytosis, cell motility and multicellular development

BACKGROUND

Dictyostelium possesses a surprisingly large number of Ras proteins and little is known about their activators, the guanine nucleotide exchange factors (GEFs). It is also unclear, in Dictyostelium or in higher eukaryotes, whether Ras pathways are linear, with each Ras controlled by its own GEF, or networked, with multiple GEFs acting on multiple Ras proteins.

RESULTS

We have identified the Dictyostelium gene that encodes RasGEFB, a protein with homology to known RasGEFs such as the Son-of-sevenless (Sos) protein. Dictyostelium cells in which the gene for RasGEFB was disrupted moved unusually rapidly, but lost the ability to perform macropinocytosis and therefore to grow in liquid medium. Crowns, the sites of macropinocytosis, were replaced by polarised lamellipodia. Mutant cells were also profoundly defective in early development, although they eventually formed tiny but normally proportioned fruiting bodies. This defect correlated with loss of discoidin Igamma mRNA, a starvation-induced gene, although other genes required for development were expressed normally or even precociously. RasGEFB was able to rescue a Saccharomyces CDC25 mutant, indicating that it is a genuine GEF for Ras proteins.

CONCLUSIONS

RasGEFB appears to be the principal activator of the RasS protein, which regulates macropinocytosis and cell speed, but it also appears to regulate one or more other Ras proteins.

Dictyostelium RasD is required for normal phototaxis, but not differentiation

RasD, a Dictyostelium homolog of mammalian Ras, is maximally expressed during the multicellular stage of development. Normal Dictyostelium aggregates are phototactic and thermotactic, moving towards sources of light and heat with great sensitivity. We show that disruption of the gene for rasDcauses a near-total loss of phototaxis and thermotaxis in mutant aggregates, without obvious effects on undirected movement. Previous experiments had suggested important roles for RasD in development and cell-type determination. Surprisingly, rasD− cells show no obvious changes in these processes. These cells represent a novel class of phototaxis mutant, and indicate a role for a Ras pathway in the connections between stimuli and coordinated cell movement.

Dictyostelium RasD is required for normal phototaxis, but not differentiation

RasD, a Dictyostelium homolog of mammalian Ras, is maximally expressed during the multicellular stage of development. Normal Dictyostelium aggregates are phototactic and thermotactic, moving towards sources of light and heat with great sensitivity. We show that disruption of the gene for rasD causes a near-total loss of phototaxis and thermotaxis in mutant aggregates, without obvious effects on undirected movement. Previous experiments had suggested important roles for RasD in development and cell-type determination. Surprisingly, rasD(-) cells show no obvious changes in these processes. These cells represent a novel class of phototaxis mutant, and indicate a role for a Ras pathway in the connections between stimuli and coordinated cell movement.

Functional overlap of the dictyostelium RasG, RasD and RasB proteins

Disruption of the rasG gene in Dictyostelium discoideum results in several distinct phenotypes: a defect in cytokinesis, reduced motility and reduced growth. Reintroduction of the rasG gene restores all of the properties of the rasG(-) cells to those of the wild type. To determine whether the defects are due to impaired interactions with a single or multiple downstream effectors, we tested the ability of the highly related but non identical Dictyostelium ras genes, rasD and rasB, to rescue the defects. Introduction of the rasD gene under the control of the rasG promoter into rasG null (rasG(-)) cells corrected all phenotypes except the motility defect, suggesting that motility is regulated by a RasG mediated pathway that is different to those regulating growth or cytokinesis. Western blot analysis of RasD protein levels revealed that vegetative rasG(-)cells contained considerably more protein than the parental AX-3 cells, suggesting that RasD protein levels are negatively regulated in vegetative cells by RasG. The level of RasD was enhanced when the rasD gene was introduced under the control of the rasG promoter, and this increase in protein is presumably responsible for the reversal of the growth and cytokinesis defects of the rasG(-)cells. Thus, RasD protein levels are controlled by the level of RasG, but not by the level of RasD. Introduction of the rasB gene under the control of the rasG promoter into rasG(-) cells produced a complex phenotype. The transformants were extremely small and mononucleate and exhibited enhanced motility. However, the growth of these cells was considerably slower than the growth of the rasG(-) cells, suggesting the possibility that high levels of RasB inhibit an essential process. This was confirmed by expressing rasB in wild-type cells; the resulting transformants exhibited severely impaired growth. When RasB protein levels were determined by western blot analysis, it was found that levels were higher in the rasG(-)cells than they were in the wild-type parental, suggesting that RasG also negatively regulates rasB expression in vegetative cells. Overexpression of rasB in the rasG(-)cells also reduced the level of RasD protein. In view of the fact that alternate Ras proteins correct some, but not all, of the defects exhibited by the rasG(-) cells, we propose that RasG interacts with more than one downstream effector. In addition, it is clear that the levels of the various Ras proteins are tightly regulated in vegetative cells and that overexpression can be deleterious.

The Dictyostelium RasS protein is required for macropinocytosis, phagocytosis and the control of cell movement

Endocytosis and cell migration both require transient localised remodelling of the cell cortex. Several lines of evidence suggest a key regulatory role in these activities for members of the Ras family of small GTPases. We have generated Dictyostelium cells lacking one member of this family, RasS, and the mutant cells are perturbed in endocytosis and cell migration. Mutant amoebae are defective in phagocytosis and fluid-phase endocytosis and are impaired in growth. Conversely, the rasS(-)cells show an enhanced rate of cell migration, moving three times faster than wild-type controls. The mutant cells display an aberrant morphology, are highly polarised, carry many elongated actin protrusions and show a concomitant decrease in formation of pinocytic crowns on the cell surface. These morphological aberrations are paralleled by changes in the actin cytoskeleton, with a significant proportion of the cortical F-actin relocalised to prominent pseudopodia. Rapid migration and endocytosis appear to be mutually incompatible and it is likely that RasS protein is required to maintain the normal balance between these two actin-dependent processes.

A mutation that separates the RasG signals that regulate development and cytoskeletal function in Dictyostelium

The expression of an activated RasG, RasG-G12T, in vegetative cells of Dictyostelium discoideium produced an alteration in cell morphology. Cells underwent a transition between an extensively flattened form that exhibited lateral membrane ruffling to a less flattened form that exhibited prominent dorsal membrane ruffling. These rasG-G12T transformants exhibited a redistribution of F-actin at the cell periphery and did not undergo the rapid contraction upon refeeding that is characteristic of wild-type cells. These results suggest a role for RasG in regulating cytoskeletal rearrangement in D. discoideum. We had shown previously that expression of rasG-G12T inhibited starvation induced aggregation (M. Khosla et al., 1996, Mol. Cell. Biol. 16, 4156-4162). rasG-G12T genes containing secondary mutations were transformed into cells to test whether the effects of rasG-G12T were transmitted through a single downstream effector. Cells expressing rasG-G12T/T35S or rasG-G12T/Y40C (secondary mutations within the effector domain) exhibited normal morphology and underwent normal aggregation, suggesting that signaling through the effector domain was required for both the morphological and the development changes induced by rasG-G12T. In contrast, cells expressing rasG-G12T/T45Q (a secondary mutation in the effector distal flanking domain) exhibited normal aggregation but a morphology indistinguishable from that of rasG-G12T transformants. This result suggests that RasG regulates developmental and cytoskeletal functions by direct interaction with more than one downstream effector.

Signaling in unicellular eukaryotes

Aspects of intercellular and intracellular signaling systems in cell survival, proliferation, differentiation, chemosensory behavior, and programmed cell death in free-living unicellular eukaryotes have been reviewed. Comparisons have been made with both bacteria and metazoa. The central organisms were flagellates (Trypanosoma, Leishmania, and Crithidia), slime molds (Dictyostelium), yeast cells (Saccharomyces cerevisiae), and ciliates (Paramecium, Euplotes, and Tetrahymena). There are two novel aspects in this review. First, cellular responses are viewed in an evolutionary perspective, rather than from the more prevailing one, in which the unicellular eukaryotes are seen by the mammalian organisms. Second, results obtained with cell cultures in minimal, chemically defined nutrient media at low cell densities where intercellular signaling is strongly reduced are discussed. These results shed light on control mechanisms and their cooperation inside the living cell. Intracellular systems have many common features in unicellular and multicellular organisms.

Rap1 overexpression reveals that activated RasD induces separable defects during Dictyostelium development

One of the Dictyostelium ras genes, rasD, is expressed preferentially in prestalk cells at the slug stage of development and overexpression of this gene containing a G12T activating mutation causes the formation of aberrant multitipped aggregates that are blocked from further development (Reymond et al., 1986, Nature, 323, 340-343). The ability of the Dictyostelium rap1 gene to suppress this abnormal developmental phenotype was investigated. The rap1 gene and G12V activated and G10V negative mutant forms of the rap1 gene were independently linked to the rasD promoter and each construct used to transform M1, a Dictyostelium cell line expressing RasD[G12T]. Transformants of M1 that expressed Rap1 or Rap1[G12V] protein still formed multitipped aggregates, but most tips were able to complete development and form fruiting bodies. Cell lines showing this modified phenotype were designated ME (multitipped escape). The rap1[G10V] construct did not modify the M1 phenotype. These data suggest that overexpression of RasD[G12T] has two effects, the formation of a multitipped aggregate and a block in subsequent differentiation and that the expression of Rap1 or Rap1[G12V] reverses only the latter. Differentiation of ME cells in low density monolayers showed the identical low level of stalk and spore cell formation seen for M1 cells under the same conditions. Thus the cell autonomous defect in monolayer differentiation induced in the M1 strain was not corrected in the ME strain. Cell type-specific gene expression during the development of M1 cells is dramatically altered: prestalk cell-specific gene expression is greatly enhanced, whereas prespore-specific gene expression is almost suppressed (Louis et al., 1997, Mol. Biol. Cell, 8, 303-312). During the development of ME cells, ecmA mRNA levels were restored to those seen for Ax3, and tagB mRNA levels were also markedly reduced, although not to Ax3 levels. cotC expression in ME cells was enhanced severalfold relative to M1, although levels were still lower than those observed during the development of Ax3. The low expression of car1 mRNA during early development of the M1 strain remained low during the development of ME cells. These data are consistent with the idea that the expression of RasD[G12T] affects two independent and temporally separated events and that only the later defect is reversed by rap1.

Dictyostelium RasG is required for normal motility and cytokinesis, but not growth

RasG is the most abundant Ras protein in growing Dictyostelium cells and the closest relative of mammalian Ras proteins. We have generated null mutants in which expression of RasG is completely abolished. Unexpectedly, RasG- cells are able to grow at nearly wild-type rates. However, they exhibit defective cell movement and a wide range of defects in the control of the actin cytoskeleton, including a loss of cell polarity, absence of normal lamellipodia, formation of unusual small, punctate polymerized actin structures, and a large number of abnormally long filopodia. Despite their lack of polarity and abnormal cytoskeleton, mutant cells perform normal chemotaxis. However, rasG- cells are unable to perform normal cytokinesis, becoming multinucleate when grown in suspension culture. Taken together, these data suggest a principal role for RasG in coordination of cell movement and control of the cytoskeleton.

Two ras genes in Dictyostelium minutum show high sequence homology, but different developmental regulation from Dictyostelium discoideum rasD and rasG genes

The social amoeba Dictyostelium discoideum expresses five ras genes at different stages of development. One of them, DdrasD is expressed during postaggregative development and transcription is induced by extracellular cAMP. A homologue of DdrasD, the DdrasG gene, is expressed exclusively during vegetative growth. We cloned two ras homologues Dmras1 and Dmras2 from the primitive species D. minutum, which show high homology to DdrasD and DdrasG and less homology to the other Ddras genes. In contrast to the DdrasD and DdrasG genes, both the Dmras1 and Dmras2 genes are expressed during the entire course of development. The expression levels are low during growth, increase at the onset of starvation and do not decrease until fruiting bodies have formed. Expression of neither Dmras1 or Dmras2 is regulated by cAMP. So even though the high degree of homology between the ras genes of different species suggests conservation of function, this function is apparently not associated with a specific developmental stage.

Expression of an activated rasD gene changes cell fate decisions during Dictyostelium development

It has been previously demonstrated that the expression of an activated rasD gene in wild-type Dictyostelium cells results in formation of aggregates with multitips, instead of the normal single tips, and a block in further development. In an attempt to better understand the role of activated RasD development, we examined cell-type-specific gene expression in a strain stably expressing high levels of RasD[G12T]. We found that the expression of prestalk cell-specific genes ecmA and tagB was markedly enhanced, whereas the expression of the prespore cell-specific gene cotC was reduced to very low levels. When the fate of cells in the multitipped aggregate was monitored with an ecmA/lacZ fusion, it appeared that most of the cells eventually adopted prestalk gene expression characteristics. When mixtures of the [G12T]rasD cells and Ax3 cells were induced to differentiate, chimeric pseudoplasmodia were not formed. Thus, although the [G12T]rasD transformant had a marked propensity to form prestalk cells, it could not supply the prestalk cell population when mixed with wild-type cells. Both stalk and spore cell formation occurred in low cell density monolayers of the [G12T]rasD strain, suggesting that at least part of the inhibition of stalk and spore formation during multicellular development involved inhibitory cell interactions within the cell mass. Models for the possible role of rasD in development are discussed.

Overexpression of an activated rasG gene during growth blocks the initiation of Dictyostelium development

Transformants that expressed either the wild-type rasG gene, an activated rasG-G12T gene, or a dominant negative rasG-S17N gene, all under the control of the folate-repressible discoidin (dis1gamma) promoter, were isolated. All three transformants expressed high levels of Ras protein which were reduced by growth in the presence of folate. All three transformants grew slowly, and the reduction in growth rate correlated with the amount of RasG protein produced, suggesting that RasG is important in regulating cell growth. The pVEII-rasG transformant containing the wild-type rasG gene developed normally despite the presence of high levels of RasG throughout development. This result indicates that the down regulation of rasG that normally occurs during aggregation of wild-type strains is not essential for the differentiation process. Dictyostelium transformants expressing the dominant negative rasG-S17N gene also differentiated normally. Dictyostelium transformants that overexpressed the activated rasG-G12T gene did not aggregate. The defect occurred very early in development, since the expression of car1 and pde, genes that are normally induced soon after the initiation of development, was repressed. However, when the transformant cells were pulsed with cyclic AMP, expression of both genes returned to wild-type levels. The transformants exhibited chemotaxis to cyclic AMP, and development was synergized by mixing with wild-type cells. Furthermore, cells that were pulsed with cyclic AMP for 4 h before being induced to differentiate by plating on filters produced small, but otherwise normal, fruiting bodies. These results suggest that the rasG-G12T transformants are defective in cyclic AMP production and that RasG - GTP blocks development by interfering with the initial generation of cyclic AMP pulses.

Dual role of cAMP and involvement of both G-proteins and ras in regulation of ERK2 in Dictyostelium discoideum

Dictyostelium discoideum expresses two Extracellular signal Regulated Kinases, ERK1 and ERK2, which are involved in growth, multicellular development and regulation of adenylyl cyclase. Binding of extracellular cAMP to cAMP receptor 1, a G-protein coupled cell surface receptor, transiently stimulates phosphorylation, activation and nuclear translocation of ERK2. Activation of ERK2 by cAMP is dependent on heterotrimeric G-proteins, since activation of ERK2 is absent in cells lacking the Galpha4 subunit. The small G-protein rasD also activates ERK2. In cells overexpressing a mutated, constitutively active rasD, ERK2 activity is elevated prior to cAMP stimulation. Intracellular cAMP and cAMP-dependent protein kinase (PKA) are essential for adaptation of the ERK2 response. This report shows that multiple signalling pathways are involved in regulation of ERK2 activity in D.discoideum.

The aimless RasGEF is required for processing of chemotactic signals through G-protein-coupled receptors in Dictyostelium

BACKGROUND

Ras proteins are small GTP-binding proteins that play an essential role in a wide range of processes, particularly in mammalian growth control. They act as molecular switches, being inactive when GDP is bound, and active when associated with GTP. Activation is accomplished by guanine nucleotide exchange factors (RasGEFs); when RasGEFs interact with Ras proteins, GDP is allowed to escape, and is replaced by GTP. Dictyostelium responds to chemoattractants through typical seven transmembrane domain receptors and heterotrimeric G proteins. There are at least five different Dictyostelium Ras genes, whose functions are not yet known.

RESULTS

We have isolated the aimless gene, which encodes the Dictyostelium homologue of RasGEFs, during a screen for insertional mutants that fail to aggregate. We found that aimless null mutants grew at a normal rate, but were severely impaired in both chemotaxis and activation of adenylyl cyclase, both of which are critical for the early stages of development. Although coupling between receptors and their G proteins is unaffected, and several cyclic AMP (cAMP)-mediated responses appear normal, activation of adenylyl cyclase by receptors and GTP gamma S (a non-hydrolyzable GTP analogue) is reduced by up to 95%. The motility of mutant cells appears normal, suggesting a true defect in gradient sensing.

CONCLUSIONS

The discovery of the aimless gene adds an interesting new member to the family of RasGEFs. Our data suggest an unforeseen role for a RasGEF, and therefore presumably a complete Ras pathway, in the processing of chemotactic signals through G-protein-coupled receptors.

Comparison of the Dictyostelium rasD and ecmA genes reveals two distinct mechanisms whereby an mRNA may become enriched in prestalk cells

The Dictyostelium ras gene, rasD, encodes an mRNA that is more abundant in prestalk than prespore cells in the migratory slug. Its expression is inducible by extracellular cAMP but is not inducible by the prestalk and stalk cell morphogen differentiation inducing factor (DIF). We show that a rasD-lacZ fusion gene is first expressed in approximately one half of the cells in the aggregate, including some cells that also express a prespore-specific marker. The amount of rasD-lacZ fusion protein in prespore cells then diminishes as the slug is formed. Analysis of a rasD-lacZ fusion protein with an N terminal substitution that reduces protein stability within the cell provides strong confirmatory evidence that the ras gene product becomes enriched in prestalk cells by selective repression of gene expression in prespore cells. In contrast, the DIF-inducible ecmA gene is expressed only in those cells that will become prestalk cells in the migratory slug. These results show that there are two different ways in which an mRNA may become enriched in prestalk cells and support the view that DIF is the inducer of prestalk cell differentiation.

A Rab4-like GTPase in Dictyostelium discoideum colocalizes with V-H(+)-ATPases in reticular membranes of the contractile vacuole complex and in lysosomes

In the course of screening a cDNA library for ras-related Dictyostelium discoideum genes, we cloned a 0.7 kb cDNA (rabD) encoding a putative protein that was 70% identical at the amino acid level to human Rab4. Rab4 is a small M(r) GTPase, which belongs to the Ras superfamily and functions to regulate endocytosis in mammalian cells. Southern blot analysis indicated that the rabD cDNA was encoded by a single copy gene while Northern blot analysis revealed that the rabD gene was expressed at relatively constant levels during growth and differentiation. Affinity-purified antibodies were prepared against a RabD fusion protein expressed in bacteria; the antibodies recognized a single 23 kDa polypeptide on western blots of cell extracts. Density gradient fractionation revealed that the RabD antigen co-distributed primarily with buoyant membranes rich in vacuolar protons pumps (V-H(+)-ATPases) and, to a lesser extent, with lysosomes. This result was confirmed by examining cell lines expressing an epitope-tagged version of RabD. Magnetically purified early endocytic vesicles and post-lysosomal vacuoles reacted more weakly with anti-RabD antibodies than did lysosomes. Other organelles were negative for RabD. Double-label indirect immunofluorescence microscopy revealed that RabD and the 100 kDa V-H(+)-ATPase subunit colocalized in a fine reticular network throughout the cytoplasm. This network was reminiscent of spongiomes, the tubular elements of the contractile vacuole system. Immunoelectron microscopy confirmed the presence of RabD in lysosome fractions and in the membranes rich in V-H(+)-ATPase. We conclude that a Rab4-like GTPase in D. discoideum is principally associated with the spongiomes of contractile vacuole complex.

RasG protein accumulation occurs just prior to amoebae emergence during spore germination in Dictyostelium discoideum

RasG protein levels in dormant and germinating spores of Dictyostelium discoideum strains JC1 and SG1 were estimated by Western blotting. RasG levels were very low in dormant spores and remained low during the lag period, regardless of whether spores were heat activated or treated with autoactivator during the early stages of spore germination. RasG levels increased late during spore swelling just prior to the emergence stage of germination. These data are consistent with a requirement for RasG during vegetative growth.

Basidiomycetous ras cDNA functionally replaces its homolog genes in yeast

It was shown by a plasmid exchange procedure that the Ras-encoding cDNA of the basidiomycete Lentinus edodes (named Leras cDNA) can functionally replace its homolog genes (ScRAS1 and ScRAS2) in the yeast Saccharomyces cerevisiae to maintain the viability of an yeast strain containing genetic disruptions of both RAS genes. The strain replaced by a Leras-cDNA-carrying plasmid, however, grew slower than the strains replaced by a ScRAS1- or a ScRAS2-carrying plasmid. The intracellular level of cAMP in the strain harboring the Leras-cDNA-carrying plasmid was clearly higher than that of a parental strain which maintains a plasmid carrying the S. cerevisiae cAMP-dependent protein kinase catalytic subunit C1 gene, TPK1, but was lower than that in a strain harboring an ScRAS2-carrying plasmid. These results suggest that the Leras cDNA can complement the ras1- ras2- mutation of yeast by virture of the stimulation of adenylate cyclase activity, although the complementation is not as efficient as that obtained by expressing the ScRAS2 gene.

Altered morphology of vegetative amoebae induced by increased expression of the Dictyostelium discoideum ras-related gene rap1

The rap1 gene of Dictyostelium discoideum is a member of the ras-gene superfamily of low molecular weight GTPase proteins. The rap1 gene is expressed both during growth and development in D. discoideum. To examine the action of the Rap1 protein in D. discoideum, the rap1 cDNA was expressed under the control of the inducible discoidin promoter. Treatment with conditioned media, which induces the discoidin promoter, increased Rap1 protein levels in vegetative cells approximately six fold. Overexpression of the Rap1 protein correlated with the appearance of morphologically aberrant vegetative amoebae: cells were extensively spread and flattened. The distribution of F-actin was altered in these cells, with an increase in actin staining around the cell periphery. Induction of the discoidin promoter by starvation in the rap1 transformants also resulted in spread flat cells. When starved D. discoideum amoebae are refed with HL5 media, the cells rapidly respond by rounding up. By contrast, the rap1 transformant cells showed a pronounced delay in rounding up. Rapid tyrosine phosphorylation of a p45 protein occurred in both control cells and the rap1 transformant upon refeeding, implying that the signal transduction pathway leading to tyrosine phosphorylation remained functional in the rap1 transformant. We propose that the Rap1 protein functions in the regulation of cell morphology in D. discoideum.

Ligand-independent reduction of cAMP receptors in Dictyostelium discoideum cells over-expressing a mutated ras gene

Drug-resistance selection in Dictyostelium discoideum transformants resulted in up to eight-times-higher ras protein levels. Over-production of the wild-type ras protein did not lead to an aberrant phenotype. Increased levels of the mutated [G12T]ras protein, however, were correlated with severe deficiencies in aggregation and development. This aberrant phenotype is associated with reduced cAMP binding, due to a lower number of cell-surface receptors. We show that both RNA and cAMP-receptor-protein levels are reduced. These results indicate that ras in Dictyostelium discoideum seems to be involved in regulating cAMP-receptor-gene expression.

Regulation of the Dictyostelium cAMP-induced, prestalk-specific DdrasD gene: identification of cis-acting elements

We have shown previously that expression of the Dictyostelium ras gene DdrasD (previously denoted Ddras) is induced during multicellular development and in single-cell shaking culture in response to cAMP (1). Analysis of transformants carrying DdrasD/lacZ reporter constructs showed DdrasD expression to be prestalk-specific (2). The gene is transcribed from three start sites with transcription from the distal site producing an approximately 1.2 kb transcript, which is expressed at low levels in growing cells and is subsequently induced late in aggregation. This promoter is also induced to high levels by cAMP. Transcription from the two more proximal sites is coregulated and is induced during development, resulting in approximately 1.0 kb transcripts. In this study, we examine cis-acting regions required for proper regulation of DdrasD expression using a DdrasD/beta-glucuronidase reporter gene construct. We have identified distinct sequence elements required for developmental and vegetative expression of DdrasD. A domain containing a CA repeat, similar to ones found in other late, cAMP-induced Dictyostelium genes, is required for cAMP-induced and developmental expression.

Two independent promoters as well as 5' untranslated regions regulate Dd ras expression in Dictyostelium

The Dd ras gene produces three transcripts during Dictyostelium development. The largest transcript (L-) can be induced by external addition of cAMP even in cells prevented from aggregating, whereas shorter transcripts (S1- and S2-) expression requires cell aggregate formation. We show the presence of two independent promoters for L- and S-transcripts by deletion analysis of Dd ras fragments fused to CAT reporter genes reintroduced in Dictyostelium. A direct repeat upstream of S-transcript start sites which seems involved in S-promoter function, modulates also L-RNA accumulation. Furthermore removal of sequences between this repeat and the AUG protein start codon reduces the level of L-transcripts in aggregates. This study allowed to uncover the intricate pattern of sequences participating in the regulation of Dd ras expression.

A developmentally regulated gene encodes the dictyostelium homolog of yeast ribosomal protein S4 and mammalian LLRep3 proteins

We report the sequence and expression of a single-copy gene from Dictyostelium discoideum which encodes the homolog of yeast ribosomal protein S4, a protein located on the small ribosomal subunit and known to play an important role in maintaining translational fidelity. Over a highly conserved central region, the Dictyostelium protein has 78% sequence similarity to the yeast protein and 83% sequence similarity to mammalian S4 protein homologs, the LLRep3 proteins. The Dictyostelium gene encodes a polypeptide 28,717 Da in size and hence this ribosomal protein has been named rp29. The N-terminal sequence of the Dictyostelium rp29 protein is extended by 61 amino acids and 14 amino acids compared to the mammalian and yeast proteins, respectively, and the C-terminus is correspondingly 15 amino acids or 2 amino acids shorter. Although the coding region of the rp29 gene is present on a single exon, a 157bp intron interrupts the 5' untranslated region and unusually contains four direct repeats of the sequence TCAATCT. The gene is expressed maximally during vegetative growth but a second peak of expression also occurs late in development which is restricted to prestalk cells; rp29 is the first Dictyostelium ribosomal protein gene reported which shows prestalk-specific developmental expression. During each round of expression, only a single 0.9kb transcript is produced which is similar in size to the yeast S4 ribosomal protein transcript (0.8kb) but markedly smaller than the mammalian LLRep3 mRNA (1.7kb) due to a much shorter 5' untranslated region.

Expression of ras-like proteins in embryonic and adult cells of Xenopus laevis

A polyclonal antibody raised against v-Ha-ras p21 was purified and its specificity was checked on Ha-ras transformed cell lines. It was used to immunoprecipitate p21 from different Xenopus laevis cell types: brain cells, blood cells, and embryonic material. By one-dimensional Western blot analysis, we show that ras p21 is synthesized very early in oogenesis and accumulates throughout vitellogenesis. The ras p21 content, estimated to be 1.1 ng in the full-grown oocyte, remains constant during oocyte maturation and egg cleavage. Increase in the amount of ras p21 occurs at the beginning of neurulation. Two-dimensional Western blot patterns reveal the presence of multiple molecular forms of p21 in all Xenopus cell types studied. The numerous resolved polypeptides were ascribed to the expression of at least two different ras genes. Furthermore, specific charge modifications of the ras polypeptides are observed in brain, blood, and embryonic cells. During oogenesis and early embryonic development, differences in two-dimensional patterns mainly concern variations in the relative amounts of the different polypeptides. The results are discussed in relation to the well documented synthesis activities of the growing oocyte and of the early developing embryo.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

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.

Analysis of the multiple transcripts of the Dd ras gene during Dictyostelium discoideum development

Transcripts from the Dd ras gene can only be detected once starved cells have begun to aggregate (Reymond et al., Cell 39: 141-148, 1984). We show in this report that the three transcripts which originate from Dd ras during normal development differ in their 5' ends. In suspension of starved single cells, one major Dd ras RNA accumulates upon addition of cAMP. It seems that the cAMP regulation of Dd ras expression happens both at the transcriptional and post-transcriptional level. An RNA secondary structure present in the 5' untranslated region of the gene is proposed to be important in this post-transcriptional regulation.

Evidence for the presence of a growth factor in Dictyostelium discoideum

Polypeptide hormones, recognized for their ability to regulate cell growth and differentiation, have been classified as growth factors. These growth factors have been extensively described in higher eukaryotic organisms and cell lines [Hedin and Westermark, Cell 37:9-20, 1984]. Here we report the identification and partial characterization of a putative growth factor present in vegetative amoebae of the cellular slime mold Dictyostelium discoideum. A mutant was selected and found to be temperature sensitive due to the absence of an extracellular protein suggestive of a growth factor. The putative growth factor (DGF) is a protein resistant to both heat and strong detergent treatment but sensitive to reducing agents. The physiological significance of DGF is as yet unknown. DGF is of interest both in relation to understanding the events which control cell proliferation in Dictyostelium and in its relationship to other known growth factors.

cAMP and cell sorting control the spatial expression of a developmentally essential cell-type-specific ras gene in Dictyostelium

The Dictyostelium ras gene (Dd-ras) is expressed at a low level in vegetative cells, is not expressed between the onset of development and aggregation, and is then re-expressed in the multicellular aggregate stages from the distal, now cAMP-responsive, promoter and from two more proximal promoters. Expression of activated Dd-ras (G12----T12) (Reymond et al. 1986) results in an abnormal developmental phenotype with the formation of aggregates having multiple tips and an inhibition of further development. In this report we investigate the spatial expression of Dd-ras by fusing the 5'-flanking region to the Escherichia coli lacZ gene and by staining aggregates for beta-galactosidase (beta-gal) activity. We show that fusions using 5'-flanking sequences that include all promoters are expressed in approximately 10-20% of the cells randomly scattered within the early aggregate. Our data indicate that these beta-gal-expressing cells migrate to newly formed tips of aggregates and localize in the region that becomes the prestalk zone. Staining is also seen in the very posterior of the organism. The anterior staining appears to be specific for the prestalk A population, and beta-gal activity is subsequently present in stalk cells as developmental proceeds. When only the two more proximal promoters are used to drive lacZ expression, localized staining is seen in the anterior prestalk region, although it is weaker than with the construct carrying all promoters. Moreover, staining is not seen in the posterior domain in the first finger stage, suggesting differences in the spatial expression from the different promoters. Staining is also observed in some cells within the prespore region, which could be anterior-like cells. The pattern of Dd-ras/lacZ staining during tip formation suggests a directed, spiral pattern of cell migration, possibly in response to the proposed spiral gradient of cAMP within the developing aggregate. The pattern of Dd-ras is consistent with the abnormal developmental phenotype caused by expressing an activated Dd-ras Thr12 gene and suggests an essential role for Dd-ras in controlling spatial differentiation.

Increased conversion of phosphatidylinositol to phosphatidylinositol phosphate in Dictyostelium cells expressing a mutated ras gene

Dictyostelium discoideum cells that overexpress a ras gene with a Gly12----Thr12 mutation (Dd-ras-Thr12) have an altered phenotype. These cells were labeled with [3H]inositol and the incorporation of radioactivity into inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was analyzed and found to be higher than in control cells. In contrast, the total mass of Ins(1,4,5)P3, as assessed with an assay using a specific Ins(1,4,5)P3-binding protein, was not significantly different between control and Dd-ras-Thr12 cells. Cells were labeled with [3H]inositol and the incorporation of radioactivity in all inositol metabolites was analyzed. Increased levels of radioactivity were observed for phosphatidylinositol phosphate (PtdInsP), phosphatidylinositol bisphosphate (PtdInsP2), Ins(1,4,5)P3, inositol 1,4-bisphosphate, inositol 4,5-bisphosphate, and inositol 4-monophosphate in Dd-ras-Thr12 cells relative to control cells. Decreased levels were found for phosphatidylinositol (PtdIns) and inositol 1-monophosphate. Calculations on the substrate/product relationships [i.e., Ins(1,4,5)P3/PtdInsP2] demonstrate that the observed differences are due only to the increased conversion of PtdIns to PtdInsP; other enzyme reactions, including phospholipase C, are not significantly different between the cell lines. The activity of PtdIns kinase in vitro is not different between Dd-ras-Thr12 and control cells, suggesting that either the regulation of this enzyme is altered or that the translocation of substrate from the endoplasmic reticulum to the kinase in the plasma membrane is modified. The results suggest multiple metabolic compartments of Ins(1,4,5)P3 in Dictyostelium cells. In Dd-ras-Thr12 transformants the increased conversion of PtdIns to PtdInsP leads to increased levels of Ins(1,4,5)P3 in the compartment with a high metabolic turnover. This Ins(1,4,5)P3 compartment is suggested to be involved in the regulation of cytosolic Ca2+ levels.

A ras-related gene from the lower eukaryote Dictyostelium that is highly conserved relative to the human rap genes

The cellular slime mold Dictyostelium discoideum contains two ras genes, DdrasG and Ddras that are differentially expressed during development. We have characterized a gene that hybridized to both Ddras and DdrasG under low, but not under high stringency conditions. The deduced amino acid sequence is highly conserved with respect to the human rap (Krev-1, smg21) proteins and the corresponding gene has been designated Ddrap1. The Ddrap1 gene is expressed at all stages during development but is expressed maximally during the aggregation and culmination periods when the expression of Ddras and DdrasG is declining. During vegetative growth and early development Ddrap1 cDNA hybridizes to a single mRNA of 1.1 kb. As development progresses the level of this mRNA declines and messages of 1.0 and 1.3 kb appear.