bagpipe-Dependent expression of vimar, a novel Armadillo-repeats gene, in Drosophila visceral mesoderm

Two homeobox-containing genes, tinman and bagpipe, play important roles during the specification of the midgut visceral musculature from the mesoderm during Drosophila embryogenesis. Expression of tinman in the dorsal mesoderm activates the expression of the bagpipe gene in segmental subsets of those cells, which then become determined to form the midgut visceral mesoderm. Understanding how the bagpipe gene affects this specification requires the isolation and characterization of its downstream target genes. Using an enhancer trap line that expresses its marker in the midgut visceral mesoderm, we have cloned and characterized a novel gene (vimar) that is expressed embryonically in the mid and hindgut visceral mesoderm, as well as in the CNS and PNS. The expression of this gene in the midgut visceral mesoderm initiates shortly after bagpipe expression and depends on bagpipe function. Maternal and zygotic transcripts are produced from this gene by alternative polyadenylation, and encode the same 634-amino acid protein. The vimar protein contains 15 tandem copies of the Armadillo repeat, a protein interaction domain, and is similar to mammalian Smg guanine dissociation stimulator protein, which stimulates the activity of a number of different p21 small G-proteins. These results, together with the observed lethality of vimar mutations, indicate that vimar is one of the bagpipe target genes that are required for normal development and differentiation of the midgut visceral mesoderm.

Identification of a specific effector of the small GTP-binding protein Rap2

Rap2 is a small GTP-binding protein that belongs to the Ras superfamily and whose function is still unknown. To elucidate Rap2 function, we searched for potential effectors by screening a mouse brain cDNA library in a yeast two-hybrid system using as a bait a Rap2A protein bearing a mutation of Gly to Val at position 12. This strategy lead to the identification of a protein that interacts specifically with Rap2A complexed with GTP, and requires an intact effector domain of Rap2A for interaction; we designated this protein Rap2-interacting protein 8 (RPIP8). Biochemical data obtained from in vitro studies with purified proteins confirmed the genetic results. Mouse RPIP8 consists of 446 amino acids, bears a coiled-coil domain between residues 265 and 313, and is expressed principally in brain. Its human counterpart, of 400 amino acids, exhibits 93.7% identity in their common region. A search for similar sequences in expressed-sequence-tags databanks revealed the presence in human and rodents of mRNAs encoding the 400-residue and 446-residue forms of RPIP8. Furthermore a doublet of 45-50 kDa, corresponding to the 400-residue and 446-residue forms of the protein, was detected by western blotting of mouse brain extracts and lysates from pheochromocytoma PC12 cells and the pancreatic beta-cell lines HIT-T15 and RIN-m5F. Using transient transfections of HIT-T15 cells it was possible to demonstrate that [Val12]Rap2 and wild-type Rap2 could be immunoprecipitated with RPIP8. These data therefore argue for RPIP8 being a specific effector of the Rap2 protein in cells exhibiting neuronal properties.

Costimulation through CD28 suppresses T cell receptor-dependent activation of the Ras-like small GTPase Rap1 in human T lymphocytes

Members of the Ras superfamily of small molecular weight GTPases play diverse and critical roles in mediating cellular responses to extracellular stimuli, including mitogenesis, cytoskeletal maintenance and rearrangement, and integrin activation. In T lymphocytes, biochemical and genetic evidence demonstrate that Ras plays an essential role in coupling T cell receptor ligation to signaling cascades required for T cell proliferation and development. Recent observations that C3G, a guanine nucleotide exchange factor specific for the Ras-related GTPase Rap1, is recruited into tyrosine-phosphorylated protein signaling complexes in activated T cells have suggested that Rap1 may also play a role in T cell activation. Utilizing a recently developed technique for detection of endogenous, GTP-bound Rap1, we have found that Rap1, but not Rap2, is transiently activated following T cell receptor stimulation of normal human T lymphocytes. Increases in intracellular calcium is both necessary and sufficient to induce Rap1 activation. Remarkably, costimulation of T cells with mitogenic anti-CD28 antibody completely abolished T cell receptor-dependent activation of Rap1. This report demonstrates a potential role for Rap1 in T cell receptor signaling and suggests inactivation of Rap1 as a candidate target of CD28-dependent costimulatory signals required for T cell antigen responsiveness.

The C-terminal silencing domain of Rap1p is essential for the repression of ribosomal protein genes in response to a defect in the secretory pathway

We have previously shown that a functional secretory pathway is essential for continued ribosome synthesis in Saccharomyces cerevisiae. When a temperature-sensitive mutant defective in the secretory pathway is transferred to the non-permissive temperature, transcription of both rRNA genes and ribosomal protein genes is nearly abolished. In order to define the cis -acting element(s) of ribosomal protein genes sensitive to a defect in the secretory pathway, we have constructed a series of fusion genes containing the CYH2 promoter region, with various deletions, fused to lacZ. Each fusion gene for which transcription is detected is subject to the repression. Rap1p is the transcriptional activator for most ribosomal protein genes, as well as having an important role in silencing in the vicinity of telomeres and at the silent mating-type loci. To assess its role in the repression of transcription by the defect in the secretory pathway, we have introduced rap1 mutations. The replacement of wild-type Rap1p by Rap1p truncated at the C-terminal region caused substantial attenuation of the repression. Furthermore, we have demonstrated that the Rap1p-truncation affects the repression of TCM1 , encoding ribosomal protein L3, which has no Rap1p-binding site in its upstream regulatory region. These results suggest that the repression of transcription of ribosomal protein genes by a secretory defect is mediated through Rap1p, but does not require a Rap1p-binding site within the UAS.

Transformation suppressor activity of C3G is independent of its CDC25-homology domain

The guanine nucleotide releasing protein C3G was initially identified as a Crk SH3-binding protein and recently shown to exhibit exchange activity on Rap1 proteins. Overexpression in NIH3T3 cells of a full-length C3G cDNA isolated from human placenta markedly reduced the focus forming activity of cotransfected, malignantly activated, ras oncogenes (5-7-fold). C3G also had a reverting effect on sis-mediated transformation, decreasing the number of c-sis-induced foci by a factor of 5-10-fold. The observed inhibitory effect of C3G on focus-forming activity of Ras and Sis was always higher than that observed with Rap1A, a known target of C3G. The inhibition of focus formation observed in the presence of C3G was not due to toxic effects on cell viability, since transfected C3G cells exhibited the same survival and growth rates as untransfected NIH3T3 cells or cells transfected with plasmid vector alone. Surprisingly, as opposed to Rap1A, which has no effect on Raf-1 oncogene-mediated transformation, C3G also reduced dramatically (6-8-fold) the number of v-raf-induced foci in transfected NIH3T3 cells. The inhibitory effect on Raf-induced transformation suggests that C3G has other functional targets in addition to Rap1. A C3G mutant (C3G deltaCat) lacking the catalytic domain (CDC25-H) but retaining the rest of the N-terminal sequences, including the Crk-binding domain, exhibited similar ability than full length C3G to inhibit focus formation. In contrast, a C3G mutant (C3G Cat), containing the catalytic domain only but lacking the rest of the N-terminal sequences, did not have any inhibitory effect on transformation mediated by the oncogenes tested. The C3G-derived gene products overexpressed in our transfected cell lines localized to the cytoplasm and did not change the basal MAPK or JNK activity of those cell lines nor their ability to activate the kinases in response to agonists. Our results suggest that the N-terminal region of C3G, and not its catalytic domain, may be responsible for the inhibitory effects observed.

The PEA3 Ets transcription factor is a downstream target of the HER2/Neu receptor tyrosine kinase

The HER2/neu gene, which is overexpressed in 20-30% of human breast tumors, encodes a receptor tyrosine kinase that functions through multiple signaling pathways to regulate the activity of nuclear transcription factors. We have reported that PEA3, an Ets family transcription factor, is overexpressed in HER2/Neu-induced breast tumors and their metastases. To account for the increased levels of PEA3 in these tumors we have suggested that HER2/Neu enhances PEA3 transcriptional activity, which then acts to stimulate expression of the PEA3 gene. This hypothesis is consistent with the occurrence of PEA3 binding sites in the PEA3 promoter and with the ability of PEA3 to transactivate this promoter. To learn whether HER2/Neu indeed regulates PEA3 activity we measured the capacity of constitutively-activated HER2/Neu to affect PEA3-dependent reporter gene expression. Coexpression of PEA3 and HER2/Neu stimulated PEA3-dependent reporter gene expression to a much greater extent than did either protein alone suggesting that HER2/Neu upregulates the transcriptional activity of PEA3. To define the pathway whereby HER2/Neu functions we employed dominant-negative mutants of signaling proteins known to be downstream of HER2/Neu. Overexpression of Rap1a, a Ras-related protein capable of antagonizing Ras function, completely inhibited the ability of HER2/Neu to stimulate PEA3-dependent gene expression. Ras is known to stimulate at least two mitogen-activated protein kinase (MAPK) cascades, the extracellular-regulated kinase (ERK) cascade and the stress-activated kinase (SAPK) or Jun kinase (JNK) cascade. Similarly, HER2/Neu activated both ERKs and SAPKs/JNKs in a Ras-dependent fashion. Dominant-inhibitory mutants in either the ERK or SAPK/JNK cascades partially inhibited HER2/Neu activation of PEA3-dependent gene expression. These findings suggest that HER2/Neu regulates PEA3 activity through two different Ras-dependent MAPK pathways.

The type of basal promoter determines the regulated or constitutive mode of transcription in the common control region of the yeast gene pair GCY1/RIO1

The yeast genes, GCY1 and RIO1, are transcribed divergently from the 869-base pair intergenic region. GCY1 is inducible by galactose about 25-fold due to Gal4p-binding to a single UASGAL, whereas RIO1 is constitutively expressed. GCY1 has a TATA box obeying the consensus TATAAA, whereas the RIO1 5'-upstream region lacks such a motif. In vitro mutagenesis of the TATA motif of GCY1, on the one hand, and introduction of a TATA-element into the promoter of RIO1, on the other hand, as well as inversion of the intergenic region have revealed that transcription of GCY1 and RIO1 is only regulated by Gal4p when a consensus TATA motif is included in their core promoters but not in its absence. The data imply that only transcription complexes that assemble at a consensus TATA box are compatible with specific transactivators, such as Gal4p. As a result, the adjacent gene is subject to regulated expression. By contrast, if a consensus TATA sequence is absent, the initiation complex does not respond to regulatory transcription factors, and consequently, the respective gene is constitutively transcribed. On the other hand, we show that two blocks of homo-oligomeric (dA.dT) sequences do not function as boundary sequences that might confine regulatory action of Gal4p to GCY1.

Colocalization of Ras and Ral on the membrane is required for Ras-dependent Ral activation through Ral GDP dissociation stimulator

Ral GDP dissociation stimulator (RalGDS), a putative effector protein of Ras, stimulated the GDP/GTP exchange reaction of the post-tanslationally lipid-modified but not the unmodified form of Ral in response to epidermal growth factor in COS cells. The RalGDS action on Ral was enhanced by an active form of Ras but not a Ras mutant which was not post-translationally modified in the cells. The RalGDS activity was inhibited by acidic membrane phospholipids such as phosphatidylinositol and phosphatidylserine but not by phosphatidylcholine or phosphatidylethanolamine in vitro. The post-translationally modified form but not unmodified form of Ras, Ral, and Rap were incorporated in liposomes consisting of these phospholipids. When Ral was incorporated alone in the liposomes, RalGDS did not stimulate the dissociation of GDP from Ral. When Ral was incorporated with the GTP-bound form of Ras in the liposomes, RalGDS stimulated the dissociation of GDP from Ral, while the GDP-bound form of Ras did not affect the RalGDS action. The Ras-dependent Ral activation through RalGDS required the Ras-binding domain of RalGDS. Rap, which shared the same effector loop as Ras, also stimulated the dissociation of GDP from Ral through RalGDS in the liposomes, although Rap did not enhance the RalGDS action in COS cells. Taken together with our previous observations that Ras recruits RalGDS to the membrane, these results indicate that the post-translational modifications of Ras and Ral are important for Ras-dependent Ral activation through RalGDS and that colocalization of Ras and Ral on the membrane is necessary for Ral activation in intact cells.

Biological characterization of Drosophila Rapgap1, a GTPase activating protein for Rap1

The activity of Ras family proteins is modulated in vivo by the function of GTPase activating proteins, which increase their intrinsic rate of GTP hydrolysis. We have isolated cDNAs encoding a GAP for the Drosophila Rap1 GTPase. Drosophila Rapgap1 encodes an 850-amino acid protein with a central region that displays substantial sequence similarity to human RapGAP. This domain, when expressed in Escherichia coli, potently stimulates Rap1 GTPase activity in vitro. Unlike Rap1, which is ubiquitously expressed, Rapgap1 expression is highly restricted. Rapgap1 is expressed at high levels in the developing photoreceptor cells and in the optic lobe. Rapgap1 mRNA is also localized in the pole plasm in an oskar-dependent manner. Although mutations that completely abolish Rapgap1 function display no obvious phenotypic abnormalities, overexpression of Rapgap1 induces a rough eye phenotype that is exacerbated by reducing Rap1 gene dosage. Thus, Rapgap1 can function as a negative regulator of Rap1-mediated signaling in vivo.

Genetic interactions with Rap1 and Ras1 reveal a second function for the fat facets deubiquitinating enzyme in Drosophila eye development

The Drosophila fat facets gene encodes a deubiquitinating enzyme that regulates a cell communication pathway essential very early in eye development, prior to facet assembly, to limit the number of photoreceptor cells in each facet of the compound eye to eight. The Fat facets protein facilitates the production of a signal in cells outside the developing facets that inhibits neural development of particular facet precursor cells. Novel gain-of-function mutations in the Drosophila Rap1 and Ras1 genes are described herein that interact genetically with fat facets mutations. Analysis of these genetic interactions reveals that Fat facets has an additional function later in eye development involving Rap1 and Ras1 proteins. Moreover, the results suggest that undifferentiated cells outside the facet continue to influence facet assembly later in eye development.

Insulin and epidermal growth factor stimulate a conformational change in Rap1 and dissociation of the CrkII-C3G complex

Insulin and epidermal growth factor (EGF) stimulation of Chinese hamster ovary cells expressing the human insulin and EGF receptors resulted in a time-dependent decrease in the ability of a Rap1 antibody (amino acid epitope 121-136) to immunoprecipitate Rap1 from whole cell detergent extracts. This was due to an apparent masking of Rap1 as heat denaturation of the whole cell detergent extracts (5 min at 100 degrees C) resulted in equal immunoprecipitation of Rap1 with this epitope-specific antibody. The time-dependent change in Rap1 immunoreactivity was paralleled with an insulin-stimulated dissociation of the CrkII-C3G complex. Similarly, EGF treatment also resulted in a time-dependent dissociation of the CrkII-C3G complex that occurred concomitant with the masking of the 121-136 Rap1 epitope. Furthermore, pretreatment of the cells with the tyrosine kinase inhibitor, genistein, decreased both the basal and insulin-stimulated tyrosine phosphorylation of CrkII that directly correlated with the amount of CrkII that was immunoprecipitated with C3G. Together, these data suggest that insulin and EGF stimulation result in the dissociation of the CrkII-C3G complex, thereby inducing an apparent conformation change in Rap1.

The general regulatory factor Reb1p controls basal, but not Gal4p-mediated, transcription of the GCY1 gene in yeast

Expression of the gene GCY1 in Saccharomyces cerevisiae is induced by about 25-fold in the presence of galactose as a result of activation by Gal4p. In contrast to other Gal4p-regulated genes, such as GAL1 or GAL10, GCY1 is transcribed at a relatively high basal level. We have analysed the basis of this behaviour and have found that, in addition to a UASGAL, a binding site for the general regulatory factor Reb1p is localized 100 bp upstream of the TATA sequence and about 140 bp 3' to the UASGAL. Reb1p binds to this site with low affinity. Reb1p, an abundant, multifunctional DNA-binding protein in yeast, acts as a weak transcriptional activator in the control regions of several genes encoding unrelated functions. The action of Reb1p is assumed to be strongly position dependent. In the control region of GCY1. Reb1p acts independently of position and stimulates basal expression of GCY1 about threefold, whereas Gal4p-mediated activation is not influenced significantly. Promoter-proximal insertion of an additional Reb1p recognition site enhances basal transcription only marginally, but can largely compensate for deletion of the natural Reb1p-binding site. Either an Abf1p- or a Rap1p-binding site can substitute for the Reb1p recognition sequence, indicating that these general regulatory factors fulfill related functions in basal transcription, without affecting Gal4p-mediated activation. In addition to Reb1p, the sequence of the Gal4p-binding site influences basal transcription. This effect is independent of the Gal4 protein, as it operates in a gal4 mutant background as well. This finding suggests that the nucleotide sequence of the UASGAL in the GCY1 promoter has intrinsic properties, presumably a particular DNA structure, that influence basal transcription and act synergistically with Reb1p.

Human SPA-1 gene product selectively expressed in lymphoid tissues is a specific GTPase-activating protein for Rap1 and Rap2. Segregate expression profiles from a rap1GAP gene product

Mouse Spa-1 gene with a region homologous to the human rap1GAP gene is transcriptionally induced in the lymphocytes by mitogenic stimulation. Herein we have cloned a cDNA for its human counterpart. SPA-1 cDNA encodes a 130-kDa protein (p130(SPA-1)) consisting of proline-rich regions and rap1GAP-related domain followed by a coiled-coil stretch. Baculovirally expressed p130(SPA-1) exhibited GTPase-activating protein (GAP) activity for Rap1 and Rap2, but not for Ras, Rho, Cdc42, Rac, and Ran, with comparable specific activity to the rap1GAP gene product (p85/95(rap1GAP)). In the cells, p130(SPA-1) was mostly localized at the perinuclear membranous region co-localizing with Rap1 and Rap2. Expression of SPA-1 and rap1GAP genes tended to be segregate in various tissues, lymphoid tissues expressing abundant SPA-1 transcript without rap1GAP, while those such as brain, kidney, and pancreas exhibiting rap1GAP mRNA with little SPA-1. Promyelocytic HL-60 cells, which expressed p130(SPA-1) with little p85/95(rap1GAP) in uninduced state, showed progressive decline in p130(SPA-1) and conversely drastic increase in p85/95(rap1GAP) as they ceased from proliferation and differentiated into macrophages by 12-O-tetradecanoylphorbol-13-acetate. These results suggested that products of SPA-1 and rap1GAP genes, albeit comparable GAP activity for Rap1 and Rap2, functioned in the distinct contexts depending on cell types and/or states.

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.

Maintenance of human T cell anergy: blocking of IL-2 gene transcription by activated Rap1

In the absence of costimulation, T cells activated through their antigen receptor become unresponsive (anergic) and do not transcribe the gene encoding interleukin-2 (IL-2) when restimulated with antigen. Anergic alloantigen-specific human T cells contained phosphorylated Cbl that coimmunoprecipitated with Fyn. The adapter protein CrkL was associated with both phosphorylated Cbl and the guanidine nucleotide-releasing factor C3G, which catalyzes guanosine triphosphate (GTP) exchange on Rap1. Active Rap1 (GTP-bound form) was present in anergic cells. Forced expression of low amounts of Rap1-GTP in Jurkat T cells recapitulated the anergic defect and blocked T cell antigen receptor (TCR)- and CD28-mediated IL-2 gene transcription. Therefore, Rap1 functions as a negative regulator of TCR-mediated IL-2 gene transcription and may be responsible for the specific defect in IL-2 production in T cell anergy.

The GAP-related domain of tuberin, the product of the TSC2 gene, is a target for missense mutations in tuberous sclerosis

Tuberous sclerosis is an autosomal dominant trait in which the dysregulation of cellular proliferation and differentiation results in the development of hamartomatous growths in many organs. The TSC2 gene is one of two genes determining tuberous sclerosis. Inactivating germline mutations of TSC2 in patients with tuberous sclerosis and somatic loss of heterozygosity at the TSC2 locus in the associated hamartomas indicate that TSC2 functions as a tumour suppressor gene and that loss of function is critical to expression of the tuberous sclerosis phenotype. The TSC2 product, tuberin, has a region of homology with the GTPase activating protein rap1GAP and stimulates the GTPase activity of rap1a and rab5a in vitro. Here we show that the region of homology between tuberin and human rap1GAP and the murine GAP mSpa1 is more extensive than previously reported and spans approximately 160 amino acid residues encoded within exons 34-38 of the TSC2 gene. Single strand conformation polymorphism analysis of these exons in 173 unrelated patients with tuberous sclerosis and direct sequencing of variant conformers together with study of additional family members enabled characterisation of disease associated mutations in 14 cases. Missense mutations, which occurred in exons 36, 37 and 38 were identified in eight cases, four of whom shared the same recurrent change P1675L. Each of the five different missense mutations identified was shown to occur de novo in at least one sporadic case of tuberous sclerosis. The high proportion of missense mutations detected in the region of the TSC2 gene encoding the GAP-related domain supports its key role in the regulation of cellular growth.

Specialized Rap1p/Gcr1p transcriptional activation through Gcr1p DNA contacts requires Gcr2p, as does hyperphosphorylation of Gcr1p

The multifunctional regulatory factor Rap1p of Saccharomyces cerevisiae accomplishes one of its tasks, transcriptional activation, by complexing with Gcr1p. An unusual feature of this heteromeric complex is its apparent capacity to contact simultaneously two adjacent DNA elements (UASRPG and the CT box, bound specifically by Rap1p and Gcr1p, respectively). The complex can activate transcription through isolated UASRPG but not CT elements. In promoters that contain both DNA signals its activity is enhanced, provided the helical spacing between the two elements is appropriate; this suggests that at least transient DNA loop formation is involved. We show here that this CT box-dependent augmentation of Rap1p/Gcr1p activation requires the presence of a third protein Gcr2p; the Gcr2- growth defect appears to result from a genome-wide loss of the CT box effect. Interestingly, a hyperphosphorylated form of Gcr1p disappears in delta gcr2 cells but reappears if they harbor a doubly point-mutated GCR1 allele that bypasses the Gcr2- growth defect. Gcr2p therefore appears to induce a conformation change in Gcr1p and/or stimulate its hyperphosphorylation; one or both of these effects can be mimicked in the absence of GCR2 by mutation of GCR1. This improved view of Rap1p/Gcr1p/Gcr2p function reveals a new aspect of eukaryotic gene regulation: modification of an upstream activator, accompanied by at least transient DNA loop formation, mediates its improved capacity to activate transcription.

Identification and characterization of mutations in Ha-Ras that selectively decrease binding to cRaf-1

The oncoprotein Ras transforms cells by binding to one or more effector proteins. Effector proteins have been identified by their ability to bind to Ras in the GTP but not GDP form, and by their requirement for the Ras effector domain for binding. The best understood Ras effectors are serine/threonine kinases of the Raf family, but other candidate Ras effectors, including a Ral guanine nucleotide dissociation stimulator and phosphatidylinositol 3-kinase (PI3 kinase) have also been identified. To investigate the mechanism of binding of cRaf-1 to Ras, and to investigate the roles of other candidate Ras effectors in transformation, we have isolated and characterized mutants of activated Ras with decreased binding to cRaf-1 relative to other candidate effectors. Examination of these mutants indicates that surface-exposed residues of Ras outside the minimal effector domain interact differentially with cRaf-1 and other Ras-binding proteins, and that fibroblast transformation correlates with cRaf-1 binding and mitogen-activated protein (MAP) kinase activation. Furthermore, activation of PI3 kinase can occur in the absence of significant MAP kinase activation, suggesting that PI3 kinase activation is a primary effect of Ras.

Alterations in the rap1 signaling pathway are common in human gliomas

Several inherited predisposition to cancer syndromes are associated with the development of nervous system tumors. Tuberous sclerosis complex (TSC) is an autosomal dominant disorder in which affected individuals are at risk for developing astrocytomas. One of the genes responsible for this disorder is TSC2, located on chromosome 16p, and encoding a 180 kDa protein (tuberin) that functions in part as a negative regulator of rap1. Previous studies from our laboratory demonstrated that 30% of sporadic astrocytomas have reduced or absent tuberin expression. In addition to loss of tuberin in sporadic astrocytomas, aberrant rap1 mediated signaling may also result from overexpression of rap1. In this study, we test the hypothesis that alterations in the rap1 signaling pathway are frequently observed in certain subsets of gliomas compared to other tumors of the nervous system. Analysis of sporadic astrocytomas and ependymomas demonstrated either increased rap1 or reduced/absent tuberin protein expression in 50-60% of different cohorts of these gliomas, compared to 30-33% of sporadic schwannomas and meningiomas and none of eight oligodendrocyte tumors. These results suggest that alterations in the rap1 signaling pathway are important in the development of certain sporadic human gliomas.

Crystal structures of the small G protein Rap2A in complex with its substrate GTP, with GDP and with GTPgammaS

The small G protein Rap2A has been crystallized in complex with GDP, GTP and GTPgammaS. The Rap2A-GTP complex is the first structure of a small G protein with its natural ligand GTP. It shows that the hydroxyl group of Tyr32 forms a hydrogen bond with the gamma-phosphate of GTP and with Gly13. This interaction does not exist in the Rap2A-GTPgammaS complex. Tyr32 is conserved in many small G proteins, which probably also form this hydrogen bond with GTP. In addition, Tyr32 is structurally equivalent to a conserved arginine that binds GTP in trimeric G proteins. The actual participation of Tyr32 in GTP hydrolysis is not yet clear, but several possible roles are discussed. The conformational changes between the GDP and GTP complexes are located essentially in the switch I and II regions as described for the related oncoprotein H-Ras. However, the mobile segments vary in length and in the amplitude of movement. This suggests that even though similar regions might be involved in the GDP-GTP cycle of small G proteins, the details of the changes will be different for each G protein and will ensure the specificity of its interaction with a given set of cellular proteins.

Structure of the Ras-binding domain of RalGEF and implications for Ras binding and signalling

The solution structure of the Ras-binding domain (RBD) of Ral guanine-nucleotide exchange factor RalGEF was solved by NMR spectroscopy. The overall structure is similar to that of Raf-RBD, another effector of Ras, although the sequence identity is only 13%. 15N chemical shifts changes in the complex of RalGEF-RBD with Ras indicate an interaction similar to the intermolecular beta-sheet observed for the complex between Ras and Raf-RBD.

Inhibition of the prenylation of K-Ras, but not H- or N-Ras, is highly resistant to CAAX peptidomimetics and requires both a farnesyltransferase and a geranylgeranyltransferase I inhibitor in human tumor cell lines

The farnesyltransferase (FTase) inhibitor FTI-277 is highly effective at blocking oncogenic H-Ras but not K-Ras4B processing and signaling. While inhibition of processing and signaling of oncogenic K-Ras4B is more sensitive to the geranylgeranyltransferase I (GGTase I) inhibitor GGTI-286 than it is to FTI-277 in K-Ras4B-transformed NIH3T3 cells, the sensitivity of K-Ras as well as H- and N-Ras to the CAAX peptidomimetics in human tumor cell lines is not known. Here, we report that a panel of five human carcinoma cell lines from pancreatic, pulmonary, and bladder origins all express H-, N-, and K-Ras, and their respective prenylation sensitivities to the FTase and GGTase I inhibitors is variable. In all of the cell lines investigated, the prenylation of N-Ras was highly sensitive to FTI-277, and in two of the cell lines, N-Ras showed slight sensitivity to GGTI-298, an analog of GGTI-286. Although the prenylation of H-Ras was also sensitive to FTI-277, complete inhibition of H-Ras processing even at high concentrations of FTI-277 and/or GGTI-298 was never achieved. The prenylation of K-Ras, on the other hand, was highly resistant to FTI-277 and GGTI-298. Most significantly, treatment of human tumor cell lines with both inhibitors was required for inhibition of K-Ras prenylation. In one cell line, the human lung adenocarcinoma A-549, prenylation of K-Ras was highly resistant even when co-treated with both inhibitors. Furthermore, soft agar experiments demonstrated that in all the human tumor cell lines tested inhibition of K-Ras prenylation was not necessary for inhibition of anchorage-independent growth. In addition, although GGTI-298 had very little effect on soft agar growth, the combination of FTI-277 and GGTI-298 resulted in significant growth inhibition. Therefore, the results demonstrate that while FTI-277 inhibits N-Ras and H-Ras processing in the human tumor cell lines evaluated, inhibition of K-Ras processing requires both an FTase inhibitor as well as a GGTase I inhibitor, and that inhibition of human tumor growth in soft agar does not require inhibition of oncogenic K-Ras processing.

[Activation of Rap1, antagonist to ras, by Crk-C3G]

Rap1 was identified as gene whose overexpression suppressed transformation by ras. Rap1 belongs to the Ras family. The amino acid sequences of Rap1 and Ras show 55% identity to each other. Due to this high sequence similarity, Rap1 binds to effector molecules of Ras, however, Rap1 does not activate them. Thus, Rap1 functions are antagonistic to Ras in the cells. C3G was identified as a Crk SH3-binding guanine nucleotide exchange factor. Biochemical and cell biological analyses revealed that C3G is a Rap1 activator. Since it has been considered that Crk transduces signals from tyrosine kinases, this finding suggests that the activity of Rap1 is also under the control of tyrosine kinases. Overexpression of C3G in ras-transformed cells caused the morphology of the cells to revert to that of normal cells. Moreover, a mutant cell line that was resistant to EGF-dependent transformation was isolated. In the cell line a mutation was found in crk gene that was the cause of the resistance. These findings suggest that Crk-C3G-Rap1 pathway may function as an anti-transformation machinery.

Clinicopathological and prognostic relevance of Rap1-GAP expression in melanocytic tumors

Rap1-GAP protein has been identified as an inactivator of Rap1 activity, a putative endogenous antagonist of Ras proteins. The Rap1-GA1 locus maps to 1p36.1-35, the region which may harbor a gene for familial melanoma. In the present immunohistochemical study we analyzed the clinicopathological and prognostic relevance of Rap1-GAP expression in 60 benign and 103 malignant melanocytic tumors. Cytoplasmic immunoreactivity was detected in the cells of 27/60 nevi (45%) and 59/103 melanomas (57%). In the latter group the frequency of Rap1-GAP expression increased (P < 0.05) with the thickness of primary tumors and was highest in metastatic lesions. Rap1-GAP protein was detected in 15/19 subsequently recurring primary melanomas (79%) but only in 32/67 tumors (47%) of patients who remained free of disease (P < 0.05) for at least 6 years. Five out of six recurring thin melanomas (< 2 mm) were found to be immunoreactive. Although being no indicator for malignant transformation of melanocytic lesions, Rap1-GAP overexpression may represent a useful marker for identifying thin high-risk melanomas. Cytoplasmic expression of Rap1-GAP has also been observed in the cells of skin appendages and in keratinocytes, particularly in suprabasal layers of the epidermis. Therefore, Rap1-GAP is likely to be associated with cellular growth and/or differentiation. However, the present study did not provide evidence that this gene, despite its chromosomal localization, represents an early melanoma gene.