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PubMed1994

Critical binding and regulatory interactions between Ras and Raf occur through a small, stable N-terminal domain of Raf and specific Ras effector residues.

Genetic and biochemical evidence suggests that the Ras protooncogene product regulates the activation of the Raf kinase pathway, leading to the proposal that Raf is a direct mitogenic effector of activated Ras. Here we report the use of a novel competition assay to measure in vitro the relative affinity of the c-Raf-1 regulatory region for Ras-GTP, Ras-GDP, and 10 oncogenic and effector mutant Ras proteins. c-Raf-1 associates with normal Ras and the oncogenic V12 and L61 forms of Ras with equal affinity. The moderately transforming mutant Ras[E30K31] also bound to the c-Raf-1 regulatory region with normal affinity. Transformation-defective Ras effector mutants Ras[N33], Ras[S35], and Ras[N38] bound poorly. In contrast, the transformation defective Ras[G26I27] and Ras[E45] mutants bound to the c-Raf-1 regulatory region with nearly wild-type affinity. A stable, high-affinity Ras-binding region of c-Raf-1 was mapped to a 99-amino-acid subfragment of the first 257 residues. The smallest Ras-binding region identified consisted of N-terminal residues 51 to 131, although stable expression of the domain and high-affinity binding were improved by the presence of residues 132 to 149. Deletion of the Raf zinc finger region did not reduce Ras-binding affinity, while removal of the first 50 amino acids greatly increased affinity. Phosphorylation of Raf[1-149] by protein kinase A on serine 43 resulted in significant inhibiton of Ras binding. demonstrating that the mechanism of cyclic AMP downregulation results through structural changes occurring exclusively in this small Ras-binding domain.

Cyclic AMP-Dependent Protein Kinases↗

PubMed1999

Genetic alterations in Ki-ras and Ha-ras genes in juvenile nasopharyngeal angiofibromas and head and neck cancer.

CONTEXT: Ras gene mutations have been associated to a wide range of human solid tumors. Members of the ras gene family (Ki-ras, Ha-ras and N-ras) are structurally related and code for a protein (p21) known to play an important role in the regulation of normal signal transduction and cell growth. The frequency of ras mutations is different from one type of tumor to another, suggesting that point mutations might be carcinogen-specific. OBJECTIVES: To study the occurrence of Ki-ras and Ha-ras mutations. We also studied the relative level of Ha-ras mRNA in 32 of the head and neck tumors. DESIGN: Case series. SETTING: University referral unit. PARTICIPANTS: 60 head and neck tumors and in 28 Juvenile Nasopharyngeal Angiofibromas (JNA). DIAGNOSTIC TEST: Using PCR-SSCP we examined the occurrence of Ki-ras and Ha-ras mutations. The relative level of Ha-ras mRNA was examined by Northern blot analysis. RESULTS: None of the head and neck tumors or JNA samples showed evidence of mutations within codons 12, 13, 59 and 61 of Ki-ras or Ha-ras genes. However, 17 (53%) of the tumors where gene expression could be examined exhibited increased levels of Ha-ras mRNA compared with the normal tissue derived from the same patient. CONCLUSIONS: Our results demonstrate for the first time that mutations of Ki-ras and Ha-ras genes are not associated with the development of JNA and confirm previous reports indicating that activating ras mutations are absent or rarely involved in head and neck tumors from western world patients. Furthermore, our findings suggest that overexpression of Ha-ras, rather than mutations, might be an important factor in the development and progression of head and neck tumors.

Adult↗

PubMed1997

Cloning and characterization of Ras-GRF2, a novel guanine nucleotide exchange factor for Ras.

Conversion of Ras proteins into an activated GTP-bound state able to bind effector proteins is catalyzed by specific guanine nucleotide exchange factors in response to a large number of extracellular stimuli. Here we report the isolation of mouse cDNAs encoding Ras-GRF2, a multidomain 135-kDa protein containing a COOH-terminal Cdc25-related domain that stimulates release of GDP from Ras but not other GTPases in vitro. Ras-GRF2 bound specifically to immobilized Ras lacking bound nucleotides, suggesting stabilization of the nucleotide-free form of Ras as a mechanism of catalyzing nucleotide exchange. The NH2-terminal region of Ras-GRF2 is predicted to contain features common to various signaling proteins including two pleckstrin homology domains and a Dbl homology region. Ras-GRF2 also contains an IQ motif which was required for its apparent constitutive association with calmodulin in epithelial cells ectopically expressing Ras-GRF2. Transient expression of Ras-GRF2 in kidney epithelial cells stimulated GTP binding by Ras and potentiated calcium ionophore-induced activation of mitogen-activated protein kinase (ERK1) dependent upon the IQ motif. Calcium influx caused Ras-GRF2 subcellular localization to change from cytosolic to peripheral, suggesting a possible mechanism for controlling Ras-GRF2 interactions with Ras at the plasma membrane. Epithelial cells overexpressing Ras-GRF2 are morphologically transformed and grow in a disorganized manner with minimal intercellular contacts. Northern analysis indicated a 9-kb GRF2 transcript in brain and lung, where p135 Ras-GRF2 is known to be expressed, and RNAs of 12 kb and 2.2 kb were detected in several tissues. Thus, Ras-GRF2 proteins with different domain structures may be widely expressed and couple diverse extracellular signals to Ras activation.

Amino Acid Sequence↗

PubMed1996

Maintenance of growth factor signaling through Ras in human colon carcinoma cells containing K-ras mutations.

Fifty percent of human colon carcinomas contain activating mutations in the K-ras gene. However, whether these alterations in K-ras affect the function of Ras proteins in growth factor (GF) signal transduction is now known. Here we have characterized a previously defined human colon carcinoma cell model system for K-ras gene mutations and for altered levels of Ras protein expression and have examined whether these alterations affect Ras function in GF signal transduction. Sequence analysis of PCR-amplified K-ras gene fragments indicated that among the more aggressive cell lines, four had a normal K-ras sequence, whereas 3 others (isolated from the same human tumor) contained a mutation at codon 13. In contrast, all 7 of the less aggressive cell lines contained a mutation at either codon 12 or 13. In addition to the presence of a K-ras mutation, one cell line expressed higher levels of the K-Ras protein and displayed elevated Ras-GTP loading (in the absence of GF addition) compared with the other cell lines examined. Despite these alterations, the mitogenic GF combination epidermal growth factor + insulin + transferrin resulted in an activation of Ras and extracellular signal-regulated kinase 2. Collectively, our results indicate that the malignant phenotype of the cell lines was not correlated with the presence of K-ras mutations or with higher levels of Ras protein expression. Furthermore, K-ras mutations, high levels of K-Ras protein expression, and elevated Ras-GTP loading, as they occur naturally in human colon carcinomas, do not abolish the function of Ras in GF signaling.

Base Sequence↗

PubMed1998

p21Ras downstream effectors are increased in activity or expression in mouse liver tumors but do not differ between ras-mutated and ras-wild-type lesions.

Mouse liver tumors frequently harbor activating ras gene mutations. Downstream effector molecules of p21Ras include Raf-1 kinase which mediates external signals via kinase signaling pathways to nuclear transcription factors including c-Fos and c-Jun. Mouse liver tumors with differing ras-mutational status were analyzed for alterations in Ras/Raf-1 signal transduction. Tumors were characterized with respect to the presence of base substitutions in the 3 known hot-spot positions at codons 12, 13, and 61 of Ha-ras, Ki-ras, and N-ras. Ha-ras codon 61 or Ki-ras codon 13 mutations, but no N-ras mutations, were detected in 23 out of 33 tumors analyzed, while no ras-mutations were found in 10 of the tumors. There was no significant difference in the expression of p21RaS proteins between ras-mutated tumors and tumors without detectable ras mutations. To allow for determination of Raf-1 kinase activity in tumors, a sensitive and specific assay was developed for measurements with tissue homogenates. Raf-1 kinase activity was increased about four-fold in liver tumors as compared with normal liver tissue. No significant differences in kinase activity, however, were evident between ras-mutated and ras-wild-type tumors. The same was true with respect to the levels of c-fos and c-jun mRNAs. Moreover, there were no significant differences in cell division (5-bromo-2'-deoxyuridine-labeling indices) of hepatocytes from ras-mutated and ras-wild-type tumors. The similar degree of constitutive activation of the Ras/Raf-1 signaling pathway in liver tumors, with and without detectable ras mutations, suggests that other molecules within the signaling pathway may substitute for ras-mutations during oncogenic conversion of ras-wild-type hepatocytes.

Animals↗

PubMed1991

Different patterns of gene expression in ras-resistant and ras-sensitive cells.

We have shown previously that nontumorigenic NIH 3T3 cells can be made tumorigenic and metastatic by transfection and expression of activated ras, whereas in LTA cells, which are tumorigenic but nonmetastatic, the degree of malignancy is not altered by ras. To investigate possible mechanisms of natural ras resistance, we compared the expression patterns of several genes thought to be involved in ras-induced metastatic progression in LTA (ras-resistant) and NIH 3T3 (ras-sensitive) cells, before and after constitutive expression of transfected T24-H-ras. We examined the expression of the nuclear "early-response" genes jun and fos and the "tumor-suppressor" retinoblastoma (Rb) gene, as well as genes involved in invasion (major excreted protein [MEP], tissue inhibitor of metalloproteinases [TIMP]), and cell adhesion (secreted phosphoprotein 1 [SPP1; also known as osteopontin]). We found distinct differences in both the basal and ras-induced levels of expression of most of these genes in LTA versus NIH 3T3 cells. High levels of MEP and low levels of TIMP were induced in ras-transfected NIH 3T3 cells, whereas LTA cells showed intermediate levels of MEP and high levels of TIMP that were only marginally affected by the expression of transfected ras. Similarly, SPP1 expression was strongly induced by ras in NIH 3T3 cells but was repressed by ras in LTA cells. Enzymogram assays for functional gelatinase activity showed an increase in 67-kd and 62-kd bands in NIH 3T3 cells in the presence of ras. LTA cells showed no gelatinolytic activity in the presence or absence of ras. Data from an in vitro assay for chemoinvasiveness showed a pattern as predicted from the expression of invasion-related genes; chemoinvasiveness in ras-transfected NIH 3T3 was greater than in LTA and ras-transfected LTA cells, which was greater than in NIH 3T3 cells. Differences in expression of the genes examined are believed to contribute to the ras responsiveness of NIH 3T3 cells and the ras resistance of LTA cells.

Animals↗

PubMed2004

Ras activation in Jurkat T cells following low-grade stimulation of the T-cell receptor is specific to N-Ras and occurs only on the Golgi apparatus.

Ras activation is critical for T-cell development and function, but the specific roles of the different Ras isoforms in T-lymphocyte function are poorly understood. We recently reported T-cell receptor (TCR) activation of ectopically expressed H-Ras on the the Golgi apparatus of T cells. Here we studied the isoform and subcellular compartment specificity of Ras signaling in Jurkat T cells. H-Ras was expressed at much lower levels than the other Ras isoforms in Jurkat and several other T-cell lines. Glutathione S-transferase-Ras-binding domain (RBD) pulldown assays revealed that, although high-grade TCR stimulation and phorbol ester activated both N-Ras and K-Ras, low-grade stimulation of the TCR resulted in specific activation of N-Ras. Surprisingly, whereas ectopically expressed H-Ras cocapped with the TCRs in lipid microdomains of the Jurkat plasma membrane, N-Ras did not. Live-cell imaging of Jurkat cells expressing green fluorescent protein-RBD, a fluorescent reporter of GTP-bound Ras, revealed that N-Ras activation occurs exclusively on the Golgi apparatus in a phospholipase Cgamma- and RasGRP1-dependent fashion. The specificity of N-Ras signaling downstream of low-grade TCR stimulation was dependent on the monoacylation of the hypervariable membrane targeting sequence. Our data show that, in contrast to fibroblasts stimulated with growth factors in which all three Ras isoforms become activated and signaling occurs at both the plasma membrane and Golgi apparatus, Golgi-associated N-Ras is the critical Ras isoform and intracellular pool for low-grade TCR signaling in Jurkat T cells.

Amino Acid Sequence↗

PubMed2004

Opposite effects of Ha-Ras and Ki-Ras on radiation-induced apoptosis via differential activation of PI3K/Akt and Rac/p38 mitogen-activated protein kinase signaling pathways.

It has been well known that Ras signaling is involved in various cellular processes, including proliferation, differentiation, and apoptosis. However, distinct cellular functions of Ras isozymes are not fully understood. Here we show the opposing roles of Ha-Ras and Ki-Ras genes in the modulation of cell sensitivity to ionizing radiation. Overexpression of active isoform of Ha-Ras (12V-Ha-Ras) in Rat2 cells increases resistance to the ionizing radiation. Constitutive activation of phosphoinositide-3-kinase (PI3K) and Akt is detected specifically in 12V-Ha-Ras-overexpressing cells. The specific PI3K inhibitor LY294002 inhibits PI3K/Akt signaling and potentiates the radiation-induced apoptosis, suggesting that activation of the PI3K/Akt signaling pathway is involved in the increased radio-resistance in cells overexpressing 12V-Ha-Ras. Overexpression of activated Ki-Ras (12V-Ki-Ras), on the other hand, markedly increases radiation sensitivity. The p38 mitogen-activated protein kinase (MAPK) activity is selectively enhanced by ionizing radiation in cells overexpressing 12V-Ki-Ras. The specific p38 MAPK inhibitor, PD169316, or dominant-negative p38 MAPK decreases radiation-induced cell death. We further show that the mechanism that underlies potentiation of cell death in cells overexpressing 12V-Ki-Ras involves Bax translocation to the mitochondrial membrane. Elevated Bax translocation following ionizing irradiation in 12V-Ki-Ras-overexpressing cells is completely inhibited by PD169316 or dominant-negative p38 MAPK. In addition, introduction of cells with RacN17, a dominant-negative mutant of Rac, resulted in a marked inhibition of radiation-induced Bax translocation and apoptotic cell death as well as p38 MAPK activation. Taken together, these findings explain the opposite effects of Ha-Ras and Ki-Ras on modulation of radiosensitivity, and suggest that differential activation of PI3K/Akt and Rac/p38 MAPK signaling by Ha-Ras and Ki-Ras may account for the opposing response to the ionizing radiation. These data provide an explanation for the diverse biological functions of Ras isozymes, and partly accounts for the differential response of transformed cells to anticancer treatments.

Animals↗

PubMed1996

Ras interaction with two distinct binding domains in Raf-1 may be required for Ras transformation.

Although Raf-1 is a critical Ras effector target, how Ras mediates Raf-1 activation remains unresolved. Raf-1 residues 55-131 define a Ras-binding domain essential for Raf-1 activation. Therefore, our identification of a second Ras-binding site in the Raf-1 cysteine-rich domain (residues 139-184) was unexpected and suggested a more complex role for Ras in Raf-1 activation. Both Ras recognition domains preferentially associate with Ras-GTP. Therefore, mutations that impair Ras activity by perturbing regions that distinguish Ras-GDP from Ras-GTP (switch I and II) may disrupt interactions with either Raf-1-binding domain. We observed that mutations of Ras that impaired Ras transformation by perturbing its switch I (T35A and E37G) or switch II (G60A and Y64W) domain preferentially diminished binding to Raf-1-(55-131) or the Raf-1 cysteine-rich domain, respectively. Thus, these Ras-binding domains recognize distinct Ras-GTP determinants, and both may be essential for Ras transforming activity. Finally, since Ha-Ras T35A and E37G mutations prevent Ras interaction with full-length Raf-1, we suggest that Raf-Cys is a cryptic binding site that is unmasked upon Ras interaction with Raf-1-(55-131).

3T3 Cells↗

PubMed2002

Galectin-1 augments Ras activation and diverts Ras signals to Raf-1 at the expense of phosphoinositide 3-kinase.

Ras proteins activate diverse effector molecules. Depending on the cellular context, Ras activation may have different biological consequences: induction of cell proliferation, senescence, survival, or death. Augmentation and selective activation of particular effector molecules may underlie various Ras actions. In fact, Ras effector-loop mutants interacting with distinctive effectors provide evidence for such selectivity. Interactions of active Ras with escort proteins, such as galectin-1, could also direct Ras selectivity. Here we show that in comparison with Ras transfectants, H-Ras/galectin-1 or K-Ras4B/galectin-1 co-transfectants exhibit enhanced and prolonged epidermal growth factor (EGF)-stimulated increases in Ras-GTP, Raf-1 activity, and active extracellular signal-regulated kinase. Galectin-1 antisense RNA inhibited these EGF responses. Conversely, Ras and galectin-1 co-transfection inhibited the EGF-stimulated increase in phosphoinositide 3-kinase (PI3K) activity. Galectin-1 transfection also inhibited Ras(G12V)-induced PI3K but not Raf-1 activity. Galectin-1 co-immunoprecipitated with Ras(G12V) or with Ras(G12V/T35S) that activate Raf-1 but not with Ras(G12V/Y40C) that activates PI3K. Thus, galectin-1 binds active Ras and diverts its signal to Raf-1 at the expense of PI3K. This demonstrates a novel mechanism controlling the duration and selectivity of the Ras signal. Ras gains selectivity when it is associated with galectin-1, mimicking the selectivity of Ras(T35S), which activates Raf-1 but not PI3K.

Animals↗

PubMed2006

Growth factor-dependent AKT activation and cell migration requires the function of c-K(B)-Ras versus other cellular ras isoforms.

K-Ras-negative fibroblasts are defective in their steady-state expression of MMP-2. This occurs through c-K(B)-Ras dependent regulation of basal levels of AKT activity. In this report, we have extended those studies to demonstrate that in the absence of K-Ras expression, PDGF-BB fails to induce significant AKT activation, although this was not the case in N-Ras-negative cells. This phenotype was directly linked to PDGF-dependent cell migration. All of the independently immortalized K-Ras-negative cells failed to migrate upon the addition of PDGF. Only ectopic expression of c-K(B)-Ras, not c-K(A)-Ras nor oncogenic N-Ras, could restore both PDGF-dependent AKT activation and cell migration. Since most Ras binding partners can interact with all Ras isoforms, the specificity of PDGF-dependent activation of AKT and enhanced cell migration suggests that these outcomes are likely to be regulated through a c-K(B)-Ras-specific binding partner. Others have published that of the four Ras isoforms, only K(B)-Ras can form a stable complex with calmodulin (CaM). Along those lines, we provide evidence that 1) PDGF addition results in increased levels of a complex between c-K(B)-Ras and CaM and 2) the biological outcomes that are strictly dependent on c-K(B)-Ras (AKT activation and cell migration) are blocked by CaM antagonists. The PDGF-dependent activation of ERK is unaffected by the absence of K(B)-Ras and presence of CaM antagonists. This is the first example of a linkage between a specific biological outcome, cell migration, and the activity of a single Ras isoform, c-K(B)-Ras.

Animals↗

PubMed2003

While K-ras is essential for mouse development, expression of the K-ras 4A splice variant is dispensable.

In mammals, the three classical ras genes encode four highly homologous proteins, N-Ras, H-Ras, and the isoforms K-Ras 4A and 4B. Previous studies have shown that K-ras is essential for mouse development and that while K-ras 4A and 4B are expressed during development, K-ras 4A expression is regulated temporally and spatially and occurs in adult kidney, intestine, stomach, and liver. In the present study, the pattern of K-ras 4A expression was examined in a wide range of wild-type adult mouse tissues, and gene targeting was used to generate K-ras 4A-deficient mice to examine its role in development. It was found that K-ras 4A is also expressed in uterus, lung, pancreas, salivary glands, seminal vesicles, bone marrow cells, and cecum, where it was the major K-Ras isoform expressed. Mating between K-ras(tmDelta4A/+) mice produced viable K-ras(tmDelta4A/tmDelta4A) offspring with the expected Mendelian ratios of inheritance, and these mice expressed the K-ras 4B splice variant only. K-ras(tmDelta4A/tmDelta4A) mice were fertile and showed no histopathological abnormalities on inbred (129/Ola) or crossbred (129/Ola x C57BL/6) genetic backgrounds. The results demonstrate that K-Ras 4A, like H- and N-Ras, is dispensable for normal mouse development, at least in the presence of functional K-Ras 4B.

Alternative Splicing↗

PubMed1990

H-ras activation and ras p21 expression in bladder tumors induced in F344/NCr rats by N-butyl-N-(4-hydroxybutyl)nitrosamine.

Bladder tumors were induced in male F344/NCr rats by administration of N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) at 500 p.p.m. in their drinking water for 12 weeks. Twenty-one bladder tumors that developed between 25 and 50 weeks after BBN administration was begun were evaluated for immunoreactivity with polyclonal or monoclonal antibodies raised against ras p21, for amplification of ras genes by Southern blotting, and for activating point mutations in ras genes by selective oligonucleotide hybridization of products from polymerase chain reaction (PCR). Increased expression of ras p21 was detected by avidin-biotin immunohistochemistry in 18/21 (85%) of the neoplastic bladder lesions. By Southern analysis, there was no significant amplification of H-ras, K-ras or N-ras in any of the tumors except one that showed a 5-fold amplification of K-ras. Point mutations in ras genes were detected by selective oligonucleotide hybridization of the products of PCR. Of the 21 bladder tumors, three tumors were shown to have mutations in codon 12 (GGA----GAA), six tumors in codon 61 (two CAA----CTA, four CAA----CGA), and one in both codon 12 (GGA----GAA) and codon 61 (CAA----CGA), all in H-ras. Thus 10 of 21 tumors has ras gene mutations in a portion of the tumor cells. The variable pattern of point mutation in H-ras suggests that these mutations may not all be a direct consequence of interaction of BBN metabolites with H-ras. Enhanced expression of ras p21 was always focal and was not necessarily associated with transforming ras mutations. It is therefore suggested that tumorigenesis in BBN-initiated bladder cells might involve H-ras activation as part of a multistep pathway; however, H-ras involvement is not obligatory for tumor development.

Animals↗

PubMed2005

p75-Ras-GRF1 is a c-Jun/AP-1 target protein: its up regulation results in increased Ras activity and is necessary for c-Jun-induced nonadherent growth of Rat1a cells.

The c-Jun/AP-1 transcription complex is associated with diverse cellular processes such as differentiation, proliferation, transformation, and apoptosis. These different biological endpoints are likely achieved by the regulation of specific target gene expression. We describe the identification of Ras guanine nucleotide exchange factor 1, Ras-GRF1, by microarray analysis as a c-Jun/AP-1 regulated gene essential for anchorage-independent growth of immortalized rat fibroblasts. Increased Ras-GRF1 expression, in response to inducible c-Jun expression in Rat1a fibroblasts, was confirmed by both real-time PCR and Northern blot analysis. We show that c-Jun/AP-1 can bind and activate the Ras-GRF1 promoter in vivo. A 75-kDa c-Jun/AP-1-inducible protein, p75-Ras-GRF1, was detected, and the inhibition of its expression with antisense oligomers significantly blocked c-Jun-regulated anchorage-independent cell growth. p75-Ras-GRF1 expression occurred with a concomitant increase in activated Ras (GTP bound), and the activation of Ras was significantly inhibited by antisense Ras-GRF1 oligomers. Moreover, p75-Ras-GRF1 could be coprecipitated with a Ras dominant-negative glutathione S-transferase (GST) construct, GST-Ras15A, demonstrating an interaction between p75-Ras-GRF1 and Ras. A downstream target of Ras activation, Elk-1, had increased transcriptional activity in c-Jun-expressing cells, and this activation was inhibited by dominant-negative Ras. In addition, c-Jun overexpression resulted in an increase in phospho-AKT while phosphorylation of ERK1/2 remained largely unaffected. The inhibition of phosphatidylinositol 3-kinase (PI3K)-AKT signal transduction by Ly294002 and wortmannin significantly blocked c-Jun-regulated morphological transformation, while inhibition of basal MEK-ERK activity with PD98059 and U0126 had little effect. We conclude that c-Jun/AP-1 regulates endogenous p75-Ras-GRF1 expression and that c-Jun/AP-1-regulated anchorage-independent cell growth requires activation of Ras-PI3K-AKT signal transduction.

Animals↗

PubMed1999

Double-mutant analysis of the interaction of Ras with the Ras-binding domain of RGL.

RalGDS is a guanine nucleotide dissociation stimulator for Ral, and one of its homologues is RGL (RalGDS-like). In this study, the effects of mutations of Ras and the Ras-binding domains (RBDs) of RalGDS and RGL on their binding have been systematically examined. The D33A mutation of Ras reduces the abilities to bind RGL-RBD and RalGDS-RBD. To identify the RGL residue interacting with Asp33 of Ras, double-mutant analyses between Ras and RGL-RBD were conducted. For example, the K685A mutation of RGL-RBD has a much smaller effect on the RGL-RBD binding ability of the D33A mutant than on those of other mutants of Ras. Accordingly, it is indicated that the attractive interaction of Asp33 in Ras with Lys685 in RGL-RBD (Lys816 in RalGDS-RBD) contributes to the Ras.RBD association. This interaction is consistent with the crystal structure of the complex of RalGDS-RBD and the E31K Ras mutant [Huang, L., Hofer, F., Martin, G. S., and Kim, S.-H. (1998) Nat. Struct. Biol. 5, 422-426]. This crystal structure exhibits interactions of the mutation-derived Lys31 side chain with three RalGDS residues. Glu31 of Ras discriminates Ras from a Ras-homologue, Rap1, with Lys31, with respect to RalGDS and RGL binding; the E31K mutation of Ras potentiates the abilities to bind RGL-RBD and RalGDS-RBD. To examine the role of Glu31 of the wild-type Ras in the interaction with RGL and RalGDS, double-mutant analyses were conducted. The Ras binding ability of the E689A mutant of RGL-RBD is much stronger than that of the wild-type RGL-RBD, and the E31K mutation of Ras no longer potentiates the Ras binding ability of the E689A mutant. Therefore, the repulsive interaction between Glu31 in Ras and Glu689 in RGL-RBD (Asp820 in RalGDS-RBD) may keep the Ras.RBD association weaker than the Rap1.RBD association, which might be relevant to the regulation of the signaling network.

Alanine↗

PubMed1993

GTP-dependent association of Raf-1 with Ha-Ras: identification of Raf as a target downstream of Ras in mammalian cells.

Ras is involved in signal transduction of various factors for growth, differentiation, and oncogenesis. Recent studies have revealed several proteins that function upstream and downstream of the Ras signaling pathway. However, its immediate downstream target molecular has not yet been identified. In an effort to identify the Ras-associated downstream proteins, we added recombinant Ha-Ras in a GTP-bound form to cell-free lysates and used several antibodies against Ras to immunoprecipitate Ras complexes. We found that a serine/threonine kinase, Raf-1, was coimmunoprecipitated with Ha-Ras by two anti-Ras antibodies (LA069 and Y13-238), whereas a neutralizing antibody against Ras (Y13-259) could not precipitate Raf-1. The coimmunoprecipitation was observed with a complex of Ras and guanosine 5'-[gamma- thio]triphosphate but not with a complex of Ras and guanosine 5'-[beta-thio]diphosphate. The GTP-dependent association of Ha-Ras with Raf-1 was observed with lysates of various types of cultured cells, including NIH 3T3, pheochromocytoma (PC) 12, Ba/F3, and Jurkat T cells, and also with crude extracts from rat brain. Furthermore, Raf-1 was precipitated with a transforming Ha-Ras mutant ([Val12]Ras) and wild-type Ha-Ras but not with an effector-region mutant ([Leu35,ARg37]Ras) that lacks transforming activity. These results indicate that Ras.GTP physically associates with Raf either directly or through other component(s) and strongly suggest that Raf functions in close downstream proximity to Ras in mammalian cells.

3T3 Cells↗

PubMed1997

Coassociation of Rap1A and Ha-Ras with Raf-1 N-terminal region interferes with ras-dependent activation of Raf-1.

Raf-1 is a major downstream effector of mammalian Ras. Binding of the effector domain of Ras to the Ras-binding domain of Raf-1 is essential for Ras-dependent Raf-1 activation. However, Rap1A, which has an identical effector domain to that of Ras, cannot activate Raf-1 and even antagonizes several Ras functions in vivo. Recently, we identified the cysteine-rich region (CRR) of Raf-1 as another Ras-binding domain. Ha-Ras proteins carrying mutations N26G and V45E, which failed to bind to CRR, also failed to activate Raf-1. Since these mutations replace Ras residues with those of Rap1A, we examined if Rap1A lacks the ability to bind to CRR. Contrary to the expectation, Rap1A exhibited a greatly enhanced binding to CRR compared with Ha-Ras. Enhanced CRR binding was also found with Ha-Ras carrying another Rap1A-type mutation E31K. Both Rap1A and Ha-Ras(E31K) mutant failed to activate Raf-1 and interfered with Ha-Ras-dependent activation of Raf-1 in Sf9 cells. Enhanced binding of Rap1A to CRR led to co-association of Rap1A and Ha-Ras with Raf-1 N-terminal region through binding to CRR and Ras-binding domain, respectively. These results suggest that Rap1A interferes with Ras-dependent Raf-1 activation by inhibiting binding of Ras to Raf-1 CRR.

Amino Acid Sequence↗

PubMed2000

Regulatory proteins of R-Ras, TC21/R-Ras2, and M-Ras/R-Ras3.

We studied the regulation of three closely related members of Ras family G proteins, R-Ras, TC21 (also known as R-Ras2), and M-Ras (R-Ras3). Guanine nucleotide exchange of R-Ras and TC21 was promoted by RasGRF, C3G, CalDAG-GEFI, CalDAG-GEFII (RasGRP), and CalDAG-GEFIII both in 293T cells and in vitro. By contrast, guanine nucleotide exchange of M-Ras was promoted by the guanine nucleotide exchange factors (GEFs) for the classical Ras (Ha-, K-, and N-), including mSos, RasGRF, CalDAG-GEFII, and CalDAG-GEFIII. GTPase-activating proteins (GAPs) for Ras, Gap1(m), p120 GAP, and NF-1 stimulated all of the R-Ras, TC21, and M-Ras proteins, whereas R-Ras GAP stimulated R-Ras and TC21 but not M-Ras. We did not find any remarkable difference in the subcellular localization of R-Ras, TC21, or M-Ras when these were expressed with a green fluorescent protein tag in 293T cells and MDCK cells. In conclusion, TC21 and R-Ras were regulated by the same GEFs and GAPs, whereas M-Ras was regulated as the classical Ras.

Amino Acid Sequence↗