Complexes of Ras.GTP with Raf-1 and mitogen-activated protein kinase kinase.

The guanosine triphosphate (GTP)-binding protein Ras functions in regulating growth and differentiation; however, little is known about the protein interactions that bring about its biological activity. Wild-type Ras or mutant forms of Ras were covalently attached to an insoluble matrix and then used to examine the interaction of signaling proteins with Ras. Forms of Ras activated either by mutation (Gly12Val) or by binding of the GTP analog, guanylyl-imidodiphosphate (GMP-PNP) interacted specifically with Raf-1 whereas an effector domain mutant, Ile36Ala, failed to interact with Raf-1. Mitogen-activated protein kinase (MAP kinase) activity was only associated with activated forms of Ras. The specific interaction of activated Ras with active MAP kinase kinase (MAPKK) was confirmed by direct assays. Thus the forming of complexes containing MAPKK activity and Raf-1 protein are dependent upon the activity of Ras.

Guanosine triphosphatase activating protein (GAP) interacts with the p21 ras effector binding domain.

A cytoplasmic protein that greatly enhances the guanosine triphosphatase (GTPase) activity of N-ras protein but does not affect the activity of oncogenic ras mutants has been recently described. This protein (GAP) is shown here to be ubiquitous in higher eukaryotes and to interact with H-ras as well as with N-ras proteins. To identify the region of ras p21 with which GAP interacts, 21 H-ras mutant proteins were purified and tested for their ability to undergo stimulation of GTPase activity by GAP. Mutations in nonessential regions of H-ras p21 as well as mutations in its carboxyl-terminal domain (residues 165-185) and purine binding region (residues 117 and 119) did not decrease the ability of the protein to respond to GAP. In addition, an antibody against the carboxyl-terminal domain did not block GAP activity, supporting the conclusion that GAP does not interact with this region. Transforming mutations at positions 12, 59, and 61 (the phosphoryl binding region) abolished GTPase stimulation by GAP. Point mutations in the putative effector region of ras p21 (amino acids 35, 36, and 38) were also insensitive to GAP. However, a point mutation at position 39, shown previously not to impair effector function, did not alter GAP-p21 interaction. These results indicate that GAP interaction may be essential for ras p21 biological activity and that it may be a ras effector protein.

Regulation of p21ras activity.

The ras genes encode GTP/GDP-binding proteins that participate in mediating mitogenic signals from membrane tyrosine kinases to downstream targets. The activity of p21ras is determined by the concentration of GTP-p21ras, which is tightly regulated by a complex array of positive and negative control mechanisms. GAP and NF1 can negatively regulate p21ras activity by stimulating hydrolysis of GTP bound to p21ras. Other cellular factors can positively regulate p21ras by stimulating GDP/GTP exchange.

p21-ras effector domain mutants constructed by "cassette" mutagenesis.

A series of mutations encoding single-amino-acid substitutions within the v-rasH effector domain were constructed, and the ability of the mutants to induce focal transformation of NIH 3T3 cells was studied. The mutations, which spanned codons 32 to 40, were made by a "cassette" mutagenesis technique that involved replacing this portion of the v-rasH effector domain with a linker carrying two BspMI sites in opposite orientations. Since BspMI cleaves outside its recognition sequence, BspMI digestion of the plasmid completely removed the linker, creating a double-stranded gap whose missing ras sequences were reconstructed as an oligonucleotide cassette. Based upon the ability of the mutants to induce focal transformation of NIH 3T3 cells, a range of phenotypes from virtually full activity to none (null mutants) was seen. Three classes of codons were present in this segment: one which could not be altered, even conservatively, without a loss of function (codons 32 and 35); one which retained detectable biologic activity with conservative changes but which lost function with more drastic substitutions (codons 36 and 40); and one which retained function even with a nonconservative substitution (codon 39).

The bovine papillomavirus E5 oncogene can cooperate with ras: identification of p21 amino acids critical for transformation by c-rasH but not v-rasH.

We have previously used a series of insertion-deletion mutants of the mutationally activated v-rasH gene to identify several regions of the encoded protein that are dispensable for cellular transformation (B. M. Willumsen, A. G. Papageorge, H.-F. Kung, E. Bekesi, T. Robins, M. Johnsen, W. C. Vass, and D. R. Lowy, Mol. Cell. Biol. 6:2646-2654, 1986). To determine if some of these amino acids are more important for the biological activity of c-rasH, we have now tested many of the same insertion-deletion mutants in the c-rasH form for their ability to transform NIH 3T3 cells. Since the transforming activity of c-rasH is low, we have used cotransfection with the bovine papillomavirus (BPV) genome to develop a more sensitive transformation assay for c-rasH mutants. The increased sensitivity of the assay, which is seen both in focal transformation and in anchorage-independent growth, is mediated by cooperation between the BPV E5 gene and ras. E5-dependent cooperation was seen for v-rasH as well as for c-rasH, which suggests that the major effect of E5 was to increase the susceptibility of the cell to transformation to a given level of ras activity. The cooperation assay was used to test the potential importance, in c-rasH, of codons 93 to 108, 123 to 130, and 166 to 183, which were nonessential for v-rasH transformation. Relative to the respective transforming activity of wild-type c-rasH and v-rasH, mutants with lesions in codons 102 and 103 were significantly less active in their c-rasH forms than in their v-rasH forms. We conclude that a region including amino acids 102 and 103 encodes a function that is more critical to c-rasH than to v-rasH. Guanine nucleotide exchange is one function that is compatible with such a phenotype.

The Ras-specific exchange factors mouse Sos1 (mSos1) and mSos2 are regulated differently: mSos2 contains ubiquitination signals absent in mSos1.

We have compared aspects of the mouse sos1 (msos1) and msos2 genes, which encode widely expressed, closely related Ras-specific exchange factors. Although an msos1 plasmid did not induce phenotypic changes in NIH 3T3 cells, addition of a 15-codon myristoylation signal to its 5' end enabled the resulting plasmid, myr-sos1, to induce approximately one-half as many foci of transformed cells as a v-H-ras control. By contrast, an isogenic myr-sos2 plasmid, which was made by fusing the first 102 codons from myr-sos1 at homologous sequences to an intact msos2 cDNA, did not induce focal transformation directly, although it could form foci in cooperation with c-H-ras. Pulse-chase experiments indicated that the half-life of Sos1 in NIH 3T3 cells was greater than 18 h, while that of Sos2 was less than 3 h. While in vitro-translated Sos1 was stable in a rabbit reticulocyte lysate, Sos2 was degraded in the lysate, as were each of two reciprocal chimeric Sos1-Sos2 proteins, albeit at a slower rate. In the lysate, Sos2 and the two chimeric proteins could be stabilized by ATPgammaS. Unlike Sos1, Sos2 was specifically immunoprecipitated by antiubiquitin antibodies. In a myristoylated version, the chimeric gene encoding Sos2 at its C terminus made a stable protein in NIH 3T3 cells and induced focal transformation almost as efficiently as myr-msos1, while the myristoylated protein encoded by the other chimera was unstable and defective in the transformation assay. We conclude that mSos2 is much less stable than mSos1 and is degraded by a ubiquitin-dependent process. A second mSos2 degradation signal, mapped to the C terminus in the reticulocyte lysate, does not seem to function under the growth conditions of the NIH 3T3 cells.

Mutational analysis of a ras catalytic domain.

We used linker insertion-deletion mutagenesis to study the catalytic domain of the Harvey murine sarcoma virus v-rasH transforming protein, which is closely related to the cellular rasH protein. The mutants displayed a wide range of in vitro biological activity, from those that induced focal transformation of NIH 3T3 cells with approximately the same efficiency as the wild-type v-rasH gene to those that failed to induce any detectable morphologic changes. Correlation of transforming activity with the location of the mutations enabled us to identify three nonoverlapping segments within the catalytic domain that were dispensable for transformation and six other segments that were required for transformation. Segments that were necessary for guanosine nucleotide (GDP) binding corresponded to three of the segments that were essential for transformation; two of the three segments share strong sequence homology with other purine nucleotide-binding proteins. Loss of GDP binding was associated with apparent instability of the protein. Lesions in two of the three other required regions significantly reduced GDP binding, while small lesions in the last required region did not impair GDP binding or membrane localization. We speculate that this latter region interacts with the putative cellular target of ras. The results suggest that transforming ras proteins require membrane localization, guanosine nucleotide binding, and an additional undefined function that may represent interaction with their target.

A transforming ras gene can provide an essential function ordinarily supplied by an endogenous ras gene.

Microinjection of monoclonal antibody Y13-259, which reacts with all known mammalian and yeast ras-encoded proteins, has previously been shown to prevent NIH 3T3 cells from entering the S phase (L. S. Mulcahy, M. R. Smith, and D. W. Stacey, Nature [London] 313:241-243, 1985). We have now found several transformation-competent mutant v-rasH genes whose protein products in transformed NIH 3T3 cells are not immunoprecipitated by this monoclonal antibody. These mutant proteins are, however, precipitated by a different anti-ras antibody. Each of these mutants lacks Met-72 of v-rasH. In contrast to the result for cells transformed by wild-type v-rasH, Y13-259 microinjection of NIH 3T3 cells transformed by these mutant ras genes did not prevent the cells from entering the S phase. These results imply that a transformation-competent ras gene can supply a normal essential function for NIH 3T3 cells. When the proteins encoded by the mutant ras genes were overproduced in Escherichia coli, several mutant proteins that lacked Met-72 failed to bind Y13-259 in a Western blot. However, a ras protein from a mutant lacking amino antibody, but a ras protein from a mutant lacking amino acids 72 to 84 did not. These results suggest that Y13-259 may bind to a higher ordered structure that has been restored in the mutant lacking amino acids 72 to 82.

Inhibition of cell growth by lovastatin is independent of ras function.

We have investigated the inhibition of cell growth by lovastatin (previously known as mevinolin), an antagonist of hydroxymethylglutaryl coenzyme A reductase which blocks the processing and membrane localization of ras proteins via inhibition of polyisoprenylation. A series of NIH 3T3 cells transformed by oncogenes with activities that are dependent or independent of isoprenylated ras were studied, including cells transformed by myristylated ras protein that is isoprenylation independent. Treatment with lovastatin at concentrations ranging from 5 to 15 microM for up to 96 h resulted in a time- and dose-dependent inhibition of cell growth in all lines tested. The inhibition ranged from 25 to 50% when cells were treated with 5 microM lovastatin for 48 h, to 72-90% for cells treated with 15 microM lovastatin for 96 h. Cells transformed by c-ras, v-ras, v-src, v-raf, and the myristylated ras genes displayed similar sensitivities; the parental NIH 3T3 line was the most resistant of the lines tested. Metabolic labeling of control and lovastatin-treated cells with [35S]methionine or tritiated lipids revealed that 15 microM lovastatin blocked the processing of both endogenous ras and v-ras proteins yet had no effect on the lipidation of myristylated ras proteins. Addition of 300 microM mevalonic acid overcame the inhibition induced by 15 microM lovastatin. Thus the inhibition of cell growth in vitro by lovastatin did not show specificity for cells the transformation of which is dependent upon isoprenylated ras protein. It is therefore likely that the inhibition of other pathways affected by lovastatin, such as cholesterol biosynthesis or the processing of other cellular proteins, are responsible for the growth inhibition by lovastatin.

Sensitivity of wild type and mutant ras alleles to Ras specific exchange factors: Identification of factor specific requirements.

We have investigated the productive interaction between the four mammalian Ras proteins (H-, N-, KA- and KB-Ras) and their activators, the mammalian exchange factors mSos1, GRF1 and GRP, by using a modified Saccharomyces cerevisiae whose growth is dependent on activation of a mammalian Ras protein by its activator. All four mammalian Ras proteins were activated with similar efficiencies by the individual exchange factors. The H-Ras mutant V103E, which is competent for membrane localization, nucleotide binding, intrinsic and stimulated GTPase activity as well as intrinsic exchange, was defective for activation by all factors tested, suggesting that the integrity of this residue is necessary for catalyzed exchange. However, when other H-Ras mutants were studied, some distinct sensitivities to the exchange factors were observed. GRP-mediated, but not mSos1-mediated, exchange was blocked in additional mutants, suggesting different structural requirements for GRP. Analysis of Ras-mediated gene activation in murine fibroblasts confirmed these results.

Different structural requirements within the switch II region of the Ras protein for interactions with specific downstream targets.

Ras proteins function through the formation of specific complexes with Raf-1, B-raf, PI-3 kinase and RalGDS. These interactions all require Ras-GTP with an intact effector binding domain (Switch I region). We have examined the requirements of the Switch II region (amino acids 60-72) for the production of stable interactions between Ras and its downstream effectors. A point mutation at position 65 or 64 combined with additional mutations at either position 65 or 71 rendered nucleotide-free Ras protein unable to stably interact with Ras specific guanine nucleotide exchange factors. Ha-Ras containing point mutations at positions 65 and 71 possessed a twofold higher affinity for B-raf and consequently MEK1. The point mutation at 64, in combination with additional point mutations at either position 65 or 71, resulted in a protein which failed to interact with either PI-3 kinase or neurofibromin, though these Ras mutants effectively bound both Raf-1 and B-raf. An activated form of Ras, Q61L-Ras, associated with all effector proteins independent of the bound guanine nucleotide. Q61L-Ras-GDP was almost as effective as wild type Ras-GMPPNP in the in vitro activation of MEK1 and MAP kinase. Competitive studies with the catalytic domain if neurofibromin, NF1-GRD, demonstrated that its interaction with Ras-GMPPNP is mutually exclusive with both Raf-1 and B-raf. These data suggest that rasGAP and neurofibromin are unable to downregulate Ras-GTP complexed to Raf-1 or B-raf.

Rasp21 sequences opposite the nucleotide binding pocket are required for GRF-mediated nucleotide release.

The substrate requirements for the catalytic activity of the mouse Cdc25 homolog Guanine nucleotide Release Factor, GRF, were determined using the catalytic domain of GRF expressed in insect cells and E. coli expressed H-Ras mutants. We found a requirement for the loop 7 residues in Ras (amino acids 102 to 105) for stimulation of guanine nucleotide release. The dependence on the switch I and II regions of Rasp21 (encompassing the residues that shift position in the GTP- versus GDP-bound protein), which had been seen with Sdc25-mediated exchange, was also found for GRF. In addition, the sensitivity of H-Ras to GRF was abolished when residues 130-139 were replaced by proline-aspartic acid-glutamine, whereas substitution of the entire loop 8 (residues 123-130 replaced by leucine-isoleucine-arginine) had no effect on the stimulation of guanine nucleotide release by GRF. Substrate activity of Ras proteins were independent of their post-translational processing, GDP release was stimulated threefold more effectively by GRF than was GTP release, and no major differences were found between the mammalian N-, H- and K-Ras proteins. Examining the responsiveness of the Ras protein from S. pombe and the human Ras like proteins RhoA, Rap1A, Rac1 and G25K revealed a strict Ras specificity; of these only S. pombe Ras was GRF sensitive.

Novel determinants of H-Ras plasma membrane localization and transformation.

Although it is well-established that modification of Ras by farnesol is a critical step for its membrane association and transforming activity, the contribution of other C-terminal sequences and palmitate modification to Ras localization and function remains unclear. We have characterized H-Ras mutant proteins with alterations in the palmitoylated cysteines or in sequences flanking these residues. We found that non-palmitoylated proteins were impaired not only in membrane association but also in transforming activity. Mutations which drastically altered residues adjacent to the palmitoylated cysteine did not abolish palmitoylation. However, despite continued lipid modification the mutant proteins failed to bind to plasma membranes and instead accumulated on internal membranes and, importantly, were not transforming. Addition of an N-terminal myristoylation signal to these defective mutants, or to proteins entirely lacking the C-terminal 25 residues restored both plasma membrane association and transforming activity. Thus, H-Ras does not absolutely require prenylation or palmitoylation nor indeed its hypervariable domain in order to interact with effectors that ultimately cause transformation. However, in this native state, the C-terminus appears to provide a combination of lipids and a previously unrecognized signal for specific plasma membrane targeting that are essential for the correct localization and biological function of H-Ras.

Kinetics of expression of inducible beta-galactosidase in murine fibroblasts: high initial rate compared to steady-state expression.

We have constructed a murine fibroblast cell line in which synthesis of beta-galactosidase can be induced by incubation with isopropyl-beta-D-thiogalactopyranoside (IPTG). This was obtained by transfection by both a plasmid expressing lacI and a second plasmid expressing lacZ from a modified simian virus 40 (SV40) promoter containing a lac operator. We have measured the induction kinetics as well as the basal and induced differential rate of synthesis of beta-galactosidase. The steady-state rate of synthesis is tenfold higher in the presence than in the absence of inducer; we calculate an average of 1200 lacZ polypeptides are synthesized per minute per cell in the induced cultures. However, immediately after induction, the rate of accumulation of beta-galactosidase is up to 50-fold higher than the basal level. Based on our measurements of stability of beta-galactosidase, we suggest that induction may result in a subsequent down-modulation of the transcriptional activity from the induced gene. We hypothesize this inhibition may result from structural changes in DNA components, such as nucleosomes.

Antigenic modifications associated with 'spontaneous' malignant alterations of mouse fibroblasts propagated in vitro.

Two types of 'spontaneous' malignant alteration in vitro of ST/a mouse lung fibroblasts (ST-L) have been observed. In contrast to cells which retained their fibroblastic appearance (R- ST-L cells), cells showing morphological signs of transformation (R+ ST-L cells) developed strong isoimmunizing properties. Both types of cells expressed MuLV antigens which were found to be responsible for serum as well as cell-mediated immune reactions in vitro. The higher concentration of gp70 in R+ ST-L cells as compared to R- cells and possibly also the morphological differences in surface structure between the two cell types may account for the differences in immunogenicity. Preimmunization with R+ ST-L cells protected ST/a mice against secondary challenge with two ascites tumors (STABAL leukemia and Ehrlich). R- ST-L cells did not have a similar protective effect. However, the two ascites tumors only showed weak or no cross-reactions in vitro with sera and lymphoid cells from ST/a mice sensitized to R+ ST-L cells, and the in vitro reaction between the latter cells and sera or lymphoid cells from mice immunized against the two ascites tumors was moderate. This discrepancy between in vivo and in vitro observations is discussed.

Identification of a c-Jun N-terminal kinase-2-dependent signal amplification cascade that regulates c-Myc levels in ras transformation.

Ras is one of the most frequently activated oncogenes in cancer. Two mitogen-activated protein kinases (MAPKs) are important for ras transformation: extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase 2 (JNK2). Here we present a downstream signal amplification cascade that is critical for ras transformation in murine embryonic fibroblasts. This cascade is coordinated by ERK and JNK2 MAPKs, whose Ras-mediated activation leads to the enhanced levels of three oncogenic transcription factors, namely, c-Myc, activating transcription factor 2 (ATF2) and ATF3, all of which are essential for ras transformation. Previous studies show that ERK-mediated serine 62 phosphorylation protects c-Myc from proteasomal degradation. ERK is, however, not alone sufficient to stabilize c-Myc but requires the cooperation of cancerous inhibitor of protein phosphatase 2A (CIP2A), an oncogene that counteracts protein phosphatase 2A-mediated dephosphorylation of c-Myc. Here we show that JNK2 regulates Cip2a transcription via ATF2. ATF2 and c-Myc cooperate to activate the transcription of ATF3. Remarkably, not only ectopic JNK2, but also ectopic ATF2, CIP2A, c-Myc and ATF3 are sufficient to rescue the defective ras transformation of JNK2-deficient cells. Thus, these data identify the key signal converging point of JNK2 and ERK pathways and underline the central role of CIP2A in ras transformation.

In vitro selection of high-infectious, leukemogenic virus from low-infectious, non-leukemogenic type C virus from a malignant ST/a mouse cell line.

Low-infectious, nontransforming type C virus was isolated from an in vitro spontaneously transformed ST/a mouse cell line, ST-L1. The virus released by ST-L1 cells was NB-tropic and XC(-). It gave rise to very small peroxidase antibody plaques (PAP) in cultures which initially were nonproducing. Sodium dodecyl sulfate (SDS)-polyacrylamide gels of the structural proteins of the ST-L1 virus showed an envelope glycoprotein with an apparent mass of 65 kilodaltons (kdal). The mouse cells SC-1, BALB/3T3, and NIH/3T3 could be productively infected with cell-free supernatants from the ST-L1 cell line; however, virus was detected in supernatant fluids only after two to four subcultures of the infected cells. The virus thus produced was XC(+) and a large plaque former. The virus released from infected SC-1 cells was N-tropic, whereas the viruses from infected NIH/3T3 and BALB/3T3 cells were NB-tropic. The structural proteins of the N- and NB-tropic viruses could be distinguished on SDS polyacrylamide gels, the major dissimilarity being a difference in the mobility of the p30. All these viruses had an envelope glycoprotein with an apparent mass of 70 kdal. The infectivity of the viruses, measured as PAP per nanogram of p30, was 30- to 60-fold lower for the virus released from the ST-L1 cell line than that of the viruses after passage in SC-1, NIH/3T3, and BALB/3T3 cells. None of the viruses could infect rabbit or mink cells. Inoculation of the viruses into newborn mice showed that the ST-L1 virus was non-leukemogenic, whereas the NB-tropic virus selected from this after passage in BALB/3T3 or NIH/3T3 cells was highly leukemogenic. Viruses isolated from leukemic animals were indistinguishable with respect to host range and protein mobilities in SDS gels from the ones with which the mice were inoculated. Although the SC-1-selected virus was highly infectious in vitro, it was only weakly, if at all, leukemogenic.