Mechanism of activation of an N-ras gene in the human fibrosarcoma cell line HT1080.

A full length N-ras gene has been cloned from both the human fibrosarcoma cell line HT1080 and from normal human DNA. N-ras isolated from HT1080 will efficiently induce morphological transformation of NIH/3T3 cells in a transfection assay, whereas N-ras isolated from normal human DNA has no effect on NIH/3T3 cells. The coding regions of the normal N-ras gene have been sequenced and the predicted amino acid sequence of the N-ras product is very similar to that of the c-Ha-ras1 and c-Ki-ras2 products. By making chimeric molecules between the two cloned genes the activating alteration in the HT1080 N-ras gene has been localised to a single base change that results in an amino acid alteration at position 61 of the p21 N-ras product.     

Biochemical and biological properties of the human N-ras p21 protein.

We characterized the normal (Gly-12) and two mutant (Asp-12 and Val-12) forms of human N-ras proteins produced by Escherichia coli. No significant differences were found between normal and mutant p21 proteins in their affinities for GTP or GDP. Examination of GTPase activities revealed significant differences between the mutant p21s: the Val-12 mutant retained 12% of wild-type GTPase activity, whereas the Asp-12 mutant retained 43%. Both mutant proteins, however, were equally potent in causing morphological transformation and increased cell motility after their microinjection into quiescent NIH 3T3 cells. This lack of correlation between transforming potency and GTPase activity or guanine nucleotide binding suggests that position 12 mutations affect other aspects of p21 function.     

Use of gene transfer and a novel cosmid rescue strategy to isolate transforming sequences.

Mouse Lewis Lung tumor DNA was ligated to a cosmid containing a geneticin (G418)/kanamycin resistance gene and transferred into NIH3T3 cells. Recipient cells were first selected for geneticin resistance and subsequently for their ability to grow as a tumour when injected into nude mice. By repeating this transfection procedure with DNA from resultant tumours, geneticin-resistant NIH3T3 cells were obtained which were tumorigenic and contained approximately 1-5 copies of the transferred cosmid. The functional oncogene was cloned by preparing cosmid libraries of third round tumour DNAs, using a cosmid which does not contain a kanamycin resistance gene. Due to the original linkage of the oncogene with the cosmid containing the kanamycin resistance gene, a series of kanamycin-resistant cosmids were isolated, five of which contained an active oncogene. Subsequent analysis showed that the oncogene present was highly related to the human N-ras gene. Using a DNA probe from the MLL N-ras gene, a non-transforming counterpart was isolated from mouse liver DNA. A comparison between the two N-ras genes showed that a mutation at the amino acid position corresponding to 61 in the human gene is responsible for transforming activity of the rescued gene.     

Highly frequent detection of transforming genes in acute leukemias by transfection using in vivo selection assays

DNAs from nine out of ten acute leukemia cases that were negative by in vitro focus forming assays exhibited transforming activity tested by in vivo selection assays in nude mice using transfected NIH3T3 cells. Of the nine cases, six cases contained activated N-ras genes, and one case exhibited activation of the c-K-ras gene. None of the ras gene family showed homology with the transforming genes derived from the other two cases. Our observations indicate that in vivo selection assays detect transforming genes including ras oncogenes at high frequency, and that activated N-ras genes are frequently detected in human acute leukemias.     

Human oncogene-transfected tumor cells display differential susceptibility to lysis by lymphokine-activated killer cells (LAK) and natural killer cells

NIH 3T3 tertiary transfectants containing the N-ras or c-Ha-ras oncogenes derived from human tumors were tested for susceptibility to lymphokine-activated killer (LAK) cell and natural killer (NK) cell lysis. N-ras tertiary transfectants contained a human acute lymphocytic leukemia-derived N-ras oncogene. C-Ha-ras transfectants contained either the position 61-activated form of the oncogene (45.342, 45.322, and 45.3B2) or the position 12-activated form (144-162). In 4 hr 51Cr release assays, seven of seven in vivo grown human oncogene transfected NIH 3T3 fibroblasts were lysed by murine LAK effectors, whereas six of seven were lysed by human LAK effectors. There was no difference in susceptibility to lysis between cells transfected with the N-ras oncogene, the position 61 activated c-Ha-ras oncogene, or the position 12 activated c-Ha-ras oncogene. Cultured NIH 3T3 fibroblasts, as well as in vitro and in vivo grown NIH 3T3 tertiary transfectants were resistant to lysis by murine NK effectors and were relatively resistant (4/6 were not lysed) to lysis by human NK effectors. We conclude that human oncogene-transfected tumors are susceptible to lysis by both murine and human LAK cells while being relatively resistant to lysis by murine and human NK cells. Different oncogenes or the same oncogene activated by different point mutations do not specifically determine susceptibility to lysis by LAK or NK. Also the presence of an activated oncogene does not appear to be sufficient for inducing susceptibility to these cytotoxic lymphocyte populations.     

mos gene transforming efficiencies correlate with oocyte maturation and cytostatic factor activities.

The mos proto-oncogenes from different vertebrate species transform mouse NIH 3T3 cells with markedly different efficiencies. v-mos, mouse (c-mosmu), and chicken (c-mosch) mos transform NIH 3T3 cells 10- to 100-fold more efficiently than do human (c-moshu) and Xenopus (c-mosxc) mos. The mos genes with the highest transforming activity efficiently induce maturation in Xenopus oocytes and mimic cytostatic factor (CSF) by causing mitotic cleavage arrest in embryos. Chimeric v-mos/c-moshu proteins that had high transforming efficiencies in NIH 3T3 cells were also effective in the induction of oocyte maturation and CSF cleavage arrest. We measured the in vitro autophosphorylation activities of the different mos proteins and found that the levels of kinase activity of v-mos, c-mosmu, and c-mosch were much higher than that of c-mosxc. These data indicate that mos gene transforming efficiency and the ability to induce oocyte maturation or mimic CSF activity are correlated with in vitro autophosphorylation activity and suggest that the mos protein plays a similar role in transformed cells and normal oocytes.     

Structural significance of the GTP-binding domain of ras p21 studied by site-directed mutagenesis.

Point mutations of p21 proteins were constructed by oligonucleotide-directed mutagenesis of the v-rasH oncogene, which substituted amino acid residues within the nucleotide-binding consensus sequence, GXG GXGK. When the glycine residue at position 10, 13, or 15 was substituted with valine, the viral rasH product p21 lost its GTP-binding and autokinase activities. Other substitutions at position 33, 51, or 59 did not impair its binding activity. G418-resistant NIH 3T3 cell lines were derived by transfection with constructs obtained by inserting the mutant proviral DNA into the pSV2neo plasmid. Clones with a valine mutation at position 13 or 15 were incapable of transforming cells, while all other mutants with GTP-binding activity were competent. A mutant with a substitution of valine for glycine at position 10 which had lost its ability to bind GTP and its autokinase activity was fully capable of transforming NIH 3T3 cells. These cells grew in soft agar and rapidly formed tumors in nude mice. The p21 of cell lines derived from tumor explants still lacked the autokinase activity. These findings suggest that the glycine-rich consensus sequence is important in controlling p21 activities and that certain mutations may confer to p21 its active conformation without participation of ligand binding.     

rasH mutants deficient in GTP binding.

Single amino acid substitutions were introduced into a region of the rasH protein (residues 116, 117, and 119) homologous to a variety of diverse GTP-binding proteins. Each of the mutant p21 proteins displayed a significant reduction (10- to 5,000-fold) in GTP binding affinity. Activated rasH proteins deficient in GTP binding were unaltered in their ability to morphologically transform NIH 3T3 cells.     

N-terminal amino acid sequences of the polyoma middle-size T antigen are important for protein kinase activity and cell transformation.

We constructed deletion mutations which removed N-terminal coding sequences of various lengths from a cloned polyoma middle-size T antigen (MT antigen) gene. We introduced the MT antigen genes into a simian virus 40 expression vector so that they were expressed at high levels under the control of the simian virus 40 late promoter in COS-1 cells. The deletion mutant genes synthesized truncated MT antigens whose size was consistent with the deletion of either 70 or 106 amino acids from N termini, owing to initiation of translation at internal methionine codons in the MT antigen-coding region. The truncated MT antigens were found in cell membrane fractions but failed to show MT antigen-associated protein kinase activity. The cloned deletion mutant DNAs failed to transform rat F2408 or mouse NIH 3T3 cells. Therefore, N-terminal amino acid sequences of the polyoma MT antigen, as well as C-terminal sequences, are important for protein kinase activity and cell transformation.     

Activation of N-ras in a human melanoma cell line.

DNA isolated from cell line Mel Swift, a human melanoma cell line, transforms NIH3T3 cells. Southern blot analysis of DNA from secondary foci revealed conserved 8.8- and 7.8-kilobase EcoRI fragments which hybridized with a human repetitive sequence clone, blur 8. The activated transforming gene was identified as N-ras, and the 8.8-kilobase EcoRI fragment from a secondary transformant was cloned. Synthetic 17-mer oligonucleotides which spanned either the normal codon 61 (CAA) or a mutant codon 61 (AAA) were used for hybridization. Cloned N-ras from melanoma cell line Mel Swift hybridized to the mutant (AAA) oligonucleotide. From this we predicted a glutamine-to-lysine substitution in amino acid 61, a change confirmed by conventional sequencing of the first and second exons of N-ras from cell line Mel Swift. Transfection experiments showed that only those recombinant clones with the mutation in position 61 were biologically active.     

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.     

A synthetic peptide corresponding to a sequence in the GTPase activating protein inhibits p21ras stimulation and promotes guanine nucleotide exchange

Amino acid sequence homology between the GTPase Activating Protein (GAP) and the GTP-binding regulatory protein, Gs alpha, suggests that a specific region of GAP primary structure (residues 891-898) may be involved in its stimulation of p21ras GTP hydrolytic activity (McCormick, F. [1989] Nature 340, 678-679). A peptide, designated p891, corresponding to GAP residues 891-906 (M891RTRVVSGFVFLRLIC906) was synthesized and tested for its ability to inhibit GAP-stimulated p21ras GTPase activity. At a concentration of 25 microM, p891 inhibited GAP activity approximately 50%. Unexpectedly, p891 also stimulated GTP binding to p21N-ras independent of GAP. This stimulation correlated with an enhancement of p21N-ras.GDP dissociation; an approximate 15-fold increase in the presence of 10 microM p891. In contrast, dissociation of the p21N-ras.GTP gamma S complex was unaffected by 10 microM p891. The p21N-ras.GDP complex was unresponsive to 100 microM mastoparan, a peptide toxin shown previously to accelerate GDP dissociation from the guanine nucleotide regulatory proteins, Gi and Go. p21H-ras, as well as the two p21H-ras effector mutants, Ala-38, and Ala-35, Leu-36, also exhibited increased rates of GDP dissociation in the presence of p891. Also tested were three ras-related GTP-binding proteins; rap, G25K and rac. The rap.-GDP complex was unaffected by 10 microM p891. Dissociation of the G25K- and rac.GDP complexes were enhanced slightly; approximately 1.3- and 1.8-fold over control, respectively. Thus, the inhibitory effect of p891 on GAP stimulation of p21ras suggests that amino acids within the region 891-906 of GAP may be essential for interaction with p21ras. In addition, p891 independently affects the nucleotide exchange properties of p21ras.     

Deletion mutants of Harvey ras p21 protein reveal the absolute requirement of at least two distant regions for GTP-binding and transforming activities.

Deletions of small sequences from the viral Harvey ras gene have been generated, and resulting ras p21 mutants have been expressed in Escherichia coli. Purification of each deleted protein allowed the in vitro characterization of GTP-binding, GTPase and autokinase activity of the proteins. Microinjection of the highly purified proteins into quiescent NIH/3T3 cells, as well as transfection experiments utilizing a long terminal repeat (LTR)-containing vector, were utilized to analyze the biological activity of the deleted proteins. Two small regions located at 6-23 and 152-165 residues are shown to be absolutely required for in vitro and in vivo activities of the ras product. By contrast, the variable region comprising amino acids 165-184 was shown not to be necessary for either in vitro or in vivo activities. Thus, we demonstrate that: (i) amino acid sequences at positions 5-23 and 152-165 of ras p21 protein are probably directly involved in the GTP-binding activity; (ii) GTP-binding is required for the transforming activity of ras p21 and by extension for the normal function of the proto-oncogene product; and (iii) the variable region at the C-terminal end of the ras p21 molecule from amino acids 165 to 184 is not required for transformation.     

Structure and activation of the human N-ras gene

The normal human N-ras gene has been cloned. In structure and sequence it closely resembles the human H-ras and K-ras genes. The three genes share regions of nucleotide homology and nucleotide divergence within coding sequences and have a common intron/exon structure, indicating that they have evolved from a similarly spliced ancestral gene. The N-ras gene of SK-N-SH neuroblastoma cells has transforming activity, while the normal N-ras gene does not, the result of a single nucleotide change substituting lysine for glutamine in position 61 of the N-ras gene product. From previous studies we conclude that amino acid substitutions in two distinct regions can activate the transforming potential of ras gene products.     

Phorbol ester- and protein kinase C-mediated phosphorylation of the cellular Kirsten ras gene product

The effect of phorbol 12-myristate 13-acetate on the phosphorylation of the ras p21 protein was studied by metabolically labeling cultured cells with [32P]orthophosphate and using a monoclonal antibody to immunoprecipitate the protein. Phorbol 12-myristate 13-acetate (100 nM) induced phosphorylation of cKi-ras p21 in a mouse adrenocortical cell line (Yl) expressing high levels of cKi-ras with exon 4B. Phosphorylation was detected at 10 min and was maximal at 2 h. The ras protein was not phosphorylated in response to phorbol 12-myristate 13-acetate in NIH 3T3 cells expressing activated cHa-ras or vHa-ras. In vitro, protein kinase C phosphorylated cKi-ras in a phosphatidylserine and diolein-dependent manner. Both in intact cells and in vitro the amino acid phosphorylated was serine. Analysis of p21 from NIH 3T3 cells expressing a variety of ras proteins indicated that phosphorylation occurs within a domain encoded by exon 4B of cKi-ras. Phosphorylation affected neither the binding nor the GTPase activity of the ras protein. We conclude that cKi-ras is a substrate for protein kinase C and that the site of phosphorylation is likely to be serine 181 encoded by exon 4B.     

MAP, a protein interacting with a tumor suppressor, merlin, through the run domain

Merlin (or schwannomin) is a tumor suppressor encoded by the neurofibromatosis type 2 gene. Many studies have suggested that merlin is involved in the regulation of cell growth and proliferation through interactions with various cellular proteins. To better understand the function of merlin, we tried to identify the proteins that bind to merlin using the yeast two-hybrid screening. Characterization of the positive clones revealed a protein of 749 amino acids named merlin-associated protein (MAP), which showed wide tissue distribution in Northern blot analysis. Sequence analysis revealed that MAP is a potential homologue of a yeast check-point protein, BUB2, and contains TBC, SH3, and RUN domains, thereby implicating its role in the Ras-like GTPase signal pathways. MAP and merlin were directly associated in vitro and in vivo, and colocalized in NIH3T3 cells. The RUN domain of MAP and the C-terminus of merlin appeared to be responsible for their interaction. MAP decreased the AP-1-dependent promoter activity additively with merlin in NIH3T3 cells. In addition, merlin and MAP synergistically reduced the colony formation of NIH3T3 cells. These results suggest that MAP may play a cooperative role in the merlin-mediated growth suppression of cells.     

Wild type N-ras displays anti-malignant properties, in part by downregulating decorin

Previously, we have shown that wild type N-ras (wt N-ras) harbors an anti-malignant effect against mutated Ras and in tumors without Ras mutations. To investigate the molecular bases of this anti-malignant activity, we have studied the potency of this anti-malignant effect in a model system against SV40 large T antigen (SV40T). We show that wild-type N-ras (wt N-ras) counteracts the effects of SV40T in NIH3T3 cells as seen by a decrease in proliferation, anchorage independence and changes in migration. We also show that wt N-ras elicits the same anti-malignant effects in some human tumor cell lines (HT1080 and MDA-MB-231). Through mRNA and microRNA (miRNAs) expression profiling we have identified genes (decorin) and miRNAs (mir-29A, let-7b) modulated by wt N-ras potentially responsible for the anti-malignant effect. Wt N-ras appears to mediate its anti-malignant effect by downregulating some of the targets of the TGFβ pathway and decorin, which are able to reverse the inhibition of migration induced by wt N-ras. Our experiments show that the molecules that mediate the anti-malignant effect by wt N-ras appear to be different from those modulated by transforming N-ras. The components of the pathways modulated by wt N-ras mediating its anti-malignant effects are potential targets for therapeutic intervention in cancer.     

Activation of cellular p21ras by myristoylation

p21ras is palmitoylated on a cysteine residue near the C-terminus. Changing Cys-186 to Ser in oncogenic forms produces a non-palmitoylated protein that fails to associate with membranes and does not transform NIH 3T3 cells. To examine whether palmitate acts in a general way to increase ras protein hydrophobicity, or is involved in more specific interactions between p21ras and membranes, we constructed genes that encode non-palmitoylated ras proteins containing myristic acid at their N-termini. Myristoylated, activated ras, without palmitate (61Leu/186Ser) exhibited both efficient membrane association and full transforming activity. Unexpectedly, we found that myristoylated forms of normal cellular ras were also potently transforming. Myristoylated c-ras retained the high GTP binding and GTPase characteristic of the cellular protein and, moreover, bound predominantly GDP in vivo. This implied that it continued to interact with GAP (GTPase-activating protein). While the membrane binding induced by myristate permitted transformation, only palmitate produced a normal (non-transforming) association of ras with membranes and must therefore regulate ras function by some unique property that myristate does not mimic. Myristoylation thus represents a novel mechanism by which the ras proto-oncogene protein can become transforming.     

Sequence and complementation analysis of recF genes from Escherichia coli, Salmonella typhimurium, Pseudomonas putida and Bacillus subtilis: evidence for an essential phosphate binding loop.

We have compared the recF genes from Escherichia coli K-12, Salmonella typhimurium, Pseudomonas putida, and Bacillus subtilis at the DNA and amino acid sequence levels. To do this we determined the complete nucleotide sequence of the recF gene from Salmonella typhimurium and we completed the nucleotide sequence of recF gene from Pseudomonas putida begun by Fujita et al. (1). We found that the RecF proteins encoded by these two genes contain respectively 92% and 38% amino acid identity with the E. coli RecF protein. Additionally, we have found that the S. typhimurium and P. putida recF genes will complement an E. coli recF mutant, but the recF gene from Bacillus subtilis [showing about 20% identity with E. coli (2)] will not. Amino acid sequence alignment of the four proteins identified four highly conserved regions. Two of these regions are part of a putative phosphate binding loop. In one region (position 36), we changed the lysine codon (which is essential for ATPase, GTPase and kinase activity in other proteins having this phosphate binding loop) to an arginine codon. We then tested this mutation (recF4101) on a multicopy plasmid for its ability to complement a recF chromosomal mutation and on the E. coli chromosome for its effect on sensitivity to UV irradiation. The strain with recF4101 on its chromosome is as sensitive as a null recF mutant strain. The strain with the plasmid-borne mutant allele is however more UV resistant than the null mutant strain. We conclude that lysine-36 and possibly a phosphate binding loop is essential for full recF activity. Lastly we made two chimeric recF genes by exchanging the amino terminal 48 amino acids of the S. typhimurium and E. coli recF genes. Both chimeras could complement E. coli chromosomal recF mutations.