Identification of a precursor in the biosynthesis of the p21 transforming protein of harvey murine sarcoma virus

Structural insight into the rearrangement of the switch I region in GTP-bound G12A K-Ras

K-Ras, a molecular switch that regulates cell growth, apoptosis and metabolism, is activated when it undergoes a conformation change upon binding GTP and is deactivated following the hydrolysis of GTP to GDP. Hydrolysis of GTP in water is accelerated by coordination to K-Ras, where GTP adopts a high-energy conformation approaching the transition state. The G12A mutation reduces intrinsic K-Ras GTP hydrolysis by an unexplained mechanism. Here, crystal structures of G12A K-Ras in complex with GDP, GTP, GTPγS and GppNHp, and of Q61A K-Ras in complex with GDP, are reported. In the G12A K-Ras-GTP complex, the switch I region undergoes a significant reorganization such that the Tyr32 side chain points towards the GTP-binding pocket and forms a hydrogen bond to the GTP γ-phosphate, effectively stabilizing GTP in its precatalytic state, increasing the activation energy required to reach the transition state and contributing to the reduced intrinsic GTPase activity of G12A K-Ras mutants.

Ras oncogenes: split personalities

Extensive research on the Ras proteins and their functions in cell physiology over the past 30 years has led to numerous insights that have revealed the involvement of Ras not only in tumorigenesis but also in many developmental disorders. Despite great strides in our understanding of the molecular and cellular mechanisms of action of the Ras proteins, the expanding roster of their downstream effectors and the complexity of the signalling cascades that they regulate indicate that much remains to be learnt.

Downregulation of Ras C-terminal processing by JNK inhibition

After translation, Ras proteins undergo a series of modifications at their C-termini. This post-translational C-terminal processing is essential for Ras to become functional, but it remains unknown whether and how Ras C-terminal processing is regulated. Here we show that the C-terminal processing and subsequent plasma membrane localization of H-Ras as well as the activation of the downstream signaling pathways by H-Ras are prevented by JNK inhibition. Conversely, JNK activation by ultraviolet irradiation resulted in promotion of C-terminal processing of H-Ras. Furthermore, increased cell density promoted C-terminal processing of H-Ras most likely through an autocrine/paracrine mechanism, which was also blocked under JNK-inhibited condition. Ras C-terminal processing was sensitive to JNK inhibition in the case of H- and N-Ras but not K-Ras, and in a variety of cell types. Thus, our results suggest for the first time that Ras C-terminal processing is a regulated mechanism in which JNK is involved.

Thematic review series: Lipid Posttranslational Modifications. Farnesyl transferase inhibitors

Some proteins undergo posttranslational modification by the addition of an isoprenyl lipid (farnesyl- or geranylgeranyl-isoprenoid) to a cysteine residue proximal to the C terminus. Protein isoprenylation promotes membrane association and contributes to protein-protein interactions. Farnesylated proteins include small GTPases, tyrosine phosphatases, nuclear lamina, cochaperones, and centromere-associated proteins. Prenylation is required for the transforming activity of Ras. Because of the high frequency of Ras mutations in cancer, farnesyl transferase inhibitors (FTIs) were investigated as a means to antagonize Ras function. Evaluation of FTIs led to the finding that both K- and N-Ras are alternatively modified by geranylgeranyl prenyltransferase-1 in FTI-treated cells. Geranylgeranylated forms of Ras retain the ability to associate with the plasma membrane and activate substrates. Despite this, FTIs are effective at inhibiting the growth of human tumor cells in vitro, suggesting that activity is dependent on blocking the farnesylation of other proteins. FTIs also inhibit the in vivo growth of human tumor xenografts and sensitize these models to chemotherapeutics, most notably taxanes. Several FTIs have entered clinical trials for various cancer indications. In some clinical settings, primarily hematologic malignancies, FTIs have displayed evidence of single-agent activity. Clinical studies in progress are exploring the antitumor activity of FTIs as single agents and in combination. This review will summarize the basic biology of FTIs, their antitumor activity in preclinical models, and the current status of clinical studies with these agents.

Ras activity regulates cyclin E degradation by the Fbw7 pathway

The Skp1-Cullin1 F-box protein-Fbw7 ubiquitin ligase regulates phosphorylation-dependent cyclin E degradation, and disruption of this pathway is associated with genetic instability and tumorigenesis. Fbw7 is a human tumor suppressor that is targeted for mutation in primary cancers. However, mechanisms other than mutation of Fbw7 may also disrupt cyclin E proteolysis in cancers. We show that oncogenic Ha-Ras activity regulates cyclin E degradation by the Fbw7 pathway. Activated Ras impairs Fbw7-driven cyclin E degradation, and, conversely, inhibition of normal Ras activity decreases cyclin E abundance. Moreover, activation of the mitogen-activated protein kinase pathway is the essential Ras function that inhibits cyclin E turnover, and activated Ha-Ras expression inhibits both the binding of cyclin E to Fbw7 and cyclin E ubiquitination. Last, we found that oncogenic Ras activity potentiates cyclin E-induced genetic instability but only when cyclin E is susceptible to degradation by Fbw7. Thus, we conclude that Ras activity regulates Fbw7-mediated cyclin E proteolysis and suggest that impaired cyclin E proteolysis is a mechanism through which Ras mutations promote tumorigenesis.

In vitro and in vivo metabolism of the anti-cancer agent CI-1040, a MEK inhibitor, in rat, monkey, and human

PURPOSE

The use of in vitro and in vivo models using both rodent and non-rodent species plays an important role with regard to metabolism during the drug development process. In this study, we compared the metabolism of a MEK inhibitor (CI-1040) using in vitro and in vivo models with that observed in a cancer patient.

METHODS

Radiolabeled CI-1040 was assessed for metabolism using rat and monkey liver microsomes and hepatocytes, as well as in Wistar rats and cynomolgus monkeys via oral administration. Human bile and plasma samples were obtained immediately after administration of CI-1040 to a patient with advanced colon cancer. A combination of HPLC-radiochromatography (HPLC-RAM), LC/MS and LC/MS/MS experiments were used to analyze all resulting metabolites. Unlabeled CI-1040 was administered (100 mg/day, QD) for 15 days to a patient suffering from colon cancer. Bile was collected by the insertion of a T-tube directly into the bile duct over a 14-h period. Metabolites were also monitored in the patient's plasma.

RESULTS

Analysis of the metabolites in all species using in vitro and animal models demonstrated that CI-1040 undergoes extensive oxidative metabolism (14 metabolites identified) with subsequent glucuronidation of the hydroxylated metabolites. Metabolites were predominantly excreted through the bile in the animal models.

CONCLUSIONS

Overall, the in vitro and animal models in combination provided comprehensive coverage for all metabolites observed in human bile and plasma. In conclusion, the results obtained in this study demonstrate the utility of conducting investigations across species in order to gain complete coverage for successfully predicting human metabolites of new compounds in development.

Ras inhibitors and radiation therapy

Local recurrence after a definitive course of radiation therapy remains a significant clinical problem and represents a common pattern of failure for many solid tumors. The sensitivity of tumor cells to the cytotoxicity of ionizing radiation is thought to be one of the major determinants of local control for tumors in patients treated with radiation therapy. There is substantial experimental evidence to demonstrate that increased radiation resistance is associated with the expression of activated oncogenes, including Ras. Mutated forms of Ras are found in 30% of human cancers including a substantial proportion of pancreatic and colon adenocarcinomas. Mutated Ras produces proteins that remain locked in a constitutively active state, thereby relaying uncontrolled signals. Ras proteins are guanosine triphosphate-binding proteins that play a pivotal role in the control of many cellular processes, including growth and differentiation. Preclinical studies have shown that expression of mutant Ras increases cellular radioresistance. Ras function is dependent on its localization to the plasma membrane. This is achieved by posttranslational modifications, including the addition of a farnesyl isoprenoid moiety in a reaction catalyzed by the enzyme protein farnesyltransferase (FTase). This enzyme has become an important target for the design of new agents that target Ras. FTase inhibitors (FTIs) block the farnesylation of Ras and reverse Ras-mediated radioresistance in human cell lines. FTIs have been well tolerated in animal studies and appear not to cause generalized cytotoxicity. There are ongoing clinical trials to determine the optimal therapeutic schedules and dose for FTIs. A phase 1 trial of the FTI L778-123 and radiotherapy has recently been completed.

Farnesyltransferase as a target for anticancer drug design

The currently understood function for Ras in signal transduction is in mediating the transmission of signals from external growth factors to the cell nucleus. Mutated forms of this GTP-binding protein are found in 30% of human cancers with particularly high prevalence in colon and pancreatic carcinomas. These mutations destroy the GTPase activity of Ras and cause the protein to be locked in its active, GTP bound form. As a result, the signaling pathways are activated, leading to uncontrolled tumor growth. Ras function in signaling requires its association with the plasma membrane. This is achieved by posttranslational farnesylation of a cysteine residue present as part of the CA1A2X carboxyl terminal tetrapeptide of all Ras proteins. The enzyme that recognizes and farnesylates the CA1A2X sequence, Ras farnesyltransferase (FTase), has become an important target for the design of inhibitors that might be interesting as antitumor agents. Several approaches have been taken in the search for in vivo active inhibitors of farnesyltransferase. These include the identification of natural products such as the chaetomellic and zaragozic acids that mimic farnesylpyrophosphate, bisubstrate transition state analogs combining elements of the farnesyl and tetrapeptide substrates and peptidomimetics that reproduce features of the carboxyl terminal tetrapeptide CA1A2X sequence. This last group of compounds has been most successful in showing highly potent inhibition of FTase and selective blocking of Ras processing in a range of Ras transformed tumor cell lines at concentrations as low as 10 nM. Certain peptidomimetics will also block tumor growth in various mouse models, with apparently few toxic side effects. These results suggest that farnesyltransferase inhibitors hold considerable promise as anticancer drugs in the clinic.

Purification of a protein palmitoyltransferase that acts on H-Ras protein and on a C-terminal N-Ras peptide

Mammalian H-Ras and N-Ras are GTP-binding proteins that must be post-translationally lipidated to function as molecular switches in signal transduction cascades controlling cell growth and differentiation. These proteins contain a C-terminal farnesyl-cysteine alpha-methyl ester and palmitoyl groups attached to nearby cysteines. Data is presented showing that rat liver microsomes contain an enzyme that transfers the palmitoyl group from palmitoyl-coenzyme A to cysteine residues of H-Ras protein and of a synthetic peptide having the structure of the C terminus of N-Ras. This protein palmitoyltransferase (PPT) was solubilized from membranes and purified 10,500-fold to apparent homogeneity with an overall yield of 10%. On an SDS gel, PPT appears as two proteins of molecular masses of approximately 30 and approximately 33 kDa. If the palmitoylation sites of the N-Ras peptide (the non-farnesylated cysteine) or H-Ras protein (cysteines 181 and 184) are changed to serine, palmitoylation by PPT does not occur. Non-farnesylated H-Ras produced in bacteria as well as in vitro farnesylated bacterial H-Ras are not substrates for PPT nor is the non-farnesylated, methylated N-Ras peptide. These results suggest, but do not prove, that farnesylation and possibly C-terminal methylation are prerequisites for Ras palmitoylation. PPT shows a large preference for palmitoyl-coenzyme A over myristoyl-coenzyme as the acyl donor. Values of Km for palmitoyl-CoA and H-Ras are 4.3 +/- 1.2 and 0.8 +/- 0.3 microM, respectively. PPT is the first protein palmitoyltransferase to be purified, and the availability of pure enzyme should contribute to our understanding of the function and regulation of Ras palmitoylation in cells.

Normal and transforming Ras are differently regulated for posttranslational modifications

Point mutation of the c-H-ras gene significantly increases cellular transforming activities of Ras. Since posttranslational modification and subsequent membrane localization are essential for the biological activities of Ras, we examined whether or not the mutation also affects these two factors. The normal (Gly(12)) or the transforming (Val(12)) c-H-ras gene was expressed in NIH3T3 cells using a metallothionein promoter. Expression of either type of Ras was efficiently induced by the cadmium treatment of these cells, and immunoprecipitation of metabolically labeled cell extracts revealed that both normal and transforming Ras were expressed as four differently migrating forms on SDS-polyacrylamide gels, two of which were slower migrating cytosolic precursors and the other two were faster migrating membrane-bound forms. There was no significant difference in half lives between normal and transforming Ras; however, posttranslational modification was quite different between the two types of Ras. Transforming Ras was processed and became membrane-bound forms much more efficiently than normal Ras. Interestingly, posttranslational modification and membrane localization of Ras was significantly inhibited when the c-myc oncogene was co-expressed with Ras. In contrast to the c-myc oncogene, expression of either wild type or mutant p53 did not affect the posttranslational modification of Ras, suggesting that the c-myc oncogene specifically impairs the posttranslational modification of Ras.

Guanine-nucleotide binding activity, interaction with GTPase-activating protein and solution conformation of the human c-Ha-Ras protein catalytic domain are retained upon deletion of C-terminal 18 amino acid residues

A truncated human c-Ha-Ras protein that lacks the C-terminal 18 amino acid residues and the truncated Ras protein with the amino acid substitution Gly-->Val in position 12 were prepared by an E. coli overexpression system. The truncated Ras protein showed the same guanine-nucleotide binding activity and GTPase activity as those of the full-length Ras protein. Further, the same extent of GTPase activity enhancement due to GTPase-activating protein was observed for the truncated and full-length Ras proteins. In fact, two-dimensional proton NMR analyses indicated that the tertiary structure of the truncated Ras protein (GDP-bound or GMPPNP-bound) was nearly the same as that of the corresponding catalytic domain of the full-length Ras protein. Moreover, a conformational change around the effector region upon GDP-->GMPPNP exchange occurred in the same manner for both proteins. These observations indicate that the C-terminal flanking region (18 amino acid residues) of the Ras protein does not appreciably interact with the catalytic domain. Therefore, the truncated Ras protein is suitable for studying the molecular mechanism involved in the GTPase activity and the interaction with the GTPase-activating protein. On the other hand, an active form of the truncated Ras protein, unlike that of the full-length Ras protein, did not induce neurite outgrowth of rat pheochromocytoma PC12 cells. Thus, membrane anchoring of the Ras protein through its C-terminal four residues is not required for the interaction of Ras and GAP, but may be essential for the following binding of the Ras-GAP complex with the putative downstream target.

Expression of c-Ki-ras in developing rat liver

1. Two major c-Ki-ras transcripts are present in rat liver throughout development. 2. Both transcripts are functional, i.e. translatable into ras protein. 3. They exhibit low levels in early foetuses and increase gradually towards term. 4. Peak levels of transcripts are detected on the fourth day of postnatal life. 5. Results from nuclear run-off assays demonstrate that the rise in level of transcripts is a result of increase in rate of transcription with the highest rate detected on the fourth day of postnatal life. 6. Western blot analysis reveals that the accumulation of c-Ki-ras protein clearly follows the pattern of changes in transcription of c-Ki-ras gene.

Genetic factors and suppression of metastatic ability of v-Ha-ras-transfected rat mammary cancer cells

Following v-Ha-ras transfection of nonmetastatic dimethylbenz(( a ))anthracene-induced rat mammary cancer (RMC1) cells, occasional transfectants were isolated that acquired high metastatic ability. High metastatic ability is not a simple process regulated by v-Ha-ras p21 levels alone in these v-Ha-ras transfectants but involves the development of cytogenetic changes. If such cytogenetic changes involve only gain in gene expression, then all hybrids formed by fusing highly metastatic v-Ha-ras RMC1 transfectants with the parental nonmetastatic RMC1 should be highly metastatic. If loss of a metastatic suppressor gene(s) is also involved, then such hybrids should be nonmetastatic since chromosomes from the nonmetastatic parental cells should supply the suppressor function. To test this possibility, a highly metastatic cloned v-Ha-ras transfectant was fused with the nonmetastatic parental RMC1 cells. Five hybrid clones were isolated that conserved the chromosomes from their parental cells. When these hybrid clones were injected into animals, primary tumors developed with the same tumor-doubling time as that of the highly metastatic parental v-Ha-ras transfectant (i.e., approximately 2 days). High metastatic ability was, however, suppressed in these hybrid clones. All hybrid clones continued to express v-Ha-ras p21. Thus, suppression of metastatic ability in the hybrids can occur even in the presence of an elevated v-Ha-ras p21 level. This suggests that the acquisition of metastatic ability following v-Ha-ras transfection involves loss of metastasis suppressor gene function in rat mammary cancer cells.

Ras-related genes in Dictyostelium discoideum

Dictyostelium discoideum, like other eukaryotes, has been shown to express several ras-related genes. Two gene products, Ddras and DdrasG, are highly conserved relative to the human ras proteins. Ddras is expressed at the pseudoplasmodial stage of development, whereas DdrasG is expressed in vegetative cells and during early development. In addition, Dictyostelium possesses three ras-related genes, SAS1, SAS2 and Ddrap1, whose gene products are only partially conserved relative to those of the ras genes. The expression of these three genes is also developmentally regulated.

Yeast RAS2 affects cell viability, mitotic division and transient gene expression in Nicotiana species

Overexpression of the budding yeast RAS2 gene in Nicotiana plumbaginifolia cells revealed that RAS2 acted as 'suicide' gene in freshly isolated protoplasts from leaves and blocked cell proliferation in cell suspension-derived protoplasts. Among a series of genes tested (such as npt II, CDC35, PDE2), RAS2 was the only one to block the expression of the cat gene, as measured in a transient gene expression assay. Another ras gene, v-Ha-ras, had similar effects. Furthermore, the RAS2 effect was species-specific and depended on the modulation of hormonal metabolism in the transfected cells, while no differences were noticed between the normal and the activated val19 gene. Transfected plant cells are shown to synthesize a RAS2 protein of the same electrophoretic mobility as the yeast RAS2 product. The results are discussed in the broader context of the evolutionarily conserved ras genes involved in vital cellular functions.

Characterization and expression of a Xenopus ras during oogenesis and development

We have characterized a cDNA which contains the entire coding sequence of a Xenopus laevis ras protein. The deduced amino acid sequence reveals a strong homology (92%) to human Ki-ras 2B protein. ras expression has been studied both qualitatively and quantitatively during Xenopus development. ras is expressed as a maternal mRNA in oocytes and early embryos at a level up to 1.5 x 10(7) copies per mature oocyte, corresponding to the level of ras mRNA found in 4 x 10(5) somatic growing cells. This level remains constant throughout the first rapid cleavage stages of the blastula before the midblastula transition (MBT). After this stage, the amount of ras RNA decreases gradually until the hatching tadpole stage, when a new zygotic expression is detected in the embryo. From that stage, a constitutive amount of 30-50 ras RNA transcripts per embryonic cell is registered, as observed in Xenopus proliferative somatic cells. The 23-kDa Xenopus ras protein has also been identified by both specific monoclonal antibody and in vitro transcription-translation experiments. It is expressed in oocytes before maturation, indicating that maturation is not the trigger for ras expression. The expression of Xenopus ras at a high level during oogenesis and early development suggests a major function of this gene both in meiosis and in mitosis events during embryonic development.

Diacylglycerol production in Xenopus laevis oocytes after microinjection of p21ras proteins is a consequence of activation of phosphatidylcholine metabolism

Microinjection of p21Ha-ras proteins into Xenopus laevis oocytes induces a rapid increase of 1,2-diacylglycerol (DAG) levels. The observed alterations in DAG levels were consistent with the ability of the protein to induce maturation, measured by germinal vesicle breakdown (GVBD). Both the increase in DAG levels and GVBD activity were dependent on the ability of the proteins to undergo membrane translocation. Alterations of DAG levels or GVBD activity did not correlate with changes in the levels of inositol phosphates. However, at minimal doses sufficient to achieve maximal biological response, a biphasic increase in the amounts of phosphocholine and CDP-choline was observed. The first burst of phosphocholine and CDP-choline preceded the increase in DAG levels. The second peak paralleled the appearance of DAG. Choline kinase activity was also increased in oocyte extracts after p21ras microinjection. These results suggest that both the synthesis and degradation of phosphatidylcholine are activated after microinjection of ras proteins into Xenopus oocytes, resulting in a net production of DAG.

Diacylglycerol production in Xenopus laevis oocytes after microinjection of p21ras proteins is a consequence of activation of phosphatidylcholine metabolism

Microinjection of p21Ha-ras proteins into Xenopus laevis oocytes induces a rapid increase of 1,2-diacylglycerol (DAG) levels. The observed alterations in DAG levels were consistent with the ability of the protein to induce maturation, measured by germinal vesicle breakdown (GVBD). Both the increase in DAG levels and GVBD activity were dependent on the ability of the proteins to undergo membrane translocation. Alterations of DAG levels or GVBD activity did not correlate with changes in the levels of inositol phosphates. However, at minimal doses sufficient to achieve maximal biological response, a biphasic increase in the amounts of phosphocholine and CDP-choline was observed. The first burst of phosphocholine and CDP-choline preceded the increase in DAG levels. The second peak paralleled the appearance of DAG. Choline kinase activity was also increased in oocyte extracts after p21ras microinjection. These results suggest that both the synthesis and degradation of phosphatidylcholine are activated after microinjection of ras proteins into Xenopus oocytes, resulting in a net production of DAG.

Modification of nuclear lamin proteins by a mevalonic acid derivative occurs in reticulocyte lysates and requires the cysteine residue of the C-terminal CXXM motif

The C-terminus of nuclear lamins (CXXM) resembles a C-terminal motif (the CAAX box) of fungal mating factors and ras-related proteins. The CAAX box is subject to different types of post-translational modifications, including proteolytic processing, isoprenylation and carboxyl methylation. By peptide mapping we show that both chicken lamins A and B2 are processed proteolytically in vivo. However, whereas the entire CXXM motif is cleaved from lamin A, at most three C-terminal amino acids are removed from lamin B2. Following translation of cDNA-derived RNAs in reticulocyte lysates, lamin proteins specifically incorporate a derivative of [14C]mevalonic acid (MV), i.e. the precursor of a putative isoprenoid modification. Remarkably, no MV is incorporated into lamin B2 translated from a mutant cDNA encoding alanine instead of cysteine in the C-terminal CXXM motif. These results implicate this particular cysteine residue as the target for modification of lamin proteins by an isoprenoid MV derivative, and they indicate that isoprenylation is amenable to studies in cell-free systems. Moreover, our observations suggest that C-terminal processing of newly synthesized nuclear lamins is a multi-step process highly reminiscent of the pathway elaborated recently for ras-related proteins.

Ras protein expression is developmentally regulated in embryonic chicken brain

Immunoblot analysis using a panreactive monoclonal antibody directed against ras p21 proteins detects two differentially regulated ras pools in embryonic chicken brain: a membrane pool that is not changed and a microsomal pool that starts at a low level but is strongly increased from E6 to E16 chicken brain. In order to study the distribution of ras proteins in different cell types of the nervous system, immunoblot analysis was performed on total cell proteins. In contrast to histochemical data showing the absence or low levels of ras proteins in glial cells, comparable amounts of ras proteins were found in cell lysates from purified chicken sympathetic neurons, cultured rat Schwann cells and mouse brain astrocytes.

Regulation of hepatocyte growth: alpha-1 adrenergic receptor and ras p21 changes in liver regeneration

Catecholamines, acting via the alpha-1 adrenergic receptor, have been demonstrated to influence adult rat hepatocyte DNA synthesis in primary culture and in vivo during liver regeneration following partial hepatectomy (PHX). Earlier investigations have suggested that the alpha-1 effect on DNA synthesis is significant only during the first day following PHX. We examined receptor binding at several early and late time points after surgery, and we observed a significant loss of specific [3H]-prazosin binding to cells isolated from rat livers 48 and 72 hr after PHX. In contrast, the ability of norepinephrine to stimulate inositol phosphate production in isolated cells prelabeled with [3H]-myo-inositol was transiently reduced between 8 and 16 hr, when alpha-1 binding capacity was virtually unchanged. This uncoupling of phosphoinositide turnover from binding was preceded by a drop in hepatic membrane ras p21 content, as assayed by liquid competition radioimmunoassay. The loss of immunoreactive p21 from membranes was significant by 2 hr after PHX. These findings suggest a role for alpha-1 receptors and ras protein in the early events of liver regeneration.

Genetic and pharmacological suppression of oncogenic mutations in ras genes of yeast and humans

The activity of an oncoprotein and the secretion of a pheromone can be affected by an unusual protein modification. Specifically, posttranslational modification of yeast a-factor and Ras protein requires an intermediate of the cholesterol biosynthetic pathway. This modification is apparently essential for biological activity. Studies of yeast mutants blocked in sterol biosynthesis demonstrated that the membrane association and biological activation of the yeast Ras2 protein require mevalonate, a precursor of sterols and other isoprenes such as farnesyl pyrophosphate. Furthermore, drugs that inhibit mevalonate biosynthesis blocked the in vivo action of oncogenic derivatives of human Ras protein in the Xenopus oocyte assay. The same drugs and mutations also prevented the posttranslational processing and secretion of yeast a-factor, a peptide that is farnesylated. Thus, the mevalonate requirement for Ras activation may indicate that attachment of a mevalonate-derived (isoprenoid) moiety to Ras proteins is necessary for membrane association and biological function. These observations establish a connection between the cholesterol biosynthetic pathway and transformation by the ras oncogene and offer a novel pharmacological approach to investigating, and possibly controlling, ras-mediated malignant transformations.

Differential expression of ras oncogene products among the types of human melanomas and melanocytic nevi

Ras oncogene expression of malignant melanoma and melanocytic nevus have been immunohistochemically analyzed on formalin-fixed and paraffin-embedded tissues from 26 melanomas and 24 melanocytic nevi with a monoclonal antibody that was generated against Harvey sarcoma virus-derived ras oncogene products (p21ras). We found distinct differences of p21ras expressions by the type of melanoma. Nodular melanoma, epithelioid cell type melanoma, and deeply invading melanoma revealed higher reactivity with anti-p21ras monoclonal antibody than the other types. The reactivity of melanomas appeared to correlate with the degree of malignancy of the melanoma. It was also demonstrated, however, that part of melanocytic nevi reacted with anti-p21ras monoclonal antibody with a relatively strong intensity. Melanocytic nevi with junctional activity and nevus cells located in the epidermis in compound nevi did not show the positive reaction in contrast to dermally located nevus cells that had relatively strong reactivity. The different p21ras expression among the type of tumors may represent the state of tumor cells differentiation with greater expression with more immaturity in the melanocyte lineage. p21ras expression does not appear to represent a marker of malignant transformation.

The ras oncogene--an important regulatory element in lower eucaryotic organisms

The ras proto-oncogene in mammalian cells encodes a 21-kilodalton guanosine triphosphate (GTP)-binding protein. This gene is frequently activated in human cancer. As one approach toward understanding the mechanisms of cellular transformation by ras, the function of this gene in lower eucaryotic organisms has been studied. In the yeast Saccharomyces cerevisiae, the RAS gene products serve as essential function by regulating cyclic adenosine monophosphate metabolism. Stimulation of adenylyl cyclase is dependent not only on RAS protein complexed to GTP, but also on the CDC25 and IRA gene products, which appear to control the RAS GTP-guanosine diphosphate cycle. Although analysis of RAS biochemistry in S. cerevisiae has identified mechanisms central to RAS action, RAS regulation of adenylyl cyclase appears to be strictly limited to this particular organism. In Schizosaccharomyces pombe, Dictyostelium discoideum, and Drosophila melanogaster, ras-encoded proteins are not involved with regulation of adenylyl cyclase, similar to what is observed in mammalian cells. However, the ras gene product in these other lower eucaryotes is clearly required for appropriate responses to extracellular signals such as mating factors and chemoattractants and for normal growth and development of the organism. The identification of other GTP-binding proteins in S. cerevisiae with distinct yet essential functions underscores the fundamental importance of G-protein regulatory processes in normal cell physiology.

Post-translational processing of p21ras is two-step and involves carboxyl-methylation and carboxy-terminal proteolysis

We have studied the post-translational processing of p21ras proteins. The primary translation product pro-p21 is cytosolic and is rapidly converted to a cytosolic form (c-p21) of higher mobility on SDS-PAGE. c-p21 is converted in turn to the membrane-bound mature palmitoylated form (m-p21) of slightly higher mobility. These processing steps are accompanied by increases in isoelectric point and in hydrophobicity as judged by Triton X-114 partitioning. Although the increases in electrophoretic mobility and hydrophobicity precede acylation we show that mutation of Cys186, which has been shown to block acylation, also abolishes the pro-p21 to c-p21 conversion. Thus the Cys186 residue is involved in the processing steps prior to acylation. We have identified two processing events which contribute to the pro-p21 conversion. Site-directed mutagenesis to insert tryptophan, which is not present in the wild type, followed by metabolic labelling with [3H]tryptophan has allowed us to map a proteolytic processing event which removes the three C-terminal residues. In addition, both the c-p21 and m-p21 forms are carboxyl-methylated. Approximately one methyl group is incorporated per molecule of p21 at steady state, which can partially account for the increase in isoelectric point. Unlike palmitate, methyl group turnover is not observed.

Activation of the cellular proto-oncogene product p21Ras by addition of a myristylation signal

The 21-kD proteins encoded by ras oncogenes (p21Ras) are modified covalently by a palmitate attached to a cysteine residue near the carboxyl terminus. Changing cysteine at position 186 to serine in oncogenic forms produces a nonpalmitylated protein that fails to associate with membranes and does not transform NIH 3T3 cells. Nonpalmitylated p21Ras derivatives were constructed that contained myristic acid at their amino termini to determine if a different form of lipid modification could restore either membrane association or transforming activity. An activated p21Ras, altered in this way, exhibited both efficient membrane association and full transforming activity. Surprisingly, myristylated forms of normal cellular Ras were also transforming. This demonstrates that Ras must bind to membranes in order to transmit a signal for transformation, but that either myristate or palmitate can perform this role. However, the normal function of cellular Ras is diverted to transformation by myristate and therefore must be regulated ordinarily by some unique property of palmitate that myristate does not mimic. Myristylation thus represents a novel mechanism by which Ras can become transforming.

Inhibition of NIH 3T3 cell proliferation by a mutant ras protein with preferential affinity for GDP

Substitution of asparagine for serine at position 17 decreased the affinity of rasH p21 for GTP 20- to 40-fold without significantly affecting its affinity for GDP. Transfection of NIH 3T3 cells with a mammalian expression vector containing the Asn-17 rasH gene and a Neor gene under the control of the same promoter yielded only a small fraction of the expected number of G418-resistant colonies, indicating that expression of Asn-17 p21 inhibited cell proliferation. The inhibitory effect of Asn-17 p21 required its localization to the plasma membrane and was reversed by coexpression of an activated ras gene, indicating that the mutant p21 blocked the endogenous ras function required for NIH 3T3 cell proliferation. NIH 3T3 cells transformed by v-mos and v-raf, but not v-src, were resistant to inhibition by Asn-17 p21, indicating that the requirement for normal ras function can be bypassed by these cytoplasmic oncogenes. The Asn-17 mutant represents a novel reagent for the study of ras function by virtue of its ability to inhibit cellular ras activity in vivo. Since this phenotype is likely associated with the preferential affinity of the mutant protein for GDP, analogous mutations might also yield inhibitors of other proteins whose activities are regulated by guanine nucleotide binding.

Localization and subcellular distribution of cellular ras gene products in rat brain

Localization and subcellular distribution of the cellular ras gene products (c-ras p21s) in rat brain were studied by immunofluorescence and immunoblotting using a monoclonal antibody recognizing all of Ki-, Ha- and N-ras p21s. In immunohistochemical analysis, strong immunoreactivity for ras p21s was observed in the neuropile of cerebral and cerebellar cortex. On the other hand, the immunoreactivity of the neuronal perikarya and that of white matter were weak and that of non-neuronal cells was undetectable. In subcellular fractionation analysis of cerebrum, c-ras p21s were found mostly in the particulate fractions and almost half of the particulate-bound c-ras p21s were recovered in the P2 fraction containing myelin, synaptosomes and mitochondria, approximately one-third were in the P3 fraction containing microsomes, and the rest were in the P1 fraction containing nuclei and cell debris. In further fractionation of the P2 fraction, most of c-ras p21s were associated with synaptosomal fraction. In the synaptosomal fraction, c-ras p21s were highly concentrated in the fractions rich in synaptic plasma membranes and were poorly present in the other fractions rich in synaptic vesicles, intrasynaptosomal mitochondria or postsynaptic densities. The content of c-ras p21s of the original homogenate was calculated to be 0.05% of the total protein and c-ras p21s were distributed in the fractions rich in synaptic plasma membranes with approximately 4-fold enrichment over the original homogenate. These results indicate that c-ras p21s are mainly localized in the synaptic plasma membranes and microsomes and suggest that they may participate in some specific neuronal functions at these sites.

Structure and expression of yeast DPR1, a gene essential for the processing and intracellular localization of ras proteins

The ras proteins represent a unique example of membrane proteins which apparently do not utilize the secretory pathway for their membrane localization. Instead, it is believed that palmitic acid, covalently attached to the protein, acts as an anchor to the membranes. Recent identification of yeast mutants defective in the processing of the ras proteins has provided a novel approach for defining these biosynthetic processes. We report here the characterization of yeast DPR1, a gene essential for the processing of the ras proteins. The sequence of the gene indicates that it encodes a protein of 431 amino acids which contains no significant homology with any known proteins. It is a relatively hydrophilic protein with no apparent hydrophobic stretches. The C-terminal half of the encoded protein has an unusually high content of cysteine. The DPR1 gene product has been identified in a cell-free translation system as a protein having an apparent molecular weight of 43 kd. This represents the first step in the investigation of a novel protein-processing pathway, one that is distinct from the secretory pathway.

Inhibition of NIH 3T3 cell proliferation by a mutant ras protein with preferential affinity for GDP

Substitution of asparagine for serine at position 17 decreased the affinity of rasH p21 for GTP 20- to 40-fold without significantly affecting its affinity for GDP. Transfection of NIH 3T3 cells with a mammalian expression vector containing the Asn-17 rasH gene and a Neor gene under the control of the same promoter yielded only a small fraction of the expected number of G418-resistant colonies, indicating that expression of Asn-17 p21 inhibited cell proliferation. The inhibitory effect of Asn-17 p21 required its localization to the plasma membrane and was reversed by coexpression of an activated ras gene, indicating that the mutant p21 blocked the endogenous ras function required for NIH 3T3 cell proliferation. NIH 3T3 cells transformed by v-mos and v-raf, but not v-src, were resistant to inhibition by Asn-17 p21, indicating that the requirement for normal ras function can be bypassed by these cytoplasmic oncogenes. The Asn-17 mutant represents a novel reagent for the study of ras function by virtue of its ability to inhibit cellular ras activity in vivo. Since this phenotype is likely associated with the preferential affinity of the mutant protein for GDP, analogous mutations might also yield inhibitors of other proteins whose activities are regulated by guanine nucleotide binding.

Purification of a RAS-responsive adenylyl cyclase complex from Saccharomyces cerevisiae by use of an epitope addition method

We developed a method for immunoaffinity purification of Saccharomyces cerevisiae adenylyl cyclase based on creating a fusion with a small peptide epitope. Using oligonucleotide technology to encode the peptide epitope we constructed a plasmid that expressed the fusion protein from the S. cerevisiae alcohol dehydrogenase promoter ADH1. A monoclonal antibody previously raised against the peptide was used to purify adenylyl cyclase by affinity chromatography. The purified enzyme appeared to be a multisubunit complex consisting of the 200-kilodalton adenylyl cyclase fusion protein and an unidentified 70-kilodalton protein. The purified protein could be activated by RAS proteins. Activation had an absolute requirement for a guanine nucleoside triphosphate.

A GTP-binding protein required for secretion rapidly associates with secretory vesicles and the plasma membrane in yeast

SEC4, one of the 10 genes involved in the final stage of the yeast secretory pathway, encodes a ras-like, GTP-binding protein. In wild-type cells, Sec4 protein is located on the cytoplasmic face of both the plasma membrane and the secretory vesicles in transit to the cell surface. In all post-Golgi blocked sec mutants, Sec4p is predominantly associated with the secretory vesicles that accumulate as a result of the secretory block. Sec4p is synthesized as a soluble protein that rapidly (t1/2 less than or equal to 1 min) and tightly associates with secretory vesicles and the plasma membrane by virtue of a conformational change of a covalent modification. These data suggest that Sec4p may function as a "G" protein on the vesicle surface to transduce an intracellular signal needed to regulate transport between the Golgi apparatus and the plasma membrane.

Both N-ras and c-myc are activated in the SHAC human stomach fibrosarcoma cell line

A transforming N-ras gene was isolated from the SHAC human stomach fibrosarcoma cell line. A single-point mutation resulting in the substitution of histidine for glutamine at codon 61 was found in the SHAC transforming allele. The N-ras gene is overexpressed in the tumor cells and transformant cells. The N-ras p21 product was studied by immunoprecipitation and showed no alteration in mobility as compared to the normal p21 protein. The c-myc gene is amplified and overexpressed in these cells. This report gives evidence that an amplified c-myc and a mutated N-ras gene are both present in this tumor cell line and provides support for the idea that co-operation of at least 2 activated cellular oncogenes is required for carcinogenesis.

Purification of a RAS-responsive adenylyl cyclase complex from Saccharomyces cerevisiae by use of an epitope addition method

We developed a method for immunoaffinity purification of Saccharomyces cerevisiae adenylyl cyclase based on creating a fusion with a small peptide epitope. Using oligonucleotide technology to encode the peptide epitope we constructed a plasmid that expressed the fusion protein from the S. cerevisiae alcohol dehydrogenase promoter ADH1. A monoclonal antibody previously raised against the peptide was used to purify adenylyl cyclase by affinity chromatography. The purified enzyme appeared to be a multisubunit complex consisting of the 200-kilodalton adenylyl cyclase fusion protein and an unidentified 70-kilodalton protein. The purified protein could be activated by RAS proteins. Activation had an absolute requirement for a guanine nucleoside triphosphate.

A carboxyl-terminal cysteine residue is required for palmitic acid binding and biological activity of the ras-related yeast YPT1 protein

The Saccharomyces cerevisiae YPT1 gene codes for a ras-like, guanine nucleotide-binding protein which is essential for cell viability. The functional significance of two consecutive cysteines at the very carboxyl-terminal end of this protein and in ypt homologues of other eukaryotic species was examined. YPT1 gene mutations were generated that either led to substitutions by serine or the deletion of one or both C-terminal cysteines. The consequences of the mutations were checked in cells after replacing the wild type with the mutant genes. It was found that as long as one of the cysteines was retained, the protein was fully functional. The YPT1 protein could be labelled with [3H]palmitic acid that appeared to be bound in an ester linkage. The wild-type protein was evenly distributed between soluble and membrane-associated proteins, the palmitoylated form was predominantly in the crude membrane fraction. The mutant protein lacking the C-terminal cysteines was not palmitoylated and was exclusively found in the soluble fraction. The extension by three residues, -Val-Leu-Ser, generating a ras-typical C-terminal end, did not interfere with the mutant YPT1 protein's function although it resulted in a reduced labelling with palmitic acid.

The int-1 proto-oncogene products are glycoproteins that appear to enter the secretory pathway

The int-1 proto-oncogene encodes a primary product of 370 amino acids, is normally expressed in mid-gestational embryos and adult testis, and is activated by proviral insertions during mammary carcinogenesis. Polyclonal and monoclonal antibodies directed against int-1-specific synthetic peptides immunoprecipitate up to five forms of int-1 protein, ranging in size from 36,000 to 44,000 Mr, from cell lines that express cloned int-1 DNA introduced by transfection or infection with retroviral vectors. Pulse-chase labeling experiments and glycosidase digestions suggested that the smallest of the int-1 proteins is the primary translation product lacking its signal peptide and that it is modified to produce the larger species of sequential glycosylation. Subcellular fractionations demonstrated that all immunoprecipitable forms of int-1 are mainly associated with membranes. int-1 proteins in crude microsomal preparations are resistant to proteolysis and extractable at elevated pH, suggesting that they are sequestered within cytoplasmic vesicles in a manner consistent with the behavior of secretory products. However, we were unable to identify secreted int-1 products in extracellular fluids.

Posttranslational modification of ras proteins: detection of a modification prior to fatty acid acylation and cloning of a gene responsible for the modification

Products of ras genes are synthesized as precursors in the cytosol and transported to the plasma membrane by a process which involves posttraslational modification by fatty acid. In this paper, we present evidence for the occurrence in the cytosol of an intermediate modification of ras proteins prior to the fatty acid acylation. The modification is detected by a slight shift in the mobility of the protein on SDS polyacrylamide gel. The fatty acid acylation does not contribute to this mobility shift. This modification is affected by the dprl mutation which has recently been shown to affect the processing of yeast RAS proteins. To further characterize the nature of the modification event, we have cloned DPR1 gene from the DNA of Saccharomyces cerevisiae. The gene is actively transcribed in yeast cells producing mRNA of approximately 1.6 kb. Genes related to the DRP1 appear to be present in a distantly related yeast, Schizosaccharomyces pombe as well as in guinea pig and human cells.

Functional modification of a 21-kilodalton G protein when ADP-ribosylated by exoenzyme C3 of Clostridium botulinum

Exoenzyme C3 from Clostridium botulinum types C and D specifically ADP-ribosylated a 21-kilodalton cellular protein, p21.bot. Guanyl nucleotides protected the substrate against denaturation, which implies that p21.bot is a G protein. When introduced into the interior of cells, purified exoenzyme C3 ADP-ribosylated intracellular p21.bot and changed its function. NIH 3T3, PC12, and other cells rapidly underwent temporary morphological alterations that were in certain respects similar to those seen after microinjection of cloned ras proteins. When injected into Xenopus oocytes, C3 induced migration of germinal vesicles and potentiated the cholera toxin-sensitive augmentation of germinal vesicle breakdown by progesterone, also as caused by ras proteins. Nevertheless, p21.bot was immunologically distinct from p21ras.

A yeast-derived ras-gene-related protein expressed in human tumor cells. I. Detection by polyclonal antibodies

Rabbit polyclonal antibodies (pAb) were raised against a yeast ras-related protein YP20 and shown to be immunoreactive with human normal as well as altered Ha-ras and Ki-ras p21 gene products using immunoblotting and immunoprecipitation techniques. The p21 protein revealed by anti-YP20 antibodies comigrates with p21 protein detected by anti-p21 monoclonal antibody (Cetus Diagnostics). These pAbs were tested against a panel of human acetone-fixed tumor cell lines and malignant effusions and nonfixed fresh-frozen tissue sections obtained from cancer patients by the indirect immunofluorescence assay (IFA). Twelve of sixteen (75%) sarcoma and carcinoma cells lines and one fibroblast cell line were stained by the anti-YP20 pAb. The binding occurred most commonly in the cytoplasm. Six of eight fresh-frozen colon and breast cancer tissue sections were immunostained and normal sections from these organs or skin showed only low level of binding to the pAbs. Four of five malignant effusions were distinctively immunostained. These antibodies are suggested to serve as additional probes for assessing the expression of ras gene-related proteins in human malignancy.

Functional modification of a 21-kilodalton G protein when ADP-ribosylated by exoenzyme C3 of Clostridium botulinum

Exoenzyme C3 from Clostridium botulinum types C and D specifically ADP-ribosylated a 21-kilodalton cellular protein, p21.bot. Guanyl nucleotides protected the substrate against denaturation, which implies that p21.bot is a G protein. When introduced into the interior of cells, purified exoenzyme C3 ADP-ribosylated intracellular p21.bot and changed its function. NIH 3T3, PC12, and other cells rapidly underwent temporary morphological alterations that were in certain respects similar to those seen after microinjection of cloned ras proteins. When injected into Xenopus oocytes, C3 induced migration of germinal vesicles and potentiated the cholera toxin-sensitive augmentation of germinal vesicle breakdown by progesterone, also as caused by ras proteins. Nevertheless, p21.bot was immunologically distinct from p21ras.

Structural and functional relationships of guanosine triphosphate binding proteins

Information available at present documents the existence of three well-defined classes of guanine nucleotide binding proteins functioning as signal transducers: Gs and Gi which stimulate and inhibit adenylate cyclase, respectively, and transducin which transmits and amplifies the signal from light-activated rhodopsin to cGMP-dependent phosphodiesterase in ROS membranes. Go is a fourth member of this family. Its function is the least known among GTP binding signal transducing proteins. The family of G proteins has a number of properties in common. All are heterotrimers consisting of three subunits, alpha, beta, and gamma. Each of the subunits may be heterogeneous depending on species and tissue of origin and may be posttranslationally modified covalently. The alpha subunits vary in size from 39 to 52 kDa. The sequences for Gs alpha and transducin alpha have 42% overall homology and those of Gi alpha and Gs alpha 43%, whereas those of Gi alpha and transducin alpha have a higher degree (68%) of homology. All alpha subunits bind guanine nucleotides and are ADP-ribosylated by either pertussis toxin (Gi, transducin, Go) or cholera toxin (Gs, Gi, transducin). Thus, transducin and Gi, which have the highest degree of sequence homology, are also ADP-ribosylated by both toxins. The beta subunits have molecular weights of 36 and 35 kDa, respectively. While Gs, Gi, and Go contain a mixture of both, transducin contains only the larger (36-kDa) beta-polypeptide. The relationship of the 36- and the 35-kDa beta subunits is not defined. Although the complete sequence of the 36-kDa beta subunit of transducin has been deduced from the cDNA sequence, complete sequences of other beta subunits are not yet available so that detailed comparisons cannot be made at present. However, the proteolytic profiles of each class of the beta subunits of different G proteins are indistinguishable. The gamma subunit of bovine transducin has been completely sequenced. It has a Mr of 8400. Again complete sequences of other gamma subunits are not yet available. While the gamma subunits of Gs, Gi, and Go have identical electrophoretic mobility in SDS gels, they differ significantly in this respect from the gamma subunit of transducin. Moreover, crossover experiments point to functional differences between gamma subunits from G protein and transducin complexes. In addition, a role for beta, gamma in anchoring guanine nucleotide binding proteins to membranes has been postulated.(ABSTRACT TRUNCATED AT 400 WORDS)