Analysis of ras oncogene products by two-dimensional gel electrophoresis: evidence for protein families with distinctive molecular forms

Identification of isoprenyl modified proteins metabolically labeled with [3H]farnesyl- and [3H]geranylgeranyl-pyrophosphate

Here we describe a direct approach for two-dimensional (2-D) gel mapping of proteins that are modified by post-translational isoprenylation in mammalian cells. Briefly, transformed human amnion cells (AMA) and transfected COS-1 cells were metabolically labeled with either [3H]farnesyl-pyrophosphate or [3H]geranylgeranyl-pyrophosphate following treatment with lovastatin, which blocks the synthesis of mevalonic acid. The proteins were then separated by 2-D gel electrophoresis and electrotransferred to nitrocellulose filters. The membranes were immersed in dimethyl ether, containing 10% of 2,5-diphenyloxazole prior to fluorography. Over 40 [3H]farnesyl-labeled proteins and over 25 [3H]geranylgeranylated proteins were identified on the 2-D autoradiograms. Several [3H]farnesyl-labeled proteins exhibited the same coordinates (M(r) and pI) as their [3H]geranylgeranylated counterparts, raising the possibility that they may be substrates for both farnesyl and geranylgeranyl transferase(s). The approach offers high resolution of both farnesylated and geranylgeranylated proteins and it may serve as a powerful tool for the identification of hitherto unknown prenylated proteins as well as for the determination of prenylated protein levels, type of isoprenoid modification, and possible changes in protein prenyltransferase activity.

Relative steady-state expression of the different post-translational products of p21-H-ras from normal rat tissues. A 2D-western immunoblot study

Proteins extracts from rat cell lines or tissues expressing normal or activated c-H-ras genes, or normal N-ras gene were submitted to westernblot analysis with an anti-H, K, N p21-ras antibody. This showed that p21-H-ras products resolved into four spots (a, b, c, d) that are readily distinguishable from the normal p21-N-ras products (spots e, f, g), and also from two other products (spots a', b') present in extracts from cells which overexpress a Val12-mutated H-ras gene. Considering metabolic isotopic labeling and cell fractionation, we were able to establish the correspondance of spots a, b, c, d with the known steps of the sequential post-translational processing (farnesylation, further carboxymethylation and ultimate palmitoylation) of p21-H-ras. The palmitoylated product predominates in normal brain and still more in normal adult liver tissues, whereas its relative level decreases in proliferating liver cells.

Quantitative and/or qualitative changes in the p21-H-ras post-translational products in regenerating liver and during hepatocarcinogenesis

Using Western immunoblot analysis with the Ras-10 monoclonal antibody (MAb), we characterized 4 post-translational products of the c-H-ras gene in rat tissues, and showed that they were readily distinguished from the normal p21-N-ras or Val12 mutant p21-H-ras products. In the present study, we used this approach to compare the electrophoretic pattern of p21-H-ras during chemically induced hepatocarcinogenesis in rats with that in control liver samples. Three types of pattern were defined in the liver samples taken at various stages of malignant progression. The first type, like all the 18 normal samples analyzed, was characterized by a predominant intensity of spot a (corresponding to the palmitoylated p21-H-ras product). This was observed in all the samples at the stage of foci and only in a proportion of nodular or tumoral tissues. The second type of pattern deviates from this normal basal pattern by a higher relative level in one or more of the precursors of the fully processed p21-H-ras product. It was observed in all liver samples at the stage of nodules and in a proportion of tumors, but also in all samples from fetal or regenerative liver, thus suggesting an association with high proliferative activity of the hepatocytes. The third type of pattern was characterized by the presence of spots never detected in any of the normal rat tissue or cells that we investigated. Abnormal spots were observed in nodular liver samples and in hepatocarcinomas, indicating that they probably correspond to mutant p21-H-ras products. The fact that the 2 types of abnormal products were not constantly associated with neoplasms indicates that if they play a role in their induction or maintenance this may also be achieved in an independent way, perhaps, but not necessarily, involving another mutation of the ras gene.

Expression of ras-like proteins in embryonic and adult cells of Xenopus laevis

A polyclonal antibody raised against v-Ha-ras p21 was purified and its specificity was checked on Ha-ras transformed cell lines. It was used to immunoprecipitate p21 from different Xenopus laevis cell types: brain cells, blood cells, and embryonic material. By one-dimensional Western blot analysis, we show that ras p21 is synthesized very early in oogenesis and accumulates throughout vitellogenesis. The ras p21 content, estimated to be 1.1 ng in the full-grown oocyte, remains constant during oocyte maturation and egg cleavage. Increase in the amount of ras p21 occurs at the beginning of neurulation. Two-dimensional Western blot patterns reveal the presence of multiple molecular forms of p21 in all Xenopus cell types studied. The numerous resolved polypeptides were ascribed to the expression of at least two different ras genes. Furthermore, specific charge modifications of the ras polypeptides are observed in brain, blood, and embryonic cells. During oogenesis and early embryonic development, differences in two-dimensional patterns mainly concern variations in the relative amounts of the different polypeptides. The results are discussed in relation to the well documented synthesis activities of the growing oocyte and of the early developing embryo.

Localization of ras antigenicity in rat hepatocyte plasma membrane and rough endoplasmic reticulum fractions

We have examined the antigenicity of plasma membrane (PM) and rough microsomal (RM) fractions from rat liver using anti-ras monoclonal antibodies 142-24EO5 and Y13-259 and immunochemistry as well as electron microscope immunocytochemistry. Proteins immunoprecipitated with monoclonal antibody 142-24E05 were separated using single-dimensional gradient-gel electrophoresis. The separated proteins were then blotted onto nitrocellulose sheets and incubated with [alpha-32P]GTP. Radioautograms of blots indicated the presence of specific 21.5- and 22-kDa labeled proteins in the PM fraction. A 23.5-kDa [alpha-32P] GTP-binding protein was detected in immunoprecipitates of both PM and RM fractions. Monoclonal antibody Y13-259 reacted only with the 21.5-kDa [alpha-32P] GTP-binding protein in the plasma membrane fraction. When anti-ras monoclonal antibody 142-24E05 and the immunogold technique were applied to membrane fractions using a preembedding immunocytochemical method, specific labeling was observed in association with both vesicular structures and membrane sheets in the PM fraction but only with electron-dense vesicular structures in the RM fraction. Thus ras antigenicity is associated with hepatocyte plasma membranes and ras-like antigenicity is probably associated with vesicular (secretory/endocytic) elements in both plasma membrane and rough microsomal preparations.

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.

Evidence of enhancement of the ras oncogene protein product (p21) in a spectrum of human tumors

Using a direct binding liquid competition radioimmunoassay, the amount of the ras oncogene protein product, p21, was quantitated in a variety of human tumors and adjacent apparently normal tissues. In 48 of 50 matched tumor and normal tissue biopsy specimens from 50 patients, more ras p21 was detected in the tumor than in its normal counterpart. Twenty-five of 28 breast tumors demonstrated more ras p21 than the average of the values obtained for fibroadenomas. Furthermore, in 17 of the 19 cases studied, over 20% more ras p21 was observed in breast carcinomas compared with their respective normal counterparts. More ras p21 was also demonstrated in the majority of tumors of the stomach, lung, colon and bladder compared with their respective adjacent normal tissues. Our data therefore indicate that ras p21 expression is quantitatively enhanced in many human tumors originating from several different tissue types.

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.

Sensitive ELISA for the gp120 and gp160 surface glycoproteins of HIV-1

We have used a panel of polyclonal and monoclonal antibodies against gp120 and gp160, the envelope glycoproteins of human immunodeficiency virus type 1, to create rapid, simple, and sensitive twin-site sandwich ELISA specific for gp120 and gp160 or for gp160 alone. These assays can detect 500 COS cells in a population transiently transfected with a construct encoding gp120 and gp160, or 50 pg of recombinant gp160. We estimate that the mean amount of gp120 + gp160 in the transfected population is equivalent to 2.5 x 10(6) molecules per cell, 40-50% of which can be recovered from the culture medium as gp120 after 24 hours. The ELISA can be adapted to assess whether gp120 is detectable in the sera of HIV-1-infected persons: we show that gp120/gp160 is completely stable in normal human serum for at least 24 hours at 37 degrees C.

Increased amount of a mitochondria-associated 21 x 10(3) dalton protein in human gastrointestinal tumors

Differences in the structure and number of mitochondria in tumor cells were found. Using isoelectrofocusing two-dimensional polyacrylamide gel electrophoresis which allows detection of alterations in the protein pattern of tumor mitochondria, we studied both quantitative and qualitative changes in the mitochondrial protein pattern of human gastrointestinal tumors and corresponding normal matrix tissues. One low molecular protein spot was found to be quantitatively changed in the tumors. The approximate molecular weight was 21 x 10(3) daltons and the pI value 5.7.

Two-dimensional polyacrylamide gel electrophoresis analysis of phosphorylated, membrane-localized ras p21 proteins

The ras p21 oncogene product migrates as a heterogeneous series of polypeptides as resolved by both one- and two-dimensional polyacrylamide gel electrophoresis (PAGE). We have prepared polyclonal rat serum antibody to ras p21 and used this as well as monoclonal antibodies to immunoprecipitate forms of p21 synthesized in vivo in transformed NIH3T3 cells. Two-dimensional PAGE of p21 resolved two distinct groups of polypeptides, one acidic (pI 4.8-5.3) which we call the "A" forms, and one less acidic or more basic (pI 6.5-7.0) which we call the "B" forms. It is the membrane-localized, B forms of v-ras-Ha p21 that are predominantly phosphorylated in vivo.

Absolute values of ras p21 defined by direct binding liquid competition radioimmunoassays

Several distinct and high-conserved genes comprise the ras gene family of rodents and humans, i.e., rodent Harvey and Kirsten, and human Harvey, Kirsten and neuroblastoma. Transformation, either by a point-mutation resulting in a change in one amino acid of the 21 kDa ras gene product (p21), or by increased expression of ras p21, has been demonstrated to be mediated by members of this gene family. We report here the development of direct binding liquid competition radioimmunoassays for the detection and quantitation of the ras oncogene and proto-oncogene products. Using these radioimmunoassays and ras p21 purified from Escherichia coli containing the full-length T24 mutant human Harvey ras gene protein product as a standard, we have defined the actual amount of ras p21 per micrograms of total cellular protein, or per cell, in various ras transformed and 'normal' mammalian cell lines. One of the radioimmunoassays developed is group-specific, since the antigenic determinant recognized is shared by both the point-mutated and proto-forms of Harvey, Kirsten and neuroblastoma members of the ras gene family, while the second may be termed type-selective, since it recognizes an antigenic determinant localized primarily on the Harvey ras p21. Both radioimmunoassays are interspecies, since they detect a ras p21 antigenic determinant common to cells of human and rodent origin. These studies thus describe the first means for defining absolute values of any oncogene or proto-oncogene protein product. The assays described, when used in combination with specific c-DNA probes to define specific ras proto-oncogenes or point-mutated oncogenes being expressed, will now permit truly quantitative analyses of ras p21 expression in experimental cell culture systems, animal models and human biopsy material.

Nucleotide sequence and characterization of the 5' flanking region of the rat Ha-ras protooncogene

Thyrotropin and cAMP stimulate growth of FRTL5 rat thyroid cells and increase c-Ha-ras mRNA levels. To study the mechanism by which thyrotropin enhances c-Ha-ras expression in the thyroid, we constructed a genomic library of FRTL5 DNA in the bacteriophage vector EMBL3. Using a v-Ha-ras probe (0.7-kilobase Sst I-Pst I fragment), we isolated eight clones containing portions of the FRTL5 c-Ha-ras gene. Restriction mapping of one of these clones revealed a structure very similar to that previously reported for the rat c-Ha-ras gene. We determined the nucleotide sequence of exon 1 as well as 1.15 kilobases upstream from exon 1. Blot-hybridization analysis of FRTL5 thyroid cell mRNA was performed with three DNA fragments upstream of exon 1. Two of these probes were Pst I-Pst I fragments 0.4 and 0.55 kilobases long, 1.15 and 0.6 kilobases upstream of exon 1, respectively. The third probe, a 0.6-kilobase Pst I-HindIII fragment, was immediately upstream of exon 1 and included 54 base pairs of the 5' end of exon 1. The data revealed an upstream ("-1") exon, consistent with the homology between the nucleotide sequences of our clone and the human c-Ha-ras gene in this region. Primer extension of a synthetic 30-mer oligonucleotide probe complementary to exon +1 on a FRTL5 mRNA template revealed three transcription start (cap) sites, 182, 169, and 153 bases upstream of the ATG codon. "CCAAT boxes" are present 65 and 100 bases upstream from the initial cap site. Three "GC boxes" and two C + G-rich inverted repeats characteristic of the binding site for the transcription regulation factor Sp1 are present in the 177 base pairs upstream of the initial cap site. Comparison of the rat and human c-Ha-ras -1 exons showed an area of poor homology immediately upstream of the human cap sites, followed further upstream by a region of close homology (approximately equal to 60 base pairs). The nucleotide sequence of the rat -1 exon is more similar to the v-ras sequence than is the human. The v-ras gene is truncated relative to both human and rat -1 exons.