Only membrane-associated RSV src proteins have amino-terminally bound lipid

N-myristoyl transferase assay using phosphocellulose paper binding

N-myristoyl-CoA:protein N-myristoyl transferase is the enzyme that catalyzes the covalent transfer of myristic acid to the NH2-terminal glycine residue of a protein, or peptide, substrate. We have established a new, rapid, reliable, and inexpensive myristoyl-CoA:protein N-myristoyl transferase assay. This N-myristoyl transferase assay is based on the binding of the [3H]myristoylated peptide to a P81 phosphocellulose paper matrix and is more convenient for assaying multiple samples than existing procedures. Two peptides, derived from the N-terminal sequences of the type II catalytic subunit of cAMP-dependent protein kinase and pp60src, were used as substrates. A survey of rat and bovine tissue extracts demonstrated that in both cases brain contained the highest NMT activity (i.e., brain greater than spleen greater than heart greater than liver). Under the assay conditions used, the rate of myristoylation was linear for 10 min and with up to 4.0 mg/ml of brain extract.

Molecular characterization of a protein-tyrosine-phosphatase enriched in striatum

A cDNA clone encoding a neural-specific putative protein-tyrosine-phosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48) has been isolated from a rat striatal cDNA library. The deduced amino acid sequence predicts a protein of approximately 369 amino acids with a strong homology to other members of the family of protein-tyrosine-phosphatases. In vitro translation produces a protein with an apparent molecular mass of 46 kDa. A potential attachment mechanism to the cytoplasmic membrane is suggested by a myristoylation amino acid-consensus sequence at the N terminus of the protein. RNA analyses of various regions of rat brain reveal a 3-kilobase (kb) and a 4.4-kb mRNA. The 3-kb mRNA is highly enriched within the striatum relative to other brain areas and has been termed a "striatum enriched phosphatase" (STEP). In contrast, the 4.4-kb message is most abundant in the cerebral cortex and rare in the striatum. These two messages appear to be alternatively processed RNA transcripts of a single gene.

Cell/substratum adhesions in RSV-transformed rat fibroblasts

Cell/substratum adhesions have been studied in rat fibroblasts transformed by a ts-mutant of Rous sarcoma virus (LA-29) using light and electron microscopy and a variety of preparative methods including immunolabeling. Cells were studied both during the process of transformation, i.e., shifting from 39 degrees to 35 degrees C, and in a fully transformed state (passaged at 35 degrees C continuously). The typical focal contacts observed at 39 degrees C (restrictive temperature) were replaced by "point-contacts" (100-200 per cell) which were classified by immunolabeling as podosome-like adhesions containing actin, beta 1 integrin subunit, vinculin, talin, alpha-actinin, and small membrane patches containing clathrin and integrin. Tyrosine-phosphorylated proteins and pp60src were found in association with groups of small particles on the protoplasmic surface of ventral membranes by gold immunolabeling. Both types of point-contacts were visualized by electron microscopy of ultrathin sections and shadowed replicas and characterized by gold immunolabeling wherever possible. The overall composition of podosome-like adhesions is similar to focal contacts but there are differences in the three-dimensional organization of the microfilaments and in the topography of vinculin which is associated more with actin filaments than with the plasma membrane. The presence of talin and extracellular matrix receptor in podosomes together with the adhesive properties of these actin-containing structures argues against the hypothesis that pp60src affects the interaction of actin with the plasma membrane by phosphorylating the fibronectin receptor and/or other associated proteins.

Site-directed mutagenesis of the SH2- and SH3-coding domains of c-src produces varied phenotypes, including oncogenic activation of p60c-src

The products of the viral and cellular src genes, p60v-src and p60c-src, appear to be composed of multiple functional domains. Highly conserved regions called src homology 2 and 3 (SH2 and SH3), comprising amino acid residues 88 to 250, are believed to modulate the protein-tyrosine kinase activity present in the carboxy-terminal halves of the src proteins. To explore the functions of these regions more fully, we have made 34 site-directed mutations in a transformation-competent c-src gene encoding phenylalanine in place of tyrosine 527 (Y527F c-src). Twenty of the new mutations change only one or two amino acids, and the remainder delete small or large portions of the SH2-SH3 region. These mutant alleles have been incorporated into a replication-competent Rous sarcoma virus vector to examine the biochemical and biological properties of the mutant proteins after infection of chicken embryo fibroblasts. Four classes of mutant proteins were observed: class 1, mutants with only slight differences from the parental gene products; class 2, mutant proteins with diminished transforming and specific kinase activities; class 3, mutant proteins with normal or enhanced specific kinase activity but impaired biological activity, often as a consequence of instability; and class 4, mutant proteins with augmented biological and catalytic activities. In general, there was a strong correlation between total kinase activity (or amounts of intracellular phosphotyrosine-containing proteins) and transforming activity. Deletion mutations and some point mutations affecting residues 109 to 156 inhibited kinase and transforming functions, whereas deletions affecting residues 187 to 226 generally had positive effects on one or both of those functions, confirming that SH2-SH3 has complex regulatory properties. Five mutations that augmented the transforming and kinase activities of Y527F c-src [F172P, R175L, delta(198-205), delta(206-226), and delta(176-226)] conferred transformation competence on an otherwise normal c-src gene, indicating that mutations in SH2 (like previously described lesions in SH3, the kinase domain, and a carboxy-terminal inhibitory domain) can activate c-src.

Isolation and characterization of fra-2, an additional member of the fos gene family

Antiserum raised against a Fos peptide (amino acids Lys-127 to Arg-140 of chicken c-Fos) recognizes a 46-kDa Fos-related protein in cell lysates of growth-stimulated chicken embryo fibroblasts. Induction of the 46-kDa protein is transient but is slightly prolonged relative to c-Fos following growth stimulation. Using a mixed oligonucleotide probe encoding the peptide antigen, we have cloned the chicken genomic locus that encodes this protein and have determined its gene structure. This locus consists of four exons, each of which has some homology with the corresponding exons of the chicken c-fos gene, and it expresses a 6-kilobase mRNA after growth stimulation. The deduced amino acid sequence of the gene (323 amino acids) contains a "leucine zipper" and includes five distinct regions that exhibit strong sequence homology to the recently identified fos-related antigens Fra-1 (rat) and FosB (mouse) as well as c-Fos. Since the other regions of the gene have little homology to any of these three proteins, this gene was named "fra-2." When the fra-2 gene was overexpressed by an avian retrovirus vector, it caused transformation of chicken embryo fibroblasts. The fra-2 gene product formed a complex in cells with the c-Jun protein, indicating that c-fos and fra-2 share biological and biochemical functions.

Site-directed mutagenesis of the SH2- and SH3-coding domains of c-src produces varied phenotypes, including oncogenic activation of p60c-src

The products of the viral and cellular src genes, p60v-src and p60c-src, appear to be composed of multiple functional domains. Highly conserved regions called src homology 2 and 3 (SH2 and SH3), comprising amino acid residues 88 to 250, are believed to modulate the protein-tyrosine kinase activity present in the carboxy-terminal halves of the src proteins. To explore the functions of these regions more fully, we have made 34 site-directed mutations in a transformation-competent c-src gene encoding phenylalanine in place of tyrosine 527 (Y527F c-src). Twenty of the new mutations change only one or two amino acids, and the remainder delete small or large portions of the SH2-SH3 region. These mutant alleles have been incorporated into a replication-competent Rous sarcoma virus vector to examine the biochemical and biological properties of the mutant proteins after infection of chicken embryo fibroblasts. Four classes of mutant proteins were observed: class 1, mutants with only slight differences from the parental gene products; class 2, mutant proteins with diminished transforming and specific kinase activities; class 3, mutant proteins with normal or enhanced specific kinase activity but impaired biological activity, often as a consequence of instability; and class 4, mutant proteins with augmented biological and catalytic activities. In general, there was a strong correlation between total kinase activity (or amounts of intracellular phosphotyrosine-containing proteins) and transforming activity. Deletion mutations and some point mutations affecting residues 109 to 156 inhibited kinase and transforming functions, whereas deletions affecting residues 187 to 226 generally had positive effects on one or both of those functions, confirming that SH2-SH3 has complex regulatory properties. Five mutations that augmented the transforming and kinase activities of Y527F c-src [F172P, R175L, delta(198-205), delta(206-226), and delta(176-226)] conferred transformation competence on an otherwise normal c-src gene, indicating that mutations in SH2 (like previously described lesions in SH3, the kinase domain, and a carboxy-terminal inhibitory domain) can activate c-src.

Suppression of Rous sarcoma virus-induced tumor formation by preinfection with viruses encoding src protein with novel N termini

Two recovered avian sarcoma viruses (rASVs), rASV157 and rASV1702, encode src products which contain novel, nonmyristoylated N-terminal amino acids. These viruses transform chicken embryo fibroblasts and cause tumors in chicks. However, the tumors rASVs induce are small and regress within 2 weeks. To determine whether this regression results from weak tumorigenicity or from the active immunity of the host, we injected 1-week-old chicks with rASV and several days later injected the chicks with challenge virus of a different subgroup. Of the rASV1702-preinfected chicks challenged 5 days later with Rous sarcoma virus (RSV), 40% showed no subsequent tumor formation and 60% formed tumors which regressed within 1 week. The potency of this protective effect depended on the dosage of preinfection virus used and increased as the interval between preinfection and challenge infection was lengthened (when the interval was 9 days, none of the challenged chicks formed tumors). rASV157-preinfected chicks challenged with RSV after 9 days showed only partial protection: 42% formed tumors which regressed, whereas 58% formed tumors which continued to grow. Challenging rASV-preinfected chicks with Fujinami sarcoma virus or a RSV vector encoding the v-fps oncogene or polyomavirus middle T resulted in no suppression of tumor formation. Preinfection with src mutants or a RSV vector encoding polyomavirus middle T antigen, both of which induce slow-growing tumors, failed to elicit the protective effect. Finally, a novel N-terminal domain encoded by rASV1702 src was shown to be involved in but not sufficient for full protection. These data indicate that determinants on or induced by rASV157 and rASV1702 can elicit a potent protection against the tumorigenic potential of RSV-encoded p60v-src.

Acylation of viral and eukaryotic proteins

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A rat mutant cell clone showing temperature-dependent transformed phenotypes with functional expression of the src gene product

The cellular mutant B814 isolated from a Fischer rat cell line shows temperature-sensitivity of focus formation on infection with Moloney murine sarcoma virus (Mo-MSV) and Rous sarcoma virus (RSV). An RSV-transformed clone (S814-2) isolated from B814 cells shows temperature-sensitive transformed phenotypes for morphology, growth in soft agar, and glucose uptake. The expression, phosphorylation, and tyrosine kinase activity of pp60v-src in S814-2 were not affected at the nonpermissive temperature, and virus rescued from this clone had wild-type transforming ability, suggesting that a cellular factor altered in S814-2 is responsible for the cellular steps of transformation after the function of pp60v-src. In addition, the cellular 36K protein, a possible candidate as a target of pp60v-src, was phosphorylated at the nonpermissive temperature in S814-2, indicating that phosphorylation of the 36K protein is not correlated with transformed phenotypes.

N-myristoylation of p60src. Identification of a myristoyl-CoA:glycylpeptide N-myristoyltransferase in rat tissues

A 16-residue synthetic peptide corresponding to the N-terminal sequence of p60src was used as the acyl acceptor in an assay for myristoyl-CoA:glycylpeptide N-myristoyltransferase in rat tissues. An additional C-terminal tyrosine amide was added to this peptide to facilitate radioiodination and enhance detectability. Reverse-phase h.p.l.c. enabled the simultaneous detection and quantification of the peptide substrate and its N-myristoylated product. N-Myristoyltransferase activity was highest in the brain with decreasing activities in lung, small intestine, kidney, heart, skeletal muscle and liver. Brain activity was distributed approximately equally between the 100,000 g pellet and supernatant fractions. The soluble enzyme exhibited a Kappm of 20 microM for the src peptide and an optimum between pH 7.0 and 7.5. Maximum N-acylating activity was seen with myristoyl (C14:0)-CoA with lower activities found with the C10:0-CoA and C12:0-CoA homologues. No activity was obtained with palmitoyl (C18:0)-CoA but this derivative inhibited N-myristoyltransferase activity greater than 50% at equimolar concentrations with myristoyl-CoA. With a decapeptide corresponding to the N-terminal sequence of the cyclic AMP-dependent protein kinase catalytic subunit as the acyl acceptor, the brain enzyme displayed a Kapp.m of 117 microM and was about 14-fold less catalytically effective than with the p60src acyl acceptor. Transferase activity was also seen with a 16-residue peptide corresponding to the N-terminal sequence of the HIV p17gag structural protein. Inhibition studies with shorter src peptide analogues indicated an enzyme specificity for the p60src acyl acceptor beyond 9 residues.

Avian sarcoma viruses

Twelve independent isolates of avian sarcoma viruses (ASVs) can be divided into four groups according to the transforming genes harbored in the viral genomes. The first group is represented by viruses containing the transforming sequence, src, inserted in the viral genome as an independent gene; the other three groups of viruses contain transforming genes fps, yes or ros fused to various length of the truncated structural gene gag. These transforming sequences have been obtained by avian retroviruses from chicken cellular DNA by recombination. The src-containing viruses code for an independent polypeptide, p60src; and the representative fps, yes and ros-containing ASVs code for P140/130gag-fps, P90gag-yes and P68gag-ros fusion polypeptides respectively. All of these transforming proteins are associated with the tyrosine-specific protein kinase activity capable of autophosphorylation and phosphorylating certain foreign substrates. p60src and P68gag-ros are integral cellular membrane proteins and P140/130gag-fps and P90gag-yes are only loosely associated with the plasma membrane. Cells transformed by ASVs contain many newly phosphorylated proteins and in most cases have an elevated level of total phosphotyrosine. However, no definitive correlation between phosphorylation of a particular substrate and transformation has been established except that a marked increase of the tyrosine phosphorylation of a 34,000 to 37,000 dalton protein is observed in most ASV transformed cells. The kinase activity of ASV transforming proteins appears to be essential, but not sufficient for transformation. The N-terminal domain of p60src required for myristylation and membrane binding is also crucial for transformation. By contrast, the gag portion of the FSV P130gag-fps is dispensable for in vitro transformation and removal of it has only an attenuating effect on in vivo tumorigenicity. The products of cellular src, fps and yes proto-oncogenes have been identified and shown to also have tyrosine-specific protein kinase activity. The transforming potential of c-src and c-fps has been studied and shown that certain structural changes are necessary to convert them into transforming genes. Among the cellular proto-oncogenes related to the four ASV transforming genes, c-ros most likely codes for a growth factor receptor-like molecule. It is possible that the oncogene products of ASVs act through certain membrane receptor(s) or enzyme(s), such as protein kinase C, in the process of cell transformation.

Acylation of proteins with myristic acid occurs cotranslationally

Several proteins of viral and cellular origin are acylated with myristic acid early during their biogenesis. To investigate the possibility that myristylation occurred cotranslationally, the BC3H1 muscle cell line, which contains a broad array of myristylated proteins, was pulse-labeled with [3H]myristic acid. Nascent polypeptide chains covalently associated with transfer RNA were isolated subsequently by ion-exchange chromatography. [3H]Myristate was attached to nascent chains through an amide linkage and was identified by thin-layer chromatography after its release from nascent chains by acid methanolysis. Inhibition of cellular protein synthesis with puromycin resulted in cessation of [3H]myristate-labeling of nascent chains, in agreement with the dependence of this modification on protein synthesis in vivo. These data represent a direct demonstration that myristylation of proteins is a cotranslational modification.

The CML-specific P210 bcr/abl protein, unlike v-abl, does not transform NIH/3T3 fibroblasts

The v-abl oncogene of the Abelson murine leukemia virus (A-MuLV) is known to efficiently transform NIH/3T3 fibroblasts in vitro and to cause an acute lymphosarcoma in susceptible murine hosts. The role of its relative, the bcr/abl gene product, in the etiology of human chronic myelogenous leukemia (CML) remains speculative. To assess the transforming properties of the bcr/abl gene product, complementary DNA clones encoding the CML-specific P210 bcr/abl protein were expressed in NIH/3T3 fibroblasts. In contrast to the v-abl oncogene product P160, the P210 bcr/abl gene product did not transform NIH/3T3 cells. Cell lines were isolated that expressed high levels of the P210 bcr/abl protein but were morphologically normal. During the course of these experiments, a transforming recombinant of bcr/abl was isolated which fuses gag determinants derived from helper virus to the NH2-terminus of the bcr/abl protein. This suggests that a property of viral gag sequences, probably myristylation-dependent membrane localization, must be provided to bcr/abl for it to transform fibroblasts.

Rat c-raf oncogene activation by a rearrangement that produces a fused protein

In a previous study, activated rat c-raf was detected by an NIH 3T3 cell transfection assay, and a rearrangement was demonstrated in the 5' half of the sequence of the gene. In the present study, the cDNAs of normal and activated rat c-raf were analyzed. Results showed that the activated c-raf gene is transcribed to produce a fused mRNA, in which the 5' half of the sequence is replaced by an unknown rat sequence. This mRNA codes a fused c-raf protein. The normal and activated c-raf cDNAs were each connected to the long terminal repeat of Rous sarcoma virus and transfected into NIH 3T3 cells. Only the activated form had transforming activity. We conclude that the rearrangement is responsible for the activation of c-raf.

Studies on the cellular location, physical properties and endogenously attached lipids of acylated proteins in human squamous-carcinoma cell lines

The location of cell proteins with covalently attached lipid was examined in two human squamous-carcinoma cell lines. Cells were labelled with either palmitic acid or myristic acid and disrupted by sonication, followed by differential centrifugation of the cell lysates. SDS/polyacrylamide-gel electrophoresis of the resulting cell fractions indicated that most of the palmitate-labelled proteins were found in cell membranes, whereas most of the myristate-labelled proteins were found in the cytosol, although some were located in cell membranes. Experiments with lipid-labelled proteins extracted with the phase-separable detergent Triton X-114 showed that palmitate-labelled proteins behaved as hydrophobic proteins, partitioning into the lower phase of the detergent, whereas most of the myristate-labelled proteins remained in the upper phase. Although one of these cell lines expressed large amounts of epidermal-growth-factor receptor, this could not be labelled by either myristic acid or palmitic acid, whereas transferrin receptor was labelled by palmitic acid. The lipids normally attached to cell proteins in these two human squamous-carcinoma cell lines were characterized by labelling the cells with [3H]acetate. The labelled cell proteins were exhaustively extracted with organic solvents, and subjected to sequential alkaline and acid hydrolyses to release the attached lipids, which were then analysed by h.p.l.c. Most of the lipid released by the alkaline treatment chromatographed as palmitic acid or stearic acid, whereas the subsequent acid treatment released myristic acid as well as some palmitic acid and stearic acid. No other fatty acids apart from these were found attached to cell proteins.

Myristylation is required for intracellular transport but not for assembly of D-type retrovirus capsids

The role of myristylation, a fatty acid modification of nascent polypeptides, in the assembly and intracellular transport of D-type retroviral capsids was investigated through the use of oligonucleotide-directed mutagenesis. Myristic acid is normally esterified through an amide linkage to a glycine residue at the amino terminus of the Mason-Pfizer monkey virus gag gene products. Mutant pA-1, which has a codon for valine substituted for that of the normally myristylated glycine, is completely noninfectious. While the mutant gag polyprotein precursors are synthesized at normal levels, they are not myristylated and are not cleaved to the mature virion proteins. No extracellular virus particles are released from mutant pA-1-infected cells, but intracytoplasmic A-type particles (capsids) accumulate in the cytoplasm. Since none of the intracellular capsids can be found associated with the plasma membrane, these results strongly suggest that myristylation is a critical signal for intracytoplasmic transport of completed viral capsids to their normal site of budding and release.

Association of p60src with Triton X-100-resistant cellular structure correlates with morphological transformation

More than 70% of wild-type Rous sarcoma virus p60v-src was found to be associated with a cellular structure resistant to nonionic detergent extraction that consists primarily of cytoskeletal proteins. On the other hand, nontransforming src proteins, including cellular p60c-src, nonmyristoylated forms, and those inactive in protein kinase, were found in the fraction solubilized by the detergent extraction. p60c-src was detergent-soluble even in transformed cells, suggesting that the association of p60v-src is not a result of cell transformation. Analyses with a variety of Rous sarcoma virus mutants showed a good correlation between the degree of association with the detergent-resistant structure and the extent of cell transformation caused by mutant src proteins, suggesting that this association may be significant for the process of cell transformation by Rous sarcoma virus.

Rat c-raf oncogene activation by a rearrangement that produces a fused protein

In a previous study, activated rat c-raf was detected by an NIH 3T3 cell transfection assay, and a rearrangement was demonstrated in the 5' half of the sequence of the gene. In the present study, the cDNAs of normal and activated rat c-raf were analyzed. Results showed that the activated c-raf gene is transcribed to produce a fused mRNA, in which the 5' half of the sequence is replaced by an unknown rat sequence. This mRNA codes a fused c-raf protein. The normal and activated c-raf cDNAs were each connected to the long terminal repeat of Rous sarcoma virus and transfected into NIH 3T3 cells. Only the activated form had transforming activity. We conclude that the rearrangement is responsible for the activation of c-raf.

Intramembrane translocation and posttranslational palmitoylation of the chloroplast 32-kDa herbicide-binding protein

The 32-kDa herbicide-binding protein, a component of photosystem II, is synthesized as a membrane-associated 33.5-kDa precursor within the chloroplast. We show that membrane attachment of the precursor and processing to the 32-kDa form occur in the unstacked stromal lamellae. Once processed, the 32-kDa protein translocates, within the thylakoids, to the topologically distinct stacked granal lamellae. Posttranslational palmitoylation of the processed 32-kDa protein is also shown to occur. This modification takes place in a membrane-protected domain and is mainly confined to the protein assembled in the granal lamellae, where functional photosystem II centers are concentrated.

The cytoskeletal protein vinculin contains transformation-sensitive, covalently bound lipid

Vinculin, which is associated with the cytoskeleton of many cells, has been suggested as a possible linker between microfilament bundles and the plasma membrane. Here it will be shown that fatty acid is covalently attached to vinculin in vivo. Furthermore, in chicken embryo fibroblasts infected with a temperature-sensitive mutant of Rous sarcoma virus, tsNY68, the acylation of vinculin at the permissive temperature was less than one-third that at the nonpermissive temperature. Thus, the covalent binding of lipid to vinculin is a transformation-sensitive event. The covalent modification of vinculin by lipids could be directly or indirectly involved in its reversible association with membranes. This modification may also provide a mechanism to alter the organization of vinculin within cells and thereby play a regulatory role in anchoring or stabilizing microfilament bundles at plasma membranes.

NH2-terminal sequences of two src proteins that cause aberrant transformation

Two isolates of recovered avian sarcoma viruses (rASVs), rASV157 and rASV1702, transform cells in culture, but have greatly reduced in vivo tumorigenicity. The src proteins of rASV157 and rASV1702 have alterations within their NH2 termini, are not myristoylated, and have an altered subcellular localization. We have molecularly cloned and determined the nucleotide sequences of the src genes of rASV157 and rASV1702. We found that their src proteins have unusual NH2 termini: the rASV157 src protein NH2 terminus consists of 30 amino acids of the env signal peptide attached to Ser-6 of the src sequence, while the rASV1702 src protein NH2 terminus consists of 45 amino acids of the env signal peptide attached to Ala-76 of the src sequence. Expression of recombinant Rous sarcoma virus constructs containing the molecularly cloned rASV src genes produced src proteins with the same properties as those of the parental viruses. Our results suggest that the NH2-terminal structures are responsible for many unusual properties of the mutant src proteins.

Avian sarcoma virus gag-fps and gag-yes transforming proteins are not myristylated or palmitylated

The transforming proteins of several avian sarcoma viruses were examined for evidence of covalently attached fatty acids. While the product of the viral src gene could be readily labeled biosynthetically with [3H]myristic acid, the gag-onc transforming proteins of Fujinami sarcoma virus, PRCII, PRCIIp, and Y73 avian sarcoma viruses were not readily labeled with either [3H]myristate or [3H]palmitate. Thus, avian gag-onc proteins appear to lack modifications shared by mammalian gag and gag-onc proteins, and the products of the oncogenes src, tck, and ras.

Regulation of cellular morphology by the Rous sarcoma virus src gene: analysis of fusiform mutants

We have been interested in how Rous sarcoma virus (RSV) influences transformed cell morphology and compared the molecular properties of chicken embryo cells (CEC) infected with mutants of RSV that induce the fusiform transformed cell morphology with those of CEC infected by wild-type RSV, which induces the more normal round transformed cell morphology. We looked for properties shared by all fusiform mutant-infected cells, because these may be responsible for maintaining the fusiform morphology. Five different fusiform mutants, two wild-type RSVs, and one wild-type back revertant of a fusiform mutant were studied. In the fusiform mutant-infected cells, the localization and myristylation of pp60src were determined and the extent of expression of the extracellular matrix protein fibronectin was examined at both the mRNA and protein levels. The phosphorylation of vinculin on tyrosine also was examined in the same CEC. Within all fusiform mutant-transformed CEC, pp60src was dramatically absent from the adhesion plaque sites normally seen in cells transformed with wild-type RSV, and these transformed CEC all expressed more fibronectin mRNA and protein in the extracellular matrix than did the wild-type RSV-transformed CEC. The absence of pp60src from the adhesion plaques was not due to lack of myristylation of the src protein, and tyrosine phosphorylation of vinculin was not related to fibronectin expression. These results suggest that the inverse relationship between pp60src in the adhesion plaques and fibronectin expression in the extracellular matrix may be interconnected phenomena and could be related to the maintenance of the fusiform transformed morphology.

Phosphatidylinositol kinase activity in virus-transformed and nontransformed cells

We assayed phosphatidylinositol (PI) kinase (EC 2.7.1.67) activity in detergent extracts of nontransformed or virus-transformed cells. Nontransformed chicken embryo fibroblasts (CEF) contain PI kinase activity with an apparent specific activity of 20 pmol/min per mg of protein. This activity sedimented as a single peak with a molecular weight of approximately 60,000 in a glycerol gradient, although immunoprecipitation with anti-p60src sera showed that the PI kinase activity is distinct from p60c-src. Extracts from CEF transformed by Rous sarcoma virus, Fujinami sarcoma virus, or avian sarcoma virus UR2 showed no elevation of PI kinase activity over nontransformed CEF. Removal of the oncogene products from extracts by immunoprecipitation did not change the level of PI kinase activity in extracts, suggesting that putative virus-coded PI kinases do not make a significant contribution to overall levels of PI kinase activity in transformed cells. Additionally, P140gag-fps was separated from cellular PI kinase by phosphocellulose chromatography. This partially purified fraction contained low PI kinase activity distinct from P140gag-fps, indicating that P140gag-fps has no detectable PI kinase activity.

Processing of p60v-src to its myristylated membrane-bound form

p60src of wild-type Rous sarcoma virus is myristylated at its N-terminal glycine residue. We have shown previously that this myristylation is necessary for p60src membrane association and for cell transformation by using src mutants with alterations within the N-terminal 30 kilodaltons of p60src. In this study we analyzed the process of p60src myristylation in wild type- and mutant-infected cells. All myristylated src proteins examined lack the initiator methionine, but two mutant src proteins lacking the initiator methionine are not myristylated, indicating that removal of the initiator methionine and myristylation are not obligatorily coupled. Analysis of the kinetics of myristylation and the association of p60src with cellular proteins p50 and p90 indicated that myristylation occurs before p60src becomes membrane associated and that transient association with p50 and p90 occurs regardless of myristylation. Myristylation is required for stable association of p60src with the plasma membrane but is not sufficient for membrane association. A mutant with an src deletion of amino acids 169 through 264 has an src protein that is myristylated but not membrane bound, remaining stably associated with p50 and p90. This mutant is transformation defective. Several N-terminal deletion mutants possessing tyrosine kinase activity have myristylated and membrane-bound src proteins but are not fully active in cell transformation, suggesting that additional N-terminal functional domains exist.

Intracellular localization and processing of pp60v-src proteins expressed by two distinct temperature-sensitive mutants of Rous sarcoma virus

The transforming protein of Rous sarcoma virus, pp60v-src, is known to be a tyrosine protein kinase, but the mechanism of cell transformation remains unclear. In further investigating pp60v-src structure and function, we have analyzed two temperature-sensitive (ts) Rous sarcoma virus src gene mutants, tsLA29 and tsLA32. The mutations in tsLA29 and tsLA32 map in the carboxy-terminal region and the amino-terminal half of pp60v-src, respectively, and encode mutant proteins with either temperature-labile (tsLA29) or -stable (tsLA32) kinase activities. Here we examined the intracellular processing and localization of these pp60v-src mutants and extended our characterization of transformation parameters expressed by cells infected by the Rous sarcoma virus variants. No obvious defects in functional integrity of the tsLA32 pp60v-src could yet be demonstrated, whereas the tsLA29 pp60v-src was perturbed not only in kinase activity, but also in aspects of protein processing and localization. Analysis of transformation parameters expressed by infected cells demonstrated the complete temperature lability of both mutants.

Processing and fatty acid acylation of RAS1 and RAS2 proteins in Saccharomyces cerevisiae

We demonstrate the pathway for the biosynthesis of RAS1 and RAS2 gene products of Saccharomyces cerevisiae leading to their localization in membranes. The primary translation products of these genes are detected in a soluble fraction. Shortly after synthesis, these precursor molecules are converted to forms that migrate slightly faster than the precursor forms on a NaDodSO4/polyacrylamide gel. These processed proteins are further modified by fatty acid acylation, which is detected by [3H]palmitic acid labeling. The acylated derivatives are found exclusively in cell membranes, indicating the translocation of the RAS proteins from cytosol to membranes during maturation process. The attached fatty acids can be released by mild alkaline hydrolysis, suggesting that the linkage between the fatty acid and the protein is an ester bond. The site of the modification by fatty acid is presumably localized to the COOH-terminal portion of the RAS proteins. Fractionation of the membranes by sucrose gradient demonstrates that a majority of the fatty-acylated RAS proteins are localized in plasma membrane.

The 36-kilodalton substrate of pp60v-src is myristylated in a transformation-sensitive manner

A primary intracellular substrate for pp60v-src kinase in a variety of avian and mammalian cells is a protein of 34 to 39 kilodaltons (kD). After incubation of chicken embryo fibroblasts (CEF) with [3H]myristic acid for 4 hours, the 36-kD protein contained covalently bound myristic acid by several criteria: (i) the radioactively labeled material comigrated with the 36-kD protein on sodium dodecyl sulfate-polyacrylamide gels in one and two dimensions, (ii) the labeled material was insoluble in chloroform-methanol, and (iii) radioactively labeled myristate could be recovered from the purified 36-kD protein. The resistance of the acyl fatty acid moiety to hydrolysis by hydroxylamine suggested that the covalent linkage to the 36-kD protein may be through an amide linkage. The [3H]myristic-acid labeling of the 36-kD protein in Rous sarcoma virus-transformed CEF showed a reduction of up to 45 percent when compared to an identical amount of 36-kD protein derived from normal cells; this reduction was not due to general changes in myristic acid metabolism in transformed cells.

Regulation of cellular morphology by the Rous sarcoma virus src gene: analysis of fusiform mutants

We have been interested in how Rous sarcoma virus (RSV) influences transformed cell morphology and compared the molecular properties of chicken embryo cells (CEC) infected with mutants of RSV that induce the fusiform transformed cell morphology with those of CEC infected by wild-type RSV, which induces the more normal round transformed cell morphology. We looked for properties shared by all fusiform mutant-infected cells, because these may be responsible for maintaining the fusiform morphology. Five different fusiform mutants, two wild-type RSVs, and one wild-type back revertant of a fusiform mutant were studied. In the fusiform mutant-infected cells, the localization and myristylation of pp60src were determined and the extent of expression of the extracellular matrix protein fibronectin was examined at both the mRNA and protein levels. The phosphorylation of vinculin on tyrosine also was examined in the same CEC. Within all fusiform mutant-transformed CEC, pp60src was dramatically absent from the adhesion plaque sites normally seen in cells transformed with wild-type RSV, and these transformed CEC all expressed more fibronectin mRNA and protein in the extracellular matrix than did the wild-type RSV-transformed CEC. The absence of pp60src from the adhesion plaques was not due to lack of myristylation of the src protein, and tyrosine phosphorylation of vinculin was not related to fibronectin expression. These results suggest that the inverse relationship between pp60src in the adhesion plaques and fibronectin expression in the extracellular matrix may be interconnected phenomena and could be related to the maintenance of the fusiform transformed morphology.

Processing of p60v-src to its myristylated membrane-bound form

p60src of wild-type Rous sarcoma virus is myristylated at its N-terminal glycine residue. We have shown previously that this myristylation is necessary for p60src membrane association and for cell transformation by using src mutants with alterations within the N-terminal 30 kilodaltons of p60src. In this study we analyzed the process of p60src myristylation in wild type- and mutant-infected cells. All myristylated src proteins examined lack the initiator methionine, but two mutant src proteins lacking the initiator methionine are not myristylated, indicating that removal of the initiator methionine and myristylation are not obligatorily coupled. Analysis of the kinetics of myristylation and the association of p60src with cellular proteins p50 and p90 indicated that myristylation occurs before p60src becomes membrane associated and that transient association with p50 and p90 occurs regardless of myristylation. Myristylation is required for stable association of p60src with the plasma membrane but is not sufficient for membrane association. A mutant with an src deletion of amino acids 169 through 264 has an src protein that is myristylated but not membrane bound, remaining stably associated with p50 and p90. This mutant is transformation defective. Several N-terminal deletion mutants possessing tyrosine kinase activity have myristylated and membrane-bound src proteins but are not fully active in cell transformation, suggesting that additional N-terminal functional domains exist.

The nucleotide sequence and the tissue-specific expression of Drosophila c-src

We have examined the coding capability and expression of the Drosophila homolog of the vertebrate proto-oncogene c-src. Sequence analysis of a cDNA clone representing the Drosophila c-src locus suggests that the gene encodes a 62 kd protein that is remarkably similar to the protein product of chicken c-src. The Drosophila c-src locus is transcribed into three mRNAs that are each regulated independently during development. Drosophila c-src RNA is abundant in embryos and pupae but rare in larvae and adults. In situ hybridization reveals that after the first 8 hr of development, c-src RNA accumulates almost exclusively in neural tissues such as the brain, ventral nerve chord, and eye-antennal discs, and in differentiating smooth muscle. We conclude that c-src may not be a mitotic signal but instead may play a role in the development of neural tissue and smooth muscle.

Phosphatidylinositol kinase activity in virus-transformed and nontransformed cells

We assayed phosphatidylinositol (PI) kinase (EC 2.7.1.67) activity in detergent extracts of nontransformed or virus-transformed cells. Nontransformed chicken embryo fibroblasts (CEF) contain PI kinase activity with an apparent specific activity of 20 pmol/min per mg of protein. This activity sedimented as a single peak with a molecular weight of approximately 60,000 in a glycerol gradient, although immunoprecipitation with anti-p60src sera showed that the PI kinase activity is distinct from p60c-src. Extracts from CEF transformed by Rous sarcoma virus, Fujinami sarcoma virus, or avian sarcoma virus UR2 showed no elevation of PI kinase activity over nontransformed CEF. Removal of the oncogene products from extracts by immunoprecipitation did not change the level of PI kinase activity in extracts, suggesting that putative virus-coded PI kinases do not make a significant contribution to overall levels of PI kinase activity in transformed cells. Additionally, P140gag-fps was separated from cellular PI kinase by phosphocellulose chromatography. This partially purified fraction contained low PI kinase activity distinct from P140gag-fps, indicating that P140gag-fps has no detectable PI kinase activity.

Low level of cellular protein phosphorylation by nontransforming overproduced p60c-src

We have previously found that Rous sarcoma virus variants in which the viral src (v-src) gene is replaced by the cellular src (c-src) gene have no transforming activity. In this study, we analyzed the basis for the inability of the p60c-src overproduced by these variants to transform cells. Phosphorylations of tyrosine residues in total cell protein or in cellular 34K protein are known to be markedly enhanced upon infection with wild-type Rous sarcoma virus. We found that these tyrosine phosphorylations were only slightly increased in the c-src-containing virus-infected cells, whereas both levels were significantly increased by infection with wild-type Rous sarcoma virus, or transforming mutant viruses which are derived from c-src-containing viruses by spontaneous mutation. Phosphorylation at tyrosine 416 of p60 itself was also extremely low in overproduced p60c-src and high in p60s of transforming mutant viruses. In immunoprecipitates with monoclonal antibody, the overproduced p60c-src had much lower casein tyrosine kinase activity than did p60v-src. We previously showed that p60 myristylation and plasma membrane localization may be required for cell transformation. p60c-src was similar to transforming p60s in these properties. These results strongly suggest that the low level of tyrosine phosphorylation by overproduced p60c-src accounts for its inability to transform cells.

Rous sarcoma virus-transformed fibroblasts adhere primarily at discrete protrusions of the ventral membrane called podosomes

Rous sarcoma virus-transformed BHK cells (RSV/B4-BHK) adhere to a fibronectin-coated substratum primarily at specific dot-shaped sites. Such sites contain actin and vinculin and represent close contacts with the substratum as revealed by interference reflection microscopy. Only a few adhesion plaques and actin filament bundles can be detected in these cells as compared to untransformed parental fibroblasts. In thin sections examined with transmission electron microscopy (TEM) these adhesion sites correspond to short protrusions of the ventral cell surface that contact the substratum at their apical portion. These structures, which may represent cellular feet, are therefore called podosomes. By screening a number of different transformed fibroblasts plated on a fibronectin-coated substratum we find that podosomes are common to mammalian and avian cell lines transformed either by Rous sarcoma virus (RSV) or by Fujinami avian sarcoma virus (FSV), whose oncogenes encode specific tyrosine kinases. Using antibodies reacting with phosphotyrosine in immunofluorescence experiments, we show that phosphotyrosine-containing molecules are concentrated in podosomes. Podosomes are not detected in fibroblasts transformed by other retroviruses (Snyder-Theilen sarcoma virus, Abelson leukemia virus and Kirsten sarcoma virus) or by DNA tumor viruses (polyoma, SV40), indicating that podosome-mediated adhesion in transformed fibroblasts is related to the peculiar properties of some oncoproteins and possibly to their tropism for adhesion systems. Podosomes and adhesion plaques, although similar in cytoskeletal protein composition, have different mechanisms and kinetics of formation. Assembly of podosomes, in fact (i) does not require fetal calf serum (FCS) in the adhesion medium, that is necessary for the organization of adhesion plaques; (ii) does not require protein synthesis; and (iii) is insensitive to the ionophore monensin, that prevents adhesion plaque formation. Moreover, during attachment to fibronectin-coated dishes, podosomes appear in the initial phase (60 min) of attachment, while adhesion plaques require a minimum of 180 min. In conclusion podosomes of RSV- and FSV-transformed fibroblasts represent a phenotypic variant of adhesion structures.

Development and analysis of a transformation-defective mutant of Harvey murine sarcoma tk virus and its gene product

The Harvey murine sarcoma virus has been cloned and induces focus formation on NIH 3T3 cells. Recombinants of this virus have been constructed which include the thymidine kinase gene of herpes simplex virus type 1 in a downstream linkage with the p21 ras gene of Harvey murine sarcoma virus. Harvey murine sarcoma tk virus rescued from cells transfected with this construct is both thymidine kinase positive and focus inducing in in vitro transmission studies. The hypoxanthine-aminopterin-thymidine selectability of the thymidine kinase gene carried by this virus has been exploited to develop three mutants defective in the p21 ras sequence. All three are focus negative and thymidine kinase positive when transmitted to suitable cells. Of these, only one encodes a p22 that is immunologically related to p21. This mutant has been used to explore the relationship between the known characteristics of p21 and cellular transformation. Data presented herein indicate that the p21 of Harvey murine sarcoma virus consists of at least two domains, one which specifies the guanine nucleotide-binding activity of p21 and the other which is involved in p21-membrane association in transformed cells.

Two classes of fatty acid acylated proteins exist in eukaryotic cells

Labelling of cultured cells with [3H]palmitic and [3H]myristic acids demonstrates that each of these fatty acids modifies a substantially different subset of cellular proteins. Hydroxylamine treatment can be used to differentiate sensitive thioester linkages to palmitate from insensitive amide linkages to myristate. Several palmitoylated proteins are surface-oriented glycoproteins while all of the myristylated proteins appear to be internal. Myristate addition is much more tightly coupled to protein synthesis than palmitoylation, which is able to continue at a reduced level even in the prolonged absence of protein synthesis. Acyl proteins patterns were affected both qualitatively and quantitatively by transformation and growth status. The preferential addition of palmitate to the transferrin receptor and myristate to pp60src, and the absence of these modifications from several other proteins is reported. We propose a nomenclature for fatty acyl proteins based on these observations.