Characterization of avian and viral p60src proteins expressed in yeast

Avian and viral p60src proteins were expressed from a galactose-inducible promoter in the yeast Saccharomyces cerevisiae. Both the viral and cellular src proteins produced in yeast cells were myristoylated at their amino termini, as is the case for src proteins expressed in chicken embryo fibroblasts. The viral src protein produced in yeast autophosphorylated at tyrosine-416 in vivo and had approximately the same level of in vitro kinase activity as p60v-src expressed in Rous sarcoma virus-transformed cells. Unlike p60c-src expressed in chicken cells, which is phosphorylated on tyrosine in vivo almost exclusively at tyrosine-527, p60c-src expressed in yeast was phosphorylated 2.5-3 times more at tyrosine-416 than at tyrosine-527. The specific activity of the p60c-src produced in yeast was 2.5-5.0 times higher than that of p60c-src overexpressed from a retroviral vector in chicken cells, implicating the altered state of in vivo phosphorylation in modulation of the in vitro kinase activity. The expression of p60v-src substantially slowed down the growth of the yeast cells, suggesting that phosphorylation of yeast proteins essential for cell growth may have interfered with their proper functioning.

Human T-cell lymphotropic virus IIIB glycoprotein (gp120) bound to CD4 determinants on normal lymphocytes and expressed by infected cells serves as target for immune attack

The lymphocyte differentiation antigen CD4 serves as a receptor for human retroviruses associated with acquired immunodeficiency syndrome (AIDS) through its interaction with the major envelope virion glycoprotein, gp120, which is also expressed on the surface of infected cells. In these experiments, purified gp120 was shown to bind to normal human T-lymphocyte populations. The gp120-CD4 complex served as a target antigen for antibody-dependent complement-mediated cytolysis by a goat serum raised against native gp120. However, patient sera that bound to gp120-adsorbed cells failed to direct their destruction in the presence of complement. In contrast, these sera were potent mediators of antibody-dependent cellular cytotoxicity. These studies demonstrate that gp120 situated on the cell surface can serve as an effective target for immune destruction by patient antibodies and effector lymphocytes. The possible contribution of this type of immunity to control of disease progression, on the one hand, and to lymphocyte destruction and immunopathology observed in AIDS, on the other, is discussed.

Avian sarcoma and leukosis virus pol-endonuclease recognition of the tandem long terminal repeat junction: minimum site required for cleavage is also required for viral growth

Integration of retroviral DNA is a site-specific reaction involving an endonuclease encoded by the viral pol gene (pol-endo). In vitro the pol-endo from avian sarcoma and leukosis viruses (ASLVs) cleaves both DNA strands near the U5-U3 junction of tandem long terminal repeats (LTR-LTR junction) in single-stranded and replicative form (RF)-I substrates. We have reported previously that the sequences that are required for cleavage of single-stranded substrates by the alpha beta form of the pol-endo differ for the plus and minus strands (G. Duyk, M. Longiaru, D. Cobrinik, R. Kowal, P. deHaseth, A. M. Skalka, and J. Leis, J. Virol. 56:589-599, 1985). This is not the case with RF-I substrates, in which a maximum of 22 base pairs of U5 and 8 base pairs of U3 were required for alpha beta pol-endo cleavage in each strand. Insertion of a palindromic octanucleotide (CATCGATG) at the LTR-LTR junction abolished cleavage in RF-I but not in single-stranded DNA substrates. Deletion of the four nucleotides (TTAA) at the junction prevented cleavage in the plus strand of RF-I DNA, but did not affect cleavage of single-stranded DNA. Furthermore, the alpha beta form of ASLV pol-endo did not recognize heterologous LTR-LTR junction sequences from the reticuloendotheliosis virus or Moloney murine leukemia virus in either substrate form, despite their sequence and structural similarities to the ASLV junction. These results support a role for a sequence-specific interaction between the ASLV pol-endo and the LTR-LTR junction domains that are required for cleavage. By using the infectious Rous sarcoma virus clone pATV8-K, we introduced a set of deletions into the U5 region that would be incorporated into the LTR-LTR junction on viral replication. In the unintegrated provirus, the deletions started 43 base pairs from the LTR-LTR junction and extended various lengths toward the junction. Results of transfection studies with these clones indicated that the U5 sequences that are required for virus production in vivo correspond to those that are required for cleavage of RF-I DNA in vitro.

Role of cell cytoskeleton in Mo-MuLV env transport and processing: implications in ts1 neuropathology

Treatment of Mo-MuLV-infected cells with cytochalasin B (CB), a microfilament disrupting drug, caused a reduction in virus yield as judged by infectivity assay and reverse transcriptase activity. Pulse-chase experiments with [3H]leucine showed that the env precursor, gPr80env, was inefficiently processed in cells treated with CB. In the presence of monensin, an inhibitor of glycoprotein transport, gPr80env accumulated intracellularly and no gp70 was observed on the cell surface, indicating a complete block in the processing of gPr80env. Pulse-chase studies also showed that gPr80env was not processed in the presence of monensin. SDS-PAGE analysis of TX-100-extracted cell cytoskeletons (TX-insoluble fraction) iodinated and immunoprecipitated with goat anti-gp70 antiserum showed that CB or monensin treatment caused a marked increase of gPr80env in the cytoskeleton-rich fraction. However, the amount of gPr80env associated with the TX-soluble fraction in both CB or monensin-treated and untreated cells labeled with [3H]leucine was about the same. The gPr80env in the TX-100-soluble fraction of the cell was the endoglycosidase H (Endo-H) sensitive mannose-rich form, whereas the cytoskeleton-associated gPr80env was the partially Endo-H-resistant complex carbohydrate form. In the presence of CB or monensin, the complex carbohydrate form of gPr80env accumulated in the cytoskeleton-rich cell fraction. Examination of Mo-MuLV ts1 mutant, which is defective in the processing of env precursor polyprotein, also revealed an accumulation of the complex carbohydrate form of gPr80env in the cytoskeleton-rich fraction and an absence of gp70 on the surface of the cell at the restrictive temperature (39 degrees C). These studies suggest that the cytoskeleton plays a role in the transport and processing of MuLV gPr80env and that oligosaccharide conversion is an important factor in this process. Further, the accumulation of gPr80env on the cytoskeleton of ts1 infected cells at restrictive temperature may play a role in the neurological disorder caused by Mo-MuLV ts1 mutant.

A protein kinase antigenically related to pp60v-src possibly involved in yeast cell cycle control: positive in vivo regulation by sterol

The effects of ergosterol, yeast's natural sterol, on cell cycling and a protein kinase antigenically related to pp60v-src were examined in a sterol auxotroph of Saccharomyces cerevisiae. Sterol-depleted cells accumulate in an unbudded, G1 state. Cell budding and proliferation are reinitiated upon addition of nonlimiting ergosterol or cholesterol with trace ergosterol, whereas cholesterol or trace ergosterol alone is less effective. Stimulation of a protein kinase associated with immune complexes of yeast protein and anti-pp60v-src shows a positive correlation with exit from the G1 phase following ergosterol addition. Ergosterol-stimulated cells also demonstrate an increase in phosphatidylinositol kinase activity. The data suggest that hormonal levels of ergosterol (effective concentration, approximately equal to 1 nM) participate in a signaling process associated with a protein kinase possibly involved in yeast cell cycle control.

Forms of pp60v-src isolated from Rous sarcoma virus-transformed cells

It has previously been shown that an electrophoretic variant form of the Rous sarcoma virus transforming protein, pp60v-src, exists in src-transformed cells. This variant, which was readily observed in vanadate-treated cells, was characterized as possessing extensive amino-terminal domain phosphotyrosine modification. Its appearance was further correlated with increased src-specific protein kinase activity. In this study, we used a src-specific monoclonal antibody (MAb) to resolve immunologic forms of pp60v-src. The MAb was able to distinguish between two populations of typical lower-band pp60v-src and was unreactive with the electrophoretic variant upper-band pp60v-src species. Using serial immunoprecipitations, we resolved four populations of pp60v-src: src protein either immunoreactive or unreactive with the MAb from both untreated and vanadate-treated transformed cells. The pp60v-src in each fraction displayed a distinct phosphoamino acid composition and tryptic phosphopeptide profile. However, analysis of their tyrosyl kinase specific activities showed that the immunologically resolved populations of pp60v-src from a given culture did not differ. Both pp60v-src fractions from vanadate-treated cells exhibited similar kinase specific activities, which were greatly enhanced over those of enzyme preparations from untreated cells. Since the MAb-reactive pp60v-src fraction from vanadate-treated cells lacked the electrophoretic variant upper-band pp60v-src species yet still possessed enhanced enzymatic specific activity, the initially stated correlation between the appearance of the electrophoretic variant src form and increased src kinase activity breaks down. These results suggest that yet to be defined modifications of the src protein may be involved in its functional regulation.

The membrane-binding domain and myristylation of p60v-src are not essential for stimulation of cell proliferation

Previous studies showed that the amino-terminal domain of Rous sarcoma virus p60v-src involved in myristylation and membrane association of the protein is required for morphological transformation and anchorage independence. Analysis of src delection mutants revealed that the amino-terminal one-third of p60v-src, including the membrane-binding domain, is not essential for induction of cell proliferation. These results demonstrated that, in contrast to the cellular target(s) involved in morphological transformation and anchorage independence, the target(s) involved in mitogenic activity is accessible to nonmyristylated src proteins.

Genistein, a specific inhibitor of tyrosine-specific protein kinases

Tyrosine-specific protein kinase activity of the epidermal growth factor (EGF) receptor, pp60v-src and pp110gag-fes was inhibited in vitro by an isoflavone genistein. The inhibition was competitive with respect to ATP and noncompetitive to a phosphate acceptor, histone H2B. By contrast, genistein scarcely inhibited the enzyme activities of serine- and threonine-specific protein kinases such as cAMP-dependent protein kinase, phosphorylase kinase, and the Ca2+/phospholipid-dependent enzyme protein kinase C. When the effect of genistein on the phosphorylation of the EGF receptor was examined in cultured A431 cells, EGF-stimulated serine, threonine, and tyrosine phosphorylation was decreased. Phosphoamino acid analysis of total cell proteins revealed that genistein inhibited the EGF-stimulated increase in phosphotyrosine level in A431 cells.

HTLV-III large envelope protein (gp120) suppresses PHA-induced lymphocyte blastogenesis

The addition of inactivated preparations of purified human T cell lymphotropic virus (HTLV-III) was found to inhibit normal human lymphocyte phytohemagglutinin (PHA)-induced blastogenesis but had no effect on concanavalin A (Con A), pokeweek mitogen, or allogeneic stimulation. The inhibition was concentration-dependent and also dependent on adding the virus preparation before or at the same time as PHA. The CD4 molecule is the receptor for HTLV-III binding. Immunopurified large envelope protein (gp120) from HTLV-III was found to bind to the CD4 molecule and also inhibited PHA-induced lymphocyte blastogenesis. These results suggest that the gp120 viral protein may alter immune function by binding to the CD4 molecule, which in turn serves as an "off" signal to lymphocyte response to PHA stimulation. Alternatively, by binding to the CD4 molecule, gp120 may interfere with the interaction of this molecule with class II histocompatibility antigens on accessory cells, thus selectively suppressing PHA response.

Tyrosine phosphorylation regulates the biochemical and biological properties of pp60c-src

To investigate the importance of tyrosine phosphorylation in the regulation of pp60c-src, we have substituted phenylalanine for tyrosine at positions 416, 519, and 527. Cells expressing the 527 or the 519/527 mutant but not the 416 or the 519 mutant were morphologically transformed, grew in soft agar, and formed foci. In addition, the 527 and 519/527 mutants had elevated kinase activities in vitro. Modifying Tyr 416 to phenylalanine in the 527 or the 519/527 mutants only partially inhibited their kinase activities yet abolished their ability to induce focus formation and promote growth in soft agar. These results suggest that two events must occur to activate the full transforming potential of pp60c-src: hypophosphorylation at Tyr 527 and hyperphosphorylation at Tyr 416.

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.

Nonmitogenic morphoregulatory action of pp60v-src on multicellular epithelial structures

Madin-Darby canine kidney (MDCK) cells form polarized, multicellular epithelial structures in vitro. Low-level expression of pp60v-src in MDCK cells elicits plasticity in these multicellular structures. Plasticity was revealed by the displacement of cells from mechanically stressed regions of the epithelial monolayers; however, the two-dimensional relationship between the cells in the remainder of the monolayer was maintained. Electron microscopy of multicellular structures revealed abnormal separation of the lateral membranes of adjacent cells and selective uncoupling of the junctional complex; the zonula adherens was disrupted, but the zonula occludens and desmosomes were retained. Significantly, this result was not accompanied by transformation of the cells, as judged by the absence of anchorage-independent growth potential. These results demonstrate a nonmitogenic biological activity of pp60v-src which is experimentally dissociable from transformation. This morphoregulatory action on higher-order epithelial structures may reflect a function of related cellular tyrosine kinases.

Characterization of a virus-specific proteolytic activity processing the gag precursor of the simian sarcoma-associated virus

The proteolytic processing of the gag precursor polypeptide pr65gag of simian sarcoma-associated virus (SSAV) has been studied in vivo and in vitro. In SSAV-infected cells (i.e., in vivo) proteins of 52 and 38 kDa and the viral protein p30 could be immunoprecipitated with anti-p30 serum. This cleavage pattern is only in part imitated by in vitro cleavage of the isolated pr65gag with avian myeloblastosis virus (AMV) protease p15. However, in vitro incubation of isolated pr65gag with detergent-disrupted SSAV particles generated products identical in size to those found in vivo, i.e., proteins of 52 and 38 kDa and p30. The extent of cleavage is dependent on the concentration of the disrupted virions added to the incubation mixture. Studies with protease inhibitors suggest that the SSAV enzyme is a serine-type protease like that of other mammalian retroviruses and unlike the protease of avian viruses. The SSAV protease activity eluted from a molecular sieve column in a range of about 10-15 kDa reflecting the molecular weight of the murine leukemia virus (MuLV) protease (Mr = 13.5K). Thus, it appears that there is a close similarity between the proteolytic enzymes present in different mammalian retroviruses such as MuLV and SSAV.

Amino-terminal processing of proteins by N-myristoylation. Substrate specificity of N-myristoyl transferase

Using synthetic octapeptides, we examined the amino-terminal sequence requirements for substrate recognition by myristoyl-CoA:protein N-myristoyl transferase (NMT). NMT is absolutely specific for peptides with amino-terminal Gly residues. Peptides with Asn, Gln, Ser, Val, or Leu penultimate to the amino-terminal Gly were substrates, whereas peptides with Asp, D-Asn, Phe, or Tyr at this position were not myristoylated. Peptides with aromatic residues at this position competitively inhibited myristoylation of substrates, introducing the possibility of developing specific in vivo inhibitors of NMT. Peptides having sequences which correspond to those of known N-myristoyl proteins, including p60src, appear to be recognized by a single enzyme, and yeast and murine NMT have identical substrate specificities. The catalytic selectivity of NMT for myristoyl transfer accounts for the remarkable acyl chain specificity of this enzyme.

Interferon-gamma inhibits cell replication, but not pp60src activity of RSV-transformed fibroblasts

The present study analyzes the effect of murine IFN-gamma on DNA synthesis, ornithine decarboxylase activity and phosphorylation of pp60src of Rouse sarcoma virus transformed cells. When natural or recombinant IFN-gamma was added to quiescent SR-BALB or L1210 cells, stimulated with fetal calf serum, only the highest IFN-gamma concentrations (2000 U/ml) inhibited DNA synthesis of both tumor lines. By contrast, lower IFN-concentrations (20-200 U/ml) inhibited the DNA synthesis of L1210 but not of SR-BALB tumor cells. A similar pattern of inhibition was observed when ornithine decarboxylase activity was analyzed. Finally, when the levels of phosphorylation in SR-BALB cells were analyzed after IFN-gamma treatment, no difference with untreated controls was observed, even at the highest concentrations. These results suggest that SR-BALB tumor cells are insensitive to IFN-gamma and that src oncogene activity is not affected at IFN-concentrations inhibiting cell growth.

In vitro transformation of osteoblasts: putative formation of osteosarcoma in vitro

The study of bone cancer has been difficult in part due to a lack of appropriate in vitro osteosarcoma model systems. The development of such systems is essential if a clearer understanding of the biology of and mechanisms behind the formation and progression of bone cancers is to be obtained. We report here the development of an in vitro model system which demonstrates important characteristics generally associated with osteosarcoma. The chick periosteal osteogenesis model was infected with the Fujinami Sarcoma Virus (FSV) containing the v-fps oncogene which encodes for a P140gag-fps protein-tyrosine kinase. Under the appropriate conditions FSV infected cultures developed bone and cartilaginous tissues which showed histopathological findings consistent with osteosarcoma. Biochemical data indicating massive increases in alkaline phosphatase activity, protein content, 3H-Thymidine incorporation as well as expression of active P140gag-fps confirm that transformation has occurred in FSV infected cultures. This novel in vitro model system should prove most useful in the study of bone cancer.

Differential effects of phorbol ester on tumor cells induced by avian sarcoma virus

Avian sarcoma virus-induced tumors usually grow progressively for several weeks and then regress. We have injected phorbol myristate acetate (PMA) directly into tumors in an effort to stimulate neoplastic growth. The results show instead that PMA exerted an inhibitory effect in this regard and, in fact, caused an acceleration of tumor regression. At the same time, treatment of cultured avian sarcoma cells with PMA resulted in greatly diminished levels of the kinase activity associated with the src gene product, pp60src. PMA-treated tumor cells from regressing sarcomas were, however, stimulated to express viral antigens at their surface and produced more progeny virus than did untreated tumor cells.

Binding of proteins, including pp60src, to activated CH-sepharose 4B

Activated CH-Sepharose 4B and protein A Sepharose CL-4B can bind, selectively and non-specifically, polypeptides from chick embryo cells. The major polypeptides bound have apparent molecular masses of 57-60 kDa and 47-49 kDa and cannot be eluted by extensive washing with buffers containing detergents. One of the 57-60 kDa polypeptides was identified by immunoblotting as the transforming protein of Rous Sarcoma Virus (RSV), pp60src. This polypeptide could be removed from the solid phase immunoabsorbent with 60% dimethylsulfoxide, but not with 2% SDS, 5% beta-mercaptoethanol, 1 M NaCl or 0.1% Tween 20.

Evidence that mannosyl residues are involved in human immunodeficiency virus type 1 (HIV-1) pathogenesis

The Human Immunodeficiency Virus (HIV), the causative agent of AIDS, is thought to bind to T4+ (CD4+) target cells through the heavily glycosylated gp120 envelope glycoprotein. Plant lectins bind glycoproteins through noncovalent interaction with specific hexose residues; therefore, lectins were evaluated for their ability to inactivate HIV in vitro. The mannose-specific lectins concanavalin-A and succinyl concanavalin-A completely inactivated HIV while lentil lectin, wheat germ agglutinin, and phytohemagglutinin-P substantially inactivated HIV. BS-II, Vicia villosa (hairy vetch), and Ptilota plumosa (red marine algae) failed to alter the infectibility of HIV. Neither simple stearic hindrance, viral aggregation, nor lectin-cell interactions served to explain this phenomenon. Glycoprotein glycosylation was evaluated by differential lectin binding as well as molecular weight changes in gp120 when virus was produced in the presence of swainsonine, a glycosylation inhibitor. Lentil lectin bound gp120 better than concanavalin-A, suggesting the majority of glycosylation sites are fucosylated. The apparent molecular weight of gp120 was reduced by swainsonine, although HIV infectivity and concanavalin-A inactivation were retained. Thus, at least some N-glycosylation sites are complex-type glycoproteins but regions external to the (GlcNAc)2(Man)3 "core" pentasaccharide region are not required for HIV infectivity. It appears that the site or sites involved are nonfucosylated, high mannose and/or biantennary, nonsialylated, N-glycosylated regions of gp120 or gp41. Alternatively, they may be in close approximation to such carbohydrate regions.

Changes in the pattern of expression of pp60c-src in cerebellar mutants of mice

Cultures of neurons from rat embryos have been shown previously to express high levels of a unique form of pp60c-src [Brugge et al (1985): Nature 316:524-526], the cellular homologue of the transforming protein of Rous sarcoma virus. This altered form of pp60c-src, designated pp60c-src(+), displays a retarded electrophoretic mobility due to a structural alteration within the aminoterminal region of the molecule [Brugge et al, 1985]. In order to investigate the distribution and possible role of pp60c-src(+) in intact brain, we have examined the expression of pp60c-src(+) in extracts from developing cerebella from wild-type mice and mutant mice that display progressive degeneration of specific classes of cerebellar neurons. The loss of pp60c-src(+) generally correlated with the loss of granule cells and Purkinje cells from the cerebella of mice carrying the staggerer (sg/sg) and Lurcher (Lc/+) mutations, with the most pronounced changes observed in cerebella from the more severely affected sg/sg mice. The expression of pp60c-src(+) in weaver wv/wv mice is qualitatively and quantitatively quite different. From the earliest time points, there was a significant reduction in the levels of pp60c-src(+), with no further loss of this form during the period of maximal neuronal differentiation. This suggests an early, predegenerative absence of pp60c-src(+) in this mutant strain, which is defective in granule-cell migration.

Biosynthesis and chemical and immunological characterization of avian reticuloendotheliosis virus env gene-encoded proteins

Two glycosylated proteins designated gp90 and gp20 were purified from replication-competent avian reticuloendotheliosis associated virus (REV-A). The N-terminal sequences of gp90 and gp20 were determined and found to match the REV-A-env-gene sequence. The alignments of the determined amino acid sequences with the predicted sequence indicate that gp20 and gp90 are the REV-A-encoded viral transmembrane and surface glycoprotein, respectively, and predict a signal peptide of 36 residues on the 5' end of the env-gene. Furthermore, gp90 of REV-A was detected by Western blot analysis with antibodies to a tridecapeptide corresponding to an env-gene nucleotide segment immediately preceding gp20 and thus representing the C-terminal portion of gp90. The env-gene precursor polyprotein gPr75-79env and Pr22(E), the precursor to gp20 and p2(E) were identified in the infected cells by monospecific antibodies raised against purified gp20. Thus the organization of gPR75-79env is likely to be N-gp90-gp20-p2(E), resembling that of M-MuLV gp85env. Sequence comparisons showed that the env gene of REV-A is highly related to both baboon endogenous virus and Type D retroviruses. In Western blot analyses, antibodies to REV-A gp20 cross-reacted with a panel of mammalian Type C and Type D viruses. Evolutionary aspects of these findings are discussed.

Cytoplasmic localization of the HTLV-III 3' orf protein in cultured T cells

HTLV-III, the etiological agent of the acquired immunodeficiency syndrome, contains in its genome coding regions for several novel proteins. One of these, the 3' open reading frame (3'orf) encodes proteins of 26-27 kDa which are expressed in infected cells both in vivo and in vitro. A specific antiserum has been raised against the recombinant 3'orf protein synthesized in bacteria and used to localize the viral proteins by subcellular fractionation and immunofluorescence on HTLV-III infected cells. The antiserum specifically immunoprecipitated the 26- to 27-kDa proteins from both the cytoplasmic (S100) and the membrane fractions, with an enrichment in the latter. The proteins were not detected in the nucleus or organelle (S100 pellet) fractions. These proteins were also recognized in the same subcellular fractions by human sera from patients with AIDS. Indirect immunofluorescence on fixed infected cells confirmed the presence of the proteins in the cytoplasm. Immunoprecipitation and Western blot analysis of total proteins from disrupted HTLV-III virions with the specific antiserum failed to detect the 3'orf protein products, suggesting that they are not a major component of mature virions and may be involved in the intracellular regulation of viral replication.

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.

Expression and characterization of the trans-activator of HTLV-III/LAV virus

The human T-lymphotropic retrovirus HTLV-III/LAV encodes a trans-activator that increases viral gene expression. We expressed this trans-activator in animal cells and studied its structural and functional characteristics. The putative trans-activator protein was immunoprecipitated from overproducing stable cell lines and shown to migrate as a 14-kilodalton polypeptide on sodium dodecyl sulfate-polyacrylamide gels. S1 nuclease mapping experiments showed that the trans-activator increases the levels of steady-state messenger RNA transcribed from the viral long terminal repeat promoter. Sequences within the R region of the HTLV-III/LAV long terminal repeat are essential for trans-activation. Quantitations of messenger RNA and protein showed that the protein increase was greater than the messenger RNA increase in CV1 and HeLa cells, indicating that more than one mechanism was responsible for the trans-activation and that cell type-specific factors may determine the final level of trans-activation.

Induction of altered c-src product during neural differentiation of embryonal carcinoma cells

The expression of the cellular src gene product pp60c-src was examined in an embryonal carcinoma cell line that differentiates in vitro into neuronlike cells after being treated with retinoic acid. Quantitative and qualitative changes in c-src expression accompanied the events associated with neuronal differentiation. The levels of pp60c-src increased 8- to 20-fold during the period when the cells elaborated neuritic processes and expressed neuron-specific proteins. The electrophoretic mobility of pp60c-src induced in these cells was retarded in comparison with that in untreated cells or in treated cells before neurite elaboration. The shift in electrophoretic mobility was due to an alteration in the amino terminal 16,000 daltons of pp60c-src and similar to an alteration of c-src protein found in neural tissues and in pure primary cultures of neuronal cells. These results indicate that expression of pp60c-src induced by retinoic acid in these embryonal carcinoma cells mimics the expression of c-src in developing neurons. Therefore, this embryonal carcinoma cell line provides a model system to investigate the function of the src protein in neuronal differentiation.

Calcium-dependent isolation of the 36-kilodalton substrate of pp60src-kinase. Fractionation of the phosphorylated and unphosphorylated species

In this paper, we present a new and simple purification of the 36-kDa protein, a major substrate of both viral and growth factor-receptor associated tyrosine protein kinases, and its complex from normal and Schmidt-Ruppin strain Rous sarcoma virus-transformed chicken embryo fibroblasts that employs a DEAE-Sephacel column and introduces the calcium-dependent adsorption of 36-kDa protein. The use of EGTA step gradients differentially elutes the 36-kDa molecules from the DEAE-Sephacel column. An average total yield of the 36-kDa protein in all fractions approached 80% and represented 0.78% of the [35S] methionine-labeled cellular protein. A purity of 95-99% was obtained with a yield of 60% in the central elution fractions from normal or Rous sarcoma virus-transformed chicken embryo fibroblasts. Furthermore, 2 mM EGTA elutes poorly phosphorylated molecules while heavily phosphorylated 36-kDa protein requires 4 or 6 M EGTA; a small residual fraction is released at 8-10 mM EGTA. If the EGTA step gradients were neutralized with Ca2+ ion, elution of the 36-kDa protein is inhibited. The complex of the 36-kDa protein and the 6-10-kDa protein may not be dependent on the phosphorylation as the associated 6-10-kDa peptide is seen in all fractions containing the 36-kDa protein. Tyrosine phosphorylation of the 36-kDa protein is increased 2-3-fold following a short term incubation of whole cells with micromolar vanadate. The elution pattern (but not intensity) of the 36-kDa protein obtained from lysates of vanadate-treated cells was identical to untreated cell lysates. The additional phosphorylation appears to result from a recruitment of unphosphorylated 36-kDa protein as the position (but not intensity) of the phosphorylated tryptic peptides is unchanged. We conclude that the function of the 36-kDa protein may be calcium ion-dependent and may be influenced by the phosphorylation state of the protein.

A glial fibrillary acidic protein-expressing and tumorigenic cell line derived from an avian sarcoma virus-induced rat astrocytoma

A permanent cell line, S635c15, was derived from an anaplastic astrocytoma induced by the Schmidt-Ruppin strain of avian sarcoma virus (ASV) in a female F-344 rat. Persistent expression of the astrocytic differentiation protein, glial fibrillary acidic protein (GFAP), was detected both in cultured cells after 100 passages in vitro and in transplanted tumors. Subcutaneous and intracerebral transplantation of S635c15 cells in syngeneic rats resulted in a 100% tumor incidence and a reproducible mortality distribution. S635c15 cells formed discrete masses after subcutaneous injection but grew intracranially as infiltrative lesions. Tumor blood flow and blood-to-tissue transport studies yield comparable values to other rat glioma models; S635c15 intracranial tumors proved to be a homogeneous model with little variation within and between tumors with respect to morphology, GFAP expression, blood flow, and permeability. This cell line provides a GFAP-expressing brain tumor model that extends the use of autochthonous ASV-induced astrocytomas by allowing in vitro and in vivo studies. It may be useful for further studies in neurobiology and brain tumor biology, diagnosis, and therapy.

Regulation of fibronectin receptor distribution by transformation, exogenous fibronectin, and synthetic peptides

Recent studies have shown that fibronectin and its 140K membrane receptor complex are spatially associated with microfilaments to form cell surface linkage complexes which are thought to mediate adhesive interactions between fibroblasts and their substrata. We examined the regulation of the organization of these cell surface structures in transformed and fibronectin-reconstituted cells as well as in cells treated with a competitive synthetic peptide inhibitor of fibronectin binding to its receptor. Correlative localization experiments with interference reflection microscopy and double-label or triple-label immunofluorescence revealed a concomitant loss of fibronectin, 140K receptor, and alpha-actinin colocalization at cell substratum extracellular matrix contact sites after transformation of chick fibroblasts by wild-type or temperature-sensitive Rous sarcoma viruses (RSV). Western and dot immunoblot analyses established that although similar total quantities of intact 140K molecules were present in the transformed cell cultures, significantly more was released into the culture medium of transformed cells. The 140K molecules on transformed cells were available for interaction with exogenously added fibronectin, which could reconstitute fibronectin-140K linkage complexes. In such fibronectin reconstitution experiments, many cells expressed both fibronectin-140K-actin linkage complexes and RSV pp60src, indicating that the morphological reversion could occur even in the continued presence of RSV transformation. The synthetic peptide Gly-Arg-Gly-Asp-Ser derived from the sequence of the cell-binding region of fibronectin could also prevent the organization of fibronectin-140K linkage complexes. Our results suggest that fibronectin interaction with cells regulates the organization of fibronectin receptor complexes and cytoskeletal components at the cell surface.

Immunocolloidal gold electron microscopy of viral antigens and cellular actin in C3H/He mouse mammary tumors

Mouse mammary tumor virus (MMTV) antigens and cellular actin were visualized with Protein A-coated colloidal gold at the ultrastructural level in C3H/He mouse mammary tumors using various antisera. Observations with the postembedding method and statistical analyses of these data showed: (a) a glycoprotein with a molecular weight of 52,000 resided on the envelope of mature virions (B-particles), on the surface of budding particles, and on the vacuolar membrane, but not on intracytoplasmic A-particles (A-particles); (b) B-particle antigens were shared by A-, B-, and budding particles with more reacting gold particles on B-particles; and (c) a protein with a molecular weight of 27,000, p27 and A-particle antigens were also shared equally by all MMTV-related particles with preferential localization on the nucleoid of B-particles and the double ring of A-particles. All of these observations are consistent with and further confirm the proposal that A-particles are pronucleocapsids of MMTV. Cellular actin was concentrated in the outermost thin cytoplasmic layer and in microvilli. Single gold particles were also found on budding and B-particles implying that actin plays a role in the MMTV budding process.

Rous sarcoma virus nucleic acid-binding protein p12 is necessary for viral 70S RNA dimer formation and packaging

To study the function(s) of the Rous sarcoma virus nucleic acid-binding protein p12, we constructed mutants by using two restriction sites in the p12 proviral coding sequence of the Prague C strain to insert KpnI synthetic linkers. The two restriction sites are in the same reading frame, which allowed us to construct a deletion mutant lacking the two conserved Cys-His regions and a duplication mutant containing three intact Cys-His boxes. These mutant DNAs were transfected into chicken embryo fibroblasts, and the viral particles produced in a transient assay were characterized biochemically and for infectivity. Our results indicate that the Rous sarcoma virus nucleic acid-binding protein p12 is necessary for genomic RNA packaging but not for particle assembly and is implicated in the formation of a stable 70S dimeric RNA. Moreover, the fact that one mutant was apparently able to package normal 70S RNA but was not infectious suggests a role for p12 during the infection process.

Analysis of pp60c-src protein kinase activity in human tumor cell lines and tissues

We have evaluated the level of pp60c-src protein kinase activity in a variety of human tumor tissues and human tumor cell lines, and have estimated the abundance of the c-src protein in several of these tissues and cell lines. All cell lines derived from tumors of neuroectodermal origin that express a neural phenotype were found to possess c-src molecules with high levels of tyrosine-specific protein kinase activity. In contrast, cell lines derived from tumors of neuroectodermal origin that do not express neural characteristics, such as glioblastomas and melanomas, were found to have pp60c-src molecules with low levels of protein kinase activity. A similar pattern was observed when we analyzed the activity of c-src molecules extracted directly from corresponding tumor tissues. Analysis of human tumor cell lines derived from tissues other than those of neuroectodermal origin revealed that pp60c-src protein kinase activity was low in most cases. Exceptions to this observation were all rhabdomyosarcoma, osteogenic sarcoma, Ewing's sarcoma, and colon carcinoma lines tested. Comparison of pp60c-src kinase activity in normal skeletal muscle and rhabdomyosarcoma tissue and in normal breast tissue and breast adenocarcinoma tissue revealed that pp60c-src kinase activity was specifically elevated in the tumor tissues in both cases. However, the amount of pp60c-src protein in both normal and tumor tissues was found to be similar. These observations suggest that increases in the specific activity of the pp60c-src phosphotransferase in some rhabdomyosarcomas and breast carcinomas may be a characteristic acquired during the malignant transformation of the cells that is retained in cell lines established from these tumors.

Glutamine starvation of murine leukaemia virus-infected cells inhibits the readthrough of the gag-pol genes and proteolytic processing of the gag polyprotein

The production of Moloney murine leukaemia virus from chronically infected cells was inhibited after starvation of glutamine. While the rate of synthesis of the precursor of the core proteins, Pr65gag, was not affected in the starved cells, its proteolytic processing was blocked. Pulse-chase experiments indicated that glutamine was required during the synthesis of Pr65gag to facilitate its subsequent processing. In addition, the synthesis of Pr200gag-pol, the precursor of the protease, reverse transcriptase and endonuclease, was inhibited in the glutamine-starved cells. Starvation for other essential amino acids such as tyrosine and isoleucine affected neither the synthesis nor the processing of the virus proteins. These results suggest that the readthrough mechanism which enables synthesis of the Pr200gag-pol polyprotein is modulated in the chronically infected cells by glutamine levels. Since the viral protease is part of the pol gene, its synthesis may be inhibited in the glutamine-starved cells and Pr65gag is therefore not processed.

Identification and characterization of the tyrosine protein kinases of rat spleen

High levels of tyrosine protein kinase have been recently detected in the membranes of rat spleen. In the present report the tyrosine protein kinase activity of the 30,000 x g pellet of rat spleen has been solubilized and partially purified by ion exchange and gel permeation chromatography. Two peaks of tyrosine protein kinase of Mr 35,000 (TPK-I) and Mr 40,000 (TPK-II) have been resolved. These kinases were free of the EGF receptor and insulin receptor tyrosine protein kinases. Although TPK-I and TPK-II phosphorylated angiotensin II, casein, histone, tubulin, phosphorylase b, and p36 they differed from each other in preference for the substrates. Both tyrosine protein kinases did not phosphorylate anti-pp60v-src IgG.

A p60v-src-related tyrosine kinase in the acetylcholine receptor-rich membranes of Narke japonica: association and dissociation of phosphatidylinositol kinase activity

We have isolated a tyrosine-specific protein kinase from the acetylcholine receptor (AChR)-rich membranes of the electric ray Narke japonica. The enzyme is immunologically related to p60v-src, the product of the transforming gene of Rous sarcoma virus. A substantial phosphatidylinositol (PI) kinase activity was associated with this enzyme when it was purified through tyrosine-agarose affinity chromatography used previously for the purification of p60v-src. However, by subsequent chromatography on casein-agarose, most of the associated PI kinase activity was separated from the tyrosine kinase activity. The results suggest that the tyrosine-specific protein kinase in the AChR-rich membranes of N. japonica has no intrinsic PI kinase activity.

Characterization of a progressive neurodegenerative disease induced by a temperature-sensitive Moloney murine leukemia virus infection

A progressive neurodegenerative disease occurred following infection of mice with a temperature-sensitive (ts) isolate of Moloney (Mo) murine leukemia virus (MuLV), ts Mo BA-1 MuLV. This NB-tropic ecotropic MuLV, which was ts for a late function, induced a syndrome of tremor, weakness of the hind limbs, and spasticity following infection of several strains of laboratory neonatal mice, including NFS, C3H/He, CBA, SJL, and BALB/c. The latent period of 8 to 16 weeks was considerably longer than that observed for the acute paralytic diseases observed following neonatal infection with other ts Mo-MuLV, rat-passaged Friend MuLV, and some wild mouse ecotropic MuLVs. Spongiform pathology without inflammation and degeneration of neurons devoid of budding virions occurred in the cerebellar grey matter, brain stem, and upper spinal cord; but lower spinal cord anterior horn cells were less obviously affected than in other MuLV-associated neuroparalytic syndromes. ts Mo BA-1 MuLV differed from other ts Mo-MuLV mutants that are capable of inducing a neuroparalytic syndrome in that while infected nervous system tissue contained high levels of MuLV p30 and gp70, no evidence of precursor accumulation or abnormal processing of MuLV p30 or gp70 could be demonstrated. The localization of virus within the nervous system suggests that direct neuronal infection may not be the etiologic mechanism in this MuLV-induced neurodegenerative disease.

Apparent activation of the MgATP-dependent protein phosphatase by pp60v-src. Identification of an activity like that of glycogen synthase kinase 3 in immunoaffinity purified pp60v-src preparations

Immunoaffinity purified pp60v-src was found to activate the MgATP-dependent protein phosphatase in the presence of MgATP. Although preliminary evidence suggested that phosphorylation of the inhibitor-2 subunit on tyrosine residues was responsible for the activation, preincubation of the pp60v-src preparation at 41 degrees C resulted in a rapid loss of its protein kinase activities towards both casein and inhibitor-2 while its ability to activate the protein phosphatase complex was relatively insensitive to this treatment. This result demonstrated that pp60v-src was not responsible for activation of the MgATP-dependent protein phosphatase. A protein kinase activity which phosphorylated glycogen synthase on serine residues was detected in the pp60v-src preparation. The protein kinase was active in the presence of inhibitors of phosphorylase kinase, glycogen synthase kinase 5/casein kinase II, and cAMP-dependent protein kinase. It is, therefore, likely that activation of the MgATP-dependent protein phosphatase resulted from the presence of a glycogen synthase kinase 3 like activity in the pp60v-src preparation. Our results illustrate the importance of applying multiple criteria to link the phosphorylation of a protein with an observed change in its activity.

Two human 35 kd inhibitors of phospholipase A2 are related to substrates of pp60v-src and of the epidermal growth factor receptor/kinase

We have purified two 35 kd phospholipase A2 inhibitors from human placenta, which we refer to as lipocortin I and II. Both proteins exhibit similar biochemical properties and occur in placenta at about 0.2% of the total protein. By peptide mapping, sequence, and immunological analyses, we show that lipocortin I and the 35 kd substrate for the EGF-receptor/kinase from A431 cells are the same protein. By similar criteria, we determine that lipocortin II is the human analogue of pp36, a major substrate for pp60src, which has been characterized in chicken embryo fibroblasts and in bovine brush border preparations. The amino acid sequences of lipocortin I and II that we deduced from cDNA clones share 50% homology, indicating that they probably evolved from a common gene.

The use of Rous sarcoma virus transformation mutants with differing tyrosine kinase activities to study the relationships between vinculin phosphorylation, pp60v-src location and adhesion plaque integrity

Tyrosine-specific phosphorylation of cellular proteins has been implicated in the neoplastic transformation of cells by Rous sarcoma virus (RSV). One of the putative substrates for the src gene product (pp60v-src) of RSV is the cytoskeletal protein vinculin, giving rise to the hypothesis that tyrosine-specific phosphorylation of vinculin disrupts adhesion plaque integrity, leading to the characteristic rounded morphology of RSV-transformed cells. We have investigated this hypothesis by analysing the properties of fibroblasts transformed by conditional and non-conditional mutants of RSV which confer different morphologies on infected cells, with respect to formation of microfilament bundles, formation of vinculin-containing adhesion plaques, the deposition of a fibronectin-containing extracellular matrix, the localization of pp60v-src and the tyrosine-specific phosphorylation of vinculin. Cells transformed by the temperature-sensitive (ts) RSV mutant LA32 cultured at 41 degrees C were morphologically normal, and contained prominent microfilament bundles and well-developed adhesion plaques. However, these cells had a fully active pp60v-src kinase, had pp60v-src concentrated in their adhesion plaques and contained vinculin which was heavily phosphorylated on tyrosine residues. Cells transformed by a recovered avian sarcoma virus, rASV 2234.3 exhibited a markedly fusiform morphology with pp60v-src concentrated in well-developed adhesion plaques and an elevation of the phosphotyrosine content of vinculin. Cells transformed by LA32 at restrictive temperature comprise morphologically normal cells, indistinguishable from untransformed CEF, yet which contain tyrosine-phosphorylated vinculin and suggest that neither tyrosine-specific phosphorylation of vinculin nor pp60v-src concentration in adhesion plaques is sufficient for the rounded morphology of RSV-transformed cells.

Modulation of p36 phosphorylation in human cells: studies using anti-p36 monoclonal antibodies

We have characterized two monoclonal antibodies which recognize human p36. These have been used to examine the sites and extent of serine and tyrosine phosphorylation of p36 in human cells treated with epidermal growth factor and platelet-derived growth factor and in human cells transformed with viruses whose oncogenes encode protein-tyrosine kinases.

Localization of the feline sarcoma virus fgr gene product (P70gag-actin-fgr): association with the plasma membrane and detergent-insoluble matrix

The v-fgr oncogene codes for a unique transforming protein (P70gag-actin-fgr) that contains virus-specific determinants and cell-derived sequences for both a tyrosine-specific kinase domain and an actin domain. We examined the subcellular distribution of the v-fgr protein by immunofluorescence microscopy and various cell fractionation techniques. By immunofluorescence, the v-fgr protein was localized in a diffuse cytoplasmic pattern within transformed cells. The v-fgr protein was not detectable at substratum adhesion sites. Crude membrane preparations (P100) obtained from fgr-transformed cells contained elevated levels of P70gag-actin-fgr. Further analysis of membranes on discontinous sucrose gradients revealed that P70gag-actin-fgr cofractionated with plasma membranes. Using an alternate method of fractionation, we found that the majority of the v-fgr protein remained with the insoluble matrix obtained by treating cells with a buffer containing Triton X-100. When membranes were similarly treated with detergent, nearly all of v-fgr protein remained with the residual insoluble matrix. These results suggest that the transforming activity of P70gag-actin-fgr may be directed to subcellular cytoskeletal targets at or near the cytoplasmic face of the plasma membrane.

Intranuclear degradation of the transformation-inducing protein encoded by avian MC29 virus

The nuclear protein, p110, encoded by the avian MC29 virus degrades with a half-life of 30 to 40 min in virus-transformed cells. Inhibitors of lysosomal proteolysis had no effect on this degradation. When inhibitors of RNA or protein synthesis were added immediately after pulse-labeling the p110 with [35S]methionine, degradation was impeded. Treatment of cells with cycloheximide prior to, and after, the pulse extended the half-life of p110 further than post-treatment alone, and addition of both actinomycin D and cycloheximide to cells pretreated with cycloheximide extended the half-life even further. In cells depleted of cellular ATP using a glucose-deficient medium containing oligomycin, degradation of p110 was only partially inhibited, indicating no direct involvement of ATP in degradation. Isolation of nuclei or nuclear matrices containing labeled p110, with subsequent incubation, resulted in minimal loss of p110 during several hours. These results suggest that p110 is degraded by a protease which is itself labile and freely diffusible from the nucleus, and, in addition, degradation may involve interaction of p110 with newly synthesized RNA.

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.

Phosphorylation of tyrosine residues of calmodulin in Rous sarcoma virus-transformed cells

Calmodulin, a wide-spread eukaryotic Ca2+-binding protein, was phosphorylated at its tyrosine residues in Rous sarcoma virus (RSV)-transformed chicken and rat cells but not in normal chicken embryo fibroblasts. In contrast, serine and threonine phosphorylation of calmodulin was found to occur in both normal and virus-transformed cells. In an in vitro system containing purified src kinase from RSV-transformed cells, tyrosine phosphorylation of calmodulin by the src kinase was inhibited by Ca2+. Furthermore, the tyrosine-phosphorylated calmodulin showed slower mobility than that of nonphosphorylated calmodulin in NaDodSO4/polyacrylamide gel electrophoresis when Ca2+ was present. These results suggest that the structure of calmodulin Ca2+ complex may be altered by tyrosine phosphorylation. It is thus inferred that Ca2+ may regulate the level of tyrosine phosphorylation of calmodulin in RSV-transformed cells, and phosphorylation in turn may attenuate the function of this protein in vivo.

Two related but distinct forms of the Mr 36,000 tyrosine kinase substrate (calpactin) that interact with phospholipid and actin in a Ca2+-dependent manner

A method was devised that allows the identification of proteins related to the Mr 36,000 tyrosine kinase substrate calpactin based on their ability to interact with actin and phospholipid in a calcium-dependent manner. Two distinct proteins, detected in human A431 cells and fibroblasts, were resolved by two-dimensional gel electrophoresis. One of these proteins (calpactin I) appears identical to the Mr 34,000-39,000 substrate of the pp60src tyrosine kinase and the second (calpactin II) reacts with antibodies to the Mr 35,000 substrate of the epidermal growth factor receptor. Both proteins interact with phospholipid and actin, are rather basic, and share structural and antigenic determinants. A major difference between the two proteins is noted in their state of association with the Mr 10,000 light chain; i.e., calpactin I is associated with the light chain while calpactin II is not.

Phenotypic change from transformed to normal induced by benzoquinonoid ansamycins accompanies inactivation of p60src in rat kidney cells infected with Rous sarcoma virus

Three benzenoid ansamycin antibiotics (herbimycin, macbecin, and geldanamycin) were found to reduce the intracellular phosphorylation of p60src at a permissive temperature (33 degrees C) in a rat kidney cell line infected with a temperature-sensitive mutant of Rous sarcoma virus. This effect was accompanied by morphological changes from the transformed to the normal phenotype. The filamentous staining pattern of actin fibers was observed in the cells treated with these antibiotics at 33 degrees C. Removal of the antibiotics allowed the cells to revert to the transformed morphology. Ansamitocin, another benzenoid ansamycin, and naphthalenoid ansamycins such as streptovaricin and rifamycins did not show this effect. Pulse-labeling of the antibiotic-treated cultures with 32Pi showed a marked reduction of 32P radioactivity incorporated into p60src. A parallel experiment with [35S]methionine showed that synthesis of p60src was slightly inhibited. The immune complex prepared by mixing the herbimycin-treated cell extracts with antibody against p60src was inactive in vitro in phosphorylating the complex itself. On the contrary, the immune complex derived from untreated cells was active in vitro even in the presence of the antibiotics. These results suggest that benzoquinonoid ansamycins have no direct effect on src kinase but destroy its intracellular environment, resulting in an irreversible alteration of p60src and loss of catalytic activity.

Protein kinase C promotes the phosphorylation of immunoprecipitated middle T antigen from polyoma-transformed cells

Stimulation of protein kinase C in polyoma virus-transformed cells increased the phosphorylation of tyrosine residues of the viral middle T (mT) antigen in mT:pp60c-src complexes precipitated by anti-mT antibodies. This increase might have been due to a stimulation of the complex's pp60c-src tyrosine kinase activity or to an increased ability of the mT protein to be phosphorylated by pp60c-src. These observations suggest that cellular protein kinase C might control the ability of polyoma virus to transform its host cell.

The absence of myristic acid decreases membrane binding of p60src but does not affect tyrosine protein kinase activity

We have constructed two point mutants of Rous sarcoma virus in which the amino-terminal glycine residue of the transforming protein, p60src, was changed to an alanine or a glutamic acid residue. Both mutant proteins failed to become myristylated and, more importantly, no longer transformed cells. The lack of transformation could not be attributed to defects in the catalytic activity of the mutant p60src proteins. In vitro phosphorylation of the peptide angiotensin or of the cellular substrate proteins enolase and p36 revealed no significant differences in the Km or specific activity of the mutant and wild-type p60src proteins. However, when cellular fractions were prepared, less than 12% of the nonmyristylated p60src proteins was bound to membranes. In contrast, more than 82% of the wild-type protein was associated with membranes. Wild-type p60src was phosphorylated by protein kinase C, a protein kinase which associates with membranes when activated. The mutant proteins were not. This finding supports the idea that within the intact cell the nonmyristylated p60src proteins are cytoplasmic and suggests that this apparent solubility is not an artifact of the cell fractionation procedure. The myristyl groups of p60src apparently encourages a tight association between protein and membranes and, by determining the cellular location of the enzyme, allows transformation to occur.

Phosphorylation of pp60v-src by the TPA receptor kinase (protein kinase C)

Incubation of pp60v-src with the purified 12-O-tetradecanoyl-13-acetate (TPA) receptor kinase (protein kinase C) resulted in an increase in the phosphorylation of pp60v-src. Two-dimensional tryptic phosphopeptide mapping showed that three major phosphoserine containing peptides were labeled which were localized within the amino terminal 18,000 Da of the src protein. Based on comparative tryptic mapping, one of these major phosphopeptides was identical to the peptide labeled on pp60v-src immunoprecipitated from cells labeled with [32P]orthophosphate and treated with TPA. These data suggest that a direct interaction takes place between protein kinase C and pp60v-src.