Identification of a transformation-specific protein induced by a Rous sarcoma virus

Targeting protein phosphatase PP2A for cancer therapy: development of allosteric pharmaceutical agents

Tumor initiation is driven by oncogenes that activate signaling networks for cell proliferation and survival involving protein phosphorylation. Protein kinases in these pathways have proven to be effective targets for pharmaceutical inhibitors that have progressed to the clinic to treat various cancers. Here, we offer a narrative about the development of small molecule modulators of the protein Ser/Thr phosphatase 2A (PP2A) to reduce the activation of cell proliferation and survival pathways. These novel drugs promote the assembly of select heterotrimeric forms of PP2A that act to limit cell proliferation. We discuss the potential for the near-term translation of this approach to the clinic for cancer and other human diseases.

Stress proteins, autoimmunity, and autoimmune disease

At birth, the immune system is biased toward recognition of microbial antigens in order to protect the host from infection. Recent data suggest that an important initial line of defense in this regard involves autologous stress proteins, especially conserved peptides of hsp60, which are presented to T cells bearing gamma delta receptors by relatively nonpolymorphic class lb molecules. Natural antibodies may represent a parallel B cell mechanism. Through an evolving process of "physiological" autoreactivity and selection by immunodominant stress proteins common to all prokaryotes, B and T cell repertoires expand during life to meet the continuing challenge of infection. Because stress proteins of bacteria are homologous with stress proteins of the host, there exists in genetically susceptible individuals a constant risk of autoimmune disease due to failure of mechanisms for self-nonself discrimination. That stress proteins actually play a role in autoimmune processes is supported by a growing body of evidence which, collectively, suggests that autoreactivity in chronic inflammatory arthritis involves, at least initially, gamma delta cells which recognize epitopes of the stress protein hsp60. Alternate mechanisms for T cell stimulation by stress proteins undoubtedly also exist, e.g., molecular mimicry of the DR beta third hypervariable region susceptibility locus for rheumatoid arthritis by a DnaJ stress protein epitope in gram-negative bacteria. While there still is confusion with respect to the most relevant stress protein epitopes, a central role for stress proteins in the etiology of arthritis appears likely. Furthermore, insight derived from the work thus far in adjuvant-induced arthritis already is stimulating analyses of related phenomena in autoimmune diseases other than those involving joints. Only limited data are available in the area of humoral autoimmunity to stress proteins. Autoantibodies to a number of stress proteins have been identified in SLE and rheumatoid arthritis, but their pathogenetic significance remains to be established. Nevertheless, the capacity of certain stress proteins to bind to multiple proteins in the nucleus and cytoplasm both physiologically and during stress or injury to cells, suggests that stress proteins may be important elements in the "immunogenic particle" concept of the origin of antinuclear and other autoantibodies. In short, this fascinating group of proteins, so mysterious only a few years ago, has impelled truly extraordinary new lines of investigation into the nature of autoimmunity and autoimmune disease.

Localization of a tumor cell adhesion domain of laminin by a monoclonal antibody

Monoclonal antibodies were prepared to localize the domain(s) of laminin to which tumor cells adhere. Rat Y3-Ag 1.2.3 myeloma cells were fused with spleen cells from a rat immunized with a purified 440-kDa fragment of chymotrypsin-digested laminin. Three monoclonal antibodies (AL-1 to AL-3) that bound to intact laminin in a solid-phase radioimmunoassay were chosen for further analysis. The epitopes recognized by these antibodies were characterized by radioimmunoassays, immunoblotting, radioimmunoprecipitation, and immunoaffinity chromatography. In cell adhesion assays, monoclonal antibody AL-2 inhibited the binding of the highly metastatic melanoma cell line, K-1735-M4, to both intact laminin and the 440-kDa fragment of laminin. Electron microscopic examination of laminin-monoclonal antibody interactions showed that monoclonal antibody AL-2 reacted with the long arm of laminin directly below the cross-region. Two monoclonal antibodies that failed to inhibit tumor cell adhesion to laminin reacted with epitopes on the lateral short arms or cross-region of laminin as seen by electron microscopy. These results suggest that a new tumor cell binding domain of laminin may be located close to the cross-region on the long arm of laminin.

p53 content in relation to cell growth and proliferation in murine L1210 leukemia and normal lymphocytes

The protein p53 has been reported to be associated with cell transformation and/or proliferation. Using p53 monoclonal antibodies we estimated by flow cytometry the relative content of this protein in individual L1210 leukemic cells from exponentially growing and plateau-phase cultures and compared it with that in normal thymocytes of parental DBA/2 mice and in mitogen-stimulation and nonstimulated human lymphocytes. Simultaneous differential staining of p53 vs DNA and p53 vs RNA, followed by bivariate analysis, made it possible to estimate p53 with respect to cell position in the cell cycle and correlate it with RNA (predominantly rRNA) content. The data show that in exponentially growing L1210 cells p53 is being progressively accumulated during the G1, S and G2 phases and that the content of p53 and RNA are highly correlated. In plateau L1210 cultures most cells are arrested in G1, some cells, however, still continue to progress through S and G2. In these cultures the p53 content of all cells, regardless of the phase of the cell cycle, is diminished and the decrease in p53 is more pronounced than that of RNA or total protein content. The normal thymocytes as well as the stimulated lymphocytes show bimodal distribution with respect to p53 expression, compatible with the assumption that the cycling cells have increased expression of this protein related to the G0 cells. Some cycling cells, however, have minimal p53. The quantitative p53 immunofluorescence data were confirmed by the immunoprecipitation and gel electrophoresis. The results suggest that expression of p53 in leukemic and normal cells is more correlated with cell growth than with entrance to the cell cycle or progression through particular phases of the cycle.

Occurrence of a unique MHC class I gene in distantly related members of the genus Mus

There is unequivocal evidence that a relatively nonpolymorphic class I gene (designated Q10) from the Qa region of inbred mice encodes a secreted class I molecule. We have used a cDNA probe specific for this gene and an antiserum specific for its secreted protein product to investigate the occurrence and expression of this gene in different species of wild mice broadly representing the entire genus Mus. Evidence is presented that a Q10-like gene has been conserved and is transcribed and translated throughout the genus, suggesting that it serves an important function. However, the data also show that some differences have appeared in this gene over the period of evolutionary time covered by this sampling of wild mice. These results indicate that a specific class I DNA probe isolated from inbred mice can be used to discriminate a particular gene among the multiple class I genes present in other species.

Detection of a secreted form of the murine H-2 class I antigen with an antibody against its predicted carboxyl terminus

Analysis of H-2 class I-specific cDNA clones has suggested the synthesis by the liver of a class I molecule that is secreted rather than membrane bound. To detect this putative class I-related molecule, we have predicted a unique region of amino acid sequence located toward the carboxyl terminus of the molecule that is not expected to be shared with any of the classical H-2 class I antigens, and we have generated specific antibodies to a synthetic peptide corresponding to this sequence. Indirect immunoprecipitation with this antibody led to the identification of a Mr 40,000 polypeptide in association with beta 2-microglobulin in the serum of mice of five different H-2 haplotypes. This class I molecule is also detected in the liver together with lower molecular weight components, which are presumably underglycosylated precursors. Synthesis of this molecule is not detected in thymus, spleen, kidney, or testis. This class I serum component has no detectable reactivity with either a broad-specificity alloantiserum against H-2b or a xenoantiserum against purified H-2a class I molecules. The availability of a specific antibody against the secreted class I molecule offers a means to purify this protein for structural and functional studies.

Protein p53 and inducer-mediated erythroleukemia cell commitment to terminal cell division

Inducer-mediated murine erythroleukemia cell (MELC) differentiation provides a model for examining factors determining terminal cell differentiation. The nuclear protein, p53, has been implicated as a potential determinant of cell cycle progression and cell differentiation. In this study p53 content and synthesis, during inducer-mediated MELC differentiation, has been examined with monoclonal antibodies to p53. A decrease in p53 synthesis and content was demonstrated during induced differentiation. As determined by cell cycle fractionation, the decrease in p53 is manifest at all stages of the cell cycle. Hemin, which induces globin mRNA accumulation but not terminal cell division, fails to decrease p53 content. A MELC variant resistant to inducer-mediated commitment to terminal cell division also fails to decrease p53 levels in response to inducers. These experiments suggest that p53 is implicated in MELC cell proliferation and that an induced decrease in p53 may be responsible for G1 phase prolongation and terminal G1 arrest.

Analysis of cell-surface protein changes accompanying differentiation of HL-60 cells

The cell-surface proteins of HL-60 human promyelocytic leukemia cells have been compared to those of normal human neutrophils. Proteins of HL-60 cells surface labeled with 125I differed markedly from those of normal neutrophils, as shown by immunoprecipitation and polyacrylamide electrophoresis. Differentiation of HL-60 cells by treatment with dimethylformamide, trans-retinoic acid, or 12-O-tetradecanoylphorbol acetate did not modify the predominant surface-labeled proteins of HL-60 cells to produce a pattern similar to that of normal, mature neutrophils. However, the agents did induce greater quantities of minor cell-surface proteins immunoprecipitated by hyperimmune anti-human neutrophil serum. These immunoprecipitated proteins resembled several of the surface-labeled polypeptides of normal human neutrophils.

Secretion of a transplantation-related antigen

Analysis of mouse cDNA clones has led to the identification of a class I (H-2)-related gene that encodes a truncated transplantation-like antigen. Unlike the products of the class I genes (H-2K, H-2D, and H2-L), which are synthesized and displayed on the surface of all cells, the class I-related gene product is expressed only in liver cells and is secreted. The region of the secreted molecule corresponding to the extracellular domain of the membrane-bound class I antigens shows unusual amino acid substitutions at positions otherwise invariably conserved. There is also loss of a glycosylation site that is used in all class I antigens. Within the region corresponding to the transmembrane domain are multiple nonconservative substitutions of hydrophobic residues, alterations that render the encoded protein incapable of inserting into the plasma membrane. Toward the end of the same domain, the polypeptide chain terminates abruptly and thus lacks the intracellular domain present on all class I antigens. A candidate for this secreted molecule, detected using various heteroantisera against class I antigens, has been identified. A potential role for this serum protein in mediating active tolerance is discussed.

Simian virus 40 tumor antigen: isolation of the origin-specific DNA-binding domain

To localize the origin-specific DNA-binding domain on the simian virus 40 tumor (T) antigen molecule, we used limited proteolysis with trypsin to generate fractional peptides for analysis. A 17,000-Mr peptide was found to be capable of binding not only to calf thymus DNA, but also specifically to the simian virus 40 origin of DNA replication. This approximately 130-amino-acid peptide was derived from the extreme N-terminus of the T antigen and represented less than one-fifth of the entire molecule. The coding sequence for this tryptic peptide was located approximately between 0.51 and 0.67 map units (excluding the intron, which maps between 0.54 and 0.59). Since the first 82 amino acids are shared between large T and small t antigens, and since the latter does not bind DNA, it can be concluded that the sequence between isoleucine 83 and approximately arginine 130 is necessary for origin-specific binding by the T antigen. We also observed that in vivo phosphorylation of the T antigen within this region completely abolished the ability of the 17,000-Mr peptide to bind DNA. This observation is consistent with the idea that DNA binding by the T antigen is regulated by posttranslational modifications.

Nucleotide sequence of Rous sarcoma virus

We present the 9312 nucleotide sequence of the Prague C (Pr-C) strain of Rous sarcoma virus (RSV). A comparison of known protein sequences with the nucleotide sequence allows assignment of the coding regions for the gag, pol, env and src genes. The gag gene is terminated by an amber stop codon and is contained within a different reading frame than is the pol gene. The pol and env genes overlap. The sequences surrounding the src gene in the Pr-C and Schmidt-Ruppin (SR-A) strains of RSV have been compared, and they reveal that an element, E, of approximately 153 nucleotides is present on the 3' side of the src gene in Pr-C, and on the 5' side in SR-A. We hypothesize that E was part of a duplicated region of over 250 nucleotides flanking the src gene in an ancestral RSV, and that differential deletion of one copy of E led to its positional difference in Pr-C and SR-A.

Mouse Lyt-2 antigen: evidence for two heterodimers with a common subunit

Immunoprecipitation of [35S]methionine-labeled extracts of BALB/c thymocytes with mouse Lyt-2.2 monoclonal antibody yielded three components with subunit Mrs of 37,000, 32,000, and 28,000 on NaDodSO4/polyacrylamide gel in the presence of a reducing agent. Two-dimensional polyacrylamide gel analysis revealed that, in the absence of a reducing agent, the three polypeptide chains exist in the form of two heterodimers, each consisting of one molecule of a Mr 28,000 subunit covalently associated through disulfide bonds with either one Mr 37,000 subunit or one Mr 32,000 subunit. These two molecular structures are present in about equimolar ratios in the immunoprecipitate. Immunoautoradiographic analysis after electrophoretic transfer of proteins from a NaDodSO4/polyacrylamide gel to a nitrocellulose membrane indicated that the Lyt-2.2 determinant detected by the monoclonal antibody resides on the Mr 28,000 component, the common subunit of both heterodimeric structures. Lyt-2 precipitated from extracts of different T-cell growth factor-dependent cloned T-cell lines also showed similar structures, although the exact apparent Mrs of the respective components varied somewhat. The structure of the Lyt-2 antigen is of importance, particularly in the light of recent suggestions that it may be involved in the construction of one class of T-cell receptors.

Transformation of MMC-E epithelial cells by acute 3611-MSV: inhibition of collagen synthesis and induction of novel polypeptides

Mouse embryo epithelial cells MMC-E were transformed by novel fibrosarcoma-inducing murine sarcoma virus 3611-MSV. The cells were analyzed for the production and deposition of pericellular glycoproteins by immunofluorescence and by radioactive metabolic and cell surface labeling techniques followed by analysis in polyacrylamide gels and fluorography. The pericellular fibronectin matrix was lost, but unlike in virus-transformed fibroblastic cells, the production of fibronectin was not affected. The major differences detected were decrease in collagen production and initiation of synthesis of two major glycoproteins with Mr 58,000 and 60,000. Cell surface carbohydrate labeling indicated that after 3611-MSV transformation the cells expressed Mr 100,000 and 68,000 polypeptides. The present and previous results show that viral transformation of epithelial cells induces different transformed phenotypes that are associated with distinct alterations in pericellular glycoproteins.

Construction of a general vector for efficient expression of mammalian proteins in bacteria: use of a synthetic ribosome binding site

With the premise that mRNAs transcribed in Escherichia coli from cloned eukaryotic DNA inserts do not possess the necessary regulatory signals for recognition by prokaryotic ribosomes, we have constructed a general plasmid vector carrying a chemically synthesized prokaryotic ribosome binding site that will ensure the efficient expression of eukaryotic proteins in E. coli. In addition to the regulatory signals necessary for ribosome recognition, the synthetic segment contains, at one end, a Pst I cleavage site which will direct its insertion to pBR322 DNA and, at the other end, a HindIII site to facilitate attachment of the passenger eukaryotic gene. Using simian virus 40 (SV40) tumor (t) antigen as a model system, we have ligated the SV40 DNA fragment containing the entire t antigen gene in tandem with the synthetic ribosome binding site to pBR322 DNA at the Pst I site, which lies within the coding sequence of the beta-lactamase gene. Initiation of transcription at the beta-lactamase promoter would produce a chimeric mRNA with the synthetic ribosome binding signals and the SV40 sequence flanked by beta-lactamase coding sequences. Utilization of the synthetic regulatory signals for initiation of translation is demonstrated by the efficient synthesis, in bacterial transformants, of authentic SV40 t antigen. Excision of the entire SV40 insert by HindIII from those clones that have retained intact HindIII sites at the junction between the ribosome binding site and the SV40 sequence would allow insertion of other heterologous DNAs by using HindIII linkers. The efficient expression of any DNA insert would require that the entire coding sequence be contiguous and that its termini be randomized by treatment with exonuclease III and nuclease S1 to vary the distance between the translational initiation codon and the synthetic ribosome binding site.

p53 transformation-related protein: detection of an associated phosphotransferase activity

Malignant cells of the mouse transformed by a variety of different agents have been found to express high levels of a 53,000 Mr phosphoprotein (designated p53). Little or no p53 can be detected in normal mouse cells. The nucleus appears to be the predominant site of p53 localization in transformed cells. p53-related antigens are also found in transformed cells of rat, hamster, rabbit, and human. In cells transformed by simian virus 40 (SV40), p53 forms a complex with SV40 tumor (t) antigen, resulting in the coprecipitation of T antigen by monoclonal p53 antibodies. Immune complexes of p53 precipitated from extracts of SV40- or methylcholanthrene-transformed cells by monoclonal p53 antibodies have protein kinase activity. This enzymatic activity is dependent upon divalent cations, utilizing Mn2+ more effectively than Mg2+. The phosphorylation of p53 in this kinase reaction has been found to involve serine and threonine, but not tyrosine residues. In view of the finding that the transforming proteins of several different oncogenic viruses have kinase activity, the association of this activity with p53 is important with regard to the possibility of a common pathway of transformation by diverse agents.

Control of simian virus 40 gene expression at the levels of RNA synthesis and processing: thermally induced changes in the ratio of the simian virus 40 early mRNA's and proteins

Examination of the simian virus 40 early mRNA's from infected AGMK or CV-1 cells showed that the ratio of large T- to small t-antigen mRNA's increased with an increased incubation temperature. In tsA58 mutant-infected cells, an increased incubation temperature resulted in the overproduction of early RNAs'; however, the ratio of the early mRNA's was the same, at any temperature, in both wild-type- and tsA58-infected cells. Thus, the thermally induced alteration in the early mRNA ratios was apparently not affected by the tsA mutation or by the overproduction of early RNA in tsA mutant-infected cells. Time course studies at various temperatures showed that, although the ratio of large T- to small t-antigen mRNA's increased with temperature, at any one temperature it was consistent from early to late times of infection. Furthermore, the ratio of the early mRNA's adjusted in temperature shift experiments. Thus, the ratio of the early mRNA's appeared to be intrinsic to the thermodynamic environment of the cell. The thermally induced alterations in the early mRNA's were reflected at the protein level by parallel changes in the ratio of large T- to small t-antigens. These data suggest a level of gene expression control which may function at the stage of splicing.

Transformation-related antigens identified by monoclonal antibodies

Tumor-cell proteins that were antigenic in a syngeneic animal were identified by immunoprecipitation with monoclonal antibodies. Spleen cells of BALB/c mice immunized with plasma membranes of Kirsten RNA sarcoma virus-transformed BALB/3T3 cells were fused with NS-1 myeloma cells. Antibodies secreted into the culture fluid from these hybridomas were distinguished by their reactivity against proteins of different target cells. A total of 191 cultures were established; 143 produced antibodies that bound to BALB/3T3 cells transformed by the RNA sarcoma virus, of which antibodies from 82 bound to BALB/3T3 transformed with simian virus 40, and antibodies from 56 bound to BALB/3T3 cells. Thus, more than 50% of the cultures produced antibodies that possibly were specific to antigens of the transformed cell. Twenty different hybridomas have been cloned, and antibodies, from eight of these were found to immunoprecipitate five different proteins. A protein of approximately 32,000 daltons was precipitated from BALB/3T3 cells transformed by the RNA sarcoma virus, simian virus 40, or methylcholanthrene but not from untransformed BALB/3T3 cells. A protein of about 300,000 daltons was precipitated from all four cell lines; precipitation was enhanced in the viral transformed cells. Proteins of approximately 57,000, 54,000, and 8500 daltons were immunoprecipitated from all four cell lines.

Effect of a tsA mutation of simian virus 40 late gene expression: variations between host cell lines

Infection of AGMK or CV-1 cells by the early simian virus 40 mutant tsA58 at the permissive temperature (32 degrees C) followed by a shift to the nonpermissive temperature (41 degrees C) caused a substantial decrease in the levels of late viral RNA in the cytoplasm of AGMK cells but not CV-1 cells. At the translational level, this depression of late viral RNA levels was reflected by a decrease in late viral protein synthesis. Thus, in AGMK cells, an early region gene product (presumably large T-antigen) appeared to be continuously required for efficient expression of the late viral genes. In contrast, late simian virus 40 gene expression, once it is initiated in CV-1 cells, continued efficiently regardless of the tsA mutation. The difference in expression of the late simian virus 40 genes in these tsA mutant-infected monkey kidney cell lines may reflect a difference in host cell proteins which regulate viral gene expression in conjunction with early viral proteins.

Plasma membrane phosphoproteins in normal and Rous sarcoma virus transformed chick embryo fibroblasts: characterization by in vitro phosphorylation

Plasma membranes isolated from normal and RSV transformed chick embryo fibroblasts were phosphorylated in vitro using endogenous protein kinase and ATP (gamma32P) and the labeled phosphoproteins were analyzed by SDS-PAGE. A number of protein phosphorylation changes were observed following transformation, however in most cases they were relatively small quantitative differences. The four major changes were in proteins of 47,000, 58,000, 75,000 and 135,000 daltons. Decreased phosphorylation of the 47,000 dalton polypeptide was found in transformed cell membranes but this alteration was shown to be due to differences in cell growth rather than transformation. Increase phosphorylation of the 75,000 dalton protein was at least partially related to virus infection. However, increased phosphorylation of the 58,000 and 135,000 dalton polypeptides were entirely transformation specific.

Immunofluorescence on avian sarcoma virus-transformed cells: localization of the src gene product

The localization of the avian sarcoma virus src gene product (termed p60src) was examined by indirect immunofluorescence in cells transformed by the Schmidt-Ruppin strain of Rous sarcoma virus, subgroup D (SR-RSV-D). Antiserum to p60src was obtained from rabbits bearing SR-RSV-D-induced tumors, and immunofluorescence was performed on chicken embryo fibroblasts (CEF) transformed with SR-RSV-D, as well as normal rat kidney (NRK) cells transformed by the same virus (termed SR-RK cells). Both acetone and formaldehyde fixation were used for the immunofluorescence tests. The specificity of the anti-tumor serum was first demonstrated in both cell systems by gel electrophoresis of immunoprecipitates prepared from 35S--methionine-labeled cells. Anti-tumor serum precipitated p60src from SR-RSV-D-transformed CEF but not from CEF infected with a transformation-defective mutant of SR-RSV-D. All viral structural proteins and precursors contained in these immunoprecipitates could be eliminated by competition with unlabeled virus. Similar experiments on SR-RK cells indicated that no viral proteins other than p60src were expressed in these cells, and this observation was supported by immunofluorescence tests using antiserum to whole virus. For immunofluorescence localization of p60src, reactions with viral structural proteins were blocked with unlabeled virus. This presaturation step, obligatory for p60src detection in the SR-RSV-D-transformed CEF, was unnecessary when antitumor serum was tested on SR-RK cells, since p60src was the only viral protein detectable in these cells. With acetone-fixed cells, p60src-specific immunofluorescence revealed a characteristic fluorescence pattern which was similar in both cell systems. The principal pattern was diffuse and situated in the cytoplasm. A clear nuclear fluorescence was never observed. Immunofluorescence on formaldehyde-fixed cells also indicated the cytoplasmic location of p60src and revealed a specific subcytoplasmic concentration of the fluorescence. With both fixation methods, an additional fluorescence pattern was seen between cells in contact, and was also found in both SR-RK cells and SR-RSV-D-transformed CEF. Immunofluorescence on viable cells suggested that p60src was not on the surface of these transformed cells. The fluorescence patterns were specific for avian sarcoma virus-transformed cells and were not found in uninfected cells, cells infected with a transformation-defective mutant of SR-RSV-D or cells transformed by an antigenically unrelated murine sarcoma virus. Furthermore, anti-tumor serum did not contain antibodies to proteins of the microtubules or intermediate filaments.

Evidence that the transforming gene of avian sarcoma virus encodes a protein kinase associated with a phosphoprotein

Avian sarcoma virus (ASV) induces sarcomas in animals and transforms fibroblasts to a neoplastic state in cell culture. A single viral gene (src) is responsible for both the induction and maintenance of neoplastic transformation. Recent work has identified a protein with a molecular weight of 60,000 daltons that is apparently encoded in src and may be the effector molecule for the gene (Brugge and Erikson, 1977; Purchio et al, 1978). The putative product of src can be immunoprecipitated by antisera obtained from rabbits bearing tumors induced by ASV. We have used this approach to isolate the protein to characterize further its genetic origins and possible function. Our rabbit tumor antisera precipitated a protein with a molecular weight of 60,000 daltons; according to serological, biochemical and genetic criteria, this protein is encoded in src. We found that this protein is phosphorylated and therefore denoted it pp60. Phosphorylation of pp60 could be accomplished in vitro with extracts of ASV-infected cells. A temperature-sensitive conditional mutation in src had no demonstrable effect on either the production or stability of pp60 in the infected cell, but phosphorylation of the protein was temperature-sensitive. Since the mutant src is not expressed at the restrictive temperature, our findings raise the possibility that phosphorylation of pp60 is required for its function as the putative effector of src. Immunoprecipitates prepared with extracts of ASV-infected cells and the rabbit tumor antisera contained a protein kinase activity that catalyzed phosphorylation of the heavy chains of immunoglobulin molecules, using either ATP or GTP as phosphate donor. The kinase activity immunoprecipitated in parallel with pp60 was obtained only from cells that contained a functioning product of src and could not be precipitated with antisera directed against structural proteins of ASV. A temperature-sensitive conditional mutation in src caused the kinase activity to be thermally inactivated in vitro far more rapidly than the activity from cells infected with wild-type virus. We conclude that both the protein kinase and pp60 are encoded in src, and that the enzymatic activity may be an intrinsic property of pp60. Phosphorylation of pp60 in cellular extracts was inhibited by calcium ion, whereas the immunoprecipitable kinase activity was not, suggesting that the kinase responsible for pp60 phosphorylation may be distinct from that encoded in src. Collett and Erikson (1978) have also identified a protein kinase activity associated with pp60. These findings raise the possibility that phosphorylation of specific cellular targets might account for transformation of the host cell by src.

Cellular transformation and the 'morphologic phenotype' of transformed cells

Expression of the product of the transforming gene (src) of RNA tumour viruses promotes growth and usually alters the adhesion, appearance and surface properties of cultured fibroblasts. The latter group of properties termed the 'morphologic phenotype' of transformed cells is largely due to diminished cell-to-substratum adhesion. The role of cyclic AMP, cell surface protein (CSP), and other factors in producing the 'morphologic phenotype' are discussed. The effects of src expression bear a striking resemblance to the action of peptide hormones such as insulin on appropriate target cells.