Esh avian sarcoma virus codes for a gag-linked transformation-specific protein with an associated protein kinase activity

Expression of the yes proto-oncogene in cerebellar Purkinje cells

To identify the kinds of cells in the brain that express the yes proto-oncogene, we examined chicken brains by using immunofluorescent staining and in situ hybridization. Both approaches showed that the highest level of the yes gene product was in cerebellar Purkinje cells. In addition, we analyzed Purkinje cell degeneration (pcd) mutant mice. The level of yes mRNA in cerebella of pcd mutants was four times lower than that found in cerebella of normal littermates. Our studies point to Purkinje cells as an attractive model for functional studies of the yes protein.

Expression of the yes proto-oncogene in cerebellar Purkinje cells

To identify the kinds of cells in the brain that express the yes proto-oncogene, we examined chicken brains by using immunofluorescent staining and in situ hybridization. Both approaches showed that the highest level of the yes gene product was in cerebellar Purkinje cells. In addition, we analyzed Purkinje cell degeneration (pcd) mutant mice. The level of yes mRNA in cerebella of pcd mutants was four times lower than that found in cerebella of normal littermates. Our studies point to Purkinje cells as an attractive model for functional studies of the yes protein.

Neurooncogenes

Using monospecific antibodies raised against the amino terminal domain of viral-yes protein, we precipitated a 58 kD protein-tyrosine kinase from the brain of Xenopus laevis frog. By a number of criteria, including pattern of expression in various tissues and one-dimensional peptide mapping, we concluded that pp58 is very likely the authentic amphibian yes protein because it is more similar to the chicken yes protein than to any other known tyrosine kinases. The pp58c-yes is expressed in adult brain at elevated levels. In contrast, its level of expression in follicular and denuded oocytes is 30-50 times lower. Because of the low endogenous expression of the oocyte-associated yes kinase, we were able to transiently express and analyze pp62c-yes by injecting oocytes with 27S size fraction of poly(A)+ mRNA isolated from chicken cerebella. The pp62c-yes expressed in oocytes from the exogenous message comigrated on SDS polyacrylamide gel with pp62c-yes from cerebellum and was indistinguishable from it by one-dimensional peptide mapping. The pp62c-yes expressed in oocytes was enzymatically active. A number of phosphotyrosine-containing proteins were detected specifically in the c-yes mRNA-injected oocytes. The usefulness of the Xenopus oocyte expression system to study the functional aspects of c-yes protein and other tyrosine kinases was evaluated.

Avian sarcoma viruses

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

A simple method for immunoaffinity purification of nondenatured avian sarcoma and leukemia virus gag-containing proteins

We have developed a one-step purification procedure for proteins containing the N-terminal portion of the gag protein of avian sarcoma and leukemia viruses. In this procedure, a resin with a covalently attached monoclonal antibody to the gag protein p19 is used to bind gag-containing proteins from crude extracts. After washing of the resin, the bound proteins are eluted with 2 M MgCl2. For the transforming protein kinase encoded by Fujinami sarcoma virus p130gag-fps, this procedure gave an enrichment of several thousand-fold, a yield of over 10%, a final purity of over 20%, and no significant loss of protein kinase activity. Similar purifications were obtained with three other gag-containing proteins. The immunoaffinity purification described may be of general utility as a first step in purification of the several other avian retroviral transforming proteins that are synthesized from fusions of an oncogene with the viral gag gene.

Expression of the v-erbA oncogene in chicken embryo fibroblasts stimulates their proliferation in vitro and enhances tumor growth in vivo

In contrast to uninfected chicken embryo fibroblasts (CEFs), CEFs infected with a retroviral vector that carries the v-erbA gene of avian erythroblastosis virus displayed new properties. These included limited anchorage-independent growth in soft agar, growth without latency in serum-supplemented medium, ability to overcome quiescence induced by serum deprivation, growth at low cell density, and an extended life span in vitro. Furthermore, when explanted in vivo onto the chorioallantoic membrane of chicken embryo, the transformed CEFs expressing v-erbA in addition to v-erbB exhibited a high proliferative rate, giving rise to fibrosarcoma tumors that were ten times larger than those developed from transformed CEFs expressing v-erbB alone. All these data show that CEFs expressing the v-erbA oncogene display activated growth and suggest that the v-erbA product interferes with the mechanisms regulating the growth and/or differentiation of primary CEFs.

Cellular proteins homologous to the viral yes gene product

We raised antibodies in rabbits against the amino-terminal portion of the viral yes protein produced in bacteria with the use of an expression vector based on the lac operon. The anti-yes serum thus obtained precipitated P90gag-yes from Yamaguchi 73 virus-transformed chicken embryo fibroblasts, and this immunoprecipitation was blocked by the purified antigen. The anti-yes serum did not recognize viral src, fps, or fgr proteins. Affinity-purified anti-yes immunoglobulin G (IgG) precipitated two proteins of 59 and 62 kilodaltons from lysates of normal chicken embryo fibroblasts. Two-dimensional tryptic peptide mapping showed that these proteins are closely related to P90gag-yes and that they are different from pp60c-src. Similar to P90gag-yes, the 59- and 62-kilodalton proteins were phosphorylated exclusively on tyrosine in an in vitro kinase reaction, whereas in vivo they were phosphorylated on serine and, to a lesser extent, on tyrosine as well. Expression of the 59- and 62-kilodalton proteins, determined by the immune complex kinase assay, was relatively high in brain, retina, kidney, and liver. The presence in normal chicken embryo fibroblasts and in chicken kidney of two transcripts, 3.7 and 3.9 kilobases in length, that hybridize with a yes-specific DNA probe, as well as the two proteins recognized by anti-yes IgG, suggests either differential splicing of cellular yes gene transcripts or the existence of another yes-related gene.

Cellular proteins homologous to the viral yes gene product

We raised antibodies in rabbits against the amino-terminal portion of the viral yes protein produced in bacteria with the use of an expression vector based on the lac operon. The anti-yes serum thus obtained precipitated P90gag-yes from Yamaguchi 73 virus-transformed chicken embryo fibroblasts, and this immunoprecipitation was blocked by the purified antigen. The anti-yes serum did not recognize viral src, fps, or fgr proteins. Affinity-purified anti-yes immunoglobulin G (IgG) precipitated two proteins of 59 and 62 kilodaltons from lysates of normal chicken embryo fibroblasts. Two-dimensional tryptic peptide mapping showed that these proteins are closely related to P90gag-yes and that they are different from pp60c-src. Similar to P90gag-yes, the 59- and 62-kilodalton proteins were phosphorylated exclusively on tyrosine in an in vitro kinase reaction, whereas in vivo they were phosphorylated on serine and, to a lesser extent, on tyrosine as well. Expression of the 59- and 62-kilodalton proteins, determined by the immune complex kinase assay, was relatively high in brain, retina, kidney, and liver. The presence in normal chicken embryo fibroblasts and in chicken kidney of two transcripts, 3.7 and 3.9 kilobases in length, that hybridize with a yes-specific DNA probe, as well as the two proteins recognized by anti-yes IgG, suggests either differential splicing of cellular yes gene transcripts or the existence of another yes-related gene.

Identification and preferential expression in thymic and bursal lymphocytes of a c-ets oncogene-encoded Mr 54,000 cytoplasmic protein

The avian retrovirus E26 is unique among acute leukemia viruses in its ability to induce transformation of cells belonging to either the myeloid or erythroid lineage. The genome of E26 carries two oncogenes, v-myb and v-ets, that are derived from distinct cellular loci, c-myb and c-ets. We have constructed a plasmid vector that allows expression of part of the coding region of v-ets in a bacterial host. Antisera to the bacterially synthesized ets protein specifically precipitated the E26-encoded P135gag-myb-ets transforming protein. These antisera permitted us to identify a chicken c-ets-encoded protein of Mr 54,000 (P54c-ets) that shares 7 out of 10 of its major [35S]methionine-containing tryptic peptides with the v-ets-encoded domain of P135gag-myb-ets. Unlike P135gag-myb-ets and the Mr 75,000 translation product of c-myb (P75c-myb), which are nuclear proteins, P54c-ets was found to be predominantly cytoplasmic. P54c-ets is expressed at low levels in most cell lines and tissues tested, including bone marrow cells and circulating lymphocytes. P54c-ets, together with a minor but closely related Mr 56,000 protein, was found to be expressed at high levels in chicken thymocytes and bursal lymphocytes.

Use of site-specific antipeptide antibodies to perturb the serine kinase catalytic activity of p37mos

The mos oncogene of Moloney murine sarcoma virus encodes a protein of approximately 37,000 daltons (designated p37mos). We have detected a serine protein kinase activity which is closely associated with p37mos in immune complexes obtained with antibodies [anti-mos(37-55) serum] that were generated with a peptide containing amino acids 37 through 55 of the v-mos protein (S. A. Maxwell and R. B. Arlinghaus, Virology 143:321-333, 1985). Immune complexes that were derived with antibodies generated against peptides representing the C-terminal 8 or 12 amino acids of v-mos (anti-C2 and anti-C3 serum, respectively) exhibited very little kinase activity capable of phosphorylating p37mos. Treatment of anti-mos(37-55) complexes containing active v-mos kinase with anti-C3 or anti-C2 serum resulted in a dramatic reduction of the in vitro phosphorylation of p37mos. Antiserum blocked with the appropriate C-terminal peptide had no inhibitory effect on the phosphorylation of p37mos in anti-mos(37-55) complexes which indicated that the inhibition of v-mos kinase activity was a specific effect of these antibodies. The specific inhibition of the in vitro phosphorylation of p37mos by antibodies directed against the C terminus of the v-mos protein provides strong evidence that the v-mos gene encodes a serine protein kinase. In addition, the extreme C terminus of p37mos may be critical for an active v-mos kinase.

Induction of proliferation or transformation of neuroretina cells by the mil and myc viral oncogenes

The genome of the avian retrovirus MH2 contains, in addition to the v-myc oncogene shared with three other avian retroviruses (MC29, CMII and OK-10), a second cell-derived oncogene, v-mil (refs 1-3). Like the three other viruses, which contain only v-myc, MH2 induces mainly liver and kidney carcinomas in fowl and transforms fibroblasts and macrophages in vitro. However, MH2 and MC29 differ in their biological properties when assayed on cultures of chicken embryo neuroretina (NR) cells. Indeed, NR cells, which normally do not multiply in vitro, are induced to proliferate and become transformed upon infection with MH2, whereas infection with MC29 has no apparent effect on these cells. To analyse the functions of the two oncogenes of MH2, we isolated spontaneous and in vitro-constructed mutants of this virus and investigated their effects on NR cell multiplication and transformation. We report here that expression of v-mil is sufficient to induce NR cell proliferation, although it does not result in cell transformation. In addition, viruses expressing only the v-myc oncogene fail to induce any detectable change in NR cells. However, cooperation of the two oncogenes is required to achieve transformation of NR cells by MH2.

Serine kinase activity associated with Maloney murine sarcoma virus-124-encoded p37mos

An antiserum directed against amino acid residues 37-55 [anti-mos (37-55) serum] of the predicted v-mos sequence was used to precipitate p37mos from Moloney murine sarcoma virus-124 (Mo-MuSV-124) acutely infected 3T3 cells. Proteins with sizes ranging from p37mos to 43 kDa (p43) were found to be phosphorylated when anti-mos (37-55) immune complexes containing p37mos were incubated with [gamma-32P]ATP and Mn2+. The phosphorylation of p37mos and p43 could be specifically blocked when the anti-mos (37-55) serum was incubated with 37-55 cyclic mos peptide prior to immunoprecipitation, but not if the serum was preincubated with an unrelated peptide representing amino acids of the myc protein sequence. Anti-mos (37-55) immune complexes from uninfected 3T3 cells did not produce any phosphorylated proteins the size of p37mos or p43. However, a 50-kDa protein (p50) was phosphorylated in both unblocked and mos peptide-blocked anti-mos (37-55) immune complexes from infected 3T3 cells, and in immune complexes from uninfected cells. Quercetin, an inhibitor of some protein kinases, inhibited the kinase phosphorylating p50 but not the kinase phosphorylating p37mos and p43. Preabsorption of the cell extract prior to immunoprecipitation with an excess of formalin-fixed Staphylococcus aureus, complexed with preimmune normal rabbit serum IgG, specifically removed the kinase phosphorylating p50. The amount of in vitro phosphorylated p37mos and p43 in the immune-complex kinase assay reached a maximum in extracts of 3T3 cells 2-3 days postinfection with Mo-MuSV 124 but decreased to trace levels after 5 days. Metabolically and in vitro phosphorylated p37mos generated an identical pattern of phosphopeptides upon partial V8 protease digestion. Based on peptide mapping and a kinetic analysis of the in vitro phosphorylation reaction, p37mos appears to be a precursor to the p43 phosphorylated species. Phosphoamino acid analyses revealed only phosphoserine in in vitro phosphorylated p37mos and p43mos. It was concluded that p37mos is closely associated with a serine kinase activity and that the in vitro phosphorylation of p37mos may lead to formation of a highly modified mos protein (p43) by way of superphosphorylation.

Common features of the yes and src gene products defined by peptide-specific antibodies

Anti-peptide antibodies generated against a hydrophilic domain of pp60src comprising amino acid residues 498 through 512 were shown to be cross-reactive with the corresponding region in the yes transforming proteins encoded by Yamaguchi 73 and Esh sarcoma viruses. This cross-reactivity was demonstrated by immunoblot and immunoprecipitation analyses, and the identity of the proteins was verified by partial proteolytic mapping. By utilizing a combination of immunofluorescence and interference-reflection microscopy, these cross-reactive anti-peptide antibodies were shown to produce an immunofluorescence staining pattern in Yamaguchi 73 and Esh sarcoma virus-transformed chicken embryo fibroblasts remarkably similar to that pp60src in Rous sarcoma virus-infected chicken cells. Like the src gene products, the yes transformation-specific polyproteins were found to be concentrated within adhesion plaque structures and needle-like interdigitating cell-cell junctions. This analogous subcellular distribution suggests that these onc proteins are functionally related and may share common intracellular targets.

Tyrosine-specific protein kinases of normal tissues

Tyrosine-specific protein kinases from normal tissue have been studied using synthetic peptides as substrate. Spleen had much higher activity of the enzyme in the particulate fraction than any other normal tissue (except purified T lymphocytes). The tyrosine protein kinase from the particulate fraction of rat spleen was partially purified and characterized. The kinase could phosphorylate src-related as well as unrelated peptides and casein at tyrosine residues. The enzyme in the membrane seemed to have somewhat different substrate specificity than the solubilized, partially purified enzyme. Serum containing antibody to pp60v-src did not precipitate the kinase; however, the protein kinase could phosphorylate the heavy chain of IgG from TBR serum (but not from normal serum). The possible relationship of the tyrosine-specific protein kinase of spleen with pp60c-src and other tyrosine-specific protein kinases is discussed.

Avian sarcoma viruses, protein kinases and cell transformation

The first RNA tumour virus to be isolated and identified as such was the Rous sarcoma virus (RSV), which causes the transformation of cells in culture as well as fibrosarcomas when injected into suitable host animals (for reviews see Hanafusa (1977) and Bishop (1978)). The genome of RSV has been studied intensively for the past 10-12 years, and it has been shown that the virus itself carries a gene responsible for malignant transformation. This gene, denoted src for sarcoma, was identified genetically through the isolation of temperature-sensitive mutants that were conditional for cell transformation in culture. These mutants are able to transform cells at a temperature of 35 °C, the permissive temperature, but are unable to transform cells morphologically at 41 °C, the non-permissive temperature. The existence of such temperature sensitive mutants implied that the product of the viral transforming gene, in RSV, was a protein (Kawai & Hanafusa 1971). In addition to temperature-sensitive mutants, non-conditional mutants were isolated that had deletions of the src gene. These mutants are unable to transform cells in culture or to cause fibrosarcomas under most conditions. About 4 years ago, the product of the src gene was identified as a phosphoprotein ( M t = 60000); this protein was denoted pp60 src (Purchio et al. 1978). The RSV genome and the expression of the src gene is illustrated in figure 1.

Translational stability of plant viral RNAs microinjected into living cells. Influence of a 3'-poly(A) segment

Three different alternative structural features have been shown to be present at the 3' terminus of plant viral RNAs: (a) a poly(A) track, (b) a tRNA-like structure, (c) no special structural or sequence characteristic. We have compared the translational stability after injection into frog oocytes of a representative of each type: (a) the small genomic RNA (M-RNA) of cowpea mosaic virus (CPMV), (b) the subgenomic mRNA for coat protein (RNA 4) of brome mosaic virus (BMV), (c) the subgenomic mRNA for coat protein (RNA 4) of alfalfa mosaic virus (AIMV). It has been shown that CPMV M-RNA exhibits the highest translational stability. However, the stability of AIMV RNA 4 is remarkably high and moreover significantly higher than that of BMV RNA 4. We demonstrate that, for all three viral RNA species considered, the presence of a poly(A) segment at the 3' end of the molecules improves the translational stability. From a comparative investigation in which AIMV RNA 4 was also injected into HeLa cells, it is concluded that the stability of a given non-adenylylated mRNA depends on the nature of the cytoplastic environment.

Association of the transforming proteins of the ST and GA strains of feline sarcoma virus and their in vitro associated protein kinase activities with cellular membranes

The translation products of the Snyder-Theilen (ST) and Gardner-Arnstein (GA) strains of feline sarcoma virus (FeSV), termed gag-fes proteins, are high molecular weight polyproteins containing different amounts of the amino terminus of the feline leukemia virus (FeLV) gag gene-coded precursor protein linked to a similar sarcoma virus-specific polypeptide. Both polyproteins are phosphoproteins with indistinguishable in vitro associated tyrosine-specific protein kinase activities. The polyproteins are extremely hydrophobic proteins which are intimately associated with the plasma membrane fraction of transformed cells. Approximately 10% of the proteins are modified by glycosylation and expressed on the cell surface where they are accessible to lactoperoxidase-mediated radio-iodination and trypsinization. Cell surface localization of the polyproteins does not appear to be necessary for transformation. However, preliminary evidence suggests that the amount of FeLV p30 sequences at the amino end of the proteins may have some effect on the intracellular distribution of the gag-fes polyproteins and on the phenotype of the transformed cell.

The transforming proteins of PRCII virus and Rous sarcoma virus form a complex with the same two cellular phosphoproteins

P105 and P110, the presumptive transforming proteins of PRCII avian sarcoma virus, have been found to be present in transformed chicken cells in two forms: as monomers and as part of a complex which contains both a 50,000-dalton and a 90,000-dalton cellular phosphoprotein. The 90,000-dalton cellular protein was found to be identical to one of the proteins in chicken cells whose synthesis is induced by stress. The 50,000-dalton protein was found to contain phosphotyrosine when isolated from the complex and therefore may be a substrate for the tyrosine protein kinase activity which is associated with P105 and P110. These same two cellular phosphoproteins have previously been shown to be present in a complex with pp60src, the tyrosine protein kinase which is the transforming protein of Rous sarcoma virus. However, not all avian sarcoma virus transforming proteins with associated tyrosine protein kinase activities form a complex efficiently with these cellular proteins. Little if any of P90, the putative transforming protein of Yamaguchi 73 virus, was found in a complex with the 50,000-dalton and 90,000-dalton cellular phosphoproteins.

Platelet-derived growth factor stimulates tyrosine-specific protein kinase activity in Swiss mouse 3T3 cell membranes

Platelet-derived growth factor (PDGF) stimulates the incorporation of 32P from [gamma-32P]ATP into a Mr approximately 170,000 protein by an endogenous tyrosine-specific protein kinase in membrane preparations of Swiss mouse 3T3 cells. Epidermal growth factor (EGF), but not fibroblast growth factor (FGF) or insulin, stimulates limited incorporation of 32P into a protein of similar molecular weight. The ligand concentration required for half-maximal activity (S0.5) for PDGF stimulation of phosphorylation is 50 ng/ml; saturation is achieved at 300 ng/ml. The S0.5 for ATP is 15 microM. Mg2+ or Mn2+ is required for protein kinase activity. Stimulation of PDGF results in the preferential phosphorylation of tyrosine residues in this Mr approximately 170,000 membrane protein. The Mr approximately 170,000 protein can be resolved into Mr approximately 180,000 and 160,000 components in 4% NaDodSO4 gels. PDGF stimulates 32P incorporation preferentially into the Mr approximately 180,000 and less extensively into the Mr approximately 160,000 protein. EGF stimulates 32P incorporation predominantly into a protein of Mr approximately 160,000. The similarity of PDGF and EGF in stimulating phosphotyrosine-specific protein kinase activity and the stimulation of a similar activity by viral transformation (src) genes suggest that a common mechanism may exist for the phenotypic expression of increased DNA synthesis and cell growth stimulated by these separate factors.

Avian sarcoma virus UR2 encodes a transforming protein which is associated with a unique protein kinase activity

UR2 is a newly characterized avian sarcoma virus whose genome contains a unique sequence that is not related to the sequences of other avian sarcoma virus transforming genes thus far identified. This unique sequence, termed ros, is fused to part of the viral gag gene. The product of the fused gag-ros gene of UR2 is a protein of 68,000 daltons (P68) immunoprecipitable by antiserum against viral gag proteins. In vitro translation of viral RNA and in vivo pulse-chase experiments showed that P68 is not synthesized as a large precursor and that it is the only protein product encoded in the UR2 genome, suggesting that it is involved in cell transformation by UR2. In vivo, P68 was phosphorylated at both serine and tyrosine residues. Immunoprecipitates of P68 with anti-gag antisera had a cyclic nucleotide-independent protein kinase activity that phosphorylated P68, rabbit immunoglobulin G in the immune complex, and alpha-casein. The phosphorylation by P68 was specific to tyrosine of the substrate proteins. P68 was phosphorylated in vitro at only one tyrosine site, and the tryptic phosphopeptide of in vitro-labeled P68 was different from those of Fujinami sarcoma virus P140 and avian sarcoma virus Y73-P90. A comparison of the protein kinases encoded by UR2, Rous sarcoma virus, Fujinami sarcoma virus, and avian sarcoma virus Y73 revealed that UR2-P68 protein kinase is distinct from the protein kinases encoded by those viruses by several criteria. Our results suggest that several different protein kinases encoded by viral transforming genes have the same functional specificity and cause essentially the same cellular alterations.

Synthetic peptide fragment of src gene product inhibits the src protein kinase and crossreacts immunologically with avian onc kinases and cellular phosphoproteins

All the known avian sarcoma viruses have associated protein kinase activities that phosphorylate tyrosine residues of their target proteins. A decapeptide fragment of pp60src of Rous sarcoma virus (RSV), residues 415-424, and an analog of that sequence have been chemically synthesized by solid-phase methods. The two decapeptides were not phosphorylated by pp60src of RSV, P90 of Y73 avian sarcoma virus, or P140 of Fujinami sarcoma virus. However, both peptides were able to inhibit competitively the kinase activities associated with the transforming proteins. Antiserum was raised against one of the peptides and IgG was purified from the serum by affinity chromatography. The antibody was able to precipitate pp60src of RSV as well as P90 of Y73 virus from cells infected with these viruses. The antibody also precipitated a number of high molecular weight phosphoproteins from normal chicken and rat fibroblasts and from several lines of virus-transformed cells.

Proposal for naming host cell-derived inserts in retrovirus genomes

We propose a system for naming inserted sequences in transforming retroviruses (i.e., onc genes), based on using trivial names derived from a prototype strain of virus.

Cleavage of four avian sarcoma virus polyproteins with virion protease p15 removes gag sequences and yields large fragments that function as tyrosine phosphoacceptors in vitro

The transformation-specific polyproteins of avian sarcoma viruses PRCII, PRCII-p, Fujinami sarcoma virus (FSV), and Esh sarcoma virus (ESV) consist of two domains, one derived from a partial viral gag gene and the other representing an apparently cell-derived insert in the defective viral genome. These gag-linked proteins were cleaved with retrovirion protease p15. Cleavage of PRCII-p polyprotein P170, P105 of PRCII, and P140 of FSV occurred within the gag domain and generated fragments of Mr 130,000, 70,000, and 115,000, respectively, containing all of the transformation-specific sequences linked to a remnant of the original gag sequences. ESV P80 was cleaved inside the transformation-specific domain, yielding a Mr 35,000--38,000 fragment from the NH2-terminal half of the molecule consisting of the entire gag portion and some no-gag sequences and a Mr 48,000 fragment containing most of the transformation-specific sequences. The tyrosine phosphorylation sites of the polyproteins were found in every case in the transformation-specific fragments. The major serine phosphorylation site of ESV P80 was found to reside in the Mr 35,000--38,000 gag-containing fragment, probably within the transformation-specific sequences of that cleavage product. Removal of all of the gag domain of ESV P80 or most of the gag domain in PRCII-p P170, PRCII P105, and FSV P140 does not affect their ability to be phosphorylated by the polyprotein-associated tyrosine-specific protein kinase activities. This observation suggests that the gag sequences of the polyproteins of classes II (PRCII-p, PRCII, and FSV) and III (ESV) avian sarcoma viruses may not be required for this enzymatic function, which appears to be of importance in transformation.