Four Rous sarcoma virus mutants which affect transformed cell morphology exhibit altered src gene products

Transforming properties and substrate specificities of the protein tyrosine kinase oncogenes ros and src and their recombinants

To determine the sequences of the oncogenes src (encoded by Rous sarcoma virus [RSV]) and ros (encoded by UR2) that are responsible for causing different transformation phenotypes and to correlate those sequences with differences in substrate recognition, we constructed recombinants of the two transforming protein tyrosine kinases (PTKs) and studied their biological and biochemical properties. A recombinant with a 5' end from src and a 3' end from ros, called SRC x ROS, transformed chicken embryo fibroblasts (CEF) to a spindle shape morphology, mimicking that of UR2. Neither of the two reverse constructs, ROS x SRC I and ROS x SRC II, could transform CEF. However, a transforming variant of ROS x SRC II appeared during passages of the transfected cells and was called ROS x SRC (R). ROS x SRC (R) contains a 16-amino-acid deletion that includes the 3' half of the transmembrane domain of ros. Unlike RSV, ROS x SRC (R) also transformed CEF to an elongated shape similar to that of UR2. We conclude that distinct phenotypic changes of RSV- and UR2-infected cells do not depend solely on the kinase domains of their oncogenes. We next examined cellular proteins phosphorylated by the tyrosine kinases of UR2, RSV, and their recombinants as well as a number of other avian sarcoma viruses including Fujinami sarcoma virus Y73, and some ros-derived variants. Our results indicate that the UR2-encoded receptorlike PTK P68gag-ros and its derivatives have a very restricted substrate specificity in comparison with the nonreceptor PTKs encoded by the rest of the avian sarcoma viruses. Data from ros and src recombinants indicate that sequences both inside and outside the catalytic domains of ros and src exert a significant effect on the substrate specificity of the two recombinant proteins. Phosphorylation of most of the proteins in the 100- to 200-kDa range correlated with the presence of the 5' src domain, including the SH2 region, but not with the kinase domain in the recombinants. This corroborates the conclusion given above that the kinase domain of src or ros per se is not sufficient to dictate the transforming morphology of these two oncogenes. High-level tyrosyl phosphorylation of most of the prominent substrates of src is not sufficient to cause a round-shape transformation morphology.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

Deletions and insertions within an amino-terminal domain of pp60v-src inactivate transformation and modulate membrane stability

We previously showed (V. W. Raymond and J. T. Parsons, Virology 160:400-410, 1987) that variants of the Prague A strain of Rous sarcoma virus containing large deletions impinging on a region of the src gene encoding amino acid residues 143 to 169 were defective for transformation of chicken cells in culture. Here we report that introduction of small (tri-and tetrapeptide) deletions into a region of pp60v-src containing amino acid residues 155 to 175 was found to inactivate transformation. In addition, insertion of four, but not one, amino acid residues at position 161 also inhibited transformation. Biochemical analysis of the src proteins encoded by individual transformation-defective variants revealed that the structural alterations introduced into this domain had only marginal effects upon src tyrosine-specific protein kinase activity. However, the src proteins encoded by defective variants exhibited a significantly shorter half-life within the cell, although these proteins efficiently and rapidly associated with cellular membranes. Our results suggest that the structural domain encompassing residues 155 to 177 may influence the stability of pp60src in the cellular membrane, possibly via the interaction of src with a cellular membrane component(s) or substrate(s).

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.

Rous sarcoma virus mutant dlPA105 induces different transformed phenotypes in quail embryonic fibroblasts and neuroretina cells

dlPA105 is a spontaneous variant of Rous sarcoma virus, subgroup E, which carries a deletion in the N-terminal portion of the v-src gene coding sequence. This virus was isolated on the basis of its ability to induce proliferation of quiescent quail neuroretina cells. The altered v-src gene encodes a phosphoprotein of 45,000 daltons which possesses tyrosine kinase activity. DNA sequencing of the mutant v-src gene has shown that deletion extends from amino acid 33 to 126 of wild-type p60v-src. We investigated the tumorigenic and transforming properties of this mutant virus. dlPA105 induced fibrosarcomas in quails with an incidence identical to that induced by wild-type virus. Quail neuroretina cells infected with the mutant virus were morphologically transformed and formed colonies in soft agar. In contrast, dlPA105 induced only limited morphological alterations in quail fibroblasts and was defective in promoting anchorage-independent growth of these cells. Synthesis and tyrosine kinase activity of the mutant p45v-src were similar in both cell types. These data indicate that the portion of the v-src protein deleted in p45v-src is dispensable for the mitogenic and tumorigenic properties of wild-type p60v-src, whereas it is required for in vitro transformation of fibroblasts. The ability of dlPA105 to induce different transformation phenotypes in quail fibroblasts and quail neuroretina cells is a property unique to this Rous sarcoma virus mutant and provides evidence for the existence of cell-type-specific response to v-src proteins.

DNA sequence encoding the amino-terminal region of the human c-src protein: implications of sequence divergence among src-type kinase oncogenes

We sequenced the 5'-coding region of the human c-src gene, exons 2 through 5, corresponding to one-third of the human c-src protein consisting of 536 amino acids. Sequence analysis of the src type of protein kinases revealed that the amino-terminal region encoded by exon 2 contains sequences specific for the src proteins and raised the possibility that this region is involved in the recognition of a src-specific substrate(s) or receptor(s).

Morphf mutants of Rous sarcoma virus: nucleotide sequencing analysis suggests that a class of morphf mutants was generated through splicing of a cryptic intron

The nature of the lesions involved in producing the fusiform phenotype of three mutants (WO101, WO201, and tsST529) of the Schmidt-Ruppin A strain of Rous sarcoma virus (RSV) was determined by molecular cloning and DNA sequencing. WO101 and WO201 contained an in-frame deletion of the v-src region coding for amino acids 116 to 140 of p60v-src. The deleted segment was flanked by consensus splice donor and acceptor sequences and contained an appropriately positioned branchpoint acceptor consensus sequence, suggesting that the deletion occurred through an aberrant RNA splicing event. S1 mapping experiments performed on RNA isolated from chicken cells infected with molecularly cloned wild-type RSV DNA suggested that the splice acceptor involved in the generation of this deletion was utilized at a low frequency (less than 1.0%) in wild-type RSV-infected cells. These results suggested that stable mutations may have arisen in the coding sequence of a eucaryotic viral transforming gene as a result of a probable aberrant RNA splicing event followed by reverse transcription into DNA. ST529 was found to harbor the same deletion present in WO101 and WO201 but also contained a point mutation which resulted in the substitution of lysine for glutamic acid at position 93. This change and the resulting large change in local charge were presumably required for the temperature-sensitive transformation phenotype of ST529. These results, together with other known deletions that produce fusiform mutants, suggested that a region within the amino-terminal one-third coding region of the src gene contributed to a structural domain of p60v-src that was important for controlling some morphological parameters of transformation in cells infected with RSV.

Creation of a chimeric oncogene: analysis of the biochemical and biological properties of v-erbB/src fusion polypeptide

A novel gene was created that linked complementary portions of two different tyrosine kinase oncogenes: v-erB and v-src. The v-erbB/src chimera encoded a glycoprotein exhibiting the subcellular distribution of the v-erbB protein but containing the kinase catalytic domain of the v-src parent. Fibroblasts expressing the v-erbB/src gene product became transformed to an oncogenic state and closely resembled cells expressing the v-erbB parent oncogene. Our results indicated that v-erbB sequences can be functionally replaced by sequences derived from a different oncogene, v-src, and that important determinants of the transformed phenotype appear to be encoded in oncogene sequences distinct from those defining the kinase catalytic domain itself.

DNA sequence encoding the amino-terminal region of the human c-src protein: implications of sequence divergence among src-type kinase oncogenes

We sequenced the 5'-coding region of the human c-src gene, exons 2 through 5, corresponding to one-third of the human c-src protein consisting of 536 amino acids. Sequence analysis of the src type of protein kinases revealed that the amino-terminal region encoded by exon 2 contains sequences specific for the src proteins and raised the possibility that this region is involved in the recognition of a src-specific substrate(s) or receptor(s).

Genetic analysis of p60v-src domains involved in the induction of different cell transformation parameters

The expression of p60v-src in chicken cells infected with Rous sarcoma virus causes stimulation of cell proliferation, morphological alteration, and anchorage independence. PA101 and PA104 are temperature-sensitive variants encoding mutant p60v-src proteins that are partially defective in the induction of these transformation parameters. To define the structural basis for the transformation defectiveness of the p60v-src mutants, the v-src genes of PA101 and PA104 were molecularly cloned and analyzed. Amino- and carboxy-terminal coding regions of the cloned mutant genes were exchanged with the corresponding regions of cloned wild-type v-src and chicken c-src genes, reconstructed into viral DNA, and expressed in infected cells maintained at various temperatures. This analysis revealed that lesions within the tyrosine kinase domains of the two mutant proteins confer temperature sensitivity on all three transformation functions of p60v-src. An amino-terminal region of the PA101 mutant protein, which coincides with the proposed modulatory domain and appears to interact with the kinase domain, affects morphological alteration in a temperature-independent manner. Our results suggest that the function of the kinase domain is essential to all three parameters examined, whereas the amino-terminal domain is important in determining cell morphology.

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

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

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

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

N-terminal deletions in Rous sarcoma virus p60src: effects on tyrosine kinase and biological activities and on recombination in tissue culture with the cellular src gene

We have constructed deletions within the region of cloned Rous sarcoma virus DNA coding for the N-terminal 30 kilodaltons of p60src. Infectious virus was recovered after transfection. Deletions of amino acids 15 to 149, 15 to 169, or 149 to 169 attenuated but did not abolish transforming activity, as assayed by focus formation and anchorage-independent growth. These deletions also had only slight effects on the tyrosine kinase activity of the mutant src protein. Deletion of amino acids 169 to 264 or 15 to 264 completely abolished transforming activity, and src kinase activity was reduced at least 10-fold. However, these mutant viruses generated low levels of transforming virus by recombination with the cellular src gene. The results suggest that as well as previously identified functional domains for p60src myristylation and membrane binding (amino acids 1 to 14) and tyrosine kinase activity (amino acids 250 to 526), additional N-terminal sequences (particularly amino acids 82 to 169) can influence the transforming activity of the src protein.

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

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

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

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

N-terminal deletions in Rous sarcoma virus p60src: effects on tyrosine kinase and biological activities and on recombination in tissue culture with the cellular src gene

We have constructed deletions within the region of cloned Rous sarcoma virus DNA coding for the N-terminal 30 kilodaltons of p60src. Infectious virus was recovered after transfection. Deletions of amino acids 15 to 149, 15 to 169, or 149 to 169 attenuated but did not abolish transforming activity, as assayed by focus formation and anchorage-independent growth. These deletions also had only slight effects on the tyrosine kinase activity of the mutant src protein. Deletion of amino acids 169 to 264 or 15 to 264 completely abolished transforming activity, and src kinase activity was reduced at least 10-fold. However, these mutant viruses generated low levels of transforming virus by recombination with the cellular src gene. The results suggest that as well as previously identified functional domains for p60src myristylation and membrane binding (amino acids 1 to 14) and tyrosine kinase activity (amino acids 250 to 526), additional N-terminal sequences (particularly amino acids 82 to 169) can influence the transforming activity of the src protein.

Human cellular src gene: nucleotide sequence and derived amino acid sequence of the region coding for the carboxy-terminal two-thirds of pp60c-src

The nucleotide sequence of the 3' two-thirds of a highly conserved, molecularly cloned human cellular src gene (c-src) has been determined. This region of the c-src gene encodes the tyrosine kinase domain of the cellular src protein (pp60c-src) and corresponds to exons 6 through 12 of the chicken c-src gene, as well as nucleotides 545 to 1542 of the Rous sarcoma virus src gene (v-src). The human c-src sequence is very strongly conserved with respect to both the chicken c-src and the Rous sarcoma virus v-src genes, with nearly 90% nucleotide homology observed in this region. Amino acid sequence conservation in this region is even greater; 98% of the amino acids are conserved between human and chicken c-src. Furthermore, the exon sizes and the locations of the exon-intron boundaries are identical in the human and chicken c-src genes. However, sequences within the introns have not been conserved, and the introns within the human c-src gene are significantly larger than the corresponding introns within the chicken c-src gene. The strong amino acid conservation between the carboxy-terminal two-thirds of pp60c-src of species as divergent as humans and chickens suggests that this portion of the pp60c-src protein specifies one or more functional domains that are of great importance to some aspect of normal cellular growth or differentiation.

Human cellular src gene: nucleotide sequence and derived amino acid sequence of the region coding for the carboxy-terminal two-thirds of pp60c-src

The nucleotide sequence of the 3' two-thirds of a highly conserved, molecularly cloned human cellular src gene (c-src) has been determined. This region of the c-src gene encodes the tyrosine kinase domain of the cellular src protein (pp60c-src) and corresponds to exons 6 through 12 of the chicken c-src gene, as well as nucleotides 545 to 1542 of the Rous sarcoma virus src gene (v-src). The human c-src sequence is very strongly conserved with respect to both the chicken c-src and the Rous sarcoma virus v-src genes, with nearly 90% nucleotide homology observed in this region. Amino acid sequence conservation in this region is even greater; 98% of the amino acids are conserved between human and chicken c-src. Furthermore, the exon sizes and the locations of the exon-intron boundaries are identical in the human and chicken c-src genes. However, sequences within the introns have not been conserved, and the introns within the human c-src gene are significantly larger than the corresponding introns within the chicken c-src gene. The strong amino acid conservation between the carboxy-terminal two-thirds of pp60c-src of species as divergent as humans and chickens suggests that this portion of the pp60c-src protein specifies one or more functional domains that are of great importance to some aspect of normal cellular growth or differentiation.

Small deletion in src of Rous sarcoma virus modifying transformation phenotypes: identification of 207-nucleotide deletion and its smaller product with protein kinase activity

Partial deletion in the src gene and the gene product were characterized in a deletion mutant, dl5, isolated from the Prague strain of Rous sarcoma virus. The mutant induced fusiform-like transformed cells, unlike the parental Prague strain, which induced round transformed cells. Determination of the total nucleotide sequences of src in dl5 and the Prague strain of Rous sarcoma virus demonstrated that in the former two deletions of 196 and 11 nucleotides had occurred at positions 403 and 696, respectively, from the 5' end of src. A protein with a molecular weight of 52,000 (p52src) was detected in cells infected with dl5, as predicted from the deletion size in src. From the nucleotide sequence, it was predicted that p52src had two deletions of 65 and 4 amino acids at positions 135 and 232, respectively, from the N-terminal methionine of p60src and also had 33 amino acid changes between these two deletion sites due to alteration of the reading frame. p52src, which contained deletions and alterations of amino acids near the N-terminus, showed protein kinase activity similar to that of p60src and functioned in the infected cells. These results strongly suggest that changes in the N-terminal region of p60src modified its transforming ability, causing induction of the fusiform-like transformation phenotype.

Analysis of the pathogenicity of transformation defective partial deletion mutants of avian sarcoma virus: characterization of recovered viruses which encode novel src specific proteins

Several transformation defective (td) mutants of the Prague strain of Rous sarcoma virus, which had been previously shown to have deletions of varying sizes and positions within the src gene, were tested for their ability to induce disease in chickens. Several of the mutants induced sarcomas after long latency, in particular two mutants which had deletions spanning the presumed active site (i.e., the phosphotyrosine residue) of the RSV transforming protein, pp60src. Viruses recovered from these tumors, as well as the tumors themselves, were analyzed to study the mechanism of tumor induction. In some examples proviral DNA structurally similar to wild-type virus was found in tumors and virus recovered from these tumors was shown to transform chick cells in vitro. Transformation specific proteins of 55,000 Da immunoprecipitable with antisera against pp60src were encoded by the recovered viruses. These proteins displayed a protein kinase activity, appeared to have small deletions in the amino termini, and by phosphotryptic peptide mapping appeared to contain novel phosphotyrosine tryptic peptides, when compared to wild-type virus, which were presumably derived from endogenous c-src.

Site-directed mutagenesis of the src gene of Rous sarcoma virus: construction and characterization of a deletion mutant temperature sensitive for transformation

Transformation of cells by Rous sarcoma virus results from the expression of the viral src gene product, pp60src. Site-directed mutagenesis techniques have been used to construct defined deletion mutations within the src gene of Prague A strain of Rous sarcoma virus. The deletion of DNA sequences at the Bg/II restriction site in the src gene yielded both transformation-defective mutants (tdCH4, 64, and 146) and a mutant temperature sensitive for morphological transformation (tsCH119). The genome of tsCH119 contains an in-phase deletion of approximately 160 base pairs, which mapped to the immediate 3' side of the Bg/II restriction site. Upon infection of chicken cells, tsCH119 encoded a structurally altered src protein, pp53src, containing a deletion of amino acid residues 202 to 255. Immune complexes containing pp53src isolated from tsCH119-infected cells grown at 41 degrees C exhibited only 50% less tyrosine-specific kinase activity than immune complexes isolated from cells grown at 35 degrees C. pp53src immunoprecipitated from tsCH119-infected cells grown at either 35 or 41 degrees C contained phosphoserine and phosphotyrosine. We suggest that tsCH119 represents a class of mutants containing mutations mapping within a functionally important domain of the src protein, distinct from the domain specifying the protein kinase activity.