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

Transformation-defective mutants with 5' deletions of the src gene are frequently generated during replication of Rous sarcoma virus in established quail fibroblasts

Replication of Rous sarcoma virus (RSV) in avian fibroblasts leads to the generation of replication-competent variants that are defective for cell transformation (td virus). These td variants contain deletions affecting various portions of the v-src gene. We compared the rate of td virus production in Q3B cells, a quail cell line established by mutagen treatment, and in normal quail fibroblasts. Twenty-five days after infection with an RSV stock containing only transforming virions, Q3B cells harbor similar amounts of v-src-containing and v-src-deleted proviruses. However, these cells synthesize very low levels of p60v-src and generate large excess of td variants, as determined by biological assays. Unlike Q3B cells, normal quail fibroblasts infected with the same virus stock produce td variants only after multiple passages of undiluted virus on fresh cells. Restriction analysis showed that the td virus produced by Q3B cells is composed of two types of genomes: one lacking the entire v-src gene and the other carrying partial deletions of this gene predominantly located in the amino-terminal portion of the coding region of v-src. To study the mechanisms of these partial deletions, we molecularly cloned and sequenced the v-src genes of several td proviruses. We show that these mutants carry single or multiple v-src deletions of limited size, presumably generated by multiple mechanisms. Two deletions of 170 and 112 bp located in the 5' portion of v-src are frequently generated during RSV replication in Q3B cells and may represent preferential sites for v-src deletion in these cells.

Transduction of proto-src sequences in tissue culture by a molecular clone of transformation-defective Rous sarcoma virus with an internal src deletion

The sporadic appearance of nondefective (nd) Rous sarcoma virus (RSV) from cells in tissue culture infected with a molecular clone of transformation-defective RSV was examined. Southern analysis of extrachromosomal, virus-specific DNA of three independent ndRSV isolates in each case indicated restoration of an isogenic src by homologous recombination with cellular proto-src. The frequency of transduction was estimated by fluctuation analysis to vary between one transduction per 0.4 x 10(7) to 1.6 x 10(7) infected cells.

A single point mutation has pleiotropic effects on pp60v-src function

The Rous sarcoma virus mutant tsLA29 encodes a pp60v-src molecule that is temperature sensitive for both tyrosine kinase activity and its ability to locate at the cell periphery. The defect in localization appears to be due to a perturbation in events following complex dissociation, since the mutant enzyme shows a rapidly reversible association with the cytoskeleton when shifted between permissive and restrictive temperatures. Although tsLA29 pp60v-src differs from the wild type at three amino acid residues, studies with chimeric proteins show that only one of the mutations, an alanine-for-proline substitution at residue 507, accounts for all the temperature-sensitive characteristics. Moreover, a single second site mutation, at residue 427, can restore the wild phenotype. Cells infected with a chimeric virus encoding only the alanine substitution at position 507 have a conspicuously fusiform morphology, suggesting that this mutation also has subtle effects on pp60v-src function that are apparently compensated for by the other mutations in native tsLA29.

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.

The mechanism of transduction of proto-oncogene c-src by avian retroviruses

Chicken c-src sequences have been transduced by avian leukosis viruses (ALV) and by partial src-deletion (td) mutants of Rous sarcoma virus in several independent events. Analyses of the recombination junctions in the genomes of src-containing viruses and the c-src DNA have shed light on the mechanism of transduction, which involves at least two steps of recombination. The initial recombination between a viral genome and the 5' region of c-src appears to occur at the DNA level. This step does not require extensive homology and can be mediated by stretches of sequences with only partial homology. The 5' recombination junction can also be formed by splicing between viral and c-src sequences. The second recombination is presumed to occur between the transducing ALV or td viral RNA and the viral-c-src hybrid RNA molecule generated from the initial recombination. This step involving recombination at the 3' ends of those molecules restores the 3' viral sequences essential for replication to the viral-c-src hybrid molecule. High frequency of c-src transduction by partial td mutants suggests that the second recombination is greatly enhanced when there is sequence homology between the transducing virus and the 3' region of c-src. Incorporation of the c-src sequences into an ALV genome results in greatly elevated expression of the gene. However, increased expression of c-src alone is insufficient to activate its transforming potential. Structural changes in c-src are necessary to convert it into a transforming gene. The changes can be as small as single nucleotide changes resulting in single amino aid substitutions at certain positions. Mutations can occur rapidly during viral replication after c-src is incorporated into the viral genome. Therefore, it is most likely that transduction of c-src by ALV is followed by subsequent mutation and selection for the sarcomagenic virus. In the case of transduction by td viruses that retain certain src sequences, joining of these sequences with the transduced c-src apparently is sufficient to activate its transforming potential.

Transduction of c-src coding and intron sequences by a transformation-defective deletion mutant of Rous sarcoma virus

The mechanism of cellular src (c-src) transduction by a transformation-defective deletion mutant, td109, of Rous sarcoma virus was studied by sequence analysis of the recombinational junctions in three td109-derived recovered sarcoma viruses (rASVs). Our results show that two rASVs have been generated by recombination between td109 and c-src at the region between exons 1 and 2 defined previously. Significant homology between td109 and c-src sequences was present at the sites of recombination. The viral and c-src sequence junction of the third rASV was formed by splicing a cryptic donor site at the 5' region of env of td109 to exon 1 of c-src. Various lengths of c-src internal intron 1 sequences were incorporated into all three rASV genomes, which resulted from activation of potential splice donor and acceptor sites. The incorporated intron 1 sequences were absent in the c-src mRNA, excluding its being the precursor for recombination with td109 and implying that initial recombinations most likely took place at the DNA level. A potential splice acceptor site within the incorporated intron 1 sequences in two rASVs was activated and was used for the src mRNA synthesis in infected cells. The normal env mRNA splice acceptor site was used for src mRNA synthesis for the third rASV.

Transmission of (LTR, v-src, LTR) without recombination with a helper virus

We have analyzed transmission of (LTR, v-src, LTR) cryptic structure integrated in the H-19 mammalian tumor cell line. From this cell line different isolates of transforming virus were rescued in heterokaryons produced by fusion with chicken fibroblasts infected by replication-competent avian leukosis virus RAV-1. One of them (F6) was used for the transformation of avian cells in the absence of the helper virus. In four transformed cell lines studied, the (LTR, v-src, LTR) structure was again integrated at a unique position in the cell DNA of each line. This indicated that the (LTR, v-src, LTR) structure is transmitted by the helper virus without recombination. This point has been further supported by the finding that a src-containing species corresponding in size to the nonpolyadenylated src mRNA is present in the RNA isolated from the rescued F6 transforming virus which might serve as template for the synthesis of (LTR, v-src, LTR) structure by the reverse transcriptase provided by RAV-1.

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

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

Role of a membrane glycoprotein in Friend virus-induced erythroleukemia: studies of mutant and revertant viruses

We previously reported the isolation and characterization of spontaneous, transmissible mutants of Friend spleen focus-forming virus (SFFV) that are nonpathogenic in adult NIH/Swiss mice and that contain abnormalities in nonoverlapping regions of their envelope glycoprotein (env) genes (M. Ruta, R. Bestwick, C. Machida, and D. Kabat, 1983, Proc. Natl. Acad. Sci. USA 80, 4704-4708). In newborn NIH/Swiss mice, these mutant SFFVs form revertants that are pathogenic in mice of all ages. At least two of three studied revertants contain second site env mutations which affect the sizes and proteolytic fragmentation patterns of their encoded glycoproteins. A variety of structural and genetic evidence suggests that the xenotropic- and ecotropic-related regions of the SFFV glycoprotein fold into separate globular domains that are connected by a flexible proline-rich joint. A glutamyl peptide bond within this joint is exceptionally susceptible to cleavage with Staphylococcus aureus V8 protease. Moreover, disulfide bonds occur within the xenotropic-related domain, but not between the globular domains. These results provide strong additional evidence that the env gene is required for SFFV pathogenesis, and they provide a new system for identifying the features of glycoprotein structure and localization which are essential for its leukemogenic activity.

Functional domains of the pp60v-src protein as revealed by analysis of temperature-sensitive Rous sarcoma virus mutants

Four temperature-sensitive (ts) Rous sarcoma virus src gene mutants with lesions in different parts of the gene represent three classes of alteration in pp60src. These classes are composed of mutants with (i) heat-labile protein kinase activities both in vitro and in vivo (tsLA27 and tsLA29), (ii) heat-labile kinases in vivo but not in vitro (tsLA33), and (iii) neither in vivo nor in vitro heat-labile kinases (tsLA32). The latter class indicates the existence of structural or functional pp60src domains that are required for transformation but do not grossly affect tyrosine kinase activity.

Mechanisms for the generation of src-deletion mutants and recovered sarcoma viruses: identification of viral sequences involved in src deletions and in recombination with c-src sequences

The precise src deletions in six transformation-defective (td) deletion mutants derived from the Schmidt-Ruppin strain of Rous sarcoma virus were determined by sequence analysis. Examination of the parental viral sequences neighboring the junctions of deletions in these td mutants revealed that these regions contained either directly repeated or inverted complementary sequences ranging from 9 to 28 nucleotides. Five td mutants were found to contain deletions flanked by directly repeated sequences, of which the 3' direct repeat was retained whereas the 5' direct repeat was deleted in the resulting td viral RNA. In the deletions of two td mutants where inverted complementary sequences were present at junctions of the deletions, both copies of the inverted complementary sequence were deleted in the td viral RNA. It is proposed from these observations that deletions of these mutants have been generated during the synthesis of minus-strand viral DNA by reverse transcriptase by jumping over a sequence flanked by direct repeats or by skipping a stem-and-loop structure formed via inverted complementary sequences on the viral RNA template. Data provide further information on the sequences in the td viral genome that are required for the generation of recovered sarcoma viruses (rASVs) by recombination with c-src. Sequence data of td viruses revealed that retaining as few as 82 nucleotides of the 3' src coding sequence is sufficient, whereas retaining as much as one-third of the 5' src but none of the 3' src coding sequences is not sufficient, for the generation of rASVs. Those that generate replication-competent rASVs retain, in addition to the 3' src region, a portion of the 5' src and/or its immediate upstream sequence that is homologous to exon 1 of the c-src DNA. These two sequence domains apparently provided 5' and 3' homologous regions for recombination between td viral genome and c-src DNA resulting in nondefective rASVs. Td109, which was shown previously to generate only replication-defective rASVs, retains 296 nucleotides of the 3' src sequence but lacks all the 5' src and 316 nucleotides of its immediate upstream region. It is concluded that the 5' src coding sequence and its immediate upstream region are not essential for the generation of rASVs. However, retaining a portion of those sequences is required for the generation of replication-competent rASVs.

Functional domains of the pp60v-src protein as revealed by analysis of temperature-sensitive Rous sarcoma virus mutants

Four temperature-sensitive (ts) Rous sarcoma virus src gene mutants with lesions in different parts of the gene represent three classes of alteration in pp60src. These classes are composed of mutants with (i) heat-labile protein kinase activities both in vitro and in vivo (tsLA27 and tsLA29), (ii) heat-labile kinases in vivo but not in vitro (tsLA33), and (iii) neither in vivo nor in vitro heat-labile kinases (tsLA32). The latter class indicates the existence of structural or functional pp60src domains that are required for transformation but do not grossly affect tyrosine kinase activity.

Induction of tumors and generation of recovered sarcoma viruses by, and mapping of deletions in, two molecularly cloned src deletion mutants

td108 , a transformation-defective (td) deletion mutant of the Schmidt-Ruppin strain of Rous sarcoma virus of subgroup A (SR-A), was molecularly cloned. Two isolates of td viruses, td108 -3b and td108 -4a, obtained by transfection of the molecularly cloned td108 DNAs into chicken embryo fibroblasts, were tested for their ability to induce tumors and generate recovered avian sarcoma viruses ( rASVs ) in chickens. Both td viruses were able to induce tumors with a latency and frequency similar to those observed previously with biologically purified td mutants of SR-A. rASVs were isolated from most of the tumors examined. The genomic RNAs of those newly obtained rASVs were analyzed by RNase T1 oligonucleotide fingerprinting. The results showed that they had regained the deleted src sequences and contained the same set of marker src oligonucleotides as those of rASVs analyzed previously. The src oligonucleotides of rASVs are distinguishable from those present in SR-A. We conclude that those rASVs must have been generated by recombination between the molecularly cloned td mutants and the c-src sequence. The deletions in the td mutants were mapped by restriction enzyme analysis and nucleotide sequencing. td108 -3b was found to contain an internal src deletion of 1,416 nucleotides and to retain 57 and 105 nucleotides of the 5' and 3' src coding sequences, respectively. td108 -4a contained a src deletion of 1,174 nucleotides and retained 180 and 225 nucleotides of the 5' and 3' src sequences, respectively. Comparison of sequences in the 5' src and its upstream region of td108 -3b with those of SR-A, rASV1441 (a td108 -derived rASV analyzed previously), and c-src suggested that the 5' recombination between td108 and c-src occurred from 7 to 20 nucleotides upstream from the beginning of the src coding sequence.

Identification of the viral sequence required for the generation of recovered avian sarcoma viruses and characterization of a series of replication-defective recovered avian sarcoma viruses

The ability of transformation-defective deletion mutants of Schmidt-Ruppin Rous sarcoma virus to induce tumors and generate recovered sarcoma viruses (rASVs) was correlated with the partial src sequences retained in the transformation-defective viral genomes. Since all the transformation-defective viruses that were capable of generating rASVs retained a portion of the 3' src sequence, regardless of the extent of the 5' src deletion, and those lacking the 3' src were unable to generate rASVs, it appears that the 3', but most likely not the 5', src sequence retained in the transformation-defective viral genome is essential for rASV formation. However, rASVs derived from a particular mutant, td109, which retained a portion of the 3' src sequence, but lacked most (if not all) of the 5' src sequence, were all found to be defective in replication. Analyses of the genomic sequences of 13 isolates of td109-derived rASVs revealed that they contained various deletions in viral envelope (env), polymerase (pol), and structural protein (gag) genes. Ten isolates of rASVs contained env deletions. One isolate (rASV3812) contained a deletion of env and the 3' half of pol, and one isolate (rASV398) contained a deletion of env and pol. The one with the most extensive deletion (rASV374) had a deletion from the p12-coding sequence through pol and env. In addition, the 5' src region of td109-derived rASVs were heterogeneous. Among the 7 isolates analyzed in detail, one isolate of rASV had a small deletion of the 5' src sequence, whereas three other isolates contained extra new sequences upstream from src. Both env- and env- pol- rASVs were capable of directing the synthesis of precursor and mature gag proteins in the infected nonproducer cells. We attribute the deletions in the replication-defective rASVs to the possibility that the 5' recombination site between the td109 and c-src sequence, involved regions of only partial homology due to lack of sufficient 5' src sequence in the td109 genome for homologous recombination. A model of recombination between the viral genome and the c-src sequence is proposed to account for the requirement of the 3' src sequence and the basis for the generation of deletions in td109-derived rASVs.

A recovered avian myelocytomatosis virus that induces lymphomas in chickens: pathogenic properties and their molecular basis

The avian myelocytomatosis virus MC29 induces neoplastic diseases in chicken, including myelocytomas and tumors of kidney and liver, which are due to the action of the v-myc gene. However, MC29 has never been reported to cause lymphoid tumors, the disease associated with activation of the c-myc gene by the insertion of a lymphoid leukosis virus genome. We have analyzed a recovered MC29 virus, HBI, which has a myc gene containing c-myc sequences, acquired by recombination with the cellular gene, and some v-myc sequences. Inoculation of HBI into chickens resulted in lymphoid tumors independent of the bursa. Antigenically these tumors were made up of T and B cells. Molecular analysis showed HBI proviral DNA in 36 of 39 tumors analyzed, with no obvious alteration of c-myc, and the HBI gag-myc fusion protein, p 108, could be detected in tumor cells. These data are discussed in terms of the mechanism of target-cell specificity for transformation by the myc gene.