Class II defective avian sarcoma viruses: comparative analysis of genome structure

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.

Single amino acid substitution, from Glu1025 to Asp, of the fps oncogenic protein causes temperature sensitivity in transformation and kinase activity

We have sequenced 2.7 kilobases of v-fps DNA encoding the transforming protein, p140, of the temperature-sensitive (ts) FL-15 clone of avian Fujinami sarcoma virus. Ten single nucleotide differences were found when compared with the v-fps sequence of the temperature-resistant (tr) clone, FSV-2. Of these differences, five encoded altered amino acids within the 5' fps domain, only one encoded an altered amino acid in the 3' kinase domain, and four were silent. Among the five amino acid changes in the 5' fps domain, four were identical to the corresponding residues of c-fps, and the remaining one, a change from His to Arg at amino acid number 559, was located in the middle of a stretch of five consecutive histidine residues. These sequence comparisons suggested that only two amino acid changes, His to Arg at amino acid 559 and Glu to Asp at amino acid 1025, were likely to be responsible for the temperature sensitivity of the v-fps protein. Two recombinants, pFL-11 containing the 5' alterations and pFL-12 containing the single 3' mutation, were constructed in vitro to determine the precise ts lesion. It was found that both the recombinant pFL-12 and the parental pFL-5 were ts by three criteria: cell morphology, colony formation, and kinase activity. In contrast, the recombinant pFL-11 was ts in morphology, but not in colony formation, and was partially ts in kinase activity. pFSV 2-2 itself was temperature resistant by these criteria. We conclude that, first, the mutation of Glu to Asp at amino acid number 1025 can cause a complete ts phenotype, implying that this residue is located at a critical position of the v-fps oncogenic protein. Secondly, the change from His to Arg at amino acid position 559 results in a partial temperature sensitivity, providing the genetic evidence for a second functional domain of the v-fps oncogenic protein.

Cytoskeletal organization, vinculin-phosphorylation, and fibronectin expression in transformed fibroblasts with different cell morphologies

Neoplastic transformation of fibroblasts results in widely different cell morphologies. We have attempted to correlate cell morphology with cytoskeletal organization and fibronectin expression in murine and avian fibroblasts transformed by a diverse group of viral and chemical agents. The distribution of vinculin, alpha-actinin, actin, and surface fibronectin was studied, and, where appropriate, also the extent of phosphotyrosine modification of vinculin. Irrespective of the transforming agent we found that increased cell rounding was generally correlated with a reduction in vinculin-containing focal adhesions, a dissolution of microfilament bundles, and a reduction of extracellular fibronectin. In contrast, spindle-shaped fibroblasts expressed relatively high levels of surface fibronectin. Reorganization of vinculin, actin, and alpha-actinin into rosette-like structures was observed in polygonal or rounded cells transformed by viruses encoding tyrosine kinases, but was not seen in fibroblasts transformed by agents without associated tyrosine kinase activity or in spindle-shaped cells. No correlation was found between the extent of phosphotyrosine modification of vinculin and the extent of cell rounding. Irrespective of cell morphology, the extent of tyrosine phosphorylation of vinculin was high in all cells transformed by viruses carrying the src gene, but low in those transformed by viruses expressing the fps gene. Our results indicate that the morphology of a transformed cell is determined by a combination of several factors which are affected to different extents by different transforming agents.

Defining the borders of the chicken proto-fps gene, a precursor of Fujinami sarcoma virus

The transforming (onc) genes of retroviruses contain specific sequences, derived from as yet poorly defined, normal cellular genes, termed proto-onc genes. Proto-onc genes must be defined to explain their docility compared to the oncogenicity of the viral derivatives. Here we set out to determine the borders of the chicken proto-fps gene from which the onc genes of avian Fujinami (FSV) and PRC sarcoma viruses (PRCSV) are derived. These onc genes are hybrids of an element from the gag gene of retroviruses (delta gag) linked to a 2.8-kb domain from proto-fps. To identify the 5' border of proto-fps we have sequenced 1.5 kb beyond the 5' border of overlap with viral fps utilizing a proto-fps clone derived previously. A possible promoter was identified that maps 736 nucleotides from this border. The 736 nucleotides contain two possible exons with 121 codons, and short regions of homology with the delta gag termini of FSV and PRCII. A translation stop codon and an adjacent polyadenylation signal were identified just prior to the 3' border of overlap with viral fps within a 1.15-kb sequence of a newly isolated proto-fps clone. Comparing four exons within this 1.15 kb proto-fps sequence with known fps equivalents of FSV and PRCSV, we have detected strain-specific, but no common point mutations in each viral genome. A 3.3-kb polyadenylated proto-fps mRNA was detected in chicken liver RNA by gel electrophoresis and hybridization with proto-fps DNA. We conclude that the coding capacity of proto-fps is just over 3 kb, consistent with the size of the putative proto-fps protein of 98 kDa and hence slightly larger than that of viral fps. Thus proto-fps and the viral delta gag-fps genes each contain distinct 5' regulatory and coding sequences and share the 3' terminal fps domains. It is suggested that this difference, rather than scattered point mutations, is responsible for the oncogenic function of the viral genes and the unknown cellular function of proto-fps.

Cloned DNA of defective avian sarcoma virus mutant LA46 encodes the cis-acting temperature-sensitive defect in replication

Avian defective sarcoma virus mutant LA46 carries a temperature-sensitive defect in replication and transformation. To elucidate this defect, we cloned the integrated provirus of LA46. By DNA-mediated transfection, the cloned DNA induced fusiform-transformed foci in chick embryo fibroblasts without helper virus. LA46-encoded transformation-specific protein p105 was expressed in these transformants in the absence of helper virus-encoded proteins. Superinfection of the transformed cells with different helper viruses resulted in the rescue of pseudotypes. All the rescued pseudotypes retained the temperature-sensitive phenotype in virus replication and transformation, suggesting that the defect was due to a cis-acting lesion in the LA46 genome. Restriction enzyme comparison between LA46 and wild-type virus revealed sequence differences near the 5' and 3' termini of the LA46 genome, including the long terminal repeat regions.

Transforming proteins of fujinami and PRCII avian sarcoma viruses have different subcellular locations

The subcellular locations of transforming proteins encoded by the related avian sarcoma viruses, PRCII and Fujinami sarcoma virus (FSV), were compared by cell fractionation and by indirect immunofluorescence. Whereas both viruses encode gag-fps proteins associated with tyrosine-specific kinase activity, FSV is more highly tumorigenic than PRCII in vivo. Cell fractionation studies showed that the PRCII transforming protein, P105, became associated with the high-speed particulate fraction shortly after synthesis. However, PRCII P105 did not fractionate with the plasma membrane marker, but rather with high-density membranes. It is unique in this subcellular localization among viral tyrosine kinases. This membrane association was found to be relatively insensitive to salt concentration and did not require divalent cations. Immunofluorescent studies, using anti-fps serum, showed that the PRCII protein was present in discrete, large, cytoplasmic patches, as well as in a juxtanuclear location. In contrast, FSV-encoded P130 was found to fractionate with the plasma membrane marker when cells were analyzed in low salt in the presence of magnesium. However, at higher salt concentrations and in the absence of magnesium, the bulk of P130 was found to be soluble. Immunofluorescent staining of FSV P130 revealed a diffuse, cytoplasmic pattern that was distinct from that of the PRCII product. The observed difference in the subcellular localization of these transforming proteins may be the cause of the difference in tumorigenicity between the two viruses.

Monoclonal antibodies to the transforming protein of Fujinami avian sarcoma virus discriminate between different fps-encoded proteins

Two monoclonal antibodies have been obtained that recognize antigenic determinants within the C-terminal fps-encoded region of P140gag-fps, the transforming protein of Fujinami avian sarcoma virus (FSV). The hybridomas which secrete these antibodies (termed 88AG and p26C) were isolated after the fusion of NS-1 mouse myeloma cells with B lymphocytes from Fischer rats that had been immunized with FSV-transformed rat-1 cells. FSV P140gag-fps immunoprecipitated by either antibody is active as a tyrosine-specific kinase and is able to autophosphorylate and to phosphorylate enolase in vitro. The fps-encoded proteins of all FSV variants, including the gag- p91fps protein of F36 virus, are recognized by both monoclonal antibodies. However, the product of the avian cellular c-fps gene. NCP98, and the transforming proteins of the recently isolated fps-containing avian sarcoma viruses 16L and UR1 are recognized only by the p26C antibody. The 88AG antibody therefore defines an epitope specific for FSV fps, whereas the epitope for p26C is conserved between cellular and viral fps proteins. The P105gag-fps protein of the PRCII virus is not precipitated by p26C (nor by 88AG), presumably as a consequence of the deletion of N-terminal fps sequences. These data indicate that the fps-encoded peptide sequences of 16L P142gag-fps and UR1 P150gag-fps are more closely related to NCP98 than that of FSV P140gag-fps. This supports the view that 16L and UR1 viruses represent recent retroviral acquisitions of the c-fps oncogene. The P85gag-fes transforming protein of Snyder-Theilen feline sarcoma virus is not precipitated by either monoclonal antibody but is recognized by some antisera from FSV tumor-bearing rats, demonstrating that fps-specific antigenic determinants are conserved in fes-encoded proteins.

The avian sarcoma virus PRCII lacks 1020 nucleotides of the fps transforming gene

Fujinami sarcoma virus (FSV) and PRCII avian sarcoma virus both encode gag-fps transforming proteins associated with tyrosine-specific protein kinase activity; however, PRCII has a lower oncogenic potential than does FSV. In this study, the genomes of PRCII and FSV have been compared. By hybridization of PRCII [32P]RNA to FSV DNA on Southern blots, a large internal deletion in the 5' half of the fps gene in PRCII has been mapped. To determine the exact size and location of the deletion in PRCII, dideoxy sequencing of PRCII RNA with FSV DNA fragments as primers was used. The FSV sequence corresponding to the deletion in PRCII was flanked by 6-base direct repeats ( AGCTGG ) at 1614-1619 and 2634-2639 nucleotides. One copy of the direct repeat was retained in the PRCII genome. The length of the deleted region was 1020 nucleotides. The deletion in fps did not alter the kinase domain or ATP-binding site of the P105 transforming protein of PRCII. It was shown that the specific kinase activity of P105 was as high as that of FSV P130 . The sequence deleted from PRCII was found to encode part of a large hydrophilic domain. In the accompanying paper [J. Woolford and K. Beemon (1984) Virology 135, 168-180], evidence that the PRCII and FSV proteins have different subcellular locations and solubility properties, possibly due to the loss of this domain, is presented. These alterations in the structure and location of the PRCII protein may prevent it from phosphorylating certain substrates involved in oncogenic transformation.

The low tumorigenic potential of PRCII, among viruses of the Fujinami sarcoma virus subgroup, corresponds to an internal (fps) deletion of the transforming gene

The avian sarcoma viruses FSV, PRCII, PRCIIp, and PRCIV share a related class of hybrid onc genes (delta gag-fps) defined by a specific nucleotide sequence fps and by delta gag-fps proteins of different sizes. Among these viruses, PRCII appears to have a lower tumorigenic potential than the others. Here we have compared fibroblast-transforming function and onc gene structure of these viruses. The fibroblast transforming ability of PRCII was lower than those of FSV, PRCIIp, and PRCIV. By gel electrophoresis the genomic RNA of PRCII measured 3.5 kb and those of FSV, PRCIIp, and PRCIV 4.5 kb; the delta gag-fps protein of PRCII measured 105 kilodaltons (kd), that of FSV 140 kd, and those of PRCIIp and PRCIV about 150 kd. By fingerprinting viral RNAs hybridized with molecularly cloned viral DNA the delta gag regions of PRCII and PRCIIp were defined to be 1.45 kb and that of FSV to be 1.3 kb. Fingerprint analysis of viral RNA-proto fps DNA hybrids showed the fps regions (approximately 2.8 kb) of FSV and PRCIIp to be isogenic. Compared to FSV and PRCIIp, the fps sequence of PRCII lacked a 1-kb region which maps between 0.3 and 1.3 kb from the 5' end of fps in FSV and PRCIIp. Based on oligonucleotide analysis, the shared fps complements of PRCII and PRCIIp were indistinguishable while that of FSV differed from those of the PRC viruses in scattered point mutations amounting to 1-2% of the RNA. Since all other regions of PRCII are isogenic with those of the highly tumorigenic variants PRCIIp, PRCIV, and FSV, it is concluded that the low fibroblast-transforming and oncogenic potential of PRCII reflects the internal fps deletion. Since the fps deletion reduces but does not eliminate transforming function, we suggest that the complete onc genes of viruses in the FSV subgroup include either several functional, or a regulatory and a functional fibroblast transforming domain. It has been reported that the 3' domains of the onc genes of viruses in the Fujinami subgroup and the onc genes of certain feline sarcoma viruses are distantly related. Since full transforming potential of the avian viruses depends on the 5' fps region not shared with the feline sarcoma viruses, we suggest that despite their structural homology, the avian and feline onc genes must have functionally different domains.

Structural relationship between the chicken DNA locus, proto-fps, and the transforming gene of Fujinami sarcoma virus, delta gag-fps

The avian Fujinami sarcoma virus (FSV) contains a hybrid transforming gene (delta gag-fps) with a 5' 1.3-kb portion derived from the gag gene of avian retroviruses and a 3' 2.8-kb portion (fps) derived from a cellular prototype. A lambda recombinant DNA clone carrying fps sequences within a 16-kb insert of cellular DNA, termed lambda proto-fps clone 12, has been selected from a chicken DNA library for comparison with the viral onc gene. Mapping of endonuclease-resistant proto-fps DNA fragments and hybridization with cloned viral DNA located FSV-related sequences at the 3' end of the insert within a region of about 4.25 kb. Alignment of endonuclease-resistant proto-fps and viral DNA fragments relative to common RNase T1-resistant oligonucleotide sequences of viral RNA, identified by fingerprinting DNA-RNA hybrids, indicated: (i) that proto-fps is colinear with viral fps but is interrupted by 1.75 kb of scattered sequences unrelated to viral fps; (ii) that among the nine endonuclease sites compared, proto-fps and viral fps share one PvuII, one BamHI, and possibly a Kpn1 site at homologous locations, and that they each have unique endonuclease sites and common sites at unique locations; (iii) that within 12 kb upstream from the 5' boundary of overlap with viral fps, proto-fps lacks gag-related sequences; and (iv) that proto-fps clone 12, like several others isolated by us, lacks at the 3' end an equivalent of the 3' 10 to 20% of viral fps. The eight endonuclease site-map coordinates of proto-fps and viral DNA also divided 44 fps oligonucleotides of viral RNA into 9 map segments. We conclude that the onc gene of FSV differs from proto-fps in delta gag and in multiple point mutations, compatible with a transforming function for the viral gene and a normal function for the cellular sequence homolog. Since proto-fps is unrelated to essential virion genes, the onc gene of FSV must have originated from cellular proto-fps by rare, illegitimate recombination.

Mapping of multiple phosphorylation sites within the structural and catalytic domains of the Fujinami avian sarcoma virus transforming protein

The phosphorylation sites of the P140gag-fps gene product of Fujinami avian sarcoma virus have been identified and localized to different regions of this transforming protein. FSV P140gag-fps isolated from transformed cells is phosphorylated on at least three distinct tyrosine residues and one serine residue, in addition to minor phosphorylation sites shared with Pr76gag. Partial proteolysis with virion protease p15 or with Staphylococcus aureus V8 protease has been used to generate defined peptide fragments of P140gag-fps and thus to map its phosphorylation sites. The amino-terminal gag-encoded region of P140gag-fps contains a phosphotyrosine residue in addition to normal gag phosphorylation sites. The two major phosphotyrosine residues and the major phosphorserine residue are located in the carboxy-terminal portion of the fps-encoded region of P140gag-fps. P140gag-fps radiolabeled in vitro in an immune complex kinase reaction is phosphorylated at only one of the two C-terminal tyrosine residues phosphorylated in vivo and weakly phosphorylated at the gag-encoded tyrosine and at a tyrosine site not detectably phosphorylated in vivo. Thus, the in vitro tyrosine phosphorylation of P140gag-fps is distinct from that seen in the transformed cell. A comparative tryptic phosphopeptide analysis of the gag-fps proteins of three Fujinami avian sarcoma virus variants showed that the phosphotyrosine-containing peptides are invariant, and this high degree of sequence conservation suggests that these sites are functionally important or lie within important regions. The P105gag-fps transforming protein of PRCII avian sarcoma virus lacks one of the C-terminal phosphotyrosine sites found in Fujinami avian sarcoma virus P140gag-fps. Partial trypsin cleavage of FSV P140gag-fps immunoprecipitated with anti-gag serum releases C-terminal fragments of 45K and 29K from the immune complex that retain an associated tyrosine-specific protein kinase activity. This observation, and the localization of the major P140gag-fps phosphorylation sites to the C-terminal fps region, indicate that the kinase domain of P140gag-fps is located at its C terminus. The phosphorylation of P140gag-fps itself is complex, suggesting that it may itself interact with several protein kinases in the transformed cell.

Localization and characterization of phosphorylation sites of the Fujinami avian sarcoma virus and PRCII virus transforming proteins

Fujinami sarcoma virus (FSV) and PRCII are avian sarcoma viruses which share cellularly derived v-fps transforming sequences. The FSV P140gag-fps gene product is phosphorylated on three distinct tyrosine residues in transformed cells or in an in vitro kinase reaction. Three variants of FSV, and the related virus PRCII which lacks about half of the v-fps sequence found in FSV, encode gene products which are all phosphorylated at tyrosine residues contained within identical tryptic peptides. This indicates a stringent conservation of amino acid sequence at the tyrosine phosphorylation sites which presumably reflects the importance of these sites for the biologic activity of the transforming proteins. Under suitable conditions the proteolytic enzymes p15 and V8 protease each introduce one cut into FSV P140, p15 in the N-terminal gag-encoded region and V8 protease in the middle of the fps-encoded region. Using these enzymes we have mapped the major site of tyrosine phosphorylation to the C-terminal end of the fps region of FSV P140gag-fps. A second tyrosine phosphorylation site is found in the fps region of FSV P140 isolated from transformed cells, and a minor tyrosine phosphorylation site is found in the N-terminal gag-encoded region. Our results suggest that the C-terminal fps-encoded region is required for expression of the tyrosine-specific protein kinase activity.