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

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.

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.

Construction and biological analysis of deletion mutants of Fujinami sarcoma virus: 5'-fps sequence has a role in the transforming activity

Fujinami sarcoma virus (FSV) genome codes for the gag-fps fusion protein FSV-P130. The amino acid sequence of the 3' one-third portion in v-fps is partially homologous to the 3' half of pp60src, or the kinase domain, but the sequence of the 5' portion is unique to v-fps. To identify a possible domain structure in the v-fps sequence responsible for cell transformation, we constructed various deletion mutants of FSV with molecularly cloned viral DNA. Their transforming activities were assayed by measuring focus formation on chicken embryo fibroblasts and rat 3Y1 cells and tumor formation in chickens. The mutants carrying a deletion at the 3' portion in v-fps, the kinase domain, lost transforming activity. The mutants carrying an approximately 1-kilobase deletion within the 5' portion of the v-fps sequence retained focus-forming activity and tumorigenicity in the chicken system, but the efficiency of focus formation was about 10 times lower than that of the wild type. The morphology of these transformed cells was distinct from that observed in cells infected with wild-type FSV. Furthermore, these mutants could not transform rat 3Y1 cells, although wild-type FSV DNA transformed rat 3Y1 cells at a high frequency. The mutants carrying a larger deletion in the 5' portion of fps completely lacked the transforming activity. These results suggest that the 3' portion of the v-fps sequence is necessary but not sufficient for cell transformation and that the 5' portion of v-fps has a role in the transforming activity.

Activated proto-onc genes: sufficient or necessary for cancer?

Proto-onc genes are normal cellular genes that are related to the transforming (onc) genes of retroviruses. Because of this relationship these genes are now widely believed to be potential cancer genes. In some tumors, proto-onc genes are mutated or expressed more than in normal cells. Under these conditions, proto-onc genes are hypothesized to be active cancer genes in one of two possible ways: The one gene-one cancer hypothesis suggests that one activated proto-onc gene is sufficient to cause cancer. The multigene-one cancer hypothesis suggests that an activated proto-onc gene is a necessary but not a sufficient cause of cancer. However, mutated or transcriptionally activated proto-onc genes are not consistently associated with the tumors in which they are occasionally found and do not transform primary cells. Further, no set of an activated proto-onc gene and a complementary cancer gene with transforming function has yet been isolated from a tumor. Thus, there is still no proof that activated proto-onc genes are sufficient or even necessary to cause cancer.

Molecular cloning of the PRCII sarcoma viral genome and the chicken proto-oncogene c-fps

The class II avian sarcoma viruses comprise PRCII, PRCIIp, PRCIV, URI, 16L, and Fujinami. The members of this class are all replication-defective viruses containing various amounts of a transforming sequence called v-fps. PRCII contains the smallest amount of fps-specific sequences, transforms fibroblasts in tissue culture, but is only weakly tumorigenic. As a first step in understanding variations in pathogenicity among the class II avian sarcoma viruses and the mechanism by which the oncogene of these viruses was transduced from a single cellular locus, we have molecularly cloned the viral genome of PRCII, its related helper PRCII-AV, and the chicken proto-oncogene (c-fps) from which v-fps derived. The fps-specific region within the cloned PRCII genome was shown to be 0.8-1.0 kb smaller than that of the Fujinami fps-specific region, in agreement with previous studies. Transfection of the cloned DNAs into primary chicken cells demonstrated that both clones (PRCII and PRCII-AV) are biologically active. The cloned PRCII genome is helper dependent and produces a gag-fusion phosphoprotein (P105) which is phosphorylated on a tyrosine residue. The cloned PRCII-AV genome produces infectious virus and can function as a helper for the cloned PRCII genome in transfection assays. Three overlapping recombinant lambda clones homologous to v-fps from a chicken genomic library have been isolated. One of these, lambda-c-fps(2), contains all of the cellular sequences homologous to v-fps. In the aggregate, the three molecular clones may represent the entirety of c-fps.

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 5' ends of the transforming gene of Fujinami sarcoma virus and of the cellular proto-fps gene are not colinear

The sequence of 500 nucleotides of a cellular proto-fps region that includes the 5' boundary of overlap with the fps sequence of Fujinami sarcoma virus (FSV) was determined. It was found that the potential reading frame of the proto-fps region continues over the 5' boundary of overlap with viral fps for 44 codons which do not include an AUG translation initiation codon. A potential upstream exon of proto-fps of over 91 codons and a potential splice acceptor at the site of fps/proto-fps overlap have been identified. It is concluded that the boundary of overlap between proto-fps and viral fps cannot represent the 5' end of the coding region of proto-fps. The proto-fps gene starts either in the upstream exon(s) not shared with FSV or at least 51 codons downstream of the boundary of overlap with FSV. In addition, the 5' coding region of proto-fps differs from the (delta gag-fps) hybrid transforming gene of FSV in the absence of the retroviral gag gene-derived sequence.

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.