The 28 S genomic RNA of avian sarcoma virus PRCII codes for the transformation-specific polyprotein P105

From Viruses to Genes to Cells

I always loved biology and to do experiments. This passion and a great deal of good fortune and serendipity landed me in the field of retrovirology at the time when it opened to experimental analysis. I became involved in viral replication, genetics, and viral oncogenes. In more recent years, I have applied what I learned in tumor virology to human cancer. The early years of my personal life were marked by displacements and migration: deportation into East Germany, escape to the West, and emigration to the United States. As a young man I faced heartbreaking personal tragedies but attained a peaceful and steady course in the second half of my life. I am fortunate to have found my home in Southern California and to continue in cancer research.

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.

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.

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.