Mechanism of c-erbB transduction: newly released transducing viruses retain poly(A) tracts of erbB transcripts and encode C-terminally intact erbB proteins

Non-canonical nematode endogenous retroviruses resulting from RNA virus glycoprotein gene capture by a metavirus

Reverse-transcribing retroviruses exist as horizontally transmitted infectious agents or vertically transmitted endogenous retroviruses (ERVs) resident in eukaryotic genomes, and they are phylogenetically related to the long terminal repeat (LTR) class of retrotransposons. ERVs and retrotransposons are often distinguished only by the presence or absence of a gene encoding the envelope glycoprotein (env). Endogenous elements of the virus family Metaviridae include the insect-restricted Errantivirus genus of ERVs, for which some members possess env, and the pan-eukaryotic Metavirus genus that lacks an envelope glycoprotein gene. Here we report a novel Nematoda endogenous retrovirus (NERV) clade with core retroviral genes arranged uniquely as a continuous gag-env-pro-pol ORF. Reverse transcriptase sequences were phylogenetically related to metaviruses, but envelope glycoprotein sequences resembled those of the Nyamiviridae and Chrysoviridae RNA virus families, suggesting env gene capture during host cell infection by an RNA virus. NERVs were monophyletic, restricted to the nematode subclass Chromadoria, and included additional ORFs for a small hypothetical protein or a large Upf1-like RNA-dependent AAA-ATPase/helicase indicative of viral transduction of a host gene. Provirus LTR identity, low copy number, ORF integrity and segregation of three loci in Meloidogyne incognita, taken together with detection of NERV transcriptional activity, support potential infectivity of NERVs, along with their recent emergence and integration. Altogether, NERVs constitute a new and distinct Metaviridae lineage demonstrating retroviral evolution through sequential heterologous gene capture events.

Characterization of Rous sarcoma virus polyadenylation site use in vitro

Polyadenylation of Rous sarcoma virus (RSV) RNA is inefficient, as approximately 15% of RSV RNAs represent read-through transcripts that use a downstream cellular polyadenylation site (poly(A) site). Read-through transcription has implications for the virus and the host since it is associated with oncogene capture and tumor induction. To explore the basis of inefficient RSV RNA 3'-end formation, we characterized RSV polyadenylation in vitro using HeLa cell nuclear extracts and HEK293 whole cell extracts. RSV polyadenylation substrates composed of the natural 3' end of viral RNA and various lengths of upstream sequence showed little or no polyadenylation, indicating that the RSV poly(A) site is suboptimal. Efficiently used poly(A) sites often have identifiable upstream and downstream elements (USEs and DSEs) in close proximity to the conserved AAUAAA signal. The sequences upstream and downstream of the RSV poly(A) site deviate from those found in efficiently used poly(A) sites, which may explain inefficient RSV polyadenylation. To assess the quality of the RSV USEs and DSEs, the well-characterized SV40 late USEs and/or DSEs were substituted for the RSV elements and vice versa, which showed that the USEs and DSEs from RSV are suboptimal but functional. CstF interacted poorly with the RSV polyadenylation substrate, and the inactivity of the RSV poly(A) site was at least in part due to poor CstF binding since tethering CstF to the RSV substrate activated polyadenylation. Our data are consistent with poor polyadenylation factor binding sites in both the USE and DSE as the basis for inefficient use of the RSV poly(A) site and point to the importance of additional elements within RSV RNA in promoting 3' end formation.

Membrane localization of v-ErbB is required but not sufficient for ligand-independent transformation

The v-ErbB retroviral oncogene is a transduced, mutated copy of the avian EGF receptor gene, and its expression is sufficient to induce tumor formation in vivo. The structural alterations that release the oncogenic potential of the v-ErbB oncogene are similar to EGFR gene mutations described in human tumors. Thus, the study of v-ErbB tumor biology offers a useful model through which we can gain insight into the mechanism of EGFR-induced malignancies. Despite years of study, however, questions remain regarding the domains of v-ErbB required for oncogenicity. We sought to clarify the role of the transmembrane domain of v-ErbB during transformation using S3-v-ErbB, an acutely transforming retroviral oncogene isolated from avian sarcomas. Infection of primary fibroblasts with a retroviral vector containing S3-v-ErbB results in the formation of a transformation-associated phosphoprotein signaling complex, soft agar colony formation, and the rapid induction of highly vascularized sarcomas in vivo. To address contribution of the transmembrane domain of S3-v-ErbB during these processes, we constructed a mutant version of this oncogene with a precise deletion in this domain. Specifically, the S3-v-ErbB-TM- mutant was created through an in-frame deletion of the entire transmembrane domain. Primary fibroblasts expressing this S3-v-ErbB-TM- mutant fail to form a characteristic transformation-associated phosphoprotein complex and do not grow in an anchorage-independent manner. In addition, day-old chicks injected with a helper-independent retrovirus expressing the S3-v-ErbB-TM- mutant exhibit only limited tumor formation in vivo. These results demonstrate that the transmembrane domain and, consequently membrane localization, are essential for S3-v-ErbB-mediated transformation.

Human immunodeficiency virus type 1 transductive recombination can occur frequently and in proportion to polyadenylation signal readthrough

One model for retroviral transduction suggests that template switching between viral RNAs and polyadenylation readthrough sequences is responsible for the generation of acute transforming retroviruses. For this study, we examined reverse transcription products of human immunodeficiency virus (HIV)-based vectors designed to mimic postulated transduction intermediates. For maximization of the discontinuous mode of DNA synthesis proposed to generate transductants, sequences located between the vectors' two long terminal repeats (vector "body" sequences) and polyadenylation readthrough "tail" sequences were made highly homologous. Ten genetic markers were introduced to indicate which products had acquired tail sequences by a process we term transductive recombination. Marker segregation patterns for over 100 individual products were determined, and they revealed that more than half of the progeny proviruses were transductive recombinants. Although most crossovers occurred in regions of homology, about 5% were nonhomologous and some included insertions. Ratios of encapsidated readthrough and polyadenylated transcripts for vectors with wild-type and inactivated polyadenylation signals were compared, and transductive recombination frequencies were found to correlate with the readthrough transcript prevalence. In assays in which either vector body or tail could serve as a recombination donor, recombination between tail and body sequences was at least as frequent as body-body exchange. We propose that transductive recombination may contribute to natural HIV variation by providing a mechanism for the acquisition of nongenomic sequences.

Grb2 regulation of the actin-based cytoskeleton is required for ligand-independent EGF receptor-mediated oncogenesis

Mutations within members of the EGF/ErbB receptor family frequently release the oncogenic potential of these receptors, resulting in the activation of downstream signaling events independent of ligand regulatory constraints. We previously have demonstrated that the signal transduction events originating from S3-v-ErbB, a ligand-independent, oncogenic EGF receptor mutant, are qualitatively distinct from the ligand-dependent mitogenic signaling pathways associated with the wild-type EGF receptor. Specifically, expression of S3-v-ErbB in primary fibroblasts results in anchorage-independent growth, increased invasive potential, and the formation of a transformation-specific phosphoprotein signaling complex, all in a Ras-independent manner. Here we demonstrate the transformation-specific interaction between two components of this complex: the adaptor protein Grb2 and the cytoskeletal regulatory protein caldesmon. This interaction is mediated via both the amino-terminal SH3 and central SH2 domains of Grb2, and the amino-terminal (myosin-binding) domain of caldesmon. Expression of a dominant-negative Grb2 deletion mutant, which lacks the carboxy-terminal SH3 domain, in fibroblasts expressing S3-v-ErbB results in a reduction in phosphoprotein complex formation, the loss of anchorage-independent growth, and a reduction in invasive potential. Together, these results demonstrate a Ras-independent role for Grb2 in modulating cytoskeletal function during ligand-independent EGF receptor-mediated transformation, and provide further support for the hypothesis that ligand-independent oncogenic signaling is qualitatively distinct from ligand-dependent mitogenic signaling by the EGF receptor.

The viral envelope is a major determinant for the induction of lymphoid and myeloid tumours by avian leukosis virus subgroups A and J, respectively

Among the six envelope subgroups of avian leukosis virus (ALV) that infect chickens, subgroups A (ALV-A) and J (ALV-J) are the most pathogenic and widespread among commercial chicken populations. While ALV-A is predominantly associated with lymphoid leukosis (LL) and less frequently with erythroblastosis (EB), ALV-J mainly induces tumours of the myeloid lineage. In order to examine the basis for the lineage specificity of tumour induction by these two ALV subgroups, we constructed two chimeric viruses by substituting the env genes into the reciprocal proviral clones. The chimeric HPRS-103(A) virus carrying the subgroup A env gene is identical to ALV-J prototype virus HPRS-103 except for the env gene, and the chimeric RCAS(J) virus carrying the subgroup J env gene is identical to the parent replication-competent ALV-A vector RCAS except for the env gene. In experimentally inoculated chickens, HPRS-103(A) virus induced LL and EB similar to ALV-A isolates such as RAV-1, while RCAS(J) virus induced myeloid leukosis (ML) and EB, similar to ALV-J, suggesting that the env gene is the major determinant for the lineage-specific oncogenicity. There were genetic differences in susceptibility to tumour induction between line 0 and line 15(I) chickens, indicating that in addition to the env gene, other viral or host factors could also serve as determinants for oncogenicity. Induction of both LL and ML by the two chimeric viruses occurred through the activation of c-myc, while the EB tumours were induced by activation of the c-erbB oncogene.

Ligand-independent oncogenic transformation by the EGF receptor requires kinase domain catalytic activity

The retroviral oncogene S3-v-erbB is a transduced, truncated form of the avian EGF (ErbB-1) receptor. Infection of avian fibroblasts with a retroviral vector expressing S3-v-ErbB results in ligand-independent cell transformation, which is accompanied by the assembly of a transformation-specific phosphoprotein signaling complex and anchorage-independent cell growth. It previously had been reported, using lysine-721 mutants (K721), that kinase domain function was required for ErbB-mediated cell transformation. However, since these initial reports, several studies using aspartate-813 mutants (D813) have demonstrated the ability of kinase-impaired ErbB receptors to induce mitogenic signal transduction pathways and cell transformation in a ligand-dependent manner. To determine the necessity of ErbB receptor kinase domain catalytic activity in ligand-independent cell transformation, we created S3-v-ErbB-K(-), a kinase-impaired oncoprotein constructed by replacing aspartate-813 with alanine (D813A). Subcellular routing as well as cell surface membrane and nuclear localization of the S3-v-ErbB-K(-) mutant receptor were unaffected by impairment of kinase activity. In contrast, avian fibroblasts expressing S3-v-ErbB-K(-) do not form the characteristic transformation-specific phosphoprotein complex, or induce soft agar colony growth in vitro. These results suggest that in contrast to ligand-dependent oncogenic signaling, ligand-independent cell transformation by a constitutively activated mutant form of the EGF receptor requires receptor kinase catalytic activity. In addition, these results demonstrate that phosphorylation and assembly of downstream signaling complexes require tyrosine phosphorylation events that are directly mediated by oncogenic forms of the EGF receptor.

Transduction of cellular sequence by a human immunodeficiency virus type 1-derived vector

During studies examining the rate of human immunodeficiency virus type 1 (HIV-1) mutation in a single cycle of replication, the 5' long terminal repeat of one progeny provirus was found to contain an insertion of 147 bp including an entire tRNA sequence as well as an additional 66 bp insertion of nonviral origin. Database searches revealed that 65 of 66 bp aligned with the human CpG island sequence found on chromosomes 6, 14, and 17. Therefore it seems probable that it is of human cellular sequence origin and was transduced by HIV-1. This is the first demonstration that HIV-1 can capture a cellular sequence. The site of integration of the parental provirus was mapped to chromosome 1p32.1. Sequence with homology to the transduced CpG island was not found on chromosome 1, suggesting that the transduced cellular sequence was not linked to the site of viral integration.

Ras-independent oncogenic transformation by an EGF-receptor mutant

Mutations in the ligand-binding domain of the epidermal growth factor receptor have been identified in several types of human cancers, including malignant gliomas. These mutations render signaling by this receptor to be constitutively ligand-independent. In fibroblasts transformed with ligand-independent epidermal growth factor receptor mutants, there is a correlation between the formation of a unique phosphotyrosine protein complex and oncogenic transformation. This phosphoprotein complex includes Grb2, Shc, Sos, tyrosine-phosphorylated form of caldesmon, and two, as yet, unidentified proteins. The presence of Grb2, Shc, and Sos in this complex implicates Ras in ligand-independent signaling by these oncogenic epidermal growth factor receptor mutants. We, therefore, have used retroviral co-infections of cultured primary fibroblasts to determine if Ras activation is required for phosphoprotein complex formation, stress fiber loss, or transformation. As predicted, expression of a dominant-negative Ras mutant (N17Ras) completely abrogates ligand-stimulated soft agar colony growth of primary fibroblasts. In contrast, N17Ras expression has no effect on v-ErbB mediated stress fiber disassembly, soft agar colony growth, or phosphoprotein complex assembly. In addition, our data suggest that ligand-dependent Ras activation may be suppressed by oncogenic v-ErbB expression. Together these observations suggest that oncogenic signaling by v-ErbB does not require Ras activation, and implicate an alternative signal transduction pathway in ligand-independent epidermal growth factor receptor oncogenic signaling.

Most retroviral recombinations occur during minus-strand DNA synthesis

Retroviral RNA molecules are plus, or sense in polarity, equivalent to mRNA. During reverse transcription, the first strand of the DNA molecule synthesized is minus-strand DNA. After the minus strand is polymerized, the plus-strand DNA is synthesized using the minus-strand DNA as the template. In this study, a helper cell line that contains two proviruses with two different mutated gfp genes was constructed. Recombination between the two frameshift mutant genes resulted in a functional gfp. If recombination occurs during minus-strand DNA synthesis, the plus-strand DNA will also contain the functional sequence. After the cell divides, all of its offspring will be green. However, if recombination occurs during plus-strand DNA synthesis, then only the plus-strand DNA will contain the wild-type gfp sequence and the minus-strand DNA will still carry the frameshift mutation. The double-stranded DNA containing this mismatch was subsequently integrated into the host chromosomal DNA of D17 cells, which were unable to repair the majority of mismatches within the retroviral double-strand DNA. After the cell divided, one daughter cell contained the wild-type gfp sequence and the other daughter cell contained the frameshift mutation in the gfp sequence. Under fluorescence microscopy, half the cells in the offspring were green and the other half of the cells were colorless or clear. Thus, we demonstrated that more than 98%, if not all, retroviral recombinations occurred during minus-strand DNA synthesis.

Tissue- and transformation-specific phosphotyrosyl proteins in v-erbB-transformed cells

To understand the mechanism of tissue-specific and transformation-specific signaling by the v-ErbB oncoprotein, we have investigated signaling pathways downstream of this transmembrane tyrosine kinase. In this report, we describe tissue-specific patterns of phosphotyrosyl proteins in three distinct cell types transformed by the v-erbB oncogene: fibroblasts, erythroblasts, and endothelial cells. In addition, we describe transformation-specific tyrosine phosphorylation events and signal complex formation in v-erbB-transformed fibroblasts. Two patterns of phosphotyrosyl proteins have been detected in v-erbB-transformed cells. The first is a fibroblast-specific pattern which includes unique phosphotyrosyl proteins of 170 kDa (c-ErbB1), 158 kDa, and 120 kDa (the catenin-like protein p120cas). The second is an erythroblast/endothelial cell-specific pattern which includes a prominent unidentified phosphotyrosyl protein of 120 kDa. Evaluation of the phosphotyrosyl proteins p120cas and SHC in chicken embryo fibroblasts infected with transforming and nontransforming v-erbB mutants reveals transformation-specific patterns of tyrosine phosphorylation. One corollary of these phosphorylation events in v-erbB-transformed fibroblasts is the formation of a complex involving SHC, growth factor receptor-bound protein 2, and a novel 75-kDa phosphotyrosyl protein. The results of these studies suggest that the v-ErbB oncoprotein can couple to multiple signal transduction pathways, that these pathways are tissue specific, and that v-erbB-mediated transformation involves specific tyrosine phosphorylation events.

Modulation of erbB kinase activity and oncogenic potential by single point mutations in the glycine loop of the catalytic domain

Avian c-erbB is activated to a leukemia oncogene following truncation of its amino-terminal ligand-binding domain by retroviral insertion. The insertionally activated transcripts encode protein products which have constitutive tyrosine kinase activity and can induce erythroleukemia but not sarcomas. We have previously found that a valine-to-isoleucine point mutation at position 157 (V157I mutant) within the tyrosine kinase domain of this truncated erbB can dramatically activate the sarcomagenic potential of the oncogene and increase the kinase activity of this oncoprotein. This mutation lies at position 157 of the insertionally activated c-erbB product, affecting a highly conserved valine residue of the glycine loop involved in ATP binding and phosphate transfer. To investigate the functional importance of this residue in the catalytic activity of kinases, we have introduced at this position, by site-directed mutagenesis, codons representing the remaining 18 amino acid residues. Most of the mutants have diminished activity, with six of them completely devoid of kinase activity, indicating the sensitivity of this region to conformational changes. Some of these mutants displayed increased kinase activity and greater transforming potential in comparison with IA c-erbB, but none had levels as high as those of the V157I mutant. In general, the sarcomagenic potential of the various erbB mutants correlated with their autophosphorylation state and their ability to cause phosphorylation of MAP kinase. However, there are important exceptions such as the V157G mutant, which lacks enhanced autophosphorylation but is highly sarcomagenic. Studies of this and other autophosphorylation site mutants point to the existence of an autophosphorylation-independent pathway in sarcomagenesis. The requirement for leukemogenic potential is much less stringent and correlates with positivity of kinase activity. When the valine-to-isoleucine substitution was put in context of the full-length erbB protein, the mutation relaxed the ligand dependence and had a positive effect on the transforming potential of the full-length c-erbB.

Tyrosine kinase activity may be necessary but is not sufficient for c-erbB1-mediated tissue-specific tumorigenicity

Expression of mutant avian c-erbB1 genes results in tissue-specific transformation in chickens. Site-directed mutagenesis was used to generate kinase-defective mutants of several tissue-specific v-erbB transforming mutants by replacement of the ATP-binding lysine residue in the kinase domain with an arginine residue. These kinase-defective v-erbB mutants were analyzed for their in vitro and in vivo transforming potentials. Specifically, kinase-defective mutants of erythroleukemogenic, hemangioma-inducing, and sarcomagenic v-erbB genes were assessed for their oncogenic potential. In vitro transformation potential was assessed by soft-agar colony formation in primary cultures of chick embryo fibroblasts (CEF). In vivo transformation potential was determined by infection of 1-day-old line 0 chicks with concentrated recombinant retrovirus and then monitoring of birds for tumor formation. These transformation assays demonstrate that kinase activity is absolutely essential for transformation by tissue-specific transforming mutants of the avian c-erbB1 gene. Since all of the tissue-specific v-erbB mutants characterized to date exhibit tyrosine kinase activity in vitro but do not transform all tissues in which they are expressed, we conclude that v-erbB-associated tyrosine kinase activity may be necessary but is not sufficient to induce tumor formation.

Modulation of erbB kinase activity and oncogenic potential by single point mutations in the glycine loop of the catalytic domain

Avian c-erbB is activated to a leukemia oncogene following truncation of its amino-terminal ligand-binding domain by retroviral insertion. The insertionally activated transcripts encode protein products which have constitutive tyrosine kinase activity and can induce erythroleukemia but not sarcomas. We have previously found that a valine-to-isoleucine point mutation at position 157 (V157I mutant) within the tyrosine kinase domain of this truncated erbB can dramatically activate the sarcomagenic potential of the oncogene and increase the kinase activity of this oncoprotein. This mutation lies at position 157 of the insertionally activated c-erbB product, affecting a highly conserved valine residue of the glycine loop involved in ATP binding and phosphate transfer. To investigate the functional importance of this residue in the catalytic activity of kinases, we have introduced at this position, by site-directed mutagenesis, codons representing the remaining 18 amino acid residues. Most of the mutants have diminished activity, with six of them completely devoid of kinase activity, indicating the sensitivity of this region to conformational changes. Some of these mutants displayed increased kinase activity and greater transforming potential in comparison with IA c-erbB, but none had levels as high as those of the V157I mutant. In general, the sarcomagenic potential of the various erbB mutants correlated with their autophosphorylation state and their ability to cause phosphorylation of MAP kinase. However, there are important exceptions such as the V157G mutant, which lacks enhanced autophosphorylation but is highly sarcomagenic. Studies of this and other autophosphorylation site mutants point to the existence of an autophosphorylation-independent pathway in sarcomagenesis. The requirement for leukemogenic potential is much less stringent and correlates with positivity of kinase activity. When the valine-to-isoleucine substitution was put in context of the full-length erbB protein, the mutation relaxed the ligand dependence and had a positive effect on the transforming potential of the full-length c-erbB.

Characterization of unintegrated retroviral DNA with long terminal repeat-associated cell-derived inserts

We have used a replication-competent shuttle vector based on the genome of Rous sarcoma virus to characterize genomic rearrangements that occur during retrovirus replication. The strategy involved cloning circular DNA that was generated during an acute infection. While analyzing a class of retroviral DNA clones that are greater than full length, we found several clones which had acquired nonviral inserts in positions adjacent to the long terminal repeats (LTRs). There appear to be two distinct mechanisms leading to the incorporation of cellular sequences into these clones. Three of the molecules contain a cell-derived insert at the circle junction site between two LTR units. Two of these molecules appear to be the results of abortive integration attempts, because of which, in each case, one of the LTRs is missing 2 bases at its junction with the cell-derived insert. In the third clone, pNO220, the cellular sequences are flanked by an inappropriately placed copy of the tRNA primer-binding site on one side and a partial copy of the U3 sequence as part of the LTR on the other side. A fourth molecule we characterized, pMD96, has a single LTR with a U5-bounded deletion of viral sequences spanning gag and pol, with cell-derived sequences inserted at the site of the deletion; its origin may be related mechanistically to pNO220. Sequence analysis indicates that all of the cellular inserts were derived from the cell line used for the acute infection rather than from sequences carried into the cell as part of the virus particle. Northern (RNA) analysis of cellular RNA demonstrated that the cell-derived sequences of two clones, pNO220 and pMD96, were expressed as polyadenylated RNA in uninfected cells. One mechanism for the joining of viral and cellular sequences suggested by the structures of pNO220 and pMD96 is recombination occurring during viral DNA synthesis, with cellular RNA serving as the template for the acquisition of cellular sequences.

Two distant upstream regions containing cis-acting signals regulating splicing facilitate 3'-end processing of avian sarcoma virus RNA

Retroviruses, pararetroviruses, and related retrotransposons generate terminally redundant RNAs by transcription of a template flanked by long terminal repeats in which initiation occurs within the 5' long terminal repeat sequences and 3'-end processing occurs within the 3' long terminal repeat sequences. Processing of avian sarcoma virus RNA is relatively inefficient; approximately 15% of the viral RNA transcripts are read-through products; i.e., they are not processed at the viral poly(A) addition site but at sites in the cellular sequence further downstream. In this report, we show that the efficiency of processing at the viral site is further reduced by deletion of two distant upstream sequences: (i) a 606-nucleotide sequence in the gag gene containing a cis-acting negative regulator of splicing and (ii) a 136-nucleotide sequence spanning the env 3' splice site. The deletion of either or both upstream regions increases the levels of read-through products of both unspliced and spliced viral RNA. In contrast, deletion of the src 3' splice site does not affect the efficiency of processing at the viral poly(A) addition site. The effects on 3'-end processing are not correlated either with distance from the promoter to the poly(A) addition site or with the overall level of viral RNA splicing. Substitution of the avian sarcoma virus poly(A) signal with the simian virus 40 early or late poly(A) signal relieves the requirement for the distant upstream sequences. We propose that cellular factors, which may correspond to splicing factors, bound to the upstream viral sequences may interact with factors bound at the avian sarcoma virus poly(A) signal to stabilize the polyadenylation-cleavage complex and allow for more efficient 3'-end processing.

Large-scale purification and characterisation of a recombinant epidermal growth-factor receptor protein-tyrosine kinase. Modulation of activity by multiple factors

The human epidermal-growth-factor receptor (EGF-R) is a 170-kDa transmembrane glycoprotein that mediates the mitogenic response of cells to EGF and transforming growth factor alpha. Culture conditions have been developed for the large-scale expression of the cytoplasmic domain of the EGF-R in insect cells using a recombinant baculovirus. From 61 Sf9 cells, grown to high density using a bioreactor, 20 mg of the EGF-R kinase was purified to greater than 95% purity. Purification, which was carried out in the absence of detergents using classical purification methods, yielded an EGF-R protein that was not phosphorylated on tyrosine. This procedure has enabled us to produce high quality enzyme for both structural and biochemical studies on the EGF-R kinase. The in vitro activity of the cytoplasmic domain of the EGF-R kinase was modulated by multiple assay factors which include substrates, divalent cations and conformational modulators. Kinetic analysis in the presence of Mn2+ gave an apparent Vmax value of 20 nmol min-1 mg-1 and Km values of 4.5 microM for ATP and 1.43 mM for angiotensin II. This corresponds to a turnover number of 1.4 mol min-1 mol-1. Ammonium sulfate (1 M) resulted in an eightfold stimulation of kinase activity when assayed using angiotensin II as substrate. The specific activity of the intracellular domain of the EGF-R, when assayed at 20 degrees C in the presence of 1M ammonium sulfate, was 160 nmol min-1 mg-1. Activation of the EGF-R kinase by ammonium sulfate was found to be substrate-specific. No activation was found when assayed using polymeric substrates. Addition of Me(2+)-ATP to the purified enzyme resulted in autophosphorylation and was accompanied by retardation of SDS/PAGE migration. Kinetic constants and metal ion preferences of a number of co-polymers and peptide substrates have been compared. Dramatic differences in kinetic constants were found which were dependent on both the substrate and metal ion used. Activation of EGF-R autophosphorylation was found to be influenced by the use of charged polymers. The random polymer of Glu, Lys, Ala, Tyr (2:5:6:1), which was not phosphorylated by the EGF-R kinase, dramatically activates autophosphorylation of the EGF-R. Thus the intracellular domain of the EGF-R appears to be in a low-activity conformation which, under appropriate assay conditions, can be activated to a similar specific activity to that reported for the purified EGF-R holoenzyme.

Mechanism of transduction by retroviruses

Retroviruses can capture cellular sequences and express them as oncogenes. Capture has been proposed to be a consequence of the inefficiency of polyadenylation of the viral genome that allows the packaging of cellular sequences flanking the integrated provirus in virions; after transfer into virions, these sequences could be incorporated into the viral genome by illegitimate recombination during reverse transcription. As a test for this hypothesis, a tissue culture system was developed that mimics the transduction process and allows the analysis and quantitation of capture events in a single step. In this model, transduction of sequences adjacent to a provirus depends on the formation of readthrough transcripts and their transmission in virions and leads to various recombinant structures whose formation is independent of sequence similarity at the crossover site. Thus, all events in the transduction process can be attributed to the action of reverse transcriptase on readthrough transcripts without involving deletions of cellular DNA.

Dissecting the activating mutations in v-erbB of avian erythroblastosis virus strain R

The v-erbB oncogene isolated from the R (or ES4) strain of avian erythroblastosis virus is capable of inducing erythroleukemia and fibrosarcomas. This oncogene differs from the proto-oncogene c-erbB, the avian homolog of the epidermal growth factor receptor, by its lack of an intact ligand-binding domain as well as additional alterations in its cytoplasmic coding sequences. By contrast, the insertionally activated c-erbB, a variant oncogene, which encodes a product that also lacks the ligand-binding domain but is otherwise unaltered in its cytoplasmic coding sequences, is capable of inducing leukemia but cannot induce sarcomas. In this report, we show that the critical changes for activating the sarcomagenic potential displayed by v-erbB R are two point mutations within the tyrosine kinase domain and an internal deletion of 21 amino acids in the carboxyl-terminal regulatory domain. The removal of the carboxyl-terminal autophosphorylation sites is not obligatory. These activating mutations (Arg-263 to His, Ile-384 to Ser, and the deletion of residues 494 to 514), when introduced singly into the insertionally activated c-erbB, all dramatically increase fibroblast-transforming potential. Arg-263 resides near the highly conserved HRD motif of the kinase domain, and its mutation to His increases the autophosphorylation activity. The other two mutations do not alter the intrinsic kinase activity and presumably affect other aspects of the receptor involved in growth signaling. Therefore, the high transforming potential of v-erbB R is a consequence of synergism among multiple activating mutations.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Common mechanism of retrovirus activation and transduction of c-mil and c-Rmil in chicken neuroretina cells infected with Rous-associated virus type 1

We previously described the isolation of the IC10 retrovirus which transduced the v-Rmil oncogene, a new member of the mil/raf gene family. This virus was generated during serial passaging of Rous-associated virus type 1 (RAV-1) in chicken embryo neuroretina (NR) cells and was selected for its ability to induce proliferation of these nondividing cells. IC10 was isolated after six passages of culture supernatants but was not detected in proliferating NR cells during early virus passages. In this study, we molecularly cloned and sequenced another v-Rmil-containing provirus, designated IC11, from NR cells infected at the third virus passage of the same experiment. Both IC11 and IC10 transduced only the serine/threonine kinase domain of c-Rmil. Comparison of v-Rmil and c-Rmil sequences indicated that amino-terminal truncation is sufficient to activate the mitogenic properties of c-Rmil. IC11 and IC10 have identical 3' ends but differ by their 5' RAV-1-Rmil junctions. The 3' ends of both viruses were generated by recombination between Rmil and env genes, involving partial sequence identity. The 5' RAV-1-Rmil junction of IC11 was formed by a splicing process between the RAV-1 leader and a 37-bp c-Rmil exon located upstream of the kinase domain. NR cells infected with this virus synthesize a unique Rmil protein. IC10 contains most of the gag gene recombined with v-Rmil and encodes a gag-Rmil hybrid protein. Serial passaging of IC11 in NR cells led to the formation of a gag-Rmil-containing retrovirus. These results indicate that IC11 represents an early step in transduction and that this virus further recombined with RAV-1 to generate IC10. They confirm our previously proposed model for the multistep generation of v-mil-transducing retroviruses. Therefore, activation and transduction of c-mil and c-Rmil, in NR cells infected with RAV-1, result from a common mechanism.

Tissue-specific transformation by epidermal growth factor receptor: a single point mutation within the ATP-binding pocket of the erbB product increases its intrinsic kinase activity and activates its sarcomagenic potential

Avian c-erbB is activated to a leukemia oncogene following truncation of its amino-terminal, ligand-binding domain by retroviral insertion. The insertionally activated transcripts encode protein products that have constitutive tyrosine kinase activity and that can induce erythro-leukemia but not sarcomas. We have found that a single point mutation within the ATP-binding pocket of the tyrosine kinase domain in this truncated molecule can increase the ability of this oncogene to induce anchorage-independent growth of fibroblasts in vitro and fibrosarcoma formation in vivo. Associated with this increased transforming potential is a corresponding increase in the kinase activity of the mutant erbB protein product. The mutation, which converts a valine to isoleucine at position 157 of the insertionally activated c-erbB product, is at a residue that is highly conserved within the protein kinase family. To our knowledge, this is the first demonstration of a point mutation in the ATP-binding pocket that activates a tyrosine kinase.

Transduction of cellular neo mRNA by retrovirus-mediated recombination

Transduction of cellular oncogenes by retroviruses is thought to be a multistep process, involving transcriptional activation of a cellular gene by upstream proviral integration and joining of cellular DNA to retroviral transcriptional signals, followed by copackaging and recombination with a helper virus genome during reverse transcription. To examine the molecular mechanism of the reverse transcriptase-mediated recombination, we introduced into mouse fibroblast cells a variety of constructs in which the neo selectable marker was joined to flanking retroviruslike or cell-like sequences. After superinfection and copackaging with a replication-competent Mo-MuLVsupF virus, the formation of recombinant neo transducing viruses was assessed in a second round of virus infection by the ability to confer G418 resistance to infected cells. Our results showed that recombinant neo proviruses were generated from neo RNA containing either a 5' or 3' retroviral end, implying that one recombination event with helper virus RNA was sufficient to incorporate the neo gene into proviral DNA. Recombination occurred with an apparent frequency of 10(-4) to 10(-5) per replication cycle in the absence of homology between the two recombining partners. This frequency, however, increased at least 100-fold if homology was provided at the site of recombination. Our results support the hypothesis that neo-transducing viruses arise via reverse transcriptase-mediated recombination of RNA rather than by recombination proceeding through DNA intermediates. Unexpectedly, removal of the retroviral packaging site psi reduced the number of neo recombinants only slightly. Our data indicated that although RNAs lacking the psi site are poorly packaged into virions, those RNAs that are included in the virions undergo frequent recombination, even if there is no selection for recombination. Many of the neo recombinants formed with the psi- constructs had undergone additional recombinations and often incorporated the psi site from the helper RNA.

Infrequent involvement of c-fos in avian leukosis virus-induced nephroblastoma

To determine whether c-fos is involved in avian leukosis virus-induced nephroblastoma, 28 tumors from chickens were analyzed for novel fos fragments. DNA from 1 of 16 clonal outgrowths (in chicken 6561) contained novel fos-related EcoRI and KpnI fragments which hybridized to both v-fos and viral probes. Oncogenicity tests using filtered 6561 tumor cell homogenates did not reveal a tumor-inducing transduction of c-fos. We conclude that c-fos is only an occasional target for proviral insertions or new transductions in avian leukosis virus-induced nephroblastoma. The results also identify a polymorphism in c-fos in K28 chickens and demonstrate that unintegrated viral DNA is not a general characteristic of avian leukosis virus-induced nephroblastoma.

Disease tropism of c-erbB: effects of carboxyl-terminal tyrosine and internal mutations on tissue-specific transformation

Avian leukosis virus induces erythroleukemia in chickens by proviral insertional mutation of the protooncogene c-erbB. The product of the insertionally activated c-erbB locus lacks the extracellular ligand-binding domain and is strictly leukemogenic. It has previously been demonstrated that the disease spectrum associated with aberrant c-erbB expression can be expanded by structural perturbation of the cytoplasmic domain of this protein. In this report, we use mutagenesis and retroviral vectors to identify specific mutations in the carboxyl-terminal domain of the insertionally activated c-erbB product that are sufficient to activate the sarcomagenic potential of this protein. Interestingly, a point mutation in the kinase domain appears to be sufficient for sarcomagenic activation. However, removal of the terminal tyrosine residue of the c-erbB product, implicated in modulating kinase activity, does not lead to a fully transforming phenotype. These studies suggest that there are multiple ways to activate the fibroblast-transforming potential of the insertionally activated c-erbB product. The conformation of this protein may play a more significant role in oncogenic activation than the phosphorylation status of the putative carboxyl-terminal autophosphorylation site.

Proviral insertional activation of c-erbB: differential processing of the protein products arising from two alternate transcripts

Proviral insertional activation of c-erbB results in the expression of two alternate transcripts (ENV+ and ENV-). We used cDNA clones representing the two alternate transcripts to generate stably transformed quail fibroblast cell lines which express the products of these transcripts independently. Analysis of the co- and posttranslational processing of the insertionally activated c-erbB products expressed in these cell lines revealed that the protein products of the ENV+ and ENV- transcripts were processed differently. The ENV+ transcript produced a primary translation product which was rapidly cotranslationally cleaved near the amino terminus to form a 79,000-Mr product. This protein product was efficiently converted to a higher-molecular-weight form, of between 82,000 and 88,000 (gp82-88), which was terminally glycosylated and expressed on the cell surface. A small portion of the ENV+ primary translation product underwent a second proteolytic cleavage to generate an unglycosylated 53,000-Mr species. In contrast, the primary translation product of the ENV- transcript, p80, was not proteolytically processed; this precursor form was rapidly converted to two discrete glycosylation intermediates, gp82 and go84. Only a small portion (less than 10%) of the total ENV- insertionally activated c-erbB product was slowly converted to the terminally glycosylated cell surface form, gp85-88. The processing differences that distinguished the ENV+ and ENV- products were similar to processing differences that we observed in parallel studies on the viral erbB products of the avian erythroblastosis viruses AEV-H and AEV-R, respectively. Since all four erbB protein products shared the same number, position, and sequence context of potential N-linked glycosylation sites, yet differed in the extent of their carbohydrate maturation, these data suggest that the mechanisms used by these truncated receptor molecules to associate with cellular membranes may be distinct.

Proviral insertional activation of c-erbB: differential processing of the protein products arising from two alternate transcripts

Proviral insertional activation of c-erbB results in the expression of two alternate transcripts (ENV+ and ENV-). We used cDNA clones representing the two alternate transcripts to generate stably transformed quail fibroblast cell lines which express the products of these transcripts independently. Analysis of the co- and posttranslational processing of the insertionally activated c-erbB products expressed in these cell lines revealed that the protein products of the ENV+ and ENV- transcripts were processed differently. The ENV+ transcript produced a primary translation product which was rapidly cotranslationally cleaved near the amino terminus to form a 79,000-Mr product. This protein product was efficiently converted to a higher-molecular-weight form, of between 82,000 and 88,000 (gp82-88), which was terminally glycosylated and expressed on the cell surface. A small portion of the ENV+ primary translation product underwent a second proteolytic cleavage to generate an unglycosylated 53,000-Mr species. In contrast, the primary translation product of the ENV- transcript, p80, was not proteolytically processed; this precursor form was rapidly converted to two discrete glycosylation intermediates, gp82 and go84. Only a small portion (less than 10%) of the total ENV- insertionally activated c-erbB product was slowly converted to the terminally glycosylated cell surface form, gp85-88. The processing differences that distinguished the ENV+ and ENV- products were similar to processing differences that we observed in parallel studies on the viral erbB products of the avian erythroblastosis viruses AEV-H and AEV-R, respectively. Since all four erbB protein products shared the same number, position, and sequence context of potential N-linked glycosylation sites, yet differed in the extent of their carbohydrate maturation, these data suggest that the mechanisms used by these truncated receptor molecules to associate with cellular membranes may be distinct.

Molecular characterization of three erbB transducing viruses generated during avian leukosis virus-induced erythroleukemia: extensive internal deletion near the kinase domain activates the fibrosarcoma- and hemangioma-inducing potentials of erbB

Three new erbB transducing viruses generated during avian leukosis virus-induced erythroblastosis have been cloned and sequenced, and their transforming abilities have been analyzed. Provirus 9134 E1 expresses an amino-terminally truncated erbB product that is analogous to the proviral insertionally activated c-erbB gag-erbB fusion product. This virus efficiently induces erythroblastosis, but does not transform fibroblasts in vitro or induce sarcomas in vivo. In contrast, virus 9134 S3 expresses an erbB product identical to the erbB product of 9134 E1, with the exception of a large internal deletion located between the kinase domain and the putative autophosphorylation site, P1. Interestingly, this virus is no longer capable of inducing erythroblastosis, but can induce both fibrosarcomas and hemangiomas in vivo. Provirus 9134 F3 has sustained an approximately 23-amino-acid carboxy-terminal truncation and is capable of inducing both erythroblastosis and sarcomagenesis. This virus expresses an erbB product with the shortest carboxy-terminal truncation sufficient to reveal the sarcomagenic potential of this protein. The distinct transforming properties of these viruses indicate that different structural domains of the erbB product confer distinct disease specificities.