β-Catenin acts in a position-independent regeneration response in the simple eumetazoan Hydra

Wnt/β-Catenin signaling plays crucial roles in regenerative processes in eumetazoans. It also acts in regeneration and axial patterning in the simple freshwater polyp Hydra, whose morphallactic regenerative capacity is unparalleled in the animal kingdom. Previous studies have identified β-catenin as an early response gene activated within the first 30min in Hydra head regeneration. Here, we have studied the role of β-Catenin in more detail. First, we show that nuclear β-Catenin signaling is required for head and foot regeneration. Loss of nuclear β-Catenin function blocks head and foot regeneration. Transgenic Hydra tissue, in which β-Catenin is over-expressed, regenerates more heads and feet. In addition, we have identified a set of putative β-Catenin target genes by transcriptional profiling, and these genes exhibit distinct expression patterns in the hypostome, in the tentacles, or in an apical gradient in the body column. All of them are transcriptionally up-regulated in the tips of early head and foot regenerates. In foot regenerates, this is a transient response, and expression starts to disappear after 12-36h. ChIP experiments using an anti-HydraTcf antibody show Tcf binding at promoters of these targets. We propose that gene regulatory β-Catenin activity in the pre-patterning phase is generally required as an early regeneration response. When regenerates are blocked with iCRT14, initial local transcriptional activation of β-catenin and the target genes occurs, and all these genes remain upregulated at the site of both head and foot regeneration for the following 2-3 days. This indicates that the initial regulatory network is followed by position-specific programs that inactivate fractions of this network in order to proceed to differentiation of head or foot structures. brachyury1 (hybra1) has previously been described as early response gene in head and foot regeneration. The HyBra1 protein, however, appears in head regenerating tips not earlier than about twelve hours after decapitation, and HyBra1 translation does not occur in iCRT14-treated regenerates. Foot regenerates never show detectable levels of HyBra1 protein at all. These results suggest that translational control mechanisms may play a decisive role in the head- and foot-specific differentiation phase, and HyBra1 is an excellent candidate for such a key regulator of head specification.

Beiträge Zur Chemie Des Hydrazins Und Seiner Derivate XXIII / Contributions To The Chemistry Of Hydrazine And Its Derivatives XXIII

With barium hydrazide, which is easily to prepare and relatively harmless to handle, some addition-, substitution- and reduction-reactions were carried out. Partially we could get the same, partially however other products than by the analogous reactions with the normally used, but highly explosive sodium hydrazide. Especially the formation of the azo-compound H5C6-CH2-CH2-N = N-CH2-CH2-C6H5 and of H5C6-C(NH)-NH-NH-C(NH)-C6H5 by the addition of barium hydrazide to styrene resp. benzoic nitrile was remarkable.

WS5, a direct target of oncogenic transcription factor Myc, is related to human melanoma glycoprotein genes and has oncogenic potential

We have isolated a gene (WS5) that is specifically expressed at the mRNA and protein level in avian fibroblasts transformed by the v-myc oncogene of avian acute leukemia virus MC29. In a conditional cell transformation system, WS5 gene expression was tightly correlated with v-myc activation. The WS5 gene contains 11 exons, encoding a 733-amino acid protein with a transmembrane region and a polycystic kidney disease (PKD) domain. Near the transcriptional start site, the WS5 promoter contains a cluster of four binding sites for the Myc-Max complex and a binding site for transcription factor C/EBPalpha. Electrophoretic mobility shift assays and chromatin immunoprecipitation showed that Myc, Max and C/EBPalpha bind specifically to these sites. Functional promoter analyses revealed that both the Myc-binding site cluster and the C/EBPalpha-binding site are essential for strong transcriptional activation, and that Myc and C/EBPalpha synergistically activate the WS5 promoter. Ectopic expression of WS5 led to cell transformation documented by anchorage-independent growth. The human melanoma antigen Pmel17, a type I transmembrane glycoprotein, is the mammalian protein with the highest amino acid sequence identity (38%) to WS5. The Pmel17 gene is regulated by the MITF protein, a bHLHZip transcription factor with DNA binding specificities similar to those of Myc/Max. WS5 is also related to human glycoprotein GPNMB expressed in metastatic melanoma cells and implicated in the progression of brain and liver tumors.

Cooperative cell transformation by Myc/Mil(Raf) involves induction of AP-1 and activation of genes implicated in cell motility and metastasis

Avian fibroblasts transformed simultaneously by the v-myc and v-mil(raf) oncogenes of acute leukemia and carcinoma virus MH2 contain elevated levels of c-Fos and c-Jun, major components of the transcription factor complex AP-1. To define specific transcriptional targets in these cells, subtractive hybridization techniques were employed leading to the identification of strongly upregulated genes including OPN (osteopontin), 126MRP, and rac2. OPN is a cytokine and cell attachment protein which has been implicated in human tumor progression and metastasis, the calcium binding 126MRP protein is related to the human S100 protein family involved in invasive cell growth, and the Rac2 protein belongs to the Rho family of small GTPases regulating actin reorganization and cell migration. Promoter analysis indicated that OPN activation is mediated by a non-consensus AP-1 binding site located close to the transcription start site. Electrophoretic mobility shift assays, chromatin immunoprecipitation and transcriptional reporter gene analyses showed that c-Fos and c-Jun bind specifically to this site and that c-Fos efficiently transactivates the OPN promoter. High-level expression of OPN, 126MRP, or Rac2 proteins from a retroviral vector led to partial cell transformation, documented by morphological changes and anchorage-independent growth. The specific activation in v-myc/v-mil(raf)-transformed cells of target genes with intrinsic oncogenic potential may provide an explanation for the longstanding observation that concomitant expression of these oncogenes leads to strongly enhanced oncogenicity in vivo and in vitro compared to cell transformation by v-myc or v-mil(raf) alone.

Molecular targets of the oncogenic transcription factor jun

The Jun oncoprotein is a major component of the transcription factor complex AP-1, which regulates the expression of multiple genes essential for cell proliferation, differentiation and apoptosis. Constitutive activation of endogenous AP-1 is required for tumor formation in avian and mammalian cell transformation systems, and also occurs in distinct human tumor cells suggesting that AP-1 plays an important role in human oncogenesis. The highly oncogenic v-jun allele capable of inducing neoplastic transformation in avian fibroblasts and fibrosarcomas in chicken as a single oncogenic event, was generated by mutation of the cellular c-jun gene during retroviral transduction. Hence, avian cells represent an excellent model system to investigate molecular mechanisms underlying jun-induced cell transformation. Approaches aimed at the identification of genes specifically deregulated in jun-transformed fibroblasts have led to the identification of several genes targeted by oncogenic Jun. Some of the activated genes represent direct transcriptional targets of Jun encoding proteins, which are presumably involved in cell growth and differentiation. Genes suppressed in v-jun-transformed cells include several extracellular proteins like components of the extracellular matrix or proteins involved in extracellular signalling. Due to aberrant regulation of multiple genes by the Jun oncoprotein, it is assumed that only the combined differential expression of Jun target genes or of a subset thereof contributes to the conversion of a normal fibroblast into a tumor cell displaying a phenotype typical of jun-induced cell transformation. It has already been shown that distinct activated targets exhibit partial transforming activity upon over-expression in avian fibroblasts. Also, distinct target genes silenced by v-Jun inhibit tumor formation when re-expressed in v-jun-transformed cells. The protein products of these transformation-relevant genes may thus represent potential drug targets for interference with jun-induced tumorigenesis.

JAC, a direct target of oncogenic transcription factor Jun, is involved in cell transformation and tumorigenesis

Using subtractive hybridization techniques, we have isolated a gene termed JAC that is strongly and specifically activated in avian fibroblasts transformed by the v-jun oncogene of avian sarcoma virus 17 (ASV17), but not in cells transformed by other oncogenic agents. Furthermore, JAC is highly expressed in cell lines derived from jun-induced avian fibrosarcomas. Kinetic analysis using a doxycycline-controlled conditional cell transformation system showed that expression of the 0.8-kb JAC mRNA is induced rapidly upon activation of the oncogenic v-jun allele. Nucleotide sequence analysis and transcriptional mapping revealed that the JAC gene contains two exons, with the longest ORF confined to exon 2. The deduced 68-amino acid chicken JAC protein is rich in cysteine residues and displays 37% sequence identity to mammalian high-sulfur keratin-associated proteins. The promoter region of JAC contains a consensus (5'-TGACTCA-3') and a nonconsensus (5'-TGAGTAA-3') AP-1 binding site in tandem, which are both specifically bound by the Gag-Jun hybrid protein encoded by ASV17. Mutational analysis revealed that the two AP-1 sites confer strong transcriptional activation by Gag-Jun in a synergistic manner. Ectopic expression of JAC in avian fibroblasts leads to anchorage-independent growth, strongly suggesting that deregulation of JAC is an essential event in jun-induced cell transformation and tumorigenesis.

TOJ3, a target of the v-Jun transcription factor, encodes a protein with transforming activity related to human microspherule protein 1 (MCRS1)

Using the established quail cell line Q/d3 conditionally transformed by the v-jun oncogene, cDNA clones (TOJ2, TOJ3, TOJ5, TOJ6) were isolated by representational difference analysis (RDA) that correspond to genes which were induced immediately upon conditional activation of v-jun. One of these genes, TOJ3, is immediately and specifically activated after doxycycline-mediated v-jun induction, with kinetics similar to the induction of well characterized direct AP-1 target genes. TOJ3 is neither activated upon conditional activation of v-myc, nor in cells or cell lines non-conditionally transformed by oncogenes other than v-jun. Sequence analysis revealed that the TOJ3-specific cDNA encodes a 530-amino acid protein with significant sequence similarities to the murine or human microspherule protein 1 (MCRS1, MSP58), a nucleolar protein that directly interacts with the ICP22 regulatory protein from herpes simplex virus 1 or with p120, a proliferation-related protein expressed at high levels in most human malignant tumor cells. Similar to its mammalian counterparts, the TOJ3 protein contains a bipartite nuclear localization motif and a forkhead associated domain (FHA). Using polyclonal antibodies directed against a recombinant amino-terminal TOJ3 protein segment, the activation of TOJ3 in jun-transformed fibroblasts was also demonstrated at the protein level by specific detection of a polypeptide with an apparent M(r) of 65 000. Retroviral expression of the TOJ3 gene in quail or chicken embryo fibroblasts induces anchorage-independent growth, indicating that the immediate activation of TOJ3 in fibroblasts transformed by the v-jun oncogene contributes to cell transformation.

Application of cross-correlated NMR spin relaxation to the zinc-finger protein CRP2(LIM2): evidence for collective motions in LIM domains

The solution structure of quail CRP2(LIM2) was significantly improved by using an increased number of NOE constraints obtained from a 13C,15N-labeled protein sample and by applying a recently developed triple-resonance cross-correlated relaxation experiment for the determination of the backbone dihedral angle psi. Additionally, the relative orientation of the 15N(i)-1HN(i) dipole and the 13CO(i) CSA tensor, which is related to both backbone angles phi and psi, was probed by nitrogen-carbonyl multiple-quantum relaxation and used as an additional constraint for the refinement of the local geometry of the metal-coordination sites in CRP2(LIM2). The backbone dynamics of residues located in the folded part of CRP2(LIM2) have been characterized by proton-detected 13C'(i-1)-15N(i) and 15N(i)-1HN(i) multiple-quantum relaxation, respectively. We show that regions having cross-correlated time modulation of backbone isotropic chemical shifts on the millisecond to microsecond time scale correlate with residues that are structurally altered in the mutant protein CRP2(LIM2)R122A (disruption of the CCHC zinc-finger stabilizing side-chain hydrogen bond) and that these residues are part of an extended hydrogen-bonding network connecting the two zinc-binding sites. This indicates the presence of long-range collective motions in the two zinc-binding subdomains. The conformational plasticity of the LIM domain may be of functional relevance for this important protein recognition motif.

Structure, function, and dynamics of the dimerization and DNA-binding domain of oncogenic transcription factor v-Myc

The protein product (c-Myc) of the protooncogene c-myc is a transcriptional regulator playing a key role in cellular growth, differentiation, and apoptosis. Deregulated myc genes, like the transduced retroviral v-myc allele, are oncogenic and cause cell transformation. The C-terminal bHLHZip domain of v-Myc, encompassing protein dimerization (helix-loop-helix, leucine zipper) and DNA contact (basic region) surfaces, was expressed in bacteria as a highly soluble p15(v-myc )recombinant protein. Dissociation constants (K(d)) for the heterodimer formed with the recombinant bHLHZip domain of the Myc binding partner Max (p14(max)) and for the Myc-Max-DNA complex were estimated using circular dichroism (CD) spectroscopy and quantitative electrophoretic mobility shift assay (EMSA). Multi-dimensional NMR spectroscopy was used to characterize the solution structural and dynamic properties of the v-Myc bHLHZip domain. Significant secondary chemical shifts indicate the presence of two separated alpha-helical regions. The C-terminal leucine zipper region forms a compact alpha-helix, while the N-terminal basic region exhibits conformational averaging with substantial alpha-helical content. Both helices lack stabilizing tertiary side-chain interactions and represent exceptional examples for loosely coupled secondary structural segments in a native protein. These results and CD thermal denaturation data indicate a monomeric state of the v-Myc bHLHZip domain. The (15)N relaxation data revealed backbone mobilities which corroborate the existence of a partially folded state, and suggest a "beads-on-a-string" motional behaviour of the v-Myc bHLHZip domain in solution. The preformation of alpha-helical regions was confirmed by CD thermal denaturation studies, and quantification of the entropy changes caused by the hydrophobic effect and the reduction of conformational entropy upon protein dimerization. The restricted conformational space of the v-Myc bHLHZip domain reduces the entropy penalty associated with heterodimerization and allows rapid and accurate recognition by the authentic Myc binding partner Max.

Conditional cell transformation by doxycycline-controlled expression of the ASV17 v-jun allele

To investigate the molecular basis of oncogenesis induced by the v-jun oncogene of avian sarcoma virus 17 (ASV17), we developed a conditional cell transformation system in which transcription of the ASV17 v-jun allele is controlled by a doxycycline-sensitive transactivator (tTA) or a reverse (doxycycline-dependent) transactivator (rtTA), respectively. Permanent cell lines of quail embryo fibroblasts conditionally transformed by a doxycycline-controlled v-jun allele revert to the normal phenotype within 3 days and lose their ability to grow in soft agar, strictly dependent on the addition or removal of the drug, respectively. The reverted cells are rapidly retransformed on conditional activation of v-jun. While full-level synthesis of v-jun mRNA and v-Jun protein in these cells is established within 2 and 14 h, respectively, after switching to the permissive conditions, the first morphological alterations are observed after 24 h, and as early as 2 days later the morphology has changed entirely from flat cells resembling normal fibroblasts to spindle-shaped fusiform cells showing a typical jun-transformed phenotype. Kinetic expression analysis revealed that transcriptional activation of the direct jun target gene BKJ precisely coincides with the establishment of full-level v-Jun protein synthesis. Furthermore, we have analyzed the expression of a novel candidate v-jun target gene, termed JAC, which shows no sequence homology to known genes. Similar to BKJ, JAC is specifically activated in jun-transformed fibroblasts, and induction of JAC is tightly linked to the conditional expression of oncogenic v-Jun. These results demonstrate the high stringency of the doxycycline-controlled v-jun expression system, and they also indicate that expression of v-jun in these cells is indispensable for enhanced proliferation, cell transformation, and the induction of specific expression patterns of downstream target genes.

Mutational analysis and NMR spectroscopy of quail cysteine and glycine-rich protein CRP2 reveal an intrinsic segmental flexibility of LIM domains

The LIM domain is a conserved cysteine and histidine-containing structural module of two tandemly arranged zinc fingers. It has been identified in single or multiple copies in a variety of regulatory proteins, either in combination with defined functional domains, like homeodomains, or alone, like in the CRP family of LIM proteins. Structural studies of CRP proteins have allowed a detailed evaluation of interactions in LIM-domains at the molecular level. The packing interactions in the hydrophobic core have been identified as a significant contribution to the LIM domain fold, whereas hydrogen bonding within each single zinc binding site stabilizes zinc finger geometry in a so-called "outer" or "indirect" coordination sphere. Here we report the solution structure of a point-mutant of the carboxyl-terminal LIM domain of quail cysteine and glycine-rich protein CRP2, CRP2(LIM2)R122A, and discuss the structural consequences of the disruption of the hydrogen bond formed between the guanidinium side-chain of Arg122 and the zinc-coordinating cysteine thiolate group in the CCHC rubredoxin-knuckle. The structural analysis revealed that the three-dimensional structure of the CCHC zinc binding site in CRP2(LIM2)R122A is adapted as a consequence of the modified hydrogen bonding pattern. Additionally, as a result of the conformational rearrangement of the zinc binding site, the packing interactions in the hydrophobic core region are altered, leading to a change in the relative orientation of the two zinc fingers with a concomitant change in the solvent accessibilities of hydrophobic residues located at the interface of the two modules. The backbone dynamics of residues located in the folded part of CRP2(LIM2)R122A have been characterized by proton-detected(15)N NMR spectroscopy. Analysis of the R2/R1ratios revealed a rotational correlation time of approximately 6.2 ns and tumbling with an axially symmetric diffusion tensor (D parallel/D perpendicular=1.43). The relaxation data were also analyzed using a reduced spectral density mapping approach. As in wild-type CRP2(LIM2), significant mobility on a picosecond/nanosecond time-scale was detected, and conformational exchange on a microsecond time-scale was identified for residues located in loop regions between secondary structure elements. In summary, the relative orientation of the two zinc binding sites and the accessibility of hydrophobic residues is not only determined by hydrophobic interactions, but can also be modified by the formation and/or breakage of hydrogen bonds. This may be important for the molecular interactions of an adaptor-type LIM domain protein in macromolecular complexes, particularly for the modulation of protein-protein interactions.

Conditional cell transformation by doxycycline-controlled expression of the MC29 v-myc allele

To investigate the molecular basis of oncogenesis induced by the v-myc oncogene of avian myelocytomatosis virus MC29, we developed a conditional cell transformation system in which expression of the MC29 v-myc allele is dependent on a doxycycline-sensitive transactivator (tTA). Clonal lines of quail embryo fibroblasts transformed by doxycycline-controlled v-myc revert to the normal phenotype and lose their ability to grow in soft agar after the addition of doxycycline. Repression of v-myc causes the cells to withdraw from the cell cycle, and long-term survival in culture requires reexpression of v-myc. Although complete repression of v-myc mRNA and v-Myc protein in these cells occurs within 14 h after the addition of doxycycline, the first morphological alterations are observed after 24 h, and after 3 days, the morphology changed entirely from small rounded cells showing a typical myc-transformed phenotype to large flat cells resembling normal fibroblasts. Cells exposed to doxycycline for 3 days reexpressed v-myc within 24 h after withdrawal of the drug from the culture medium, partial retransformation occurred after 2 days, and complete morphological transformation was reestablished after 6 days. Analogous results were obtained with a cell line in which expression of the v-myc allele is dependent on a reverse transactivator (rtTA) that is activated by doxycycline. The striking differential expression of known transformation-sensitive genes and of new candidate v-myc target genes revealed the tightness of the doxycycline-controlled v-myc expression system. The data also indicate that expression of v-myc in these cells is indispensable for enhanced proliferation, transformation, and immortalization.

Structure and transcriptional regulation of BKJ, a novel AP-1 target gene activated during jun- or fos-induced fibroblast transformation

The BKJ gene was originally identified based on its specific transcriptional activation in jun-transformed avian fibroblasts. We now show that BKJ is a direct transcriptional target of the AP-1 transcription factor components Jun and Fos. The complete structural organization of the quail BKJ gene was determined by nucleotide sequence analysis and transcriptional mapping. The gene contains three exons with the coding region confined to the third exon. A major mRNA species of 0.8 kb and a minor one of 1.3 kb are produced by variable usage of two transcriptional initiation sites. The BKJ promoter region contains two authentic AP-1 binding sites. By transactivation of reporter gene constructs and direct binding of Jun recombinant protein, the proximal AP-1 element was shown to be essential for BKJ promoter activation. Using polyclonal antiserum directed against recombinant BKJ protein, the activation of BKJ in jun-transformed avian fibroblasts was also demonstrated at the protein level. BKJ is a novel gene related to the avian beta-keratin gene family whose members display highly specific expression patterns during embryogenesis and epidermal development. Activation of BKJ in fibroblasts by retroviral or deregulated cellular jun or fos alleles may contribute to cell transformation.

Structure of cysteine- and glycine-rich protein CRP2. Backbone dynamics reveal motional freedom and independent spatial orientation of the lim domains

Members of the cysteine- and glycine-rich protein family (CRP1, CRP2, and CRP3) contain two zinc-binding LIM domains, LIM1 (amino-terminal) and LIM2 (carboxyl-terminal), and are implicated in diverse cellular processes linked to differentiation, growth control, and pathogenesis. Here we report the solution structure of full-length recombinant quail CRP2 as determined by multi-dimensional triple-resonance NMR spectroscopy. The structural analysis revealed that the global fold of the two LIM domains in the context of the full-length protein is identical to the recently determined solution structures of the isolated individual LIM domains of quail CRP2. There is no preference in relative spatial orientation of the two domains. This supports the view that the two LIM domains are independent structural and presumably functional modules of CRP proteins. This is also reflected by the dynamic properties of CRP2 probed by 15N relaxation values (T1, T2, and nuclear Overhauser effect). A model-free analysis revealed local variations in mobility along the backbone of the two LIM domains in the native protein, similar to those observed for the isolated domains. Interestingly, fast and slow motions observed in the 58-amino acid linker region between the two LIM domains endow extensive motional freedom to CRP2. The dynamic analysis indicates independent backbone mobility of the two LIM domains and rules out correlated LIM domain motion in full-length CRP2. The finding that the LIM domains in a protein encompassing multiple LIM motifs are structurally and dynamically independent from each other supports the notion that these proteins may function as adaptor molecules arranging two or more protein constituents into a macromolecular complex.

Structure and intramodular dynamics of the amino-terminal LIM domain from quail cysteine- and glycine-rich protein CRP2

Members of the cysteine and glycine-rich protein (CRP) family (CRP1, CRP2, and CRP3) contain two zinc-binding LIM domains, LIM1 and LIM2, and are implicated in diverse cellular processes linked to differentiation, growth control and pathogenesis. The solution structure of an 81-amino acid recombinant peptide encompassing the amino-terminal LIM1 domain of quail CRP2 has been determined by 2D and 3D homo- and heteronuclear NMR spectroscopy. The LIM1 domain consists of two zinc binding sites of the CCHC and the CCCC type, respectively, which both contain two orthogonally arranged antiparallel beta-sheets and which are packed together by a hydrophobic core composed of residues from the zinc finger loop regions. The CCCC zinc finger is followed by a short alpha-helical stretch. The structural analysis revealed that the global fold of LIM1 closely resembles the recently determined solution structures of the carboxyl-terminal LIM2 domains of quail CRP2 and chicken CRP1, and that LIM1 and LIM2 are independently folded structural and presumably functional domains of CRP proteins. To explore the dynamical properties of CRP proteins, we have used 15N relaxation values (T1, T2, and nuclear Overhauser effect (NOE) to describe the dynamical behavior of a LIM domain. A model-free analysis revealed local variations in mobility along the backbone of the quail CRP2 LIM1 motif. Slow motions are evident in turn regions located between the various antiparallel beta-sheets or between their strands. By use of an extended motional model, fast backbone motions were detected for backbone amide NH groups of hydrophobic residues located in the core region of the LIM1 domain. These findings point to a flexible hydrophobic core in the LIM1 domain allowing residual relative mobility of the two zinc fingers, which might be important to optimize the LIM1 interface for interaction with its physiological target molecule(s) and to compensate enthalpically for the entropy loss upon binding.

Cloning, structural analysis, and chromosomal localization of the human CSRP2 gene encoding the LIM domain protein CRP2

The CSRP2 gene encoding the LIM domain protein CRP2 was originally identified in quail based on its strong transcriptional suppression in transformed avian fibroblasts. Here we have isolated a human CSRP2 cDNA clone encoding a 193-amino-acid human CRP2 (hCRP2) protein with 96.4% amino acid sequence identity to the avian homolog. The CSRP2 cDNA clone was used to isolate CSRP2-related clones from gamma EMBL3 and P1 libraries of human genomic DNA. The complete organization of the CSRP2 gene was determined by nucleic acid hybridization, transcriptional mapping, and nucleotide sequence analysis. The gene spans a total of approximately 22 kb and contains six exons. The coding region is confined to exons 2-6 and predicts a hCRP2 protein identical in its amino acid sequence to the protein deduced from the CSRP2 cDNA clone. By fluorescence in situ hybridization using both lambda EMBL3 and P1 library clones as hybridization probes and a new method for computerized signal localization, CSRP2 was mapped to chromosome subband 12q21.1, a region frequently affected by deletion or breakage events in various tumor types. The library screens also led to the isolation of a CSRP2-related pseudogene, CSRP2P, which carried several extensive deletions and nucleotide substitutions but no intervening sequences in comparison to the CSRP2 cDNA sequence. By physical linkage and fluorescence in situ hybridization, CSRP2P was mapped to chromosome subband 3q21.1.

Solution structure of the carboxyl-terminal LIM domain from quail cysteine-rich protein CRP2

Proteins of the cysteine-rich protein (CRP) family (CRP1, CRP2, and CRP3) are implicated in diverse processes linked to cellular differentiation and growth control. CRP proteins contain two LIM domains, each formed by two zinc-binding modules of the CCHC and CCCC type, respectively. The solution structure of the carboxyl-terminal LIM domain (LIM2) from recombinant quail CRP2 was determined by multidimensional homo- and heteronuclear magnetic resonance spectroscopy. The folding topology retains both independent zinc binding modules (CCHC and CCCC). Each module consists of two orthogonally arranged antiparallel beta-sheets, and the carboxyl-terminal CCCC module is terminated by an alpha-helix. 15N magnetic relaxation data indicate that the modules differ in terms of conformational flexibility. They pack together via a hydrophobic core region. In addition, Arg122 in the CCHC module and Glu155 in the CCCC module are linked by an intermodular hydrogen bond and/or salt bridge. These residues are absolutely conserved in the CRP family of LIM proteins, and their interaction might contribute to the relative orientation of the two zinc-binding modules in CRP LIM2 domains. The global fold of quail CRP2 LIM2 is very similar to that of the carboxyl-terminal LIM domain of the related but functionally distinct CRP family member CRP1, analyzed recently. The carboxyl-terminal CCCC module is structurally related to the DNA-binding domain of the erythroid transcription factor GATA-1. In the two zinc-binding modules of quail CRP2 LIM2, flexible loop regions made up of conserved amino acid residues are located on the same side of the LIM2 domain and may cooperate in macromolecular recognition.

Suppression in transformed avian fibroblasts of a gene (CO6) encoding a membrane protein related to mammalian potassium channel regulatory subunits

Gene expression patterns in normal and v-myc-transformed quail embryo fibroblasts were compared by mRNA differential display. Displaying approximately 2500 mRNA species by reverse transcription/PCR, reamplification of 73 differential cDNA fragments and rescreening by Northern analysis led to the isolation of a clone, termed CO6, that hybridized to an mRNA species present only in the normal but not in the transformed fibroblasts. Further analyses revealed that the 0.95-kb CO6 mRNA was present in all normal quail and chicken embryo fibroblasts tested, but that it was undetectable in a variety of established quail cell lines transformed by the v-myc, v-myc/v-mil, v-jun/junD or v-src oncogenes or by a chemical carcinogen. Furthermore, CO6 mRNA was not detectable in fibroblasts newly transformed by retroviral constructs carrying v-myc or v-jun alleles or by the avian sarcoma virus ASV17. In fibroblasts transformed by a temperature-sensitive v-src mutant, expression of CO6 was strongly induced at the non-permissive temperature and reduced at the permissive temperature. Nucleotide sequence analysis of quail CO6 cDNA indicated that the corresponding gene encodes a 200-amino acid protein with 46 to 48% amino acid sequence identity to the regulatory beta subunits (K(VCa)beta) of the bovine, human and canine high conductance Ca2+-activated K+ channels. No sequence homology to other ion channel subunits or to any other proteins in the databases was found. Like the K(VCa)beta subunits, the CO6 protein contains two putative transmembrane segments. Based on the relationship to mammalian K(VCa)beta both in primary structure and domain topology, the CO6 protein may represent the regulatory subunit of a yet unidentified avian Ca2+-activated potassium channel or a related membrane protein possibly involved in the regulation of cell proliferation.

A novel function for Myc: inhibition of C/EBP-dependent gene activation

We have investigated the effect of the v-Myc oncoprotein on gene expression in myelomonocytic cells. We find that v-Myc dramatically down-regulates the expression of myelomonocytic-specific genes, such as the chicken mim-1 and lysozyme genes, both of which are known targets for C/EBP transcription factors. We present evidence that Myc downregulates these genes by inhibiting the function of C/EBP transcription factors. Detailed examination of the inhibitory mechanism shows that amino-terminal sequences of v-Myc, but not its DNA-binding domain, are required for the suppression of C/EBP-dependent transactivation. Our findings identify a new function for Myc and reveal a novel mechanism by which Myc affects the expression of other genes.

Specific activation in jun-transformed avian fibroblasts of a gene (bkj) related to the avian beta-keratin gene family

We have analyzed differential gene expression in normal versus jun-transformed avian fibroblasts by using subtracted nucleic acid probes and differential nucleic acid hybridization techniques for the isolation of cDNA clones. One clone corresponded to a gene that was strongly expressed in a previously established quail (Coturnix japonica) embryo fibroblast line (VCD) transformed by a chimeric jun oncogene but whose expression was undetectable in normal quail embryo fibroblasts. Furthermore, the gene was expressed in quail or chicken fibroblast cultures that were freshly transformed by retroviral constructs carrying various viral or cellular jun alleles and in chicken fibroblasts transformed by the avian retrovirus ASV17 carrying the original viral v-jun allele. However, its expression was undetectable in a variety of established avian cell lines or freshly prepared avian fibroblast cultures transformed by other oncogenes or a chemical carcinogen. The nucleotide and deduced amino acid sequences of the cDNA clone were not identical to any sequence entries in the data bases but revealed significant similarities to avian beta-keratin genes; the highest degree of amino acid sequence identity was 63%. The gene, which we termed bkj, may represent a direct or indirect target for jun function.

The cysteine-rich protein family of highly related LIM domain proteins

Here we describe a family of closely related LIM domain proteins in avian cells. The LIM motif defines a zinc-binding domain that is found in a variety of transcriptional regulators, proto-oncogene products, and proteins associated with sites of cell-substratum contact. One type of LIM-domain protein, called the cysteine-rich protein (CRP), is characterized by the presence of two LIM domains linked to short glycine-rich repeats and a potential nuclear localization signal. We have identified and characterized two evolutionarily conserved members of the CRP family, CRP1 and CRP2, in chicken and quail. Expression of the genes encoding both CRP1 and CRP2 is differentially regulated in normal versus transformed cells, raising the possibility that members of the CRP family may function in control of cell growth and differentiation.

A quail long-term cell culture transformed by a chimeric jun oncogene

A chimeric construct (VCD) consisting of parts from viral jun, chicken c-jun, and chicken junD was cloned into the replication-competent retroviral RCAS vector. This construct, RCAS-VCD, was found to have a higher focus forming potential in quail fibroblasts than the equivalent construct RCAS-VJ-1, expressing viral jun. DNAs from RCAS-VCD and RCAS-VJ-1 were transfected into primary quail embryo fibroblasts. Cells derived from one RCAS-VCD-induced focus survived cell crisis, which became manifest after 15 passages, and could be expanded into a long-term culture. This cell line, termed VCD, has been passaged for over 100 times so far. The cells grow to very high densities and then pile up into clumps of rounded cells. The culture releases a transforming virus with a titer of 10(5) FFU/ml, as tested on primary quail embryo fibroblasts. The transformed phenotype of VCD cells was verified by agar colony formation. VCD cells are capable of anchorage-independent growth with a cloning efficiency of 10%. Southern blot analysis of genomic DNA from VCD cells showed proviral integration of the RCAS construct without detectable rearrangements. Northern and Western blot analyses confirmed correct expression from integrated RCAS-VCD of predicted RNAs and of the chimeric Jun(VCD) protein. jun(VCD)-transformed cells provide a constant source of homogeneous cellular material for the investigation of the molecular mechanisms of jun-induced cell transformation and for the identification of direct and indirect targets of Jun protein function.

Identification of amino acid-base contacts in the Myc-DNA complex by site-specific bromouracil mediated photocrosslinking

Myc binds to a 6 bp 2-fold symmetric DNA site: 5'-C-3A-2C-1G+1T+2G+3-3'. Using site-specific 5-bromouracil mediated photocrosslinking, we show that His336 of Myc contacts, or is close to, the thymine 5-methyl group at 2-fold symmetry-related positions -2 and +2 of the DNA site in the Myc-DNA complex. Our results strongly suggest that homologous amino acids of Myc and Max make equivalent contacts in the respective protein-DNA complexes.

Suppression in transformed avian fibroblasts of a gene (crp) encoding a cysteine-rich protein containing LIM domains

Using cDNA subtraction and differential hybridization techniques, a cDNA library derived from normal quail embryo fibroblasts was screened for clones corresponding to genes whose expression was suppressed in v-myc-transformed, as compared with normal, quail embryo fibroblasts. One of the isolated cDNA clones corresponded to a 0.9-kb mRNA that was present in normal quail and chicken embryo fibroblasts, but was virtually absent from all transformed avian cells tested: quail embryo fibroblasts transformed by the v-myc, v-myc/v-mil or v-src oncogenes, cells derived from a methylcholanthrene-induced quail fibrosarcoma or v-myc-transformed chicken macrophages. Nucleotide sequence analysis of the original and supplementary cDNA clones indicated that the corresponding gene encodes a 194 amino acid cysteine-rich protein (M(r) 20,911). A database search revealed that the gene is the avian homolog of a human primary response gene (crp) of unknown function. Both the quail and human CRP proteins contain two copies of a cysteine-rich amino acid sequence motif (LIM) with putative zinc-binding activity that was previously identified in several proteins with presumed regulatory functions essential for cell growth or differentiation.

Sequence and expression of a glyceraldehyde-3-phosphate dehydrogenase-encoding gene from quail embryo fibroblasts

Using differential hybridization techniques, a cDNA library derived from a line of v-myc-transformed quail embryo fibroblasts was screened for clones whose expression was elevated in transformed, as compared with normal, cells. One of the isolated clones contained the entire coding region of the quail glyceraldehyde-3-phosphate dehydrogenase-encoding gene (GAPDH). A comparison of the deduced 333-amino-acid (aa) sequence of quail GAPDH with that of the only other avian (chicken) GAPDH sequence known, and with those of mammalian counterparts indicates the strong aa sequence conservation of this glycolytic enzyme. GAPDH is expressed in all transformed and non-transformed quail and chicken embryo fibroblasts and macrophages tested, with a moderate elevation of expression in most of the transformed cell lines. In the avian genomes, GAPDH is present in a single copy, in contrast to the high number of GAPDH-related sequences in mammalian species.

Sequence-specific DNA binding by Myc proteins

Myc proteins have a tripartite carboxyl-terminal domain containing specific amino acid sequence motifs: a basic motif, a helix-loop-helix motif, and a leucine heptad repeat. Similar sequence motifs have been identified in several eukaryotic transcription factors and were shown to facilitate protein-DNA and protein-protein interactions. By using recombinant v-Myc proteins obtained by bacterial expression of full-length or partially deleted avian v-myc alleles, the functional relevance of these sequence motifs for Myc protein oligomerization and for DNA binding was investigated. All recombinant v-Myc proteins that have retained the carboxyl-terminal domain dimerize and specifically bind to double-stranded DNA containing the palindromic core sequence CACGTG. This and a closely related DNA sequence element have been defined previously as part of the binding sites for human transcription factors USF and TFE3, which specifically bind to the adenovirus major late promoter or the muE3 motif within the immunoglobulin heavy-chain enhancer, respectively. It is shown that a 61-amino-acid peptide sequence containing only the bipartite basic motif/helix-loop-helix domain of Myc is necessary and sufficient for dimerization and sequence-specific DNA binding of v-Myc recombinant proteins.

Myc protein structure: localization of DNA-binding and protein dimerization domains

Wildtype and mutant v-Myc proteins were overexpressed in Escherichia coli using the T7 RNA polymerase system, and the in vitro DNA-binding activities of partially or highly purified proteins were analysed by native DNA-cellulose chromatography. For the construction of the expression plasmids, cloned proviral DNA from wildtype MC29 or from its spontaneous deletion mutant Q10C was used, the latter lacking internal v-myc sequences. Both the wildtype (p59) and the mutant (p42) recombinant protein contain at their amino termini 12 amino acids encoded by the vector, followed by 11 gag amino acids and 9 amino acids encoded by v-myc sequences derived from noncoding c-myc sequences. In addition, p59 contains 416 amino acids encoded by v-myc sequences derived from the complete chicken c-myc coding region, whereas p42 lacks 120 amino acids from the central region of the Myc protein including the highly acidic domain. Two additional proteins were engineered which contain the first 309 (p53) or the last 107 (p16) amino acids, respectively, of the Myc protein sequence in addition to vector-encoded amino acids. The p16 protein represents the carboxyl terminus of the Myc protein sequence containing both a muscle determination gene (MyoD1) homology region, including a basic motif and an amphipathic helix-loop-helix motif, and a leucine heptad repeat. All proteins, except p53 which lacks the carboxyl-terminal Myc protein sequences, bound to native DNA-cellulose and were eluted with 200-500 mM NaCl. Based on the DNA-binding activities of recombinant or spontaneous mutant v-Myc proteins extracted from bacterial or from transformed avian cells, we conclude that the DNA-binding domain of avian Myc proteins is confined within the last 86 carboxyl-terminal amino acids. The same region is also shown to be necessary and sufficient for Myc protein dimerization. This 86-amino acid region essentially encompasses a putative basic DNA contact surface and a tandem array of two presumed protein dimerization motifs, helix-loop-helix and leucine repeat.

Structural analysis of normal and transforming mil(raf) proteins: effect of 5'-truncation on phosphorylation in vivo or in vitro

The phosphorylation sites of the cellular proto-oncogene product p71/73c-mil(raf) from quail and from human cells were analyzed by two-dimensional peptide mapping and compared to the sites phosphorylated in proteins encoded by three transforming alleles of c-mil(raf). These alleles all were 5'-truncated resulting from either retroviral transduction (v-mil, v-raf) or promoter insertion mutagenesis (LTR-c-raf). The normal cellular proteins each were phosphorylated in vivo on three major sites, two of which were identical in the two protein species. MH2 p100gag-mil, murine sarcoma virus 3611 p75gag-raf, and LTR-c-raf p45-50 delta c-raf were phosphorylated in vivo on several sites. One site was shared between these transforming proteins and was also conserved in both avian and human p71/73c-mil(raf). All normal and transforming mil(raf) proteins were phosphorylated on serine in vivo while p100gag-mil and p75gag-raf occasionally also contained low levels of phosphothreonine. No specific phosphorylation of p71/73c-mil(raf) was detected in vitro under conditions that readily revealed presumed autophosphorylation of p100gag-mil, p75gag-raf, and p45-50 delta c-raf. However, the in vitro phosphorylated sites of these proteins were different to each other and to the sites phosphorylated in vivo. In contrast to the predominant threonine phosphorylation of the two viral proteins, only phosphoserine could be detected in p45-50 delta c-raf phosphorylated in vitro.

Primary structure of the chicken c-mil protein:identification of domains shared with or absent from the retroviral v-mil protein

The complete primary structure of the protein product of the proto-oncogene c-mil was deduced from the nucleotide sequence of chicken c-mil cDNA clones. The c-mil protein contains 647 amino acid residues and has a calculated molecular weight of 73,132. Based on sequence comparisons with proteins of known or presumed biochemical function, two domains were recognized on the c-mil protein. In the carboxyl-terminal half of the protein, a 250-amino acid segment displays significant homology to the protein kinase domains of the src oncogene protein or of protein kinase C. In the amino-terminal half, a cysteine-rich segment (Cys-X2-Cys-X9-Cys-X2-Cys-X7-Cys-X7-Cys) of the c-mil protein shares significant homology with two similar repetitive domains of protein kinase C. Of the two structural and presumably functional domains of the c-mil protein, only the kinase domain is contained within the carboxyl-terminal 379-amino acid polypeptide encoded by the transduced v-mil allele of avian oncogenic retrovirus MH2. Hence, truncation of the 5' coding region in the course of the transduction and the resulting lack of the authentic amino-terminal domain in the protein product of the transduced allele may be a critical event in changing mil function from physiologic to oncogenic.

Structure of mutant and wild-type MC29 v-myc alleles and biochemical properties of their protein products

Proviral DNAs of three fibroblast-transforming MC29 deletion mutants (MC29-10A, MC29-10C, MC29-10H) with defects in hemopoietic cell transformation and tumor induction were molecularly cloned and their deletions were defined by nucleotide sequence analysis. The MC29-10C and MC29-10H v-myc alleles have identical internal deletions overlapping with a smaller one in the MC29-10A v-myc allele, and MC29-10H has an additional internal deletion in the partial gag complement. All deletions are in frame, and the deduced sequences of the mutant gag-myc hybrid proteins lack 56 (MC29-10A) or 120 (MC29-10C, MC29-10H) myc-specific and 44 gag-specific (MC29-10H) amino acid residues. The deleted v-myc nucleotide sequences correspond to the 3' end of exon 2 and the 5' end of exon 3 of the cellular c-myc gene including a region that encodes a high number of acidic amino acid residues. Based on these structural analyses, biochemical properties of mutant and wild-type gag-myc hybrid proteins were compared. Tryptic digests of all three mutant proteins lack a large myc-specific peptide that is present in digests of the wild-type protein and is extensively phosphorylated at serine and threonine residues. Concordantly, the sequence analyses predict that such a large tryptic peptide with putative phosphorylation sites at serine and threonine residues is present in the wild-type gag-myc protein but absent in all three mutant proteins due to the v-myc deletions. Chromatography of wild-type and mutant gag-myc proteins on DNA-cellulose revealed that their in vitro DNA affinities are indistinguishable from each other. Correspondingly, the sequence analyses predict that the carboxyl-terminal region rich in basic amino acid residues and with putative DNA affinity is conserved in wild-type and mutant gag-myc proteins. We conclude that the internal v-myc protein sequences defined by the deletions are necessary for hemopoietic cell transformation and complete phosphorylation, but dispensable for fibroblast transformation and in vitro DNA binding.

Nucleotide sequence of the CMII v-myc allele

The entire nucleotide sequence of the transduced v-myc allele in the genome of avian oncogenic retrovirus CMII was determined. The CMII v-myc and the chicken c-myc alleles differ in their shared coding sequences by a single nucleotide substitution causing a glutamic acid/alanine exchange in the predicted sequences of the corresponding protein products. This mutation has not been found in the transduced v-myc alleles of avian oncogenic retroviruses MC29, MH2, and OK10. We conclude that no specific, if any, missense mutation of the c-myc coding sequence is necessary for oncogenic activation upon transduction of the cellular gene.

Structure and transforming function of transduced mutant alleles of the chicken c-myc gene

A small retroviral vector carrying an oncogenic myc allele was isolated as a spontaneous variant (MH2E21) of avian oncovirus MH2. The MH2E21 genome, measuring only 2.3 kilobases, can be replicated like larger retroviral genomes and hence contains all cis-acting sequence elements essential for encapsidation and reverse transcription of retroviral RNA or for integration and transcription of proviral DNA. The MH2E21 genome contains 5' and 3' noncoding retroviral vector elements and a coding region comprising the first six codons of the viral gag gene and 417 v-myc codons. The gag-myc junction corresponds precisely to the presumed splice junction on subgenomic MH2 v-myc mRNA, the possible origin of MH2E21. Among the v-myc codons, the first 5 are derived from the noncoding 5' terminus of the second c-myc exon, and 412 codons correspond to the c-myc coding region. The predicted sequence of the MH2E21 protein product differs from that of the chicken c-myc protein by 11 additional amino-terminal residues and by 25 amino acid substitutions and a deletion of 4 residues within the shared domains. To investigate the functional significance of these structural changes, the MH2E21 genome was modified in vitro. The gag translational initiation codon was inactivated by oligonucleotide-directed mutagenesis. Furthermore, all but two of the missense mutations were reverted, and the deleted sequences were restored by replacing most of the MH2E21 v-myc allele by the corresponding segment of the CMII v-myc allele which is isogenic to c-myc in that region. The remaining two mutations have not been found in the v-myc alleles of avian oncoviruses MC29, CMII, and OK10. Like MH2 and MH2E21, modified MH2E21 (MH2E21m1c1) transforms avian embryo cells. Like c-myc, it encodes a 416-amino-acid protein initiated at the myc translational initiation codon. We conclude that neither major structural changes, such as in-frame fusion with virion genes or internal deletions, nor specific, if any, missense mutations of the c-myc coding region are necessary for activation of the basic oncogenic function of transduced myc alleles.

v-mil induces autocrine growth and enhanced tumorigenicity in v-myc-transformed avian macrophages

MH2, an avian retrovirus containing the v-myc and v-mil oncogenes, rapidly transforms chick hematopoietic cells in vitro. The transformed cells belong to the macrophage lineage and proliferate in the absence of exogenous growth factors. Here we analyze a series of MH2 deletion mutants and show that these two oncogenes together establish an autocrine growth system in which v-myc stimulates cell proliferation, while v-mil induces the production of chicken myelomonocytic growth factor (cMGF). We also demonstrate that these two oncogenes cooperate in vivo. MH2 efficiently induces monocytic leukemias and liver tumors, while deletion mutants lacking either a functional v-mil or v-myc do not.

Protein product of proto-oncogene c-mil

Using antipeptide antibodies with specificity for the carboxyl termini of v-raf and v-mil protein products, two proteins with apparent molecular weights of approximately 71,000/73,000 and 215,000 were detected in immunoprecipitates from normal uninfected chicken cells. The 71,000/73,000-molecular-weight protein was identified as the product of the c-mil proto-oncogene by the close structural relationship of its 42,000-molecular-weight carboxyl-terminal domain to the v-mil-encoded domain of the hybrid protein p100gag-mil specified by the avian retrovirus MH2. The amino-terminal domain of the cellular protein is encoded by 5' c-mil sequences that have not been transduced into the genome of MH2. The c-mil protein (p71/73c-mil) was found to be phosphorylated in vivo, and homologous proteins were detected at variable levels in a variety of vertebrate cells, including human cells.

Interaction between Raf and Myc oncogenes in transformation in vivo and in vitro

3611 MSV, a raf-oncogene-transducing murine retrovirus, induces fibrosarcomas and erythroid hyperplasia in newborn mice after a latency of 4-8 wk. In contrast, new recombinant murine retroviruses carrying the myc oncogene (J-3, J-5 construct viruses) do not induce tumors before greater than 9 wk. A combination of both oncogenes in an infectious murine retrovirus (J-2) induces hematopoietic neoplasms in addition to less prominent fibrosarcomas and pancreatic adenocarcinoma 1-3 wk after inoculation. The hematologic neoplasms consist of immunoblastic lymphomas of T and B cell lineage and erythroblastosis. If animals were inoculated with a variant of the J-3 virus, which induces altered foci in cultures of NIH 3T3 cells, carcinoma developed in the pancreas with a 2-6 mo latency. In parallel to the synergistic action of both oncogenes on hematopoietic cells in vivo, we find that raf-oncogene-induced transformation of bone marrow cells in culture is enhanced by the addition of myc, which by itself does not transform these cells when grown in standard media. We conclude that concomitant expression of raf and myc oncogenes in hematopoietic and epithelial cells alters their respective transforming activities. The contribution of v-myc in this synergism was examined by use of a series of recombinant murine retroviruses capable of expressing the avian v-myc to study the effect of altered myc expression on hematopoietic/lymphoid cells. With either interleukin 3- or interleukin 2-dependent cell lines, introduction of the recombinant viruses abrogated the requirement for IL 3 or IL 2 for growth, and associated with this was the suppression of c-myc expression. The findings suggest that myc is a component in the signal transduction pathway for IL 3 and IL 2 and support an autoregulatory mechanism of c-myc expression. In contrast to v-myc, expression of v-raf in primary lymphoid/hematopoietic cells has an immortalizing function without abrogating the requirement for IL 3 for growth. This suggests that v-raf and v-myc affect different components of growth regulation, as, for example, commitment (v-myc) and cell cycle progression (v-raf).

Rapid induction of hemopoietic neoplasms in newborn mice by a raf(mil)/myc recombinant murine retrovirus

3611 MSV, a raf oncogene-transducing murine retrovirus, induced fibrosarcomas in newborn mice after a latency of 4 to 8 weeks. In contrast, newly constructed recombinant murine retroviruses carrying the myc oncogene did not induce tumors before greater than or equal to 9 weeks. A combination of both oncogenes in an infectious murine retrovirus induced hematopoietic neoplasms in addition to less prominent fibrosarcomas and pancreatic acinar dysplasia 1 to 3 weeks after inoculation. The hematological neoplasms consisted of immunoblastic lymphomas of T- and B-lineage cells and erythroblastosis. Cell lines from these tumors could be readily established in culture in regular medium, whereas culture of cells from raf oncogene-induced tumors required the addition of interleukin 3. In parallel to the synergistic action of both oncogenes on hematopoietic cells in vivo, we found that raf oncogene-induced transformation of fibroblast cell lines in culture was enhanced by the addition of myc, which by itself did not morphologically transform these permanent cell lines. We conclude that concomitant expression of raf and myc oncogenes in hematopoietic cells and fibroblastic cell lines enhances their respective transforming activities.

Nucleotide sequence analysis of the chicken gene c-mil, the progenitor of the retroviral oncogene v-mil

The nucleotide sequence of the chicken gene c-mil was determined within and around all regions homologous to the oncogene v-mil of avian retrovirus MH2. The regions of homology to the previously determined v-mil sequence, ranging in size from 28 to 177 base pairs (bp), are distributed over 14 kilobase pairs (kbp) of the chicken genome and are organized in 11 exons. All exon-intron boundaries of c-mil, except the 5' boundary of exon 1 and the 3' boundary of exon 11, were unambiguously defined by the identification of consensus splice donor and acceptor sites precisely at positions where homology to v-mil ceases or resumes. The homology to v-mil starts within the coding sequence of exon 1 and ends within the 3' untranslated region of exon 11, 12 nucleotides downstream from the nonsense codon terminating the large open reading frame shared between c-mil and v-mil. The c-mil and v-mil sequences differ at only 7 out of 1153 nucleotide positions, and the predicted sequences of v-mil and c-mil proteins differ by one conservative and four nonconservative substitutions among 379 amino acid residues. Hence, the carboxy-terminal domains of the MH2 gag-mil hybrid protein and of the putative c-mil protein are very similar. However, the amino-terminal domain of the cellular protein is possibly encoded by additional 5' c-mil sequences not present in the transduced v-mil oncogene, while that of the MH2 hybrid protein is encoded by viral gag sequences. The sequence analysis also revealed that c-mil and c-myc derived sequences are immediately adjacent on the MH2 genome carrying both the v-mil and the v-myc oncogene. Hence, transduction of c-mil into MH2 involved recombination, at the 3' site, with either the c-myc locus or a previously transduced v-myc gene, and, at the 5' site, with gag sequences of the transducing virus. At both sites, no significant homologies were found between the sequence elements involved in the recombination.

Molecular and biological properties of MH2D12, a spontaneous mil deletion mutant of avian oncovirus MH2

Avian oncogenic retrovirus MH2 carries two cell-derived oncogenes, v-mil and v-myc. From an infectious stock of MH2 a spontaneous deletion mutant, MH2D12, that has lost most of the v-mil gene but has retained a complete and functional v-myc gene, has been isolated. Nonproducer quail embryo cells transformed by MH2D12 in the absence of helper virus contain two virus-specific proteins: a gag-related protein of 53,000 Da (p53gag), and a v-myc gene product of 59,000/61,000 Da (p59/61v-myc) indistinguishable from the v-myc protein encoded by MH2. MH2D12 viral RNA contains all T1-oligonucleotides specific for the MH2 v-myc gene but none of those characteristic for the v-mil gene. The genetic structure of molecularly cloned proviral DNA of MH2D12 was revealed by restriction mapping, blot hybridization, heteroduplex analysis, and nucleotide sequencing. The MH2D12 provirus is homologous to the MH2 genome but has suffered a deletion of 1271 nucleotides from the central region encompassing the 3' end of delta gag and all of v-mil except the very 3' 31 nucleotides directly adjacent to the v-myc gene. A nine-nucleotide overlap of homology to gag or mil at the delta gag/delta mil junction suggests that recombination between homologous sequence elements of the delta gag and v-mil domains of MH2 was involved in the genesis of MH2D12. The nucleotide sequence analysis predicts that the carboxyterminal 17 amino acids of p53gag are encoded by the residual v-mil sequences and by intron-derived v-myc sequences. Transformation of quail embryo cells by MH2D12 can be assayed by focus and colony formation of transformed cells. This indicates that the v-mil gene is not essential for these activities. However, size and morphology of foci and colonies, and cellular morphology of cultured MH2D12-transformed cell lines can easily be distinguished from those observed in cell transformation by MH2 and resemble more those seen in cell transformation by viruses containing the myc oncogene only.

3'-Terminal region of avian carcinoma virus MH2 shares sequence elements with avian sarcoma viruses Y73 and SR-A

We determined the nucleotide sequence of the acute transforming avian retrovirus MH2 from an HgiAI site within the coding region of its oncogene, v-myc, to the KpnI site within the long terminal repeat. Comparison with published sequences from other retroviruses allowed us to identify all sequence elements in this region. We conclude that MH2 contains a unique assembly of 3'-terminal sequences, which includes part of the helper virus-derived SPC region of avian sarcoma virus Y73 and the complete F3 and F1 segments of Rous sarcoma virus strain SR-A.

Structural relationship between the chicken protooncogene c-mil and the retroviral oncogene v-mil

The avian leukemia and carcinoma inducing retrovirus MH2 contains the novel oncogene v-mil in addition to the cell-derived oncogene v-myc. High-molecular-weight chicken DNA contains sequences closely related to v-mil and also sequences more distantly related to this gene. Several phage clones were isolated by screening of a chicken recombinant DNA library with a v-mil-specific probe in stringent conditions. These clones contain overlapping segments of v-mil-related chicken DNA. Hence, the sequences closely related to v-mil, termed c-mil, appear to represent a single-copy locus of the chicken genome. The close relationship between the cellular and the viral gene was demonstrated by hybridization between c-mil DNA and MH2 viral RNA, or between c-mil DNA and cloned v-mil DNA, and by a comparison of their restriction maps, which revealed total conservation in c-mil DNA of all restriction sites found in v-mil DNA. The c-mil locus spans at least 10 kb, with nine regions of homology to v-mil, ranging in size from about 0.07 to 0.17 kb, interrupted by eight intervening sequences with complexities of about 0.5 to 3.5 kb. Analyses of the cloned chicken c-mil and c-myc loci by nucleic acid hybridization employing specific probes from the mil-myc junction of MH2 proviral DNA revealed that c-mil- and c-myc-related sequences are directly adjacent in the viral genome and that MH2 contains additional 5' c-myc-related sequences not present in the genomes of other leukemia viruses carrying the v-myc oncogene.

Immunological analysis of v-myc gene products using antibodies against a myc-specific synthetic peptide

Rabbit antisera were prepared against a synthetic peptide corresponding to the carboxyterminal amino acid sequences of the transforming protein p110gag-myc of avian oncovirus MC29. Analysis of immunoprecipitates formed with these sera (anti-mycC) demonstrated that the gag-myc hybrid proteins encoded by the avian leukemia viruses MC29, CMII, and OK10 were recognized by the anti-peptide sera, but not the gag or pol precursor proteins of helper viruses or the MH2-encoded p100gag-mil protein. In cells transformed by OK10 or MH2, putative v-myc proteins of 60K (OK10) and 59/61K (MH2) were also precipitated by anti-mycC. In addition, the anti-peptide sera reacted specifically with the gag-myc proteins encoded by three partially transformation-defective mutants of MC29, td10A, td10C, and td10H, and by MC29 variant HBI.

Nucleotide sequence of avian retroviral oncogene v-mil: homologue of murine retroviral oncogene v-raf

Eukaryotic cells contain genes termed proto-oncogenes (c-onc) which have the potential to transform cells in culture and induce tumours in vivo. Most of these genes have been identified by their occasional incorporation into retroviral genomes which can act as natural transducing vectors for these and perhaps other cellular genes. Cell-derived oncogenes of retroviruses (v-onc) are associated mostly with the induction of mesenchymal tumours whereas carcinoma induction is rare. One of these rare carcinoma-inducing viruses is the acutely transforming avian retrovirus MH2 (refs 3-5). Recently we and others have shown that this virus carries a novel putative oncogene, v- mil , in addition to the known oncogene v-myc. While the transforming ability of v- mil has not been directly established, we have recently discovered by hybridization analysis that v- mil is homologous to v-raf (ref. 9), the transforming gene of the murine retrovirus 3611 MSV (ref. 10). Both viruses express the mil /raf oncogene product as a gag-fusion polyprotein, while the myc oncogene of MH2 is expressed via a subgenomic mRNA. Here we report the complete nucleotide sequence of v- mil and compare it with that of v-raf. The 80% homology between the nucleotide sequences and the 94% homology between the predicted amino acid sequences of the two viral genes clearly indicate that these are the avian and murine forms of the same gene. Comparison of the two sequences with that of the human cellular homologue (T. I. Bonner et al., manuscript in preparation) indicates that v-raf has more 3' untranslated sequences while v- mil has additional sequences from two 5' exons of the cellular homologue. Although the mil /raf amino acid sequences reveal partial homology to that of the v-src product, no tyrosine-specific protein kinase activity is detected for the gag- mil and the gag-raf hybrid proteins.

Homologous cell-derived oncogenes in avian carcinoma virus MH2 and murine sarcoma virus 3611

Retroviral oncogenes (v-onc) are derived from cellular genes (c-onc) which are highly conserved among different species. Retrovirus-transduced oncogenes are most commonly associated with the induction of haematopoietic tumours and sarcomas. The avian retrovirus Mill Hill no. 2 (MH2) was isolated from a spontaneous ovarian tumour of a chicken and is distinguished by the predominant induction of liver and kidney carcinomas in fowl. MH2 also induces transformation of fibroblasts, macrophages and epithelial cells in culture. The genome of MH2 contains two unrelated and independently expressed cell-derived oncogenes, v-mil and v-myc. Three other viral isolates among avian acute transforming retroviruses contain the v-myc oncogene, but only MH2 contains both v-myc and v-mil. Hence, some of the pathogenic specificities of MH2 may be due to the simultaneous expression of two oncogenes. The murine sarcoma virus 3611 (3611-MSV) isolated from a mouse carrying lung carcinoma and peritoneal tumours, induces fibrosarcomas in newborn mice and the transformation of fibroblasts and epithelial cells in culture. The oncogenic properties of 3611-MSV are due to the presence in its genome of a cell-derived oncogene termed v-raf. We report here that the two independently transduced oncogenes v-mil and v-raf are closely related and that they were apparently derived from homologous cellular genes of avian and mammalian species.

Two unrelated cell-derived sequences in the genome of avian leukemia and carcinoma inducing retrovirus MH2

Molecularly cloned proviral DNA of avian replication-defective retrovirus Mill Hill No. 2 (MH2) was analyzed. The MH2 provirus measures 5.5 kb including two long terminal repeats (LTR), and contains a partial complement of the structural gene gag, 1.5 kb in size, near the 5' terminus, and a 1.3-kb segment of the v-myc transforming gene near the 3' terminus. These v-myc sequences are closely related to the v-myc transforming gene of avian acute leukemia virus MC29, and to the cellular chicken gene c-myc. The gag and myc domains on the MH2 provirus are separated by unique sequences, 1.3 kb in size and termed v-mil, which are unrelated to v-myc, or to other oncogenes or structural genes of the avian leukemia-sarcoma group of retroviruses. Normal chicken DNA contains sequences closely related to v-mil, termed c-mil. Analyses of chicken c-mil clones isolated from a recombinant DNA library of the chicken genome reveal that c-mil is a single genetic locus with a complex split gene structure. In the MH2 genome, v-mil is expressed via genome-sized mRNA as a gag-related hybrid protein, p100gag-mil, while v-myc is apparently expressed via subgenomic mRNA independently from major coding regions of structural genes. The presence in the MH2 genome of two unrelated cell-derived sequences and their independent expression may be significant for the oncogenic specificities of this virus.

Avian oncovirus MH2: molecular cloning of proviral DNA and structural analysis of viral RNA and protein

Viral RNA, molecularly cloned proviral DNA, and virus-specific protein of avian retrovirus MH2 were analyzed. The complexity and sequence conservation of the transformation-specific v-myc sequences of MH2 RNA were compared with those of the other members of the MC29 subgroup of acute leukemia viruses, MC29, CMII, and OK10, and with chicken cellular c-myc sequences. All T1 oligonucleotides mapping within the 1.3-kilobase coding region of MC29 v-myc have homologous counterparts in the RNAs of all MC29 subgroup viruses and in c-myc. These counterparts are either identical in composition or altered by single point mutations. Hence, the 47,000-dalton carboxy-terminal sequences of the transforming proteins of these viruses and of the cellular gene product are probably highly conserved but may contain single amino acid substitutions. T1 oligonucleotide mapping of MH2 RNA indicated that the MH2 v-myc sequences map close to the 3' end of viral RNA. A genomic library of an MH2-transformed quail cell line was prepared by using the Charon 4A vector system. By screening with an myc-specific probe, a clone containing the entire MH2 provirus (lambda MH2-1) was isolated. Digestion of cloned DNA with KpnI yielded a 5.1-kilobase fragment hybridizing to both gag- and myc-specific probes. Further restriction mapping of lambda MH2-1 DNA showed that about 1.6 kilobases of the gag gene are present near the 5' end of proviral DNA, and the conserved part of v-myc, i.e., 1.3 kilobases, is present near the 3' end of proviral DNA. These two domains are separated by a segment of at least 1 kilobase of different genetic origin, including additional unique sequences unrelated to virion genes. Tryptic peptide analysis of the gag-related protein of MH2, p100, revealed gag-specific peptides and several unique methionine-containing peptides. One of the latter is possibly shared with the polymerase precursor protein Pr180gag-pol, but no myc-specific peptides, defined for the MC29 protein p110gag-myc, appear to be present in MH2 p100. The data on viral RNA, proviral DNA, and protein of MH2 reveal a unique genetic structure for this virus of the MC29 subgroup and suggest that its v-myc gene is not expressed as a gag-related protein.

Genome structure of HBI, a variant of acute leukemia virus MC29 with unique oncogenic properties

We have analyzed the viral RNA of a variant of avian acute leukemia virus MC29, termed HBI. This virus was isolated during in vitro passage of a partially transformation-defective (td) mutant of MC29 (td10H-MC29) in chicken macrophages. While td10H-MC29 has a reduced ability to transform macrophages in vitro or to induce tumors in vivo, HBI-MC29 transforms macrophages efficiently and induces in vivo a high incidence of lymphoid tumors. Electrophoretic analysis of HBI-MC29 genomic RNA revealed that it has a complexity of 5.7 kilobases, like the RNA of wild-type (wt) MC29, and that it is 0.6 kilobases longer than the 5.1-kilobase RNA of the deletion mutant td10H-MC29. Analysis of the viral RNAs of two clonal isolates of HBI-MC29 by T1 oligonucleotide fingerprinting showed that sequences from the viral transformation-specific region, v-myc, which are deleted in td10H RNA, are present in HBI RNA. Moreover, hybridization of HBI RNA to molecularly cloned subgenomic fragments of wtMC29 proviral DNA, followed by fingerprint analysis of hybridized RNA, showed that the entire v-myc-specific RNA sequences defined previously are present. Hybridization to cloned DNA of the normal chicken locus c-myc shows a close relationship between HBI v-myc RNA and c-myc DNA, especially in the sequences which were deleted from td10H-MC29. T1 oligonucleotide maps of HBI and td10H RNAs were prepared and compared. Total conservation of the oligonucleotide pattern is observed in the overlapping v-myc regions, while the partial structural genes gag and env show some variations, most of which can be directly proven to be due to point mutations or recombination with helper viral RNAs that were analyzed in parallel. Recombination of td10H-MC29 with c-myc, followed by recombinational and mutational changes in the structural genes during passage with helper virus, could be a possible explanation for the origin of HBI.