The avian erythroblastosis virus erbA oncogene encodes a DNA-binding protein exhibiting distinct nuclear and cytoplasmic subcellular localizations

Thyroid hormone receptor localization in target tissues

The thyroid hormone receptors, TRα1, TRβ1 and other subtypes, are members of the nuclear receptor superfamily that mediate the action of thyroid hormone signaling in numerous tissues to regulate important physiological and developmental processes. Their most well-characterized role is as ligand-dependent transcription factors; TRs bind thyroid hormone response elements in the presence or absence of thyroid hormone to facilitate the expression of target genes. Although primarily residing in the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. We have identified multiple nuclear localization signals and nuclear export signals within TRα1 and TRβ1 that interact with importins and exportins, respectively, to mediate translocation across the nuclear envelope. More recently, enigmatic cytoplasmic functions have been ascribed to other TR subtypes, expanding the diversity of the cellular response to thyroid hormone. By integrating data on localization signal motifs, this review provides an overview of the complex interplay between TR's dynamic transport pathways and thyroid hormone signaling activities. We examine the variation in TR subtype response to thyroid hormone signaling, and what is currently known about regulation of the variety of tissue-specific localization patterns, including targeting to the nucleus, the mitochondria and the inner surface of the plasma membrane.

Cancer promoted by the oncoprotein v-ErbA may be due to subcellular mislocalization of nuclear receptors

The retroviral v-ErbA oncoprotein is a highly mutated variant of the thyroid hormone receptor alpha (TRalpha), which is unable to bind T(3) and interferes with the action of TRalpha in mammalian and avian cancer cells. v-ErbA dominant-negative activity is attributed to competition with TRalpha for T(3)-responsive DNA elements and/or auxiliary factors involved in the transcriptional regulation of T(3)-responsive genes. However, competition models do not address the altered subcellular localization of v-ErbA and its possible implications in oncogenesis. Here, we report that v-ErbA dimerizes with TRalpha and the retinoid X receptor and sequesters a significant fraction of the two nuclear receptors in the cytoplasm. Recruitment of TRalpha to the cytoplasm by v-ErbA can be partially reversed in the presence of ligand and when chromatin is disrupted by the histone deacetylase inhibitor trichostatin A. These results define a new mode of action of v-ErbA and illustrate the importance of cellular compartmentalization in transcriptional regulation and oncogenesis.

Nuclear export of the oncoprotein v-ErbA is mediated by acquisition of a viral nuclear export sequence

v-ErbA, an oncogenic derivative of the thyroid hormone receptor alpha (TRalpha) carried by the avian erythroblastosis virus, contains several alterations including fusion of a portion of avian erythroblastosis virus Gag to its N terminus, N- and C-terminal deletions, and 13 amino acid substitutions. Nuclear export of v-ErbA occurs through a CRM1-mediated pathway. In contrast, nuclear export of TRalpha and another isoform, TRbeta, is CRM1-independent. To determine which amino acid changes in v-ErbA confer CRM1-dependent nuclear export, we expressed a panel of green and yellow fluorescent protein-tagged mutant and chimeric proteins in mammalian cells. The sensitivity of subcellular trafficking of these mutants to leptomycin B (LMB), a specific inhibitor of CRM1, was assessed by fluorescence microscopy. Our data showed that a nuclear export sequence resides within a 70-amino acid domain in the C-terminal portion of the p10 region of Gag, and in vitro binding assays demonstrated that Gag interacts directly with CRM1. However, a panel of ligand-binding domain mutants of v-ErbA lacking the Gag sequence exhibited greater nuclear localization in the presence of LMB, suggesting that the various amino acid substitutions/deletions may cause a conformation shift, unmasking an additional CRM1-dependent nuclear export sequence. In contrast, the altered DNA-binding domain of the oncoprotein did not contribute to CRM1-dependent nuclear export. Heterokaryon experiments revealed that v-ErbA did not undergo nucleocytoplasmic shuttling when the CRM1 export pathway was blocked by LMB treatment, suggesting that the ability to follow the export pathway used by TRalpha has been lost by the oncoprotein during its evolution. Our findings thus point to the intriguing possibility that acquisition of altered nuclear export capabilities contributes to the oncogenic properties of v-ErbA.

The v-erbA oncogene blocks expression of alpha2/beta1 integrin a normal inhibitor of erythroid progenitor proliferation

T2EC are chicken erythrocytic progenitors that balance between self-renewal and differentiation as a function of response to specific growth factors. Their transformation by the v-erbA oncogene locks them into the self-renewal program. We show here that the expression of the VLA-2 integrin alpha2 subunit mRNA is downregulated by v-erbA and that VLA-2 engagement and clustering, brought about by treatment with an alpha2-specific antibody or by culture on the VLA-2 ligand collagen I, inhibits T2EC proliferation. From competition studies using antibodies, VLA-2 was shown to be involved in the collagen-induced response. While engagement of VLA-2 inhibited proliferation, it was not sufficient to induce differentiation. The transformation of T2EC by v-erbA decreased their interaction with collagen I and the VLA-2 brake on cell proliferation, which may account for the increased proliferation potential of transformed erythrocytic progenitors and for their shedding into the blood of infected chickens. Our data suggest that the interaction between erythroid progenitors and collagen, mediated by VLA-2, play a major role in the control of erythropoiesis in vitro and that this pathway is a target of the v-erbA oncogene.

The role of hinge domain in heterodimerization and specific DNA recognition by nuclear receptors

Four structural domains are characteristic of the members of the nuclear receptor superfamily. The hinge (D) domain which is located between the DNA binding (C) domain and the ligand binding (EF) domain, is less conserved among the nuclear receptors. In this study, we investigated the effects of the D domain on receptor function with regard to ligand binding, protein-protein interaction and DNA recognition. We found that EF domain of TR lacked T3 binding activity and additional D domain was required for its ligand binding. Using pull down assays and two-hybrid assays, we also demonstrated that the EF domain of TR did not dimerize with TR or RXR in solution, while the DEF domain was able to homo-and heterodimerize with RXR. In contrast, the RXR EF domain alone was able to heterodimerize with TR. The D domain of TR is required but that of RXR is not necessary for the interaction. We further demonstrated that the D domain was required for receptor specific DNA recognition. The ABC domain of vitamin D receptor (VDR) and TR(DEF) chimeric receptor could not bind to VDR response element (VDRE). Addition of own D domain of VDR to the ABC domain enables the chimeric receptor to bind VDRE and transactivate. The D domain of TR cannot substitute for that of VDR in context of specific DNA recognition. These data suggest that the D domain is important to maintain the integrity of the functional structure of the nuclear receptors.

Nucleocytoplasmic shuttling of the thyroid hormone receptor alpha

The thyroid hormone receptor alpha (TR alpha) exhibits a dual role as an activator or repressor of gene transcription in response to thyroid hormone (T(3)). Our studies show that TR alpha, formerly thought to reside solely in the nucleus tightly bound to DNA, actually shuttles rapidly between the nucleus and cytoplasm. The finding that TR alpha shuttles reveals an additional checkpoint in receptor control of gene expression. Using Xenopus oocyte microinjection assays, we show that there are two coexisting mechanisms for nuclear entry of TR alpha. First, nuclear import of TR alpha (molecular mass 46 kDa) was not sensitive to general inhibitors of signal-mediated transport, indicating that TR alpha can enter the oocyte nucleus by passive diffusion. Second, when TR alpha was tagged with glutathione-S:-transferase, import of the fusion protein (molecular mass 73 kDa) was completely blocked by these inhibitors, demonstrating that an alternative, signal-mediated import pathway exists for TR alpha. Nuclear retention of TR alpha in oocytes is enhanced in the presence of T(3), suggesting that more intranuclear binding sites are available for the ligand-bound receptor. Using mammalian cells, we show that shuttling of green fluorescent protein (GFP)-tagged and untagged TR alpha is inhibited in both chilled and energy-depleted cells, suggesting that there is an energy-requiring step in the nuclear retention/export process. Nuclear export of TR alpha is not blocked by leptomycin B, a specific inhibitor of the export receptor CRM1, indicating that TR alpha does not require the CRM1 pathway to exit the nucleus. Dominant negative mutants of TR with defects in DNA binding and transactivation accumulate in the cytoplasm at steady state, illustrating that even single amino acid changes in functional domains may alter the subcellular distribution of TR. In contrast to TR alpha, nuclear export of its oncogenic homolog v-ErbA is sensitive to leptomycin B, suggesting that the oncoprotein follows a CRM1-mediated export pathway. Acquisition of altered nuclear export capabilities may contribute to the oncogenic properties of v-ErbA.

The v-ErbA oncoprotein quenches the activity of an erythroid-specific enhancer

v-ErbA is a mutated variant of thyroid hormone receptor (TRalpha/NR1A1) borne by the Avian Erythroblastosis virus causing erythroleukemia. TRalpha is known to activate transcription of specific genes in the presence of its cognate ligand, T3 hormone, while in its absence it represses it. v-ErbA is unable to bind ligand, and hence is thought to contribute to leukemogenesis by actively repressing erythroid-specific genes such as the carbonic anhydrase II gene (CA II). In the prevailing model, v-ErbA occludes liganded TR from binding to its cognate elements and constitutively interacts with the corepressors NCoR/SMRT. We previously identified a v-ErbA responsive element (VRE) within a DNase I hypersensitive region (HS2) located in the second intron of the CA II gene. We now show that HS2 fulfils all the requirements for a genuine enhancer that functions independent of its orientation and position with a profound erythroid-specific activity in normal erythroid progenitors (T2ECs) and in leukemic erythroid cell lines. We find that the HS2 enhancer activity is governed by two adjacent GATA-factor binding sites. v-ErbA as well as unliganded TR prevent HS2 activity by nullifying the positive function of factors bound to GATA-sites. However, v-ErbA, in contrast to TR, does not convey active repression to silence the transcriptional activity intrinsic to a heterologous tk promoter. We propose that depending on the sequence and context of the binding site, v-ErbA contributes to leukemogenesis by occluding liganded TR as well as unliganded TR thereby preventing activation or repression, respectively.

Thyroid hormone receptor coactivators and corepressors

In the absence of triiodothyronine (T3), thyroid hormone receptors (TRs) repress transcription of many genes; in the presence of T3, TRs activate transcription of those same genes. Both of these events are dependent on interactions between TRs and other nuclear proteins. TRs bind to specific DNA sequences, generally found in the 5' flanking regions of target genes. In the unliganded state, TRs interact with one of several corepressor proteins. These proteins, in turn, interact with a series of other proteins, which includes histone deacetylases. Histone deacetylation tightens chromatin structure, thus impairing access of critical transcription factors and thereby repressing transcription. In addition, corepressors may invoke mechanisms of gene repression independent of histone deacetylation. The binding of T3 causes a conformational change in the TR that results in release of the corepressor and recruitment of coactivator proteins. Several coactivator proteins appear to bind the ligand-occupied TR as a multiprotein complex. Opposite to corepressors, coactivators acetylate histones, thereby loosening chromatin structure and facilitating access of key transcription factors. Again, mechanisms independent of histone acetylation also may be involved. Overall, gene activation by T3 is a two-step process; removal of active repression, and induction of transcription to levels above the "neutral" state.

Chicken thyroid hormone receptor alpha requires the N-terminal amino acids for exclusive nuclear localization

The subcellular localization of natural and engineered forms of the chicken thyroid hormone receptor (cTR alpha) is dependent on amino acids encoded in the N-terminal region. The full length receptor protein, cTR alpha-p46, was found to localize exclusively to the nucleus, whereas the N-terminally shorter variant, cTR alpha-p40, localizes to both the nucleus and the cytoplasm. The exclusive nuclear localization of cTR alpha-p46 is dependent on the presence of the first 11 N-terminal amino acids, but independent of the phosphorylation of the serine at position 12. Our data identify a novel role for an N-terminal domain of the full length thyroid hormone receptor.

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Identification of a domain required for oncogenic activity and transcriptional suppression by v-erbA and thyroid-hormone receptor alpha

v-erbA, a mutated version of the chicken thyroid hormone (TH) receptor type alpha, can inhibit hormonal induction of target genes. In addition, v-erbA acts as a constitutive repressor of the basal promoter activity. In vivo, v-erbA can arrest the differentiation of erythroid precursor cells and suppresses transcription of erythrocyte-specific genes. We show that the v-erbA protein of the transformation-defective avian erythroblastosis virus mutant (AEVtd359) fails to suppress basal transcription level and exhibits impaired ability in antagonizing the TH and retinoic acid response. The inactivating mutation is a 1-nt change leading to a Pro-->Arg replacement in the "hinge region" of v-erbA protein. Introducing this mutation in the context of TH receptor alpha selectively inactivates the suppressor function, while hormone-binding and transcriptional-activation properties are unaffected. These data suggest that trans-repression rather than a dominant negative block of TH-receptor or retinoic acid-receptor activation may represent the primary molecular property underlying erbA oncogenesis.

Biological actions of oncogenes

Cancer, in many cases, results from multistep genetic mutation. Certain genes can have a predisposed susceptibility to mutations that lead to cancer because of chromosome location or their importance in the control of cell cycles. Mutations that deregulate the expression or activity of enzymes involved in the biochemical pathways of growth and differentiation or that suppress the expression of negative cell cycle control factors result in activation of oncogenesis. The study of oncogenes and tumor suppressor genes has greatly influenced our understanding of the molecular origins of cancer. We focus here on the normal biological action of proto-oncogenes compared with the transforming activities of oncogenes and tumor suppressor genes, and we discuss possible mechanisms of oncogenic transformation.

Retinoid and thyroid hormone receptors: ligand-regulated transcription factors as proto-oncogenes

The retroviral v-erb A locus is derived from a cellular gene, c-erb A, encoding a thyroid hormone receptor. The v-erb A and c-erb A proteins are, in turn, members of a larger family of structurally and functionally interrelated polypeptides that includes the steroid, retinoic acid, and vitamin D3 receptors. These nuclear hormone receptors act by binding to specific sites on the cell genome and, in response to cognate hormone, modulating the transcription of adjacent 'target' genes. The expression, properties, and mechanisms of action of the thyroid hormone receptors (c-erb A proteins) and the closely related retinoic acid receptors are discussed.

Structural analysis and transcriptional mapping of the Marek's disease virus gene encoding pp38, an antigen associated with transformed cells

The gene encoding a Marek's disease virus (MDV) pp38 phosphoprotein has been identified, sequenced, and localized to the BamHI H fragment to the left of the putative MDV origin of replication. The open reading frame was defined by sequencing of a lacZ-pp38 fusion protein gene from a lambda gt11 expression library. The entire open reading frame is 290 amino acids long and codes for a protein with a calculated molecular weight of 31,169, compared with the size of 38 kDa of the phosphorylated form estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. S1 nuclease protection analysis showed that the pp38 gene is transcribed leftward as an unspliced mRNA. On the basis of transcriptional mapping studies, the pp38 transcript is predicted to be about 1.8 kb in length without a poly(A) sequence. Its promoter-enhancer region overlaps that of the major rightward BamHI H 1.8-kb transcript implicated in tumor induction. This region contains Oct-1, Sp1, and CCAAT motifs as well as the putative origin of replication. The pp38 protein is the only presently known antigen that is consistently associated with the transformation state. It may play a significant role in MDV transformation.

v-erbA overexpression is required to extinguish c-erbA function in erythroid cell differentiation and regulation of the erbA target gene CAII

The v-erbA oncoprotein represents a retrovirus-transduced oncogenic version of the thyroid hormone (T3/T4) receptor c-erbA (type alpha). It contributes to virus-induced erythroleukemia by efficiently arresting differentiation of red cell progenitors and by suppressing transcription of erythrocyte-specific genes. Here, we show that v-erbA and c-erbA bind directly to sequences within the promoter of the erythrocyte-specific carbonic anhydrase II (CAII), a gene whose transcription is efficiently suppressed by v-erbA. This erbA-binding site confers thyroid hormone responsiveness to a heterologous promoter in transient expression experiments and is a target for efficient down-regulation of CAII transcription by the v-erbA oncoprotein. In stably transformed erythroblasts coexpressing the v-erbA oncoprotein and the c-erbA/T3 receptor at an approximately equimolar ratio, c-erbA activity is dominant over v-erbA. T3 efficiently induced erythroid differentiation in these cells, thus overcoming the v-erbA-mediated differentiation arrest. Likewise, T3 activated CAII transcription as well as transient expression of a T3-responsive reporter gene containing the CAII-specific erbA-binding site. The c-erbA-dependent activation of this CAII reporter construct could only be suppressed by very high amounts of v-erbA. Our results suggest that overexpression of v-erbA is required for its function as an oncoprotein.

Ontogeny of the v-erbA oncoprotein from the thyroid hormone receptor: an alteration in the DNA binding domain plays a role crucial for v-erbA function

The avian erythroblastosis virus v-erbA oncogene is imprecisely derived from a cellular gene (c-erbA) encoding a thyroid hormone receptor: the v-erbA protein has sustained both small terminal deletions and internal amino acid sequence changes relative to c-erbA. We report here that one of these missense differences between v- and c-erbA proteins, located in a zinc finger DNA binding domain, has dramatic effects on the biological activities of the encoded protein. Back mutation of the viral coding sequence to resemble c-erbA at this site severely impairs erythroid transformation and produces subtle changes in DNA binding by the encoded protein, suggesting that differences in DNA binding by the viral and cellular proteins may be involved in the activation of v-erbA as an oncogene.

Xenopus laevis alpha and beta thyroid hormone receptors

The Xenopus laevis genome encodes two genes for the alpha (TR alpha) and two genes for the beta (TR beta) thyroid hormone receptors. The two TR alpha genes closely resemble their rat, human, and chicken counterparts. No alternatively spliced TR alpha cDNA clones were found in the 5' untranslated region (5' UTR). In contrast, complex alternative splicing of TR beta mRNA occurs within the 5' UTR as well as possible alternative transcriptional start sites. As many as eight exons encoding mainly the 5' UTR are alternatively spliced, giving rise to at least two amino termini for each of the two TR beta proteins. The 5' UTR of transcripts from both TR alpha and TR beta genes contain multiple AUG sequences with short open reading frames suggesting translational control mechanisms might play a role in expression of TR genes.

The lack of transcriptional activation of the v-erbA oncogene is in part due to a mutation present in the DNA binding domain of the protein

Using a transient co-transfection system we have demonstrated that response elements for estrogen (ER), thyroid hormone (TR) and retinoic acid receptors (RAR) are closely related. Thyroid hormone-induced activation of transcription was observed in CV1 cells and not in HeLa cells, suggesting the existence of cell-specific transcription factors necessary for the response. By contrast to its cellular counterpart (c-erbA/cTR alpha) the oncogene protein gag v-erbA is unable to activate gene transcription from different response elements derived from the rat growth hormone (rGH) gene promoter. A chimeric construct consisting of the ER in which the DNA binding domain has been replaced by that of cTR alpha was able to stimulate the reporter gene. In contrast, a construct in which ER DNA binding domain has been replaced by that of gag v-erbA did not activate gene transcription. These results lead us to the conclusion that the mutated DNA binding domain of v-erbA is in part responsible for the lack of transcriptional activation and in repression of gene expression. This is due in large part to the Gly73----Ser mutation which corresponds to the position of one of the three discriminating amino acids that are thought to interact with a specific base of the response element.

v-erbA oncogene activation entails the loss of hormone-dependent regulator activity of c-erbA

The v-erbA oncogene, one of the two oncogenes of the avian erythroblastosis virus, efficiently blocks erythroid differentiation and suppresses erythrocyte-specific gene transcription. Here we show that the overexpressed thyroid hormone receptor c-erbA effectively modulates erythroid differentiation and erythrocyte-specific gene expression in a T3-dependent fashion, when introduced into erythroid cells via a retrovirus. In contrast, the endogenous thyroid hormone receptor does not detectably affect erythroid differentiation. The analysis of a series of chimeric v-/c-erbA proteins suggests that the v-erbA oncoprotein has lost one type of thyroid hormone receptor function (regulating erythrocyte gene transcription in response to T3), but constitutively displays another function: it represses transcription in the absence of T3. The region responsible for the loss of hormone-dependent regulator activity of v-erbA has been mapped to the very C-terminus of c-erbA, encompassing a cluster of highly conserved amino acid residues with the potential to form an amphipathic alpha-helix.

Sequence-specific DNA binding by the v-erbA oncogene protein of avian erythroblastosis virus

The v-erbA oncogene, a transduced copy of a thyroid hormone receptor, plays an important role in establishment of the transformed cell phenotype induced by avian erythroblastosis virus. The ability of thyroid hormone receptors to bind to specific sites on chromatin and to thereby modify the expression of adjacent target genes is a crucial element in their mechanism of action in the normal cell. The v-erbA protein also bound at high affinity to a set of DNA fragments recognized by the rat thyroid hormone receptor, but the relative affinity of the v-erbA protein for the different binding sites was distinct from that previously reported for the thyroid hormone receptors.

A subpopulation of the avian erythroblastosis virus v-erbA protein, a member of the nuclear hormone receptor family, is glycosylated

The v-erbA oncogene of avian erythroblastosis virus is derived from a cellular gene for a thyroid hormone (T4/T3 thyronine) receptor and encodes a DNA-binding protein found principally in the nucleus of the infected cell. I report here that a subpopulation of the v-erbA protein is glycosylated. The v-erbA protein, therefore, is another member of the newly recognized family of eucaryotic transcription factors and related polypeptides which are glycoproteins.

An enhancer within the divergent promoter of Epstein-Barr virus responds synergistically to the R and Z transactivators

The EA-R and NotI repeat genes of Epstein-Barr virus (EBV) are oriented head to head and separated by a 1,000-base-pair (bp) divergent promoter region. We have identified functional domains within this divergent promoter which are important for regulation of the rightward EA-R gene. Both the R transactivator (Rta) and the Z transactivator (Zta) increase the abundance of correctly initiated EA-R transcripts. A 258-bp fragment (-114 to -372 from the EA-R cap site) contained the primary Rta and Zta response elements and was capable of transferring Rta and Zta activity to a heterologous promoter in an orientation- and position-independent manner. Rta activated this 258-bp enhancer region in both EBV-positive and EBV-negative cells. However, Zta activity appeared to be dependent on another EBV gene product, since Zta activated the enhancer efficiently (500- to 2,000-fold) in EBV-positive cells but had little or no activity in EBV-negative cells. The combination of Rta and Zta produced a striking synergistic effect on the enhancer in the absence of any additional EBV components, suggesting that the interaction between Zta and Rta accounts for the Zta response observed in EBV-positive cells. An Rta response element was mapped to a domain located 60 bp away from a Zta-binding site within the enhancer. Although Rta activated the enhancer and other early promoters without additional EBV- or B-cell-specific factors, it did not activate the lytic cycle of EBV, in contrast to Zta. Immunofluorescence patterns of Rta and Zta with antipeptide antisera indicated that they have overlapping but different subcellular localizations. Both transactivators were found in the nucleus, but Rta was also found in the cytoplasm.

Expression of the v-erbA product, an altered nuclear hormone receptor, is sufficient to transform erythrocytic cells in vitro

We investigated the effect of the v-erbA oncogene product, an altered thyroid hormone receptor, in chicken erythrocyte progenitor cells. Bone marrow cells were infected with a retrovirus vector (XJ12) carrying the v-erbA gene in association with the neoR gene. XJ12-infected erythrocyte progenitor cells gave rise to G418-resistant clones. Some were composed of blast cells identified as transformed CFU-Es blocked in their differentiation. These cells could be grown in culture for at least 25 generations and required anemic chicken serum as a source of erythropoietic growth factors. XJ12 can infect erythrocyte progenitor cells in vivo but is not sufficient to induce erythroleukemia. These data suggest that the activation of a nuclear hormone receptor might represent one step toward the development of neoplasms.

Avian erythroblastosis virus transforms a novel mast cell-basophil precursor target in the Japanese quail

Hematopoietic cells of the Japanese quail were transformed by avian erythroblastosis virus in vivo and in vitro. In both circumstances, the infected hematopoietic tissues exhibited a dual oncogenic response of erythroid and mast cell-basophil elements. The erythroid transformants escaped the avian erythroblastosis virus block in differentiation and progressed to hemoglobinization. Resulting basophilic cells were morphologically, biochemically, and ultrastructurally identical to mast cell-basophils observed in other species. None of the virally transformed cells actively produced reverse transcriptase activity. Nonproducer cell lines synthesized viral RNA and both v-erbA and v-erbB proteins. These results indicate that the Japanese quail has a viral target cell different from that of the chicken. The implications of a single bipotential transformation target yielding both erythroid and mast cell-basophil colonies is discussed.

c-erbA encodes multiple proteins in chicken erythroid cells

To identify and characterize the proteins encoded by the erbA proto-oncogene, we expressed the C-terminal region of v-erbA in a bacterial trpE expression vector system and used the fusion protein to prepare antiserum. The anti-trp-erbA serum recognized the P75gag-erbA protein encoded by avian erythroblastosis virus and specifically precipitated six highly related proteins ranging in size from 27 to 46 kilodaltons from chicken embryonic erythroid cells. In vitro translation of a chicken erbA cDNA produced essentially the same pattern of proteins. Partial proteolytic maps and antigenicity and kinetic analyses of the in vivo and in vitro proteins indicated that they are related and that the multiple bands are likely to arise from internal initiations within c-erbA to generate a nested set of proteins. All of the c-erbA proteins are predominantly associated with chicken erythroblast nuclei. However, Nonidet P-40 treatment resulted in extraction of the three smaller proteins, whereas the larger proteins were retained. During differentiation of erythroid cells in chicken embryos, we found maximal levels of c-erbA protein synthesis at days 7 to 8 of embryogenesis. By contrast, c-erbA mRNA levels remained essentially constant from days 5 to 12. Together, our results indicate that posttranscriptional or translational mechanisms are involved in regulation of c-erbA expression and in the complexity of its protein products.

Genetic dissection of functional domains within the avian erythroblastosis virus v-erbA oncogene

The avian erythroblastosis virus v-erbA locus potentiates the oncogenic transformation of erythroid and fibroblast cells and is derived from a host cell gene encoding a thyroid hormone receptor. We report here the use of site-directed mutagenesis to identify and characterize functional domains within the v-erbA protein. Genetic lesions introduced into a putative hinge region or at the extreme C-terminus of the v-erbA coding domain had no significant effect on the biological activity of this polypeptide. In contrast, mutations introduced within the cysteine-lysine-arginine-rich center of the v-erbA coding region, a DNA-binding domain in the thyroid and steroid hormone receptors, abolished or severely compromised the ability of the viral protein to function. Our results suggest that the mechanism of action of the v-erbA protein in establishing the neoplastic phenotype is closely related to its ability to interact with DNA, presumably thereby altering expression of host target genes by either mimicking or interfering with the action of the normal c-erbA gene product.

Genetic dissection of functional domains within the avian erythroblastosis virus v-erbA oncogene

The avian erythroblastosis virus v-erbA locus potentiates the oncogenic transformation of erythroid and fibroblast cells and is derived from a host cell gene encoding a thyroid hormone receptor. We report here the use of site-directed mutagenesis to identify and characterize functional domains within the v-erbA protein. Genetic lesions introduced into a putative hinge region or at the extreme C-terminus of the v-erbA coding domain had no significant effect on the biological activity of this polypeptide. In contrast, mutations introduced within the cysteine-lysine-arginine-rich center of the v-erbA coding region, a DNA-binding domain in the thyroid and steroid hormone receptors, abolished or severely compromised the ability of the viral protein to function. Our results suggest that the mechanism of action of the v-erbA protein in establishing the neoplastic phenotype is closely related to its ability to interact with DNA, presumably thereby altering expression of host target genes by either mimicking or interfering with the action of the normal c-erbA gene product.

c-erbA encodes multiple proteins in chicken erythroid cells

To identify and characterize the proteins encoded by the erbA proto-oncogene, we expressed the C-terminal region of v-erbA in a bacterial trpE expression vector system and used the fusion protein to prepare antiserum. The anti-trp-erbA serum recognized the P75gag-erbA protein encoded by avian erythroblastosis virus and specifically precipitated six highly related proteins ranging in size from 27 to 46 kilodaltons from chicken embryonic erythroid cells. In vitro translation of a chicken erbA cDNA produced essentially the same pattern of proteins. Partial proteolytic maps and antigenicity and kinetic analyses of the in vivo and in vitro proteins indicated that they are related and that the multiple bands are likely to arise from internal initiations within c-erbA to generate a nested set of proteins. All of the c-erbA proteins are predominantly associated with chicken erythroblast nuclei. However, Nonidet P-40 treatment resulted in extraction of the three smaller proteins, whereas the larger proteins were retained. During differentiation of erythroid cells in chicken embryos, we found maximal levels of c-erbA protein synthesis at days 7 to 8 of embryogenesis. By contrast, c-erbA mRNA levels remained essentially constant from days 5 to 12. Together, our results indicate that posttranscriptional or translational mechanisms are involved in regulation of c-erbA expression and in the complexity of its protein products.