The DNA binding subunit of NF-kappa B is identical to factor KBF1 and homologous to the rel oncogene product

NF-kappa B and related proteins: Rel/dorsal homologies meet ankyrin-like repeats

Molecular cloning of the subunits of the transcription factor NF-kappa B and its inhibitor l kappa B revealed regions of sequence homology with two different classes of proteins: the Rel/dorsal family and a heterogeneous group of proteins containing ankyrin-like repeats. Both the Rel/dorsal homology domain and the ankyrin-like repeats appear to play important roles in protein-protein interactions that regulate localization and activity of the NF-kappa B subunits.

tra-2 encodes a membrane protein and may mediate cell communication in the Caenorhabditis elegans sex determination pathway

The Caenorhabditis elegans sex-determining gene, tra-2, promotes female development in XX animals. In this paper we report the cDNA sequence corresponding to a 4.7 kb tra-2 mRNA and show that it is composed of 23 exons, is trans-spliced to SL2, and contains a perfect direct repeat in the 3' untranslated region. This mRNA is predicted to encode a 1475 amino acid protein, named pTra2A, that has a secretory signal and several potential membrane-spanning domains. The molecular analysis of tra-2 loss-of-function mutations supports our open reading frame identification and suggests that the carboxy-terminal domain is important for tra-2 activity. We propose that in XX animals the carboxy-terminal domain of pTra2A negatively regulates the downstream male promoting fem genes. In XO animals, tra-2 is negatively regulated by her-1, which acts cell nonautonomously. Because hydropathy predictions suggest that pTra2A is an integral membrane protein, pTra2A might act as a receptor for the her-1 protein. We propose that in XO animals, the her-1 protein promotes male development by binding and inactivating pTra2A. The role of cell communication in C. elegans sex determination might be to ensure unified sexual development throughout the animal. If so, then regulation of sexual fate by her-1 and tra-2 might provide a general model for the coordination of groups of cells to follow a single cell fate.

Mouse mammary tumor gene int-3: a member of the notch gene family transforms mammary epithelial cells

Expression of a 2.3-kb RNA species is induced in mammary tumors as a consequence of insertional mutagenesis at the int-3 locus by the mouse mammary tumor virus. The nucleotide sequence and biological activity of this mammary tumor-specific int-3 RNA species were determined. It contains an open reading frame which encodes a 57-kDa protein. The translated protein possesses six nearly contiguous 32-amino-acid repeats which are related to a similar motif in the Saccharomyces cerevisiae cdc-10-encoded cell cycle protein. In addition, the int-3 cdc-10 repeats are bounded by the PEST amino acid sequence motif which is commonly found in proteins having a rapid turnover and may represent sites for phosphorylation. The int-3 cdc-10 repeat sequences are 50% identical to a portion of the intracellular domain of the neurogenic Drosophila notch gene product. Activation of expression of a recombinant int-3 genomic DNA fragment encoding the 2.3-kb RNA species in HC11 mouse mammary epithelial cells in vitro induces anchorage-independent growth in soft agar.

The functional domains of the Drosophila morphogen dorsal: evidence from the analysis of mutants

The dorsal (dl) protein is a member of the Rel family of transcription factors. It is distributed in a nuclear concentration gradient along the dorsoventral axis of Drosophila embryos and activates or represses a discrete set of zygotic genes in a concentration-dependent manner. The nuclear uptake of the dl protein is stimulated by products of the dorsal group genes but inhibited by the cactus (cact) product. To analyze the functional domains of the dl protein, we sequenced 11 dl alleles and studied their interaction with cact. Four of these alleles were found to result in carboxy-terminal truncations of the protein. A deletion of 80 carboxy-terminal amino acids abolishes the ability of dl protein to activate the expression of mesodermal genes. Larger deletions also affect the repressor function of dl. However, a protein consisting only of the Rel homologous region still acts as a weak repressor of zerknüllt transcription. A missense mutation in the presumptive DNA-binding domain causes a complete lack-of-function phenotype in trans to a deficiency but exerts a dominant-negative effect in trans to a wild-type copy of dl. These and genetic data with the alleles that produce truncated proteins indicate that dl oligomerizes. The proteins truncated at the carboxy-terminal end show increased levels of nuclear uptake dorsally, but they still respond to the cact-mediated inhibition of nuclear transport. Therefore, carboxy-terminal sequences influence the cytoplasmic retention, although a domain of dl-cact interaction residues in the amino-terminal portion.

Intramolecular masking of the nuclear location signal and dimerization domain in the precursor for the p50 NF-kappa B subunit

We show that the non-DNA-binding precursor for the p50 subunit (p110), like NF-kappa B, is subject to control of nuclear uptake. In contrast to p50, p110 was excluded from nuclei and unable to associate detectably with p50 or p65 NF-kappa B subunits. The nuclear location signal in the N-terminal half of p110 was not accessible for monospecific antibodies. Removal of only 191 amino acids from the C-terminus of p110 restored antibody accessibility as well as nuclear uptake. The C-terminal half of p110, which is linked to the p50 portion via a glycine-rich hinge, could also noncovalently bind to p50. This helps to explain why p50, after cleavage of the precursor in intact cells, was still retained in an inactive form in the cytoplasm. Our study describes a novel mechanism of nuclear uptake control by masking of a nuclear location signal through a remote domain within a precursor molecule.

I kappa B gamma, a 70 kd protein identical to the C-terminal half of p110 NF-kappa B: a new member of the I kappa B family

A cDNA corresponding to the 2.6 kb NF-kappa B mRNA species present in a variety of lymphoid cell lines has been molecularly cloned. The deduced 607 amino acid sequence is identical to the sequence of the C-terminal region of 110 kd NF-kappa B protein. A 70 kd protein can be identified in lymphoid cells using antibodies raised against the C-terminal region of p110 NF-kappa B. Comparison of the two-dimensional tryptic peptide maps of the 70 kd protein expressed in cells and the in vitro translated product encoded by the cDNA display extensive homology. The 70 kd protein expressed in bacteria prevents sequence-specific DNA binding of p50-p65 NF-kappa B heterodimer, p50 homodimer, and c-rel. p70 also interferes with transactivation by c-rel and prevents its nuclear translocation. The 70 kd protein, predominantly found in lymphoid cells, is a new member of the I kappa B family of proteins and is referred to as I kappa B gamma.

The ankyrin repeat domains of the NF-kappa B precursor p105 and the protooncogene bcl-3 act as specific inhibitors of NF-kappa B DNA binding

The inducible pleiotropic transcription factor NF-kappa B is composed of two subunits, p50 and p65. The p50 subunit is encoded on the N-terminal half of a 105-kDa open reading frame and contains a rel-like domain. To date, no function has been described for the C-terminal portion. We show here that the C-terminal half of p105, when expressed as a separate molecule, binds to p50 and can rapidly disrupt protein-DNA complexes of p50 or native NF-kappa B. Deletion analysis of this precursor-derived inhibitor activity indicated a domain containing ankyrin-like repeats as necessary for inhibition. The protooncogene bcl-3, which contains seven ankyrin repeats, can equally inhibit p50 DNA binding. These observations identify bcl-3 as an inhibitor of NF-kappa B and strongly suggest that the ankyrin repeats in these factors are involved in protein-protein interactions with the rel-like domain of p50. Comparison with other ankyrin repeat-containing proteins suggests that a subclass of these proteins acts as regulators of rel-like transcription factors.

Activation of human immunodeficiency virus type 1 provirus in T-cells and macrophages is associated with induction of inducer-specific NF-kappa B binding proteins

We have analyzed the limiting factors involved in the induction of human immunodeficiency virus type 1 (HIV-1) provirus expression by tumor necrosis factor-alpha (TNF-alpha), phorbol-12-myristate-13-acetate (PMA), and bryostatin-1 in T-cells (ACH-2) and monocytes (U1). We have demonstrated that, while there is a correlation among the increase of 9.2-kilodalton (kDa) HIV-1 RNA, the increase of viral proteins (p24) in the cells, and the release of HIV-1 virions into the medium, there is no direct correlation between the levels of induced NF-kappa B binding proteins and the expression of HIV-1 provirus. The presence of nuclear NF-kappa B-specific proteins appears to be essential only for the initiation of viral replication, since the HIV-1 transcripts could be detected in TNF-alpha or bryostatin-1-stimulated cells also at later times postinduction, times when no NF-kappa B proteins could be detected in the nucleus. The uv crosslinking of DNA and proteins has shown that TNF-alpha, PMA, and bryostatin-1 induce different sets of NF-kappa B binding proteins with distinct kinetics of binding.

Evolutionary variation of the CCAAT-binding transcription factor NF-Y

NF-Y is a CCAAT-specific transcription factor thought to be involved in the regulation of a variety of eukaryotic genes. It shows a striking sequence similarity with the yeast factor HAP2/3. In an attempt to trace back its evolutionary history, we succeeded in isolating NF-Y cDNA clones from a plant and from several species of vertebrates. The patterns of sequence conservation delineate potential functional domains: A central, highly conserved, domain likely responsible for DNA-binding and subunit interaction; more evolutionarily flexible flanking regions, in which variability is clustered, individualizing conserved glutamine or acidic amino-acids putatively involved in protein-protein contacts.

The scalloped gene encodes a novel, evolutionarily conserved transcription factor required for sensory organ differentiation in Drosophila

The scalloped (sd) gene of Drosophila melanogaster was initially characterized by mutants affecting structures on the wing of the adult fly. The sequence of a cDNA clone of the gene reveals a predicted protein sequence homologous to that of a human transcriptional enhancer factor, TEF-1 (68% identity). The homology includes a sequence motif, the TEA domain, that was shown previously to be a DNA-binding domain of TEF-1. An sd enhancer trap strain expresses the reporter gene in a subset of neuroblasts in the central nervous system and in the peripheral sense organs of the embryo. The reporter gene is later expressed in specific regions of the imaginal discs, including regions of the wing disc destined to become structures defective in viable sd mutants. Later still, expression in the adult brain is restricted to subsets of cells, some in regions involved in the processing of gustatory information. These observations indicate that the sd gene encodes a transcription factor that functions in the regulation of cell-specific gene expression during Drosophila development, particularly in the differentiation of the nervous system.

Distinct combinations of NF-kappa B subunits determine the specificity of transcriptional activation

The nuclear factor that binds to the kappa light-chain enhancer of B cells (NF-kappa B) is a transcription factor that regulates the expression of a variety of cellular and viral genes. NF-kappa B is composed of distinct subunits, and at least four independent genes (p105, p100, p65, and c-rel) have been isolated that encode related proteins that bind kappa B sites. Because it is possible that specific interactions of different subunits can allow selective gene activation, we have characterized the specificity of transcriptional activation by various combinations of these subunits. When tested alone, an approximately 49-kDa form (p49) of the p100 protein bound weakly to kappa B, but p49 associated with p65 to bind efficiently to this site. Furthermore, p49 acted in combination with either p65 or a Rel/VP16 fusion protein to activate kappa B-dependent transcription in Jurkat T leukemia cells. The p49/p65 or p49/Rel combination stimulated transcription mediated by the canonical kappa B site but did not stimulate reporter genes containing interleukin 2 receptor alpha or major histocompatibility complex kappa B elements, despite its ability to bind to these sites. Transactivation mediated by the p49/p100 and p65 NF-kappa B proteins is therefore sensitive to minor changes in the sequence of the kappa B site. Specificity determined by the association of NF-kappa B subunits provides a mechanism to selectively regulate variant kappa B sites associated with different cellular and viral genes.

The c-rel protooncogene product c-Rel but not NF-kappa B binds to the intronic region of the human interferon-gamma gene at a site related to an interferon-stimulable response element

Interferon-gamma (IFN-gamma) is an important immunoregulatory protein that is expressed usually only in large granular lymphocytes and T cells. The gene encoding IFN-gamma was previously found to contain an intronic enhancer element that was not tissue-specific in its activity, despite the restricted expression of the intact IFN-gamma-encoding gene. Using nuclear extracts from the human T-cell line Jurkat, we have now identified two protein-binding regions in this intronic enhancer element. One of the protected regions has strong partial identify to the NF-kappa B site present in the promoter region of the human interleukin 2-encoding gene. Based on this observation and recent reports of the interaction of the c-rel protooncogene product (c-Rel) with NF-kappa B sites, we determined whether c-Rel could interact with the intronic enhancer element in the human IFN-gamma genomic DNA. Most surprisingly, gel-shift analysis, using c-Rel expressed in Escherichia coli established that c-Rel binds specifically to the IFN-gamma intronic DNA but not to the interleukin 2-like NF-kappa B site. Additional studies with antibodies prepared against c-Rel peptides verified specificity of the interaction of c-Rel with this binding site. In addition, using an affinity-purified p50 subunit of the NF-kappa B complex, we observed that the p50 protein did not bind to this additional c-Rel-binding site. Furthermore, nucleotide sequence analysis of this DNA region revealed a strong similarity of the additional c-Rel-binding site to a previously identified IFN-stimulable response element. These data show that c-Rel can interact with DNA regions distinct from that recognized by NF-kappa B and may, in fact, be involved in transcriptional regulation of the IFN-stimulable genes via the IFN-stimulable response element.

The 65-kDa subunit of human NF-kappa B functions as a potent transcriptional activator and a target for v-Rel-mediated repression

Molecular cloning of the polypeptide component of the Rel-related human p75 nucleoprotein complex has revealed its identity with the 65-kDa (p65) subunit of NF-kappa B. Functional analyses of chimeric proteins composed of NF-kappa B p65 C-terminal sequences linked to the DNA-binding domain of the yeast GAL4 polypeptide have indicated that the final 101 amino acids of NF-kappa B p65 comprise a potent transcriptional activation domain. Transient transfection of human T cells with an expression vector encoding NF-kappa B p65, but not NF-kappa B p50, produced marked transcriptional activation of a basal promoter containing duplicated kappa B enhancer motifs from the long terminal repeat of type 1 human immunodeficiency virus. These stimulatory effects of NF-kappa B p65 were synergistically enhanced by coexpression of NF-kappa B p50 but were completely inhibited by coexpression of the v-rel oncogene product. Together, these functional studies demonstrate that NF-kappa B p65 is a transactivating subunit of the heterodimeric NF-kappa B complex and serves as one cellular target for v-Rel-mediated transcriptional repression.

Extracellular Tax1 protein stimulates tumor necrosis factor-beta and immunoglobulin kappa light chain expression in lymphoid cells

The human T-cell leukemia virus type I tax1 gene product is responsible for the increased expression of several cytokine and cellular genes that contain NF-kappa B regulatory sequences. Our laboratory has previously demonstrated that purified, extracellular Tax1 protein induced the nuclear accumulation of NF-kappa B binding activity in lymphoid cells. Since HTLV-I infection causes increased levels of lymphotoxin tumor necrosis factor-beta [TNF-beta] and immunoglobulin secretion, we have studied the interaction of NF-kappa B proteins from Tax1-stimulated cells with the TNF-beta and immunoglobulin kappa (Ig kappa) light chain genes. Tax1 induction of NF-kappa B occurred in the presence of cycloheximide, and Tax1 stimulation did not result in increased levels of NF-kappa B or c-rel RNA. These results indicate that new synthesis of NF-kappa B proteins was not required for induction of NF-kappa B-binding activity. With use of the Ig kappa NF-kappa B-binding site as a probe, two distinct NF-kappa B gel shift complexes were induced by the Tax1 protein. A slower-migrating complex, C1, was inhibited by the addition of purified I kappa B. In contrast, the faster-migrating C2 complex was not inhibited by I kappa B, but C2 was increased by detergent treatment of cytoplasmic extracts, suggesting that its binding activity was also regulated by an inhibitor. The Tax1-stimulated proteins that interacted with the NF-kappa B-binding sites in the Ig kappa and TNF-beta promoters were distinct. A 75-kDa protein preferentially associated with the Ig kappa NF-kappa B-binding site. In contrast, a 59-kDa protein associated with the TNF-beta NF-kappa B-binding site. Tax1 stimulation led to increased levels of TNF-beta and Ig kappa mRNA, as measured by reverse transcription and polymerase chain reaction analysis. These results represent the first experimental evidence that extracellular Tax1 can regulate the expression of endogenous cellular genes.

Induction of monocytic differentiation and NF-kappa B-like activities by human immunodeficiency virus 1 infection of myelomonoblastic cells

The effects of human immunodeficiency virus 1 (HIV-1) infection on cellular differentiation and NF-kappa B DNA binding activity have been investigated in a new model of myeloid differentiation. PLB-985 cells represent a bipotential myelomonoblastic cell population capable of either granulocytic or monocytic differentiation after induction with appropriate inducers. By virtue of the presence of CD4 on the cell surface, PLB-985 cells were chronically infected with HIV-1 strain IIIB. PLB-IIIB cells clearly possessed a more monocytic phenotype than the parental myeloblasts, as determined by differential staining, increased expression of the myeloid-specific surface markers, and transcription of the c-fms proto-oncogene. NF-kappa B binding activity was inducible by tumor necrosis factor and phorbol myristate acetate in PLB-985. However, in PLB-IIIB cells, constitutive expression of a novel NF-kappa B complex was detected, composed of proteins ranging between 70 and 110 kD. These proteins interacted specifically with the symmetric NF-kappa B site from the interferon beta (IFN-beta) promoter. Mutations affecting the 5' guanine residues of the kappa B site were unable to compete for these NF-kappa B-related proteins. Inducibility of endogenous IFN-beta and IFN-alpha RNA was also increased in PLB-IIIB cells. These studies indicate that HIV-1 infection of myelomonoblastic cells may select for a more mature monocytic phenotype and that unique subunit associations of NF-kappa B DNA binding proteins may contribute to differential NF-kappa B-mediated gene expression.

Characterization of a novel nuclear localization signal in the HTLV-I tax transactivator protein

The Tax trans-activator protein of the type I human T-cell leukemia virus is expressed predominantly in the nuclei of cells. However, this viral trans-activator is distinguished from most other nuclear proteins by the absence of a short highly basic nuclear localization signal. Previous mutational analyses of the tax gene revealed that many of the missense mutations involving the amino terminus of Tax resulted in a predominantly cytoplasmic pattern of expression. We now report that the amino terminal 48 residues of Tax comprise a functional nuclear localization signal as demonstrated by the ability of this region to retarget expression of a large cytoplasmic protein to the nucleus.

A novel mitogen-inducible gene product related to p50/p105-NF-kappa B participates in transactivation through a kappa B site

A Rel-related, mitogen-inducible, kappa B-binding protein has been cloned as an immediate-early activation gene of human peripheral blood T cells. The cDNA has an open reading frame of 900 amino acids capable of encoding a 97-kDa protein. This protein is most similar to the 105-kDa precursor polypeptide of p50-NF-kappa B. Like the 105-kDa precursor, it contains an amino-terminal Rel-related domain of about 300 amino acids and a carboxy-terminal domain containing six full cell cycle or ankyrin repeats. In vitro-translated proteins, truncated downstream of the Rel domain and excluding the repeats, bind kappa B sites. We refer to the kappa B-binding, truncated protein as p50B by analogy with p50-NF-kappa B and to the full-length protein as p97. p50B is able to form heteromeric kappa B-binding complexes with RelB, as well as with p65 and p50, the two subunits of NF-kappa B. Transient-transfection experiments in embryonal carcinoma cells demonstrate a functional cooperation between p50B and RelB or p65 in transactivation of a reporter plasmid dependent on a kappa B site. The data imply the existence of a complex family of NF-kappa B-like transcription factors.

RelB, a new Rel family transcription activator that can interact with p50-NF-kappa B

We have identified a serum-inducible gene, relB, which encodes a protein of 558 amino acids containing a region with high similarity to c-Rel and other members of the Rel family. Transcriptional activation analysis of GAL4-RelB fusion proteins in yeast cells reveals that RelB contains in its C-terminal 180 amino acids a transcriptional activation domain. The N-terminal part including the region of similarity with the Rel family shows no detectable transcriptional activity. RelB does not bind with high affinity to NF-kappa B sites, but heterodimers between RelB and p50-NF-kappa B do bind to different NF-kappa B-binding sites with a similar affinity to that shown by p50-NF-kappa B homodimers. However, RelB/p50-NF-kappa B heterodimers, in contrast to p50-NF-kappa B homodimers, transactivate transcription of a promoter containing a kappa B-binding site.

Functional characterization of the NF-kappa B p65 transcriptional activator and an alternatively spliced derivative

The NF-kappa B transcription factor complex is composed of two proteins, designated p50 and p65, both having considerable homology to the product of the rel oncogene. We present evidence that the p65 subunit is a potent transcriptional activator in the apparent absence of the p50 subunit, consistent with in vitro results demonstrating that p65 can interact with DNA on its own. To identify the minimal activation domain, chimeric fusion proteins between the DNA binding domain of the yeast transcriptional activator protein GAL4 and regions of the carboxy terminus of p65 were constructed, and their transcriptional activity was assessed by using a GAL4 upstream activation sequence-driven promoter-chloramphenicol acetyltransferase fusion. This analysis suggests that the boundaries of the activation domain lie between amino acids 415 and 550. Moreover, single amino acid changes within residues 435 to 459 greatly diminished activation. Similar to other activation domains, this region contains a leucine zipper-like motif as well as an overall net negative charge. To identify those residues essential for DNA binding, we made use of a naturally occurring derivative of p65, lacking residues 222 to 231 (hereafter referred to as p65 delta), and produced via an alternative splice site. Gel mobility shift analysis using bacterially expressed p65, p65 delta, and various mutants indicates that residues 222 to 231 are important for binding to kappa B DNA. Coimmunoprecipitation analysis suggests that these residues likely contribute to the multimerization function required for homomeric complex formation or heteromeric complex formation with p50 in that no association of p65 delta with itself or with p50 was evident. However, p65 delta was able to form weak heteromeric complexes with p65 that were greatly reduced in their ability to bind DNA. On the basis of these findings, we suggest that subtle changes within the proposed multimerization domain can elicit different effects with the individual Rel-related proteins and that a potential role of p65 delta may be to negatively regulate NF-kappa B function through formation of nonfunctional heteromeric complexes.

Multiple extracellular activities in Drosophila egg perivitelline fluid are required for establishment of embryonic dorsal-ventral polarity

Twelve maternal effect genes (the dorsal group and cactus) are required for the establishment of the embryonic dorsal-ventral axis in the Drosophila embryo. Embryonic dorsal-ventral polarity is defined within the perivitelline compartment surrounding the embryo by the ventral formation of a ligand for the Toll receptor. Here, by transplantation of perivitelline fluid we demonstrate the presence of three separate activities present in the perivitelline fluid that can restore dorsal-ventral polarity to mutant easter, snake, and spätzle embryos, respectively. These activities are not capable of defining the polarity of the dorsal-ventral axis; instead they restore structures according to the intrinsic dorsal-ventral polarity of the mutant embryos. They appear to be involved in the ventral formation of a ligand for the Toll protein. This process requires serine proteolytic activity; the injection of serine protease inhibitors into the perivitelline space of wild-type embryos results in the formation of dorsalized embryos.

RelB, a new Rel family transcription activator that can interact with p50-NF-kappa B

We have identified a serum-inducible gene, relB, which encodes a protein of 558 amino acids containing a region with high similarity to c-Rel and other members of the Rel family. Transcriptional activation analysis of GAL4-RelB fusion proteins in yeast cells reveals that RelB contains in its C-terminal 180 amino acids a transcriptional activation domain. The N-terminal part including the region of similarity with the Rel family shows no detectable transcriptional activity. RelB does not bind with high affinity to NF-kappa B sites, but heterodimers between RelB and p50-NF-kappa B do bind to different NF-kappa B-binding sites with a similar affinity to that shown by p50-NF-kappa B homodimers. However, RelB/p50-NF-kappa B heterodimers, in contrast to p50-NF-kappa B homodimers, transactivate transcription of a promoter containing a kappa B-binding site.

A novel mitogen-inducible gene product related to p50/p105-NF-kappa B participates in transactivation through a kappa B site

A Rel-related, mitogen-inducible, kappa B-binding protein has been cloned as an immediate-early activation gene of human peripheral blood T cells. The cDNA has an open reading frame of 900 amino acids capable of encoding a 97-kDa protein. This protein is most similar to the 105-kDa precursor polypeptide of p50-NF-kappa B. Like the 105-kDa precursor, it contains an amino-terminal Rel-related domain of about 300 amino acids and a carboxy-terminal domain containing six full cell cycle or ankyrin repeats. In vitro-translated proteins, truncated downstream of the Rel domain and excluding the repeats, bind kappa B sites. We refer to the kappa B-binding, truncated protein as p50B by analogy with p50-NF-kappa B and to the full-length protein as p97. p50B is able to form heteromeric kappa B-binding complexes with RelB, as well as with p65 and p50, the two subunits of NF-kappa B. Transient-transfection experiments in embryonal carcinoma cells demonstrate a functional cooperation between p50B and RelB or p65 in transactivation of a reporter plasmid dependent on a kappa B site. The data imply the existence of a complex family of NF-kappa B-like transcription factors.

The rel family of proteins

The rel family of proteins can be defined as a group of proteins that share sequence homology over a 300 amino acid region termed the rel domain. The rel family comprises important regulatory proteins from a wide variety of species and includes the Drosophila morphogen dorsal, the mammalian transcription factor NF-kappa B, the avian oncogene v-rel, and the cellular proto-oncogene c-rel. Over the last two years it has become apparent that these proteins function as DNA-binding transcription factors, and that their activity is regulated at the level of subcellular localization.

Functional characterization of the NF-kappa B p65 transcriptional activator and an alternatively spliced derivative

The NF-kappa B transcription factor complex is composed of two proteins, designated p50 and p65, both having considerable homology to the product of the rel oncogene. We present evidence that the p65 subunit is a potent transcriptional activator in the apparent absence of the p50 subunit, consistent with in vitro results demonstrating that p65 can interact with DNA on its own. To identify the minimal activation domain, chimeric fusion proteins between the DNA binding domain of the yeast transcriptional activator protein GAL4 and regions of the carboxy terminus of p65 were constructed, and their transcriptional activity was assessed by using a GAL4 upstream activation sequence-driven promoter-chloramphenicol acetyltransferase fusion. This analysis suggests that the boundaries of the activation domain lie between amino acids 415 and 550. Moreover, single amino acid changes within residues 435 to 459 greatly diminished activation. Similar to other activation domains, this region contains a leucine zipper-like motif as well as an overall net negative charge. To identify those residues essential for DNA binding, we made use of a naturally occurring derivative of p65, lacking residues 222 to 231 (hereafter referred to as p65 delta), and produced via an alternative splice site. Gel mobility shift analysis using bacterially expressed p65, p65 delta, and various mutants indicates that residues 222 to 231 are important for binding to kappa B DNA. Coimmunoprecipitation analysis suggests that these residues likely contribute to the multimerization function required for homomeric complex formation or heteromeric complex formation with p50 in that no association of p65 delta with itself or with p50 was evident. However, p65 delta was able to form weak heteromeric complexes with p65 that were greatly reduced in their ability to bind DNA. On the basis of these findings, we suggest that subtle changes within the proposed multimerization domain can elicit different effects with the individual Rel-related proteins and that a potential role of p65 delta may be to negatively regulate NF-kappa B function through formation of nonfunctional heteromeric complexes.

Mapping of the human gene for the human immunodeficiency virus type 1 enhancer binding protein HIV-EP2 to chromosome 6q23-q24

The human gene encoding the human immunodeficiency virus type 1 enhancer binding protein HIV-EP2 has been isolated. Using Southern analysis of human-rodent somatic cell hybrid DNA with a human HIV-EP2-specific cDNA probe, the HIV-EP2 gene was assigned to chromosome 6. The gene was further localized to the region 6q23-24 by fluorescence in situ hybridization.

Molecular insights into human immunodeficiency virus type 1 pathogenesis

Infection with human immunodeficiency virus type 1 leads to a persistent but progressive cytopathic process that culminates in the near complete destruction of the CD4+ subset of T cells. The levels of human immunodeficiency virus type 1 replication and virus burden increase throughout the clinical course of disease reflecting a balance between the viral and cellular regulatory influences as well as the ability of the host immune system to eliminate infected T cells. Human immunodeficiency virus type 1 replication is dependent on the state of cellular activation and involves both inducible host cell derived transcription factors and at least three virus-derived gene products. Further study of the mechanism of action of these factors, particularly those encoded by the virus, may facilitate the future development of highly specific and effective therapies for human immunodeficiency virus type 1.

Regulation of immunoglobulin gene transcription

Analysis of the immunoglobulin gene suggests that their expression is controlled through the combinatorial action of tissue- and stage-specific factors (OTF-2, TF-microB, NF-kappa B), as well as more widely expressed E motif-binding factors such as E47/E12. Two basic issues cloud understanding of how these factors are involved in immunoglobulin gene regulation. First, cloning of these factors shows them to be members of families of proteins, all with similar DNA-binding specificities. OTF-2 is a member of the POU domain family, NF-kappa B is a related protein, and the microE5/kappa E2-binding factors are members of the bHLH family. Second, these binding sites and associated factors are involved in the regulation of many genes, not only the immunoglobulin genes, and in fact not only lymphoid-specific genes. These facts complicate understanding which member of a family is in fact responsible for interaction with, and activation of, a particular binding element in an enhancer/promoter. Recently, more detailed analysis of the interactions between such proteins and their related binding sites suggest that a certain level of specificity may in fact be encoded by the DNA element such that one family member of a protein is preferentially bound, or alternatively that the protein-DNA interactions that occur give subtle alterations in protein conformation that unmask an activation or protein-protein interactive domain. An additional level of regulation is imparted by combinatorial mechanisms such as adjacent DNA-binding elements and factors that may alter activity, as well as "cofactors" that, by forming a complex with the bound factor, affect its activation of a gene in a particular cell type. A third level of specificity may be obtained by factors such as NF-kappa B and the bHLH family due to their ability to create heterogeneous complexes, creating unique complexes in a tissue- or stage-specific manner. The multiple functions transcription factors such as NF-kappa B and OTF-2 play in the transcriptional regulation of multiple genes seems complex in contrast to a one factor, one gene regulation model. However, this type of organization may limit the number of factors lymphocytes would require if each lymphoid-specific gene were activated by a unique factor. Thus what appears to be complexity at the molecular level may reflect an economical organization at the cellular level. Investigation of the key factors controlling these genes suggests an ordered cascade of transcription factors becomes available in the cell during B cell differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Helix-loop-helix proteins in the regulation of immunoglobulin gene transcription

Transcriptional control of the Ig heavy chain and kappa light chain is dominated by single enhancers located within the body of each gene. These enhancers bind distinct, yet overlapping, sets of cell-type-specific and ubiquitous nuclear proteins. This review focuses on one particular subclass of enhancer-binding protein, termed helix-loop-helix (HLH) proteins, describing their role in the establishment of cell-type-specific transcription and suggesting how they may be regulated during B-cell maturation.

Activation of lymphokine genes in T cells: role of cis-acting DNA elements that respond to T cell activation signals

Activation of T cells is initiated by the recognition of antigen on antigen presenting cells to exert the effector functions in immune and inflammatory responses. Two types of helper T cell (Th) clones (Th1 and Th2) are defined on the basis of different patterns of cytokine (lymphokine) secretion. They determine the outcome of an antigenic response toward humoral or cell-mediated immunity. Although lymphokine genes are coordinately regulated upon antigen stimulation, they are regulated by the mechanisms common to all as well as those which are unique to each gene. For most lymphokine genes, a combination of phorbol esters (phorbol 12-myristate 13 acetate, PMA) and calcium ionophores (A23187) is required for their maximal induction. Yet phorbol ester alone or calcium ionophore alone produce several lymphokines. The production of the granulocyte-macrophage colony stimulating factor (GM-CSF) is completely dependent on the two signals. We have previously found a cis-acting region spanning the GM-CSF promoter region (positions -95 to +27) that confers inducibility to reporter genes in transient transfection assays. Further analysis identified three elements required for efficient induction, referred to as GM2, GC-box and conserved lymphokine element (CLE0). GM2 defines a binding site for protein(s) whose binding is inducible by PMA. One protein, NF-GM2 is similar to the transcription factor NF-kB. GC-box is a binding site for constitutively bound proteins. CLEO defines a binding site for protein(s) whose optimum binding is stimulated by PMA and A23187. Viral trans-activators such as Tax (human T cell leukemia virus-1, HTLV-1) and E2 (bovine papilloma virus, BPV) proteins are other agents which activate lymphokine gene expression by bypassing T cell receptor (TCR) mediated signaling. The trans-activation domain of E2 and Tax is interchangeable although they have no obvious sequence homology between them. The viral trans-activators appear to target specific DNA binding protein such as NF-kB and Sp1 to cis-acting DNA site and promote lymphokine gene expression without TCR-mediated stimulation.

Regulation of human immunodeficiency virus enhancer function by PRDII-BF1 and c-rel gene products

The human immunodeficiency virus (HIV) enhancer element is important in the regulation of HIV gene expression. A number of cellular proteins have been demonstrated to bind to the NF-kappa B motifs in this element. The genes encoding several of these proteins, including members of the rel family and PRDII-BF1, have been cloned. We characterized the binding of proteins encoded by the human c-rel and PRDII-BF1 genes to HIV NF-kappa B motifs and related enhancer elements. Both the human c-rel protein and two proteins derived from the PRDII-BF1 gene by alternative splicing bound specifically to the HIV NF-kappa B motif and related enhancer elements found in the immunoglobulin kappa, class I major histocompatibility complex, and interleukin-2 receptor genes. To determine the role of these factors in regulating HIV gene expression, we fused the cDNAs encoding either of the two proteins derived by alternative splicing of the PRDII-BF1 gene or the c-rel gene to the DNA binding region of the yeast transcription factor GAL4. GAL4 binding sites were inserted in place of the native HIV enhancer sequences in an HIV long terminal repeat chloramphenicol acetyltransferase construct. Cotransfection of these constructs revealed that c-rel was a strong activator of basal HIV gene expression but did not result in synergistic effects in the presence of tat. PRDII-BF1-derived cDNAs did not result in stimulation of either basal or tat-induced activated gene expression. These results indicate that multiple enhancer binding proteins may potentially regulate HIV in both a positive and negative manner.

A protein kinase-A recognition sequence is structurally linked to transformation by p59v-rel and cytoplasmic retention of p68c-rel

The Rel family of proteins includes a number of proteins involved in transcriptional control, such as the retroviral oncoprotein v-Rel, c-Rel, the Drosophila melanogaster developmental protein Dorsal, and subunits of the transcription factor NF-kappa B. These proteins are related through a highly conserved domain of approximately 300 amino acids, called the Rel homology domain, that contains dimerization, DNA binding, and nuclear targeting functions. Also within the Rel homology domain, there is a conserved consensus sequence (Arg-Arg-Pro-Ser) for phosphorylation by cyclic AMP-dependent protein kinase (PKA). We used linker insertion mutagenesis and site-directed mutagenesis to determine the importance of this sequence for the transformation of avian spleen cells by v-Rel and the subcellular localization of c-Rel in chicken embryo fibroblasts (CEF). The insertion of 2 amino acids (Pro-Trp) within this sequence completely abolished transformation and transcriptional repression by v-Rel and resulted in a shift in the localization of c-Rel from cytoplasmic to nuclear in CEF. When the conserved Ser within the PKA recognition sequence was replaced by Ala, there was no significant effect on transformation and transcriptional repression by v-Rel or on cytoplasmic retention of c-Rel. However, when this Ser was changed to Asp or Glu, transformation and transcriptional repression by v-Rel were significantly inhibited and c-Rel showed a diffuse nuclear and cytoplasmic localization in CEF. Although a peptide containing the recognition sequence from v-Rel can be phosphorylated by PKA in vitro, this site is not constitutively phosphorylated to a high degree in vivo in transformed spleen cells incubated with okadaic acid. Our results indicate that the transforming and transcriptional repressing activities of v-Rel and the cytoplasmic retention of c-Rel are dependent on the structure of the conserved PKA recognition motif. In addition, they suggest that phosphorylation at the conserved PKA site could have a negative effect on transformation and transcriptional repression by v-Rel and induce the nuclear localization of c-Rel.

B cell lymphoma-associated chromosomal translocation involves candidate oncogene lyt-10, homologous to NF-kappa B p50

A B cell lymphoma-associated chromosomal translocation, t(10;14)(q24;q32), juxtaposes the immunoglobulin C alpha 1 locus to a novel gene, lyt-10. The normal lyt-10 cDNA codes for a 98 kd protein which displays amino-terminal homology with the rel (DNA-binding) domain of the NF-kappa B-rel family of transcription factors and carboxy-terminal homology with the NF-kappa B p50 precursor protein, including the putative proteolytic cleavage domain (poly-G) and the ankyrin-like repeat domains. The lyt-10 protein can bind to kappa B sequences in vitro, although with different specificity from NF-kappa B p50, and in vitro DNA-binding is activated by removal of the ankyrin domain. Chromosomal translocation generates an lyt-10-C alpha 1 fusion gene coding for a protein that retains the rel effector domain, lacks the ankyrin regulatory domain, and binds kappa B sequences in vitro, suggesting its constitutive activation in vivo. Analogous rearrangements of the lyt-10 gene have been found in an additional three cases of lymphoid neoplasia. The lyt-10 gene defines a new subfamily (rel/poly-G/ankyrin) of NF-kappa B-rel transcription factors with potential for oncogenic activation in human cancer.

c-myc oncoprotein function

Genetic alterations of the c-myc locus in various malignancies and the ability of c-myc to transform cultured cells and induce tumors in transgenic animals attest to its central role in many neoplasms. By dissecting the c-Myc protein, a number of critical functional domains of c-Myc have been identified and characterized; these findings suggest a model for c-Myc function and intracellular activity (Fig. 4). c-Myc is synthesized in the cytoplasm and undergoes oligomerization another protein such as Max. Its nuclear localization signal allows c-Myc to be targeted to and retained in the nucleus, where the protein seeks out and binds to specific DNA sites, perhaps facilitated by c-Myc's ability to bind non-specifically to DNA. Once bound to specific DNA sequences, c-Myc then activates or inhibits transcription of a number of target genes, with consequent alterations in cell growth and differentiation. Continued studies of c-Myc and its partner Max should further elucidate the mechanisms by which c-Myc can contribute both to the regulation of normal cell growth and the alteration in that regulation in neoplasia.

Generation of p50 subunit of NF-kappa B by processing of p105 through an ATP-dependent pathway

The transcription factor NF-kappa B is a heterodimer consisting of two proteins encoded by different members of the rel gene family (p50 and p65). The p50 subunit is unusual among DNA-binding proteins in that its functional form is encoded in an open reading frame of relative molecular mass 105,000 (p105; ref. 4). The N-terminal region of this open reading frame encodes p50, whereas the remaining C terminus contains ankyrin repeats. Although p50 binds to DNA, full-length p105 translated in vitro does not. The mechanism by which p50 is generated in vivo, and the fate of the C-terminal region of p105 have not been established. Here we show that functional p50 is produced by ATP-dependent proteolysis of p105. Moreover, we find that the C-terminal half of p105 is not required for processing in vivo, and is rapidly degraded on processing. We propose that the C-terminal region of p105 is involved in the cytoplasmic assembly of the complex between the p50/p65 heterodimer and the inhibitor I kappa B.

A protein kinase-A recognition sequence is structurally linked to transformation by p59v-rel and cytoplasmic retention of p68c-rel

The Rel family of proteins includes a number of proteins involved in transcriptional control, such as the retroviral oncoprotein v-Rel, c-Rel, the Drosophila melanogaster developmental protein Dorsal, and subunits of the transcription factor NF-kappa B. These proteins are related through a highly conserved domain of approximately 300 amino acids, called the Rel homology domain, that contains dimerization, DNA binding, and nuclear targeting functions. Also within the Rel homology domain, there is a conserved consensus sequence (Arg-Arg-Pro-Ser) for phosphorylation by cyclic AMP-dependent protein kinase (PKA). We used linker insertion mutagenesis and site-directed mutagenesis to determine the importance of this sequence for the transformation of avian spleen cells by v-Rel and the subcellular localization of c-Rel in chicken embryo fibroblasts (CEF). The insertion of 2 amino acids (Pro-Trp) within this sequence completely abolished transformation and transcriptional repression by v-Rel and resulted in a shift in the localization of c-Rel from cytoplasmic to nuclear in CEF. When the conserved Ser within the PKA recognition sequence was replaced by Ala, there was no significant effect on transformation and transcriptional repression by v-Rel or on cytoplasmic retention of c-Rel. However, when this Ser was changed to Asp or Glu, transformation and transcriptional repression by v-Rel were significantly inhibited and c-Rel showed a diffuse nuclear and cytoplasmic localization in CEF. Although a peptide containing the recognition sequence from v-Rel can be phosphorylated by PKA in vitro, this site is not constitutively phosphorylated to a high degree in vivo in transformed spleen cells incubated with okadaic acid. Our results indicate that the transforming and transcriptional repressing activities of v-Rel and the cytoplasmic retention of c-Rel are dependent on the structure of the conserved PKA recognition motif. In addition, they suggest that phosphorylation at the conserved PKA site could have a negative effect on transformation and transcriptional repression by v-Rel and induce the nuclear localization of c-Rel.