The ZiN/POZ domain of ZF5 is required for both transcriptional activation and repression

NAC-1 is a brain POZ/BTB protein that can prevent cocaine-induced sensitization in the rat

Levels of the mRNA NAC-1 are increased in the rat forebrain weeks after cocaine exposure. This long-term neuroadaptation occurs during the expression of behavioral sensitization, a model of psychostimulant-induced paranoia. NAC-1, the protein encoded by this cocaine-regulated mRNA, contains a Pox virus and zinc finger/bric-a-brac tramtrack broad complex (POZ/BTB) motif, which mediates interactions among several transcriptional regulators. The present studies demonstrate that NAC-1 acts as a transcription factor. NAC-1 was localized to the nucleus of neurons in the brain. Transfection of NAC-1 in cell culture repressed transcription of a reporter gene. NAC-1 was also able to affect the actions of other POZ/BTB proteins in mammalian two-hybrid studies; these interactions required the presence of the POZ/BTB domain. However, NAC-1 appears to be a unique POZ/BTB transcriptional regulator because it does not contain any zinc finger regions found in these other DNA-binding proteins. Adenoviral-mediated overexpression of NAC-1 protein in the rat nucleus accumbens prevented the development but not the expression of behavioral sensitization produced by repeated administration of cocaine. Thus, NAC-1 may modify the long-term behaviors of psychostimulant abuse by regulating gene transcription in the mammalian brain.

NAC-1 is a brain POZ/BTB protein that can prevent cocaine-induced sensitization in the rat

Levels of the mRNA NAC-1 are increased in the rat forebrain weeks after cocaine exposure. This long-term neuroadaptation occurs during the expression of behavioral sensitization, a model of psychostimulant-induced paranoia. NAC-1, the protein encoded by this cocaine-regulated mRNA, contains a Pox virus and zinc finger/bric-a-brac tramtrack broad complex (POZ/BTB) motif, which mediates interactions among several transcriptional regulators. The present studies demonstrate that NAC-1 acts as a transcription factor. NAC-1 was localized to the nucleus of neurons in the brain. Transfection of NAC-1 in cell culture repressed transcription of a reporter gene. NAC-1 was also able to affect the actions of other POZ/BTB proteins in mammalian two-hybrid studies; these interactions required the presence of the POZ/BTB domain. However, NAC-1 appears to be a unique POZ/BTB transcriptional regulator because it does not contain any zinc finger regions found in these other DNA-binding proteins. Adenoviral-mediated overexpression of NAC-1 protein in the rat nucleus accumbens prevented the development but not the expression of behavioral sensitization produced by repeated administration of cocaine. Thus, NAC-1 may modify the long-term behaviors of psychostimulant abuse by regulating gene transcription in the mammalian brain.

Myoneurin, a novel member of the BTB/POZ-zinc finger family highly expressed in human muscle

Initially characterized as Drosophila developmental regulators, the BTB/POZ and zinc finger proteins (BTB/POZ-ZF) constitute a growing family of proteins with gene expression regulatory functions since they have been shown to be involved in both transcriptional activation and repression of various genes in a broad range of species, including mammals. Here we report the cloning of a novel human transcript, coding for a 68-kDa deduced BTB/POZ-ZF protein. This molecule, called myoneurin on the basis of its prevalent expression in the neuromuscular system, contains an amino-terminal BTB/POZ domain and eight tandemly repeated zinc-finger motifs of the C(2)H(2) type. The murine myoneurin, identified in the mouse embryo, is highly homologous to the human protein.

Transcriptional repression by blimp-1 (PRDI-BF1) involves recruitment of histone deacetylase

B-lymphocyte-induced maturation protein (Blimp-1) is a transcriptional repressor that is considered to be a master regulator of terminal B-cell development because it is sufficient to trigger differentiation in the BCL(1)-cell model. Transcription of the c-myc gene is repressed by Blimp-1 during B-cell differentiation. In this study, we have explored the mechanism by which Blimp-1 represses transcription by using Gal4-fusion protein assays and assays in which Blimp-1 represses the natural c-myc promoter. The results show that Blimp-1 represses the c-myc promoter by an active mechanism that is independent of the adjacently bound activator YY1. Blimp-1 contains two regions that independently associate with histone deacetylase (HDAC) and endogenous Blimp-1 in nuclear extracts binds in vitro to the c-myc Blimp-1 site in a complex containing HDAC. The functional importance of recruiting HDAC for Blimp-1-dependent repression of c-myc transcription is supported by two experiments. First, the HDAC inhibitor tricostatin A inhibits Blimp-1-dependent repression in cotransfection assays. Second, a chromatin immunoprecipitation assay shows that expression of Blimp-1 causes deacetylation of histone H3 associated with the c-myc promoter, and this deacetylation depends on the Blimp-1 binding site in the c-myc promoter.

Structural analysis of the gene encoding RP58, a sequence-specific transrepressor associated with heterochromatin

RP58, a sequence-specific transcriptional repressor sharing homology with the POZ domain of a number of zinc-finger proteins, is highly synthesized in brain and localized in condensed chromatin regions, suggesting a role in transcriptional repression in the central nervous system. In the present study, genomic clones of the human rp58 gene were isolated to determine the complete genomic organization. Sequence analyses indicated that the human rp58 gene encoding the functional protein is uninterrupted over its entire 4.2 kb length. Comparison of the human and mouse rp58 genes revealed that they share not only a high homology in the amino acid sequences of their encoded proteins, but also a high degree of structural similarity at the genomic level. RT-PCR analysis also demonstrated the existence of an alternatively spliced form of rp58 similar to the previously reported zinc-finger cDNA, C2H2-171. Chromosomal mapping by fluorescence in situ hybridization analysis allowed localization of the rp58 gene to human chromosome 1q44 ter, a genetic region associated with a number of human malignancies and neurological disorders.

Regulation of lymphocyte cell fate decisions and lymphomagenesis by BCL-6

Genetic alterations of the BCL-6 gene in mice and man have established BCL-6 as a pivotal regulator of normal differentiation of B and T lymphocytes as well as one of the most frequently translocated oncogenes in human B cell lymphomas. As an oncogene, BCL-6 has not been easy to place into existing paradigms of cellular transformation. Rather, it is likely that the function of BCL-6 as a regulator of lymphocyte differentiation is subverted in BCL-6-induced lymphomas. The lymphomas in which BCL-6 is translocated are all suspected to arise from the germinal center B lymphocyte. Given the selective expression of BCL-6 protein in normal germinal center B lymphocytes and the requirement for BCL-6 in germinal center development, the functions of BCL-6 in normal and malignant B cells are probably intertwined. The BCL-6 protein is a potent transcriptional repressor which presumably controls lymphocyte differentiation and induces lymphomas by regulating the expression of key downstream target genes.

Interrupted expression of NAC-1 augments the behavioral responses to cocaine

NAC-1 is an mRNA that is increased selectively in the nucleus accumbens after acute and repeated cocaine administration. Antisense or control oligonucleotides were microinjected into the nucleus accumbens of rats to define the role of NAC-1 in the behavioral responses to acute systemic cocaine. Antisense oligonucleotides decreased NAC-1 mRNA levels by 26% and markedly enhanced the motor stimulant response to an acute cocaine injection compared to sense oligonucleotide microinjections. The augmentation in cocaine motor behavior produced by NAC-1 antisense pretreatment in the nucleus accumbens was not associated with increased dopamine release as estimated by microdialysis. In contrast, the behavioral response to dopamine microinjection into the nucleus accumbens was increased after antisense oligonucleotide treatment, while the motor response to mu-opioid receptor stimulation was unaltered. These data suggest that the induction of NAC-1 by cocaine may be a compensatory mechanism that minimizes the behavioral impact of cocaine administration by regulating postsynaptic dopamine transmission within the nucleus accumbens.

The catenin p120(ctn) interacts with Kaiso, a novel BTB/POZ domain zinc finger transcription factor

p120(ctn) is an Armadillo repeat domain protein with structural similarity to the cell adhesion cofactors beta-catenin and plakoglobin. All three proteins interact directly with the cytoplasmic domain of the transmembrane cell adhesion molecule E-cadherin; beta-catenin and plakoglobin bind a carboxy-terminal region in a mutually exclusive manner, while p120 binds the juxtamembrane region. Unlike beta-catenin and plakoglobin, p120 does not interact with alpha-catenin, the tumor suppressor adenomatous polyposis coli (APC), or the transcription factor Lef-1, suggesting that it has unique binding partners and plays a distinct role in the cadherin-catenin complex. Using p120 as bait, we conducted a yeast two-hybrid screen and identified a novel transcription factor which we named Kaiso. Kaiso's deduced amino acid sequence revealed an amino-terminal BTB/POZ protein-protein interaction domain and three carboxy-terminal zinc fingers of the C2H2 DNA-binding type. Kaiso thus belongs to a rapidly growing family of POZ-ZF transcription factors that include the Drosophila developmental regulators Tramtrak and Bric à brac, and the human oncoproteins BCL-6 and PLZF, which are causally linked to non-Hodgkins' lymphoma and acute promyelocytic leukemia, respectively. Monoclonal antibodies to Kaiso were generated and used to immunolocalize the protein and confirm the specificity of the p120-Kaiso interaction in mammalian cells. Kaiso specifically coprecipitated with a variety of p120-specific monoclonal antibodies but not with antibodies to alpha- or beta-catenin, E-cadherin, or APC. Like other POZ-ZF proteins, Kaiso localized to the nucleus and was associated with specific nuclear dots. Yeast two-hybrid interaction assays mapped the binding domains to Arm repeats 1 to 7 of p120 and the carboxy-terminal 200 amino acids of Kaiso. In addition, Kaiso homodimerized via its POZ domain but it did not heterodimerize with BCL-6, which heterodimerizes with PLZF. The involvement of POZ-ZF proteins in development and cancer makes Kaiso an interesting candidate for a downstream effector of cadherin and/or p120 signaling.

DEF-3(g16/NY-LU-12), an RNA binding protein from the 3p21.3 homozygous deletion region in SCLC

DEF-3(g16/NY-LU-12) encodes a novel RNA binding protein isolated by positional cloning from an SCLC homozygous deletion region in 3p21.3 and, in parallel, as a differentially expressed gene during myelopoiesis from FDCPmix-A4 cells. DEF-3(g16/NY-LU-12) is ubiquitously expressed during mouse embryogenesis and in adult organs while human hematopoietic tissues showed differential expression. The mouse and human proteins are highly conserved containing two RNA recognition motifs (RRMs) and other domains associated with RNA binding and protein-protein interactions. A database search identified related proteins in human, rat, C. elegans and S. pombe including the 3p21.3 co-deleted gene, LUCA15. Recombinant proteins containing the RRMs of DEF-3(g16/NY-LU-12) and LUCA15 specifically bound poly(G) RNA homopolymers in vitro. These RRMs also show similarity to those of the Hu protein family. Since anti-Hu RRM domain antibodies are associated with an anti-tumor effect and paraneoplastic encephalomyelitis, we tested sera from Hu syndrome patients with the RRMs of DEF-3(g16/NY-LU-12) and LUCA15. These were non-reactive. Thus, DEF-3(g16/NY-LU-12) and LUCA15 represent members of a novel family of RNA binding proteins with similar expression patterns and in vitro RNA binding characteristics. They are co-deleted in some lung cancers and immunologically distinct from the Hu proteins.

ZF5, which is a Kruppel-type transcriptional repressor, requires the zinc finger domain for self-association

ZF5, which we have cloned as a transcriptional repressor on the mouse c-myc promoter, has the POZ domain at the amino-terminus and the Kruppel-type zinc finger domain at the carboxy-terminus. In this report, we showed that ZF5 has two contradictory functions in transcription: activation of human immunodeficiency virus (HIV) promoter and repression of the HSV thymidine kinase (TK) promoter. The POZ domain contributed to the repressor activity, whereas the active function resulted from the DNA-binding ability of the zinc finger domain. We demonstrated that the POZ domain has a function mediating homomeric protein-protein interaction and this interaction requires the zinc finger domain. Furthermore, the POZ domain decreased the DNA-binding activity of the zinc finger domain. These results can provide evidence indicating the important interaction between the POZ and zinc finger domains.

Co-operative DNA binding by GAGA transcription factor requires the conserved BTB/POZ domain and reorganizes promoter topology

The POZ domain is a conserved protein-protein interaction motif present in a variety of transcription factors involved in development, chromatin remodelling and human cancers. Here, we study the role of the POZ domain of the GAGA transcription factor in promoter recognition. Natural target promoters for GAGA typically contain multiple GAGA-binding elements. Our results show that the POZ domain mediates strong co-operative binding to multiple sites but inhibits binding to single sites. Protein cross-linking and gel filtration chromatography experiments established that the POZ domain is required for GAGA oligomerization into higher order complexes. Thus, GAGA oligomerization increases binding specificity by selecting only promoters with multiple sites. Electron microscopy revealed that GAGA binds to multiple sites as a large oligomer and induces bending of the promoter DNA. Our results indicate a novel mode of DNA binding by GAGA, in which a large GAGA complex binds multiple GAGA elements that are spread out over a region of a few hundred base pairs. We suggest a model in which the promoter DNA is wrapped around a GAGA multimer in a conformation that may exclude normal nucleosome formation.

The glutamine-rich domain of the Drosophila GAGA factor is necessary for amyloid fibre formation in vitro, but not for chromatin remodelling

The Drosophila GAGA factor binds specifically to the sequence GAGAG, and synergises with nucleosome remodelling factor to remodel chromatin in vitro. It consists of an N-terminal domain (POZ/BTB) which mediates protein-protein interactions, a central region which contains the DNA-binding domain, and a C-terminal glutamine-rich region. It is shown that the glutamine-rich region is responsible for the formation of fibres in vitro which, on the basis of their tinctorial properties and CD spectra, may be classified as amyloid fibres. A large structural change, probably resulting in beta-sheet structure, is observed upon fibre formation. Mutants containing the central region, either alone or together with the glutamine-rich region, are largely lacking in secondary structure but they bind specifically to the cognate DNA and are able to remodel chromatin in vitro. Consequently, neither the N-terminal domain nor the C-terminal glutamine-rich regions of the GAGA factor are necessary for chromatin remodelling in vitro.

DNA distortion and multimerization: novel functions of the glutamine-rich domain of GAGA factor

GAGA factor (GAF) is an essential protein in Drosophila melanogaster, important for the transcriptional regulation of numerous genes. The effect of GAF on chromatin structure and promoter function has been the subject of much attention, yet little is known of the actual mechanism and the specific contributions of individual GAF domains to its function. The DNA-binding activity of GAF, as specified by the single zinc finger binding domain (Zn), has been examined in some detail; however, the functions of the POZ/BTB and glutamine domain (Q) remain poorly understood. Here, we report three separate activities of the Q domain of GAF; promoter distortion, single-strand binding, and multimerization. In vitro, GAF binding to the hsp70 promoter produces extended DNase I protection and KMnO4 hypersensitivity. These activities require both the Zn domain and Q domain of GAF, and appear independent of the POZ/BTB domain. GAF also has a single-stranded DNA binding affinity, as does the Q-rich region alone. GAF forms multimers both in vitro and in vivo, and the Q domain itself forms multimers. Protein-protein interactions mediated by the Q domain may, therefore, be at least partially responsible for the multimerization capabilities of GAF. We discuss these findings in the context of their possible function in GAF mediated transcriptional regulation.

trans repression of the human metallothionein IIA gene promoter by PZ120, a novel 120-kilodalton zinc finger protein

Metallothioneins are small, highly conserved, cysteine-rich proteins that bind a variety of metal ions. They are found in virtually all eukaryotic organisms and are regulated primarily at the transcriptional level. In humans, the predominant metallothionein gene is hMTIIA, which accounts for 50% of all metallothioneins expressed in cultured human cells. The hMTIIA promoter is quite complex. In addition to cis-acting DNA sequences that serve as binding sites for trans-acting factors such as Sp1, AP1, AP2, AP4, and the glucocorticoid receptor, the hMTIIA promoter contains eight consensus metal response element sequences. We report here the cloning of a novel zinc finger protein with a molecular mass of 120 kDa (PZ120) that interacts specifically with the hMTIIA transcription initiation site. The PZ120 protein is ubiquitously expressed in most tissues and possesses a conserved poxvirus and zinc finger (POZ) motif previously found in several zinc finger transcription factors. Intriguingly, we found that a region of PZ120 outside of the zinc finger domain can bind specifically to the hMTIIA DNA. Using transient-transfection analysis, we found that PZ120 repressed transcription of the hMTIIA promoter. These results suggest that the hMTIIA gene is regulated by an additional negative regulator that has not been previously described.

RP58 associates with condensed chromatin and mediates a sequence-specific transcriptional repression

An approximately 120-amino acid domain present generally at the NH2 termini, termed the POZ domain, is highly conserved in various proteins with zinc finger DNA binding motifs. We have isolated a novel protein sharing homology with the POZ domain of a number of zinc finger proteins, including the human BCL-6 protein. By using a binding site selection technique (CAST), a high affinity binding site of the protein was determined to be (A/C)ACATCTG(G/T)(A/C), containing the E box core sequence motif. The protein was shown to repress transcription from a promoter linked to its target sequences and was hence named RP58 (Repressor Protein with a predicted molecular mass of 58 kDa). Immunogold electron microscopic study revealed that almost all RP58 is localized in condensed chromatin regions. These observations demonstrate for the first time that a protein mediating a sequence-specific transcriptional repression associates with highly condensed chromatin. We suggest that RP58 may be involved in a molecular link between sequence-specific transcriptional repression and the organization of chromosomes in the nucleus.

BAZF, a novel Bcl6 homolog, functions as a transcriptional repressor

The BCL6 gene, which has been identified from the chromosomal translocation breakpoint in B-cell lymphomas, functions as a sequence-specific transcriptional repressor. We cloned a novel Bcl6-homologous gene, BAZF (encoding Bcl6-associated zinc finger protein). The predicted amino acid sequence of BAZF indicated that the BTB/POZ domain and the five repeats of the Krüppel-like zinc finger motif are located in the NH2-terminal region and the COOH-terminal region, respectively. BAZF associated with Bcl6 at the BTB/POZ domain and localized in the nucleus. Since zinc finger motifs of BAZF were 94% identical to those of Bcl6 at the amino acid level, BAZF bound specifically to the DNA-binding sequence of Bcl6 and functioned as a transcriptional repressor. The repressor activity was associated with both the BTB/POZ domain and the middle portion of BAZF. The 17-amino-acid sequence in the middle portion was completely conserved between BAZF and Bcl6, and the conserved region was critical for the repressor activity. Expression of BAZF mRNA, like that of Bcl6 mRNA, was induced in activated lymphocytes as an immediate-early gene. Therefore, the biochemical character of BAZF is similar to that of Bcl6 although the tissue expression pattern of BAZF differs from that of Bcl6. This is apparently the first report of a gene family whose members encode zinc finger proteins with the BTB/POZ domain.

Genomic cloning and characterization of the mouse POZ/zinc-finger protein ZF5

We isolated genomic DNA containing the entire sequence of ZF5, which was originally identified by its ability to repress the mouse c-myc promoter and which was characterized as one of the POZ (Poxvirus and zinc finger) proteins. The POZ motif is a protein-protein interaction interface found at the N-terminal region of zinc finger proteins. Sequence analysis demonstrated that the ATG translation initiation codon was separately located from the remainder of the coding sequence. Using both RNase protection and primer extension assay, a single major transcription start site was determined. Promoter analysis by transient transfection assay suggested positive autoregulation by ZF5 itself. The ZF5 N-terminal region, including the POZ domain, was required for this regulation. Sp1 also activated the ZF5 promoter and this activity was repressed by addition of ZF5. ZF5 expression was stronger in mouse ovary, lung and brain than in other organs.

Functional characterization of the major late promoter of mouse adenovirus type 1

During the late phase of adenovirus infection, the major late promoter (MLP) controls the regulated expression of the genes that encode most viral structural proteins. Recently, the region of the genome of mouse adenovirus type 1 (MAV-1), predicted to contain the MLP, was sequenced and compared to that of the human virus MLP. The general organization of the transcriptional elements of the putative MAV-1 MLP is similar to that of the human virus counterpart, with some interesting differences. We wished to investigate the function of the predicted MLP of MAV-1 and to determine the significance of the differences found in the MAV-1 MLP. To test the activity of the predicted MLP of MAV-1, both Northern blot and primer extension analyses were performed on intracellular RNA isolated from cells infected with MAV-1. The results show that late RNA can be detected 48 hr postinfection and increases up to 6 days p.i. Primer extension analysis revealed that the major start sites of transcription are 28 and 31 nt downstream of the first T residue of the predicted TATA box. To analyze the functional significance of the predicted transcriptional elements, a transient transfection system, using the firefly luciferase gene controlled by the MAV-1 MLP sequence, was established. The predicted MLP sequence was capable of directing luciferase gene expression, to a level some 60% of that of the human virus MLP. Mutations were created in the inverted CAAT box, the SP1 site, and the TATA box, either singly or in combination. Each single-element mutation causes a marked reduction in luciferase gene expression, with the SP1 mutation showing the greatest effect. Double mutations were even more deficient, suggesting a level of functional redundancy among the various transcriptional elements. Finally, the putative SP1-binding site was examined by gel mobility shift assay and shown to interact with purified SP1 protein specifically, supporting the functional significance of this transcriptional element. These findings contribute to a better understanding of gene expression in MAV-1 and to its development as an appropriate model for the study of the molecular basis of pathogenesis in a natural host animal.