Apolipoprotein B gene regulatory factor-2 (BRF-2) is structurally and immunologically highly related to hepatitis B virus X associated protein-1 (XAP-1)

Hepatitis B virus X protein stimulates viral genome replication via a DDB1-dependent pathway distinct from that leading to cell death

The hepatitis B virus (HBV) X protein (HBx) is essential for virus infection and has been implicated in the development of liver cancer associated with chronic infection. HBx can interact with a number of cellular proteins, and in cell culture, it exhibits pleiotropic activities, among which is its ability to interfere with cell viability and stimulate HBV replication. Previous work has demonstrated that HBx affects cell viability by a mechanism that requires its binding to DDB1, a highly conserved protein implicated in DNA repair and cell cycle regulation. We now show that an interaction with DDB1 is also needed for HBx to stimulate HBV genome replication. Thus, HBx point mutants defective for DDB1 binding fail to complement the low level of replication of an HBx-deficient HBV genome when provided in trans, and one such mutant regains activity when directly fused to DDB1. Furthermore, DDB1 depletion by RNA interference specifically compromises replication of wild-type HBV, indicating that HBx produced from the viral genome also functions in a DDB1-dependent fashion. We also show that HBx in association with DDB1 acts in the nucleus and stimulates HBV replication mainly by enhancing viral mRNA levels, regardless of whether the protein is expressed from the HBV genome itself or supplied in trans. Interestingly, whereas HBx induces cell death in both HepG2 and Huh-7 hepatoma cell lines, it enhances HBV replication only in HepG2 cells, suggesting that the two activities involve distinct DDB1-dependent pathways.

Drosophila damaged DNA-binding protein 1 is an essential factor for development

The damaged DNA-binding protein (DDB) complex, thought to recognize (6-4) photoproducts and other lesions in DNA, has been implicated to have a role in global genomic nucleotide excision repair (NER) and E2F-1-mediated transcription. The complex consists of a heterodimer of p127 (DDB1) and p48 (DDB2), the latter also being known as XPE. We reported previously that in Drosophila expression of the DDB1 (D-DDB1) gene is controlled by the DRE/DREF system, and external injury to DNA is not essential for D-DDB1 function. In the present study of the function of D-DDB1 in a multicellular system, we prepared transgenic flies, which were knocked down for the D-DDB1 gene due to RNA interference (RNAi), and performed immunocytochemistry to ascertain the distribution of D-DDB1 in the eye imaginal disc. It was found to be abundant in the anterior of the morphogenetic furrow (MF). Whole-body overexpression of dsRNA of D-DDB1 in Drosophila using a GAL4-UAS targeted expression system induced melanotic tumors and caused complete lethality. When limited to the eye imaginal disc, a severe rough eye phenotype resulted. Correspondingly, all of the D-DDB1 gene knocked-out flies also died. D-DDB1 therefore appears to be an essential development-associated factor in a multicellular organism.

USF2 is connected to GAAAATATGATA element and associates with androgen receptor-dependent transcriptional regulation in prostate

BACKGROUND

We have previously identified a GAAAATATGATA binding site (pros) of a transcription factor involved in prostatic and androgen-dependent gene regulation. We now purified the potential factors interacting with the pros and characterized their co-operation with the androgen receptor (AR).

METHODS

Sequence-specific DNA affinity chromatography, mass-spectrometry, electromobility shift assays, supershifts, glutathione-S-transferase pull-downs, and transient transfections.

RESULTS

Several proteins bound to the pros site, but only upstream stimulatory factor 2 (USF2) was confirmed to be part of the transcription factor complex. Weak interaction was detected between AR and the transcription factor complex. Physical proximity between the androgen response element (ARE) and the pros was shown to be important for their co-operation. In the presence of pros and androgen, AR achieves its maximal efficiency even at low concentrations.

CONCLUSIONS

The protein complex binding to the GAAAATATGATA site does not have a significant independent function, but may interact with AR if GAAAATATGATA is physically close to the ARE and enhances the transactivation function of AR.

Schizosaccharomyces pombe Ddb1 is functionally linked to the replication checkpoint pathway

Schizosaccharomyces pombe Ddb1 is homologous to the mammalian DDB1 protein, which has been implicated in damaged-DNA recognition and global genomic repair. However, a recent study suggested that the S. pombe Ddb1 is involved in cell division and chromosomal segregation. Here, we provide evidence that the S. pombe Ddb1 is functionally linked to the replication checkpoint control gene cds1. We show that the S. pombe strain lacking ddb1 has slow growth due to delayed replication progression. Flow cytometric analysis shows an extensive heterogeneity in DNA content. Furthermore, the Deltaddb1 strain is hypersensitive to UV irradiation in S phase and is unable to tolerate a prolonged replication block imposed by hydroxyurea. Interestingly, the Deltaddb1 strain exhibits a high level of the Cds1 kinase activity during passage through S phase. Moreover, mutation of the cds1 gene relieves the defects observed in Deltaddb1 strain. The results suggest that many of the defects observed in Deltaddb1 cells are linked to an aberrant activation of Cds1, and that Ddb1 is functionally linked to Cds1.

Characterization of a Schizosaccharomyces pombe strain deleted for a sequence homologue of the human damaged DNA binding 1 (DDB1) gene

Human damaged DNA-binding protein (DDB) is a heterodimer of p48/DDB2 and p127/DDB1 subunits. Mutations in DDB2 are responsible for Xeroderma Pigmentosum group E, but no mutants of mammalian DDB1 have been described. To study DDB1, the Schizosaccharomyces pombe DDB1 sequence homologue (ddb1(+)) was cloned, and a ddb1 deletion strain was constructed. The gene is not essential; however, mutant cells showed a 37% impairment in colony-forming ability, an elongated phenotype, and abnormal nuclei. The ddb1Delta strain was sensitive to UV irradiation, X-rays, methylmethane sulfonate, and thiabendazole, and these sensitivities were compared with those of the well characterized rad13Delta, rhp51Delta, and cds1Delta mutant strains. Ddb1p showed nuclear and nucleolar localization, and the aberrant nuclear structures observed in the ddb1Delta strain suggest a role for Ddb1p in chromosome segregation.

De-etiolated 1 and damaged DNA binding protein 1 interact to regulate Arabidopsis photomorphogenesis

BACKGROUND

Plant development is exquisitely sensitive to light. Seedlings grown in the dark have a developmentally arrested etiolated phenotype, whereas in the light they develop leaves and complete their life cycle. Arabidopsis de-etiolated 1 (det1) mutants develop like light-grown seedlings even when grown in the dark. DET1 encodes a nuclear protein that appears to act downstream from multiple photoreceptors to regulate morphogenesis and gene expression in response to light. However, its function has remained unknown.

RESULTS

We used microarrays to examine defects in transcription in dark-grown det1 seedlings. We found extensive changes in gene expression, including many of the transcriptional responses observed in light-treated wild-type seedlings. We used an epitope-tagging approach to determine the basis of DET1 function. GFP-DET1 rescues the det1 phenotype, is localized to the nucleus, and forms an approximately 350 kDa complex, which is required for full DET1 activity. We affinity-purified the DET1 complex and identified an approximately 120 kDa copurifying protein that is the plant homolog of UV-Damaged DNA Binding Protein 1 (DDB1), a protein implicated in the human disease xeroderma pigmentosa. A null mutation in Arabidopsis DDB1A results in no obvious phenotype on its own, yet it enhances the phenotype of a weak det1 allele.

CONCLUSIONS

DET1 and DDB1 interact both biochemically and genetically. In animal cells, DDB1 interacts with histone acetyltransferase complexes. The DET1/DDB1 complex may regulate gene expression in response to light via recruitment of HAT activity. Thus, DET1, whose sequence is conserved in both animals and plants, may play a direct role in the regulation of many genes.

Xeroderma pigmentosum complementation group E and UV-damaged DNA-binding protein

UV-damaged DNA-binding protein (UV-DDB) is composed of two subunits, DDB1 (p127) and DDB2 (p48). Mutations in the DDB2 gene inactivate UV-DDB in individuals from complementation group E of xeroderma pigmentosum (XP-E), an autosomal recessive disease characterized by sun sensitivity, severe risk for skin cancer and defective nucleotide excision repair. UV-DDB is also deficient in many rodent tissues, exposing a shortcoming in rodent models for cancer. In vitro, UV-DDB binds to cyclobutane pyrimidine dimers (CPDs), 6-4 photoproducts and other DNA lesions, stimulating the excision of CPDs, and to a lesser extent, of 6-4 photoproducts. In vivo, UV-DDB plays an important role in the p53-dependent response of mammalian cells to DNA damage. When cells are exposed to UV, the resulting accumulation of p53 activates DDB2 transcription, which leads to increased levels of UV-DDB. Binding of UV-DDB to CPDs targets these lesions for global genomic repair, suppressing mutations without affecting UV survival. Apparently, cells are able to survive with unrepaired CPDs because of the activity of bypass DNA polymerases. Finally, there is evidence that UV-DDB may have roles in the cell that are distinct from DNA repair.

UV-damaged DNA-binding proteins are targets of CUL-4A-mediated ubiquitination and degradation

Cul-4A, which encodes a member of the cullin family subunit of ubiquitin-protein ligases, is expressed at abnormally high levels in many tumor cells. CUL-4A can physically associate with the damaged DNA-binding protein (DDB), which is composed of two subunits, p125 and p48. DDB binds specifically to UV-damaged DNA and is believed to play a role in DNA repair. We report here that CUL-4A stimulates degradation of p48 through the ubiquitin-proteasome pathway, resulting in an overall decrease in UV-damaged DNA binding activity. The R273H mutant of p48 identified from a xeroderma pigmentosium (group E) patient is not subjected to CUL-4A-mediated proteolysis, consistent with its inability to bind CUL-4A. p125 is also an unstable protein, and its ubiquitination is stimulated by CUL-4A. However, the abundance of p125 is not dramatically altered by Cul-4A overexpression. UV irradiation inhibits p125 degradation, which is temporally coupled to the UV-induced translocation of p125 from the cytoplasm into the nucleus. CUL-4A is localized primarily in the cytoplasm. These findings identify DDB subunits as the first substrates of the CUL-4A ubiquitination machinery and suggest that abnormal expression of Cul-4A results in reduced p48 levels, thus impairing the ability of DDB in lesion recognition and DNA repair in tumor cells.

The xeroderma pigmentosum group E gene product DDB2 is a specific target of cullin 4A in mammalian cells

The damaged-DNA binding protein DDB consists of two subunits, DDB1 (127 kDa) and DDB2 (48 kDa). Mutations in the DDB2 subunit have been detected in patients suffering from the repair deficiency disease xeroderma pigmentosum (group E). In addition, recent studies suggested a role for DDB2 in global genomic repair. DDB2 also exhibits transcriptional activity. We showed that expression of DDB1 and DDB2 stimulated the activity of the cell cycle regulatory transcription factor E2F1. Here we show that DDB2 is a cell cycle-regulated protein. It is present at a low level in growth-arrested primary fibroblasts, and after release the level peaks at the G(1)/S boundary. The cell cycle regulation of DDB2 involves posttranscriptional mechanisms. Moreover, we find that an inhibitor of 26S proteasome increases the level of DDB2, suggesting that it is regulated by the ubiquitin-proteasome pathway. Our previous study indicated that the cullin family protein Cul-4A associates with the DDB2 subunit. Because cullins are involved in the ubiquitin-proteasome pathway, we investigated the role of Cul-4A in regulating DDB2. Here we show that DDB2 is a specific target of Cul-4A. Coexpression of Cul-4A, but not Cul-1 or other highly related cullins, increases the ubiquitination and the decay rate of DDB2. A naturally occurring mutant of DDB2 (2RO), which does not bind Cul-4A, is not affected by coexpression of Cul-4A. Studies presented here identify a specific function of the Cul-4A gene, which is amplified and overexpressed in breast cancers.

The p48 subunit of the damaged-DNA binding protein DDB associates with the CBP/p300 family of histone acetyltransferase

DDB has been implicated in DNA repair as well as transcription. Mutations in DDB have been correlated with the repair-deficiency disease, xeroderma pigmentosum group E (XP-E). The XP-E cells exhibit deficiencies in global genomic repair, suggesting a role for DDB in that process. DDB also possesses a transcription stimulatory activity. We showed that DDB could function as a transcriptional partner of E2F1. But the mechanism by which DDB stimulates E2F-regulated transcription or carry out its DNA repair function is not understood. To investigate the mechanisms, we looked for nuclear proteins that interact with DDB. Here we show that DDB associates with the CBP/p300 family of proteins, in vivo and in vitro. We suggest that DDB participates in global genomic repair by recruiting CBP/p300 to the damaged-chromatin. It is possible that the histone acetyltransferase activities of the CBP/p300 proteins induce chromatin remodeling at the damaged-sites to allow recruitment of the repair complexes. The observation offers insights into both transcription and repair functions of DDB.

Nuclear transport of human DDB protein induced by ultraviolet light

Human damage-specific DNA-binding (DDB) protein can be purified as a heterodimer (p48 and p127) that binds to DNA damaged by ultraviolet light. We report here the effects of UV irradiation on the cellular localization of each DDB subunit as a function of time using green fluorescent fusion proteins in three diploid fibroblast strains: repair-proficient IMR-90 and two repair-deficient xeroderma pigmentosum group E strains (XP95TO and XP3RO). Although p48 remained in the nucleus after UV irradiation, a dynamic nuclear accumulation of p127 from the cytoplasm was found after 24 h. In IMR-90 cells, the nuclear localization of p127 corresponded to the up-regulation of p48 mRNA and protein levels and of DDB activity. XP3RO cells showed delayed but similar kinetics with less transport, whereas XP95TO cells appeared to have different kinetics, suggesting that these cells exhibit different defects in p127 translocation. We propose that p48 might act as the transporter for nuclear entry of p127 but that a third factor might be necessary for efficient transportation.

Expression of hepatitis B virus X protein does not alter the accumulation of spontaneous mutations in transgenic mice

Chronic infection with hepatitis B virus (HBV) is one of the major etiological factors in the development of human hepatocellular carcinoma. Transgenic mice that express the HBV X protein (HBx) have previously been shown to be more sensitive to the effects of hepatocarcinogens, although the mechanism for this cofactor role remains unknown. The ability of HBx to inhibit DNA repair in transiently transfected cell lines suggests one possible pathway. In the present study, primary hepatocytes isolated from transgenic mice that possess the HBV X gene under the control of the human alpha-1-antitrypsin regulatory region (ATX mice) were found to be deficient in their ability to conduct unscheduled DNA synthesis in response to UV-induced DNA damage. In order to measure the impact of HBx expression on DNA repair in vivo, double-transgenic mice that express HBx and possess a bacteriophage lambda transgene were sacrificed at 30, 90, and 240 days of age. Mutation frequency was determined for high-molecular-weight liver DNA of ATX and control mice by functional analysis of the lambda transgene. Expression of HBx did not significantly increase the accumulation of spontaneous mutations. These results are consistent with previous studies of HBx transgenic mice in which no effect of HBx on liver histology was apparent. This new animal model provides a powerful system in which to investigate the in vivo cooperation between HBx expression and environmental carcinogens.

Dissociation of DDB1-binding and transactivation properties of the hepatitis B virus X protein

The hepatitis B virus (HBV) X protein (HBx) is a transactivator encoded by mammalian hepadnaviruses, and is thought to stimulate transcription by interacting with one or more host cell factors. Numerous cellular proteins have been reported to interact with HBx including a component of the nucleotide excision repair complex called ultraviolet damaged DNA binding (UV-DDB, or DDB1) protein. Recent studies have identified a role for DDB1 in transcription, raising the possibility that HBx may acquire its broad transcriptional properties by interacting with DDB1. A panel of HBx mutant proteins, some of which no longer bind to DDB1, was used to test this hypothesis. Plasmid DNAs encoding HBx wildtype and mutant derivatives were transfected into HepG2 cells, and their ability to transactivate a cotransfected reporter plasmid tested. Results from the transactivation assays in HepG2 cells were then compared with data obtained from HBx-DDB1 binding studies performed in yeast. Several HBx mutant proteins unable to bind DDB1 remained competent for transactivation, indicating that HBx binding to DDB1 is not required for HBx transactivation of the ETS1 promoter. It remains possible that a subset of HBx transactivation function targets an as yet undefined DDB1-specific pathway.

Putative role of hepatitis B virus X protein in hepatocarcinogenesis: effects on apoptosis, DNA repair, mitogen-activated protein kinase and JAK/STAT pathways

Chronic infection with hepatitis B virus (HBV) is a major risk factor for the development of hepatocellular carcinoma (HCC). The pathogenesis of HBV-induced malignant transformation is, however, incompletely understood. HBx, the protein encoded by the X open reading frame, is a transcriptional activator that has been implicated in hepatocarcinogenesis. HBx inhibits the function of the tumour suppressor protein p53 in what is thought to be an early event in hepatocyte transformation before the later accumulation of inactivating p53 point mutations. HBx inhibits apoptosis but also exerts pro-apoptotic effects. The effects of HBx on apoptosis may be important not only for the development of HCC but also for the establishment of HBV infection. Further implication of HBx in hepatocyte transformation has been the demonstration that it inhibits the repair of damaged hepatocyte DNA. This effect may be mediated by interaction with p53 or through binding to the damaged DNA binding protein (DDB), which plays an accessory role in nucleotide excision repair. In addition, HBx activates cell signalling cascades involving mitogen-activated protein kinase (MAPK) and Janus family tyrosine kinases (JAK)/signal transducer and activators of transcription (STAT) pathways. The implications of these modulating effects of HBx are not fully understood, but they are likely to have wide-ranging effects on hepatocyte proliferation, apoptosis and the regulation of cell growth checkpoints. The cellular functions ascribed to HBx are unusually diverse, and defining the biologically important role of HBx during HBV replication will go some way to understanding the sequelae of chronic HBV infection.

Studies of the murine DDB1 and DDB2 genes

Human DDB (Damaged DNA Binding protein) is a heterodimer of 48 and 127kDa subunits whose activity is absent from cell strains derived from a subset of Xeroderma Pigmentosum (XP) complementation group E individuals (Ddb(-)) [Keeney, S., Wein, H., and Linn, S., (1992). Mut. Res. 273, 49-56]. Whereas in vivo DNA repair appears to be compromised in both Ddb(-) and Ddb(+) XPE cells, DDB activity is not necessary for nucleotide excision repair (NER) in vitro. In this study, the presence of a specific UV-damaged DNA binding activity in mouse cell-free extracts that is comparable to the activity observed in HeLa cells was demonstrated. The mouse DDB2 cDNA, coding for DDB p48 subunit, was cloned and the partial genomic structure of DDB2 was obtained. A search of current databases revealed amino acid sequences of mouse and Drosophila predicted p127 homologues, but not of a Drosophila p48 homologue. The alignment of these higher eukaryotic p127 sequences uncovered the presence of three highly conserved domains in the p127 polypeptides which we hypothesize could function in DNA binding, transcription-transactivation, and protein-protein interaction, respectively.

Cullin 4A associates with the UV-damaged DNA-binding protein DDB

The damaged DNA-binding protein (DDB) is believed to be involved in DNA repair, and it has been linked to the repair deficiency disease xeroderma pigmentosum. DDB also exhibits transcriptional activities. DDB binds to the activation domain of E2F1 and stimulates E2F1-activated transcription. Here we provide evidence that DDB or DDB-associated proteins are targets of cullin 4A (CUL-4A). CUL-4A is a member of the cullin family of proteins, which are believed to be ubiquitin-protein isopeptide ligases (type E3). The CUL-4A gene has been shown to be amplified and up-regulated in breast carcinomas. In this study, we identify CUL-4A as one of the DDB-associated proteins. CUL-4A co-immunoprecipitates with DDB, but not with a naturally occurring mutant of DDB. Moreover, CUL-4A in HeLa nuclear extracts co-purifies with DDB, suggesting they are parts of the same complex. The observation provides insights how CUL-4A, through an interaction with DDB, might be playing a role in the development of breast carcinomas.

Purified apolipoprotein B gene regulatory factor-3 is DNA topoisomerase I

Hepatic cell-specific expression of the human apolipoprotein B (apoB) gene is controlled by at least four cis-acting elements located between positions -128 and +122 [Chuang, S. S., & Das, H. K. (1996) Biochem. Biophys. Res. Commun. 220, 553-562]. A negative cis-acting element (+20 to +40) is located in the first nontranslated exon of the human apoB gene, and apoB regulatory factor-3 (BRF-3) interacts with this. In this paper, we report the purification and characterization of BRF-3 from rat liver nuclear extracts. BRF-3 has been purified to apparent homogeneity by DEAE-cellulose, heparin-agarose, and DNA-specific affinity chromatography. Purified BRF-3 produced two polypeptide bands with apparent molecular masses of 70 kDa and 67 kDa in SDS/PAGE as detected by silver staining. Both 70-kDa and 67-kDa proteins have been found to hybridize specifically with labeled double-stranded oligonucleotide containing BRF-3 binding site in a South-Western blot. Double-stranded oligonucleotide containing mutations in the BRF-3 binding site was found to abolish DNA binding by these two proteins. Amino acid sequences of tryptic peptides derived from affinity purified 70-kDa and 67-kDa rat BRF-3 proteins were found to have 100% sequence homologies with DNA topoisomerase I. These data suggest that the 70-kDa and 67-kDa forms of BRF-3 are derived by proteolytic cleavage of topoisomerase I, and therefore, topoisomerase I may play an important role in transcriptional regulation of apoB.

The naturally occurring mutants of DDB are impaired in stimulating nuclear import of the p125 subunit and E2F1-activated transcription

The human UV-damaged-DNA binding protein DDB has been linked to the repair deficiency disease xeroderma pigmentosum group E (XP-E), because a subset of XP-E patients lack the damaged-DNA binding function of DDB. Moreover, the microinjection of purified DDB complements the repair deficiency in XP-E cells lacking DDB. Two naturally occurring XP-E mutations of DDB, 82TO and 2RO, have been characterized. They have single amino acid substitutions (K244E and R273H) within the WD motif of the p48 subunit of DDB, and the mutated proteins lack the damaged-DNA binding activity. In this report, we describe a new function of the p48 subunit of DDB, which reveals additional defects in the function of the XP-E mutants. We show that when the subunits of DDB were expressed individually, p48 localized in the nucleus and p125 localized in the cytoplasm. The coexpression of p125 with p48 resulted in an increased accumulation of p125 in the nucleus, indicating that p48 plays a critical role in the nuclear localization of p125. The mutant forms of p48, 2RO and 82TO, are deficient in stimulating the nuclear accumulation of the p125 subunit of DDB. In addition, the mutant 2RO fails to form a stable complex with the p125 subunit of DDB. Our previous studies indicated that DDB can associate with the transcription factor E2F1 and can function as a transcriptional partner of E2F1. Here we show that the two mutants, while they associate with E2F1 as efficiently as wild-type p48, are severely impaired in stimulating E2F1-activated transcription. This is consistent with our observation that both subunits of DDB are required to stimulate E2F1-activated transcription. The results provide insights into the functions of the subunits of DDB and suggest a possible link between the role of DDB in E2F1-activated transcription and the repair deficiency disease XP-E.

A 127-kDa protein (UV-DDB) binds to the cytoplasmic domain of the Alzheimer's amyloid precursor protein

Alzheimer amyloid precursor protein (APP) is an integral membrane protein with a short cytoplasmic domain of 47 amino acids. It is hoped that identification of proteins that interact with the cytoplasmic domain will provide new insights into the physiological function of APP and, in turn, into the pathogenesis of Alzheimer's disease. To identify proteins that interact with the cytoplasmic domain of APP, we employed affinity chromatography using an immobilized synthetic peptide corresponding to residues 645-694 of APP695 and identified a protein of approximately 130 kDa in rat brain cytosol. Amino acid sequencing of the protein revealed the protein to be a rat homologue of monkey UV-DDB (UV-damaged DNA-binding protein, calculated molecular mass of 127 kDa). UV-DDB/p127 co-immunoprecipitated with APP using an anti-APP antibody from PC12 cell lysates. APP also co-immunoprecipitated with UV-DDB/p127 using an anti-UV-DDB/p127 antibody. These results indicate that UV-DDB/p127, which is present in the cytosolic fraction, forms a complex with APP through its cytoplasmic domain. In vitro binding experiments using a glutathione S-transferase-APP cytoplasmic domain fusion protein and several mutants indicated that the YENPTY motif within the APP cytoplasmic domain, which is important in the internalization of APP and amyloid beta protein secretion, may be involved in the interaction between UV-DDB/p127 and APP.

A single in vitro point mutation in the first non-translated exon silences transcription of the human apolipoprotein B gene in HepG2 cells

Hepatic cell-specific expression of the human apolipoprotein B (apoB) gene is controlled by at least four cis-acting elements located within the -128 to +122 promoter region (S.S. Chuang, H.K. Das, Identification of trans-acting factors that interact with cis-acting elements present in the first non-translated exon of the human apolipoprotein B gene, Biochem. Biophys. Res. Commun. 220 (1996) 553-562). Two cis-acting positive elements (-104 to -85; -84 to -60) are located upstream from the start of transcription. A negative element (+20 to +40) and a strong positive element (+43 to +53) are located in the first non-translated exon of the human apolipoprotein B gene. Trans-acting factors BRF-2, BRF-1, BRF-3, and BRF-4 interact with the above four cis-acting elements respectively. In this study, we examine the roles of the upstream positive elements -104 to -85 and -84 to -60 in modulating transcriptional regulation of the apoB gene by downstream elements +20 to +40 and +43 to +53. Using in vitro mutagenesis and transient transfection experiments in HepG2 cells, the cis-acting element -84 to -60 has been found to be absolutely necessary for the function of the upstream element -104 to -85 and downstream elements +20 to +40 and +43 to +53. In vitro mutagenesis of the downstream positive element +43 to +53 and transfection of the mutant promoter constructs in HepG2 cells reveal that nucleotide G at position +51 is essential for the strong positive activity of the element +43 to +53. A single substitution point mutation of nucleotide G to either A or T at position +51 reduces apolipoprotein B gene transcription substantially in HepG2 cells. These results suggest that a single substitution mutation in vivo, of nucleotide G to either A or T at position +51 in the downstream positive promoter element +43 to +53 may potentially cause hypobetalipoproteinemia, a heterozygous from of an autosomal-dominant disorder.

A link between secretion and pre-mRNA processing defects in Saccharomyces cerevisiae and the identification of a novel splicing gene, RSE1

Secretory proteins in eukaryotic cells are transported to the cell surface via the endoplasmic reticulum (ER) and the Golgi apparatus by membrane-bounded vesicles. We screened a collection of temperature-sensitive mutants of Saccharomyces cerevisiae for defects in ER-to-Golgi transport. Two of the genes identified in this screen were PRP2, which encodes a known pre-mRNA splicing factor, and RSE1, a novel gene that we show to be important for pre-mRNA splicing. Both prp2-13 and rse1-1 mutants accumulate the ER forms of invertase and the vacuolar protease CPY at restrictive temperature. The secretion defect in each mutant can be suppressed by increasing the amount of SAR1, which encodes a small GTPase essential for COPII vesicle formation from the ER, or by deleting the intron from the SAR1 gene. These data indicate that a failure to splice SAR1 pre-mRNA is the specific cause of the secretion defects in prp2-13 and rse1-1. Moreover, these data imply that Sar1p is a limiting component of the ER-to-Golgi transport machinery and suggest a way that secretory pathway function might be coordinated with the amount of gene expression in a cell.

The V protein of the paramyxovirus SV5 interacts with damage-specific DNA binding protein

The simian parainfluenza virus 5 (SV5) V/P gene encodes two proteins: V and the phosphoprotein P. The V and P proteins are amino coterminal for 164 residues, but they have unique carboxyl termini. The unique carboxyl terminus of V contains seven cysteine residues, resembles a zinc finger, and binds two atoms of zinc. In a glutathione-S-transferase (GST)-fusion protein selection of cell lysate assay, the GST-V protein was found to interact with the 127-kDa subunit (DDB1) of the damage-specific DNA binding protein (DDB) [also known as UV-damaged DNA binding protein (UV-DDB), xeroderma pigmentosum group E binding factor (XPE-BF), and the hepatitis B virus X-associated protein 1 (XAP-1)]. A reciprocal GST-DDB1 fusion protein selection assay of SV5-infected cell lysates showed that DDB1 and V interact, and it was found that V and DDB1 could be coimmunoprecipitated from SV5-infected cells or from cells expressing V and DDB1 using the vaccinia virus T7 expression system. The interaction of V and DDB1 involves the carboxyl-terminal domain of V in that either deletion of the V carboxyl-terminal domain or substitution of the cysteine residues (C189, C193, C205, C207, C210, C214, and C217) in the zinc-binding domain with alanine was able to disrupt binding to DDB1. The V proteins of the mumps virus, human parainfluenza virus 2 (hPIV2), and measles virus have also been found to interact with DDB1 in GST-fusion protein selection assays using in vitro transcribed and translated DDB1.

Rapid changes of nucleotide excision repair gene expression following UV-irradiation and cisplatin treatment of Dictyostelium discoideum

Organisms use different mechanisms to detect and repair different types of DNA damage, and different species vary in their sensitivity to DNA damaging agents. The cellular slime mold Dictyostelium discoideum has long been recognized for its unusual resistance to UV and ionizing radiation. We have recently cloned three nucleotide excision repair (NER) genes from Dictyostelium , the rep B, D and E genes (the homologs of the human xeroderma pigmentosum group B, D and E genes, respectively). Each of these genes has a unique pattern of expression during the multicellular development of this organism. We have now examined the response of these genes to DNA damage. The rep B and D DNA helicase genes are rapidly and transiently induced in a dose dependent manner following exposure to both UV-light and the widely used chemotherapeutic agent cisplatin. Interestingly, the rep E mRNA level is repressed by UV but not by cisplatin, implying unique signal transduction pathways for recognizing and repairing different types of damage. Cells from all stages of growth and development display the same pattern of NER gene expression following exposure to UV-light. These results suggest that the response to UV is independent of DNA replication, and that all the factors necessary for rapid transcription of these NER genes are either stable throughout development, or are continuously synthesized. It is significant that the up-regulation of the rep B and D genes in response to UV and chemical damage has not been observed to occur in cells from other species. We suggest that this rapid expression of NER genes is at least in part responsible for the unusual resistance of Dictyostelium to DNA damage.

Hepatitis B virus X protein interferes with cellular DNA repair

The hepatitis B virus X protein (HBx) is a broadly acting transactivator implicated in the development of liver cancer. Recently, HBx has been reported to interact with several different cellular proteins, including our report of its binding to XAP-1, the human homolog of the simian repair protein UVDDB. In the present study, several HBx mutants were used to localize the minimal domain of HBx required for binding to XAP-1/UVDDB to amino acids 55 to 101. The normal function of XAP-1/UVDDB is thought to involve binding to damaged DNA, the first step in nucleotide excision repair (NER); therefore, we hypothesized that this interaction may affect the cell's capacity to correct lesions in the genome. When tested in two independent assays that measure NER (unscheduled DNA synthesis and host cell reactivation), the expression of HBx significantly inhibited the ability of cells to repair damaged DNA. Under the assay conditions, HBx was expressed at a level similar to that previously observed during natural viral infection and was able to transactivate several target reporter genes. These results are consistent with a model in which HBx acts as a cofactor in hepatocarcinogenesis by preventing the cell from efficiently repairing damaged DNA, thus leading to an accumulation of DNA mutations and, eventually, cancer. An adverse effect on cellular DNA repair processes suggests a new mechanism by which a tumor-associated virus might contribute to carcinogenesis.