Ubinuclein, a novel nuclear protein interacting with cellular and viral transcription factors.

The major target tissues for Epstein-Barr virus (EBV) infection are B lymphocytes and epithelial cells of the oropharyngeal zone. The product of the EBV BZLF1 early gene, EB1, a member of the basic leucine-zipper family of transcription factors, interacts with both viral and cellular promoters and transcription factors, modulating the reactivation of latent EBV infection. Here, we characterize a novel cellular protein interacting with the basic domains of EB1 and c-Jun, and competing of their binding to the AP1 consensus site. The transcript is present in a wide variety of human adult, fetal, and tumor tissues, and the protein is detected in the nuclei throughout the human epidermis and as either grainy or punctuate nuclear staining in the cultured keratinocytes. The overexpression of tagged cDNA constructs in keratinocytes revealed that the NH(2) terminus is essential for the nuclear localization, while the central domain is responsible for the interaction with EB1 and for the phenotype of transfected keratinocytes similar to terminal differentiation. The gene was identified in tail-to-tail orientation with the periplakin gene (PPL) in human chromosome 16p13.3 and in a syntenic region in mouse chromosome 16. We designated this novel ubiquitously expressed nuclear protein as ubinuclein and the corresponding gene as UBN1.

ERK6, a mitogen-activated protein kinase involved in C2C12 myoblast differentiation.

ERK6, a mitogen-activated protein (MAP) kinase-related serine/threonine kinase, is highly expressed in human skeletal muscle and appears to function as a signal transducer during differentiation of myoblasts to myotubes. In transfected 293 cells, activation of the 45-kDa enzyme results in tyrosine-phosphorylated 46- and 56-kDa forms, which phosphorylate myelin basic protein. Overexpression of wild-type ERK6 or the inactive mutant Y185F has no effect on fibroblast and myoblast proliferation, but it enhances or inhibits C2C12 cell differentiation to myotubes, respectively. Our findings suggest ERK6 to be a tissue-specific, differentiation signal-transducing factor that is connected to phosphotyrosine-mediated signaling pathways distinct from those activating other members of the MAP kinase family such as LRK1 and ERK2.

Cloning and characterization of two related serotonergic receptors from the brain and the reproductive system of Aplysia that activate phospholipase C.

Serotonin (5-HT) plays important roles in various behavioral and physiological processes in Aplysia californica. These include feeding, locomotion, circadian rhythm, learning and memory, synaptic plasticity, and synaptic growth. Serotonin modulates these various functions by interacting with different 5-HT receptor subtypes that are coupled to various second-messenger systems. We report here the isolation and characterization of the first two serotonergic receptors from Aplysia californica, Ap5-HTB1 and Ap5-HTB2, using a strategy based on the amino acid sequence homology among G-protein-coupled biogenic amine receptors. Ap5-HTB1 and Ap5-HTB2 are both intronless and highly homologous to each other, sharing 79.5% sequence identity at the amino acid level. Sequence comparison reveals that these receptors are 33.1 to 23.3% identical to the following 5-HT receptors: 5-HTdro1 > 5-HT6 > 5-HTlym > mouse 5-HT1B > 5-HTdro2A > mouse 5-HT7 > rat 5-HT2A. Both Ap5-HTB1 and Ap5-HTB2 encode functional 5-HT receptors. When expressed in cultured cells, these receptors stimulate phospholipase C in response to 5-HT in a dose-dependent manner. This stimulation can be blocked by specific 5-HT receptor antagonists. Using RT-PCR and Western blot analysis, we have detected these receptors in the CNS (Ap5-HTB2) and in the reproductive system (Ap5-HTB1). The nucleotide sequences of Ap5-HTB1 and Ap5-HTB2 were submitted to GenBank; the accession numbers are L43557 and L43558, respectively.

Enzyme-linked immunosorbent assay for antibodies to ZEBRA, an Epstein-Barr trans-activator.

Patients with AIDS and AIDS-related complex often show symptoms of Epstein-Barr virus (EBV) reactivation. Several EBV-encoded trans-acting factors activate the EBV lytic cycle, and one, called ZEBRA (BamHI Z EBV replication activator), controls the switch of EBV from a latent to a productive cycle. We describe here a simple ELISA test using a bacterially expressed ZEBRA protein as antigen. Utilizing this technique, we evaluated the humoral response to ZEBRA antigen in 38 patients with nasopharyngeal carcinoma and 134 subjects with an asymptomatic HIV 1 infection. The control group consisted of 40 healthy adult blood donors.

The cellular oncogene c-myb can interact synergistically with the Epstein-Barr virus BZLF1 transactivator in lymphoid cells.

Regulation of replicative functions in the Epstein-Barr virus (EBV) genome is mediated through activation of a virally encoded transcription factor, Z (BZLF1). We have shown that the Z gene product, which binds to AP-1 sites as a homodimer and has sequence similarity to c-Fos, can efficiently activate the EBV early promoter, BMRF1, in certain cell types (i.e., HeLa cells) but not others (i.e., Jurkat cells). Here we demonstrate that the c-myb proto-oncogene product, which is itself a DNA-binding protein and transcriptional transactivator, can interact synergistically with Z in activating the BMRF1 promoter in Jurkat cells (a T-cell line) or Raji cells (an EBV-positive B-cell), whereas the c-myb gene product by itself has little effect. The simian virus 40 early promoter is also synergistically activated by the Z/c-myb combination. Synergistic transactivation of the BMRF1 promoter by the Z/c-myb combination appears to involve direct binding by the Z protein but not the c-myb protein. A 30-bp sequence in the BMRF1 promoter which contains a Z binding site (a consensus AP-1 site) is sufficient to transfer high-level lymphoid-specific responsiveness to the Z/c-myb combination to a heterologous promoter. That the c-myb oncogene product can interact synergistically with an EBV-encoded member of the leucine zipper protein family suggests c-myb is likely to engage in similar interactions with cellularly encoded transcription factors.

Domains of the Epstein-Barr virus (EBV) transcription factor R required for dimerization, DNA binding and activation.

In cells latently infected with EBV, the switch from latency to a productive infection is linked to the expression of two transcriptional activators, the upstream element factor EB1 and the enhancer factor R. R activates by interacting directly with specific DNA sequences called RREs (R Responsive Elements). Each binding site covers about 18 bp, where R simultaneously contacts two core sequences separated by 5 to 7 bp (1). Here we show that R binds in vitro as a homodimer to an RRE, and that stable homodimers can also form in solution in the absence of DNA. By functional analysis of deletion and insertion mutants of R, we have localized the DNA binding region within the 280 N-terminal amino acids and the dimerization region within the 232 N-terminal amino acids. As no obvious homologies were detected with other known DNA binding or dimerization motifs, R could contain novel protein structures mediating these functions. The transcriptional activation domain has been located in the C-terminal half of the protein. This domain contains two regions with structures already identified in other transcription factors: one region is rich in proline, the other rich in acidic residues.

Transcriptional interference between the EBV transcription factors EB1 and R: both DNA-binding and activation domains of EB1 are required.

The switch from latency to a productive infection in EBV-infected B cells is linked to the expression of two viral sequence-specific DNA-binding transcription factors called EB1 and R. EB1 shares sequence homologies with the bZIP family of proteins in the basic region required for specific DNA interaction. Here, we provide evidence that EB1 and R can synergistically activate specific transcription, and that overexpressed, unbound EB1, represses the R-induced transcription ('squelching'). In order to identify the EB1 domains involved in transcriptional activation, transcriptional synergy and transcriptional repression, we performed extensive mutagenesis of the EB1 protein. Results show that five segments (region 1 to region 5), localized at the N-terminus of EB1 exhibit characteristics of activating domains, since they are required for full transcriptional activity, without obvious role in DNA-binding, or the nuclear localization. Two domains rich in basic amino-acids are required for the nuclear localization of EB1. One domain is within the basic region B, also necessary for specific and stable interaction between EB1 and its cognate DNA sequences. It is also shown that the 'activation' domain, and more surprisingly the DNA-binding domain of EB1, may interact with a factor(s), essential for R-induced activation, and probably required for synergy between EB1 and R.

Epstein-Barr virus bicistronic mRNAs generated by facultative splicing code for two transcriptional trans-activators.

The Epstein-Barr virus (EBV) genome codes for several transcriptional trans-activators. One of them, the BZLF1 open reading frame (ORF)-encoded product EB1, is able to induce the productive cycle in infected B cells. From the cloning and characterization of full-length cDNAs, we found that EB1 could be made from three overlapping messenger RNAs expressed under the control of two different promoters that we call P1 and P2. The first mRNA, 1 kb long, is made from the P1 promoter and codes for EB1 alone. The two other mRNAs, respectively 3 and 4 kb long and made by facultative splicing, are bicistronic mRNAs. They code not only for the trans-activator EB1 but also for a second EBV transcriptional trans-activator R, encoded by the BRLF1 ORF. In effect, authentic EB1 and R proteins are expressed from the 3 and 4 kb long cDNAs as demonstrated by identification of the proteins with specific antisera. In addition, EB1 and R expressed from the 3 and 4 kb cDNAs activate transcription from their specific targets in the EBV early promoter DR.