A human nuclear-localized chaperone that regulates dimerization, DNA binding, and transcriptional activity of bZIP proteins

Growth factors can activate ATF2 via a two-step mechanism: phosphorylation of Thr71 through the Ras-MEK-ERK pathway and of Thr69 through RalGDS-Src-p38

Transcription factor ATF2 regulates gene expression in response to environmental changes. Upon exposure to cellular stresses, the mitogen-activated proteinkinase (MAPK) cascades including SAPK/JNK and p38 can enhance ATF2's transactivating function through phosphorylation of Thr69 and Thr71. How ever, the mechanism of ATF2 activation by growth factors that are poor activators of JNK and p38 is still elusive. Here, we show that in fibroblasts, insulin, epidermal growth factor (EGF) and serum activate ATF2 via a so far unknown two-step mechanism involving two distinct Ras effector pathways: the Raf-MEK-ERK pathway induces phosphorylation of ATF2 Thr71, whereas subsequent ATF2 Thr69 phosphorylation requires the Ral-RalGDS-Src-p38 pathway. Cooperation between ERK and p38 was found to be essential for ATF2 activation by these mitogens; the activity of p38 and JNK/SAPK in growth factor-stimulated fibroblasts is insufficient to phosphorylate ATF2 Thr71 or Thr69 + 71 significantly by themselves, while ERK cannot dual phosphorylate ATF2 Thr69 + 71 efficiently. These results reveal a so far unknown mechanism by which distinct MAPK pathways and Ras effector pathways cooperate to activate a transcription factor.

Characterization of the chicken L-Maf, MafB and c-Maf in crystallin gene regulation and lens differentiation

BACKGROUND

Members of the Maf family, including L-Maf, MafB and c-Maf, are "basic region/leucine zipper" (bZIP) transcription factors. Maf proteins contain a highly conserved acidic transactivation domain (AD), and a bZIP region that mediates DNA-binding activity. The hinge region between AD and bZIP varies considerably in length between different proteins. Recent studies reveal that L-Maf, c-Maf and MafB play key roles in vertebrate lens development.

RESULTS

We investigated the transactivation activity of individual factors in culture cells to analyse their specific functions. In transient transfection assays with a reporter gene containing Maf responsive elements, MafB and c-Maf activated higher levels of the reporter gene than L-Maf. However, L-Maf transactivated the alphaA-crystallin promoter as effectively as MafB and c-Maf, and induced the expression of the endogenous delta-crystallin gene more efficiently than the other two proteins. Domain-swapping experiments reveal that the bZIP region of MafB takes part in strong transcriptional activity, while the acidic and hinge regions (AH) of c-Maf collectively serve as a strong transactivation domain. The AH region of L-Maf (but not c-Maf) conferred transactivation activity to induce delta-crystallin gene expression.

CONCLUSIONS

These results suggest that despite their similar DNA binding properties, L-Maf, MafB and c-Maf regulate different sets of target genes by complex interactions with multiple factors that recognize cis-elements in promoters. The AH region of L-Maf has a distinct role in inducing endogenous delta-crystallin gene.

An intron in the YRA1 gene is required to control Yra1 protein expression and mRNA export in yeast

Yra1p is an essential and conserved mRNA export factor in yeast. Strikingly, removal of the intron from YRA1 causes a dominant-negative growth phenotype and a concomitant inhibition of mRNA export. However, both defects are neutralized by replacement of the intron of YRA1 by a different intron. Significantly, Yra1p is overproduced in yeast when expressed from its intronless gene, but Yra1p levels are the same as the wild type when expressed from an intron-containing YRA1 gene. Thus, an intron in YRA1 controls Yra1p expression and mRNA export.

A conserved mRNA export machinery coupled to pre-mRNA splicing

Recent advances have led to a new understanding of how mRNAs are exported from the nucleus to the cytoplasm. This process requires a heterodimeric mRNA export receptor that is part of an elaborate machinery conserved from yeast to humans. Export of mRNAs is coupled to upstream steps in gene expression, such as pre-mRNA splicing, and to downstream events, including nonsense-mediated decay.

mRNA export: travelling with DEAD box proteins

Recent studies have shown that the putative RNA helicase protein UAP56 and its yeast homologue Sub2p are not only involved in pre-mRNA splicing but also required for the export of mRNA out of the nucleus, even if the mRNA is encoded by an intron-less gene.

Plant bZIP G-box binding factors. Modular structure and activation mechanisms

In this review we sum-up the knowledge about bZIP G-box binding factors (GBFs), which possess an N-terminal, proline-rich domain. The GBF has been one of the most extensively studied transcription factor family. Based on protein sequence homology with yeast and animal basic leucine-zipper (bZIP) transcription factors, bioinformatic studies have identified their main structural domains (proline-rich, basic and leucine-zipper), which have been further functionally characterized by in vitro and in vivo experiments. Recent reports have led to the discovery of other GBF-specific short amino-acid sequences that may take part in the regulation of gene expression by post-transcriptional modifications or interaction with other proteins such as bZIP enhancing factors or plant 14-3-3-like proteins. We identified a GBF region, called the 'multifunctional mosaic region', that may be implicated in cytoplasmic retention, translocation to the nucleus and regulation of transcription. We also identified many conserved protein motifs that suggest a modular structure for GBFs. At the whole plant level, GBFs have been shown to be involved in developmental and physiological processes in response to major cues such as light or hormones. Nevertheless, it remains difficult to assign a physiological role to a particular GBF protein modular structure. Finally, bringing together these different aspects of GBF studies we propose a model describing the puzzling transduction pathway involving GBFs from cytoplasmic events of signal transduction to the regulation of gene expression in the nucleus.

Splicing factor Sub2p is required for nuclear mRNA export through its interaction with Yra1p

The yeast nuclear protein Yra1p is an essential export factor for mRNA. Yra1p interacts directly with the mRNA transport factor Mex67p/Mtr2p, which is associated with the nuclear pore. Here, we report a genetic interaction between YRA1 and SUB2, the gene for a DEAD box helicase involved in splicing. Mutation of SUB2 as well as its overexpression leads to a defect in mRNA export. Moreover, Yra1p and Sub2p bind directly to each other both in vivo and in vitro. Significantly, Sub2p and Mex67p/Mtr2p bind to the same domains of Yra1p, and the proteins compete for binding to Yra1p. Together, these data indicate that the spliceosomal component Sub2p is also important in mRNA export and may function to recruit Yra1p to the mRNA. Sub2p may then be displaced from Yra1p by the binding of Mex67p/Mtr2p, which participates in the export of mRNA through the nuclear pores.

Pre-mRNA splicing and mRNA export linked by direct interactions between UAP56 and Aly

Recent studies indicate that splicing of pre-messenger RNA and export of mRNA are normally coupled in vivo. During splicing, the conserved mRNA export factor Aly is recruited to the spliced mRNA-protein complex (mRNP), which targets the mRNA for export. At present, it is not known how Aly is recruited to the spliced mRNP. Here we show that the conserved DEAD-box helicase UAP56, which functions during spliceosome assembly, interacts directly and highly specifically with Aly. Moreover, UAP56 is present together with Aly in the spliced mRNP. Significantly, excess UAP56 is a potent dominant negative inhibitor of mRNA export. Excess UAP56 also inhibits the recruitment of Aly to the spliced mRNP. Furthermore, a mutation in Aly that blocks its interaction with UAP56 prevents recruitment of Aly to the spliced mRNP. These data suggest that the splicing factor UAP56 functions in coupling the splicing and export machineries by recruiting Aly to the spliced mRNP.

The yeast hnRNP-Like proteins Yra1p and Yra2p participate in mRNA export through interaction with Mex67p

Yra1p is an essential nuclear protein which belongs to the evolutionarily conserved REF (RNA and export factor binding proteins) family of hnRNP-like proteins. Yra1p contributes to mRNA export in vivo and directly interacts with RNA and the shuttling mRNP export receptor Mex67p in vitro. Here we describe a second nonessential Saccharomyces cerevisiae family member, called Yra2p, which is able to complement a YRA1 deletion when overexpressed. Like other REF proteins, Yra1p and Yra2p consist of two highly conserved N- and C-terminal boxes and a central RNP-like RNA-binding domain (RBD). These conserved regions are separated by two more variable regions, N-vr and C-vr. Surprisingly, the deletion of a single conserved box or the deletion of the RBD in Yra1p does not affect viability. Consistently, neither the conserved N and C boxes nor the RBD is required for Mex67p and RNA binding in vitro. Instead, the N-vr and C-vr regions both interact with Mex67p and RNA. We further show that Yra1 deletion mutants which poorly interact with Mex67p in vitro affect the association of Mex67p with mRNP complexes in vivo and are paralleled by poly(A)(+) RNA export defects. These observations support the idea that Yra1p promotes mRNA export by facilitating the recruitment of Mex67p to the mRNP.

Close encounters of many kinds: Fos-Jun interactions that mediate transcription regulatory specificity

Fos and Jun family proteins regulate the expression of a myriad of genes in a variety of tissues and cell types. This functional versatility emerges from their interactions with related bZIP proteins and with structurally unrelated transcription factors. These interactions at composite regulatory elements produce nucleoprotein complexes with high sequence-specificity and regulatory selectivity. Several general principles including binding cooperativity and conformational adaptability have emerged from studies of regulatory complexes containing Fos-Jun family proteins. The structural properties of Fos-Jun family proteins including opposite orientations of heterodimer binding and the ability to bend DNA can contribute to the assembly and functions of such complexes. The cooperative recruitment of transcription factors, coactivators and chromatin remodeling factors to promoter and enhancer regions generates multiprotein transcription regulatory complexes with cell- and stimulus-specific transcriptional activities. The gene-specific architecture of these complexes can mediate the selective control of transcriptional activity.

Cyclin A is a mediator of p120E4F-dependent cell cycle arrest in G1

E4F is a ubiquitously expressed GLI-Krüppel-related transcription factor which has been identified for its capacity to regulate transcription of the adenovirus E4 gene in response to E1A. However, cellular genes regulated by E4F are still unknown. Some of these genes are likely to be involved in cell cycle progression since ectopic p120E4F expression induces cell cycle arrest in G1. Although p21WAF1 stabilization was proposed to mediate E4F-dependent cell cycle arrest, we found that p120E4F can induce a G1 block in p21(-/-) cells, suggesting that other proteins are essential for the p120E4F-dependent block in G1. We show here that cyclin A promoter activity can be repressed by p120E4F and that this repression correlates with p120E4F binding to the cyclic AMP-responsive element site of the cyclin A promoter. In addition, enforced expression of cyclin A releases p120E4F-arrested cells from the G1 block. These data identify the cyclin A gene as a cellular target for p120E4F and suggest a mechanism for p120E4F-dependent cell cycle regulation.

Identification of the G13 (cAMP-response-element-binding protein-related protein) gene product related to activating transcription factor 6 as a transcriptional activator of the mammalian unfolded protein response

Eukaryotic cells control the levels of molecular chaperones and folding enzymes in the endoplasmic reticulum (ER) by a transcriptional induction process termed the unfolded protein response (UPR). The mammalian UPR is mediated by the cis-acting ER stress response element consisting of 19 nt (CCAATN(9)CCACG), the CCACG part of which is considered to provide specificity. We recently identified the basic leucine zipper (bZIP) protein ATF6 as a mammalian UPR-specific transcription factor; ATF6 is activated by ER stress-induced proteolysis and binds directly to CCACG. Here we report that eukaryotic cells express another bZIP protein closely related to ATF6 in both structure and function. This protein encoded by the G13 (cAMP response element binding protein-related protein) gene is constitutively synthesized as a type II transmembrane glycoprotein anchored in the ER membrane and processed into a soluble form upon ER stress as occurs with ATF6. The proteolytic processing of ATF6 and the G13 gene product is accompanied by their relocation from the ER to the nucleus; their basic regions seem to function as a nuclear localization signal. Overexpression of the soluble form of the G13 product constitutively activates the UPR, whereas overexpression of a mutant lacking the activation domain exhibits a strong dominant-negative effect. Furthermore, the soluble forms of ATF6 and the G13 gene product are unable to bind to several point mutants of the cis-acting ER stress response element in vitro that hardly respond to ER stress in vivo. We thus concluded that the two related bZIP proteins are crucial transcriptional regulators of the mammalian UPR, and propose calling the ATF6 gene product ATF6alpha and the G13 gene product ATF6beta.

REF proteins mediate the export of spliced and unspliced mRNAs from the nucleus

The REF family of evolutionarily conserved heterogeneous ribonucleoprotein (hnRNP)-like proteins consists of one central RNP-type RNA binding domain flanked by Arg-Gly-rich regions of variable length. Members of this protein family bind directly to RNA and the mRNA export factor TAP/Mex67p, and it has been suggested that they facilitate the recruitment of TAP/Mex67p to cellular mRNPs. We show that the variable regions are necessary for binding of REFs to RNA and to TAP. Antibodies specific to REFs prevent their interaction with RNA in vitro. After microinjection into Xenopus oocytes, these antibodies inhibit mRNA nuclear export. This inhibition of export is observed whether or not the mRNAs are generated by splicing. The antibodies do not interfere with pre-mRNA splicing or with the nuclear export of constitutive transport element (CTE)-containing RNAs (directly mediated by TAP), so REF proteins must play a critical role in mRNA nuclear export, acting downstream of splicing and upstream of TAP/Mex67p. We also show that recombinant REFs stimulate directly the export of mRNAs that are otherwise exported inefficiently. Together, our data indicate that REFs are directly implicated in the export of mRNAs from the nucleus. More generally, we show that spliced and unspliced mRNAs use common export factors to reach the cytoplasm.

REF proteins mediate the export of spliced and unspliced mRNAs from the nucleus

The REF family of evolutionarily conserved heterogeneous ribonucleoprotein (hnRNP)-like proteins consists of one central RNP-type RNA binding domain flanked by Arg-Gly-rich regions of variable length. Members of this protein family bind directly to RNA and the mRNA export factor TAP/Mex67p, and it has been suggested that they facilitate the recruitment of TAP/Mex67p to cellular mRNPs. We show that the variable regions are necessary for binding of REFs to RNA and to TAP. Antibodies specific to REFs prevent their interaction with RNA in vitro. After microinjection into Xenopus oocytes, these antibodies inhibit mRNA nuclear export. This inhibition of export is observed whether or not the mRNAs are generated by splicing. The antibodies do not interfere with pre-mRNA splicing or with the nuclear export of constitutive transport element (CTE)-containing RNAs (directly mediated by TAP), so REF proteins must play a critical role in mRNA nuclear export, acting downstream of splicing and upstream of TAP/Mex67p. We also show that recombinant REFs stimulate directly the export of mRNAs that are otherwise exported inefficiently. Together, our data indicate that REFs are directly implicated in the export of mRNAs from the nucleus. More generally, we show that spliced and unspliced mRNAs use common export factors to reach the cytoplasm.

In vivo requirement of activator-specific binding targets of mediator

There has been no unequivocal demonstration that the activator binding targets identified in vitro play a key role in transcriptional activation in vivo. To examine whether activator-Mediator interactions are required for gene transcription under physiological conditions, we performed functional analyses with Mediator components that interact specifically with natural yeast activators. Different activators interact with Mediator via distinct binding targets. Deletion of a distinct activator binding region of Mediator completely compromised gene activation in vivo by some, but not all, transcriptional activators. These demonstrate that the activator-specific targets in Mediator are essential for transcriptional activation in living cells, but their requirement was affected by the nature of the activator-DNA interaction and the existence of a postrecruitment activation process.

Hsp70-RAP46 interaction in downregulation of DNA binding by glucocorticoid receptor

Receptor-associating protein 46 (RAP46) is a cochaperone that regulates the transactivation function of several steroid receptors. It is transported into the nucleus by a liganded glucocorticoid receptor where it downregulates DNA binding and transactivation by this receptor. The N- and C-termini of RAP46 are both implicated in its negative regulatory function. In metabolic labelling experiments, we have shown that the N-terminus of RAP46 is modified by phosphorylation, but this does not contribute to the downregulation of glucocorticoid receptor activity. However, deletion of a sequence that binds 70 kDa heat shock protein (Hsp70) and the constitutive isoform of Hsp70 (Hsc70) at the C-terminus of RAP46 abrogated its negative regulatory action. Surface plasmon resonance studies showed that RAP46 binds the glucocorticoid receptor only when it has interacted with Hsp70/Hsc70, and confocal immunofluorescence analyses revealed a nuclear transport of Hsp70/Hsc70 by the liganded receptor. Together these findings demonstrate an important contribution of Hsp70/Hsc70 in the binding of RAP46 to the glucocorticoid receptor and suggest a role for this molecular chaperone in the RAP46-mediated downregulation of glucocorticoid receptor activity.

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.

Gene regulation: protein escorts to the transcription ball

A new way by which the potency of a eukaryotic transcription factor can be regulated has been discovered, in which nuclear factors increase the concentration of the transcription factor's active form by modulating an otherwise unfavorable equilibrium between monomeric and dimeric forms of the protein.