(1/241) Dominant negative murine serum response factor: alternative splicing within the activation domain inhibits transactivation of serum response factor binding targets.
Primary transcripts encoding the MADS box superfamily of proteins, such as MEF2 in animals and ZEMa in plants, are alternatively spliced, producing several isoformic species. We show here that murine serum response factor (SRF) primary RNA transcripts are alternatively spliced at the fifth exon, deleting approximately one-third of the C-terminal activation domain. Among the different muscle types examined, visceral smooth muscles have a very low ratio of SRFDelta5 to SRF. Increased levels of SRFDelta5 correlates well with reduced smooth muscle contractile gene activity within the elastic aortic arch, suggesting important biological roles for differential expression of SRFDelta5 variant relative to wild-type SRF. SRFDelta5 forms DNA binding-competent homodimers and heterodimers. SRFDelta5 acts as a naturally occurring dominant negative regulatory mutant that blocks SRF-dependent skeletal alpha-actin, cardiac alpha-actin, smooth alpha-actin, SM22alpha, and SRF promoter-luciferase reporter activities. Expression of SRFDelta5 interferes with differentiation of myogenic C2C12 cells and the appearance of skeletal alpha-actin and myogenin mRNAs. SRFDelta5 repressed the serum-induced activity of the c-fos serum response element. SRFDelta5 fused to the yeast Gal4 DNA binding domain displayed low transcriptional activity, which was complemented by overexpression of the coactivator ATF6. These results indicate that the absence of exon 5 might be bypassed through recruitment of transcription factors that interact with extra-exon 5 regions in the transcriptional activating domain. The novel alternatively spliced isoform of SRF, SRFDelta5, may play an important regulatory role in modulating SRF-dependent gene expression. (+info)
(2/241) Mammalian transcription factor ATF6 is synthesized as a transmembrane protein and activated by proteolysis in response to endoplasmic reticulum stress.
The unfolded protein response (UPR) controls the levels of molecular chaperones and enzymes involved in protein folding in the endoplasmic reticulum (ER). We recently isolated ATF6 as a candidate for mammalian UPR-specific transcription factor. We report here that ATF6 constitutively expressed as a 90-kDa protein (p90ATF6) is directly converted to a 50-kDa protein (p50ATF6) in ER-stressed cells. Furthermore, we showed that the most important consequence of this conversion was altered subcellular localization; p90ATF6 is embedded in the ER, whereas p50ATF6 is a nuclear protein. p90ATF6 is a type II transmembrane glycoprotein with a hydrophobic stretch in the middle of the molecule. Thus, the N-terminal half containing a basic leucine zipper motif is oriented facing the cytoplasm. Full-length ATF6 as well as its C-terminal deletion mutant carrying the transmembrane domain is localized in the ER when transfected. In contrast, mutant ATF6 representing the cytoplasmic region translocates into the nucleus and activates transcription of the endogenous GRP78/BiP gene. We propose that ER stress-induced proteolysis of membrane-bound p90ATF6 releases soluble p50ATF6, leading to induced transcription in the nucleus. Unlike yeast UPR, mammalian UPR appears to use a system similar to that reported for cholesterol homeostasis. (+info)
(3/241) Activation of ATF6 and an ATF6 DNA binding site by the endoplasmic reticulum stress response.
ATF6 is a member of the basic-leucine zipper family of transcription factors. It contains a transmembrane domain and is located in membranes of the endoplasmic reticulum. ATF6 has been implicated in the endoplasmic reticulum (ER) stress response pathway since it can activate expression of GRP78 and other genes induced by the ER stress response. ER stress appears to activate ATF6 by cleavage from the ER membrane and translocation to the nucleus. However, direct DNA binding by ATF6 had not been demonstrated. In this report, we have identified a consensus DNA binding sequence for ATF6. This site is related to but distinct from ATF1/CREB binding sites. The site was placed in a reporter gene and was specifically activated by ATF6 overexpression and was strongly induced by the ER stress response. A dominant negative form of ATF6 blocked ER stress induction of both ATF6 site and GRP78 reporter genes. We further found that GAL4-ATF6 could be activated by ER stress. These results demonstrate that ATF6 is a direct target of the ER stress response. A proximal sensor of the ER stress response, human IRE1 (hIRE1), was sufficient to activate the ATF6 reporter gene, while a dominant negative form of hIRE1 blocked ER stress activation, suggesting that hIRE1 is upstream of ATF6 in the ER stress signaling pathway. (+info)
(4/241) ATF6 as a transcription activator of the endoplasmic reticulum stress element: thapsigargin stress-induced changes and synergistic interactions with NF-Y and YY1.
ATF6, a member of the leucine zipper protein family, can constitutively induce the promoter of glucose-regulated protein (grp) genes through activation of the endoplasmic reticulum (ER) stress element (ERSE). To understand the mechanism of grp78 induction by ATF6 in cells subjected to ER calcium depletion stress mediated by thapsigargin (Tg) treatment, we discovered that ATF6 itself undergoes Tg stress-induced changes. In nonstressed cells, ATF6, which contains a putative short transmembrane domain, is primarily associated with the perinuclear region. Upon Tg stress, the ATF6 protein level dropped initially but quickly recovered with the additional appearance of a faster-migrating form. This new form of ATF6 was recovered as soluble nuclear protein by biochemical fractionation, correlating with enhanced nuclear localization of ATF6 as revealed by immunofluorescence. Optimal ATF6 stimulation requires at least two copies of the ERSE and the integrity of the tripartite structure of the ERSE. Of primary importance is a functional NF-Y complex and a high-affinity NF-Y binding site that confers selectivity among different ERSEs for ATF6 inducibility. In addition, we showed that YY1 interacts with ATF6 and in Tg-treated cells can enhance ATF6 activity. The ERSE stimulatory activity of ATF6 exhibits properties distinct from those of human Ire1p, an upstream regulator of the mammalian unfolded protein response. The requirement for a high-affinity NF-Y site for ATF6 but not human Ire1p activity suggests that they stimulate the ERSE through diverse pathways. (+info)
(5/241) ATF6 activated by proteolysis binds in the presence of NF-Y (CBF) directly to the cis-acting element responsible for the mammalian unfolded protein response.
Transcription of genes encoding molecular chaperones and folding enzymes in the endoplasmic reticulum (ER) is induced by accumulation of unfolded proteins in the ER. This intracellular signaling, known as the unfolded protein response (UPR), is mediated by the cis-acting ER stress response element (ERSE) in mammals. In addition to ER chaperones, the mammalian transcription factor CHOP (also called GADD153) is induced by ER stress. We report here that the transcription factor XBP-1 (also called TREB5) is also induced by ER stress and that induction of CHOP and XBP-1 is mediated by ERSE. The ERSE consensus sequence is CCAAT-N(9)-CCACG. As the general transcription factor NF-Y (also known as CBF) binds to CCAAT, CCACG is considered to provide specificity in the mammalian UPR. We recently found that the basic leucine zipper protein ATF6 isolated as a CCACG-binding protein is synthesized as a transmembrane protein in the ER, and ER stress-induced proteolysis produces a soluble form of ATF6 that translocates into the nucleus. We report here that overexpression of soluble ATF6 activates transcription of the CHOP and XBP-1 genes as well as of ER chaperone genes constitutively, whereas overexpression of a dominant negative mutant of ATF6 blocks the induction by ER stress. Furthermore, we demonstrated that soluble ATF6 binds directly to CCACG only when CCAAT exactly 9 bp upstream of CCACG is bound to NF-Y. Based on these and other findings, we concluded that specific and direct interactions between ATF6 and ERSE are critical for transcriptional induction not only of ER chaperones but also of CHOP and XBP-1. (+info)
(6/241) Identification of ERSE-II, a new cis-acting element responsible for the ATF6-dependent mammalian unfolded protein response.
Herp is a 54-kDa membrane protein in the endoplasmic reticulum (ER). The mRNA expression level of Herp is increased by the accumulation of unfolded proteins in the ER. Transcriptional changes designed to deal with this type of ER stress is called the unfolded protein response (UPR). Most mammalian UPR-target genes encode ER-resident molecular chaperones: GRP78, GRP94, and calreticulin. The promoter regions of these genes contain a cis-acting ER stress response element, ERSE, with the consensus sequence of CCAAT-N(9)-CCACG. Under conditions of ER stress, p50ATF6 (the active form of the transcription factor, ATF6) binds to CCACG when CCAAT is bound by the general transcription factor, NF-Y/CBF. Here, we report the genomic structure of human Herp and the presence of a new ER stress response element, ERSE-II, in its promoter region. The gene for Herp consists of eight exons, localized to chromosome 16q12.2-13. The promoter region contains a single ERSE-like sequence. In reporter gene assays, disruption of this cis-element resulted in a partial reduction of the transcriptional response to ER stress, suggesting that the element is functional for the UPR. These results also suggest the involvement of additional elements in the UPR. Further analysis, using an optimized plasmid containing an mRNA-destabilizing sequence, revealed ERSE-II (ATTGG-N-CCACG) as the second ER stress response element. Interestingly, ERSE-II was also dependent on p50ATF6, in a manner similar to that of ERSE, despite the disparate structure. The strong induction of Herp mRNA by ER stress would be achieved by the cooperation of ERSE and ERSE-II. (+info)
(7/241) Endoplasmic reticulum stress-induced formation of transcription factor complex ERSF including NF-Y (CBF) and activating transcription factors 6alpha and 6beta that activates the mammalian unfolded protein response.
The levels of molecular chaperones and folding enzymes in the endoplasmic reticulum (ER) are controlled by a transcriptional induction process termed the unfolded protein response (UPR). The mammalian UPR is mediated by the cis-acting ER stress response element (ERSE), the consensus sequence of which is CCAAT-N(9)-CCACG. We recently proposed that ER stress response factor (ERSF) binding to ERSE is a heterologous protein complex consisting of the constitutive component NF-Y (CBF) binding to CCAAT and an inducible component binding to CCACG and identified the basic leucine zipper-type transcription factors ATF6alpha and ATF6beta as inducible components of ERSF. ATF6alpha and ATF6beta produced by ER stress-induced proteolysis bind to CCACG only when CCAAT is bound to NF-Y, a heterotrimer consisting of NF-YA, NF-YB, and NF-YC. Interestingly, the NF-Y and ATF6 binding sites must be separated by a spacer of 9 bp. We describe here the basis for this strict requirement by demonstrating that both ATF6alpha and ATF6beta physically interact with NF-Y trimer via direct binding to the NF-YC subunit. ATF6alpha and ATF6beta bind to the ERSE as a homo- or heterodimer. Furthermore, we showed that ERSF including NF-Y and ATF6alpha and/or beta and capable of binding to ERSE is indeed formed when the cellular UPR is activated. We concluded that ATF6 homo- or heterodimers recognize and bind directly to both the DNA and adjacent protein NF-Y and that this complex formation process is essential for transcriptional induction of ER chaperones. (+info)
(8/241) ER stress induces cleavage of membrane-bound ATF6 by the same proteases that process SREBPs.
ATF6 is a membrane-bound transcription factor that activates genes in the endoplasmic reticulum (ER) stress response. When unfolded proteins accumulate in the ER, ATF6 is cleaved to release its cytoplasmic domain, which enters the nucleus. Here, we show that ATF6 is processed by Site-1 protease (S1P) and Site-2 protease (S2P), the enzymes that process SREBPs in response to cholesterol deprivation. ATF6 processing was blocked completely in cells lacking S2P and partially in cells lacking S1P. ATF6 processing required the RxxL and asparagine/proline motifs, known requirements for S1P and S2P processing, respectively. Cells lacking S2P failed to induce GRP78, an ATF6 target, in response to ER stress. ATF6 processing did not require SCAP, which is essential for SREBP processing. We conclude that S1P and S2P are required for the ER stress response as well as for lipid synthesis. (+info)