Latest Articles Include:
- ATM Signals miRNA Biogenesis through KSRP
- mol cell 41(4):367-368 (2011)
In this issue of Molecular Cell, Zhang and colleagues (Zhang et al., 2011) describe a critical link between the DNA damage response and the miRNA pathway, in which DNA double-strand breaks (DSBs) induce ATM-dependent KSRP phosphorylation to facilitate pri-miRNA processing. - More, More, More: Downregulation of a MK5-FoxO3a-mir34b/c Pathway Further Increases c-Myc Levels in Colorectal Cancer
- mol cell 41(4):369-370 (2011)
The c-Myc transcription factor has been shown to be central for colorectal carcinogenesis. In this issue of Molecular Cell, Kress and colleagues (Kress et al., 2011) identify an exciting new pathway that downregulates c-Myc translation which is lost in aggressive colorectal cancer. - The ATM Kinase Induces MicroRNA Biogenesis in the DNA Damage Response
- mol cell 41(4):371-383 (2011)
The DNA damage response involves a complex network of processes that detect and repair DNA damage. Here we show that miRNA biogenesis is globally induced upon DNA damage in an ATM-dependent manner. About one-fourth of miRNAs are significantly upregulated after DNA damage, while loss of ATM abolishes their induction. KH-type splicing regulatory protein (KSRP) is a key player that translates DNA damage signaling to miRNA biogenesis. The ATM kinase directly binds to and phosphorylates KSRP, leading to enhanced interaction between KSRP and pri-miRNAs and increased KSRP activity in miRNA processing. Mutations of the ATM phosphorylation sites of KSRP impaired its activity in regulating miRNAs. These findings reveal a mechanism by which DNA damage signaling is linked to miRNA biogenesis. - WAC, a Functional Partner of RNF20/40, Regulates Histone H2B Ubiquitination and Gene Transcription
- mol cell 41(4):384-397 (2011)
Histone H2B ubiquitination plays an important role in regulating chromatin organization during gene transcription. It has been shown that RNF20/40 regulates H2B ubiquitination. Here, using protein affinity purification, we have identified WAC as a functional partner of RNF20/40. Depletion of WAC abolishes H2B ubiquitination. WAC interacts with RNF20/40 through its C-terminal coiled-coil region and promotes RNF20/40 s E3 ligase activity for H2B ubiquitination. The N-terminal WW domain of WAC recognizes RNA polymerase II. During gene transcription, WAC targets RNF20/40 to associate with RNA polymerase II complex for H2B ubiquitination at active transcription sites, which regulates transcription. Moreover, WAC-dependent transcription is important for cell-cycle checkpoint activation in response to genotoxic stress. Taken together, our results demonstrate an important regulator for transcription-coupled histone H2B ubiquitination. - The Histone Chaperones Nap1 and Vps75 Bind Histones H3 and H4 in a Tetrameric Conformation
- mol cell 41(4):398-408 (2011)
Histone chaperones physically interact with histones to direct proper assembly and disassembly of nucleosomes regulating diverse nuclear processes such as DNA replication, promoter remodeling, transcription elongation, DNA damage, and histone variant exchange. Currently, the best-characterized chaperone-histone interaction is that between the ubiquitous chaperone Asf1 and a dimer of H3 and H4. Nucleosome assembly proteins (Nap proteins) represent a distinct class of histone chaperone. Using pulsed electron double resonance (PELDOR) measurements and protein crosslinking, we show that two members of this class, Nap1 and Vps75, bind histones in the tetrameric conformation also observed when they are sequestered within the nucleosome. Furthermore, H3 and H4 trapped in their tetrameric state can be used as substrates in nucleosome assembly and chaperone-mediated lysine acetylation. This alternate mode of histone interaction provides a potential means of maintaining the inte! grity of the histone tetramer during cycles of nucleosome reassembly. - Transcriptional Activators Enhance Polyadenylation of mRNA Precursors
- mol cell 41(4):409-418 (2011)
Polyadenylation of mRNA precursors is frequently coupled to transcription by RNA polymerase II. Although this coupling is known to involve interactions with the C-terminal domain of the RNA polymerase II largest subunit, the possible role of other factors is not known. Here we show that a prototypical transcriptional activator, GAL4-VP16, stimulates transcription-coupled polyadenylation in vitro. In the absence of GAL4-VP16, specifically initiated transcripts accumulated but little polyadenylation was observed, while in its presence polyadenylation was strongly enhanced. We further show that this stimulation requires the transcription elongation-associated PAF complex (PAF1c), as PAF1c depletion blocked GAL4-VP16-stimulated polyadenylation. Furthermore, knockdown of PAF subunits by siRNA resulted in decreased 3′ cleavage, and nuclear export, of mRNA in vivo. Finally, we show that GAL4-VP16 interacts directly with PAF1c and recruits it to DNA templates. Our results in! dicate that a transcription activator can stimulate transcription-coupled 3′ processing and does so via interaction with PAF1c. - Identification of an mRNP Complex Regulating Tumorigenesis at the Translational Elongation Step
- mol cell 41(4):419-431 (2011)
Transcript-selective translational regulation of epithelial-mesenchymal transition (EMT) by transforming growth factor-β (TGF-β) is directed by the hnRNP E1-containing TGF-β-activated-translational (BAT) mRNP complex. Herein, eukaryotic elongation factor-1 A1 (eEF1A1) is identified as an integral component of the BAT complex. Translational silencing of Dab2 and ILEI, two EMT transcripts, is mediated by the binding of hnRNP E1 and eEF1A1 to their 3′UTR BAT element, whereby hnRNP E1 stalls translational elongation by inhibiting the release of eEF1A1 from the ribosomal A site. TGF-β-mediated hnRNP E1 phosphorylation, through Akt2, disrupts the BAT complex, thereby restoring translation of target EMT transcripts. Attenuation of hnRNP E1 expression in two noninvasive breast epithelial cells (NMuMG and MCF-7) not only induced EMT but also enabled cells to form metastatic lesions in vivo. Thus, translational regulation by TGF-β at the elongation stage represents a crit! ical checkpoint coordinating the expression of EMT transcripts required during development and in tumorigenesis and metastatic progression. - Structural Basis of an ERAD Pathway Mediated by the ER-Resident Protein Disulfide Reductase ERdj5
- mol cell 41(4):432-444 (2011)
ER-associated degradation (ERAD) is an ER quality-control process that eliminates terminally misfolded proteins. ERdj5 was recently discovered to be a key ER-resident PDI family member protein that accelerates ERAD by reducing incorrect disulfide bonds in misfolded glycoproteins recognized by EDEM1. We here solved the crystal structure of full-length ERdj5, thereby revealing that ERdj5 contains the N-terminal J domain and six tandem thioredoxin domains that can be divided into the N- and C-terminal clusters. Our systematic biochemical analyses indicated that two thioredoxin domains that constitute the C-terminal cluster form the highly reducing platform that interacts with EDEM1 and reduces EDEM1-recruited substrates, leading to their facilitated degradation. The pulse-chase experiment further provided direct evidence for the sequential movement of an ERAD substrate from calnexin to the downstream EDEM1-ERdj5 complex, and then to the retrotranslocation channel, probabl! y through BiP. We present a detailed molecular view of how ERdj5 mediates ERAD in concert with EDEM1. - The MK5/PRAK Kinase and Myc Form a Negative Feedback Loop that Is Disrupted during Colorectal Tumorigenesis
- mol cell 41(4):445-457 (2011)
Expression of the Myc oncoprotein is downregulated in response to stress signals to allow cells to cease proliferation and escape apoptosis, but the mechanisms involved in this process are poorly understood. Cell cycle arrest in response to DNA damage requires downregulation of Myc via a p53-independent signaling pathway. Here we have used siRNA screening of the human kinome to identify MAPKAPK5 (MK5, PRAK) as a negative regulator of Myc expression. MK5 regulates translation of Myc, since it is required for expression of miR-34b and miR-34c that bind to the 3′UTR of MYC. MK5 activates miR-34b/c expression via phosphorylation of FoxO3a, thereby promoting nuclear localization of FoxO3a and enabling it to induce miR-34b/c expression and arrest proliferation. Expression of MK5 in turn is directly activated by Myc, forming a negative feedback loop. MK5 is downregulated in colon carcinomas, arguing that this feedback loop is disrupted during colorectal tumorigenesis. - TBK1 Directly Engages Akt/PKB Survival Signaling to Support Oncogenic Transformation
- mol cell 41(4):458-470 (2011)
The innate immune-signaling kinase, TBK1, couples pathogen surveillance to induction of host defense mechanisms. Pathological activation of TBK1 in cancer can overcome programmed cell death cues, enabling cells to survive oncogenic stress. The mechanistic basis of TBK1 prosurvival signaling, however, has been enigmatic. Here, we show that TBK1 directly activates AKT by phosphorylation of the canonical activation loop and hydrophobic motif sites independently of PDK1 and mTORC2. Upon mitogen stimulation, triggering of the innate immune response, re-exposure to glucose, or oncogene activation, TBK1 is recruited to the exocyst, where it activates AKT. In cells lacking TBK1, insulin activates AKT normally, but AKT activation by exocyst-dependent mechanisms is impaired. Discovery and characterization of a 6-aminopyrazolopyrimidine derivative, as a selective low-nanomolar TBK1 inhibitor, indicates that this regulatory arm can be pharmacologically perturbed independently of c! anonical PI3K/PDK1 signaling. Thus, AKT is a direct TBK1 substrate that connects TBK1 to prosurvival signaling. - Clk2 and B56β Mediate Insulin-Regulated Assembly of the PP2A Phosphatase Holoenzyme Complex on Akt
- mol cell 41(4):471-479 (2011)
Akt mediates important cellular decisions involved in growth, survival, and metabolism. The mechanisms by which Akt is phosphorylated and activated in response to growth factors or insulin have been extensively studied, but the molecular regulatory components and dynamics of Akt attenuation are poorly understood. Here we show that a downstream target of insulin-induced Akt activation, Clk2, triggers Akt dephosphorylation through the PP2A phosphatase complex. Clk2 phosphorylates the PP2A regulatory subunit B56β (PPP2R5B, B′β), which is a critical regulatory step in the assembly of the PP2A holoenzyme complex on Akt leading to dephosphorylation of both S473 and T308 Akt sites. Since Akt plays a pivotal role in cellular signaling, these results have important implications for our understanding of Akt regulation in many biological processes. - A Comprehensive Genomic Binding Map of Gene and Chromatin Regulatory Proteins in Saccharomyces
- mol cell 41(4):480-492 (2011)
Hundreds of different proteins regulate and implement transcription in Saccharomyces. Yet their interrelationships have not been investigated on a comprehensive scale. Here we determined the genome-wide binding locations of 200 transcription-related proteins, under normal and acute heat-shock conditions. This study distinguishes binding between distal versus proximal promoter regions as well as the 3′ ends of genes for nearly all mRNA and tRNA genes. This study reveals (1) a greater diversity and specialization of regulation associated with the SAGA transcription pathway compared to the TFIID pathway, (2) new regulators enriched at tRNA genes, (3) a global co-occupancy network of >20,000 unique regulator combinations that show a high degree of regulatory interconnections among lowly expressed genes, (4) regulators of the SAGA pathway located largely distal to the core promoter and regulators of the TFIID pathway located proximally, and (5) distinct mobilization of SA! GA- versus TFIID-linked regulators during acute heat shock. - Sirt3 Promotes the Urea Cycle and Fatty Acid Oxidation during Dietary Restriction
- mol cell 41(4):493 (2011)
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