Hot off the presses! Aug 05 Mol Cell

The Aug 05 issue of the Mol Cell is now up on Pubget (About Mol Cell): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

• UneCLIPsing HuR Nuclear Function
  - Mol Cell 43(3):319-321 (2011)
  The RNA-binding protein HuR, while known to stabilize cytoplasmic mRNAs, is largely nuclear. In this issue of Molecular Cell, Mukherjee et al. (2011) and Lebedeva et al. (2011) identify transcriptome-wide HuR-RNA interactions using PAR-CLIP, unveiling HuR's nuclear role in pre-mRNA processing.
• CUL4B: Trash Talking at Chromatin
  - Mol Cell 43(3):321-323 (2011)
  In this issue, Nakagawa and Xiong (2011) reveal a mechanism targeting WDR5 for proteolysis dependent on the X-linked mental retardation gene, CUL4B. This provides a link between the stability of a chromatin factor and gene expression implicated in neurological pathogenesis.
• The Ripoptosome: Death Decision in the Cytosol
  - Mol Cell 43(3):323-325 (2011)
  In this issue of Molecular Cell, Tenev et al. and Feoktistova et al. describe the Ripoptosome, a cytosolic death-inducing RIP1-, FADD-, and caspase-8-containing complex that spontaneously assembles upon cIAP depletion, challenging the view that such complexes exclusively originate from receptor activation.
• Integrative Regulatory Mapping Indicates that the RNA-Binding Protein HuR Couples Pre-mRNA Processing and mRNA Stability
  - Mol Cell 43(3):327-339 (2011)
  RNA-binding proteins coordinate the fates of multiple RNAs, but the principles underlying these global interactions remain poorly understood. We elucidated regulatory mechanisms of the RNA-binding protein HuR, by integrating data from diverse high-throughput targeting technologies, specifically PAR-CLIP, RIP-chip, and whole-transcript expression profiling. The number of binding sites per transcript, degree of HuR association, and degree of HuR-dependent RNA stabilization were positively correlated. Pre-mRNA and mature mRNA containing both intronic and 3′ UTR binding sites were more highly stabilized than transcripts with only 3′ UTR or only intronic binding sites, suggesting that HuR couples pre-mRNA processing with mature mRNA stability. We also observed HuR-dependent splicing changes and substantial binding of HuR in polypyrimidine tracts of pre-mRNAs. Comparison of the spatial patterns surrounding HuR and miRNA binding sites provided functional evidence for HuR-! dependent antagonism of proximal miRNA-mediated repression. We conclude that HuR coordinates gene expression outcomes at multiple interconnected steps of RNA processing.
• Transcriptome-wide Analysis of Regulatory Interactions of the RNA-Binding Protein HuR
  - Mol Cell 43(3):340-352 (2011)
  Posttranscriptional gene regulation relies on hundreds of RNA binding proteins (RBPs) but the function of most RBPs is unknown. The human RBP HuR/ELAVL1 is a conserved mRNA stability regulator. We used PAR-CLIP, a recently developed method based on RNA-protein crosslinking, to identify transcriptome-wide 26,000 HuR binding sites. These sites were on average highly conserved, enriched for HuR binding motifs and mainly located in 3′ untranslated regions. Surprisingly, many sites were intronic, implicating HuR in mRNA processing. Upon HuR knockdown, mRNA levels and protein synthesis of thousands of target genes were downregulated, validating functionality. HuR and miRNA binding sites tended to reside nearby but generally did not overlap. Additionally, HuR knockdown triggered strong and specific upregulation of miR-7. In summary, we identified thousands of direct and functional HuR targets, found a human miRNA controlled by HuR, and propose a role for HuR in splicing.
• The Ewing Sarcoma Protein Regulates DNA Damage-Induced Alternative Splicing
  - Mol Cell 43(3):353-368 (2011)
  The Ewing sarcoma (EWS) protein is a member of the TET (TLS/EWS/TAF15) family of RNA- and DNA-binding proteins whose expression is altered in cancer. We report that EWS depletion results in alternative splicing changes of genes involved in DNA repair and genotoxic stress signaling, including ABL1, CHEK2, and MAP4K2. Chromatin and RNA crosslinking immunoprecipitation results indicate that EWS cotranscriptionally binds to its target RNAs. This association is reduced upon irradiation of cells with ultraviolet light, concomitant with transient enrichment of EWS in nucleoli and with alternative splicing changes that parallel those induced by EWS depletion and that lead to reduced c-ABL protein expression. Consistent with the functional relevance of EWS-mediated alternative splicing regulation in DNA damage response, EWS depletion reduces cell viability and proliferation upon UV irradiation, effects that are attenuated by restoring c-ABL expression. These results provide ins! ights into posttranscriptional mechanisms of DNA damage response by a TET protein.
• Nonhistone Scm3 Binds to AT-Rich DNA to Organize Atypical Centromeric Nucleosome of Budding Yeast
  - Mol Cell 43(3):369-380 (2011)
  The molecular architecture of centromere-specific nucleosomes containing histone variant CenH3 is controversial. We have biochemically reconstituted two distinct populations of nucleosomes containing Saccharomyces cerevisiae CenH3 (Cse4). Reconstitution of octameric nucleosomes containing histones Cse4/H4/H2A/H2B is robust on noncentromere DNA, but inefficient on AT-rich centromere DNA. However, nonhistone Scm3, which is required for Cse4 deposition in vivo, facilitates in vitro reconstitution of Cse4/H4/Scm3 complexes on AT-rich centromere sequences. Scm3 has a nonspecific DNA binding domain that shows preference for AT-rich DNA and a histone chaperone domain that promotes specific loading of Cse4/H4. In live cells, Scm3-GFP is enriched at centromeres in all cell cycle phases. Chromatin immunoprecipitation confirms that Scm3 occupies centromere DNA throughout the cell cycle, even when Cse4 and H4 are temporarily dislodged in S phase. These findings suggest a model in ! which centromere-bound Scm3 aids recruitment of Cse4/H4 to assemble and maintain an H2A/H2B-deficient centromeric nucleosome.
• X-Linked Mental Retardation Gene CUL4B Targets Ubiquitylation of H3K4 Methyltransferase Component WDR5 and Regulates Neuronal Gene Expression
  - Mol Cell 43(3):381-391 (2011)
  CUL4B, encoding a scaffold protein for the assembly of Cullin4B-Ring ubiquitin ligase (CRL4B) complexes, is frequently mutated in X-linked mental retardation (XLMR) patients. Here, we show that CUL4B, but not its paralog, CUL4A, targets WDR5, a core subunit of histone H3 lysine 4 (H3K4) methyltransferase complexes, for ubiquitylation and degradation in the nucleus. Knocking down CUL4B increases WDR5 and trimethylated H3K4 (H3K4me3) on the neuronal gene promoters and induces their expression. Furthermore, CUL4B depletion suppresses neurite outgrowth of PC12 neuroendocrine cells, which can be rescued by codepletion of WDR5. XLMR-linked mutations destabilize CUL4B and impair its ability to support neurite outgrowth of PC12 cells. Our results identify WDR5 as a critical substrate of CUL4B in regulating neuronal gene expression and suggest epigenetic change as a common pathogenic mechanism for CUL4B-associated XLMR.
• Alternative Ubiquitin Activation/Conjugation Cascades Interact with N-End Rule Ubiquitin Ligases to Control Degradation of RGS Proteins
  - Mol Cell 43(3):392-405 (2011)
  Vertebrates express two enzymes for activation of ubiquitin—UBA1, which is responsible for activation of the vast majority of E2 conjugating enzymes, and UBA6, which uses the dedicated E2, USE1. However, targets and E3s for UBA6-USE1 are unknown. Here, we demonstrate that UBA6-USE1 functions with the UBR1–3 subfamily of N-recognin E3s to degrade the N-end rule substrates RGS4, RGS5, and Arg (R)-GFP. This pathway functions in the cytoplasm in parallel with the UBA1-UBE2A/B-UBR2 cascade, which promotes turnover of nuclear RGS4/5 proteins and an apparently phenotypically distinct pool of cytoplasmic RGS4/5. UBR2 promotes Lys48 (K48)-specific ubiquitin discharge from, and RGS4 ubiquitylation by, both USE1 and UBE2A in vitro. This work provides insight into the machinery employed by the UBA6-USE1 cascade to promote protein turnover and suggests that the UBA6 and UBA1 pathways can function in parallel with the same E3 to degrade the same targets in a spatially distinct m! anner.
• Quantitative Proteomics Reveals the Basis for the Biochemical Specificity of the Cell-Cycle Machinery
  - Mol Cell 43(3):406-417 (2011)
  Cyclin-dependent kinases comprise the conserved machinery that drives progress through the cell cycle, but how they do this in mammalian cells is still unclear. To identify the mechanisms by which cyclin-cdks control the cell cycle, we performed a time-resolved analysis of the in vivo interactors of cyclins E1, A2, and B1 by quantitative mass spectrometry. This global analysis of context-dependent protein interactions reveals the temporal dynamics of cyclin function in which networks of cyclin-cdk interactions vary according to the type of cyclin and cell-cycle stage. Our results explain the temporal specificity of the cell-cycle machinery, thereby providing a biochemical mechanism for the genetic requirement for multiple cyclins in vivo and reveal how the actions of specific cyclins are coordinated to control the cell cycle. Furthermore, we identify key substrates (Wee1 and c15orf42/Sld3) that reveal how cyclin A is able to promote both DNA replication and mitosis.
• LGN/mInsc and LGN/NuMA Complex Structures Suggest Distinct Functions in Asymmetric Cell Division for the Par3/mInsc/LGN and Gαi/LGN/NuMA Pathways
  - Mol Cell 43(3):418-431 (2011)
  Asymmetric cell division requires the establishment of cortical cell polarity and the orientation of the mitotic spindle along the axis of cell polarity. Evidence from invertebrates demonstrates that the Par3/Par6/aPKC and NuMA/LGN/Gαi complexes, which are thought to be physically linked by the adaptor protein mInscuteable (mInsc), play indispensable roles in this process. However, the molecular basis for the binding of LGN to NuMA and mInsc is poorly understood. The high-resolution structures of the LGN/NuMA and LGN/mInsc complexes presented here provide mechanistic insights into the distinct and highly specific interactions of the LGN TPRs with mInsc and NuMA. Structural comparisons, together with biochemical and cell biology studies, demonstrate that the interactions of NuMA and mInsc with LGN are mutually exclusive, with mInsc binding preferentially. Our results suggest that the Par3/mInsc/LGN and NuMA/LGN/Gαi complexes play sequential and partially overlapping r! oles in asymmetric cell division.
• The Ripoptosome, a Signaling Platform that Assembles in Response to Genotoxic Stress and Loss of IAPs
  - Mol Cell 43(3):432-448 (2011)
  A better understanding of the mechanisms through which anticancer drugs exert their effects is essential to improve combination therapies. While studying how genotoxic stress kills cancer cells, we discovered a large 2MDa cell death-inducing platform, referred to as "Ripoptosome." It contains the core components RIP1, FADD, and caspase-8, and assembles in response to genotoxic stress-induced depletion of XIAP, cIAP1 and cIAP2. Importantly, it forms independently of TNF, CD95L/FASL, TRAIL, death-receptors, and mitochondrial pathways. It also forms upon Smac-mimetic (SM) treatment without involvement of autocrine TNF. Ripoptosome assembly requires RIP1's kinase activity and can stimulate caspase-8-mediated apoptosis as well as caspase-independent necrosis. It is negatively regulated by FLIP, cIAP1, cIAP2, and XIAP. Mechanistically, IAPs target components of this complex for ubiquitylation and inactivation. Moreover, we find that etoposide-stimulated Ripoptosome for! mation converts proinflammatory cytokines into prodeath signals. Together, our observations shed new light on fundamental mechanisms by which chemotherapeutics may kill cancer cells.
• cIAPs Block Ripoptosome Formation, a RIP1/Caspase-8 Containing Intracellular Cell Death Complex Differentially Regulated by cFLIP Isoforms
  - Mol Cell 43(3):449-463 (2011)
  The intracellular regulation of cell death pathways by cIAPs has been enigmatic. Here we show that loss of cIAPs promotes the spontaneous formation of an intracellular platform that activates either apoptosis or necroptosis. This 2 MDa intracellular complex that we designate "Ripoptosome" is necessary but not sufficient for cell death. It contains RIP1, FADD, caspase-8, caspase-10, and caspase inhibitor cFLIP isoforms. cFLIPL prevents Ripoptosome formation, whereas, intriguingly, cFLIPS promotes Ripoptosome assembly. When cIAPs are absent, caspase activity is the "rheostat" that is controlled by cFLIP isoforms in the Ripoptosome and decides if cell death occurs by RIP3-dependent necroptosis or caspase-dependent apoptosis. RIP1 is the core component of the complex. As exemplified by our studies for TLR3 activation, our data argue that the Ripoptosome critically influences the outcome of membrane-bound receptor triggering. The differential quality of cell death m! ediated by the Ripoptosome may cause important pathophysiological consequences during inflammatory responses.
• Cordon-Bleu Uses WH2 Domains as Multifunctional Dynamizers of Actin Filament Assembly
  - Mol Cell 43(3):464-477 (2011)
  Cordon-Bleu is, like Spire, a member of the growing family of WH2 repeat proteins, which emerge as versatile regulators of actin dynamics. They are expressed in morphogenetic and patterning processes and nucleate actin assembly in vitro. Here, we show that Cordon-Bleu works as a dynamizer of actin assembly by combining many properties of profilin with weak filament nucleating and powerful filament severing activities and sequestration of ADP-actin, which altogether generate oscillatory polymerization kinetics. A short lysine-rich sequence, N-terminally adjacent to the three WH2 domains, is required for nucleation and severing. In this context, nucleation requires only one WH2 domain, but filament severing requires two adjacent WH2 domains. A model integrating the multiple activities of Cordon-Bleu and quantitatively fitting the multiphasic polymerization curves is derived. Hence, with similar structural organization of WH2 repeats, Cordon-Bleu and Spire display differe! nt functions by selecting different sets of the multifunctional properties of WH2 domains.
• Direct Membrane Binding by Bacterial Actin MreB
  - Mol Cell 43(3):478-487 (2011)
  Bacterial actin MreB is one of the key components of the bacterial cytoskeleton. It assembles into short filaments that lie just underneath the membrane and organize the cell wall synthesis machinery. Here we show that MreB from both T. maritima and E. coli binds directly to cell membranes. This function is essential for cell shape determination in E. coli and is proposed to be a general property of many, if not all, MreBs. We demonstrate that membrane binding is mediated by a membrane insertion loop in TmMreB and by an N-terminal amphipathic helix in EcMreB and show that purified TmMreB assembles into double filaments on a membrane surface that can induce curvature. This, the first example of a membrane-binding actin filament, prompts a fundamental rethink of the structure and dynamics of MreB filaments within cells.
• The TFIIH Subunit Tfb3 Regulates Cullin Neddylation
  - Mol Cell 43(3):488-495 (2011)
  Cullin proteins are scaffolds for the assembly of multisubunit ubiquitin ligases, which ubiquitylate a large number of proteins involved in widely varying cellular functions. Multiple mechanisms cooperate to regulate cullin activity, including neddylation of their C-terminal domain. Interestingly, we found that the yeast Cul4-type cullin Rtt101 is not only neddylated but also ubiquitylated, and both modifications promote Rtt101 function in vivo. Surprisingly, proper modification of Rtt101 neither correlated with catalytic activity of the RING domain of Hrt1 nor required the Nedd8 ligase Dcn1. Instead, ubiquitylation of Rtt101 was dependent on the ubiquitin-conjugating enzyme Ubc4, while efficient neddylation involves the RING domain protein Tfb3, a subunit of the transcription factor TFIIH. Tfb3 also controls Cul3 neddylation and activity in vivo, and physically interacts with Ubc4 and the Nedd8-conjugating enzyme Ubc12 and the Hrt1/Rtt101 complex. Together, these resu! lts suggest that the conserved RING domain protein Tfb3 controls activation of a subset of cullins.