Nature Cell Biology. doi:10.1038/ncb1763

Authors: Colin K. Choi, Miguel Vicente-Manzanares, Jessica Zareno, Leanna A. Whitmore, Alex Mogilner & Alan Rick Horwitz

Nature Cell Biology. doi:10.1038/ncb1762

Authors: Kaylene J. Simpson, Laura M. Selfors, James Bui, Angela Reynolds, Devin Leake, Anastasia Khvorova & Joan S. Brugge

Nature Cell Biology. doi:10.1038/ncb1764

Authors: Michael Wanzel, Annika C. Russ, Daniela Kleine-Kohlbrecher, Emanuela Colombo, Pier-Guiseppe Pelicci & Martin Eilers

Nature Cell Biology. doi:10.1038/ncb1772

Authors: Shiraz Mujtaba, Karishma L. Manzur, James R. Gurnon, Ming Kang, James L. Van Etten & Ming-Ming Zhou

Viruses recruit host proteins to secure viral genome maintenance and replication. However, whether they modify host histones directly to interfere with chromatin-based transcription is unknown. Here we report that Paramecium bursaria chlorella virus 1 (PBCV-1) encodes a functional SET domain histone Lys methyltransferase (HKMTase) termed vSET, which is linked to rapid inhibition of host transcription after viral infection. We show that vSET is packaged in the PBCV-1 virion, and that it contains a nuclear localization signal and probably represses host transcription by methylating histone H3 at Lys 27 (H3K27), a modification known to trigger gene silencing in eukaryotes. We also show that vSET induces cell accumulation at the G2/M phase by recruiting the Polycomb repressive complex CBX8 to the methylated H3K27 site in a heterologous system. vSET-like proteins that have H3K27 methylation activity are conserved in chlorella viruses. Our findings suggest a viral mechanism to repress gene transcription by direct modification of chromatin by PBCV-1 vSET.

Nature Cell Biology. doi:10.1038/ncb1765

Authors: Xian Zhang, Guoying Jiang, Yunfei Cai, Susan J. Monkley, David R. Critchley & Michael P. Sheetz

Cell spreading, adhesion and remodelling of the extracellular matrix (ECM) involve bi-directional signalling and physical linkages between the ECM, integrins and the cell cytoskeleton. The actin-binding proteins talin1 and 2 link ligand-bound integrins to the actin cytoskeleton and increase the affinity of integrin for the ECM. Here we report that depletion of talin2 in talin1-null (talin1−/−) cells did not affect the initiation of matrix-activated spreading or Src family kinase (SFK) activation, but abolished the ECM–integrin–cytoskeleton linkage and sustained cell spreading and adhesion. Specifically, focal adhesion assembly, focal adhesion kinase (FAK) signalling and traction force generation on substrates were severely affected. The talin1 head domain restored β1 integrin activation but only full-length talin1 restored the ECM–cytoskeleton linkage and normal cytoskeleton organization. Our results demonstrate three biochemically distinct steps in fibronectin-activated cell spreading and adhesion: 1) fibronectin–integrin binding and initiation of spreading, 2) fast cell spreading and 3) focal adhesion formation and substrate traction. We suggest that talin is not required for initial cell spreading. However, talin provides the important mechanical linkage between ligand-bound integrins and the actin cytoskeleton required to catalyse focal adhesion-dependent pathways.

Nature Cell Biology. doi:10.1038/ncb1770

Authors: Marco M. Candeias, Laurence Malbert-Colas, Darren J. Powell, Chrysoula Daskalogianni, Magda M. Maslon, Nadia Naski, Karima Bourougaa, Fabien Calvo & Robin Fåhraeus

The E3 ubiquitin ligase Mdm2 is a focal regulator of p53 tumour suppressor activity. It binds p53, promoting its polyubiquitination and degradation, and also controls p53 synthesis. However, it is not known how this dual function of Mdm2 on p53 synthesis and degradation is achieved. Here we show that the p53 mRNA region encoding the Mdm2-binding site interacts directly with the RING domain of Mdm2. This impairs the E3 ligase activity of Mdm2 and promotes p53 mRNA translation. We also show that introduction of cancer-derived single silent point-mutations in the p53 mRNA weakens its binding to Mdm2 and results in reduced p53 activity. These data are consistent with a mechanism by which changes in silent nucleotides can affect the function of the encoded protein, and indicate that Mdm2-mediated control of p53 synthesis and degradation has evolved in the p53 mRNA sequence and its encoded amino acids.

Nature Cell Biology. doi:10.1038/ncb1771

Authors: Miki Ebisuya, Takuya Yamamoto, May Nakajima & Eisuke Nishida

Transcriptional initiation of each gene is assumed to be independently controlled in mammals. On the other hand, recent large-scale transcriptome analyses have shown that the genome is pervasively transcribed, such that the most of its DNA gives rise to RNAs. This raises the question of whether it is possible to pinpoint and activate a particular locus without perturbing numerous neighbouring transcripts. Here we show that intensive transcription at one locus frequently spills over into its physical neighbouring loci. Rapid induction of immediate-early genes (IEGs) in response to growth factor stimulation is accompanied by co-upregulation of their neighbouring genes. Profiling the primary transcripts in the nucleus with whole-genome tiling arrays delineated simultaneous activation of transcription centred on IEGs. Even in surrounding intergenic regions, transcriptional activation took place at the same time. Acetylation levels of histone H3 and H4 are elevated along with the IEG induction and neighbouring co-upregulation. Inhibition of the mitogen-activated protein kinase (MAPK) pathway or the transcription factor SRF suppresses all transcriptional upregulation. These results suggest that transcriptional activation has a ripple effect, which may be advantageous for coordinated expression.

Nature Cell Biology. doi:10.1038/ncb1767

Authors: Zheng Fu, Liviu Malureanu, Jun Huang, Wei Wang, Hao Li, Jan M. van Deursen, Donald J. Tindall & Junjie Chen

Proper control of entry into and progression through mitosis is essential for normal cell proliferation and the maintenance of genome stability. The mammalian mitotic kinase Polo-like kinase 1 (Plk1) is involved in multiple stages of mitosis. Here we report that Forkhead Box M1 (FoxM1), a substrate of Plk1 (refs 6, 7, 8), controls a transcriptional programme that mediates Plk1-dependent regulation of cell-cycle progression. The carboxy-terminal domain of FoxM1 binds Plk1, and phosphorylation of two key residues in this domain by Cdk1 is essential for Plk1–FoxM1 interaction. Formation of the Plk1–FoxM1 complex allows for direct phosphorylation of FoxM1 by Plk1 at G2/M and the subsequent activation of FoxM1 activity, which is required for expression of key mitotic regulators, including Plk1 itself. Thus, Plk1-dependent regulation of FoxM1 activity provides a positive-feedback loop ensuring tight regulation of transcriptional networks essential for orderly mitotic progression.

Nature Cell Biology. doi:10.1038/ncb1768

Authors: Min Li, Yong-Hyun Shin, Lingfei Hou, Xingxu Huang, Zhubo Wei, Eric Klann & Pumin Zhang

The anaphase promoting complex (APC) or cyclosome is a multi-subunit E3 ubiquitin ligase. Cdc20 (fizzy (fzy)) or p55CDC, and Cdh1 (Hct1, srw1 or fizzy-related 1 (fzr1)) encode two adaptor proteins that bring substrates to the APC. Both APC–Cdc20 and APC–Cdh1 have been implicated in the control of mitosis through mediating ubiquitination of mitotic regulators, such as cyclin B1 and securin. However, the importance of Cdh1 function in vivo and whether its function is redundant with that of Cdc20 are unclear. Here we have analysed mice lacking Cdh1. We show that Cdh1 is essential for placental development and that its deficiency causes early lethality. Cdh1-deficient mouse embryonic fibroblasts (MEFs) entered replicative senescence prematurely because of stabilization of Ets2 and subsequent activation of p16Ink4a expression. These results have uncovered an unexpected role of the APC in maintaining replicative lifespan of MEFs. Further, Cdh1 heterozygous mice show defects in late-phase long-term potentiation (L-LTP) in the hippocampus and are deficient in contextual fear-conditioning, suggesting that Cdh1 has a role in learning and memory.

Nature Cell Biology. doi:10.1038/ncb1769

Authors: Xiaowen Wang, Xiaoming Zuo, Blanka Kucejova & Xin Jie Chen

Mitochondrial function degenerates with ageing and in ageing-related neuromuscular degenerative diseases, causing physiological decline of the cell. Factors that can delay the degenerative process are actively sought after. Here, we show that reduced cytosolic protein synthesis is a robust cellular strategy that suppresses ageing-related mitochondrial degeneration. We modelled autosomal dominant progressive external ophthalmoplegia (adPEO), an adult- or later-onset degenerative disease, by introducing the A128P mutation into the adenine nucleotide translocase Aac2p of Saccharomyces cerevisiae. The aac2A128P allele dominantly induces ageing-dependent mitochondrial degeneration and phenotypically tractable degenerative cell death, independently of its ADP/ATP exchange activity. Mitochondrial degeneration was suppressed by lifespan-extending nutritional interventions and by eight longevity mutations, which are all known to reduce cytosolic protein synthesis. These longevity interventions also independently suppressed ageing-related mitochondrial degeneration in the pro-ageing prohibitin mutants. The aac2A128P mutant has reduced mitochondrial membrane potential (Δψm) and is synthetically lethal to low Δψm conditions, including the loss of prohibitin. Mitochondrial degeneration was accelerated by defects in protein turnover on the inner membrane and was suppressed by cycloheximide, a specific inhibitor of cytosolic ribosomes. Reduced cytosolic protein synthesis suppressed membrane depolarization and defects in mitochondrial gene expression in aac2A128P cells. Our finding thus establishes a link between protein homeostasis (proteostasis), cellular bioenergetics and mitochondrial maintenance during ageing.

Nature Cell Biology. doi:10.1038/ncb1766

Authors: Olivier Namy, Aurélie Galopier, Cyrielle Martini, Senya Matsufuji, Céline Fabret & Jean-Pierre Rousset

Prion proteins are found in mammals and yeast, and can transmit diseases and encode heritable phenotypic traits. In Saccharomyces cerevisiae, eRF3, Rnq1, Ure2 and Swi1 are functional proteins with a soluble conformation that can switch to a non-functional, amyloid conformation denoted as [PSI+], [PIN+], [URE3] and [SWI+], respectively. The prion [PSI+] corresponds to an aggregated conformation of the translational release factor eRF3, which suppresses nonsense codons. [PSI+] modifies cellular fitness and induces several phenotypes according to the genetic background. An elegant series of studies has demonstrated that several [PSI+]-induced phenotypes occur as a consequence of decreased translational termination efficiency. However, the genes whose expression levels are controlled by [PSI+] remain largely unknown. Here, we show that [PSI+] enhances expression of antizyme, a negative regulator of cellular polyamines, by modulating the +1 frameshifting required for its expression. Our study also demonstrates that [PSI+] greatly affects cellular polyamines in yeast. We show that modification of the cellular content of polyamines by the prion accounts for half of the [PSI+]-induced phenotypes. Antizyme is the first protein to be described for which expression in its functional form is stimulated by [PSI+].