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Molecular Biology of the Cell

Katarzyna Chmielarska Masoumi, Renée Daams, Wondossen Sime, Valentina Siino, Hengning Ke, Fredrik Levander, Ramin Massoumi
The Wnt signaling pathway is essential in regulating various cellular processes. Different mechanisms of inhibition for Wnt signaling have been proposed. Besides β-catenin degradation, through the proteasome, nemo-like kinase (NLK) is another molecule that is known to negatively regulate Wnt signaling. However, the mechanism by which NLK mediates the inhibition of Wnt signaling is not yet known. In the present study, we used primary embryonic fibroblast cells isolated from NLK-deficient mice and showed that these cells proliferate faster and have a shorter cell cycle compared with wild-type cells...
November 30, 2016: Molecular Biology of the Cell
Ginger M Pocock, Laraine L Zimdars, Ming Yuan, Kevin W Eliceiri, Paul Ahlquist, Nathan M Sherer
Cis-acting RNA structural elements govern crucial aspects of viral gene expression. How these structures and other post-transcriptional signals affect RNA trafficking and translation in the context of single cells is poorly understood. Herein we describe a multi-color, long-term (>24 h) imaging strategy for measuring integrated aspects of viral RNA regulatory control in individual cells. We apply this strategy to demonstrate differential mRNA trafficking behaviors governed by RNA elements derived from three retroviruses (HIV-1, murine leukemia virus, and Mason-Pfizer monkey virus), two hepadnaviruses (hepatitis B virus and woodchuck hepatitis virus) and an intron-retaining transcript encoded by the cellular NXF1 gene...
November 30, 2016: Molecular Biology of the Cell
Sevan Mattie McNally, Erin K Karim, Mahmoud A Vali, Hojatollah Brett, Christopher L Brett
Lysosomal membrane fusion mediates the last step of the autophagy and endocytosis pathways, and supports organelle remodeling and biogenesis. Because fusogenic proteins and lipids concentrate in a ring at the vertex between apposing organelle membranes, the encircled area of membrane can be severed and internalized within the lumen as a fragment upon lipid bilayer fusion. How or why this intralumenal fragment forms during fusion, however, is not entirely clear. To better understand this process, we studied fragment formation during homotypic vacuolar lysosome membrane fusion in Saccharomyces cerevisiae Using cell-free fusion assays and light microscopy, we find that GTPase activation and trans-SNARE complex zippering have opposing effects on fragment formation, and verify that this impacts the morphology of the fusion product and regulates transporter protein degradation...
November 23, 2016: Molecular Biology of the Cell
Michelle A Baird, Neil Billington, Aibing Wang, Robert S Adelstein, James R Sellers, Robert S Fischer, Clare M Waterman
The role of non-muscle myosin 2 (NM2) pulsatile dynamics in generating contractile forces required for developmental morphogenesis has been characterized, however whether these pulsatile contractions are an intrinsic property of all actomyosin networks is not known. Here we used live-cell fluorescence imaging to show that transient, local assembly of NM2A "pulses" occur in the cortical cytoskeleton of single adherent cells of mesenchymal, epithelial, and sarcoma origin, independent of developmental signaling cues and cell-cell or cell-ECM interactions...
November 23, 2016: Molecular Biology of the Cell
Milton To, Clark W H Peterson, Melissa A Roberts, Jessica L Counihan, Tiffany T Wu, Mercedes S Forster, Daniel K Nomura, James A Olzmann
The endoplasmic reticulum (ER) mediates the folding, maturation, and deployment of the secretory proteome. Proteins that fail to achieve their native conformation are retained in the ER and targeted for clearance by ER-associated degradation (ERAD), a sophisticated process that mediates the ubiquitin-dependent delivery of substrates to the 26S proteasome for proteolysis. Recent findings indicate that inhibition of long-chain acyl-CoA synthetases with triacsin C, a fatty acid analog, impairs lipid droplet (LD) biogenesis and ERAD, suggesting a role for LDs in ERAD...
November 23, 2016: Molecular Biology of the Cell
Yue Qu, Ines Hahn, Stephen Webb, Simon P Pearce, Andreas Prokop
Axons are the cable-like neuronal processes wiring the nervous system. They contain parallel bundles of microtubules as structural backbones, surrounded by regularly-spaced actin rings termed the periodic membrane skeleton (PMS). Despite being an evolutionarily-conserved, ubiquitous, highly-ordered feature of axons, the function of PMS is unknown. Here we studied PMS abundance, organisation and function, combining versatile Drosophila genetics with super-resolution microscopy and various functional readouts...
November 23, 2016: Molecular Biology of the Cell
Valeria Padovano, Ivana Y Kuo, Lindsey K Stavola, Hans R Aerni, Benjamin J Flaherty, Hannah C Chapin, Ming Ma, Stefan Somlo, Alessandra Boletta, Barbara E Ehrlich, Jesse Rinehart, Michael J Caplan
Autosomal dominant polycystic kidney disease is caused by mutations in the genes encoding polycystin-1 (PC1) and polycystin-2 (PC2), which form an ion channel complex that may mediate ciliary sensory processes and regulate endoplasmic reticulum Ca(2+) release. Loss of PC1 expression profoundly alters cellular energy metabolism. The mechanisms that control the trafficking of PC1 and PC2 as well as their broader physiological roles remain poorly understood. We found that O2 levels regulate the subcellular localization and the channel activity of the polycystin complex through its interaction with the O2-sensing prolyl hydroxylase domain containing protein EGLN3 (or PHD3), which hydroxylates PC1...
November 23, 2016: Molecular Biology of the Cell
Travis R Ruch, David M Bryant, Keith E Mostov, Joanne N Engel
Pathogens can alter epithelial polarity by recruiting polarity proteins to the apical membrane, but how a change in protein localization is linked to polarity disruption is not clear. In this study we used chemically induced dimerization to rapidly relocalize proteins from the cytosol to the apical surface. We demonstrate that forced apical localization of Par3, which is normally restricted to tight junctions, is sufficient to alter apical membrane identity through its interactions with phosphatidylinositol 3-Kinase (PI3K) and the Rac1 guanine nucleotide exchange factor Tiam1...
November 23, 2016: Molecular Biology of the Cell
Anna Lürick, Jieqiong Gao, Anne Kuhlee, Erdal Yavavli, Lars Langemeyer, Angela Perz, Stefan Raunser, Christian Ungermann
Membrane fusion at endomembranes requires crosstalk between Rab GTPases and tethers to drive SNARE-mediated lipid bilayer mixing. Several tethers have multiple Rab binding sites with largely untested function. Here, we dissected the lysosomal HOPS complex as a tethering complex with just two binding sites for the Rab7-like Ypt7 protein to determine their relevance for fusion. Using tethering and fusion assays combined with HOPS mutants we show that HOPS-dependent fusion requires both Rab binding sites, with Vps39 being the stronger Ypt7-interactor than Vps41...
November 16, 2016: Molecular Biology of the Cell
Armin Haupt, Nicolas Minc
Surface charges at the inner leaflet of the plasma membrane may contribute to regulate the surface recruitment of key signaling factors. Phosphatidylserine (PS) is an abundant charged lipid, which may regulate charge distribution in different cell types. In here, we characterize the subcellular distribution and function of PS in the rod-shaped, polarized fission yeast. We find that PS preferably accumulates at cell tips and defines a gradient of negative charges along the cell surface. This polarization depends on actin-mediated endocytosis and contributes to the subcellular partitioning of charged polarity-regulating Rho-GTPases like Rho1 or Cdc42 in a protein charge-dependent manner...
November 16, 2016: Molecular Biology of the Cell
Kenneth B Buck, Andrew W Schaefer, Vincent T Schoonderwoert, Matthew S Creamer, Eric R Dufresne, Paul Forscher
Homophilic binding of Ig superfamily molecules, such as apCAM, leads to actin filament assembly near nascent adhesion sites. Such actin assembly can generate significant localized forces that have not been characterized in the larger context of axon growth and guidance. We used apCAM coated bead substrates applied to the surface of neuronal growth cones to characterize the development of forces evoked by varying stiffness of mechanical restraint. Unrestrained bead propulsion matched or exceeded rates of retrograde network flow and was dependent on Arp2/3 complex activity...
November 16, 2016: Molecular Biology of the Cell
Zhen Zhang, Yukako Nishimura, Pakorn Kanchanawong
Microtubule filaments form ubiquitous networks that specify spatial organization in cells. However, quantitative analysis of the microtubule networks is hampered by their complex architecture, limiting insights into the interplay between their organization and cellular functions. Although superresolution microscopy has greatly facilitated high-resolution imaging of microtubule filaments, the extraction of the complete filament networks from such dataset remains challenging. Here, we describe a computational tool for the automated retrieval of microtubule filaments from single-molecule-localization-based superresolution microscopy images...
November 16, 2016: Molecular Biology of the Cell
Mira I Pronobis, Natalie Deuitch, Vinya Posham, Yuko Mimori-Kiyosue, Mark Peifer
Negatively regulating key signaling pathways is critical to development and altered in cancer. Wnt signaling is kept off by the destruction complex, assembled around the tumor suppressors APC and Axin, which targets βcatenin for destruction. Axin and APC are large proteins with many domains and motifs that bind other partners. We hypothesized that if we could identify the essential regions required for APC:Axin cooperative function and use these data to design a minimal βcatenin-destruction machine, we would gain new insights into the core mechanisms of destruction complex function...
November 16, 2016: Molecular Biology of the Cell
Joshua P Garrett, Anthony M Lowery, Alejandro Adam, Andrew P Kowalczyk, Peter A Vincent
Endothelial p120-catenin (p120) maintains the level of vascular endothelial cadherin (VE-Cad) by inhibiting VE-cad endocytosis. Loss of p120 results in a decrease in VE-Cad levels leading to the formation of monolayers with decreased barrier function (as assessed by transendothelial electrical resistance [TEER]), while overexpression of p120 increases VE-Cad levels and promotes a more restrictive monolayer. To test whether reduced endocytosis mediated by p120 is required for VE-cad formation of a restrictive barrier, we restored VE-Cad levels using an endocytic-defective VE-Cad mutant...
November 16, 2016: Molecular Biology of the Cell
Zsófia Szíber, Hanna Liliom, Carlos O Oueslati Morales, Attila Ignácz, Anikó Erika Rátkai, Kornelia Ellwanger, Gisela Link, Attila Szűcs, Angelika Hausser, Katalin Schlett
Ras and Rab interactor 1 (RIN1) is predominantly expressed in the nervous system. RIN1 knockout animals have deficits in latent inhibition and fear extinction in the amygdala, suggesting a critical role for RIN1 in preventing the persistence of unpleasant memories. At the molecular level, RIN1 signals through Rab5 GTPases that control endocytosis of cell-surface receptors and Abl non-receptor tyrosine kinases that participate in actin cytoskeleton remodelling.Here, we report that RIN1 controls the plasticity of cultured mouse hippocampal neurons...
November 16, 2016: Molecular Biology of the Cell
Barbara J Mann, Sai K Balchand, Patricia Wadsworth
Mitotic motor proteins generate force to establish and maintain spindle bipolarity but how they are temporally and spatially regulated in vivo is unclear. Prior work demonstrated that the microtubule-associated protein, TPX2, targets Kinesin-5 and Kinesin-12 motors to spindle microtubules. The C-terminal domain of TPX2 regulates the localization and motility of Kinesin-5, Eg5, but if this domain regulates Kinesin-12, Kif15, is not known. We found that the C-terminal domain of TPX2 contributes to the localization of Kif15 to spindle microtubules in cells and suppresses motor walking in vitro Kif15 and Eg5 are partially redundant motors, and overexpressed Kif15 can drive spindle formation in the absence of Eg5 activity...
November 16, 2016: Molecular Biology of the Cell
Timothy D Matheson, Paul D Kaufman
Chromatin Assembly Factor 1 (CAF-1) deposits histones during DNA synthesis. The p150 subunit of human CAF-1 contains an N-terminal domain (p150N) that is dispensable for histone deposition but which promotes the localization of specific loci (Nucleolar-Associated Domains, or "NADs") and proteins to the nucleolus during interphase. One of the p150N-regulated proteins is proliferation antigen Ki-67, whose depletion also decreases the nucleolar association of NADs. Ki-67 is also a fundamental component of the perichromosomal layer (PCL), a sheath of proteins surrounding condensed chromosomes during mitosis...
November 2, 2016: Molecular Biology of the Cell
Yao Cheng, Dan Wang, Bin Wang, Huanan Li, Junjie Xiong, Shuyun Xu, Quan Chen, Kun Tao, Xiaoyan Yang, Yu Zhu, Sirong He
We performed studies to determine the role of High-mobility group box 1 (HMGB1) in cigarette smoke (CS)-induced pulmonary inflammation. After mice were exposed to five cigarettes four times a day for 3 days, toll-like receptor 4 (TLR4) expression and TLR4-mediated signaling were significantly up-regulated, and HMGB1 had translocated from the nucleus to the cytoplasm in lung epithelial cells and was then released into the extracellular lung space. Upon CS exposure, inflammatory cell recruitment and proinflammatory cytokine production were significantly increased in the lung tissue and BAL, and these effects depended on the TLR4 signaling pathway...
November 2, 2016: Molecular Biology of the Cell
Julien Villeneuve, Juan Duran, Margherita Scarpa, Laia Bassaganyas, Josse Van Galen, Vivek Malhotra
Golgi specific sialyltransferase (ST) expressed as a chimera with the rapamycin-binding domain of mTOR, FRB, relocates to the endoplasmic reticulum (ER) in cells exposed to rapamycin that also express invariant chain (Ii)-FKBP in the ER. This result has been taken to propose that Golgi resident enzymes cycle to the ER constitutively. We show ST-FRB is trapped in the ER even without Ii-FKBP upon rapamycin addition. This is because ER-Golgi cycling FKBP proteins contain a C-terminal KDEL-like sequence, bind ST-FRB in the Golgi and are transported together back to the ER by KDEL-receptor mediated retrograde transport...
November 2, 2016: Molecular Biology of the Cell
Meghan C Barnhart-Dailey, Prasad Trivedi, P Todd Stukenberg, Daniel R Foltz
Centromeric chromatin is required for kinetochore assembly during mitosis and accurate chromosome segregation. A unique nucleosome containing the histone H3-specific variant CENP-A is the defining feature of centromeric chromatin. In humans, CENP-A nucleosome deposition occurs in early G1 just following mitotic exit at the time when the CENP-A deposition machinery localizes to centromeres. The mechanism by which CENP-A is deposited onto an existing, condensed chromatin template is not understood. Here, we identify the selective association of the CENP-A chaperone HJURP with the condensin II complex and not condensin I...
November 2, 2016: Molecular Biology of the Cell
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