dictyNews
Electronic Edition
Volume 40, number 1
January 3, 2014
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Abstracts
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The inverse BAR-domain protein IBARa drives membrane remodelling
to control osmoregulation, phagocytosis and cytokinesis
Joern Linkner (1), Gregor Witte (2), Hongxia Zhao (3), Alexander
Junemann (1), Benjamin Nordholz (1), Petra Runge-Wollmann (2),
Pekka Lappalainen (3) and Jan Faix (1)
1) Institute for Biophysical Chemistry, Hannover Medical School,
30625 Hannover, Germany; 2) Gene Center and Department of Biochemistry
at the Ludwig-Maximilians-University, 81377, Munich, Germany;
3) Cellular Biotechnology, Institute of Biotechnology, University of Helsinki,
Helsinki 00014, Finland.
Journal of Cell Science, in press
Here, we analyzed the single I-BAR family member IBARa from D. discoideum.
The X-ray structure of the N-terminal I-BAR domain solved at 2.2 Å resolution
revealed an all-alpha helical structure that self-associates into a 165 Å
zeppelin-shaped antiparallel dimer. The structural data are consistent with its
shape in solution obtained by small-angle X-ray-scattering. Cosedimentation,
fluorescence-anisotropy as well as fluorescence and electron microscopy
revealed the I-BAR domain to bind preferentially to phosphoinositide-containing
vesicles and drive the formation of negatively curved tubules.
Immunofluorescence labelling further showed accumulation of endogenous
IBARa at the tips of filopodia, the rim of constricting phagocytic cups, in foci
connecting dividing cells during the final stage of cytokinesis, and most
prominently at the osmoregulatory contractile vacuole (CV). Consistently,
IBARa-null mutants displayed defects in CV formation and discharge, growth,
phagocytosis and mitotic cell division, whereas filopodia formation was not
compromised. Of note, IBARa-null mutants were also strongly impaired in cell
spreading. Together, these data suggest IBARa to constitute an important
regulator of numerous cellular processes intimately linked with the dynamic
rearrangement of cellular membranes.
Submitted by Jan Faix [[log in to unmask]]
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A PIP5Kinase essential for efficient chemotactic signalling
Louise Fets, John Nichols and Robert R. Kay
Current Biology, in press
In neutrophils and Dictyostelium, chemoattractant gradients generate directed
cell migration by eliciting signalling events that bias intrinsic motility and
favour the production and retention of up-gradient pseudopods[1, 2].
Phosphoinositides are actively regulated during chemotaxis in these cells,
most iconically in the production of PI(3,4,5)P3 gradients within the plasma
membrane[3, 4]. Although it is now known that PI(3,4,5)P3 signalling is non-
essential for gradient sensing[5, 6], the role of the related phosphoinositide
PI(4,5)P2 is little understood, despite its clear importance in many cell-
biological processes[7]. We describe a PIP5kinase, PikI, which produces
PI(4,5)P2 and is essential for efficient chemotaxis of Dictyostelium cells.
Without PikI, PI(4,5)P2 levels are reduced by 90%, and while pikI- cells move
at normal speeds, they are highly disorientated in cAMP gradients. Following
chemotactic stimulation, Ras is efficiently activated in pikI- cells, yet Ras-
dependent responses (including activation of PKB) are severely impaired. PikI
is phosphorylated by PKB[8], and in vitro studies of a phospho-mimic mutant
suggest that this phosphorylation increases PikI activity. We propose that
adequate PI(4,5)P2 levels are required to couple activated Ras to its
downstream effectors and that these levels are regulated by PikI, making it a
crucial player in gradient sensing.
Submitted by Louise Fets [[log in to unmask]]
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A Retinoblastoma orthologue is required for the sensing of a chalone i
n Dictyostelium
Deenadayalan Bakthavatsalam,* Michael J. V. White, Sarah E. Herlihy, J
onathan E. Phillips, and Richard H. Gomer
Department of Biology, Texas A&M University, College Station, Texas, USA
*Current address: Texas Heart Institute, Houston, Texas, USA
Eukaryotic Cell, In press
Retinoblastoma-like proteins regulate cell differentiation and inhibit cell
proliferation. The Dictyostelium Retinoblastoma orthologue RblA affects the
differentiation of cells during multicellular development, but it is unclear
whether RblA has a significant effect on Dictyostelium cell proliferation,
which is inhibited by the secreted proteins AprA and CfaD. We found that
rblA¯ cells in shaking culture proliferate to a higher density, die faster after
reaching stationary density, and after starvation have a lower spore viability
compared to wild-type cells, possibly because in shaking culture rblA¯ cells
have both increased cytokinesis and lower extracellular accumulation of CfaD.
However, rblA¯ cells have abnormally slow proliferation on bacterial lawns.
Recombinant AprA inhibits the proliferation of wild-type cells, but not that of
rblA¯ cells, whereas CfaD inhibits the proliferation of both wild type cells and
rblA¯ cells. Similar to aprA¯ cells, rblA¯ cells have a normal mass and protein
accumulation rate on a per nucleus basis, indicating that RblA affects cell
proliferation but not cell growth. AprA also functions as a chemorepellent, and
RblA is required for proper AprA chemorepellent activity despite the fact that
RblA does not affect cell speed. Together, our data indicate that an autocrine
proliferation-inhibiting factor acts through RblA to regulate cell density in
Dictyostelium, suggesting that such factors may signal through
Retinoblastoma-like proteins to control the sizes of structures such as
developing organs or tumors.
Submitted by Richard Gomer [[log in to unmask]]
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Regulation of transcriptional bursting by a naturally oscillating signal
Adam M. Corrigan and Jonathan R. Chubb
MRC Laboratory for Molecular Cell Biology, Department of Cell and
Developmental Biology, University College London, Gower Street, London,
WC1E 6BT
Current Biology, in press
Transcription is highly stochastic, occurring in irregular bursts[1-3]. For
temporal and spatial precision of gene expression, cells must somehow
deal with this noisy behaviour. To address how this is achieved, we
investigated how transcriptional bursting is entrained by a naturally
oscillating signal, by direct measurement of transcription together with
signal dynamics in living cells. We identify a Dictyostelium gene showing
rapid transcriptional oscillations with the same period as extracellular
cAMP signalling waves. Bursting approaches anti-phase to cAMP waves,
with accelerating transcription cycles during differentiation. While coupling
between signal and transcription oscillations was clear at the population
level, single cell transcriptional bursts retained considerable heterogeneity,
indicating transcription is not solely governed by signalling frequency.
Previous studies imply burst heterogeneity reflects distinct chromatin states
[4-6]. Here we show heterogeneity is determined by multiple intrinsic and
extrinsic cues, and is maintained by a transcriptional persistence.
Unusually for a persistent transcriptional behaviour, the lifetime was only
20 minutes, with rapid randomisation of transcriptional state by the
response to oscillatory signalling. Linking transcription to rapid signalling
oscillations allows reduction of gene expression heterogeneity by temporal
averaging, providing a mechanism to generate precision in cell choices
during development.
Submitted by Jonathan Chubb [[log in to unmask]]
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[End dictyNews, volume 40, number 1]
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