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dictyNews Volume 34 Number 12
dictyNews
Electronic Edition
Volume 34, number 12
April 9, 2010
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu
or by using the form at
http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit.
Back issues of dictyNews, the Dicty Reference database and other
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=========
Abstracts
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SunB, a novel SUN domain-containing protein required for development
of Dictyostelium discoideum
Nao Shimada 1, 2, Kei Inouye 3, Satoshi Sawai 2 and
Takefumi Kawata 1, *
1Department of Biology, Faculty of Science, Toho University, 2-2-1 Miyama,
Funabashi, Chiba 274-8510, Japan,
2Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba,
Meguro-ku, Tokyo 153-8902, Japan
3Department of Botany, Graduate School of Science, Kyoto University,
Sakyo-ku, Kyoto 606-8502, Japan
*Author to whom all correspondence should be addressed.
Devleop. Growrh & Differ., In press.
A gene, sunB, encoding a novel class of Sad1 and UNC-84 (SUN) domain,
was isolated from a cDNA screen for suppressors of a mutation in Dd-STATa –
a Dictyostelium homologue of metazoan STAT (signal transducers and activators
of transcription). The SunB protein localized in the area around the nucleus in
growing cells, but in the multicellular stages it was predominantly found in
prespore vacuoles (PSVs). A disruptant of sunB was multinucleated in the
vegetative phase; during development it formed mounds with multiple tips and
failed to culminate. The mutation was cell autonomous, and showed reduced
expression of the prespore marker gene pspA and elevated expression of
marker genes for prestalk AB cells. Interestingly, the level of SunB was
abnormally high in the prestalk cells of Dd-STATa mutants, which are defective
in culmination. We conclude that SunB is essential for accurate prestalk/prespore
differentiation during Dictyostelium development and that its cell-type dependent
localization is regulated by a Dd-STATa-mediated signaling pathway.
Submitted by Takefumi Kawata [tkawata@bio.sci.toho-u.ac.jp]
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An unconventional myosin required for cell polarization and chemotaxis
Laura M. Breshears, Deborah Wessels, David R. Soll, and Margaret A. Titus
Proc.Natl.Acad. Sci., in press
MyTH/FERM (myosin tail homology 4/band 4.1, ezrin, radixin, and moesin)
myosins have roles in cellular adhesion, extension of actin-filled projections
such as filopiodia and stereocilia, and directional migration. The amoeba
Dictyostelium discoideum expresses a simple complement of MyTH/FERM
myosins, a class VII (M7) myosin required for cell-substrate adhesion and a
unique myosin named MyoG. Mutants lacking MyoG exhibit a wide range of
normal actin-based behaviors, including chemotaxis to folic acid, but have a
striking defect in polarization and chemotaxis to cAMP. Although the myoG
mutants respond to cAMP stimulation by increasing persistence and weakly
increasing levels of cortical F-actin, they do not polarize; instead, they
maintain a round shape and move slowly and randomly when exposed to
a chemotactic gradient. The mutants also fail to activate and localize PI3K
to the membrane closest to the source of chemoattractant. These data
reveal a role for a MyTH/FERM myosin in mediating early chemotactic
signaling and suggest that MyTH/FERM proteins have conserved roles in
signaling and the generation of cell polarity.
Submitted by Deborah Wessels [deborah-wessels@uiowa.edu]
--------------------------------------------------------------------------------
Involvement of the Cytoskeleton in Controlling Leading Edge Function
during Chemotaxis
Susan Lee, Zhouxin Shen, Douglas N. Robinson, Steven Briggs, and
Richard A. Firtel
Mol. Biol. Cell., in press
In response to directional stimulation by a chemoattractant, cells rapidly
activate a series of signaling pathways at the site closest to the
chemoattractant source that leads to F-actin polymerization, pseudopod
formation, and directional movement up the gradient. Ras proteins are
major regulators of chemotaxis in Dictyostelium; they are activated at
the leading edge, are required for chemoattractant-mediated activation
of PI3K and TORC2, and are one of the most rapid responders with
activity peaking at ~3 sec after stimulation. We demonstrate that in
myosin II (MyoII) null cells, Ras activation is highly extended and is not
restricted to the site closest to the chemoattractant source. This causes
elevated, extended, and spatially misregulated activation of PI3K and
TORC2 and their effectors Akt/PKB and PKBR1, and elevated F-actin
polymerization. We further demonstrate that disruption of specific
IQGAP/cortexillin complexes, which also regulate cortical mechanics,
causes extended activation of PI3K and Akt/PKB but not Ras activation.
Our findings suggest that MyoII and IQGAP/cortexillin play key roles in
spatially and temporally regulating leading edge activity and, through this,
the ability of cells to restrict the site of pseudopod formation.xation of the
MTBD into the stable low affinity state.
Submitted by Rick Firtel [rafirtel@ucsd.edu]
--------------------------------------------------------------------------------
SCAR/WAVE is activated at mitosis and drives myosin-independent
cytokinesis
Jason S. King1,3, Douwe M. Veltman1, Marios Georgiou2, Buzz Baum2
and Robert H. Insall1
1Beatson Institute for Cancer Research, Garscube Estate, Switchback
Road, Bearsden, Glasgow, UK. G61 1BD.
2MRC-LMCB, University College London, Gower Street, London, UK.
WC1E 6BT
J. Cell Sci., in press
Cell division requires the tight coordination of multiple cytoskeletal
pathways. The best understood of these involves myosin II-dependent
constriction around the cell equator but both Dictyostelium and mammalian
cells also use a parallel, adhesion-dependent mechanism to generate
furrows. We show that the actin nucleation factor SCAR/WAVE is strongly
activated during Dictyostelium cytokinesis. This activation localises to large
polar protrusions, driving separation of the daughter cells. This continues for
10 minutes after division before the daughter cells revert to normal random
motility, indicating that this is a tightly regulated process. We demonstrate
that SCAR activity is essential to drive myosin II-independent cytokinesis,
and stabilises the furrow, ensuring symmetrical division. SCAR is also
responsible for the generation of MiDASes, mitosis-specific actin-rich
adhesions. Loss of SCAR in both Dictyostelium and Drosophila leads to a
similar mitotic phenotype, with severe mitotic blebbing, indicating conserved
functionality. We also find that the microtubule end-binding protein EB1 is
required to restrict SCAR localisation and direct migration. EB1-null cells
also exhibit decreased adhesion during mitosis. Our data reveal a
spindle-directed signalling pathway that regulates SCAR activity, migration
and adhesion at mitosis.
Submitted by Jason King [J.king@beatson.gla.ac.uk]
--------------------------------------------------------------------------------
Autophagic cell death in Dictyostelium requires the receptor histidine
kinase DhkM
Corinne Giusti, Marie-Françoise Luciani, Sarina Ravens, Alexandre Gillet
and Pierre Golstein
Molecular Biology of the Cell, in press
Dictyostelium constitutes a genetically tractable model for the analysis of
autophagic cell death (ACD). During ACD Dictyostelium cells first transform
into paddle cells, then become round, synthesize cellulose, vacuolize and die.
Through random insertional mutagenesis we identified the receptor histidine
kinase DhkM as being essential for ACD. Surprisingly, different DhkM mutants
showed distinct non-vacuolizing ACD phenotypes. One class of mutants
arrested ACD at the paddle cell stage, perhaps through a dominant negative
effect. Other mutants, however, progressed further in the ACD programme.
They underwent rounding and cellulose synthesis but stopped before
vacuolization. Moreover, they underwent clonogenic but not morphological
cell death. Exogenous 8-bromo-cAMP restored vacuolization and death.
A role for a membrane receptor at a late stage of the ACD pathway is
puzzling, raising questions as to which ligand it is a receptor for and which
moieties it phosphorylates. Altogether, DhkM is the most downstream known
molecule required for this model ACD, and its distinct mutants genetically
separate previously undissociated late cell death events.
Submitted by Pierre Golstein [golstein@ciml.univ-mrs.fr]
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[End dictyNews, volume 34, number 12]
