dictyNews
Electronic Edition
Volume 34, number 12
April 9, 2010
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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 [[log in to unmask]]
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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 [[log in to unmask]]
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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 [[log in to unmask]]
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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 [[log in to unmask]]
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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 [[log in to unmask]]
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[End dictyNews, volume 34, number 12]
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