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dictyNews Volume 22 Number 03
Dicty News
Electronic Edition
Volume 22, number 3
February 6, 2004
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 Dicty-News, the Dicty Reference database and other
useful information is available at dictyBase - http://dictybase.org.
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Abstracts
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The necessity of mitochondrial genome DNA for normal development of
Dictyostelium cells
Junji Chida, Hitomi Yamaguchi, Aiko Amagai and Yasuo Maeda*
Department of Developmental Biology and Neurosciences, Graduate School of
Life Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan
J. Cell Sci. (in press)
Summary
Most unexpectedly, there is now increasing evidence that mitochondria have
novel and crucial functions in the regulatory machinery of the
growth/differentiation transition, cell-type determination, cellular
movement and pattern formation. Here we created rho delta cells with a
reducedamount (about 1/4) of mitochondrial DNA (mtDNA) from Dictyostelium
discoideum Ax-2 cells, by means of exposure of Ax-2 cells to ca. 30 mg/ml of
ethidium bromide (EtBr) in axenic growth medium. Importantly, the rho delta
cellsexhibited a series of fascinating behaviors: when they were starved,
theyexhibited a marked delay of differentiation and stopped their
development atthe slug stage, thus failing to construct fruiting bodies.
Moreover, cellpatterning and cell-type proportioning were found to be greatly
modified inslugs (referred to as rho delta slugs) derived from rho delta cells.
That is,prestalk differentiation was significantly enhanced in rho delta slugs,
whileprespore differentiation was markedly inhibited. In addition, the clear
anterior prestalk/posterior prespore pattern was considerably disturbed in
rho delta slugs, presumably because of incomplete sorting between the two
typesof differentiated cells. After the assay of phototaxis, rho delta slugs
alsoexhibited highly disordered movement toward the light source. Taken
togetherthese results suggest that mtDNA might have important multiple
functions ina variety of cellular processes during Dictyostelium development.
Submitted by: Yasuo Maeda [ymaeda@mail.cc.tohoku.ac.jp]
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Structure of an activated Dictyostelium STAT
in its DNA-unbound form
Montserrat Soler-Lopez1,3, Carlo Petosa1,3, Masashi Fukuzawa2,
Raimond Ravelli1, Jeffrey G. Williams2, Christoph W. Mueller1
Molecular Cell, in press
1 European Molecular Biology Laboratory, Grenoble Outstation, B.P. 181,
38042 Grenoble Cedex 9, France
2 School of Biological Sciences, University of Dundee, MSI/WTB Complex
Dow Street, Dundee, DD1 5EH, U.K.
3 These authors contributed equally to this work.
Summary
Dd-STATa is a STAT protein which transcriptionally regulates cellular
differentiation in Dictyostelium discoideum, the only non-metazoan known
to employ SH2 domain signalling. The 2.7 Angstrom crystal structure of a tyrosine
phosphorylated Dd-STATa homodimer reveals a four-domain architecture similar
to that of mammalian STATs 1 and 3, but with an inverted orientation for the
coiled coil domain. Dimerization is mediated by SH2 domain:phosphopeptide
interactions and by a novel, direct interaction between SH2 domains. The
unliganded Dd-STATa dimer adopts a fully extended conformation remarkably
different from that of the DNA-bound mammalian STATs, implying a large
conformational change upon target site recognition. Buried hydrophilic
residues predicted to destabilize the coiled coil domain suggest how
hydrophobic residues may become exposed and mediate nuclear export.
Functional and evolutionary implications for metazoan STAT proteins are
discussed.
Submitted by: Jeff Williams [j.g.williams@dundee.ac.uk]
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THE STAT PROTEINS OF DICTYOSTELIUM
JEFFREY G. WILLIAMS - N. B. This is a review article, written some time ago,
so some aspects are out of date, e.g. a paper on Dd-STATb is now published and
the complete genome sequence reveals no additional STATs.
In 'Signal Transducers and Activators of Transcription (STATs)' edited by
Pravin B. Sehgal, M.D., Ph.D., David E. Levy, Ph.D., Toshio Hirano, M.D.Kluwer,
Acad. Publishers
School of Life Sciences
University of Dundee
MSI/WTB Complex
Dow Street
Dundee DD1 5EH, U.K.
Overview
Dictyostelium possesses at least four STATs. They all contain the three
defining STAT domains: a DNA binding domain, an SH2 domain and a site of
tyrosine phosphorylation. However, their N terminal halves are highly
diverged from those of metazoan STATs and they also appear to lack
C terminal transcriptional trans-activation domains; in all four proteins
the site of tyrosine phosphorylation is just a few amino acids from the
C terminus. Only two of the proteins, Dd-STATa and Dd-STATc, have been
characterised and they function, in different contexts, as either
activators or repressor of gene expression. Dd-STATa and Dd-STATc both
regulate the transcription of prestalk and stalk cell-specific genes and
Dd-STATc also activates specific gene trancription in response to a variety
of environmental stresses. Dd-STATa and Dd-STATc have additional, less
well-characterised roles; disruption of the Dd-STATa gene impairs cellular
chemotaxis to cAMP and retards early development, while inactivation of the
Dd-STATc gene reduces the cellular growth rate and accelerates early
development. Upon binding of extracellular cAMP to its serpentine receptor
Dd-STATa is very rapidly tyrosine phosphorylated and translocates to the
nucleus Activation and nuclear accumulation do not require G protein
function. Over a slower time scale, cAMP signalling also activates GSK-3
to phosphorylate Dd-STATa and this facilitates its export from the nucleus.
During development Dd-STATc is activated by DIF-1, a chlorinated
hexaphenone. DIF-1 induces the nuclear accumulation of Dd-STATc by reducing
its rate of export from the nucleus. The signalling steps that activate
Dd-STATa and Dd-STATc are unknown and, although the Dictyostelium genome
project is now 95% complete, there is as yet no obvious candidate for a
JAK or a receptor tyrosine kinase.
Submitted by: Jeff Williams [j.g.williams@dundee.ac.uk]
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Sensitisation of Dictyostelium chemotaxis by PI3-kinase mediated
self-organising signalling patches
Marten Postma, Jeroen Roelofs, Joachim Goedhart, Harrit M. Loovers,
Antonie J.W.G. Visser, and Peter J.M. Van Haastert
Department of Biochemistry, University of Groningen, Nijenborgh 4,
9747 AG Groningen, the Netherlands
J Cell Science, in press
The leading edge of Dictyostelium cells in chemoattractant gradients can be
visualised using GFP tagged to the PH domain of CRAC, which presumable binds
PI(3,4,5)P3. Uniform cAMP induces persistent translocation PHCrac-GFP from
the cytosol to multiple patches, which are similar to the single patch of
PHCrac-GFP at the leading edge in a cAMP gradient. We show that cAMP
determines the probability of patch formation (half-maximal effect at
0.5 nM cAMP), but not size, lifetime or intensity of patches, indicating
that patches are self-organising structures. A pseudopod is extended from
the area of the cell with a PHCrac-GFP patch at about 10 s after patch
formation. Cells treated with the F-actin inhibitor latrunculin A are round
without pseudopodia; uniform cAMP still induces localised patches of
PHCrac-GFP. Inhibition of PI3-kinase activity with LY294002 inhibits
PHCrac-GFP patches and inhibits chemotaxis towards nanomolar cAMP, but has
no effect at higher cAMP concentrations. Thus, very low cAMP concentrations
induce self-organising PHCrac-GFP patches that serve as a spatial cue for
pseudopod formation, which enhances the sensitivity and amplitude of
chemotactic movement.
Submitted by: Peter van Haastert [P.J.M.van.Haastert@chem.rug.nl]
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[End Dicty News, volume 22, number 3]
