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dictyNews Volume 20 Number 10
Dicty News
Electronic Edition
Volume 20, number 10
May 30, 2003
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu.
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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Superoxide signalling required for multicellular development of Dictyostelium.
Gareth Bloomfield and Catherine Pears
Biochemistry Department, Oxford University, South Parks Road, Oxford OX1 3QU, UK
J. Cell Science, in press
Reactive oxygen species are known to have a signalling role in many organisms.
In bacteria and yeast various response systems have evolved to combat oxidative
stress which are triggered by reactive oxygen species. Mammals and plants are
known to actively generate reactive oxygen species such as superoxide during
signalling responses to a variety of extracellular factors. We report here
the generation of superoxide as a signalling molecule in early development of
Dictyostelium discoideum. Dictyostelium grows as single amoebae but, on
starvation, the single cells aggregate to form a multicellular organism.
Superoxide is generated in response to a secreted factor during the
transition to the multicellular phase of development. Scavenging superoxide,
either pharmacologically or by overexpressing the enzyme superoxide
dismutase, inhibits the formation of the aggregate. This report of the use of
superoxide as a signalling molecule in a lower eukaryote as it switches to a
multicellular phase suggests that this signalling mechanism arose early in
the evolution of multicellular organisms, perhaps as a necessary consequence
of the need to diversify the number and type of signalling pathways available
to facilitate intercellular communication.
Submitted by: Catherine Pears [catherine.pears@bioch.ox.ac.uk]
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CF45-1, a secreted protein which participates in group size regulation in
Dictyostelium
Debra A. Brock, R. Diane Hatton, Dan-Victor Giurgiutiu, Brenton Scott, Wonhee
Jang, Robin Ammann, and Richard H. Gomer
Howard Hughes Medical Institute and Department of Biochemistry and Cell Biology,
MS-140, Rice University, 6100 S. Main Street, Houston, TX 77005 USA
Eukaryotic Cell, in press
Developing Dictyostelium cells aggregate to form fruiting bodies containing
typically 2 x 104 cells. To prevent the formation of an excessively large
fruiting body, streams of aggregating cells break up into groups if there
are too many cells. The breakup is regulated by a secreted complex of
polypeptides called counting factor (CF). Countin and CF50 are two of the
components of CF. Disrupting the expression of either of these proteins
results in cells secreting very little detectable CF activity, and as a
result aggregation streams remain intact and form large fruiting bodies,
which invariably collapse. We find that disrupting the gene encoding a
third protein present in crude CF, CF45-1, also results in the formation
of large groups when cells are grown with bacteria on agar plates and then
starve. However, unlike countinø and cf50ø cells, cf45-1ø cells sometimes
form smaller groups than wild-type cells when the cells are starved on
filter pads. The predicted amino acid sequence of CF45-1 has some
similarity to lysozyme, but recombinant CF45-1 has no detectable lysozyme
activity. In the exudates from starved cells, CF45-1 is present in a
~450 kDa fraction that also countin and CF50, suggesting that it is part of
a complex. Recombinant CF45-1 decreases group size in colonies of cf45-1ø
cells with an EC50 of ~8 ng/ ml and in colonies of wild-type and cf50ø
cells with an EC50 of ~40 ng/ml. Like countinø and cf50ø cells, cf45-1ø
cells have high levels of cytosolic glucose, high cell-cell adhesion, and
low cell motility. Together, the data suggest that CF45-1 participates in
group size regulation in Dictyostelium.
Submitted by: Richard Gomer [richard@bioc.rice.edu]
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Regulated expression of the centrosomal protein DdCP224 affects microtubule
dynamics and reveals mechanisms for the control of supernumerary centrosome
number
Ralph Graef*, Ursula Euteneuer, Thi-Hieu Ho, and Markus Rehberg
Adolf-Butenandt-Institut / Zellbiologie, Universitaet Muenchen, Schillerstr.
42, D-80336 Muenchen, Germany
Mol. Biol. Cell, in press
The Dictyostelium XMAP215-family member DdCP224 is involved in centrosome
duplication and cytokinesis and is concentrated at the centrosome and
microtubule tips. Here we have created a DdCP224 promoter replacement
mutant that allows both over- and underexpression. Overexpression led to
supernumerary MTOCs and, independently, an increase of the number of
multinuclear cells. Electron microscopy demonstrated that supernumerary
MTOCs represented bona fide centrosomes. Live cell imaging of DdCP224-GFP
mutants also expressing GFP-histone2B as a DNA label revealed that
supernumerary centrosomes were also competent of cell cycle-dependent
duplication. By contrast, underexpression of DdCP224 inhibited cell growth,
reduced the number and length of astral microtubules, and caused nocodazole
hypersensitivity. Moreover, microtubule regrowth after nocodazole removal
was dependent on DdCP224. Underexpression also resulted in a striking
disappearance of supernumerary centrosomes and multinuclear cells caused by
previous overexpression. We show for the first time by live cell
observation that the number of supernumerary centrosomes can be reduced
either by centrosome fusion (coalescence) or by the formation of cytoplasts
containing supernumerary centrosomes during cytokinesis.
Submitted by: Ralph Graef [rgraef@lrz.uni-muenchen.de]
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[End Dicty News, volume 20, number 10]
