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dictyNews Volume 32 Number 11
dictyNews
Electronic Edition
Volume 32, number 11
April 19, 2009
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
useful information is available at dictyBase - http://dictybase.org.
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Abstracts
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Novel functions of ribosomal protein S6 (RPS6) in growth and differentiation
of Dictyostelium cells
Kazutaka Ishii1, Yusaku Nakao1, Aiko Amagai2 and Yasuo Maeda1*
1Department of Developmental Biology and Neurosciences, Graduate School
of Life Sciences, Tohoku University, Sendai 980-8578
2Department of Biomolecular Science, Graduate School of Life Sciences,
Tohoku University, Katahira 2-1-1, Aoba-Ku, Sendai 980-8577, Japan
Develop. Growth Differ., in press
We have previously shown that in Dictyostelium cells a 32 kDa protein is rapidly
and completely dephosphorylated in response to starvation that is essential for
the initiation of differentiation (Akiyama and Maeda 1992). In the present work,
this phosphoprotein was identified as a homologue (Dd-RPS6) of ribosomal
protein S6 (RPS6) that is an essential member for protein synthesis. As
expected, Dd-RPS6 seems to be absolutely required for cell survival, because
we failed to obtain antisense-RNA mediated cells as well as Dd-rps6-null cells
by homologous recombination in spite of many trials. In many kinds of cell lines,
RPS6 is known to be located in the nucleus and cytosol, but Dd-RPS6 is
predominantly in the cell cortex with cytoskeletons, and in the contractile ring
of just-dividing cells. In this connection, the overexpression of Dd-RPS6 greatly
impairs cytokinesis during axenic shake-cultures in growth medium, resulting
in formation of multinucleate cells. Much severe impairment of cytokinesis was
observed when Dd-RPS6-overexpressing cells (Dd-RPS6OE cells) were
incubated on a living Escherichia coli lawn. The initiation of differentiation
triggered by starvation was also delayed in Dd-RPS6OE cells. In addition,
Dd-RPS6OE cells exhibit defective differentiation into prespore cells and
spores during late development. Thus, it is likely that the proper expression
of Dd-RPS6 may be of importance for the normal progression of late
differentiation as well as for the initiation of differentiation.
Submitted by: Yasuo Maeda [kjygy352@ybb.ne.jp]
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Transcriptional down-regulation and rRNA cleavage in Dictyostelium discoideum
mitochondria during Legionella pneumophila infection.
Chenyu Zhang and Adam Kuspa
The Departments of Biochemistry and Molecular Biology, Pharmacology, and
Molecular and Human Genetics. Baylor College of Medicine, One Baylor Plaza,
Houston TX 77030.
PLoS One, In press
Background
Bacterial pathogens employ a variety of survival strategies when they invade
eukaryotic cells. The amoeba Dictyostelium discoideum is used as a model
host to study the pathogenic mechanisms that Legionella pneumophila, the
causative agent of Legionnaire’s disease, uses to kill eukaryotic cells.
Methodology/Principal Findings
Under standard conditions, infection of D. discoideum by L. pneumophila
results in a decrease in mitochondrial messenger RNAs, beginning more
than 8 hours prior to detectable host cell death. These changes can be
mimicked by hydrogen peroxide treatment, but not by other cytotoxic agents.
The mitochondrial large subunit ribosomal RNA (LSU rRNA) is also cleaved
at three specific sites during the course of infection. Two LSU rRNA fragments
appear first, followed by smaller fragments produced by additional cleavage
events. The initial LSU rRNA cleavage site is predicted to be on the surface
of the large subunit of the mitochondrial ribosome, while two secondary sites
map to the predicted interface with the small subunit. No LSU rRNA cleavage
was observed after exposure of D. discoideum to hydrogen peroxide, or other
cytotoxic chemicals that kill cells in a variety of ways. Functional
L. pneumophila type II and type IV secretion systems are required for the
cleavage, establishing a correlation between the pathogenesis of
L. pneumophila and D. discoideum LSU rRNA destruction. LSU rRNA
cleavage was not observed in L. pneumophila infections of Acanthamoeba
castellanii or human U937 cells, suggesting that L. pneumophila uses
distinct mechanisms to interrupt metabolism in different hosts.
Conclusion/Significance
L. pneumophila infection of D. discoideum results in dramatic decrease
of mitochondrial RNAs, and in the specific cleavage of mitochondrial rRNA.
The predicted location of the cleavage sites on the mitochondrial ribosome
suggests that rRNA destruction is initiated by a specific sequence of
events. These findings suggest that L. pneumophila specifically disrupts
mitochondrial protein synthesis in D. discoideum during the course
of infection.
Submitted by: Adam Kuspa [akuspa@bcm.edu]
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Scaffolding Proteins that Regulate the Actin Cytoskeleton in Cell Movement.
S.J. Annesley and P.R. Fisher
Department of Microbiology, La Trobe University, Melbourne, Australia.
In press: Cell Movement: New Research Trends. Editors: T. Abreu and G. Silva.
Nova Science Publishers, Inc.
Actin is the main component of the microfilament system in all eukaryotic
cells and is essential for most intra- and inter-cellular movement including
muscle contraction, cell movement, cytokinesis, cytoplasmic organisation and
intracellular transport. The polymerisation and depolymerisation of actin
filaments in nonmuscle cells is highly regulated and the reorganisation of
the actin cytoskeleton can occur within seconds after chemotactic stimulation.
There are many proteins which are involved in the regulation of the actin
cytoskeleton. These include receptors which receive chemotactic stimuli,
G proteins, second messengers, signalling molecules, kinases, phosphatases
and transcription factors. These proteins are varied and numerous and are
involved in multiple pathways. Despite the large number of proteins, there
are not enough to coordinate the various responses of the cytoskeleton. An
additional level of regulation is conferred by scaffolding proteins. Due to
the presence of numerous protein interaction domains, scaffolding proteins
can tether various proteins to a certain location within the cell to
facilitate the rapid transfer of signals from one protein to the next.
This colocalisation of the components of a particular pathway also helps
to prevent unwanted crosstalk with components of other pathways. Tethering
receptors, kinases, phosphatases and cytoskeletal components to a particular
location within a cell helps ensure efficient relaying and feedback inhibition
of signals to enable rapid activation and inactivation of responses.
Scaffolding proteins are also thought to stabilise the otherwise weak
interactions between particular proteins in a cascade and to catalyse the
activation of the pathway components. There are numerous scaffolding
proteins involved in the regulation of the cytoskeleton and this chapter
has focussed on examples from several groups of scaffolding proteins
including the MAPK scaffolds, the AKAPs, scaffolds of the post synaptic
density and actin binding scaffolding proteins.
Submitted by: Paul R Fisher [P.Fisher@latrobe.edu.au]
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[End dictyNews, volume 32, number 11]
