dictyNews
Electronic Edition
Volume 32, number 10
April 19, 2009

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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 [[log in to unmask]]
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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 [[log in to unmask]]
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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 [[log in to unmask]]
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[End dictyNews, volume 32, number 11]