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dictyNews Volume 32 Number 11

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Dicty News
 · 1 year ago

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.

=========
Abstracts
=========


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]
--------------------------------------------------------------------------------


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]
--------------------------------------------------------------------------------

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]
==============================================================
[End dictyNews, volume 32, number 11]

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