dictyNews Electronic Edition Volume 32, number 10 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]