dictyNews
Electronic Edition
Volume 35, number 8
October 1, 2010

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Abstracts
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Ras Proteins Have Multiple Functions In Vegetative Cells Of
Dictyostelium

Parvin Bolourani, George Spiegelman and Gerald Weeks

Department of Microbiology and Immunology, Life Sciences Centre,
University of British Columbia, Vancouver, B.C., Canada


Eukaryotic Cell, in press

During the aggregation of Dictyostelium cells, signaling through RasG
is important in regulating cAMP chemotaxis, while signaling through
RasC is important in regulating the cAMP relay.  However, RasC is
capable of substituting for RasG for chemotaxis, since rasG- null cells
are only partially deficient in chemotaxis, while rasC-/rasG- cells are
totally incapable of chemotaxis.  In this study we have examined the
possible functional overlap between RasG and RasC in vegetative
cells by comparing the vegetative cell properties of rasG-, rasC- and
rasC-/rasG- cells.  In addition, since RasD, a protein not normally
found in vegetative cells, is expressed in vegetative rasG- and
rasC-/rasG- cells and appears to partially compensate for the absence
of RasG, we have also examined the possible functional overlap
between RasG and RasD, by comparing the properties of rasG- and
rasC-/rasG- cells with those of the mutant cells expressing higher
levels of RasD.  The results of these two lines of investigation show
that RasD is capable of totally substituting for RasG for cytokinesis
and growth in suspension, while RasC is without effect.  In contrast,
for chemotaxis to folate, RasC is capable of partially substituting for
RasG, but RasD is totally without effect.  Finally neither RasC nor
RasD is able to substitute for the role that RasG plays in regulating
actin distribution and random motility.  These specificity studies
therefore delineate three distinct and none overlapping functions for
RasG in vegetative cells.


Submitted by  Geryy Weeks [[log in to unmask]]
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Regulation of the actin cytoskeleton by an interaction of IQGAP
related protein GAPA with Filamin and Cortexillin I

Subhanjan Mondal, Bhagyashri Burgute, Daniela Rieger, Rolf Müller,
Francisco Rivero, Jan Faix, Michael Schleicher, Angelika A. Noegel


PLoS ONE, accepted

Filamin and Cortexillin are F-actin crosslinking proteins in  
Dictyostelium
discoideum allowing actin filaments to form three-dimensional networks.
GAPA, an IQGAP related protein, is required for cytokinesis and  
localizes
to the cleavage furrow during cytokinesis. Here we describe a novel
interaction with Filamin which is required for cytokinesis and  
regulation
of the F-actin content. The interaction occurs through the actin binding
domain of Filamin and the GRD domain of GAPA. A similar interaction
takes place with Cortexillin I. We further report that Filamin  
associates
with Rac1a implying that filamin might act as a scaffold for small
GTPases. Filamin and activated Rac associate with GAPA to regulate
actin remodelling. Overexpression of filamin and GAPA in the various
strains suggests that GAPA regulates the actin cytoskeleton through
interaction with Filamin and that it controls cytokinesis through  
association
with Filamin and Cortexillin.


Submitted by Angelika Noegel [[log in to unmask]]
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Phospholipase D Controls Dictyostelium Development By
Regulating G Protein Signaling


Sibnath Ray, Yi Chen, Joanna Ayoung, Rachel Hanna and
Derrick Brazill*


Journal of Cellular Signalling, in press

Dictyostelium discoideum cells normally exist as individual amoebae,
but will enter a period of multicellular development upon starvation.
The initial stages of development involve the aggregation of individual
cells, using cAMP as a chemoattractant. Chemotaxis is initiated when
cAMP binds to its receptor, cAR1, and activates the associated
G protein, Galpha2betagamma. However, chemotaxis will not occur
unless there is a high density of starving cells present, as measured
by high levels of the secreted quorum sensing molecule, CMF.  We
previously demonstrated that cells lacking PldB bypass the need for
CMF and can aggregate at low cell density, whereas cells
overexpressing pldB do not aggregate even at high cell density.
Here, we found that PldB controlled both cAMP chemotaxis and cell
sorting.  PldB was also required by CMF to regulate G protein signaling.
Specifically, CMF used PldB, to regulate the dissociation of Galpha2
from Gbetagamma.  Using fluorescence resonance energy transfer
(FRET), we found that along with cAMP, CMF increased the
dissociation of the G protein.  In fact, CMF augmented the dissociation
induced by cAMP.  This augmentation was lost in cells lacking PldB.
PldB appears to mediate the CMF signal through the production of
phosphatidic acid, as exogenously added phosphatidic acid phenocopies
overexpression of pldB.  These results suggest that phospholipase D
activity is required for CMF to alter the kinetics of cAMP-induced G
protein signaling.


Submitted by  Derrick Brazill [[log in to unmask]]
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[End dictyNews, volume 35, number 8]