dictyNews
Electronic Edition
Volume 34, number 14
May 7, 2010

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=========
Abstracts
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Complex genotype interactions influence social fitness during the  
developmental
phase of the social amoeba Dictyostelium discoideum

Neil J. Buttery1, Christopher R. L. Thompson1,* and Jason B. Wolf2,*

1Faculty of Life Sciences, The University of Manchester, Michael Smith  
Building,
Oxford Rd, Manchester, M13 9PT, UK.
2Departmemt of Biology & Biochemistry, University of Bath, Claverton  
Down,
Bath BA2 7AY, UK.

*Corresponding authors


Journal of Evolutionary Biology, in press

Abstract: When individuals interact, phenotypic variation can be  
partitioned into
direct genetic effects (DGEs) of the individuals’ own genotypes,  
indirect genetic
effects (IGEs) of their social partners’ genotypes and epistatic  
interactions between
the genotypes of interacting individuals (‘genotype-by-genotype (G×G)  
epistasis’).
These components can all play important roles in evolutionary  
processes, but few
empirical studies have examined their importance. The social amoeba  
Dictyostelium
discoideum provides an ideal system to measure these effects during  
social
interactions and development. When starved, free-living amoebae  
aggregate and
differentiate into a multicellular fruiting body with a dead stalk  
that holds aloft viable
spores. By measuring interactions among a set of natural strains we  
quantify DGEs,
IGEs and G×G epistasis affecting spore formation. We find that DGEs  
explain most
of the phenotypic variance (57.6%) while IGEs explain a smaller  
(13.3%) but highly
significant component.  Interestingly, G×G epistasis explains nearly a  
quarter of the
variance (23.0%), highlighting the complex nature of genotype  
interactions. These
results demonstrate the large impact that social interactions can have  
on
development and suggest that social effects should play an important  
role in
developmental evolution in this system.


Submitted by Chris Thompson [[log in to unmask]]
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Unconventional secretion of AcbA in Dictyostelium discoideum through a
vesicular intermediate.

Matthew Cabral, Christophe Anjard, Vivek Malhotra, William F. Loomis,
and Adam Kuspa

Verna and Marrs McLean Department Biochemistry and Molecular Biology and
Department of Molecular and Human Genetics, Baylor College of Medicine,
Houston, Texas, USA, 77030.
Center for Molecular Genetics, Division of Biological Sciences,  
University of
California San Diego, La Jolla, CA 92093.


Eukaryotic Cell, in press

The acyl-CoA binding protein, AcbA, is secreted unconventionally and  
processed
into SDF-2, a peptide that coordinates sporulation in Dictyostelium.  
We report that
AcbA is localized in vesicles that accumulate in the cortex of  
prespore cells just
prior to sporulation.  These vesicles are not observed after cells are  
stimulated to
release AcbA, but remain visible after stimulation in cells lacking  
the Golgi
reassembly stacking protein (GRASP). Acyl-CoA binding is required for  
the
inclusion of AcbA in these vesicles, and the secretion of AcbA requires
N-ethylmaleimide sensitive factor (NSF).  About 1 percent of the total  
cellular
AcbA can be purified within membrane-bound vesicles.  The yield of  
vesicles
decreases dramatically when purified from wild-type cells that were  
stimulated to
release AcbA, whereas the yield from GRASP mutant cells was only  
modestly
altered by stimulation.  We suggest these AcbA-containing vesicles are  
secretion
intermediates and that GRASP functions at a late step leading to the  
docking/fusion
of these vesicles at the cell surface.


Submitted by Adam Kuspa [[log in to unmask]]
--------------------------------------------------------------------------------


Genetic evidence for concerted control of actin dynamics in cytokinesis,
endocytic traffic, and cell motility by coronin and Aip1

Hellen C. Ishikawa-Ankerhold1, Günther Gerisch1, and
Annette Müller-Taubenberger2

1 Max Planck Institute for Biochemistry, Am Klopferspitz 18,
82152 Martinsried, Germany
2 Institute for Cell Biology and Center for Integrated Protein Science  
Munich (CIPSM),
Ludwig Maximilians University Munich, Schillerstr. 42,
80336 München, Germany


Cytoskeleton, in press
	
Coronin and actin-interacting protein 1 (Aip1) are actin-binding  
proteins that by
different mechanisms inhibit actin polymerization or enhance the  
disassembly of
actin filaments. Cells of Dictyostelium discoideum lacking both  
proteins are retarded
in growth and early development and often fail to proceed to fruiting  
body formation.
Coronin/Aip1-null cells show numerous surface protrusions enriched in  
filamentous
actin and cofilin. We show that the double-null cells are  
characterized by an increase
in filamentous actin that causes a thickening of the cell cortex. This  
imbalance has
severe consequences for processes that rely on the dynamic  
reorganization of the
actin cytoskeleton as cell motility, cytokinesis and endocytosis.  
Although cell motility
is considerably slowed down, the double-mutant cells are still capable  
of orientating
in a gradient of chemoattractant. The cytokinesis defect is caused by  
the lack of proper
cleavage furrow formation, a defect that is partially rescued by low  
concentrations of
latrunculin A, an inhibitor of actin polymerization. Furthermore, we  
demonstrate that
the disassembly of the actin coat after phagocytic or macropinocytic  
uptake is
significantly delayed in the double-mutant cells. Our results prove  
that coronin and
Aip1 are important effectors that act together in maintaining the  
balance of actin
polymerization and depolymerization in living cells.


Submitted by Annette Müller-Taubenberger [[log in to unmask]]
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[End dictyNews, volume 34, number 14]