dictyNews
Electronic Edition
Volume 39, number 19
July 5, 2013
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Abstracts
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The AmpA protein functions by different mechanisms to influence early cell
type specification and to modulate cell adhesion and actin polymerization
in Dictyostelium discoideum
Hoa N. Cost, Elizabeth F. Noratel and Daphne D. Blumberg
Department of Biological Sciences, University of Maryland Baltimore County
1000 Hilltop Circle, Baltimore, Maryland 21250
Differentiation, in press.
The Dictyostelium discoideum ampA gene encodes a multifunctional regulator
protein that modulates cell-cell and cell-substrate adhesions and actin
polymerization during growth and is necessary for correct cell type
specification and patterning during development. Insertional inactivation
of the ampA gene results in defects that define two distinct roles for the
ampA gene during development. AmpA is necessary in a non-cell autonomous
manner to prevent premature expression of a prespore gene marker. It is
also necessary in a cell autonomous manner for the anterior like cells,
which express the ampA gene, to migrate to the upper cup during
culmination. It is also necessary to prevent excessive cell-cell
agglutination when cells are developed in a submerged suspension culture.
Here we demonstrate that a supernatant source of AmpA protein, added
extracellularly, can prevent the premature mis-expression of the prespore
marker. Synthetic oligopeptides are used to identify the domain of the
AmpA protein that is important for preventing cells from mis-expressing
the prespore gene. We further demonstrate that a factor capable of
inducing additional cells to express the prespore gene marker accumulates
extracellularly in the absence of AmpA protein. While secreted AmpA acts
extracellularly to suppress prespore gene expression, the effects of AmpA
on cell agglutination and on actin polymerization in growing cells are not
due to an extracellular role of secreted AmpA protein. Rather, these
effects appear to reflect a distinct cell autonomous role of the ampA
gene. Finally, we show that secretion of AmpA protein is brought about by
elevating the levels of expression of ampA so that the protein accumulates
to an excessive level.
Submitted by Daphne Blumberg [[log in to unmask]]
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ForC lacks canonical formin activity but bundles actin filaments and is
required and multicellular development of Dictyostelium cells
Alexander Junemann (1), Moritz Winterhoff (1), Benjamin Nordholz (1),
Klemens Rottner (2,3), Ludwig Eichinger (4), Ralph Gräf (5), and Jan Faix (1)
(1) Institute for Biophysical Chemistry, Hannover Medical School,
Carl-Neuberg Straße 1, 30625 Hannover, Germany;
(2) Institute of Genetics, University of Bonn, Karlrobert-Kreiten-Strasse 13,
53115 Bonn, Germany;
(3) Helmholtz Centre for Infection Research, Inhoffenstrasse 7, 38124
Braunschweig, Germany;
(4) Centre for Biochemistry, Medical Faculty, University of Cologne,
Joseph-Stelzmann-Str. 52, 50931 Cologne, Germany;
(5) Department of Cell Biology, University of Potsdam,
Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany.
EJCB, in press
Diaphanous-related formins (DRFs) drive the nucleation and elongation
of linear actin filaments downstream of Rho GTPase signaling pathways.
Dictyostelium formin C (ForC) resembles a DRF, except that it lacks a
genuine formin homology domain 1 (FH1), raising the questions whether
or not ForC can nucleate and elongate actin filaments. We found that a
recombinant ForC-FH2 fragment does not nucleate actin polymerization,
but moderately decreases the rate of spontaneous actin assembly and
disassembly, although the barbed-end elongation rate in the presence
of the formin was not markedly changed. However, the protein bound
and crosslinked actin filaments into loose bundles of mixed polarity.
Furthermore, ForC is an important regulator of morphogenesis since
ForC-null cells are severely impaired in development resulting in the
formation of aberrant fruiting bodies. Immunoblotting revealed that ForC
is absent during growth, but becomes detectable at the onset of early
aggregation when cells chemotactically stream together to form a
multicellular organism, and peaks around the culmination stage.
Fluorescence microscopy of cells ectopically expressing a GFP-tagged,
N-terminal ForC fragment showed its prominent accumulation in the
leading edge, suggesting that ForC may play a role in cell migration. In
agreement with its expression profile, no defects were observed in
random migration of vegetative mutant cells. Notably, chemotaxis of
starved cells towards a source of cAMP was severely impaired as
opposed to control. This was, however, largely due to a marked
developmental delay of the mutant, as evidenced by the expression profile
of the early developmental marker csA. In line with this, chemotaxis was
almost restored to wild type levels after prolonged starvation. Finally, we
observed a complete failure of phototaxis due to abolished slug formation
and a massive reduction of spores consistent with forC promoter-driven
expression of ß-galactosidase in prespore cells. Together, these findings
demonstrate ForC to be critically involved in regulation of the cytoskeleton
during various stages of development.
Submitted by Jan Faix [[log in to unmask]]
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[End dictyNews, volume 39, number 19]
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