dictyNews
Electronic Edition
Volume 35, number 17
Dec 10, 2010
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Abstracts
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Molecular mechanism of Ena/VASP-mediated actin filament elongation
1 Dennis Breitsprecher, 1 Antje K. Kiesewetter, 1 Joern Linkner,
2 Marlene Vinzenz, ,4 Theresia E.B. Stradal, 2 J. Victor Small,
1 Ute Curth, 4 Richard B. Dickinson and 1 Jan Faix.
1) Institute for Biophysical Chemistry, Hannover Medical School, Germany.
2) Institute of Molecular Biotechnology, Austrian Academy of Sciences, Austria.
3) Institute for Molecular Cell Biology, University of Muenster, Germany.
4) Department of Chemical Engineering, University of Florida, USA.
EMBO Journal, in press
Ena/VASP proteins are implicated in a variety of fundamental cellular processes
including axon guidance and cell migration. In vitro, they enhance elongation of
actin filaments, but at rates differing in nearly an order of magnitude according
to species, raising questions about the molecular determinants of rate control.
Chimeras from fast and slow elongating VASP proteins were generated and their
ability to promote actin polymerization and to bind G-actin was assessed. By in
vitro TIRF microscopy as well as thermodynamic and kinetic analyses we show
that the velocity of VASP-mediated filament elongation depends on G-actin
recruitment by the WH2 motif. Comparison of the experimentally observed
elongation rates with a quantitative mathematical model moreover revealed that
Ena/VASP-mediated filament elongation displays a saturation dependence on
the actin monomer concentration, implying that Ena/VASP proteins, independent
of species, are fully saturated with actin in vivo and generally act as potent
filament elongators. Moreover, our data showed that spontaneous addition of
monomers does not occur during processive VASP-mediated filament elongation
on surfaces, suggesting that most filament formation in cells is actively controlled.
Submitted by Jan Faix [[log in to unmask]]
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The Dictyostelium Model for Mitochondrial Disease
Lisa M. Francione,1 Sarah J. Annesley1, Sergio Carilla-Latorre2,
Ricardo Escalante2, Paul R. Fisher1*
1 Department of Microbiology, La Trobe University, VIC 3086, Australia.
2 Instituto de Investigaciones Biomédicas Alberto Sols, CSIC-UAM,
Arturo Duperier 4, 28029-Madrid, Spain.
Seminars in Cell and Developmental Biology, in press.
Mitochondrial diseases are a diverse family of genetic disorders caused
by mutations affecting mitochondrial proteins encoded in either the nuclear
or the mitochondrial genome. By impairing mitochondrial oxidative
phosphorylation, they compromise cellular energy production and the
downstream consequences in humans are a bewilderingly complex array
of signs and symptoms that can affect any of the major organ systems in
unpredictable combinations. This complexity and unpredictability has
limited our understanding of the cytopathological consequences of
mitochondrial dysfunction. By contrast, in Dictyostelium the mitochondrial
disease phenotypes are consistent, measurable “readouts” of dysregulated
intracellular signalling pathways. When the underlying genetic defects
would produce coordinate, generalized deficiencies in multiple
mitochondrial respiratory complexes, the disease phenotypes are mediated
by chronic activation of an energy-sensing protein kinase, AMPK
(AMP-activated protein kinase). This chronic AMPK hyperactivity maintains
mitochondrial mass and cellular ATP concentrations at normal levels, but
chronically impairs growth, cell cycle progression, multicellular development,
photosensory and thermosensory signal transduction. It also causes the
cells to support greater proliferation of the intracellular bacterial pathogen,
Legionella pneumophila. Notably however, phagocytic and macropinocytic
nutrient uptake are impervious both to AMPK signalling and to these types
of mitochondrial dysfunction. Surprisingly, a Complex I-specific deficiency
(midA knockout) not only causes the foregoing AMPK-mediated defects,
but also produces a dramatic deficit in endocytic nutrient uptake accompanied
by an additional secondary defect in growth. More restricted and specific
phenotypic outcomes are produced by knocking out genes for nuclear-encoded
mitochondrial proteins that are not required for respiration. The Dictyostelium
model for mitochondrial disease has thus revealed consistent patterns of
sublethal dysregulation of intracellular signalling pathways that are produced
by different types of underlying mitochondrial dysfunction.
Submitted by Paul Fisher [[log in to unmask]]
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The NDR family kinase NdrA of Dictyostelium localizes to the centrosome
and is required for efficient phagocytosis
Peter M. Kastner, Michael Schleicher, Annette Müller-Taubenberger
Institute for Anatomy and Cell Biology, Ludwig Maximilian University of Munich,
Schillerstr. 42, 80336 Munich, Germany
Traffic, in press
Dictyostelium discoideum cells are professional phagocytes that provide an
easily accessible system to gain insights into the mechanisms and the regulatory
machinery controlling phagocytosis. Here, we describe a novel function for NDR
(nuclear Dbf2-related) family kinases in phagocytosis of D. discoideum. Deletion
of one of the four NDR kinases of D. discoideum, NdrA, resulted in impaired cell
growth caused by reduced phagocytosis rates. Detailed analysis of NdrA-null
cells revealed that the formation of phagocytic cups was delayed. Microscopic
investigations showed that NdrA localizes to centrosomes, and NdrA was also
identified in isolated centrosome preparations. The localization of NdrA is
regulated during the cell cycle. In prophase, NdrA disappears from the
centrosome and forms a cloud-like structure around the spindle, which is
totally absent during later stages until completion of mitosis. Our results
suggest that a signal which originates from the NdrA kinase at the centrosome
affects the efficiency of phagocytosis. We assume that in NdrA-null cells the
defects in phagocytosis may be caused by an impairment of vesicle trafficking
which is involved in providing new membrane at the sites of particle uptake.
Submitted by Annette Müller-Taubenberger [[log in to unmask]]
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Primitive agriculture in a social amoeba
Debra A. Brock, Tracy E. Douglas, David C. Queller, and Joan E. Strassmann
Department of Ecology and Evolutionary Biology, Rice University,
6100 Main Street, Houston, Texas 77005, USA
Nature, in press
Agriculture has been a large part of the ecological success of humans. A
handful of animals, notably the fungus growing ants, termites, and ambrosia
beetles, have advanced agriculture that involves dispersing and seeding of
food propagules, cultivation of the crop, and sustainable harvesting. More
primitive examples, which could be called husbandry because they involve
fewer adaptations, include marine snails farming intertidal fungi and damselfish
farming algae. Recent work has shown that microorganisms are surprisingly
like animals in having sophisticated behaviours such as cooperation,
communication, and recognition, as well as many kinds of symbioses. We now
show that the social amoeba Dictyostelium discoideum exhibits a primitive
farming symbiosis that includes dispersal and prudent harvesting of the crop.
About a third of wild-collected clones engage in husbandry of bacteria. Instead
of consuming all bacteria in their patch, they stop feeding early and incorporate
bacteria into their fruiting bodies. They then carry bacteria during spore
dispersal and can seed a new food crop, a major advantage if edible bacteria
are lacking at the new site. However, if they arrive at sites already containing
appropriate bacteria, the costs of early feeding cessation are not compensated,
which may account for the dichotomous nature of this farming symbiosis. The
striking convergent evolution between bacterial husbandry in social amoebas
and fungus farming in social insects makes sense because multigenerational
benefits of farming go to already-established kin groups.
Submitted by Debbie Brock [[log in to unmask]]
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[End dictyNews, volume 35, number 17]
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