dictyNews
Electronic Edition
Volume 39, number 2
January 18, 2013
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Abstracts
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Asymmetric PTEN Distribution Regulated by Spatial Heterogeneity in
Membrane-Binding State Transitions
Satomi Matsuoka, Tatsuo Shibata, Masahiro Ueda
PLOS Computational Biology, in press
The molecular mechanisms that underlie asymmetric PTEN distribution
at the posterior of polarized motile cells and regulate anterior pseudopod
formation were addressed by novel single-molecule tracking analysis.
Heterogeneity in the lateral mobility of PTEN on a membrane indicated
the existence of three membrane-binding states with different diffusion
coefficients and membrane-binding lifetimes. The stochastic state
transition kinetics of PTEN among these three states were suggested to
be regulated spatially along the cell polarity such that only the stable
binding state is selectively suppressed at the anterior membrane to
cause local PTEN depletion. By incorporating experimentally observed
kinetic parameters into a simple mathematical model, the asymmetric
PTEN distribution can be explained quantitatively to illustrate the
regulatory mechanisms for cellular asymmetry based on an essential
causal link between individual stochastic reactions and stable
localizations of the ensemble.
Submitted by Satomi Matsuoka [[log in to unmask]]
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Dictyostelium ACAP-A is an ArfGAP involved in cytokinesis, cell
migration and actin cytoskeleton dynamics
Marco Dias, Cédric Blanc, Nelcy Thazar-Poulot, Sabrina Ben Larbi,
Pierre Cosson, and Francois Letourneur
Journal of Cell Science, in press
ACAPs and ASAPs are Arf-GTPase-activating proteins with BAR, PH,
GAP and ankyrin repeat domains and are known to regulate vesicular
traffic and actin cytoskeleton dynamics in mammalian cells. The amoeba
Dictyostelium has only two proteins with this domain organization instead
of six in human, enabling a more precise functional analysis. Genetic
invalidation of acapA, resulted in multinucleated cells with cytokinesis
defects. Mutant acapA- cells were hardly motile and their multicellular
development was significantly delayed. In addition, formation of filopodial
protrusions was deficient in these cells. Conversely, re-expression of
ACAP-A-GFP resulted in numerous and long filopodia-like protrusions.
Mutagenesis studies showed that ACAP-A actin remodeling function was
dependent on its ability to activate its substrate, the small GTPase ArfA.
Likewise, the expression of a constitutively active ArfA•GTP mutant in
wild-type cells led to a significant reduction of filopodia length. Together
our data support a role for ACAP-A in the control of the actin cytoskeleton
organization and dynamics through an ArfA-dependent mechanism.
Submitted by François Letourneur [[log in to unmask]]
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The precision with which single cells of Dictyostelium discoideum can
locate a source of cyclic AMP.
Abha Chopra(1) and Vidyanand Nanjundiah(2)
(1)Institute for Immunology and Infectious Diseases, Murdoch University,
Perth, WA 6150, Australia;
(2)Department of Molecular Reproduction and Genetics, Indian Institute of
Science, Bangalore 560012, India)
Chaos, Solitons & Fractals, accepted
When a starved wild-type amoeba of Dictyostelium discoideum is stimulated
by cyclic AMP emanating from a nearby point source, it responds by putting
out a hollow balloon-like membrane extension followed by a pseudopod. The
effect of the stimulus is to influence the position where either of these
protrusions is made on the cell rather than to cause them to be made.
Because the pseudopod forms perpendicular to the cell surface, the site of
formation is a measure of the precision with which a cell can locate the
cAMP source. Cells beyond 1hr of starvation respond non-randomly with a
precision that improves steadily thereafter. A cell that is starved for
1-2hrs can locate the source accurately 43% of the time; the corresponding
figure for a cell starved for 6-7hrs is 87%. The response always displays
scatter; and population-level heterogeneity reflects stochasticity in the
behaviour of a single cell. From the angular distribution of the response
its maximum information content is estimated to be 2 to 3 bits. In
summary, we quantitatively demonstrate the stochastic nature of the
directional response and the increase in its accuracy over time.
Submitted by Vidyanand Nanjundiah [[log in to unmask]]
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Mass spectrometric analysis of neutral and anionic N-glycans from a
Dictyostelium discoideum model for human congenital disorder of
glycosylation CDG IL.
Hykollari A, Balog CI, Rendic; D, Braulke T, Wilson IB, Paschinger K.
J Proteome Res. 2013 Jan 15. [Epub ahead of print]
The HL241 mutant strain of the cellular slime mould Dictyostelium
discoideum is a potential model for human congenital disorder of
glycosylation type IL (ALG9-CDG) and has been previously predicted
to possess a lower degree of modification of its N-glycans with anionic
moieties than the parental wild-type. In this study, we first showed that
this strain has a premature stop codon in its alg9 mannosyltransferase
gene compatible with the occurrence of truncated N-glycans. These
were subject to an optimised analytical workflow, considering that mass
spectrometry of acidic glycans often presents challenges due to neutral
loss and suppression effects. Therefore, the protein-bound N-glycans
were first fractionated, after serial enzymatic release, by solid phase
extraction. Then primarily single glycan species were isolated by mixed
hydrophilic-interaction/anion-exchange or reversed phase HPLC and
analysed using chemical and enzymatic treatments and MS/MS. We
show that protein-linked N-glycans of the mutant are of reduced size as
compared to those of wild-type AX3, but still contain core alpha1,3-fucose,
intersecting N-acetylglucosamine, bisecting N-acetylglucosamine,
methylphosphate, phosphate and sulphate residues. We observe that a
single N-glycan can carry up to four of these six possible modifications.
Due to the improved analytical procedures, we reveal fuller details
regarding the N-glycomic potential of this fascinating model organism.
Submitted by Iain Wilson [[log in to unmask]]
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Dictyostelium development shows a novel pattern of evolutionary
conservation
Xianjun Tian, Joan E. Strassmann and David C. Queller
Molecular Biology and Evolution, in press
Von Baer’s law states that early stages of animal development are the
most conserved. More recent evidence supports a modified "hourglass"
pattern in which an early but somewhat later stage is most conserved.
Both patterns have been explained by the relative complexity of either
temporal or spatial interactions; the greatest conservation and lowest
evolvability occur at the time of the most complex interactions, because
these cause larger effects that are harder for selection to alter. This
general kind of explanation might apply universally across independent
multicellular systems, as supported by the recent finding of the hourglass
pattern in plants. We use RNA-seq expression data from the development
of the slime mold Dictyostelium to demonstrate that it does not follow either
of the two canonical patterns but instead tends to show the strongest
conservation and weakest evolvability late in development. We propose
that this is consistent with a version of the spatial constraints model,
modified for organisms that never achieve a high degree of developmental
modularity.
Submitted by David Queller [[log in to unmask]]
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A non-mitotic CENP-E homolog in Dictyostelium discoideum with slow
motor activity
Süleyman Kösem (1,3), Zeynep Ökten (1,3), Thi-Hieu Ho (1), Gudrun
Trommler (1), Michael P. Koonce (2), Matthias Samereier (1),
Annette Müller-Taubenberger (1)
(1) Institute for Anatomy and Cell Biology, Ludwig Maximilian University of
Munich, Schillerstr. 42, 80336 Munich, Germany
(2) 2Division of Translational Medicine, Wadsworth Center, Empire State
Plaza, Albany, New York 12201-0509, USA
(3) Present address: Physics Department E22, Technical University Munich,
James-Franck-Str. 1, 85748 Garching, Germany
Biochem. Biophys. Res. Comm., in press
Kinesins are ATP-dependent molecular motors that mediate unidirectional
intracellular transport along microtubules. Dictyostelium discoideum has 13
different kinesin isoforms including two members of the kinesin-7 family,
Kif4 and Kif11. While Kif4 is structurally and functionally related to
centromere-associated CENP-E proteins involved in the transport of
chromosomes to the poles during mitosis, the function of the unusually
short CENP-E variant Kif11 is unclear. Here we show that orthologs of short
CENP-E variants are present in plants and fungi, and analyze functional
properties of the Dictyostelium CENP-E version, Kif11. Gene knockout
mutants reveal that Kif11 is not required for mitosis or development.
Imaging of GFP-labeled Kif11 expressing Dictyostelium cells indicates that
Kif11 is a plus-end directed motor that accumulates at microtubule plus
ends. By multiple motor gliding assays, we show that Kif11 moves with an
average velocity of 38 nm per second, thus defining Kif11 as a very slow
motor. The activity of the Kif11 motor appears to be modulated via
interactions with the non-catalytic tail region. Our work highlights a
subclass of kinesin-7-like motors that function outside of a role in mitosis.
Submitted by [Annette Müller-Taubenberger [[log in to unmask]]
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Temporal and non-permanent division of labor during sorocarp formation in
the social amoeba Acytostelium subglobosum
Kurato Mohri, Yu Kiyota, Hidekazu Kuwayama and Hideko Urushihara*
Developmental Biology, in press
Somatic cell differentiation is crucial for the development of multicellular
organisms. While the development of a fruiting body in Dictyostelium
discoideum represents a simple model of this process with separation of
stalk cells from the spore lineage, that of Acytostelium subglobosum is not
accompanied by cell type separation. This species produces acellular stalks
and, seemingly, all aggregated amoebae become spores; however, it
possesses homologs for the stalk-cell marker genes of D. discoideum. In
this study, we analyzed the spatio-temporal expression of A. subglobosum
orthologs for D. discoideum stalk- or spore-lineage markers to clarify the
developmental process of A. subglobosum. We first found that the prespore
vesicles, which contained spore coat proteins, started to accumulate in the
tip region and were observed in the entire sorogen throughout later
development, confirming that all A. subglobosum cells became spores. The
expression of a stalk-lineage gene ortholog, As-ecmA, started at the mound
stage and was prominent in the protruding sorogen. Although two spore-
lineage gene orthologs, As-cotD1 and -cotD2, were likewise detected shortly
after cell aggregation and increased in intensity until tip formation, their
expression diminished in the protruding sorogen. Double-fluorescence
staining of these prestalk and prespore marker genes revealed that the
expression of these marker genes was mutually exclusive and that
expression switching occurred in the early tip. Our results indicate that
A. subglobosum cells become committed to the spore lineage first, and
then, while keeping this commitment intact, participate in stalk formation.
Instead of the permanent division of labor observed in D. discoideum,
A. subglobosum produces fruiting bodies by all cells contributing to the
formation of the stalk as well as forming spores.
Submitted by Hideko Urushihara [[log in to unmask]]
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[End dictyNews, volume 39, number 2]
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