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dictyNews Volume 18 Number 11
Dicty News
Electronic Edition
Volume 18, number 11
June 15, 2002
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu.
Back issues of Dicty-News, the Dicty Reference database and other useful
information is available at DictyBase--http://dictybase.org.
======================
Position Available
======================
Research Assistant 1A, Department of Biological Sciences, Imperial
College of Science, Technology and Medicine
Salary range 19,681 - 21,503 plus 2,134 London Allowance per annum
Our Cell Dynamics group is located in the Sir Alexander Fleming Building,
South Kensington campus which offers excellent research facilities in
modern laboratories, including state-of the-art instrumentation.
Applications are invited for the post of Research Assistant 1A
(postdoctoral level) in our young and dynamic group to study the molecular
and cellular functions of a myosin I in actin polymerisation, cell cortex
dynamics and motility. Our aim is to understand the cellular and molecular
mechanims of cell motility, and particularly the function of actin-
dependent molecular motors. We previously demonstrated that MyoK, a class
I myosin from Dictyostelium, plays an important role in the maintenance of
cell cortex tension, motility and phagocytosis. As the components of the
complex machineries involved are evolutionarily conserved, their molecular
and cellular dissection in Dictyostelium is directly relevant to unravel
their functional importance in higher organisms. A description of this
and other projects of the group is available at:
http://www.bio.ic.ac.uk/research/tps/
The candidate should be motivated and enthusiastic about this area of
research. Proficiency in a variety of techniques, including cell culture,
single cell assays, genetic screenings, recombinant protein expression,
biophysical and enzymatic assays, and in a range of standard molecular
biology and biochemistry methods will be a determining asset for the
successful candidate.
For further details, please contact Dr. Thierry Soldati, Department of
Biological Sciences, Imperial College of Science, Technology and Medicine,
Exhibition Road, London, SW7 2AZ. E-mail t.soldati@ic.ac.uk. To apply,
please, send a full CV, a description of current research and interests,
and the name of two referees at the same address. Closing date July 15.
The work is supported by a BBSRC Research Grant for 3 years. The position
is available immediately or upon other agreement.
The College is committed to equality of opportunity.
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Abstracts
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Visualising PI3 kinase mediated cell-cell signalling during Dictyostelium
development
Dirk Dormann, Gerti Weijer, Carole A. Parent, Peter N. Devreotes and
Cornelis J. Weijer
Current Biology, in press
Background. Starving amoebae of Dictyostelium discoideum communicate by
relaying extracellular cAMP signals, which direct chemotactic movement,
resulting in the aggregation of thousands of cells into multicellular
aggregates. Both cAMP relay and chemotaxis require the activation of PI3
kinase signalling. The spatio-temporal dynamics of PI3 kinase signalling
can be followed in individual cells via the cAMP induced membrane
recruitment of a GFP tagged PH-domain containing protein, CRAC, which is
required for the activation of adenylylcyclase.
Results. We show that polarised periodic CRAC-GFP translocation occurs
during the aggregation and mound stages of development in response to
periodic cAMP signals. The duration of CRAC translocation to the membrane
is determined by the duration of the rising phase of the cAMP signal. The
system shows fast adaptation and responds to the rate of change of the
extracellular cAMP concentration. When the cells are in close contact it
takes 10 seconds for the signal to propagate from one cell to the next.
In slugs all cells show a permanent polarised PI3 kinase signalling in
their leading edge, which is dependent on cell-cell contact.
Conclusions. Measuring the redistribution of GFP tagged CRAC has enabled
us to study the dynamics of PI3 kinase mediated cell-cell communication
at the individual cell level in the multicellular stages of Dictyostelium
development. This approach should also be useful to study the interactions
between cell-cell signalling, cell polarisation and movement in the
development other organisms.
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Dictyostelium mobile elements: strategies to amplify in a compact genome
Thomas Winckler(1), Theodor Dingermann(1) and Gernot Glckner(2)
(1) Institut fuer Pharmazeutische Biologie, Universitaett Frankfurt/M.
(Biozentrum), Marie-Curie-Strasse 9, D-60439 Frankfurt am Main, Germany and
(2) IMB Jena, Department of Genome Analysis, Beutenbergstrasse 11, D-07745
Jena, Germany
Review, Cell. Mol. Life Sci., in press
Dictyostelium discoideum is a eukaryotic microorganism that is attractive for
the study of fundamental biological phenomena such as cell-cell
communication, formation of multicellularity, cell differentiation, and
morphogenesis. Large-scale sequencing of the D. discoideum genome has
provided new insights into evolutionary strategies evolved by transposable
elements (TEs) to settle in compact microbial genomes and to maintain active
populations over evolutionary time. The high gene density (about 1 gene/2.6
kb) of the D. discoideum genome leaves limited space for selfish molecular
invaders to move and amplify without causing deleterious mutations that
eradicate their host. Targeting of tRNA gene loci appears to be a generally
successful strategy for TEs residing in compact genomes to insert away from
coding regions. In D. discoideum tRNA gene-targeted retrotransposition has
evolved independently at least three times by both non-long terminal repeat
(LTR) retrotransposons and retrovirus-like LTR retrotransposons. Unlike the
nonspecifically inserting D. discoideum TEs, which have a strong tendency to
insert into preexisting TE copies and form large and complex clusters near
the ends of chromosomes, the tRNA gene-targeted retrotransposons have managed
to occupy 75% of the tRNA gene loci spread on chromosome 2 and represent 80%
of the TEs recognized on the assembled central 6.5 Mb part of chromosome 2.
In this review we update the available information about D. discoideum TEs
which emerges both from previous work and current large-scale genome
sequencing with special emphasis on the fact that tRNA genes are principle
determinants of retrotransposon insertions into the D. discoideum genome.
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Ga-Mediated Inhibition of Developmental Signal-Response
Joseph A. Brzostowski, Cynthia Johnson + and Alan R. Kimmel
Laboratory of Cellular and Developmental Biology, NIDDK (Bldg. 50/3351)
National Institutes of Health, Bethesda, MD 20892-8028. +The University of
Texas Southwestern Medical Center Dallas, TX 75390. ark@helix.nih.gov
Current Biology, in press.
SUMMARY
Background: Seven-transmembrane receptor (7-TMR)-G protein networks are
molecular sensors of extracellular signals in all eukarya. These pathways
cycle through activated (sensitized) and inhibited (de-sensitized) states,
and, while many of the molecular components for signal activation have been
described, inhibitory mechanisms are not well characterized. In Dictyostelium,
7-TM cAMP receptors direct chemotaxis and development, but also regulate the
periodic synthesis of their own ligand, the chemoattractant/morphogen cAMP.
We now demonstrate through loss-of-function, gain-of -function studies that
the novel heterotrimeric Ga9 protein subunit regulates an inhibitory pathway
during early Dictyostelium development for cAMP signal-response.
Results: ga9-null cells form more cAMP signaling centers, are more resistant
to compounds that inhibit cAMP signaling, and complete aggregation sooner and
at lower cell densities than wild-type. These phentoypes are consistent with
the loss of an inhibitory signaling pathway during development of ga9-null
cells. Cells expressing constitutively activated Ga9 are defective in cAMP
signal center formation and development at low cell density and display an
increased sensitivity to cAMP signal inhibition that is characteristic of
enhanced suppression of cAMP signal-response. Finally, we demonstrate that
ga9-null cells, which have been co-developed with a majority of wild-type
cells, primarily establish cAMP signaling centers and are able to non-
autonomously direct wild-type cells to adopt a ga9-null-like phenotype.
Conclusions: We suggest that Ga9 functions in an inhibitory-feedback pathway
that regulates cAMP signal center formation and propagation. Ga9 may be part
of the mechanism that regulates lateral signal inhibition or that modulates
receptor de-sensitization.
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Cell polarity and locomotion, as well as endocytosis, depend on NSF.
Chris R.L. Thompson and Mark S. Bretscher
MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, England.
Development, in press
NEM-sensitive factor (NSF) is an essential protein required during membrane
transport. We replaced part of the endogenous D. discoideum NSF gene (nsfA)
by a pcr-mutagenised library and isolated 11 mutants temperature-sensitive
(ts) for growth. Two of these have been studied in detail. As expected, both
are ts for FITC-dextran uptake by macropinocytosis, for internalising their
surface membrane (monitored with FM1-43) and for phagocytosis. However,
after 10-20 minutes at 28C, they round up and cease to chemotax, move or
cap ConA receptors. They fully recover when returned to 22C. These cells
carry out a normal "cringe" reaction in response to cAMP, indicating that
the actin cytoskeleton and this signal transduction pathway are still
functional at 28C . The behaviour of these mutants shows that NSF-catalysed
processes are required not only for the different endocytic cycles but also
for the maintenance of cell polarity. As cell locomotion depends on a cell
having a polarity, the mutants stop moving at high temperature. A tentative
model is proposed to explain the surprising link between membrane recycling
and cell polarity revealed here.
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[End Dicty News, volume 18, number 11]
