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dictyNews Volume 17 Number 01
Dicty News
Electronic Edition
Volume 17, number 1
June 30, 2001
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.
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Abstracts
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Requirements for the Adenylyl Cyclases in Development of Dictyostelium
Christophe Anjard, Fredrik Sderbom, and William F. Loomis
Center for Molecular Genetics, Division of Biology
University of California San Diego, La Jolla, CA 92093
Development, in press
ABSTRACT
It has been suggested that all intracellular signaling by cAMP during
development of Dictyostelium is mediated by the cAMP dependent protein
kinase, PKA, since cells carrying null mutations in the acaA gene that
encodes adenylyl cyclase can develop so as to form fruiting bodies under
some conditions if PKA is made constitutive by overexpressing the catalytic
subunit. However, a second adenylyl cyclase encoded by acrA has recently
been found that functions in a cell autonomous fashion during late
development. We have found that expression of a modified acaA gene rescues
acrA- mutant cells indicating that the only role played by ACR is to
produce cAMP. To determine whether cells lacking both adenylyl cyclase
genes can develop when PKA is constitutive we disrupted acrA in a acaA-
PKA-C(over) strain. When developed at high cell densities, acrA- acaA-
PKA-C(over) cells form mounds, express cell type specific genes at reduced
levels and secrete cellulose coats but do not form fruiting bodies or
significant numbers of viable spores. Thus, it appears that synthesis of
cAMP is required for spore differentiation in Dictyostelium even if PKA
activity is high.
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Myosin II Dynamics and Cortical Flow during Contractile Ring Formation in
Dictyostelium Cells.
Shigehiko Yumura
Department of Biology, Faculty of Science, Yamaguchi University, Yamaguchi
753-8512, Japan.
J. Cell Biol., In press.
Abstract
Myosin II is a major component of a contractile ring. In order to
examine if myosin II turns over in contractile rings, fluorescence of
GFP-myosin II expressed in Dictyostelium cells was bleached locally by laser
illumination, and the recovery was monitored. The fluorescence recovered
with a half time of 7.01 2.62 sec. This recovery was not caused by
lateral movement of myosin II from the non-bleached area but by an exchange
with endoplasmic myosin II. Similar experiments were performed in cells
expressing GFP-3ALA myosin II, of which three phosphorylatable threonine
residues were replaced with alanine residues. In this case, recovery was not
detected within a comparable time range. These results indicate that myosin
II in the contractile ring performs dynamic turnover via its heavy chain
phosphorylation. Because GFP-3ALA myosin II did not show the recovery, it
served as a useful marker of myosin II movement, which enabled us to
demonstrate cortical flow of myosin II toward the equator for the first
time. Thus, cortical flow accompanies the dynamic exchange of myosin II
during the formation of contractile rings.
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Brief Communications Genomics: Genes lost during evolution
Jeroen Roelofs and Peter J.M. van Haastert
Department of Biochemistry, University of Groningen, Nijenborgh 4, 9747
AG Groningen, The Netherlands
Nature 411, 1013 - 1014 (2001)
One of the main conclusions presented by the International Human Genome
Sequencing Consortium is that "hundreds of gene appear to have resulted from
horizontal gene transfer from bacteria at some point in the vertebrate
lineage". We noticed that a significant proportion of these human genes have
closely related orthologues in the primitive eukaryote Dictyostelium.
This observation supports independent gene loss in multiple lineages (worm,
fly, yeast, plants) rather than horizontal gene transfer from bacteria.
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The Dictyostelium homologue of mammalian soluble adenylyl cyclase
encodes a guanylyl cyclase
Jeroen Roelofs1, Marcel Meima2, Pauline Schaap2 and Peter J.M. Van
Haastert1
1) GBB, Department of Biochemistry, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, the Netherlands
2) Department of Biochemistry, University of Dundee, Dundee DD1 5EH, UK
EMBO journal, 2001, in press
Abstract
A new Dictyostelium cyclase gene was identified that encodes a protein
(sGC) with 35% similarity to mammalian soluble adenylyl cyclase (sAC). Gene
disruption of sGC has no effect on adenylyl cyclase activity and results
in more then a ten-fold reduction of guanylyl cyclase activity. The scg-
null mutants show reduced chemotactic sensitivity and aggregate poorly under
stringent conditions. With Mn2+/GTP as substrate most of sGC activity is
soluble, but with the more physiological Mg2+/GTP the activity is
detected in membranes and stimulated by GTPgS. Unexpectedly, orthologues
of sGC and sAC are present in bacteria and vertebrates, but absent from
Drosophila, C. elegans, Arabidopsis and S. cerevisiae.
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A Diffusion-Translocation Model for Gradient Sensing by Chemotactic
Cells
Marten Postma and Peter J.M. Van Haastert
GBB, Department of Biochemistry, University of Groningen, Nijenborgh 4,
9747 AG Groningen, the Netherlands
Biophysical Journal, 2001, in press
Abstract
Small chemotactic cells like Dictyostelium and neutrophils transduce shallow
spatial chemoattractant gradients into strongly localized intracellular
responses. We show that the capacity of a second messenger to establish and
maintain localized signals, is mainly determined by its dispersion range l =
(Dm/k-1)0.5, which must be small compared to the cells length. Therefore
short living second messengers (high k-1) with diffusion coefficients Dm
in the range of 0 - 5 mm2s-1 are most suitable. Additional to short
dispersion ranges, gradient sensing may include positive feedback mechanisms
that lead to local activation and global inhibition of second messenger
production. To introduce the essential non-linear amplification we have
investigated models in which one or more components of the signal
transduction cascade translocate from the cytosol to the second messenger
in the plasma membrane. A one-component model is able to amplify a 1.5-fold
difference of receptor activity over the cell length into a 15-fold difference
of second messenger concentration. Amplification can be improved considerably
by introducing an additional activating component that translocates to the
membrane. In both models, communication between the front and the back of the
cell is mediated by partial depletion of cytosolic components, which leads to
both, local activation and global inhibition. The results suggest that a
biochemically simple and general mechanism may explain various signal
localization phenomena, not only in chemotactic cells but also those occurring
in morphogenesis and cell differentiation.
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cAMP and DIF-1 repress the expression of the Dictyostelium MADS-box gene
srfA at early stages of development
Ricardo Escalante1and Leandro Sastre
Instituto de Investigaciones Biomdicas del Consejo Superior de
Investigaciones Cientficas , C/Arturo Duperier,4. 28029 Madrid. Spain.
B.B.R.C. (in press)
1 To whom correspondence should be addressed. Fax: 34-1-5854587. e-mail:
rescalante@iib.uam.es
Abstract
The MADS-box containing gene srfA from Dictyostelium discoideum codes for
a putative transcription factor that plays multiple roles in the
development of this social amoeba. We have investigated the regulation of
srfA gene expression after disaggregation of the cells from developing
structures. The steady-state level of srfA mRNA was strongly and
transiently induced shortly after disaggregation. srfA is maximally
expressed 20 minutes after cell disaggregation and decreases thereafter.
Induction was not dependent on protein synthesis, PKA, the kinase SplA and
SrfA itself. This phenomena does not occur when cells are disaggregated in
a small volume of buffer, suggesting the presence of extracellular
molecules that repress srfA gene expression. In order to test this
hypothesis, several well-known extracellular signaling molecules were
studied. We found that srfA mRNA induction can be efficiently repressed by
addition of exogenous cAMP and DIF-1 to the buffer in which the cells were
disaggregated. Addition of other extracellular compounds such as ammonia,
adenosine, SDF-1 and SDF-2 had no effect. srfA promoter P2, specifically
induced during slug migration, was responsible for this regulation by
extracellular compounds.
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[End Dicty News, volume 17, number 1]
